US20140272614A1

US20140272614A1 - Integrated power generation and carbon capture using fuel cells

Info

Publication number: US20140272614A1
Application number: US14/197,430
Authority: US
Inventors: Paul J. Berlowitz; Timothy Andrew Barckholtz; Frank Hershkowitz
Original assignee: ExxonMobil Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 2013-03-15
Filing date: 2014-03-05
Publication date: 2014-09-18
Also published as: US20140272615A1; US20140272616A1; US9774053B2; US20140272613A1

Abstract

Systems and methods are provided for capturing CO₂from a combustion source using molten carbonate fuel cells (MCFCs). The fuel cells are operated to have a reduced anode fuel utilization. Optionally, at least a portion of the anode exhaust is recycled for use as a fuel for the combustion source. Optionally, a second portion of the anode exhaust is recycled for use as part of an anode input stream. This can allow for a reduction in the amount of fuel cell area required for separating CO₂from the combustion source exhaust and/or modifications in how the fuel cells are operated.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Application Ser. Nos. 61/788,628, 61/787,587, 61/787,697, and 61/787,879, each filed on Mar. 15, 2013; which are each incorporated by reference herein in their entirety, as well as the three U.S. non-provisional applications filed on even date herewith and also claiming priority to the four provisional applications enumerated above, each of which non-provisional applications also being to incorporated by reference herein in their entirety.

FIELD OF THE INVENTION

In various aspects, the invention is related to low emission power production with separation and/or capture of resulting emissions via integration of molten carbonate fuel cells with a combustion power source.

BACKGROUND OF THE INVENTION

Capture of gases emitted from power plants is an area of increasing interest. Power plants based on the combustion of fossil fuels (such as petroleum, natural gas, or coal) generate carbon dioxide as a by-product of the reaction. Historically this carbon dioxide has been released into the atmosphere after combustion. However, it is becoming increasingly desirable to identify ways to find alternative uses for the carbon dioxide generated during combustion.
One option for managing the carbon dioxide generated from a combustion reaction is to use a capture process to separate the CO₂from the other gases in the combustion exhaust. An example of a traditional method for capturing carbon is passing the exhaust stream through an amine scrubber. While an amine scrubber can be effective for separating CO₂from an exhaust stream, there are several disadvantages. In particular, energy is required to operate the amine scrubber and/or modify the temperature and pressure of the exhaust stream to be suitable for passing through an amine scrubber. The energy required for CO₂separation reduces the overall efficiency of the power generation process.
In order to offset the power required for CO₂capture, one option is to use a molten carbonate fuel cell to assist in CO₂separation. The fuel cell reactions that cause transport of CO₂from the cathode portion of the fuel cell to the anode portion of the fuel cell can also result in generation of electricity. However, conventional combinations of a combustion powered turbine or generator with fuel cells for carbon separation have resulted in a net reduction in power generation efficiency per unit of fuel consumed.
An article in the Journal of Fuel Cell Science and Technology (G. Manzolini et. al. J. Fuel Cell Sci. and Tech., Vol. 9, February 2012) describes a power generation system that combines a combustion power generator with molten carbonate fuel cells. The combustion output from the combustion generator is used in part as the input for the cathode of the fuel cell. This input is supplemented with a recycled portion of the anode output after passing through the anode output through a cryogenic CO₂separator.
An article by Desideri et al. (Intl. J. of Hydrogen Energy, Vol. 37, 2012) describes a method for modeling the performance of a power generation system using a fuel cell for CO₂separation. Recirculation of anode exhaust to the anode inlet and the cathode exhaust to the cathode inlet are used to improve the performance of the fuel cell. Based on the model and configuration shown in the article, increasing the CO₂utilization within the fuel cell is shown as being desirable for improving separation of CO₂.
U.S. Pat. No. 7,396,603 describes an integrated fossil fuel power plant and fuel cell system with CO₂emissions abatement. At least a portion of the anode output is recycled to the anode input after removal of a portion of CO₂from the anode output.

SUMMARY OF THE INVENTION

In one aspect, a method for capturing carbon dioxide from a combustion source is provided. The method can introducing one or more fuel streams and an O₂-containing stream into a combustion zone; performing a combustion reaction in the combustion zone to generate a combustion exhaust, the combustion exhaust comprising CO₂; processing at least a first portion of the combustion exhaust with a fuel cell array of one or more molten carbonate fuel cells to form a cathode exhaust stream from at least one cathode outlet of the fuel cell array, the one or more fuel cells each having an anode and a cathode, the molten carbonate fuel cells being operatively connected to the combustion exhaust through one or more cathode inlets in the fuel cell array; reacting carbonate from the one or more fuel cell cathodes with hydrogen within the one or more fuel cell anodes to produce electricity, an anode exhaust stream from at least one anode outlet of the fuel cell array comprising CO₂and H₂; separating CO₂from the anode exhaust stream in one or more separation stages to form a CO₂-depleted anode exhaust stream; passing at least a first portion of the CO₂-depleted anode exhaust stream to the combustion zone; and recycling at least a second portion of the CO₂-depleted anode exhaust stream to one or more of the fuel cell anodes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows an example of a combined cycle system for generating electricity based on combustion of a carbon-based fuel.

FIG. 2 schematically shows an example of the operation of a molten carbonate fuel cell.

FIG. 3 shows an example of the relation between anode fuel utilization and voltage for a molten carbonate fuel cell.

FIG. 4 schematically shows an example of a configuration for an anode recycle loop.

FIG. 5 shows an example of the relation between CO₂utilization, voltage, and power for a molten carbonate fuel cell.

FIG. 6 schematically shows another example of a combined cycle system for generating electricity based on combustion of a carbon-based fuel.

FIGS. 7, 8, and 9 show results from simulations of various configurations of a power generation system including a combustion-powered turbine and a molten carbonate fuel cell for carbon dioxide separation.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In various aspects, systems and methods are provided for capturing CO₂from a combustion source using molten carbonate fuel cells (MCFCs). The systems and methods can address one or more problems related to carbon capture from combustion exhaust stream and/or performing carbon capture using molten carbonate fuel cells.
One difficulty with conventional uses of carbon capture technology in conjunction with a combustion-based power source for power generation, such as use of molten carbonate fuel cells as part of a carbon capture scheme, is that the overall efficiency of the power generation system is reduced. Although molten carbonate fuel cells can generate electrical power, so that the net power generated by a system is increased, conventional combinations of fuel cells with combustion-powered generators result in net lower power efficiency for the power plant as a whole. In other words, the electrical power produced (watts) per unit of fuel input (lower heating value of the fuel, kJ) is reduced. This can be due in part to additional power or heating requirements for operating the additional carbon capture components. This can also be due in part to a lower efficiency of power generation for conventionally operated fuel cells in comparison with a system such as a combustion-powered turbine.
In some aspects, the overall efficiency of a carbon capture system that includes molten carbonate fuel cells can be improved by operating the fuel cells at lower anode fuel utilization values. Conventionally, molten carbonate fuel cells are operated at a fuel utilization that balances the heat needed to operate the fuel cell with the fuel consumed within the cell. The fuel utilization is made as high as possible while maintaining this heat balance. By contrast, it has been determined that for various types of power system configurations, reducing the anode fuel utilization of fuel cell array can allow for improved power generation efficiency for the overall system.
Another difficulty with using molten carbonate fuel cells for separation of CO₂from an exhaust stream can include the large area of fuel cells typically required for handling the exhaust from a commercial scale turbine or other power/heat generator. Accommodating a commercial scale exhaust flow using molten carbonate fuel cells can typically involve using a plurality of fuel cells, rather than constructing a single fuel cell of sufficient area. In order to deliver the exhaust stream to this plurality of fuel cells, additional connections can be required in order to divide the exhaust between the various fuel cells. Thus, reducing the fuel cell area required to capture a desired amount of carbon dioxide can provide a corresponding decrease in the number and/or complexity of flow connections required.
In some aspects of the invention, the area of fuel cells required for processing a CO₂-containing exhaust stream can be reduced or minimized by recycling at least a portion of the anode exhaust stream back to the anode inlet. Additionally or alternately, the fuel cells can be operated at lower fuel utilization. An exhaust stream can be passed into the cathode(s) of molten carbonate fuel cells. During operation of the fuel cell, the anode exhaust can be passed through one or more separation stages. This can include separation stages for removal of H₂O and/or CO₂. At least a portion of the remaining anode exhaust can then be recycled to the anode input. In one preferred embodiment, any recycle of the anode exhaust directly to the cathode can be avoided. By recycling at least some portion the anode exhaust to the anode inlet, at least some of the fuel not used on the first pass through the anode can be utilized in a subsequent pass.
In addition to or as an alternative to recycling the anode exhaust to the anode inlet, at least a portion of the hydrogen in the anode exhaust can be recycled to the combustion zone for a turbine or other combustion-powered generator/heat source. It is noted that any hydrogen generated via reforming as part of the anode loop can represent a fuel where the CO₂has already been “captured” by transfer to the anode loop. This can reduce the amount of CO₂needing to be transferred from the cathode side to the anode side of the fuel cell, and therefore can lead to a reduced fuel cell area.
An additional or alternative feature that can contribute to a reduced fuel cell area can be reducing and/or minimizing the amount of energy required for processes not directly involved in power generation. For example, the anode reaction in a molten carbonate fuel cell can combine H₂with CO₃ ²⁻ ions transported across the electrolyte between cathode and anode to form H₂O and CO₂. Although the anode reaction environment can facilitate some reforming of a fuel such as CH₄to form H₂, some H₂can advantageously be present in a fuel in order to maintain desirable reaction rates in the anode. As a result, prior to entering the anode itself, fuels (such as natural gas/methane) are conventionally at least partially reformed prior to entering the anode. The reforming stage prior to the anode for a fuel cell can require additional heat in order to maintain a suitable temperature for reforming.
In some aspects of the invention, recycling at least a portion of the anode exhaust to the anode inlet can allow for a reduced amount of reforming and/or elimination of the reforming stage prior to the anode inlet. Instead of reforming a fuel stream prior to entering the anode, the recycled anode exhaust can provide sufficient hydrogen for the fuel input to the anode. This can allow the input stream for the anode to be passed into the anode without passing through a separate pre-reforming stage. Operating the anode at a reduced level of hydrogen fuel utilization can further facilitate reducing and/or eliminating the pre-reforming stage by providing an anode exhaust with increased hydrogen content. Increasing the hydrogen content can allow a portion of the anode exhaust to also be used as an input to the turbine combustion zone, while still having sufficient hydrogen in the feed to the anode inlet so that pre-reforming can be reduced and/or eliminated.
Another challenge with using molten carbonate fuel cells can be due to the relatively low CO₂content of the exhaust of properly operated gas turbine. For example, a gas turbine powered by a low CO₂content natural gas fuel source can generate an exhaust, for example, with a CO₂of about 4 vol %. If some type of exhaust gas recycle is used, this value can be raised, for example, to about 6 vol %. By contrast, a typical desired CO₂content for the input to the cathode of a molten carbonate fuel cell can be about 10% or more. In some aspects of the invention, systems and methods are provided herein to allow for increased CO₂content in the exhaust gas while still efficiently operating the gas turbine or other combustion powered generator. In some aspects of the invention, systems and methods are provided for improving and/or optimizing the efficiency of carbon capture by the fuel cell when operated with a cathode exhaust having a low CO₂content.
Still another challenge can include reducing or mitigating the loss of efficiency in power generation caused by carbon capture. As noted above, conventional methods of carbon capture can result in a loss of net efficiency in power generation per unit of fuel consumed. In some aspects of the invention, systems and methods are provided for improving the overall power generation efficiency. Additionally or alternately, in some aspects of the invention, methods are provided for separating CO₂in a manner to reduce and/or minimize the energy required for generation of a commercially valuable CO₂stream.
In most aspects of the invention, one or more of the above advantages can be achieved, at least in part, by using molten carbonate fuel cells in combination with a combined cycle power generation system, such as a natural gas fired combined cycle plant, where the flue gas and/or heat from combustion reaction(s) can also be used to power a steam turbine. More generally, the molten carbonate fuel cells can be used in conjunction with various types of power or heat generation systems, such as boilers, combustors, catalytic oxidizers, and/or other types of combustion powered generators. In some aspects of the invention, at least a portion of the anode exhaust from the MCFCs can be (after separation of CO₂) recycled to the input flow for the MCFC anode(s). Additionally or alternately, a portion of the anode exhaust from the MCFCs can be recycled to the input flow for the combustion reaction for power generation. In one embodiment, a first portion of the anode exhaust from the MCFCs (after separation of CO₂) can be recycled to the input flow for the MCFC anode(s), and a second portion of the anode exhaust from the MCFCs can be recycled to the input flow for the combustion reaction for power generation. In aspects where the MCFCs can be operated with remaining (unreacted) H₂in the anode exhaust, recycling a portion of the H₂from the anode exhaust to the anode input can reduce the fuel needed for operating the MCFCs. The portion of H₂delivered to the combustion reaction can advantageously modify and/or improve reaction conditions for the combustion reaction, leading to more efficient power generation. A water-gas shift reaction zone after the anode exhaust can optionally be used to further increase the amount of H₂present in the anode exhaust while also allowing conversion of CO into more easily separable CO₂.
In various aspects of the invention, an improved method for capturing CO₂from a combustion source using a molten carbonate fuel cell can be provided. This can include, for example, systems and methods for power generation using turbines (or other power or heat generation methods based on combustion, such as boilers, combustors, and/or catalytic oxidizers) while reducing and/or mitigating emissions during power generation. This can optionally be achieved, at least in part, by using a combined cycle power generation system, where the flue gas and/or heat from combustion reaction(s) can also be used to power a steam turbine. This can additionally or alternately be achieved, at least in part, by using one or more molten carbonate fuel cells (MCFCs) as both a carbon capture device as well as an additional source of electrical power. In some aspects of the invention, the MCFCs can be operated under low fuel utilization conditions that can allow for improved carbon capture in the fuel cell while also reducing and/or minimizing the amount of fuel lost or wasted. Additionally or alternately, the MCFCs can be operated to reduce and/or minimize the total number and/or volume of MCFCs required to reduce the CO₂content of a combustion flue gas stream to a desired level, for example, 1.5 vol % or less or 1.0 vol % or less. Such aspects can be enabled, at least in part, by recycling the exhaust from the anode back to the inlet of the anode, with removal of at least a portion of the CO₂in the anode exhaust prior to returning the anode exhaust to the anode inlet. Such removal of CO₂from the anode exhaust can be achieved, for example, using a cryogenic CO₂separator. In some optional aspects of the invention, the recycle of anode exhaust to the anode inlet can be performed so that no pathway is provided for the anode exhaust to be recycled directly to the cathode inlet. By avoiding recycle of anode exhaust directly to the cathode inlet, any CO₂transported to the anode recycle loop via the MCFCs can remain in the anode recycle loop until the CO₂is separated out from the other gases in the loop.
Molten carbonate fuel cells are conventionally used in a standalone mode to generate electricity. In a standalone mode, an input stream of fuel, such as methane, can be passed into the anode side of a molten carbonate fuel cell. The methane can be reformed (either externally or internally) to form H₂and other gases. The H₂can then be reacted with carbonate ions that have crossed the electrolyte from the cathode in the fuel cell to form CO₂and H₂O. For the reactions in the anode of the fuel cell, the rate of fuel utilization is typically about 70% or 75%, or even higher. In a conventional configuration, the remaining fuel in the anode exhaust can be oxidized (burned) to generate heat for maintaining the temperature of the fuel cell and/or external reformer, in view of the endothermic nature of the reforming reaction. Air and/or another oxygen source can be added during this oxidation to allow for more complete combustion. The anode exhaust (after oxidation) can then be passed into the cathode. In this manner, a single fuel stream entering the anode can be used to provide all of the energy and nearly all of the reactants for both anode and cathode. This configuration can also allow all of the fuel entering the anode to be consumed while only requiring ˜70% or ˜75% or slightly more fuel utilization in the anode.
In the above standalone method, typical of conventional systems, the goal of operating a molten carbonate fuel cell is generally to efficiently generate electric power based on an input fuel stream. By contrast, a molten carbonate fuel cell integrated with a combustion powered turbine, engine, or other generator can be used to provide additional utility. Although high-efficiency power generation by the fuel cell is still desirable, the fuel cell can be operated, for instance, to improve and/or maximize the amount of CO₂captured from an exhaust stream for a given volume of fuel cells. This can allow for improved CO₂capture while still generating power from the fuel cell. Additionally, in some aspects of the invention, the exhaust from the anode(s) of the fuel cell(s) can still contain excess hydrogen. This excess hydrogen can advantageously be used as a fuel for the combustion reaction for the turbine, thus allowing for improved efficiency for the turbine.
FIG. 1 provides a schematic overview for the concept of some aspects of the invention. FIG. 1 is provided to aid in understanding of the general concept, so additional feeds, processes, and or configurations can be incorporated into FIG. 1 without departing from the spirit of the overall concept. In the overview example shown in FIG. 1, a natural gas turbine 110 (or another combustion-powered turbine) can be used to generate electric power based on combustion of a fuel 112. For the natural gas turbine 110 shown in FIG. 1, this can include compressing an air stream or other gas phase stream 111 to form a compressed gas stream 113. The compressed gas stream 113 can then be introduced into a combustion zone 115 along with fuel 112. Additionally, a stream 185, including a portion of the fuel (hydrogen) present in the exhaust from anode 130, can also be introduced into the combustion zone 115. This additional hydrogen can allow the combustion reaction to be operated under enhanced conditions. The resulting hot flue or exhaust gas 117 can then be passed into the expander portion of turbine 110 to generate electrical power.
After expansion (and optional clean up and/or other processing steps), the expanded flue gas can be passed into the cathode portion 120 of a molten carbonate fuel cell. The flue gas can include sufficient oxygen for the reaction at the cathode, or additional oxygen can be provided if necessary. To facilitate the fuel cell reaction, fuel 132 can be passed into the anode portion 130 of the fuel cell, along with at least a portion of the anode exhaust 135. Prior to being recycled, the anode exhaust 135 can be passed through several additional processes. One additional process can include or be a water-gas shift reaction process 170. The water gas-shift reaction 170 can be used to react H₂O and CO present in the anode exhaust 135 to form additional H₂and CO₂. This can allow for improved removal of carbon from the anode exhaust 135, as CO₂can typically be more readily separated from the anode exhaust, as compared to CO. The output 175 from the (optional) water-gas shift process 170 can then be passed through a carbon dioxide separation system 140, such as a cryogenic carbon dioxide separator. This can remove at least a portion of CO ₂ 147 from the anode exhaust, typically as well as a portion of the water 149 also. After removal of at least a portion of the CO₂and water, the recycled anode exhaust can still contain some CO₂and water, as well as unreacted fuel in the form of H₂and/or possibly a hydrocarbon such as methane. In certain embodiments of the invention, a portion 145 of the output from the CO₂separation stage(s) can be recycled for use as an input stream to anode 130, while a second portion can be used as input 185 to the combustion reaction 115. Fuel 132 can represent a hydrogen-containing stream and/or a stream containing methane and/or another hydrocarbon that can be reformed (internally or externally) to form H₂.
The exhaust from the cathode portion 120 of the fuel cell can then be passed into a heat recovery zone 150 so that heat from the cathode exhaust can be recovered. e.g., to power a steam generator 160. After recovering heat, the cathode exhaust can exit the system as an exhaust stream 156. The exhaust stream 156 can exit to the environment, or optional additional clean-up processes can be used, such as performing additional CO₂capture on stream 156, for example, using an amine scrubber.

