US20080022593A1

US20080022593A1 - Steam-carbon cell for hydrogen production

Info

Publication number: US20080022593A1
Application number: US11/497,037
Authority: US
Inventors: Turgut M. Gur; Alvin Duskin
Original assignee: Individual
Current assignee: Direct Carbon Technologies LLC
Priority date: 2006-07-31
Filing date: 2006-07-31
Publication date: 2008-01-31
Also published as: WO2008016355A2; WO2008016355A3

Abstract

This invention relates to high purity hydrogen production in a steam-carbon cell, which may be operated either in fuel cell mode thus generating electricity at the same time, or in electrolysis mode where the hydrogen production rate is augmented by an externally applied voltage. Introduction of a solid carbonaceous fuel at the anode eliminates the uphill barrier of the open circuit voltage for the reduction of H₂O to hydrogen. This novel concept nearly doubles the conversion efficiency of conventional electrolysis and offers near-zero emissions. The improved efficiency would mean that nearly half the greenhouse gases and other pollutants are produced. The product stream from the anode compartment primarily consists of CO₂and, hence, it is easier and cheaper to capture and mineralize the CO₂.

Description

FIELD

The invention relates to hydrogen production, and, more particularly, to producing hydrogen with a steam-carbon electrochemical cell.

BACKGROUND

There is a need to develop cost effective and efficient energy technologies based on hydrogen in order to meet environmentally and economically conflicting goals of energy production. Hydrogen is an efficient carrier of useful energy and the cleanest fuel from an environmental point of view where the end product is always the innocuous water. An economy structured upon hydrogen as the fuel of choice will be critical in mitigating global climate change, improving air quality and creating energy independence from petroleum-based fuels. However, the economics of currently available commercial hydrogen production technologies need to be dramatically improved to increase the competitiveness of hydrogen as a viable fuel.
Unfortunately, however, hydrogen is not a primary energy source, i.e., it is not readily available in free form. It is the most abundant (˜75 at %) element in the universe. But on earth, almost all of it is chemically bound in water. Therefore, hydrogen needs to be chemically extracted from water first. In this sense, it is an energy carrier that needs to be produced before it is used as fuel.
This invention represents a new technology that addresses the need for high-purity, low-cost, energy efficient hydrogen production. The technology is equally well suited for stationary and transportation applications due to its ability to be economically scaled to the desired capacity. It is inherently a modular technology, which makes it attractive for large-scale hydrogen production at regional facilities, such as renewable power parks, as well as distributed production such as small-scale vehicle fueling stations. Moreover, the technology disclosed in this invention produces an ultra high purity, inherently carbon-free hydrogen that makes it ideal for PEM fuel cells, which require CO-free hydrogen fuel for proper operation and long catalyst life.
At nearly $6.0/kg H₂, or higher, truck delivered hydrogen is a prohibitively expensive proposition for building a hydrogen fueling infrastructure essential for the Hydrogen Fuel Initiative announced by the Bush administration in 2003. Transition to a hydrogen-based transportation economy will require distributed generation technologies that can produce hydrogen cost-effectively and on-site. The novel technology disclosed in this invention is a major breakthrough towards materializing DOE's goal of cost effective distributed hydrogen generation.
Annually 9 million tons of hydrogen is produced centrally in a dozen plants in the U.S. mostly by steam reforming of fossil fuels, mostly natural gas. This is a mature technology accounting for 75% of the production capacity. However, steam reforming is cost effective only in large capacities for central production and for ready use, such as in petrochemical plants. Hence, these hydrogen production facilities are generally built next to or inside other chemical or petrochemical plants where hydrogen is readily used on site. Storage and transportation of hydrogen is very costly and inefficient.
Serious technical challenges confront distributed production technologies also for cost-effective hydrogen at lower capacities. This issue lies on the critical path to a hydrogen based energy economy. A radical approach to hydrogen production is clearly needed, and this invention promises just that.

Hydrogen by Steam Reforming

Conventional steam reforming is the primary technology to generate hydrogen centrally and is employed widely for large scale hydrogen production. Most of the new developing technologies for distributed production of hydrogen involve incremental adaptations of this process. Steam reforming involves the highly endothermic reaction of methane with excess steam at high pressures (up to 35 atm) and elevated temperatures (usually 800-1000° C.) in the presence of a transition metal catalyst to produce oxides of carbon and hydrogen i.e., syngas. Natural gas is the fuel of choice for steam reforming, but due to the highly endothermic nature of the reforming reaction, more than 30% of the methane is consumed just to provide the heat required for thermal management of the various processes during hydrogen production.
The syngas undergoes a two-stage water gas shift reaction where more steam is reacted with CO in the presence of ferro-chrome (Fe—Cr alloy) shift catalyst to obtain additional hydrogen. Both CO₂and CO are removed from hydrogen by scrubbing and pressure swing adsorption (PSA) processes, both of which add cost and inefficiencies. Furthermore, it is prohibitively expensive to remove all of the CO from hydrogen, which even in trace quantities is detrimental to PEM fuel cell catalytic electrodes and degrades its performance. Purified hydrogen is either liquefied, or pressurized for transportation to the points of use. Compression and transportation steps are expensive and energy intensive operations that account for a significant portion of the production cost. In short, centrally produced hydrogen may not be an effective and economically viable option to meet the distributed availability of cost effective hydrogen for fuel cell vehicles and other transportation needs.

