WO2013178430A1 - Pre-reforming of sulfur-containing fuels to produce syngas for use in fuel cell systems - Google Patents

Pre-reforming of sulfur-containing fuels to produce syngas for use in fuel cell systems Download PDF

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Publication number
WO2013178430A1
WO2013178430A1 PCT/EP2013/059356 EP2013059356W WO2013178430A1 WO 2013178430 A1 WO2013178430 A1 WO 2013178430A1 EP 2013059356 W EP2013059356 W EP 2013059356W WO 2013178430 A1 WO2013178430 A1 WO 2013178430A1
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Prior art keywords
fuel cell
reformer
solid oxide
cell system
oxide fuel
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PCT/EP2013/059356
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French (fr)
Inventor
Thomas Rostrup-Nielsen
Bøgild John HANSEN
Pedro Nehter
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Topsøe Fuel Cell A/S
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/06—Combination of fuel cells with means for production of reactants or for treatment of residues
    • H01M8/0606—Combination of fuel cells with means for production of reactants or for treatment of residues with means for production of gaseous reactants
    • H01M8/0612—Combination of fuel cells with means for production of reactants or for treatment of residues with means for production of gaseous reactants from carbon-containing material
    • H01M8/0618—Reforming processes, e.g. autothermal, partial oxidation or steam reforming
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B3/00—Hydrogen; Gaseous mixtures containing hydrogen; Separation of hydrogen from mixtures containing it; Purification of hydrogen
    • C01B3/02—Production of hydrogen or of gaseous mixtures containing a substantial proportion of hydrogen
    • C01B3/32—Production of hydrogen or of gaseous mixtures containing a substantial proportion of hydrogen by reaction of gaseous or liquid organic compounds with gasifying agents, e.g. water, carbon dioxide, air
    • C01B3/34—Production of hydrogen or of gaseous mixtures containing a substantial proportion of hydrogen by reaction of gaseous or liquid organic compounds with gasifying agents, e.g. water, carbon dioxide, air by reaction of hydrocarbons with gasifying agents
    • C01B3/38—Production of hydrogen or of gaseous mixtures containing a substantial proportion of hydrogen by reaction of gaseous or liquid organic compounds with gasifying agents, e.g. water, carbon dioxide, air by reaction of hydrocarbons with gasifying agents using catalysts
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/06—Combination of fuel cells with means for production of reactants or for treatment of residues
    • H01M8/0662—Treatment of gaseous reactants or gaseous residues, e.g. cleaning
    • H01M8/0675—Removal of sulfur
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B2203/00—Integrated processes for the production of hydrogen or synthesis gas
    • C01B2203/02—Processes for making hydrogen or synthesis gas
    • C01B2203/0205—Processes for making hydrogen or synthesis gas containing a reforming step
    • C01B2203/0227—Processes for making hydrogen or synthesis gas containing a reforming step containing a catalytic reforming step
    • C01B2203/0233—Processes for making hydrogen or synthesis gas containing a reforming step containing a catalytic reforming step the reforming step being a steam reforming step
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B2203/00—Integrated processes for the production of hydrogen or synthesis gas
    • C01B2203/02—Processes for making hydrogen or synthesis gas
    • C01B2203/0205—Processes for making hydrogen or synthesis gas containing a reforming step
    • C01B2203/0227—Processes for making hydrogen or synthesis gas containing a reforming step containing a catalytic reforming step
    • C01B2203/0244—Processes for making hydrogen or synthesis gas containing a reforming step containing a catalytic reforming step the reforming step being an autothermal reforming step, e.g. secondary reforming processes
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B2203/00—Integrated processes for the production of hydrogen or synthesis gas
    • C01B2203/02—Processes for making hydrogen or synthesis gas
    • C01B2203/025—Processes for making hydrogen or synthesis gas containing a partial oxidation step
    • C01B2203/0261—Processes for making hydrogen or synthesis gas containing a partial oxidation step containing a catalytic partial oxidation step [CPO]
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B2203/00—Integrated processes for the production of hydrogen or synthesis gas
    • C01B2203/06—Integration with other chemical processes
    • C01B2203/066—Integration with other chemical processes with fuel cells
    • C01B2203/067—Integration with other chemical processes with fuel cells the reforming process taking place in the fuel cell
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B2203/00—Integrated processes for the production of hydrogen or synthesis gas
    • C01B2203/14—Details of the flowsheet
    • C01B2203/142—At least two reforming, decomposition or partial oxidation steps in series
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B2203/00—Integrated processes for the production of hydrogen or synthesis gas
    • C01B2203/14—Details of the flowsheet
    • C01B2203/148—Details of the flowsheet involving a recycle stream to the feed of the process for making hydrogen or synthesis gas
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/10—Fuel cells with solid electrolytes
    • H01M8/12—Fuel cells with solid electrolytes operating at high temperature, e.g. with stabilised ZrO2 electrolyte
    • H01M2008/1293—Fuel cells with solid oxide electrolytes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04082—Arrangements for control of reactant parameters, e.g. pressure or concentration
    • H01M8/04089—Arrangements for control of reactant parameters, e.g. pressure or concentration of gaseous reactants
    • H01M8/04097—Arrangements for control of reactant parameters, e.g. pressure or concentration of gaseous reactants with recycling of the reactants
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/06—Combination of fuel cells with means for production of reactants or for treatment of residues
    • H01M8/0606—Combination of fuel cells with means for production of reactants or for treatment of residues with means for production of gaseous reactants
    • H01M8/0612—Combination of fuel cells with means for production of reactants or for treatment of residues with means for production of gaseous reactants from carbon-containing material
    • H01M8/0637—Direct internal reforming at the anode of the fuel cell
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/30—Hydrogen technology
    • Y02E60/50—Fuel cells