Molten Carbonate Fuel Cell

In various aspects of the invention, a molten carbonate fuel cell (MCFC) can be used to facilitate separation of CO₂from a CO₂-containing stream while also generating additional electrical power. The CO₂separation can be further enhanced by taking advantage of synergies with the combustion-based power generator that can provide at least a portion of the input feed to the cathode portion of the fuel cell.
In this discussion, a fuel cell can correspond to a single cell, with an anode and a cathode separated by an electrolyte. The anode and cathode can receive input gas flows to facilitate the respective anode and cathode reactions for transporting charge across the electrolyte and generating electricity. A fuel cell stack can represent a plurality of cells in an integrated unit. Although a fuel cell stack can include multiple fuel cells, the fuel cells can typically be connected in parallel and can function (approximately) as if they collectively represented a single fuel cell of a larger size. When an input flow is delivered to the anode or cathode of a fuel cell stack, the fuel stack can include flow channels for dividing the input flow between each of the cells in the stack and flow channels for combining the output flows from the individual cells. In this discussion, a fuel cell array can be used to refer to a plurality of fuel cells (such as a plurality of fuel cell stacks) that are arranged in series, in parallel, or in any other convenient manner (e.g., in a combination of series and parallel). A fuel cell array can include one or more stages of fuel cells and/or fuel cell stacks, where the anode/cathode output from a first stage may serve as the anode/cathode output for a second stage. It is noted that the anodes in a fuel cell stage do not have to be connected in the same way as the cathodes in a stage. For convenience, the input to the first anode stage of a fuel cell array may be referred to as the anode input for the array, and the input to the first cathode stage of the fuel cell array may be referred to as the cathode input to the array. Similarly, the output from the final anode/cathode stage may be referred to as the anode/cathode output from the array.
It should be understood that reference to use of a fuel cell herein typically denotes a “fuel cell stack” composed of individual fuel cells, and more generally refers to use of one or more fuel cell stacks in fluid communication. Individual fuel cell elements (plates) can typically be “stacked” together in a rectangular array called a “fuel cell stack”. This fuel cell stack can typically take a feed stream and distribute reactants among all of the individual fuel cell elements and can then collect the products from each of these elements. When viewed as a unit, the fuel cell stack in operation can be taken as a whole even though composed of many (often tens or hundreds) of individual fuel cells. These individual fuel cells can typically have similar voltages (as the reactant and product concentrations are similar), and the total power output can result from the summation of all of the electrical currents in all of the cells. Stacks can also be arranged in a series/parallel arrangement to result in high voltages. If a sufficiently large volume fuel cell stack is available to process a given exhaust flow, the systems and methods described herein can be used with a single molten carbonate fuel cell stack. In other aspects of the invention, a plurality of fuel cell stacks may be desirable or needed for a variety of reasons.
For the purposes of this invention, unless otherwise specified, the term “fuel cell” should be understood to also refer to and/or is defined as including a reference to a fuel cell stack composed of set of one or more individual fuel cells for which there is a single input and output, as that is the manner in which fuel cells are typically employed in practice. Similarly, the term fuel cells (plural), unless otherwise specified, should be understood to also refer to and/or is defined as including a plurality of separate fuel cell stacks. In other words, all references within this document, unless specifically noted, can refer interchangeably to the operation of a fuel cell stack as a “fuel cell”. For example, the volume of exhaust generated by a commercial scale combustion generator may be too large for processing by a fuel cell (i.e., a single stack) of conventional size. In order to process the full exhaust, a plurality of fuel cells (i.e., two or more separate fuel cells or fuel cell stacks) can be arranged in parallel, so that each fuel cell processes (roughly) an equal portion of the combustion exhaust. Although multiple fuel cells can be used, each fuel cell can be operated in a generally similar manner.
One way of characterizing the operation of a fuel cell can be to characterize the “utilization” of various inputs received by the fuel cell. For example, one common method for characterizing the operation of a fuel cell can be to specify the (anode) fuel utilization for the fuel cell.
Fuel cell fuel utilization as used herein can be computed using the flow rates and Lower Heating Value (LHV) of the fuel components entering and leaving the fuel cell anode. Lower heating value is defined as the enthalpy of combustion of a fuel component to vapor phase, fully oxidized products (i.e., vapor phase CO₂and H₂O product). As such, fuel utilization (U_f) can be computed as U_f=(LHV(in)−LHV(out))/LHV(in), where LHV(in) and LHV(out) refer to the LHV of the fuel components (such as H₂, CH₄, and/or CO) in the anode inlet and outlet streams or flows, respectively. In this definition, the LHV of a stream or flow may be computed as a sum of values for each fuel component in the input and/or output stream. The contribution of each fuel component to the sum can correspond to the fuel component's flow rate (e.g., mol/hr) multiplied by the fuel component's LHV (e.g., joules/mol). It is noted that components in the anode input flow that do not participate in a combustion reaction to form H₂O and/or CO₂are not typically considered “fuel components”.
It is noted that, for the special case where the only fuel in the anode input flow is H₂, the only reaction involving a fuel component that can take place in the anode represents the conversion of H₂into H₂O. In this special case, the fuel utilization simplifies to (H₂-rate-in minus H₂-rate-out)/H₂-rate-in. In such a case, H₂would be the only fuel component, and so the H₂LHV would cancel out of the equation. In the more general case, the anode feed may contain, for example. CH₄, H₂, and CO in various amounts. Because these species can typically be present in different amounts in the anode outlet, the summation as described above can be needed to determine the fuel utilization.
In addition to fuel utilization, the utilization for other reactants in the fuel cell can be characterized. For example, the operation of a fuel cell can additionally or alternately be characterized with regard to “CO₂utilization” and/or “oxidant” utilization. The values for CO₂utilization and/or oxidant utilization can be specified in a similar manner. For CO₂utilization, the simplified calculation of (CO₂-rate-in minus CO₂-rate-out)/CO₂-rate-in can be used if CO₂is the only fuel component present in the input stream or flow to the cathode, with the only reaction thus being the formation of CO₃ ²⁻. Similarly, for oxidant utilization, the simplified version can be used if O₂is the only oxidant present in the input stream or flow to the cathode, with the only reaction thus being the formation of CO₃ ²⁻.
Another reason for using a plurality of fuel cells can be to allow for efficient fuel cell operation while reducing the CO₂content of the combustion exhaust to a desired level. Rather than operating a fuel cell to have a high (or optimal) rate of CO₂utilization, two (or more) fuel cells can be operated at lower fuel utilization rate(s) while reducing the combustion to a desired level.
FIG. 2 shows a schematic example of the operation of an MCFC for generation of electrical power. In FIG. 2, the anode portion of the fuel cell can receive fuel and steam (H₂O) as inputs, with outputs of water, CO₂, and optionally excess H₂, CH₄(or other hydrocarbons), and/or CO. The cathode portion of the fuel cell can receive CO₂and some oxidant (e.g. air/O₂) as inputs, with an output corresponding to a reduced amount of CO₂in O₂-depleted oxidant (air). Within the fuel cell, CO₃ ²⁻ ions formed in the cathode side can be transported across the cathode (membrane), through the electrolyte in the fuel cell, and across the anode (membrane) to provide the carbonate ions needed for the reactions occurring at the anode.
In a molten carbonate fuel cell such as the example fuel cell shown in FIG. 2, there are three basic reactions that can occur. The first reaction can be optional, and can be reduced or eliminated if sufficient H₂is provided directly to the anode.
CH₄+2H₂O=>4H₂+CO₂ <anode>
4H₂+4CO₃ ²⁻=>4H₂O+4CO₂+8e ⁻ <anode>
2O₂+4CO₂+8e ⁻=>4CO₃ ²⁻ <cathode>
Reaction (1) represents a hydrocarbon reforming reaction to generate H₂for use in the anode of the fuel cell. Reaction (1) can occur external to the fuel cell, and/or the reforming can be performed internal to the fuel cell. Reaction (1) can be optional, as the primary purpose of reaction (1) is to generate H₂. The CO₂generated by reaction (1) does not generally undergo further reaction within the fuel cell, and, to a first approximation, thus does not significantly impact reaction (2).
Reactions (2) and (3), at the anode and cathode respectively, represent the reactions that can result in electrical power generation within the fuel cell. Reaction (2) combines H₂, optionally generated by reaction (1), with carbonate ions to form H₂O, CO₂, and electrons. Reaction (3) combines O₂, CO₂, and electrons to form carbonate ions. The carbonate ions generated by reaction (3) can be transported across the electrolyte of the fuel cell to provide the carbonate ions needed for reaction (2). In combination with the transport of carbonate ions across the electrolyte, a closed current loop can then be formed by providing an electrical connection between the anode and cathode.
During conventional operation of a fuel cell, such as standalone operation, the goal of operating the fuel cell can be to generate electrical power while efficiently using the “fuel” provided to the cell. The “fuel” can correspond to either hydrogen (H₂), a gas stream comprising hydrogen, and/or a gas stream comprising a substance that can be reformed to provide hydrogen (such as methane, another alkane or hydrocarbon, and/or one or more other types of compounds containing carbon and hydrogen that, upon reaction, can provide hydrogen). These reforming reactions are typically endothermic and thus usually consume some heat energy in the production of hydrogen. Carbon sources that can provide CO directly and/or upon reaction can also be utilized, as typically the water gas shift reaction (CO+H₂O═H₂+CO₂) can occur in the presence of the fuel cell anode catalyst surface. This can allow for production of hydrogen from a CO source. For such conventional operation, one potential goal of operating the fuel cell can be to consume all of the fuel provided to the cell, while maintaining a desirable output voltage for the fuel cell, which can be traditionally accomplished by operating the fuel cell anode at a fuel utilization of about 70% to about 75%, followed by oxidizing (such as burning) the remaining fuel to generate heat to maintain the temperature of the fuel cell. The fuel utilization is measured in terms of the total enthalpy of the fuel used in the fuel cell reactions divided by the enthalpy of the fuel entering the fuel cell.
By contrast, in various embodiments, the goal of operating the fuel cell can be to separate CO₂from the output stream of a combustion reaction, in addition to allowing generation of electric power. In such embodiments, the combustion reaction(s) can be used to power one or more generators or turbines, which provide the majority of the power generated by the combined generator/fuel cell system. Rather than operating the fuel cell to optimize power generation by the fuel cell, the system can instead be operated to improve the capture of carbon dioxide from the combustion-powered generator while reducing and/or minimizing the number of fuels cells required for capturing the carbon dioxide. Selecting an appropriate configuration for the input and output flows of the fuel cell, as well as selecting appropriate operating conditions for the fuel cell, can allow for a desirable combination of overall power generation efficiency and carbon capture. One aspect of selecting appropriate operating conditions can correspond to selecting operating conditions based on a factor other than fuel utilization. In terms of fuel utilization, the operating conditions can result in lower fuel utilization than in a conventional fuel cell.
In various aspects of the invention where fuel cells are operated to have a low fuel utilization, a molten carbonate fuel cell can be operated to have a fuel utilization of about 65% or less, for example, about 60% or less, about 55% or less, about 50% or less, or about 45% or less. Additionally or alternately, a molten carbonate fuel cell can be operated to have a fuel utilization of at least about 25%, for example at least about 30%, at least about 35%, or at least about 40%.
In some embodiments, the fuel cells in a fuel cell array can be arranged so that only a single stage of fuel cells (such as fuel cell stacks) can be present. In this type of embodiment, the anode fuel utilization for the single stage can represent the anode fuel utilization for the array. Another option can be that a fuel cell array can contain multiple stages of anodes and/or multiple stages of cathodes, with each anode stage having a fuel utilization within the same range, such as each anode stage having a fuel utilization within 10% of a specified value, for example within 5% of a specified value. Still another option can be that each anode stage can have a fuel utilization equal to a specified value or lower than the specified value by less than an amount, such as having each anode stage be not greater than a specified value by 10% or less, for example, by 5% or less. As an illustrative example, a fuel cell array with a plurality of anode stages can have each anode stage be within about 10% of 50% fuel utilization, which would correspond to each anode stage having a fuel utilization between about 40% and about 60%. As another example, a fuel cell array with a plurality of stages can have each anode stage be not greater than 60% anode fuel utilization with the maximum deviation being about 5% less, which would correspond to each anode stage having a fuel utilization between about 55% to about 60%. In still another example, one or more stages of fuel cells in a fuel cell array can be operated at a fuel utilization from about 30% to about 50%, such as operating a plurality of fuel cell stages in the array at a fuel utilization from about 30% to about 50%. More generally, any of the above types of ranges can be paired with any of the anode fuel utilization values specified herein.
Still another option can include specifying a fuel utilization for less than all of the anode stages. For example, in some aspects of the invention where fuel cells/stacks are arranged at least partially in one or more series arrangements, that anode fuel utilization can be specified for the first anode stage in a series, the second anode stage in a series, the final anode stage in a series, or any other convenient anode stage in a series. As used herein, the “first” stage in a series corresponds to the stage (or set of stages, if the arrangement contains parallel stages as well) to which input is directly fed from the fuel source(s), with later (“second”, “third”, “final”, etc.) stages representing the stages to which the output from one or more previous stages is fed, instead of directly from the respective fuel source(s). In situations where both output from previous stages and input directly from the fuel source(s) are co-fed into a stage, there can be a “first” (set of) stage(s) and a “last” (set of) stage(s), but other stages (“second”, “third”, etc.) can be more tricky among which to establish an order (e.g., in such cases, ordinal order can be determined by concentration levels of one or more components in the composite input feed composition, such as CO₂for instance, from highest concentration “first” to lowest concentration “last” with approximately similar compositional distinctions representing the same ordinal level.
Yet another option can be to specify the anode fuel utilization corresponding to a particular cathode stage (again, where fuel cells/stacks are arranged at least partially in one or more series arrangements). As noted above, based on the direction of the flows within the anodes and cathodes, the first cathode stage may not correspond to (be across the same fuel cell membrane from) the first anode stage. Thus, in some aspects of the invention, the anode fuel utilization can be specified for the first cathode stage in a series, the second cathode stage in a series, the final cathode stage in a series, or any other convenient cathode stage in a series.
Yet still another option can be to specify an overall average of fuel utilization over all fuel cells in a fuel cell array. In various aspects, the overall average of fuel utilization for a fuel cell array can be about 65% or less, for example, about 60% or less, about 55% or less, about 50% or less, or about 45% or less (additionally or alternately, the overall average fuel utilization for a fuel cell array can be at least about 25%, for example at least about 30%, at least about 35%, or at least about 40%). Such an average fuel utilization need not necessarily constrain the fuel utilization in any single stage, so long as the array of fuel cells meets the desired fuel utilization.
In a molten carbonate fuel cell, the transport of carbonate ions across the electrolyte in the fuel cell can provide a method for transporting CO₂from a first flow path to a second flow path, where the transport method can allow transport from a lower concentration (the cathode) to a higher concentration (the anode), which can thus facilitate capture of CO₂. For embodiments where the input to the cathode can be primarily based on the output gas from a combustion reaction for powering a turbine or another type of power generator, the CO₂content of the output gas can tend to be relatively low in comparison to the total output gas composition. For example, the CO₂content of the output from a natural gas combustion turbine can be from about 3 vol % to about 6 vol %, although higher CO₂contents can be possible, e.g., for turbine configurations including exhaust gas recovery. Coal-fired power plants can have higher CO₂concentrations, such as up to about 15 vol % or more. For such output flows, the majority of the gas in the output flow can be nitrogen, especially if the source of oxidant for the combustion reaction is air or a primarily nitrogen-containing gas. Due to the relatively low concentration of CO₂, one of the challenges in separating out the CO₂from such streams can be related to performing a cost-effective separation resulting in a relatively high purity CO₂output stream. An MCFC can be beneficial for performing this type of separation, as part of the selectivity of the fuel cell can be based on the electrochemical reactions allowing the cell to generate electrical power. For non-reactive species (such as N₂) that effectively do not participate in the electrochemical reactions within the fuel cell, there can be an insignificant amount of reaction and transport from cathode to anode. By contrast, the potential (voltage) difference between the cathode and anode can provide a strong driving force for transport of carbonate ions across the fuel cell. As a result, the transport of carbonate ions in the molten carbonate fuel cell can allow CO₂to be transported from the cathode (lower CO₂concentration) to the anode (higher CO₂concentration) with relatively high selectivity.