Hydrogen by Water Electrolysis

Hydrogen can also be produced by the electrolysis of water. A voltage that opposes and is greater than the open-circuit voltage must be applied in order to split water. This requires an externally applied potential significantly more than 1.23 V at 25° C. to overcome the uphill energy barrier for the reduction of H₂O to H₂.
In order to electrolyze water, a voltage that opposes and is greater than the open-circuit voltage must be applied in order to split H₂O. Hence, much of the electricity used in conventional water electrolysis, between 60% to 70% of the total electrical power, is wasted in overcoming this potential barrier that is the result of the steep and uphill chemical potential gradient in oxygen.
Two types of near room temperature water electrolysis technologies are commercially available. The first is an established technology that employs an alkaline electrolyte (30 wt % KOH) with a diaphragm separator (usually asbestos) placed between the electrodes. The other uses a proton-conducting polymer membrane (PEM). In both techniques water is electrochemically split into gaseous hydrogen and oxygen that separately evolve in pure form at the cathode and anode, respectively.

Hydrogen by High Temperature Electrolysis of Steam

In conventional steam electrolysis, the gas circulated in the cathode side (where water is decomposed) is typically a mixture of steam and nascent hydrogen, while the gas circulated in the anode side is usually air. At zero current, the system has an uphill open-circuit voltage around −0.9 V depending upon the hydrogen/steam ratio and the operating temperature. This open-circuit voltage is much smaller than 1.23 V at 25° C. in the case for liquid water electrolysis, favoring electrolysis at elevated temperatures. A voltage significantly larger than the open circuit voltage must be supply from an external source in order to split H₂O and pump the oxygen ions from the steam side electrode through the electrolyte to the air side. Again, a substantial portion of the electrical energy is wasted in overcoming the steep potential barrier and giving rise to considerable reduction in the system efficiency.
High temperature electrolysis of steam was developed in the 1980s by Dornier GmbH in Germany. Their electrolyzer (dubbed HOT Elly) operated at 900-1000° C. and employed yttria stabilized zirconia (YSZ) solid electrolyte tubes for selective transport of oxide ions. Steam was fed inside the YSZ tubes and air was on the outside giving rise to an uphill open circuit potential of more than −0.9 V that must be overcome for electrolysis. The YSZ membrane physically separated the anode and cathode compartments. A DC voltage applied well above 0.9 V strips the oxygen from steam on the cathode surface, transports it across the zirconia membrane, and ejects it into air as molecular oxygen at the anode. Hydrogen produced in the cathode compartment is dried for high purity product.

SUMMARY

The invention is directed to a steam-carbon cell for the production of hydrogen while cogenerating electricity. This electrochemical cell comprises an anode compartment that houses the solid carbonaceous fuel, a cathode compartment that contains steam, and a ceramic electrolyte membrane. In a preferred embodiment, there is a thin film solid oxide electrolyte which is sandwiched between a porous cathode and an outer porous anode layer. In a preferred embodiment, the cell operates at elevated temperature with direct physical contact of an anode surface with carbon-containing solid fuel particles.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows the expected open circuit voltages for the three reactions relevant to the steam-carbon cell as a function of temperature. Note the temperature independence of E for the carbon oxidation reaction, while the behavior is strongly dependent on temperature for the cases of hydrogen and CO. The solid vertical arrow indicates the magnitude of the downhill driving force for hydrogen production in the case of complete oxidation of carbon. This driving force obviously increases with increasing temperature. The driving force is also greater for partial oxidation to CO than for full oxidation to CO₂as indicated by the dashed vertical arrow.

FIG. 2. Schematic design and operating principle of the steam-carbon cell depicting the details of the cell cross-section (not to scale), ionic transport, and electrode reactions. Right: The thin film solid oxide electrolyte is sandwiched between the porous cathode support tube indicated by the inner gray shade, and the outer porous anode layer. Steam is introduced into the cathode compartment and electrolyzed into hydrogen gas, which leaves the compartment. Left: Steam is reduced to hydrogen and oxide ions at the cathode. The solid electrolyte allows selective transport of oxide ion, which oxidizes carbon at the anode and release its electrons to the external circuit generating electricity.

FIG. 3. Schematic stalactite design of the agitated bed steam-carbon cell illustrating coal as the solid carbonaceous fuel at the anode compartment and steam at the cathode. The general design features include one-end closed ceramic tubular cell and the capability to capture any entrained coal particles in a cyclone, and recycling the captured coal particles and part of the CO₂back to the coal bed, the latter in order to enhance mass transport by agitation.

FIG. 4. Schematic stalactite design of the agitated bed steam-carbon cell illustrating coal as the solid carbonaceous fuel at the anode compartment and steam at the cathode. The general design features include one-end closed ceramic tubular cell and recycling part of the CO₂back to the coal bed in order to enhance mass transport by agitation.

FIG. 5. Schematic stalactite design of the immersion bed steam-carbon cell illustrating coal as the solid carbonaceous fuel at the anode compartment and steam at the cathode. The general design include one-end closed ceramic tubular cell. There is no recycling of the CO₂back to the coal bed for agitation.

FIG. 6. Schematic stalagmite design of the immersion bed steam-carbon cell illustrating coal as the solid carbonaceous fuel at the anode compartment and steam at the cathode. The general design include one-end closed ceramic tubular cell. There is no recycling of the CO₂back to the coal bed for agitation.

FIG. 7. Shell-and-tube type design where the pulverized coal bed is outside the tube in touch with the anode surface. This particular schematic does not illustrate CO₂or captured coal recycling, but these features can easily be incorporated and falls within the scope of this invention.

