WO2004043851A1

WO2004043851A1 - Reduction of ammonia formation during fuel reforming

Info

Publication number: WO2004043851A1
Application number: PCT/US2003/035816
Authority: WO
Inventors: Abdul R. Khan; Yanlong Shi; Carlo Cioffi
Original assignee: Nuvera Fuel Cells, Inc.
Priority date: 2002-11-12
Filing date: 2003-11-12
Publication date: 2004-05-27
Also published as: AU2003291451A1; WO2004043851A8

Abstract

Method and apparatus for reducing formation of ammonia in a fuel reforming process, such as an autothermal reforming process. The process includes reacting a fuel (16) with air (20) and water (18) in a reforming unit (12) containing a precious metal catalyst bed (14) to produce a hydrogen containing reformate stream (22) substantially free of ammonia. The precious catalyst can be, for example, platinum, palladium, rhodium, ruthenium, iridium and combinations thereof. The operating temperature and pressure for the reforming unit (12) can be controlled to minimize the formation of ammonia in the reformate stream (22).

Description

REDUCTION OF AMMONIA FORMATION DURING FUEL REFORMING

RELATED APPLICATION(S)

This application claims the benefit of U.S. Provisional Application No. 60/425,841, filed on November 12, 2002, the entire teachings of which are incorporated herein by reference.

GOVERNMENT SUPPORT

The invention was supported, in whole or in part, by a grant DE-FC02- 99EE50580 from the U.S. Department of Energy. The Government has certain rights in the invention.

BACKGROUND OF THE INVENTION

Recently, proton exchange membrane (PEM) fuel cell based power systems have gained attention for possible application in stationary and transportation markets. These integrated power systems include a reformer that converts hydrocarbon fuel to a hydrogen-rich stream of reformate gas. This stream is then fed to a fuel cell that generates electric power. Auto-thermal reforming (ATR) is a widely practiced and accepted process for extracting hydrogen from hydrocarbon based fuels. This type of reforming has been extensively studied for a variety of fuels including gasoline, naptha, and other petroleum-based fuels, methanol, ethanol, and "bio-diesel." Previous studies were mainly focused on optimizing parameters for syngas production and efficiencies as a function of reforming conditions. For durable fuel cell operation, the quality of the reformate is an important factor, as the reforming process also produces some unwanted trace species, such as CO, F£₂S, NH₃, and hydrocarbons (HC) that are considered to be poisons for PEM fuel cells. The presence of these contaminants in reformate can adversely effect PEM fuel cell performance and functioning.

Ammonia, which is formed by the dissociation of nitrogen from air or fuel in the reforming process, has been shown to constitute a harmful fuel cell contaminant. The effects of ammonia on PEM fuel cell performance have previously been studied. Past studies have shown that, over time, trace levels of ammonia in the reformate will irreversibly damage the fuel cell. Contaminants like ammonia can be removed from the reformate by adding cleanup stages to the process. Depending on the number of contaminants, and the levels and capacity of cleanup material used, these cleanup stages can occupy an appreciable volume of the total system and impose pressure drops that penalize efficiencies. Such beds also add another step toward the complexity of the reforming process.

SUMMARY OF THE INVENTION

In a fuel reforming system, it would be desirable to reduce the complexity, size, and even the necessity of the clean up stages for removing fuel cell contaminants, such as ammonia. One possible alternative to the downstream cleanup of such species is to use catalyst(s) in the fuel reforming process which reduce or eliminate the formation of certain unwanted contaminants. This invention relates in one aspect to reduced ammonia formation in autothermal reformers by selection of the proper reforming catalyst(s), and optimizing the process conditions for reforming. Specifically, the present inventors have found that substitution of conventional nickel-based catalysts with precious-metal catalysts in the fuel reforming process can significantly reduce the amount of ammonia in the reformate stream. hi one aspect, the present invention relates to a method of reducing the formation of ammonia in a fuel reforming process which comprises reacting a fuel with water and air over a precious metal catalyst to produce a hydrogen-containing reformate that is substantially free of ammonia. In a preferred embodiment, the fuel reforming process is an auto-thermal reforming (ATR) process. The precious metal catalyst can be, for example, platinum, palladium, rhodium, ruthenium, indium, and combinations thereof. Preferably, the precious metal catalyst is present in an amount sufficient to produce a hydrogen-containing reformate having an ammonia content of less than about 50 ppm, even more preferably less than about 10 pp , even more preferably less than about 5 ppm, and even more preferably less than about 2 ppm. hi another aspect, the method comprises controlling the operating temperature and pressure of the fuel reforming reaction to minimize the formation of ammonia in the reformate stream.