Operation of Anode Portion and Anode Recycle Loop

In various aspects of the invention, molten carbonate fuel cells can be operated under conditions that allow for lower fuel utilization in the anode portion of the fuel cell. This is in contrast to conventional operation for fuel cells, where the fuel utilization is typically selected in order to allow all of the fuel delivered to the fuel cell to be consumed as part of operation of the fuel cell. In conventional operation, all of the fuel is typically either consumed within the anode of the fuel cell or burned to provide sensible heat for the feed streams to the fuel cell.
FIG. 3 shows an example of the relationship between fuel utilization and output power for a fuel cell operating under conventional (stand-alone) conditions. The diagram shown in FIG. 3 shows two limiting cases for operation of a fuel cell. One limiting case includes the limit of operating a fuel cell to consume an amount of fuel (such as H₂or methane reformed into H₂) that approaches 100% of the fuel delivered to the fuel cell. From an efficiency standpoint, consumption of ˜100% of the fuel delivered to a fuel cell would be desirable, so as not to waste fuel during operation of the fuel cell. However, there are two potential drawbacks with operating a fuel cell to consume more than about 80% of the fuel delivered to the cell. First, as the amount of fuel consumed approaches 100%, the voltage provided by the fuel cell can be sharply reduced. In order to consume an amount of fuel approaching 100%, the concentration of the fuel in the fuel cell (or at least near the anode) must almost by definition approach zero during at least part of the operation of the fuel cell. Operating the anode of the fuel cell with a fuel concentration approaching zero can result in a decreasingly low driving force for transporting carbonate across the electrolyte of the fuel cell. This can cause a corresponding drop in voltage, with the voltage potentially also approaching zero in the true limiting case of consuming all fuel provided to the anode.
The second drawback is also related to relatively high fuel utilization values (greater than about 80%) even if consumption does not approach 100%. As shown in FIG. 3, at fuel utilization values of about 75% or less, the voltage generated by the fuel cell has a roughly linear relationship with the fuel utilization. At about 75% fuel utilization, the voltage generated can be about 0.7 Volts, with mildly increasing voltages as the fuel utilization decreases. At fuel utilization values of about 80% or greater, the voltage versus utilization curve appears to take on an exponential or power type relationship. From a process stability standpoint, it can be preferable to operate a fuel cell in a portion of the voltage versus utilization curve where the relationship is linear.
In the other limiting case shown in FIG. 3, the voltage generated by a molten carbonate fuel cell shows a mild increase as the fuel utilization decreases. However, in conventional operation, operating a fuel cell at reduced utilization can pose various difficulties. For example, the total amount of fuel delivered to a conventionally operated fuel cell operated with lower fuel utilization may need to be reduced, so that whatever fuel remains in the anode exhaust/output stream can still provide the appropriate amount of heat (upon further combustion) for maintaining the fuel cell temperature. If the fuel utilization is reduced without adjusting the amount of fuel delivered to the fuel cell, the oxidation of the unused fuel may result in higher than desired temperatures for the fuel cell. Based at least on these limiting case considerations, conventional fuel cells are typically operated at a fuel utilization of about 70% to about 75%, so as to achieve heat balance with complete utilization of the fuel.
In contrast to conventional operation, in various embodiments of the invention, an alternative configuration can be to recycle at least a portion of the exhaust from a fuel cell anode to the input of a fuel cell anode. The output stream from an MCFC anode can include H₂O, CO₂, optionally CO, and optionally but typically unreacted fuel (such as H₂or CH₄) as the primary output components. Instead of using this output stream as a fuel source to provide heat for a reforming reaction, one or more separations can be performed on the anode output stream in order to separate out the CO₂from the components with potential fuel value, such as H₂or CO. The components with fuel value can then be recycled to the input of an anode.
This type of configuration can provide one or more benefits. First. CO₂can be separated out from the anode output, such as by using a cryogenic CO₂separator. Several of the components of the anode output (H₂, CO, CH₄) are not easily condensable components, while CO₂and H₂O can be separated individually as condensed phases. Depending on the embodiment, at least about 90 vol % of the CO₂in the anode output can be separated out to form a relatively high purity CO₂output stream. After separation, the remaining portion of the anode output can correspond primarily to components with fuel value, as well as reduced amounts of CO₂and/or H₂O. This portion of the anode output after separation can be recycled for use as part of the anode input, along with additional fuel. In this type of configuration, even though the fuel utilization in a single pass through the MCFC(s) may be low, the unused fuel can be advantageously recycled for another pass through the anode. As a result, the single-pass fuel utilization can be at a reduced level, while avoiding loss (exhaust) of unburned fuel to the environment.
Additionally or alternatively to recycling a portion of the anode exhaust to the anode input, another configuration option can be to use a portion of the anode exhaust as an input for a combustion reaction for a turbine or other combustion power source. The relative amounts of anode exhaust recycled to the anode input and/or as an input to the combustion zone can be any convenient or desirable amount. If the anode exhaust is recycled to only one of the anode input and the combustion zone, the amount of recycle can be any convenient amount, such as up to 100% of the portion of the anode exhaust remaining after any separation to remove CO₂and/or H₂O. When a portion of the anode exhaust is recycled to both the anode input and the combustion zone, the total recycled amount by definition can be 100% or less of the remaining portion of anode exhaust. Otherwise, any convenient split of the anode exhaust can be used. In various embodiments of the invention, the amount of recycle to the anode input can be at least about 10% of the anode exhaust remaining after separations, for example at least about 25%, at least about 40%, at least about 50%, at least about 60%, at least about 75%, or at least about 90%. Additionally or alternately in those embodiments, the amount of recycle to the anode input can be about 90% or less of the anode exhaust remaining after separations, for example about 75% or less, about 60% or less, about 50% or less, about 40% or less, about 25% or less, or about 10% or less. Further additionally or alternately, in various embodiments of the invention, the amount of recycle to the combustion zone (turbine) can be at least about 10% of the anode exhaust remaining after separations, for example at least about 25%, at least about 40%, at least about 50%, at least about 60%, at least about 75%, or at least about 90%. Additionally or alternately in those embodiments, the amount of recycle to the combustion zone (turbine) can be about 90% or less of the anode exhaust remaining after separations, for example about 75% or less, about 60% or less, about 50% or less, about 40% or less, about 25% or less, or about 10% or less.
Any H₂present in the anode exhaust can represent a fuel that can be combusted without generating CO₂. Because at least some H₂can be generated as part of the anode portion of the fuel cell(s), the CO₂generated during reforming can be primarily removed by the CO₂separation stage(s) in the anode portion of the system. As a result, use of H₂from the anode exhaust as part of the fuel for the combustion reaction can allow for a situation where the CO₂generated from “combustion” of the fuel can be created in the anode portion of the system, as opposed to having to transport the CO₂across the membrane.
Recycle of H₂from the anode exhaust to the combustion reaction can provide synergistic benefits for a turbine (or other combustion system) that includes an exhaust gas recycle (EGR) configuration. In an EGR configuration, a portion of the CO₂-containing exhaust gas from the combustion reaction can be recycled and used as part of the input gas flow to the turbine. During operation of a combustion-powered turbine, an input gas flow of an oxidant (such as air or oxygen-enriched air) can be compressed prior to introduction into the combustion reaction. The compressors used for the input flows to the combustion reaction can tend to be volume limited, so that a similar number of moles of gas can be compressed, typically regardless of the mass of the gas. However, gases with a higher mass can tend to have higher heat capacities and/or can allow for greater pressure ratios across the expander portion of a turbine. A CO₂-enriched exhaust stream can provide a convenient source of a gas stream with higher molecular weight components that can allow for improved conversion of the energy from the combustion reaction into electric power from the turbine. Although introducing a CO₂-enriched stream into the combustion reaction can provide some benefits, there can be effective limits to the amount of the CO₂-enriched stream that can be added without significantly (negatively) impacting the combustion reaction. Since the CO₂-enriched stream does not itself typically contain “fuel”, the stream can largely act as a diluent within the combustion reaction. As a result, the amount of recyclable CO₂can be limited based, at least in part, on maintaining the conditions in the combustion reaction within an appropriate flammability window.
Recycle of H₂from the anode exhaust can complement an EGR configuration in one or more ways. First, combustion of H₂may not directly result in generation of CO₂. Instead, as noted above, the CO₂generated when the H₂is produced can be generated in the anode loop. This can reduce the amount of CO₂needing to be transferred from cathode to anode for a given level of power generation. Additionally, H₂can also have the benefit of modifying the operation of the combustion source, such as through modifying the flammability window, so that higher concentrations of CO₂can be tolerated while still maintaining a desired combustion reaction. Being able to expand the flammability window can allow for increased concentrations of CO₂in the combustion exhaust, and therefore increased CO₂in the input to the cathodes of the fuel cell.
The benefit of being able to increase the CO₂concentration in the input to the fuel cell cathode can be related to the nature of how a molten carbonate fuel cell operates. As detailed below, there can be practical limits in the amount of CO₂separable by an MCFC from a cathode exhaust stream. Depending on the operating conditions, an MCFC can lower the CO₂content of a cathode exhaust stream to about 2.0 vol % or less, e.g., about 1.5 vol % or less or about 1.2 vol % or less. Due to this limitation, the net efficiency of CO₂removal when using molten carbonate fuel cells can be dependent on the amount of CO₂in the cathode input. For a combustion reaction using natural gas as a fuel, the amount of CO₂in the combustion exhaust can correspond to a CO₂concentration at the cathode input of at least about 4 vol %. Use of exhaust gas recycle can allow the amount of CO₂at the cathode input to be increased to at least about 5 vol %, e.g., at least about 6 vol %. Due to the increased flammability window that can be provided when using H₂as part of the fuel, the amount of CO₂added via exhaust gas recycle can be increased still further, so that concentrations of CO₂at the cathode input of at least about 7.5 vol % or at least about 8 vol % can be achieved. Based on a removal limit of about 1.5 vol % at the cathode exhaust, increasing the CO₂content at the cathode input from about 5.5 vol % to about 7.5 vol % corresponds to a ˜50% increase in the amount of CO₂that can be captured using a fuel cell and transported to the anode loop for eventual CO₂separation.
The amount of H₂present in the anode output can be increased, for example, by using a water gas shift reactor to convert H₂O and CO present in the anode output into H₂and CO₂. Water is an expected output of the reaction occurring at the anode, so the anode output can typically have an excess of H₂O relative to the amount of CO present in the anode output. CO can be present in the anode output due to incomplete carbon combustion during reforming and/or due to the equilibrium balancing reactions between H₂O, CO, H₂, and CO₂(i.e., the water-gas shift equilibrium) under either reforming conditions or the conditions present during the anode reaction. A water gas shift reactor can be operated under conditions to drive the equilibrium further in the direction of forming CO₂and H₂at the expense of CO and H₂O. Higher temperatures can tend to favor the formation of CO and H₂O. Thus, one option for operating the water gas shift reactor can be to expose the anode output stream to a suitable catalyst, such as a catalyst including iron oxide, zinc oxide, copper on zinc oxide, or the like, at a suitable temperature, e.g., between about 190° C. to about 210° C. Optionally, the water-gas shift reactor can include two stages for reducing the CO concentration in an anode output stream, with a first higher temperature stage operated at a temperature from at least about 300° C. to about 375° C. and a second lower temperature stage operated at a temperature of about 225° C. or less, such as from about 180° C. to about 210° C. In addition to increasing the amount of H₂present in the anode output, the water-gas shift reaction can also increase the amount of CO₂at the expense of CO. This can exchange difficult-to-remove carbon monoxide (CO) for carbon dioxide, which can be more readily removed by condensation (e.g., cryogenic removal), chemical reaction (such as amine removal), and/or other CO₂removal methods.
In some aspects of the invention, all or substantially all of the anode output stream remaining after separation of (portions of) the CO₂(and H₂O) can be recycled for use as an input for the fuel cell anode(s) and/or as a fuel input for the combustion-powered generator. Thus, for the portion of the anode output stream that remains after a water-gas shift reaction, removal of CO₂, and/or removal of H₂O, at least about 90% of the remaining content can advantageously be used as either an input for the fuel cell anode(s) or as a fuel input for the combustion powered generator. Alternatively, the anode output stream after separation can be used for more than one purpose, but recycle of any portion of the anode output stream for use as a direct input to a cathode and/or as an input to an oxidizer for heating of the fuel cell can advantageously be avoided.
FIG. 4 shows an example of the anode flow path portion of a generator/fuel cell system according to the invention. In FIG. 4, an initial fuel stream 405 can optionally be reformed 410 to convert methane (or another type of fuel) and water into H₂and CO₂. Alternatively, the reforming reaction can be performed in a reforming stage that is part of an assembly including both the reforming stage and the fuel cell anode 420. Additionally or alternately, at least a portion of fuel stream 405 can correspond to hydrogen gas, so that the amount of reforming needed to provide fuel to the anode 420 can be reduced and/or minimized. The optionally reformed fuel 415 can then be passed into anode 420. A recycle stream 455 including fuel components from the anode exhaust 425 can also serve as an input to the anode 420. A flow of carbonate ions 422 from the cathode portion of the fuel cell (not shown) can provide the remaining reactant needed for the anode fuel cell reactions. Based on the reactions in the anode 420, the resulting anode exhaust 425 can include H₂O, CO₂, one or more components corresponding to unreacted fuel (H₂, CO, CH₄, or others), and optionally one or more additional non-reactive components, such as N₂and/or other contaminants that are part of fuel stream 405. The anode exhaust 425 can then be passed into one or more separation stages 430 for removal of CO₂(and optionally also H₂O). A cryogenic CO₂removal system can be an example of a suitable type of separation stage. Optionally, the anode exhaust can first be passed through a water gas shift reactor 440 to convert any CO present in the anode exhaust (along with some H₂O) into CO₂and H₂in an optionally water gas shifted anode exhaust 445.
An initial portion of the separation stage(s) 430 can be used to remove a majority of the H₂O present in the anode exhaust 425 as an H₂ O output stream 432. Additionally or alternately, a heat recovery steam generator system or other heat exchangers independent of the cryogenic separation system can be used to remove a portion of the H₂O. A cryogenic CO₂removal system can then remove a majority of the CO₂as a high purity CO₂stream 434. A purge stream (not shown) can also be present, if desired, to prevent accumulation of inert gases within the anode recycle loop. The remaining components of the anode exhaust stream can then be used either as a recycled input 455 to the inlet of anode 420 or as an input stream 485 for a combustion powered turbine.
Conventionally, at least some reforming is performed prior to any fuel entering a fuel cell. This initial/preliminary reforming can be performed in a reformer that is external to the fuel cell(s) or fuel cell stack(s). Alternatively, the assembly for a fuel cell stack can include one or more reforming zones located within the stack but prior to the anodes of the fuel cells in the stack. This initial reforming typically converts at least some fuel into hydrogen prior to entering the anode, so that the stream that enters the anode can have sufficient hydrogen to maintain the anode reaction. Without this initial reforming, in certain embodiments, the hydrogen content in the anode can be too low, resulting in little or no transport of CO₂from cathode to anode. By contrast, in some embodiments the fuel cell(s) in a fuel cell array can be operated without external reforming, i.e., based only on reforming within the anode portion of the fuel cell, due to sufficient hydrogen being present in the recycled portion of the anode exhaust. When a sufficient amount of H₂is present in the anode feed, such as at least about 10 vol % of the fuel delivered to the anode in the form of H₂, the reaction conditions in the anode can allow for additional reforming to take place within the anode itself, which, depending on flow path, can reduce and/or eliminate the need for a reforming stage external (prior) to the anode input(s) in the methods according to the invention. If the anode feed does not contain a sufficient amount of hydrogen, the anode reaction can stall, and reforming activity and/or other reactions in the anode can be reduced, minimized, or halted entirely. As a result, in embodiments where the amount of H₂present in the anode feed is insufficient, it may be desirable (or necessary) for there to be a reforming stage external (prior) to the anode input(s).