FIG. 8. Shell-and-tube type design (inverted version of FIG. 7) where the pulverized coal bed is now inside the tube in touch with the anode surface that is also inside the tube.

The annulus between the metal shell and the cathode surface facing the metal shell allows a flow of air. This particular schematic does not illustrate CO₂or captured coal recycling, but these features can easily be incorporated and falls within the scope of this invention.

FIG. 9. Schematic of a two chamber flat plate fluidized bed design where the pulverized coal bed is in touch with the anode surfaces of the ceramic membrane assemblies. More chambers are possible. This particular schematic also applies to corrugated plate design of the ceramic membrane assemblies. It does not illustrate CO₂or captured coal recycling, but these features can easily be incorporated and falls within the scope of this invention.

DESCRIPTION

Unlike conventional electrolysis technologies that need to overcome large open circuit potential barriers to split water or steam, the unique process disclosed here presents a novel concept with dramatic improvement in energy requirement and cost effectiveness. The present invention involves the use of a solid carbonaceous fuel such as coal at the anode compartment and steam at the cathode compartment of a high temperature solid oxide fuel cell. Such a combination provides sufficient gradient in the oxygen activity across the oxide ion conducting ceramic electrolyte membrane that eliminates the open circuit potential barrier of conventional electrolysis processes completely, and allows the system to operate as a fuel cell. Oxygen derived from the reduction of steam at the cathode diffuses as oxide ions downhill towards the anode by the vacancy mechanism through the ceramic membrane, and reacts with the carbon fuel at the anode to form carbon dioxide. The hydrogen gas produced at the cathode leaves the compartment in pure form. The only impurity is unreacted steam, which can further be removed by desiccation or other drying methods to obtain pure hydrogen.
The major impacts of this arrangement on total energy consumption, efficiency and product quality are several. First, the oxygen activity difference across the ceramic electrolyte is sufficiently large at elevated temperatures, which allows operation in a fuel cell mode since this is thermodynamically a downhill process. In other words, the cell not only produces high purity hydrogen at the cathode but also cogenerates electricity at the same time. This desirable outcome is directly the result of the innovative chemistry of the steam-carbon couple employed here, and makes distributed hydrogen production cost effective and highly efficient. Second, the exothermic heat for the oxidation of the carbonaceous fuel at the anode provides a significant portion of the endothermic reaction heat needed for electrolysis of steam at the cathode, thus improving thermal efficiency. Third, since heat is much cheaper than electricity per unit energy basis, this process significantly reduces the cost of H₂production. Fourth, all carbon containing species reside inside the anode compartment so the hydrogen produced at the cathode is collected in pure form with no contamination from CO or CO₂. This is because the carbon fuel deployed at the anode side is physically separated by a ceramic membrane from the steam in the cathode compartment. And finally, high operating temperature of this process not only allows faster reaction kinetics but also provides high value waste heat for process make-up energy, thus increasing the thermal efficiency of the overall process.
In the case where larger volumes or higher rates of hydrogen production is desirable, this fuel cell can be run also as an electrolyzer in order to accelerate the production rate. This involves supplementing the driving force for splitting steam by application of an appropriate potential drop across the ceramic membrane from an external power source. Since the novel chemistry employed here entirely eliminates the open circuit potential barrier for electrolysis, it significantly reduces the power requirement for electrolysis and increases electrical efficiency of hydrogen production.
This invention differs also from conventional steam reforming (i.e., coal gasification) technologies in significant ways. Unlike steam reforming where, in addition to steam, the oxygen required for gas phase oxidation of coal (or other solid carbonaceous fuels) is generally supplied in the form of air, this invention employs direct electrochemical oxidation (as opposed to reforming) of the carbonaceous fuel by lattice oxygen supplied through the ceramic electrolyte at the anode. So nitrogen plays no part in this invention and is entirely excluded from the process.
Equally important is the fact that the process disclosed here represents an inherently modular technology that is scaleable up or down and is ideally suited for distributed production of hydrogen on-site at high efficiency and low cost.
A similar approach for hydrogen production from steam was recently proposed by Pham et al [3,4] who exploited the depolarizing property of natural gas deployed to the anode.

Fundamental Aspects

The electrochemistry of this invention is founded on sound principles and favorable thermodynamics. The cathode (1), anode (2) and overall cell (3) reactions and their associated Gibbs free energy (ΔG°) and enthalpy (ΔH°) changes in kJ/mole of O₂at 300 K and 1300 K are given below in Table 1.

TABLE 1

Gibbs free energies and enthalpies of cell reactions calculated from established
thermochemical data [Barin and Knacke].

	ΔG°	ΔG°	ΔH°	ΔH°
	300 K	1300 K	300 K	1300 K
Reactions	(kJ/mol O₂)	(kJ/mol O₂)	(kJ/mol O₂)	(kJ/mol O₂)

1	2 H₂O_(g)= 2 H₂+ O₂	+458.298	+353.12	+484.963	+500.573
2	C + O₂= CO₂	−394.367	−396.149	−393.509	−395.727
3	2 H₂O_(g)+ C = CO₂+ 2 H₂	+63.931	−43.029	+91.454	+104.846
1	2 H₂O_(g)= 2 H₂+ O₂	+458.298	+353.12	+484.963	+500.573
4	2 C + O₂= 2 CO	−274.654	−453.093	−221.062	−227.844
5	2 H₂O_(g)+ 2 C = 2 CO + 2 H₂	+183.644	−99.973	+263.901	+272.729

From the simple relationship ΔG=−nFE, where n is the number of electrons and F is Faraday's constant, one can easily calculate the expected open circuit potential, E, of the cell utilizing these reactions. This is presented in Table 2 below.