The invention also relates to a fuel reformer, such as an auto-thermal fuel reformer, which comprises a precious metal catalyst bed. The fuel reformer is adapted to react a fuel with water and air over the precious-metal catalyst to produce a hydrogen-containing reformate that is substantially free of ammonia. The precious-metal catalyst can be, for example, platinum, palladium, rhodium, ruthenium, iridium, and combinations thereof. Preferably, the precious metal catalyst is present in an amount sufficient to produce a hydrogen-containing reformate having an ammonia content of less than about 50 ppm, even more preferably less than about 10 ppm, even more preferably less than about 5 ppm, and even more preferably less than about 2 ppm.

Precious metal catalysts have generally not been used in conventional fuel reforming methods, primarily due to the fact that these catalysts are significantly more expensive than traditional transitional-metal catalysts (such as nickel, and nickel-iron)^'. However, the present inventors have discovered that by substituting a traditional nickel-based catalysts with a precious metal catalyst, the amount of ammonia in the reformate stream can be greatly reduced. This minimizes the need for large, complex and costly ammonia cleanup units downstream of the reformer, and can help improve efficiency and reduce cost in an integrated fuel reformer/fuel cell system.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

Fig. 1 is a schematic of a fuel reforming system of the present invention; Fig. 2 is a schematic of an auto-thermal reforming (ATR) process setup used to compare ammonia formation in nickel-based and precious metal-based catalysts;

Fig. 3 is a graph showing ammonia formation for different fuels and different catalyst materials;

Fig. 4 is a graph showing the equilibrium calculation for ammonia in an ATR reformate stream;

Fig. 5 is a graph showing the effect of temperature on ammonia formation with a precious metal-based catalyst; and

Fig. 6 is a graph showing the effect of pressure on ammonia formation with a precious metal-based catalyst.

DETAILED DESCRIPTION OF THE INVENTION

A description of preferred embodiments of the invention follows. Figure 1 is a schematic of a fuel processing system 10 according to the invention. As is well known, fuel reforming designed to produce hydrogen for use in fuel cells has three main steps. The first step, fuel reforming, occurs in reforming unit 12. The reforming unit receives inputs of fuel 16, water 18, and air 20, and reacts these fluids at a high-temperature over a catalyst bed 14. In the present invention, the catalyst bed 14 is comprised of a precious metal catalyst. The precious metal catalyst can be, for example, platinum, palladium, rhodium, ruthenium, iridium, or combinations thereof. In the reforming process, a fuel, typically a hydrocarbon or alcohol is reacted with water to form a reformate gas stream 22 containing carbon monoxide, hydrogen, and, when air is introduced, nitrogen. In autothermal reforming (and in a functionally identical process, partial oxidation reforming), a controlled quantity of air is introduced into the reformer, along with steam and fuel, to react with some of the fuel to provide the heat needed by the reforming reaction, which is endothermic.

Next, the reformate stream 22 and additional water 24 are fed to shift reactor 26. In the shift reactor, the added water 24 reacts with the carbon monoxide in the reformate 22 in the "water gas shift" reaction to produce carbon dioxide and more hydrogen. In a third stage, the reformate 22 fed to one or more cleanup modules 28 to remove unwanted contaminants and make the reformate suitable for the end use, such as in a PEM fuel cell. In particular, fuel cell poisons such as carbon monoxide and ammonia are removed in this third stage, by preferential oxidation or absorption beds, for example. An advantage of the present invention is that by using a precious metal catalyst in the reforming reaction, rather than a traditional nickel-based catalyst, the reformate 22 generally contains only trace amounts of ammonia following the fuel reforming reaction. Accordingly, the cleanup module for removing ammonia from the reformate can be made substantially smaller, or even eliminated entirely from the fuel processing system.

Experimental Setup

Figure 2 a schematic of an auto-themal reforming process setup used to compare ammonia formation in nickel-based and precious metal-based catalysts . The main focus of study was on the autothermal reforming zone, as this was shown to be the primary site for ammonia formation. This is because downstream samples did not show any additional ammonia formation in the later stages of the reforming process. As a result, the water gas shift steps were bypassed and all the analysis was done on the pre-shift reformate. Nuvera's Modular Pressurized Reactor facility was used to conduct these studies. This apparatus is capable of testing a wide variety of fuels and catalyst forms at sizes appropriate for a full scale reformer. Power input can range from 40-15 OkW thermal over an operating pressure range of 1-10 atmospheres. Each step of the reforming process occured in a module in which the conditions of temperature, pressure and space velocity that would normally be present inside a real reformer were provided. Multiple sampling ports were provided for gas and thermal analysis. Analytical equipment used for gas analysis included on-line gas analyzers (CO, CO₂), a gas chromatograph, and a Fourier transform infrared (FTIR) spectrometer. FTIR spectroscopy was used for analyzing contaminants, particularly ammonia, in the reformate stream. The FTIR was calibrated for low levels of ammonia ranging from 5 to 100 PPM. The lower detectable limit for the FTIR was about 2ppm. Gasoline, ethanol and natural gas were reformed over different ATR catalysts in this apparatus. The study was divided into two parts. The first part focused the effect of catalyst on ammonia formation. Fuels tested in this study were ethanol and sulfur free gasoline. The properties of the different fuels are listed in Tablel . Denatured ethanol was supplied by Fisher Scientific and sulfur free gasoline from Phillips Chemical Company. The two fuels were reformed over precious metal and nickel based catalysts. A direct comparison was made between reformates generated by using two different types of catalyst for the same fuel and two different fuels over the same catalyst. In the second part of the experiment, natural gas was the only fuel and the study was more focused on the effect of ATR operating parameters, mainly temperature and pressure, on ammonia formation.