Operation of Cathode Portion

In various aspects according to the invention, molten carbonate fuel cells used for carbon capture can be operated to improve or enhance the carbon capture aspects of the fuel cells, as opposed to (or even at the expense of) enhancing the power generation capabilities. Conventionally, a molten carbonate fuel cell can be operated based on providing a desirable voltage while consuming all fuel in the fuel stream delivered to the anode. This can be conventionally achieved in part by using the anode exhaust as at least a part of the cathode input stream. By contrast, the present invention uses separate/different sources for the anode input and cathode input. By removing any direct link between the composition of the anode input flow and the cathode input flow, additional options become available for operating the fuel cell to improve capture of carbon dioxide.
One initial challenge in using molten carbonate fuel cells for carbon dioxide removal can be that the fuel cells have limited ability to remove carbon dioxide from relatively dilute cathode feeds. FIG. 5 shows an example of the relationship between 1) voltage and CO₂concentration and 2) power and CO₂concentration, based on the concentration of CO₂in the cathode input gas. As shown in FIG. 5, the voltage and/or power generated by a carbonate fuel cell can start to drop rapidly as the CO₂concentration falls below about 2.0 vol %. As the CO₂concentration drops further, e.g., to below 1.0 vol %, at some point the voltage across the fuel cell can become low enough that little or no further transport of carbonate may occur. Thus, at least some CO₂is likely to be present in the exhaust gas from the cathode stage of a fuel cell, pretty much regardless of the operating conditions.
One modification of the fuel cell operating conditions can be to operate the fuel cell with an excess of available reactants at the anode, such as by operating with low fuel utilization at the anode, as described above. By providing an excess of the reactants for the anode reaction in the fuel cell, the availability of CO₂for the cathode reaction can be used as a/the rate limiting variable for the reaction.
When operating MCFCs to enhance the amount of carbon capture, the factors for balancing can be different than when attempting to improve fuel utilization. In particular, the amount of carbon dioxide delivered to the fuel cells can be determined based on the output flow from the combustion generator providing the CO₂-containing stream. To a first approximation, the CO₂content of the output flow from a combustion generator can be a minor portion of the flow. Even for a higher CO₂content exhaust flow, such as the output from a coal-fired combustion generator, the CO₂content from most commercial coal fired power plants can be about 15 vol % or less. In order to perform the cathode reaction, this could potentially include between about 5% and about 15%, typically between about 7% and about 9%, of oxygen used to react with the CO₂to form carbonate ions. As a result, less than about 25 vol % of the input stream to the cathode can typically be consumed by the cathode reactions. The remaining at least about 75% portion of the cathode flow can be comprised of inert/non-reactive species such as N₂. H₂O, and other typical oxidant (air) components, along with any unreacted CO₂and O₂.
Based on the nature of the input flow to the cathode relative to the cathode reactions, the portion of the cathode input consumed and removed at the cathode can be about 25 vol % or less, for example about 10 vol % or less for input flows based on combustion of cleaner fuel sources, such as natural gas sources. The exact amount can vary based on the fuel used, the diluent content in the input fuel (e.g., N₂is typically present in natural gas at a small percentage), and the oxidant (air) to fuel ratio at which the combustor is operated, all of which can vary, but are typically well known for commercial operations. As a result, the total gas flow into the cathode portions of the fuel cells can be relatively predictable (constant) across the total array of fuel cells used for carbon capture. Several possible configurations can be used in order to provide an array of fuel cells to enhance/improve/optimize carbon capture. The following configuration options can be used alone or in combination as part of the strategy for improving carbon capture.
A first configuration option can be to divide the CO₂-containing stream between a plurality of fuel cells. The CO₂-containing output stream from an industrial generator can typically correspond to a large flow volume relative to desirable operating conditions for a single MCFC of reasonable size. Instead of processing the entire flow in a single MCFC, the flow can be divided amongst a plurality of MCFC units, usually at least some of which are in parallel, so that the flow rate in each unit can be within a desired flow range.
A second configuration option can be to utilize fuel cells in series to successively remove CO₂from a flow stream. Regardless of the number of initial fuel cells to which a CO₂-containing stream can be distributed to in parallel, each initial fuel cell can be followed by one or more additional cells in series to further remove additional CO₂. Similar to the situation demonstrated in FIG. 3 for the H₂input to the anode, attempting to remove CO₂within a stream in a single fuel cell could lead to a low and/or unpredictable voltage output. Rather than attempting to remove CO₂to a desired level in a single fuel cell, CO₂can be removed in successive cells until a desired level can be achieved. For example, each cell in a series of fuel cells can be used to remove some percentage (e.g., about 50%) of the CO₂present in a fuel stream. In such an example, if three fuel cells are used in series, the CO₂concentration can be reduced (e.g., to about 15% or less of the original amount present, which can correspond to reducing the CO₂concentration from about 6% to about 1% or less over the course of three fuel cells in series).
In another configuration, the operating conditions can be selected in early fuel stages in series to provide a desired output voltage while the array of stages can be selected to achieve a desired level of carbon capture. As an example, an array of fuel cells can be used with three fuel cells in series. The first two fuel cells in series can be used to remove CO₂while maintaining a desired output voltage. The final fuel cell can then be operated to remove CO₂to a desired concentration.
In still another configuration, there can be separate connectivity for the anodes and cathodes in a fuel cell array. For example, if the fuel cell array includes fuel cathodes connected in series, the corresponding anodes can be connected in any convenient manner, not necessarily matching up with the same arrangement as their corresponding cathodes, for example. This can include, for instance, connecting the anodes in parallel, so that each anode receives the same type of fuel feed, and/or connecting the anodes in a reverse series, so that the highest fuel concentration in the anodes can correspond to those cathodes having the lowest CO₂concentration.

Exhaust Gas Recycle

Aside from providing exhaust gas to a fuel cell array for capture and eventual separation of the CO₂, an additional or alternate potential use for exhaust gas can include recycle back to the combustion reaction to increase the CO₂content. Increasing the CO₂content of a combustion reaction for a gas powered turbine can be used to increase the power output of the turbine. When hydrogen is available for addition to the combustion reaction, such as hydrogen from the anode exhaust of the fuel cell array, further benefits can be gained from using recycled exhaust gas to increase the CO₂content within the combustion reaction.
In various aspects of the invention, the exhaust gas recycle loop of a power generation system can receive a first portion of the exhaust gas from combustion, while the fuel cell array can receive a second portion. The amount of exhaust gas from combustion recycled to the combustion zone of the power generation system can be any convenient amount, such as at least about 15% (by volume), for example at least about 25%, at least about 35%, at least about 45%, or at least about 50%. Additionally or alternately, the amount of combustion exhaust gas recirculated back to the combustion zone can be about 65% (by volume) or less, e.g., about 60% or less, about 55% or less, about 50% or less, or about 45% or less.
In one or more aspects of the invention, a mixture of an oxidant (such as air and/or oxygen-enriched air) and fuel can be combusted and (simultaneously) mixed with a stream of recycled exhaust gas. The stream of recycled exhaust gas, which can generally include products of combustion such as CO₂, can be used as a diluent to control, adjust, or otherwise moderate the temperature of combustion and of the exhaust that can enter the succeeding expander. As a result of using oxygen-enriched air, the recycled exhaust gas can have an increased CO₂content, thereby allowing the expander to operate at even higher expansion ratios for the same inlet and discharge temperatures, thereby enabling significantly increased power production.
A gas turbine system can represent one example of a power generation system where recycled exhaust gas can be used to enhance the performance of the system. The gas turbine system can have a first/main compressor coupled to an expander via a shaft. The shaft can be any mechanical, electrical, or other power coupling, thereby allowing a portion of the mechanical energy generated by the expander to drive the main compressor. The gas turbine system can also include a combustion chamber configured to combust a mixture of a fuel and an oxidant. In various aspects of the invention, the fuel can include any suitable hydrocarbon gas/liquid, such as natural gas, methane, ethane, naphtha, butane, propane, syngas, diesel, kerosene, aviation fuel, coal derived fuel, bio-fuel, oxygenated hydrocarbon feedstock, or any combinations thereof. The oxidant can, in some embodiments, be derived from a second or inlet compressor fluidly coupled to the combustion chamber and adapted to compress a feed oxidant. In one or more embodiments of the invention, the feed oxidant can include atmospheric air and/or enriched air. When the oxidant includes enriched air alone or a mixture of atmospheric air and enriched air, the enriched air can be compressed by the inlet compressor (in the mixture, either before or after being mixed with the atmospheric air). The enriched air and/or the air-enriched air mixture can have an overall oxygen concentration of at least about 25%, e.g., at least about 30 wt %, at least about 35 wt %, at least about 40 wt %, at least about 45 wt %, or at least about 50 wt %.
The enriched air can be derived from any one or more of several sources. For example, the enriched air can be derived from such separation technologies as membrane separation, pressure swing adsorption, temperature swing adsorption, nitrogen plant-byproduct streams, and/or combinations thereof. The enriched air can additionally or alternately be derived from an air separation unit (ASU), such as a cryogenic ASU, for producing nitrogen for pressure maintenance or other purposes. In certain embodiments of the invention, the reject stream from such an ASU can be rich in oxygen, having an overall oxygen content from about 50 wt % to about 70 wt %, can be used as at least a portion of the enriched air and subsequently diluted, if needed, with unprocessed atmospheric air to obtain the desired oxygen concentration.
In addition to the fuel and oxidant, the combustion chamber can optionally also receive a compressed recycle exhaust gas, such as an exhaust gas recirculation primarily having CO₂and nitrogen components. The compressed recycle exhaust gas can be derived from the main compressor, for instance, and adapted to help facilitate combustion of the oxidant and fuel, e.g. by moderating the temperature of the combustion products. As can be appreciated, recirculating the exhaust gas can serve to increase CO₂concentration.
An exhaust gas directed to the inlet of the expander can be generated as a product of combustion reaction. The exhaust gas can have a heightened CO₂content based, at least in part, on the introduction of recycled exhaust gas into the combustion reaction. As the exhaust gas expands through the expander, it can generate mechanical power to drive the main compressor, to drive an electrical generator, and/or to power other facilities.
The power generation system can, in many embodiments, also include an exhaust gas recirculation (EGR) system. In one or more aspects of the invention, the EGR system can include a heat recovery steam generator (HRSG) and/or another similar device fluidly coupled to a steam gas turbine. In at least one embodiment, the combination of the HRSG and the steam gas turbine can be characterized as a power-producing closed Rankine cycle. In combination with the gas turbine system, the HRSG and the steam gas turbine can form part of a combined-cycle power generating plant, such as a natural gas combined-cycle (NGCC) plant. The gaseous exhaust can be introduced to the HRSG in order to generate steam and a cooled exhaust gas. The HRSG can include various units for separating and/or condensing water out of the exhaust stream, transferring heat to form steam, and/or modifying the pressure of streams to a desired level. In certain embodiments, the steam can be sent to the steam gas turbine to generate additional electrical power.
After passing through the HRSG and optional removal of at least some H₂O, the CO₂-containing exhaust stream can, in some embodiments, be recycled for use as an input to the combustion reaction. As noted above, the exhaust stream can be compressed (or decompressed) to match the desired reaction pressure within the vessel for the combustion reaction.

Example of Integrated System

FIG. 6 schematically shows an example of an integrated system including introduction of both CO₂-containing recycled exhaust gas and H₂from the fuel cell anode exhaust into the combustion reaction for powering a turbine. In FIG. 6, the turbine can include a compressor 102, a shaft 104, an expander 106, and a combustion zone 115. An oxygen source 111 (such as air and/or oxygen-enriched air) can be combined with recycled exhaust gas 198 and compressed in compressor 102 prior to entering combustion zone 115. A fuel 112, such as CH₄, and a stream containing H ₂ 187 can be delivered to the combustion zone. The fuel and oxidant can be reacted in zone 115 and optionally but preferably passed through expander 106 to generate electric power. The exhaust gas from expander 106 can be used to form two streams, e.g., a CO₂-containing stream 122 (that can be used as an input feed for fuel cell array 125) and another CO₂-containing stream 192 (that can be used as the input for a heat recovery and steam generator system 190, which can, for example, enable additional electricity to be generated using steam turbines 194). After passing through heat recovery system 190, including optional removal of a portion of H₂O from the CO₂-containing stream, the output stream 198 can be recycled for compression in compressor 102. The proportion of the exhaust from expander 106 used for CO₂-containing stream 192 can be determined based on the desired amount of CO₂for addition to combustion zone 115.
The CO₂-containing stream 122 can be passed into a cathode portion (not shown) of a molten carbonate fuel cell array 125. Based on the reactions within fuel cell array 125. CO₂can be separated from stream 122 and transported to the anode portion (not shown) of the fuel cell array 125. This can result in a cathode output stream 124 depleted in CO₂. The cathode output stream 124 can then be passed into a heat recovery (and optional steam generator) system 150 for generation of heat exchange and/or additional generation of electricity using steam turbines 154. After passing through heat recovery and steam generator system 150, the resulting flue gas stream 156 can be exhausted to the environment and/or passed through another type of carbon capture technology, such as an amine scrubber.
After transport of CO₂from the cathode side to the anode side of fuel cell array 125, the anode output 135 can optionally be passed into a water gas shift reactor 170. Water gas shift reactor 170 can be used to generate additional H₂and CO₂at the expense of CO (and H₂O) present in the anode output 135. The output from the optional water gas shift reactor 170 can then be passed into one or more separation stages 140, such as a cold box or a cryogenic separator. This can allow for separation of an H₂O stream 147 and CO₂stream 149 from the remaining portion of the anode output. The remaining portion of the anode output 185 can include unreacted H₂generated by reforming but not consumed in fuel cell array 125. A first portion 145 of the H₂-containing stream 185 can be recycled to the input for the anode(s) in fuel cell array 125. A second portion 187 of stream 185 can be used as an input for combustion zone 115. A third portion (not shown) can be vented to the atmosphere, used as is for another purpose, and/or treated for subsequent further use. Although FIG. 6 and the description herein schematically details up to three portions, it is contemplated that only one of these three portions can be exploited, only two can be exploited, or all three can be exploited according to the invention.