TABLE 2

Expected values for the open circuit potentials for the cell reactions.

	E_OCV	E_OCV
	300 K	1300 K
Reactions	(V)	(V)

1	2 H₂O_(g)= 2 H₂+ O₂	−1.187	−0.915
3	2 H₂O_(g)+ C = CO₂+ 2 H₂	−0.166	+0.111
5	2 H₂O_(g)+ 2 C = 2 CO + 2 H₂	−0.114	+0.259

Note in Table 1, reaction (3) represents the steam-carbon couple employed in this invention. It is not only energetically spontaneous, but starkly different from conventional steam electrolysis, represented by the energetically unfavorable reaction (1) with a large and positive Gibbs free energy change of 353.12 kJ/mole of O₂at 300 K. In contrast, the negative Gibbs free energy change of −43 kJ/mole of O₂for the net cell reaction (3) indicates favorable thermodynamics for the steam-carbon couple at operating temperatures around 1000° C. This large Gibbs free energy assures that there is sufficient difference in the oxygen activity between the gaseous environment at the cathode and the anode compartments, the latter being much smaller, such that the oxygen extracted from steam gets transported to the carbon at the anode when an external resistive load is connected to the cell to complete the circuit for the electrons. In other words, the system operates as a fuel cell, splitting water to make hydrogen at the cathode and oxidizing carbon to CO₂at the anode, while generating electricity through the external circuit. Furthermore, the thermal energy requirement is nearly five times less for this new process (+104.846 kJ/mole O₂) than it is for conventional steam electrolysis (+500.573 kJ/mole of O₂) as evidenced by the large difference in the respective enthalpies for reactions (1) and (3) at 1300 K.
Note that conventional steam electrolysis is represented by reaction (1), which is thermodynamically an uphill process with a large and positive Gibbs free energy change of 353.12 kJ/mole of O₂, besides being highly endothermic indicated by the large and positive enthalpy of 500.573 kJ/mole of O₂. For these reasons, not only large amount of heat is required for reaction (1) to proceed, but also an electrical DC bias significantly larger than the uphill open circuit voltage of −0.915 volt (see Table 2) must externally be supplied in order to overcome this potential barrier and electrolyze the steam.
Conversely, the steam-carbon couple of reaction (3) totally and completely eliminates this potential barrier, and in fact, tilts it in the opposite direction that allows the system to operate in a fuel cell mode with a downhill open circuit voltage of +0.111 volt. When higher production rates for hydrogen are required, an external bias of suitable magnitude, but possibly less than 2 volts, may be applied to supplement this favorable gradient in the oxygen activity across the ceramic membrane. Naturally, renewable energy resources may be used to provide 100% of this energy requirement with no loss in system efficiency. Wind, solar and hydro resources may be used to supply this supplemental electric power.
As suggested by the data of reaction (3) in Tables 1 and 2, the difference in the oxygen activity across the ceramic membrane between the anode and cathode compartments show a significant temperature dependence primarily due to the large entropy change for reaction (1). So it may be preferable to operate this system at high temperatures, even above 1300 K, so that the oxygen activity difference between the anode and cathode increases further, providing a larger open circuit voltage, a steeper activity gradient for faster transport, and proportionately higher hydrogen production rate.
Another advantage higher temperatures may provide lies in the fact that partial oxidation of carbon to CO is thermodynamically more favorable than complete oxidation to CO₂at these elevated temperatures (see FIG. 1). This further increases the activity difference, i.e., the driving force for oxygen transport, which leads to higher electrical current densities and commensurately higher production rates for hydrogen. This is presented by reactions (4) and (5) in Table 1. Note in this case that the Gibbs free energy, which is a measure of the driving force for spontaneous change, is increased to −99.973 kJ/mole of O₂for reaction (5) than the smaller value of −43 kJ/mole of O₂for reaction (3). This manifests itself with a more favorable downhill open circuit voltage of +0.259 volt for this couple. Furthermore, since both reactions (1) and (3) have significant entropy changes leading to strong temperature dependence but with opposite temperature coefficients, higher temperatures lead to larger downhill open circuit voltages and encourage higher production rates for hydrogen as well as CO. The latter is a viable fuel in itself that can be utilized for water gas shift reaction to further make more hydrogen.
A comparison of technical advantages of this novel process relative to conventional steam electrolysis is summarized below:

- A reduction in the uphill open-circuit voltage from more than −0.9 V to a downhill voltage of more than +0.1V (depending upon the hydrogen/steam ratio and the operating temperature) results in a substantial reduction in the electric power requirement for this process when compared to conventional steam electrolysis.
- By utilizing a primary energy resource (e.g., coal, or any other carbonaceous solid fuel) for the majority of the system's energy requirements, this process achieves primary energy efficiency of 70%, nearly double that of conventional steam electrolysis.
- This process generates electricity while producing hydrogen at the same time.
- This process possesses fuel flexibility and can operate on many solid carbonaceous fuels including but not limited to coal, coke, peat, char, petroleum coke, biomass, waste plastics, tar sands, oil shale etc.
- When driven by an externally supplied power to increase hydrogen production rate, this process essentially substitutes electric power with cheap and abundant coal (or biomass, or other solid fuels), thereby greatly reducing the electricity and raw material costs for hydrogen production. Per unit of energy basis, coal at less than $0.002/MJ is nearly ten times less expensive than electricity at $0.019/MJ.
- By the substitution of coal or other solid carbonaceous fuels for air on the anode side, this process demands less heating compared to conventional steam electrolysis due to the highly exothermic reaction between carbon and oxygen on the anode side.
- This process improves the thermal efficiency of the system by supplying the large exothermic heat of carbon oxidation partially for the endothermic heat required for the steam electrolysis reaction.