Table 1 : Fuel Properties

Study of Ammonia Formation Over Different ATR Catalysts

Gasoline was autothermally reformed over a nickel based ATR catalyst. The catalyst, G91, was a commercial variety supplied by SudChemie. A preheated fuel, steam and air mixture was sent to the ATR reactor where an exothermic reaction provided the heat for reforming. An equivalence ratio of 3.5 was used in the study where Equivalence ratio = (Fuel/Air)/(Fuel/Air)_{stoiclliometric}. Once steady state was achieved, gas samples were collected along the length of the reactor bed and were analyzed for ammonia using FTIR spectroscopy. The samples taken were kept hot to prevent ammonia from dropping out into water.

As shown in Figure 3, nickel based catalyst test results for gasoline showed an increasing trend for ammonia with higher residence times. Residence times shown in plots are the normalized values. Residence time is defined as Residence time= Catalyst Volume Flow rate. The study was repeated with a precious metal based ATR catalyst. The precious metal (PM) catalyst was a commercial variety, 383, obtained from the OMG Group; the PM is believed to be primarily platinum. Other precious metals believed to be effective in this substitution include, without limitation, palladium, rhodium, ruthenium and iridium. Such catalysts have not been used in more classical reforming methods, such a steam reforming, because PM- containing catalysts are significantly more expensive than traditional transition- metal based catalysts (nickel, often also containing iron).

The resulting reformate prepared over the PM catalyst showed very low levels (BDL, below detectable limit) of ammonia in the gas stream. Ethanol was also reformed under similar conditions over nickel and precious metal based ATR catalysts. A similar increasing trend as a function of higher residence times and effect of catalyst on ammonia formation was exhibited by ethanol.

The results from this testing show that ammoma formation had a strong dependence on the type of catalyst used for reforming. Compared to the precious metal catalyst, the nickel based catalyst promoted the ammoma formation reaction and the levels seen were closer to the equilibrium as shown in Figure 4. This plot shows the theoretical equilibrium values for ammonia over a range of ATR temperatures. Precious metal catalyst was less favorable to ammonia formation. As a result, reformate generated using precious metal based ATR catalysts was cleaner than conventional nickel.

In addition, it was seen from this study that catalyst behavior to promote ammonia formation exceeded the fuel structure difference as similar ammonia levels and trends were observed for the two very different fuels. This is shown clearly in Figure 4, in which the equilibrium ammonia formation at constant conditions except for varying temperature is calculated. It is also to be noted that increase in the ATR bed temperature diminishes the amount of ammonia formed. Study of ATR Operating Conditions on Ammonia Formation hi this study natural gas was reformed using a precious metal based ATR catalyst. The dependencies of temperature and pressure on ammoma formation were investigated. The reformer was operated at lower temperatures and ammonia levels measured were under detectable limits of FTIR. Once the data was collected at this operating condition ATR bed temperatures were cycled by varying preheats of steam, fuel and air mixture going to the reforming zone. The results are shown in Figure 5. A gradual increase in ammonia levels was observed with higher temperatures, ranging from the below detectable levels to a maximum of ~9 ppm. After reaching this point, ATR temperatures were again lowered back to starting conditions by reducing preheats to the feeds. This led to the reduced ammonia levels finally going below detectable limits. As shown in Figure 5, continuous ammonia measurements were made and the ammonia profile as a function of ATR temperature was generated. It showed that ammonia formation was largely controlled by ATR bed temperatures. Faster kinetics due to higher temperature shifted ammonia levels towards equilibrium. As seen in the graph, optimum conditions made it possible to achieve below detectable levels of ammonia in the natural gas reformate. hi order to observe the effect of pressure on the ammonia formation the reactor was first started under atmospheric conditions. For comparison purposes, the conditions were adjusted so as to make ammoma within detectable limits of FTIR. After collecting some steady state data at this condition the pressure was raised during the operation from atmospheric to 3atms. As shown in Figure 6 little or no significant increase was observed in the ammonia formation. The hump seen during the transition was attributed to the pressure variation caused by the reactor in the FTIR cell compartment. This study showed the operating temperature dominated over pressure and turned out to be the key parameter for controlling ammonia formation in the autothermal reforming zone.