Examples of Operating Ranges

In various embodiments of the invention, the process can be approached as starting with a combustion reaction for powering a turbine, an internal combustion engine, or another system where heat and/or pressure generated by a combustion reaction can be converted into another form of power. The fuel for the combustion reaction can comprise or be hydrogen, a hydrocarbon, and/or any other compound containing carbon that can be oxidized (combusted) to release energy. Except for when the fuel contains only hydrogen, the composition of the exhaust gas from the combustion reaction can have a range of CO₂contents, depending on the nature of the reaction (e.g. from at least about 2 vol % to about 25 vol % or less). Thus, in certain embodiments where the fuel is carbonaceous, the CO₂content of the exhaust gas can be at least about 2 vol %, for example at least about 4 vol %, at least about 5 vol %, at least about 6 vol %, at least about 8 vol %, or at least about 10 vol %. Additionally or alternately in such carbonaceous fuel embodiments, the CO₂content can be about 25 vol % or less, for example about 20 vol % or less, about 15 vol % or less, about 10 vol % or less, about 7 vol % or less, or about 5 vol % or less. Exhaust gases with lower relative CO₂contents (for carbonaceous fuels) can correspond to exhaust gases from combustion reactions on fuels such as natural gas. Higher relative CO₂content exhaust gases (for carbonaceous fuels) can correspond to optimized natural gas combustion reactions, such as those with exhaust gas recycle, or combustion reactions on fuels such as coal.
In some aspects of the invention, the fuel for the combustion reaction can contain at least about 90 wt % of compounds containing five carbons or less, e.g., at least about 95 wt %. In such aspects, the CO₂content of the exhaust gas can be at least about 4 vol %, for example at least about 5 vol %, at least about 6 vol %, at least about 7 vol %, or at least about 7.5 vol %. Additionally or alternately, the CO₂content of the exhaust gas can be about 13 vol % or less. e.g., about 12 vol % or less, about 10 vol % or less, about 9 vol % or less, about 8 vol % or less, about 7 vol % or less, or about 6 vol % or less. The CO₂content of the exhaust gas can represent a range of values depending on the configuration of the combustion powered generator. Recycle of an exhaust gas can be beneficial for achieving a CO₂content of at least about 6 vol %, while addition of hydrogen to the combustion reaction can allow for further increases in CO₂content to achieve a CO₂content of at least about 7.5 vol %.
Other components of the exhaust gas can correspond to any excess oxidant (O₂) from the combustion reaction, water vapor, any incomplete combustion products from carbonaceous material (such as CO), and/or other spectator species present in the fuel source or oxidant source. For example, if air is used as part of the oxidant source, the exhaust gas can include typical components of air such as N₂, H₂O, and other compounds in minor amounts that are present in air. Depending on the nature of the fuel source, additional species present after combustion based on the fuel source may include H₂O, H₂S, and other compounds either present in the fuel and/or that are partial or complete combustion products of compounds present in the fuel. The amount of O₂present in the exhaust can advantageously be sufficient to provide the oxygen needed for the cathode reaction in the fuel cell. Thus, the volume percentage of O can advantageously be at least 0.5 times the amount of CO₂in the exhaust. Optionally, as necessary, additional air can be added to the exhaust to provide sufficient oxidant for the cathode reaction. When some form of air is used as the oxidant, the amount of N₂in the exhaust can be at least about 50 vol %, e.g., at least about 60 vol %.
The input gas to the cathode can be similar in composition to the exhaust gas from the combustion reaction. Optionally, if the combustion reaction is performed under stoichiometric or nearly stoichiometric conditions, the exhaust gas may contain insufficient oxygen for the cathode reaction. In this situation, additional oxidant (air) may be added to either the exhaust gas or to the cathode input. The temperature and pressure of the exhaust gas from the combustion reaction may be similar or may differ from the input conditions for the fuel cell cathode. A suitable temperature for operation of an MCFC can be between about 500° C. and about 700° C., e.g., with an inlet temperature of about 550° C. and an outlet temperature of about 600° C. By contrast, the outlet temperature from the combustion reaction and/or corresponding turbine can be significantly higher. Prior to entering the cathode, heat can be removed from the combustion exhaust, if desired, e.g., to provide heat for other processes, such as reforming the fuel input for the anode.
The cathode of a fuel cell can correspond to a plurality of cathodes from an array of fuel cells, as previously described. For the cathode output from the final cathode(s) in an array sequence (typically at least including a series arrangement, or else the final cathode(s) the initial cathode(s) would be the same), the output composition can include about 2.0 vol % or less of CO₂(e.g., about 1.5 vol % or less or about 1.2 vol % or less). This relatively low concentration can reflect the loss of CO₂as carbonate ions transported across the electrolyte in the fuel cell(s) to the corresponding anode(s). The amount of O₂in the cathode output can also be reduced, typically in an amount proportional to the amount of CO₂removed, which can result in small corresponding increases in the amount(s) of the other (spectator) species at the cathode exit.
At least three input source components can be used for the anode reaction in the fuel cell. One input source is a fuel source, such as a stream containing H₂and/or a fuel that can be reformed into H₂(such as methane or another compound containing carbon and hydrogen). A second input source is a recycle feed from the anode output. A third “input” represents the carbonate ions transported across the electrolyte from the cathode.
The fuel source input can have a ratio of water to fuel appropriate for reforming the hydrocarbon (or hydrocarbon-like) compound in the fuel source used to generate hydrogen. For example, if methane is the input for reforming to generate H₂, the ratio of water to fuel can be about two to one. To the degree that H₂is a portion of the fuel, no additional water may typically be needed in the fuel. The fuel source can also optionally contain (small amounts of) components incidental to the fuel source (e.g., a natural gas feed can contain some content of CO₂as an additional component). For example, a natural gas feed can contain CO₂, N₂, and/or other inert (noble) gases as additional components.
For the anode output from the final anode(s) in an array sequence (typically at least including a series arrangement, or else the final anode(s) and the initial anode(s) would be the same), the output composition from the final anode(s) can include H₂O, CO₂, H₂, optionally CO, and optionally but typically a smaller portion of unreacted fuel (e.g., CH₄). The anode output can include at least about 25 vol % H₂O and from about 20 vol % to about 35 vol % CO₂. When the anode is operated to have a reduced fuel utilization, the amount of H₂in the anode output can additionally or alternately be from about 10 vol % H₂to about 50 vol % H₂. At the anode output, when present, the amount of CO can be from about 1 vol % or less to about 10 vol %. Optionally, a reforming stage can be included after the anode output to convert CO and H₂O in the anode output into CO₂and H₂, if desired. The anode output can further additionally or alternately include 2 vol % or less of various other components, such as N₂, CH₄(or other unreacted carbon-containing fuels), and/or other components.
After passing through the optional reforming stage, the anode output can be passed through one or more separation stages for removal of water and/or CO₂from the anode output stream. A cryogenic CO₂separator can be an example of a suitable separator. As the anode output is cooled, the majority of the water in the anode output can be separated out as a condensed (liquid) phase. Further cooling and/or pressurizing of the water-depleted anode output flow can then separate out high purity CO₂, as the other remaining components in the anode output flow (such as H₂, N₂, CH₄) do not tend to readily form condensed phases. A cryogenic CO₂separator can recover between about 90% and about 99% of the CO₂present in a flow, depending on the operating conditions.
Because both water and CO₂can be readily condensed out from the anode output flow, the stream leaving the separation stage(s) can include from about 30 vol % to about 70 vol % H₂, along with 15 vol % or less each of CO₂, H₂O, CH₄, and/or other components that can be considered spectator species during the anode reaction(s).
Applications for CO₂Output after Capture
In various aspects of the invention, the systems and methods described above can allow for production of carbon dioxide as a pressurized liquid. For example, the CO₂generated from a cryogenic separation stage can initially correspond to a pressurized CO₂liquid with a purity of at least about 90%, e.g., at least about 95%, at least about 97%, at least about 98%, or at least about 99%. This pressurized CO₂stream can be used, e.g. for injection into wells in order to further enhance oil or gas recovery such as in secondary oil recovery. When done in proximity to a facility that encompasses a gas turbine, the overall system may benefit from additional synergies in use of electrical/mechanical power and/or through heat integration with the overall system.
Alternatively, for systems dedicated to an enhanced oil recovery (EOR) application (i.e., not comingled in a pipeline system with tight compositional standards), the CO₂separation requirements may be substantially relaxed. The EOR application can be sensitive to the presence of O₂, so O₂can be absent, in some embodiments, from a CO₂stream intended for use in EOR. However, the EOR application can tend to have a low sensitivity to dissolved CO, H₂, and/or CH₄. Those dissolved gases can typically have only subtle impacts on the solubilizing ability of CO₂used for EOR. Injecting gases such as CO, H₂, and/or CH₄as EOR gases can result in some loss of fuel value recovery, but such gases can be otherwise compatible with EOR applications.
Additionally or alternately, a potential use for CO₂as a pressurized liquid can be as a nutrient in biological processes such as algae growth/harvesting. The use of MCFCs for CO₂separation can ensure that most biologically significant pollutants could be reduced to acceptably low levels, resulting in a CO₂-containing stream having only minor amounts of other “contaminant” gases (such as CO, H₂, N₂, and the like, and combinations thereof) that are unlikely to substantially negatively affect the growth of photosynthetic organisms. This can be in stark contrast to the output streams generated by most industrial sources, which can often contain potentially highly toxic material such as heavy metals.
In this type of aspect of the invention, the CO₂stream generated by separation of CO₂in the anode loop can be used to produce biofuels and/or chemicals, as well as precursors thereof. Further additionally or alternately, CO₂may be produced as a liquid, allowing for much easier pumping and transport across distances, e.g., to large fields of photosynthetic organisms. Conventional emission sources can emit hot gas containing modest amounts of CO₂(e.g., about 4-15%) mixed with other gases and pollutants. These materials would normally need to be pumped as a dilute gas to an algae pond or biofuel “farm”. By contrast, the MCFC system according to the invention can produce a concentrated CO₂stream (˜60-70% by volume on a dry basis) that can be concentrated further to 95%+(for example 96%+, 97%+, 98%+, or 99%+) and easily liquefied. This stream can then be transported easily and efficiently over long distances at relatively low cost and effectively distributed over a wide area. In these embodiments, residual heat from the combustion source/MCFC may be integrated into the overall system as well.
An alternative embodiment may apply where the CO₂source/MCFC and biological/chemical production sites are co-located. In that case, only minimal compression may be necessary (i.e. to provide enough CO₂pressure to use in the biological production, e.g., from about 15 psig to about 150 psig). Several novel arrangements can be possible in such a case. Secondary reforming may optionally be applied to the anode exhaust to reduce CH₄content, and water-gas shift may optionally additionally or alternately be present to drive any remaining CO into CO₂and H₂.

ADDITIONAL EMBODIMENTS

Embodiment

1

A method for capturing carbon dioxide from a combustion source, the method comprising: introducing one or more fuel streams and an O₂-containing stream into a combustion zone; performing a combustion reaction in the combustion zone to generate a combustion exhaust, the combustion exhaust comprising CO₂; processing at least a first portion of the combustion exhaust with a fuel cell array of one or more molten carbonate fuel cells to form a cathode exhaust stream from at least one cathode outlet of the fuel cell array, the one or more fuel cells each having an anode and a cathode, the molten carbonate fuel cells being operatively connected to the combustion exhaust through one or more cathode inlets in the fuel cell array; reacting carbonate from the one or more fuel cell cathodes with hydrogen within the one or more fuel cell anodes to produce electricity, an anode exhaust stream from at least one anode outlet of the fuel cell array comprising CO₂and H₂; separating CO₂from the anode exhaust stream in one or more separation stages to form a CO₂-depleted anode exhaust stream; passing at least a first portion of the CO₂-depleted anode exhaust stream to the combustion zone; and recycling at least a second portion of the CO₂-depleted anode exhaust stream to one or more of the fuel cell anodes.

Embodiment 2

The method of embodiment 1, wherein one or more of the following are satisfied: the fuel utilization in the one or more fuel cell anodes is about 65% or less, for example about 60% or less, about 55% or less, or about 50% or less (and optionally least about 25%, at least about 30%, or at least about 35%), which one or more fuel cell anodes can optionally correspond to at least one of the cathode stages whose inlet(s) has(have) a CO₂content as high as or higher than any other of the cathode stages; a plurality of anode stages have an anode fuel utilization of about 65% or less, for example from about 30% to about 50%; an average fuel utilization for the fuel cell array is about 65% or less, for example about 60% or less, about 55% or less, or about 50% or less (and optionally least about 25%, at least about 30%, or at least about 35%); and each anode stage has an anode fuel utilization of about 65% or less, for example about 60% or less, about 55% or less, or about 50% or less (and optionally least about 25%, at least about 30%, or at least about 35%).

Embodiment 3

The method of embodiment 1 or embodiment 2, wherein at least about 25% of the CO₂-depleted anode exhaust stream is recycled to the combustion zone and/or at least about 25% of the CO₂-depleted anode exhaust stream is recycled to the one or more fuel cell anodes.

Embodiment 4

The method of any one of the previous embodiments, further comprising passing carbon-containing fuel (such as comprising CH₄) into the one or more fuel cell anodes, without passing the carbon-containing fuel into an intervening reforming stage.

Embodiment 5

The method of embodiment 4, further comprising: reforming the carbon-containing fuel to generate hydrogen; and passing at least a portion of the generated hydrogen into the one or more fuel cell anodes.

Embodiment 6

The method of any one of the previous embodiments, wherein the combustion exhaust comprises about 10 vol %, or less (e.g., about 8 vol. % or less) of CO₂, the combustion exhaust optionally comprising at least about 4 vol % of CO₂.

Embodiment 7

The method of any one of the previous embodiments, further comprising recycling a CO₂-containing stream from the combustion exhaust to the combustion zone, which recycling can optionally comprise: exchanging heat between a second portion of the combustion exhaust and an H₂O-containing stream to form steam; separating water from the second portion of the combustion exhaust to form an H₂O-depleted combustion exhaust stream; and passing at least a portion of the H₂O-depleted combustion exhaust into the combustion zone.

Embodiment 8

The method of any one of the previous embodiments, wherein the combustion exhaust comprises at least about 6 vol % CO₂and/or wherein the anode exhaust stream comprises at least about 5.0 vol % of hydrogen (e.g., at least about 10 vol % or at least about 15 vol %).

Embodiment 9

The method of any one of the previous embodiments, further comprising exposing the anode exhaust stream to a water gas shift catalyst, a hydrogen content of the anode exhaust stream after exposure to the water gas shift being greater than a hydrogen content of the anode exhaust stream prior to the exposure.

Embodiment 10

The method of any one of embodiments 2-9, wherein the fuel utilization of the at least one anode stage is at least about 40%, such as at least about 45% or at least about 50%.

Embodiment 11

The method of any one of the previous embodiments, wherein the first portion of the CO₂-depleted anode exhaust stream is combined with a fuel stream prior to passing the first portion of the CO₂-depleted anode exhaust stream into the combustion zone.

Embodiment 12

The method of any one of the previous embodiments, wherein the cathode exhaust stream has a CO₂content of about 2.0 vol % or less (e.g., about 1.5 vol % or less or about 1.2 vol % or less).

Embodiment 13

The method of any one of the previous embodiments, wherein separating CO₂from the anode exhaust stream comprises cooling the anode exhaust stream to form a condensed phase of CO₂.

Embodiment 14

The method of embodiment 13, further comprising separating water from the anode exhaust stream prior to forming the condensed phase of CO₂.