Properties of Solid Oxide Electrolytes

An important component of the steam-carbon cell disclosed here is the solid oxide electrolyte that facilitates selective oxide ion transport. It helps slit up steam, extracts the oxygen and supplies the oxygen to the anode for the oxidation of the carbonaceous fuel and other reactants (such as hydrogen, sulfur etc present in the fuel, as in coal).
Predominantly oxide-ion conducting solids have been known to exist for almost a century. Among these solids, zirconia-based electrolytes have widely been employed as electrolyte material for solid oxide fuel cells (SOFC).
Zirconium dioxide has three well-defined polymorphs, with monoclinic, tetragonal and cubic structures. The monoclinic phase is stable up to about 1100° C. and then transforms to the tetragonal phase. The cubic phase is stable above 2200° C. with a CaF2 structure. The tetragonal-to-monoclinic phase transition is accompanied by a large molar volume (about 4%), which makes the practical use of pure zirconia impossible for high temperature refractory applications. However, addition of 8-15 m % of alkali or rare earth oxides (e.g., CaO, Y₂O₃, Sc₂O₃) stabilizes the high temperature cubic fluorite phase to room temperature and eliminates the undesirable tetragonal-to monoclinic phase transition at around 1100° C. The dopant cations substitute for the zirconium sites in the structure. When divalent or trivalent dopants replace the tetravalent zirconium, a large concentration of oxygen vacancies is generated to preserve the charge neutrality of the crystal. It is these oxygen vacancies that are responsible for the high ionic conductivity exhibited by these solid solutions. These materials also exhibit high activation energy for conduction [5] that necessitates elevated temperatures in order to provide sufficient ionic transport rates. The electronic contribution to the total conductivity is several orders of magnitude lower than the ionic component at these temperatures. Hence, these materials can be employed as solid electrolytes in high temperature electrochemical cells.
The chemical potential difference of oxygen across the solid oxide electrolyte is a measure of the open circuit potential given by the Nernst Equation,
E=−(RT/nF)ln(PO₂′/PO₂″) (6)
where E is the equilibrium potential of the fuel cell under open circuit conditions, R is the gas constant , F is Faraday's constant, n is the number of electrons per mole ( in the case of O₂, n=4), and PO₂denotes the partial pressure of oxygen. At the elevated temperatures where the steam-carbon cell normally operates, the oxygen activity at the steam side (i.e., cathode) is greater than at the carbon fuel side (i.e., anode), giving rise to a downhill open circuit potential (see Table 2) that provides the driving force for the oxide ions to diffuse from the cathode side to the anode.
The electrochemical production of hydrogen involves a high temperature steam-carbon cell that features an oxide ion selective solid electrolyte that extracts the oxygen from steam and supplies it to the anode where it is consumed for the electrochemical oxidation of carbon in the solid fuel. Granulated or pulverized carbon fuel, such as coal, is introduced into the anode compartment of the cell with or without other solid constituents, such as capturing agents for the mineralization of the CO₂and SO₂produced by the oxidation of the solid carbon fuel, which usually contains sulfur and other impurities.
The open circuit voltage of the steam-carbon cell is determined primarily by the carbon-oxygen equilibrium at the anode, and the hydrogen-steam equilibrium at the cathode. FIG. 1 shows the expected open circuit voltage as a function of temperature for the electrochemical oxidation reactions of carbon, and also compares the carbon-oxygen couple with that for hydrogen, which shows strong temperature dependence due to entropic effects.
The steam-carbon cell can be designed in various geometries. Only two will be discussed here, namely, the tubular and flat-plate geometries. Naturally other geometries are also possible, and are not excluded from the scope of this invention.
A typical schematic of the tubular cell involves a thick porous ceramic cathode that provides mechanical integrity for the multilayer structure. The thick porous cathode configuration is commonly referred to as a cathode-supported tube. Other tubular geometries, including flattened tubes etc. are also possible. A thin, impervious layer of oxide ion conducting ceramic membrane such as yttria stabilized zirconia (YSZ) solid electrolyte is coated on the outer surface of the cathode tube. Another thin but preferably porous layer that serves as the anode is then deposited on top of the YSZ as the outermost layer. A schematic of the tube structure and its operating principle is shown in FIG. 2. Typically, the YSZ and porous anode layers are each 10-50 mm thick, while the cathode support tube may be about 1-2 mm in wall thickness. The porous cathode support tube may be made of a mixed conducting perovskite or a catalytically active cermet while the porous anode layer is typically made of catalytically active cermet or a mixed conducting oxide.
FIG. 2 shows an anode 202, a solid oxide electrolyte 204, a cathode 206, oxygen ions 208, steam 210, a seal 212, and a metal shell 214.
Alternative to the cathode-supported tube geometry is the anode-supported tube configuration, where, this time the anode 202 is a thick porous structure to provide for mechanical integrity, and is usually made of a mixed conducting oxide or a suitable cermet that is catalytically active and electronically conductive enough to transport both electrons and oxide ions 208. This time, the thin YSZ and cathode 206 layers are coated on the surface of the anode-support tube to complete the cell configuration.
YSZ is the preferred solid electrolyte 204 for its high stability and ionic conductivity. However, scandia stabilized zirconia (SSZ) has an even higher conductivity than its yttria counterpart and, hence, may be more beneficial [5]. Also, it is possible to employ tetragonal zirconia which is known to possess higher conductivity and better thermal shock resistance than cubic zirconia electrolytes. Similarly, other oxide ion conductors such as doped cerates (e.g. Gd₂O₃.CeO₂) and doped gallates (e.g., La₂O₃.Ga₂O₂) can also be considered for the thin electrolyte 204 membrane.