Gasoline, ethanol, and natural gas were successfully reformed in the modular pressurized flow reactor. A nickel based catalyst promoted ammonia formation reaction(s), adding significant levels of contamination in the reformate stream generated through the auto-thermal reforming process. In comparison to nickel, reformate generated using a precious metal based ATR catalyst was seen to be much cleaner and suitable for the system. This can greatly reduce the complexity, size, and need of the clean up stages in the reforming system. Difference in the fuel structure does not have much of an effect on the ammonia formation. The ammonia formation was greatly governed by the nature of the catalyst. Effects of ATR operating conditions on ammoma formation were investigated. Both increased catalyst temperature and increased bed pressure increased the amount of ammonia formed over the PM catalyst. The effect of catalyst bed temperature was found to be much larger than the effect of pressure (in the range of a few atmospheres) for controlling ammonia formation. Hence, it has been demonstrated that the right selection of catalyst and optimization conditions can greatly reduce the ammonia levels generated in a reformer. This allows a longer life for the fuel cell stack, and minimizes the extent of cleanup apparatus required for operation of an integrated system.

While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.

Claims

CLAIMSWhat is claimed is:

1. A method of reducing the formation of ammonia in a fuel reforming process, comprising: reacting a fuel with water and air over a precious metal catalyst to produce a hydrogen-containing reformate that is substantially free of ammonia.

2. The method of claim 1 , wherein the reaction comprises an auto-thermal reaction.

3. The method of claim 1 , wherein the precious metal catalyst comprises platinum.

4. The method of claim 1, wherein the precious metal catalyst comprises palladium.

5. The method of claim 1, wherein the precious metal catalyst comprises rhodium.

6. The method of claim 1, wherein the precious metal catalyst comprises ruthenium.

7. The method of claim 1, wherein the precious metal catalyst comprises iridium.

8. The method of claim 1, wherein precious metal catalyst is present in an amount sufficient to produce a hydrogen-containing reformate having an ammonia content of less than about 50 ppm.

9. The method of claim 8, wherein the precious metal catalyst is present in an amount sufficient to produce a hydrogen-containing reformate having an ammonia content of less than about 10 ppm.

10. The method of claim 9, wherein the precious metal catalyst is present in an amount sufficient to produce a hydrogen-containing reformate having an ammonia content of less than about 5 ppm.

11. The method of claim 10, wherein the precious metal catalyst is present in an amount sufficient to produce a hydrogen-containing reformate having an ammoma content of less than about 2 ppm.

12. The method of claim 1, further comprising: controlling the temperature of the fuel reforming reaction to minimize the formation of ammonia in the reformate stream.

13. The method of claim 12, wherein controlling the temperature comprises maintaining the reaction at as low a temperature as feasible.

14. The method of claim 1, further comprising: controlling the pressure of the fuel reforming reaction to mimimize ^■ the formation of ammonia in the reformate stream.

15. The method of claim 14, wherein controlling the pressure comprises maintaining the reaction at as low an operating pressure as feasible.

16. A fuel reformer, comprising: a precious metal catalyst bed for facilitating a fuel reforming reaction to produce a hydrogen-containing reformate that is substantially free of ammonia.

17. The fuel reformer of claim 16, wherein the reformer is an auto-thermal reformer.

18. The fuel reformer of claim 16, wherein the precious metal catalyst comprises platinum.

19. The fuel reformer of claim 16, wherein the precious metal catalyst comprises palladium.

20. The fuel reformer of claim 16, wherein the precious metal catalyst comprises rhodium.

21. The fuel reformer of claim 16, wherein the precious metal catalyst comprises ruthenium.

22. The fuel reformer of claim 16, wherein the precious metal catalyst comprises iridium.

23. The fuel reformer of claim 16, wherein precious metal catalyst is present in an amount sufficient to produce a hydrogen-containing reformate having an ammonia content of less than about 50 ppm.

24. The fuel reformer of claim 23, wherein the precious metal catalyst is present in an amount sufficient to produce a hydrogen-containing reformate having an ammonia content of less than about 10 ppm.

25. The fuel reformer of claim 24, wherein the precious metal catalyst is present in an amount sufficient to produce a hydrogen-containing reformate having an ammonia content of less than about 5 ppm.

6. The fuel reformer of claim 25, wherein the precious metal catalyst is present in an amount sufficient to produce a hydrogen-containing reformate having an ammonia content of less than about 2 ppm.