EXAMPLES

A series of simulations were performed in order to demonstrate the benefits of using an improved configuration for using a fuel cell for CO₂separation. The simulations were based on empirical models for the various components in the power generation system. The simulations were based on determining steady state conditions within a system based on mass balance and energy balance considerations.
For the combustion reaction for the turbine, the model included an expected combustion energy value and expected combustion products for each fuel component in the feed to the combustion zone (such as C₁-C₄hydrocarbon, H₂, and/or CO). This was used to determine the combustion exhaust composition. An initial reforming zone prior to the anode can be operated using an “idealized” reforming reaction to convert CH₄to H₂. The anode reaction was modeled to also operate to perform further reforming during anode operation. It is noted that the empirical model for the anode did not require an initial H₂concentration in the anode for the reforming in the anode to take place. Both the anode and cathode reactions were modeled to convert expected inputs to expected outputs at a utilization rate that was selected as a model input. The model for the initial reforming zone and the anode/cathode reactions included an expected amount of heat energy needed to perform the reactions. The model also determined the electrical current generated based on the amount of reactants consumed in the fuel cell and the utilization rates for the reactants based on the Nernst equation. For species that were input to either the combustion zone or the anode/cathode fuel cell that did not directly participate in a reaction within the modeled component, the species were passed through the modeled zone as part of the exhaust or output.
In addition to the chemical reactions, the components of the system had expected heat input/output values and efficiencies. For example, the cryogenic separator had an energy that was required based on the volume of CO₂and H₂O separated out, as well as an energy that was required based on the volume of gas that was compressed and that remained in the anode output flow. Expected energy consumption was also determined for a water gas shift reaction zone, if present, and for compression of recycled exhaust gas. An expected efficiency for electric generation based on steam generated from heat exchange was also used in the model.
The basic configuration used for the simulations included a combustion turbine combine including a compressor, a combustion zone, and an expander. In the base configuration, a natural gas fuel input was provided to the combustion zone. The natural gas input included ˜93% CH₄, ˜2% C₂H₆, ˜2% CO₂, and ˜3% N₂. The oxidant feed to the compressor had a composition representative of air, including about 70% N₂and about 18% O₂. After passing through the expander, a portion of the combustion exhaust gas was passed through a heat recovery steam generation system and then recycled to the compressor. The remainder of the combustion exhaust was passed into the fuel cell cathode. After passing through the fuel cell cathode, the cathode exhaust exited the system. Unless otherwise specified, the portion of the combustion exhaust recycled back to the combustion zone was ˜35%. This recycled portion of the combustion exhaust served to increase the CO₂content of the output from the combustion zone. Because the fuel cell area was selected to reduce the CO₂concentration in the cathode output to a fixed value of ˜1.45%, recycling the combustion exhaust was found to improve the CO₂capture efficiency.
In the base configuration, the fuel cell was modeled as a single fuel cell of an appropriate size to process the combustion exhaust. This was done to represent use of a corresponding plurality of fuel cells (fuel cell stacks) arranged in parallel having the same active area as the modeled cell. Unless otherwise specified, the fuel utilization in the anode of the fuel cell was set to ˜75%. The fuel cell area was allowed to vary, so that the selected fuel utilization results in the fuel cell operating at a constant fuel cell voltage of ˜0.7 volts and a constant CO₂cathode output/exhaust concentration of ˜1.45 vol %.
In addition to the chemical reactions, the components of the system had expected heat input/output values and efficiencies. For example, the cryogenic separator had an energy that was required based on the volume of CO₂and H₂O separated out, as well as an energy that was required based on the volume of gas that was compressed and that remained in the anode output flow. Expected energy consumption was also determined for a water gas shift reaction zone, if present, and for compression of recycled exhaust gas. An expected efficiency for electric generation based on steam generated from heat exchange was also used in the model.
The basic configuration used for the simulations included a combustion turbine combine including a compressor, a combustion zone, and an expander. In the base configuration, a natural gas fuel input was provided to the combustion zone. The natural gas input included ˜93% CH₄, ˜2% C₂H₆, ˜2% CO₂, and ˜3% N₂. The oxidant feed to the compressor had a composition representative of air, including about 70% N₂and about 18% O₂. After passing through the expander, a portion of the combustion exhaust gas was passed through a heat recovery steam generation system and then recycled to the compressor. The remainder of the combustion exhaust was passed into the fuel cell cathode. After passing through the fuel cell cathode, the cathode exhaust exited the system. Unless otherwise specified, the portion of the combustion exhaust recycled back to the combustion zone was ˜35%. This recycled portion of the combustion exhaust served to increase the CO₂content of the output from the combustion zone. Because the fuel cell area was selected to reduce the CO₂concentration in the cathode output to a fixed value of ˜1.45%, recycling the combustion exhaust was found to improve the CO₂capture efficiency.
In the base configuration, the fuel cell was modeled as a single fuel cell of an appropriate size to process the combustion exhaust. This was done to represent use of a corresponding plurality of fuel cells (fuel cell stacks) arranged in parallel having the same active area as the modeled cell. Unless otherwise specified, the fuel utilization in the anode of the fuel cell was set to ˜75%. The fuel cell area was allowed to vary, so that the selected fuel utilization results in the fuel cell operating at a constant fuel cell voltage of ˜0.7 volts and a constant CO₂cathode output/exhaust concentration of ˜1.45 vol %.
In the base configuration, an anode fuel input flow provided the natural gas composition described above as a feed to the anode. Steam was also present to provide a steam to carbon ratio in the input fuel of ˜2:1. Optionally, the natural gas input can undergo reforming to convert a portion of the CH₄in the natural gas to H₂prior to entering the anode. When a prior reforming stage is present, ˜20% of the CH₄could be reformed to generate H₂prior to entering the anode. The anode output was passed through a cryogenic separator for removal of H₂O and CO₂. The remaining portion of the anode output after separation was processed depending on the configuration for each Example.
For a given configuration, a variety of values could be calculated at steady state. For the fuel cell, the amount of CO₂in the anode exhaust and the amount of O₂in the cathode exhaust was determined. The voltage for the fuel cell was fixed at ˜0.7 V within each calculation. For conditions that could result in a higher maximum voltage, the voltage was stepped down in exchange for additional current, in order to facilitate comparison between simulations. The area of fuel cell required to achieve a final cathode exhaust CO₂concentration of ˜1.45 vol % was also determined to allow for determination of a current density per fuel cell area.
Another set of values were related to CO₂emissions. The percentage of CO₂captured by the system was determined based on the total CO₂generated versus the amount of CO₂(in Mtons/year) captured and removed via the cryogenic separator. The CO₂not captured corresponded to CO₂“lost” as part of the cathode exhaust. Based on the amount of CO₂captured, the area of fuel cell required per ton of CO₂captured could also be determined.
Other values determined in the simulation included the amount of H₂in the anode feed relative to the amount of carbon and the amount of N₂in the anode feed. It is noted that the natural gas used for both the combustion zone and the anode feed included a portion of N₂, as would be expected for a typical real natural gas feed. As a result, N₂was present in the anode feed. The amount of heat (or equivalently steam) required for heating the anode feed for reforming was also determined. A similar power penalty was determined based on the power required for compression and separation in the cryogenic separation stages. For configurations where a portion of the anode exhaust was recycled to the combustion turbine, the percentage of the turbine fuel corresponding to H₂was also determined. Based on the operation of the turbine, the fuel cell, and the excess steam generated, as well as any power consumed for heating the reforming zone, compression, and/or separation, a total net power was determined for the system to allow for a net electrical efficiency to be determined based on the amount of natural gas (or other fuel) used as an input for the turbine and the anode.
FIGS. 7, 8, and 9 show results from simulations performed based on several configuration variants. FIG. 7 shows configurations corresponding to a base configuration as well as several configurations where a portion of the anode output was recycled to the anode input. In FIG. 7, a first configuration (1a) was based on passing the remaining anode output after the carbon dioxide and water separation stage(s) into a combustor located after the turbine combustion zone. This provided heat for the reforming reaction and also provided additional carbon dioxide for the cathode input. Configuration 1a was representative of a conventional system, such as the aforementioned Manzolini reference, with the exception that the Manzolini reference did not describe recycle of exhaust gas. Use of the anode output as a feed for the combustor resulted in a predicted fuel cell area of ˜208 km²in order to reduce the CO₂content of the cathode output to ˜1.45 vol %. The amount of CO₂lost as part of the cathode exhaust was ˜111 lbs CO₂/MWhr. Due to the large fuel cell area required for capturing the CO₂, the net power generated was ˜724 MW per hour. Based on these values, the amount of fuel cell area needed to capture a fixed amount of CO₂could be calculated, such as an area of fuel cell needed to capture a megaton of CO₂during a year of operation. For Configuration 1a, the area of fuel cell required was ˜101.4 km²*year/Mton-CO₂. The efficiency for generation of electrical power relative to the energy content of all fuel used in the power generation system was ˜58.9%. By comparison, the electrical efficiency for the turbine without any form of carbon capture was ˜61.1%.
In a second set of configurations (2a-2e), the anode output was recycled to the anode input. Configuration 2a represented a basic recycle of the anode output after separation to the anode input. Configuration 2b included a water gas shift reaction zone prior to the carbon dioxide separation stages. Configuration 2c did not include a reforming stage prior to the anode input. Configuration 2d included a reforming stage, but was operated with a fuel utilization of ˜50% instead of ˜75%. Configuration 2e was operated with a fuel utilization of ˜50% and did not have a reforming stage prior to the anode.
Recycling the anode output back to the anode input, as shown in Configuration 2a, resulted in a reduction of the required fuel cell area to ˜161 km². However, the CO₂loss from the cathode exhaust was increased to ˜123 lbs CO₂/MWhr. This was due to the fact that additional CO₂was not being added to the cathode input by the combustion of anode exhaust in a combustor after the turbine. Instead, the CO₂content of the cathode input was based only on the CO₂output of the combustion zone. The net result in Configuration 2a was a lower area of fuel cell per ton of CO₂captured of ˜87.5 km²*year/Mton-CO₂, but a modestly higher amount of CO₂emissions. Due to the reduced fuel cell area, the total power generated was ˜661 MW. Although the net power generated in Configuration 2a was about 10% less than the net power in Configuration 1a, the fuel cell area was reduced by more than 20%. The electrical efficiency was ˜58.9%.
In Configuration 2b, the additional water gas shift reaction zone increased the hydrogen content delivered to the anode, which reduced the amount of fuel needed for the anode reaction. Including the water gas shift reaction zone in Configuration 2b resulted in a reduction of the required fuel cell area to ˜152 km². The CO₂loss from the cathode exhaust was ˜123 lbs CO₂/MWhr. The area of fuel cell per megaton of CO₂captured was ˜82.4 km²*year/Mton-CO₂. The total power generated was ˜664 MW. The electrical efficiency was ˜59.1%.