The inner surface of the cathode 206 support tube is in contact with steam 210 to furnish the oxygen needed for the oxidation reaction at the anode 202, while the outer surface of the anode 202 is in direct, physical contact with the solid carbonaceous fuel. The YSZ solid oxide electrolyte 204 film in between serves as a selective membrane for transporting oxygen ions 208 only. The water molecule in steam 210 picks up electrons from the external circuit through the cathode 206 and is reduced to hydrogen gas and an oxide ion. The latter is then incorporated into the YSZ solid electrolyte 204 and diffuses downhill towards the anode 202 where it oxidizes the carbon to CO₂.
Using Kroger-Vink defect notation, the electrochemical reduction of steam 210 at the cathode 206 takes place as follows:
2H₂O(g)+2Vo{umlaut over ( )} (YSZ)+4e′ (electrode)=H₂(g)+2O_o ^x(YSZ) (7)
While the oxygen vacancies, Vo{umlaut over ( )}(YSZ), migrate under the influence of the chemical potential gradient through the YSZ solid electrolyte 204 film from the anode 202 to the cathode 206, oxygen ions 208 are transported in the reverse direction from the cathode 206 to the anode 202 where they participate in the electrochemical oxidation of carbon.
C+2O_o ^x(YSZ)=CO₂(g)+2Vo{umlaut over ( )} (YSZ)+4e′ (electrode) (8)
The electrons released during the oxidation reaction at the anode 202 travel through the external circuit (i.e., the power grid) towards the cathode 206, producing useful electricity. The oxygen chemical potential difference between the anode 202 and the cathode 206 provides greater than 0.1 volt of open circuit voltage (see Table 2).
For obtaining high conversion efficiency, it is desirable that the oxidation reaction of carbon primarily takes place at the anode 202 surface by lattice oxygen (i.e., Eq. (3)). The presence of lattice oxygen is preferred in embodiments involving direct physical contact of the anode 202 surface with the particles of carbon-containing fuel.
Expressed this time in ionic notation, the desired reaction (3) can be rewritten as
C(s)+2O²⁻(YSZ)=CO₂(g)+4e′(electrode) (9)
There are several other gas phase reactions possible at the solid carbon-gas interface, namely,
C(s)+½ O₂(g)=CO(g) (10)
C(s)+O₂(g)=CO₂(g) (11)
as well as the gas phase oxidation of CO by molecular oxygen supplied from the cathode 206 through the YSZ electrolyte 204.
CO(g)+½ O₂(g)=CO₂(g) (12)
and the reverse Bouduard reaction that leads to carbon precipitation
2 CO(g)=C(s)+CO₂(g) (13)
The desired reaction is (9) for obtaining maximum conversion efficiency. Therefore it is important to bring coal particles in direct physical contact with the anode 202 surface, i.e., they should be touching each other. This naturally necessitates that the coal bed and the anode 202 surface, and hence the ceramic cell tubes, must reside in the same temperature zone, and not thermally and spatially separated from one another.
This is achieved by immersing the solid electrolyte 204 containing cell tubes inside the pulverized carbon fuel bed, where the bed and the tubes reside in the same thermal zone. The coal particles touching the anode 202 surface are readily oxidized by the oxygen provided at the anode 202 surface through the solid electrolyte 204 membrane. Since the electrolyte 204 membrane is selective only to oxygen, the hydrogen gas produced from steam 210 stays behind in the cathode 206 compartment. This way, hydrogen never sees or interacts with carbon directly. In other words, this cell produces pure hydrogen with no contamination from CO₂and CO, the latter is a critically detrimental impurity that limits use of hydrogen for fuel cell applications.
Moreover, this invention makes it easy and inexpensive to capture and sequester or mineralize the CO₂since the anode flue gases from the steam-carbon cell is primarily CO₂. This point is important for compliance with Kyoto protocols regarding greenhouse gas emissions.
For cases where the abrasive action of the solid carbonaceous fuel on the anode 202 surface may be a series issue, an alternative design concept is to utilize a detached arrangement where the solid carbon fuel is physically separated from the ceramic cells, i.e., the ceramic tubes and the solid carbon fuel do not make direct physical contact.
The carbon-containing fuel comprises any carbon rich substance including: all grades and varieties of coal, charcoal, peat, petroleum coke, oil sand, tar sand, coke, char, carbon produced by pyrolysis of a carbonaceous substance, waste plastics, and biomass. For brevity, the carbon-containing fuel substances listed above may be referred to as “coal” in this document.
Optionally, an external voltage may be applied between the anode and cathode. This external voltage increases the oxygen transport rate through the solid oxide electrolyte, and drives the rate for hydrogen production higher.
Several different design alternatives are provided as examples to achieve direct, physical contact of the anode 202 surface with the coal particles. Other design alternatives are also possible. These designs may or may not involve recycling or circulation of an inert gas, such as He, Ar, N₂or preferably, CO2, to agitate the coal bed to enhance mass transport of reaction products away from the anode 202 surface so as not to block, hinder, or slow down the next unit of oxidation reaction taking place.
The steam-carbon cell operates in the temperature range 500 to 1300° C. This range provides the spectrum for the optimum operation of the cell, hydrogen production, carbon oxidation, and electricity generation. Thermodynamically, partial oxidation of carbon to CO has a strong temperature dependence and is favored at high temperatures, whereas complete oxidation to CO₂is thermodynamically favored at lower temperatures as is depicted in FIG. 1 (OCV vs. T). However, thermodynamics dictate only the natural tendency of a system to change or react, but does not govern how fast the system undergoes change. Kinetics and diffusion dictate collectively how fast a reaction or change will occur, and this is an exponential function of temperature. So higher temperatures offer faster reaction rates. Accordingly, the kinetics and product distribution of the carbon conversion and hydrogen production reactions are collectively best optimized when the operating temperature range of the steam-carbon cell lies between 500 to 1300° C.