Configuration 2c can take further advantage of the hydrogen content in the anode recycle by eliminating the reforming of fuel occurring prior to entering the anode. In Configuration 2c, reforming can still occur within the anode itself. However, in contrast to a conventional system incorporating a separate reforming stage prior to entry into the fuel cell anode. Configuration 2c relied on the hydrogen content of the recycled anode gas to provide the minimum hydrogen content for sustaining the anode reaction. Because a separate reforming stage was not required, the heat energy was not consumed to maintain the temperature of the reforming stage. Configuration 2c resulted in a reduction of the required fuel cell area to ˜149 km². The CO₂loss from the cathode exhaust was ˜122 lbs CO₂/MWhr. The area of fuel cell per ton of CO₂captured was ˜80.8 km²*year/Mton-CO₂. The total power generated was ˜676 MW. The electrical efficiency was ˜60.2%. Based on the simulation results, eliminating the reforming step seemed to have only a modest impact on the required fuel cell area, but the electrical efficiency appeared to be improved by about 1% relative to Configuration 2b. For an industrial scale power generation plant, an efficiency improvement of even only 1% is believed to represent an enormous advantage over the course of a year in power generation.
In Configuration 2d, reforming was still performed to convert ˜20% of the methane input to the anode into H₂prior to entering the anode. Instead, the fuel utilization within the anode was reduced from ˜75% to ˜50%. This resulted in a substantial reduction of the required fuel cell area to ˜113 km². The CO₂loss from the cathode exhaust was ˜123 lbs CO₂/MWhr. The area of fuel cell per ton of CO₂captured was ˜61.3 km²*year/Mton-CO₂. The total power generated was ˜660 MW. The electrical efficiency was ˜58.8%. Based on the simulation results, reducing the fuel utilization provided a substantial reduction in fuel cell area. Additionally, in comparison with Configurations 2b and 2e, Configuration 2d unexpectedly provided the lowest fuel cell area for achieving the desired level of CO₂removal.
Configuration 2e incorporated both the reduced fuel utilization of ˜50% as well as elimination of the reforming stage prior to the anode inlet. This configuration provided a combination of improved electrical efficiency and reduced fuel cell area. However, the fuel cell area was slightly larger than the fuel cell area required in Configuration 2d. This was surprising, as eliminating the reforming stage prior to the anode inlet in Configuration 2c reduced the fuel cell area in comparison with Configuration 2b. Based on this, it would have been expected that Configuration 2e would provide a further reduction in fuel cell area relative to Configuration 2d. In Configuration 2e, the CO₂loss from the cathode exhaust was ˜124 lbs CO₂/MWhr. The area of fuel cell per ton of CO₂captured of ˜65.0 km²*year/Mton-CO₂. The total power generated was ˜672 MW. The electrical efficiency was ˜59.8%. It is noted that Configuration 2d generated only 2% less power than Configuration 2e, while the fuel cell area of Configuration 2d was at least 6% lower than Configuration 2e.
The simulation results for Configurations 2b-2e provide a comparison of how reducing the anode fuel utilization can impact the total electrical efficiency in a power generation system. Even though reducing the fuel utilization to ˜50% in Configuration 2d led to a reduction in fuel cell area relative to Configuration 2b, the reduced anode fuel utilization also appeared to result in a reduction in electrical efficiency from ˜59.1% to ˜58.8%. This was in general agreement with conventional views on fuel utilization for molten carbonate fuel cells, where high fuel utilization values can be used to allow for efficient use of fuel delivered to the system. In the simulations for Configurations 2b-2e, in order to achieve an improvement in total electrical efficiency, the low fuel utilization can be combined with reducing and/or eliminating the amount of reforming, as shown in Configuration 2e.
FIG. 8 shows simulation results for additional configurations that included recycle of at least a portion of the anode exhaust to the combustion zone for the turbine. In FIG. 8, Configuration 1b was similar to Configuration 1a (shown in FIG. 7), but also included a water-gas shift reaction stage prior to the CO₂separation stages. Thus, Configuration 1b was representative of a conventional system, such as the aforementioned Manzolini reference, with the exceptions that the Manzolini reference did not describe a water-gas shift reaction stage or recycle of exhaust gas. The required fuel cell area to achieve a CO₂concentration in the cathode exhaust of ˜1.45% was ˜190 km². The amount of CO₂lost as part of the cathode exhaust was ˜117 lbs CO₂/MWhr. The area of fuel cell per ton of CO₂captured was ˜97.6 km²*year/Mton-CO₂. The total power generated was ˜702 MW. The electrical efficiency was ˜59.1%.
Configurations 3a, 3b, and 3d correspond to configurations where the anode output was used as an input for the combustion zone of the turbine. In these configurations, the H₂content of the anode output was available for use as a fuel in the turbine combustion zone. This appeared to be advantageous, as the carbon-containing fuel used to generate the H₂was generated in the anode recycle loop, where the majority of the resulting CO₂can be removed via the cryogenic separation stages. This could also result in a reduction of the amount of carbon containing fuel delivered to the combustion zone, but the reduction in carbon-containing fuel in the combustion zone could also result in the reduction of the CO₂concentration in the input to the cathode.
Configuration 3a was a configuration similar to Configuration 1a, but with recycle of the anode exhaust to the combustion zone. The required fuel cell area to achieve a CO₂concentration in the cathode exhaust of ˜1.45% was ˜186 km². The amount of CO₂lost as part of the cathode exhaust was ˜114 lbs CO₂/MWhr. The area of fuel cell per ton of CO₂captured was ˜100.3 km²*year/Mton-CO₂. The total power generated was ˜668 MW. The electrical efficiency was ˜59.7%. Relative to Configuration 1a, Configuration 3a had a lower total amount of CO₂generated (˜2.05 Mtons/year for Configuration 1a vs. ˜1.85 Mtons/year for Configuration 3a). This was believed to be due to the reduced amount of carbon-containing fuel delivered to the combustion zone. However, this also appeared to result in a reduced CO₂concentration delivered to the cathode input, which caused the model to show a reduced efficiency of CO₂removal for Configuration 3a. As a result, the net amount of CO₂exiting in the cathode exhaust was comparable for Configuration 1a and Configuration 3a. However. Configuration 3a appeared to have several advantages relative to Configuration 1a. First, Configuration 3a required a lower fuel cell area, so that the system in Configuration 3a would likely have a reduced cost. Additionally, the system in Configuration 3a appeared to have improved electrical efficiency, which can indicate lower fuel usage, even after adjusting for the different power output of the configurations.
Configuration 3b was similar to Configuration 3a, but also included a water gas shift reaction zone prior to the cryogenic separation stages. The required fuel cell area to achieve a CO₂concentration in the cathode exhaust of ˜1.45% was ˜173 km². The amount of CO₂lost as part of the cathode exhaust was ˜124 lbs CO₂/MWhr. The area of fuel cell per ton of CO₂captured was ˜96.1 km²*year/Mton-CO₂. The total power generated was ˜658 MW. The electrical efficiency was ˜59.8%. Configuration 3b appeared to have increased CO₂emission via the cathode exhaust. This was believed to be due to the additional hydrogen delivered to the combustion zone, which can result in a corresponding reduction in the amount of CO₂the combustion exhaust used for the cathode input. However, the fuel cell area was further reduced.
Configuration 3d was similar to Configuration 3b, but the anode fuel utilization was reduced from ˜75% to ˜50%. The required fuel cell area to achieve a CO₂concentration in the cathode exhaust of ˜1.45% was ˜132 km². The amount of CO₂lost as part of the cathode exhaust was ˜128 lbs CO₂/MWhr. The area of fuel cell per ton of CO₂captured was ˜77.4 km²*year/Mton-CO₂. The total power generated was ˜638 MW. The electrical efficiency was ˜60.7%. Based on the simulation results, reducing the fuel utilization in the anode appeared to result in a substantial improvement in electrical efficiency relative to Configuration 3b. This was believed to be due to the additional hydrogen delivered to the combustion zone for the turbine. For comparison, the electrical efficiency of the turbine without any carbon capture was ˜61.1%. Thus, the combination of recycling anode exhaust to the combustion zone and lower fuel utilization appeared to allow an electrical efficiency to be achieved approaching the efficiency without a carbon capture system.
FIG. 9 shows simulation results for additional configurations including recycle of at least a portion of the anode exhaust to both the combustion zone for the turbine and to the anode inlet. Configurations 4d, 4e, and 4f represent configurations where the remaining anode exhaust after separation (removal) of CO₂and H₂O was divided evenly between recycle to the anode input and recycle to the combustion zone for the turbine. In order to provide sufficient hydrogen for both the anode input and the combustion zone, the anode fuel utilization in Configurations 4d and 4e was set to ˜50%. Configurations 4d and 4e both included a water gas shift reaction zone prior to the separation stages. Configuration 4d included a separate reforming stage for reforming ˜20% of the additional fuel input to the anode prior to the fuel entering the anode. Configuration 4e did not include a reforming stage prior to the fuel entering the anode input. Configuration 4f was similar to Configuration 4e, with the exception that the anode fuel utilization in Configuration 4f was ˜33%, as opposed to the ˜50% in Configuration 4e.
Configuration 4d appeared to show the benefits of recycling the anode exhaust to both the anode input and the combustion zone. Relative to Configuration 2d, Configuration 4d appeared to provide an electrical efficiency about a full percentage point greater. Relative to Configuration 3d, Configuration 4d provided a reduced fuel cell area. In Configuration 4d, the required fuel cell area to achieve a CO₂concentration in the cathode exhaust of ˜1.45% was ˜122 km². The amount of CO₂lost as part of the cathode exhaust was ˜126 lbs CO₂/MWhr. The area of fuel cell per ton of CO₂captured was ˜63.4 km²*year/Mton-CO₂. The total power generated was ˜650 MW. The electrical efficiency was ˜59.9%.
Removing the pre-reforming stage in Configuration 4e appeared to provide further benefits. The required fuel cell area to achieve a CO₂concentration in the cathode exhaust of ˜1.45% was ˜112 km². The amount of CO₂lost as part of the cathode exhaust was ˜126 lbs CO₂/MWhr. The area of fuel cell per ton of CO₂captured was ˜63.4 km²*year/Mton-CO₂. The total power generated was ˜665 MW. The electrical efficiency was ˜61.4%. It is noted that the electrical efficiency was actually greater than the efficiency of the turbine without any type of carbon capture (˜61.1%).
Reducing the anode fuel utilization in Configuration 4f appeared to provide still further benefits with regard to both reducing fuel cell area and increasing electrical efficiency. The required fuel cell area to achieve a CO₂concentration in the cathode exhaust of ˜1.45% was ˜86 km². The amount of CO₂lost as part of the cathode exhaust was ˜126 lbs CO₂/MWhr. The area of fuel cell per ton of CO₂captured was ˜50.6 km²*year/Mton-CO₂. The total power generated was ˜654 MW. The electrical efficiency was ˜62.4%. It is noted that the electrical efficiency is actually greater than the efficiency of the turbine without any type of carbon capture (61.1%).
Configurations 5d, 5e, and 5f were similar to Configurations 4d, 4e, and 4f, with the exception that the exhaust gas recycle rate in Configurations 5d, 5e, and 5f was increased to ˜45%. Configurations 5d, 5e, and 5f had similar fuel cell areas and appeared to provide similar electrical efficiency, as compared to Configurations 4d, 4e, and 4f. However, the net amount of CO₂allowed to leave the system via the cathode exhaust was reduced by about 15% to about 209%, when the exhaust gas recycle rate was increased from about 30% to about 45%.
Although the present invention has been described in terms of specific embodiments, it is not so limited. Suitable alterations/modifications for operation under specific conditions should be apparent to those skilled in the art. It is therefore intended that the following claims be interpreted as covering all such alterations/modifications as fall within the true spirit/scope of the invention.

Claims

What is claimed is:

1. A method for capturing carbon dioxide from a combustion source, the method comprising:

introducing one or more fuel streams and an O₂-containing stream into a combustion zone;

performing a combustion reaction in the combustion zone to generate a combustion exhaust, the combustion exhaust comprising CO₂;

processing at least a first portion of the combustion exhaust with a fuel cell array of one or more molten carbonate fuel cells to form a cathode exhaust stream from at least one cathode outlet of the fuel cell array, the one or more fuel cells each having an anode and a cathode, the molten carbonate fuel cells being operatively connected to the combustion exhaust through one or more cathode inlets in the fuel cell array;

reacting carbonate from the one or more fuel cell cathodes with hydrogen within the one or more fuel cell anodes to produce electricity, an anode exhaust stream from at least one anode outlet of the fuel cell array comprising CO₂and H₂;

separating CO₂from the anode exhaust stream in one or more separation stages to form a CO₂-depleted anode exhaust stream;

passing at least a first portion of the CO₂-depleted anode exhaust stream to the combustion zone; and

recycling at least a second portion of the CO₂-depleted anode exhaust stream to one or more of the fuel cell anodes.

2. The method of claim 1, wherein the fuel utilization in the one or more fuel cell anodes is about 65% or less, such as about 60% or less.

3. The method of claim 2, wherein the at least one anode stage having an anode fuel utilization of 65% or less (such as about 60% or less) corresponds to at least one of the cathode stages whose inlet(s) has(have) a CO₂content as high as or higher than any other of the cathode stages.

4. The method of claim 1, wherein a plurality of anode stages have an anode fuel utilization of 65% or less, such as from about 30% to about 50%.

5. The method of claim 1, wherein an average fuel utilization for the fuel cell array is about 65% or less, such as about 60% or less.

6. The method of claim 1, wherein each anode stage has an anode fuel utilization of about 65% or less, such as about 60% or less.

7. The method of claim 1, wherein at least about 25% of the CO₂-depleted anode exhaust stream is recycled to the combustion zone and at least about 25% of the CO₂-depleted anode exhaust stream is recycled to the one or more fuel cell anodes.

8. The method of claim 7, further comprising passing carbon-containing fuel into the one or more fuel cell anodes.

9. The method of claim 8, further comprising:

reforming the carbon-containing fuel to generate hydrogen; and

passing at least a portion of the generated hydrogen into the one or more fuel cell anodes.

10. The method of claim 8, wherein the carbon-containing fuel is passed into the one or more fuel cell anodes without passing the carbon-containing fuel into a reforming stage prior to entering the one or more fuel cell anodes.

11. The method of claim 8, wherein the carbon-containing fuel comprises CH₄.

12. The method of claim 1, wherein the combustion exhaust comprises about 10 vol % or less of CO₂, such as about 8 vol % or less of CO₂, the combustion exhaust optionally comprising at least about 4 vol % of CO₂.

13. The method of claim 1, further comprising recycling a CO₂-containing stream from the combustion exhaust to the combustion zone.

14. The method of claim 13, wherein recycling the CO₂-containing stream from the combustion exhaust to the combustion zone comprises:

exchanging heat between a second portion of the combustion exhaust and an H₂O-containing stream to form steam;

separating water from the second portion of the combustion exhaust to form an H₂O-depleted combustion exhaust stream; and

passing at least a portion of the H₂O-depleted combustion exhaust into the combustion zone.

15. The method of claim 13, wherein the combustion exhaust comprises at least about 6 vol % CO₂.

16. The method of claim 1, wherein the anode exhaust stream comprises at least about 5.0 vol % of H₂, for example at least about 10 vol % or at least about 15 vol %.

17. The method of claim 1, further comprising exposing the anode exhaust stream to a water gas shift catalyst, a hydrogen content of the anode exhaust stream after exposure to the water gas shift being greater than a hydrogen content of the anode exhaust stream prior to the exposure.

18. The method of claim 1, wherein the fuel utilization of the at least one anode stage is at least about 40%, for example at least about 45% or at least about 50%.

19. The method of claim 1, wherein the first portion of the CO₂-depleted anode exhaust stream is combined with a fuel stream prior to passing the first portion of the CO₂-depleted anode exhaust stream into the combustion zone.

20. The method of claim 1, wherein the cathode exhaust stream has a CO₂content of about 2.0 vol % or less, for example about 1.5 vol % or less or about 1.2 vol % or less.

21. The method of claim 1, wherein separating CO₂from the anode exhaust stream comprises cooling the anode exhaust stream to form a condensed phase of CO₂.

22. The method of claim 21, further comprising separating water from the anode exhaust stream prior to forming the condensed phase of CO₂.