There is another consideration that affects the operating temperature of the system. That has to do with the transport of oxide ions 208 through the ceramic electrolyte 204 membrane, which is a highly thermally activated process as discussed earlier, and prefers high operating temperatures. The oxide ions 208 transported across the membrane oxidize the carbon at the anode 202 compartment to generate electricity. In order to produce practically significant and useful levels of electrical current, which is intimately associated with the transport rate of oxide ions 208 through the membrane via the well-known Faraday's equation, the steam-carbon cell may operate between 600 and 1100° C., where the ionic conductivity of the electrolyte 204 membrane is larger than 10⁻⁴S/cm. To obtain even better performance, the steam-carbon cell may optionally operate in a temperature range of 700 to 1000° C.
The schematic of the agitated bed steam-carbon cell shown in FIG. 3 illustrates the general design features including the stalactite design of one-end closed ceramic tubular cells. The agitated bed is preferably made of a stainless steel shell 214 with proper ports for feeding the pulverized coal into the bed, and discharging the flue gases. It also has the capability to capture any entrained coal or other solid fuel particles in a cyclone, and recycling both the captured coal particles and part of the CO₂back to the coal bed, the latter in order to enhance mass transport by agitation of the coal bed.
FIG. 3 shows coal fuel 302, a resistive load 304, a coal bed 306, electrodes 308, CO ₂ 310, a membrane assembly 312, recycled CO ₂ 314, ash and slag 316, and hydrogen 318.
Variant modes of the stalactite design are shown in FIGS. 4 and 5 as examples, where the former shows only CO₂recycling for agitation of the coal bed.
Another design concept shown in FIG. 5 is an immersion bed direct coal fuel cell where the coal bed is immobile (i.e., fixed bed) and there is no forced agitation of the bed caused by the recycling of the CO₂product gas.
Yet another design concept is the stalagmite configuration of the ceramic tube cells as depicted in FIG. 6, which also illustrates an immersion type of coal bed operation without CO₂recycling. Naturally, the stalagmite design concept is also possible for the other modes of operation described above, as well as others not mentioned here.
Other design concepts may include shell-and-tube type design where the pulverized coal bed is outside the tube in touch with the anode 202 surface as illustrated in FIG. 7. This particular schematic does not illustrate CO₂or captured coal recycling, but these features can easily be incorporated and falls within the scope of this invention.
Another variant of this is the inverted shell-and-tube type design (i.e., inverted version of FIG. 7) where the pulverized coal bed is now inside the tube in touch with the anode 202 surface that is also inside the tube as shown in FIG. 8. The annulus 804 between the metal shell 214 and the cathode 206 surface facing the metal shell 214 allows steam 210 to flow. This particular schematic does not illustrate CO₂or captured coal recycling, but these features can easily be incorporated and falls within the scope of this invention.
Although similar in operation, another design geometry involves the use of flat or corrugated planar ceramic membrane assemblies. These are multilayered structures that consist of porous anode 202 (or cathode 206) support plates coated with thin impervious layers of the oxide ion conducting solid electrolyte 204 membrane, over which there is coated another thin but porous electrode layer to complete the cell structure. The plates are stacked in parallel fashion in the cell as shown in FIG. 9 such that the anode 202 surfaces face each other. Carbon-fuel is fed in between the anode 202 surfaces in alternating pairs of plates while steam 210 is provided along the outer surfaces that act as cathodes for the reduction reaction that generates molecular hydrogen 318 and oxide ions 208 as given by (7).
The steam-carbon cell has another unique characteristic. Operationally, it can be operated in a fuel cell mode where both hydrogen 318 and electricity are generated simultaneously, or, if large production rates are required, it can be operated much like a steam electrolyzer where an externally applied voltage drives the oxidation and reduction reactions at the electrodes faster. In the electrolysis mode, this process offers dramatic savings in the electricity consumption and cost for the cell operation since the unique chemistry of the steam-carbon cell diminishes the open circuit potential entirely, which otherwise would have to be overcome to extract the oxygen from the H2O molecule.
Please note: although the figures depict single ceramic tubes or flat plates in carbon beds, in practice, there may be a multitude of such ceramic tubes or flat plates in each carbon bed. The number of tubes or plates dictated by the level of desired power produced.
Yet another mode of operating the steam-carbon cell is to couple it to CO₂and SO₂sequestration either inside the cell in the anode 202 compartment or outside downstream of the cell. Sequestration of CO₂and SO₂can be achieved inside the bed by introducing gettering agents such as calcium oxide, magnesium oxide, dolomite, or a variety of magnesium silicates (e.g., olivine, serpentine, talc) mixed with pulverized coal and fed directly into the bed. These inorganic compounds may be used to mineralize carbon dioxide. The gettering agents readily react with the oxidation products CO₂and SO₂inside the cell forming solid carbonates and sulfates which eventually settle to the bottom of the carbon bed where they can be extracted. Or the flue gas leaving the cell can be treated with these gettering agents in a separate containment outside where the reaction products CO₂and SO₂can easily be mineralized by fixing them as solid carbonates and sulfates. Some of the relevant reactions for mineralization (also called carbonization) are provided below as examples.
Lime: CaO+CO₂═CaCO₃ (14)
Magnesia: MgO+CO₂═MgCO₃ (15)
Serpentine: Mg₃Si₂O₅(OH)_4(s)+3 CO_2(g)=3 MgCO_3(s)+2 SiO_2(s)+2 H₂O (16)
Olivine Mg₂SiO_4(s)+2 CO_2(g)=2 MgCO_3(s)+SiO_2(s) (17)
There are many embodiments of the present invention:

- A cell that produces pure hydrogen with no CO₂and more importantly, no CO contamination.
- A cell where the carbon containing anode compartment is physically separated from the cathode compartment where hydrogen is produced.
- A cell using a single temperature zone.
- A cell using multiples of temperature zones.
- A cell using direct physical contact (or touching) of anode surface with the coal particles.
- A cell where the anode surfaces are physically separated from the coal particles.
- A cell using fixed, immersion, agitated or fully fluidized coal bed to materialize contact.
- A cell using the solid carbonaceous fuel directly, rather than intermediate conversion to gaseous products by gasification or steam reforming.
- A cell wherein there is a one step process for simultaneous hydrogen production and electricity generation by direct conversion of coal to electrical energy.
- A process that does not combust coal, but oxidizes it.
- A cell that utilizes a solid oxide electrolyte to extract the oxygen from H₂O.
- A cell that utilizes a solid oxide electrolyte to supply the oxygen for the electrochemical oxidation of coal.
- A cell that produces highly concentrated (85-95% CO₂) flue gas that enables easy capturing and mineralization of the carbon dioxide.
- A cell that offers single point collection of CO₂.
- A cell that utilizes mineral carbonization.
- A cell that offers potentially near-zero emissions and stackless operation.

It will be apparent to one skilled in the art that the described embodiments may be altered in many ways without departing from the spirit and scope of the invention. Accordingly, the scope of the invention should be determined by the following claims and their equivalents.

Claims

1. A system that produces hydrogen, the system comprising:

an anode;

an anode compartment;

a solid oxide electrolyte that selectively transports oxygen ions; and

a cathode,

where the system uses a solid carbon-containing fuel,

where electrical current is generated by oxidation of the carbon-containing fuel,

where the oxygen and hydrogen are produced by dissociation of steam,

where steam is introduced proximate the cathode, and

where the carbon-containing fuel is introduced into the anode compartment.

2. The system of claim 1, where there is sequestration of carbon dioxide achieved by an inorganic compound introduced into the anode compartment.

3. The system of claim 1, where there is sequestration of CO₂and SO₂achieved by introducing into an anode compartment a gettering agent selected from a group consisting of calcium oxide, magnesium oxide, dolomite, olivine, serpentine, talc, mica, clay, and zeolite.

4. The system of claim 1, where there is direct physical contact of a surface of the anode with the carbon-containing fuel.

5. The system of claim 1, where there is direct physical contact of a surface of the cathode with the steam.

6. The system of claim 1, where the oxidation of carbon-containing fuel is by lattice oxygen provided through the solid oxide electrolyte to the anode.

7. The system of claim 1, where the oxidation of carbon-containing fuel is by lattice oxygen, the lattice oxygen being extracted from splitting the steam at the cathode surface, and where the lattice oxygen is provided through the solid oxide electrolyte to the anode.

8. The system of claim 1, where the anode compartment comprises a fixed bed of carbon-fuel with no gas flow.

9. The system of claim 1, where the anode compartment comprises a carbon-fuel containing bed agitated by gas flow.

10. The system of claim 1, where the anode compartment comprises a carbon-fuel containing bed fluidized by gas flow.

11. The system of claim 1, where there is a shell-and-tube type design, where there is a bed of pulverized carbon-containing material that is outside of the tube and in contact with the anode.

12. The system of claim 1, where there is a shell-and-tube type design, where there is a bed of pulverized carbon-containing material that is inside of the tube and in contact with the anode.

13. The system of claim 1, where there is a flat plate design with alternating compartments of carbon-fuel bed and steam flow space,

where the compartments are separated by flat or corrugated ceramic membrane assemblies, and

where there is a bed of pulverized carbon-containing material that is in contact with the anode.

14. The system of claim 1, where the system has an operating temperature in the range 500 to 1300 degrees Centigrade.

15. The system of claim 14, where the operating temperature is in the range 600 to 1100 degrees Centigrade.

16. The system of claim 15, where the operating temperature is in the range 700 to 1000 degrees Centigrade.

17. The system of claim 1, where an external voltage is applied between the anode and cathode, the external voltage increasing the oxygen transport rate through the solid oxide electrolyte and driving a rate for hydrogen production higher.

18. The system of claim 1, where the solid carbon-containing fuel is selected from a group consisting of pulverized coal, charcoal, peat, coke, char, petroleum coke, oil sand, tar sand, waste plastics, biomass, and carbon produced by pyrolysis of carbonaceous substance.