Definitions

  • the present invention concerns pre-reforming of sulfur- containing fuels, such as hydrocarbon fuels, to produce syngas for use in fuel cell systems. More specifically, the invention concerns a method for the processing of hydrocarbon fuels by pre-reforming to produce an anode feed gas for use in connection with a fuel cell system. The invention also concerns a solid oxide fuel cell assembly supplied with fuel obtained by the method.
  • the basic idea underlying the present invention consists in using a pre- reformer also as a sulfur trap.
  • the off-gas from the solid oxide fuel cell downstream of the pre-reformer which typically contains mainly 3 ⁇ 4, N 2 , H 2 0, CO and CO 2 , can be used directly in a recycle loop for mixing with the incoming hy- drocarbon fuel. To avoid carbon formation, a suitable amount of H 2 is supplied.
  • the method according to the in ⁇ vention requires neither a separate hydrodesulfurization unit nor a ZnO bed.
  • the idea of mixing the anode recycle gas with incoming fuel upstream the HDS and an adiabatic reforming process is de ⁇ scribed in WO 2010/044772 Al .
  • Fuel cells are electrochemical systems which generate elec ⁇ tric current by chemically reacting a fuel gas and an oxi ⁇ dant gas on the electrode surfaces.
  • the oxi- dant gas is oxygen or air
  • the fuel gas is hydrogen or a mixture of hydrogen, carbon oxides and traces of hydro ⁇ carbons.
  • the specific fuel gas composition requirements de ⁇ pend on the type of fuel cell.
  • Low temperature fuel cells such as proton exchange membrane (PEM) cells and alkaline fuel cells (AFCs) , can only utilize hydrogen as fuel, and they contain precious metal catalysts that become poisoned by carbon monoxide.
  • High temperature fuel cells such as solid oxide fuel cells (SOFCs) and molten carbonate fuel cells (MCFCs)
  • SOFCs solid oxide fuel cells
  • MCFCs molten carbonate fuel cells
  • SOFCs solid oxide fuel cells
  • MCFCs molten carbonate fuel cells
  • Pure hydrogen is the ideal fuel for lots of fuel cell types, but it is not widely available. Moreover, storage and transportation involves large, heavy and costly means, such as compressed gas bottles. In practice fuel cells must therefore utilize commonly available and easily transported fuels including natural gas, methanol, ethanol, diesel fuel and hydrocarbon fuel. These hydrocarbons and alcohols must be reformed to a fuel gas that is suitable for the particu- lar fuel cell application. In addition, these fuels often contain sulfur that has to be removed.
  • hydrocarbon fuels which contain both H and C in various ratios.
  • hydrocarbon fuels include satu ⁇ rated hydrocarbons (e.g. methane, ethane, propane and bu ⁇ tane) , natural gas, biogas, gasoline, gasified coal or bio- mass, diesel, synthetic fuels, marine fuel and jet fuels.
  • hydrocarbon fuels also includes alcohols common ⁇ ly used as fuels, e.g. methanol, ethanol and butanol.
  • the fuels with the highest energy density such as diesel or jet fuels, contain large amounts of heavy hydro ⁇ carbons as well as more than 0.5 wt% sulfur.
  • Catalysts for high temperature reforming of these fuels are very suscep ⁇ tible to carbon formation from these higher hydrocarbon species, as well as from sulfur poisoning, and thus it is very difficult to develop a fuel processing unit that can operate directly on these fuels.
  • Sulfur is generally removed from fuel for pre-reforming by either hydrodesulfurization (HDS) with a downstream adsorption of 3 ⁇ 4S; this process requires 3 ⁇ 4 which can be supplied by an external hydrogen source or via an anode recycle: diesel + 3 ⁇ 4 ⁇ HDS ⁇ ZnO ⁇ diesel + 3 ⁇ 40 ⁇ pre-reforming or liquid desulfurization : diesel ⁇ liq. desulph. ⁇ diesel + 3 ⁇ 40 ⁇ pre-reforming .
  • HDS hydrodesulfurization
  • the zinc oxide bed requires frequent maintenance, and a hydrodesulfurization system will be rather energy intensive and also difficult to design for a small fuel pro ⁇ cessing unit. It would therefore be desirable to provide a novel method for pre-reforming of sulfur containing fuels to produce syngas for solid oxide fuel cell applications, in which method neither a hydrodesulfurization nor a zinc oxide bed will be required.
  • Solid oxide fuel cells provide promising improve- ments with regard to efficiency and emissions.
  • the choice of fuel processing method e.g. catalytic partial oxida ⁇ tion, autothermal reforming or steam reforming, strongly affects the efficiency and power density of the system.
  • Pre-reforming of hydrocarbon fuels is one of the most at- tractive solutions for SOFCs and MCFCs, both making it pos ⁇ sible to obtain high electrical system effectivities and also allowing more compact SOFC systems than hitherto pos ⁇ sible . It has now been found that it is possible to remove the sulfur from the fuel (particularly the heavy hydrocarbons thereof) via the reforming process instead of having to make use of an upstream HDS-ZnO process or an upstream liquid desulfurizer .
  • the invention therefore concerns a method for the pro ⁇ cessing of hydrocarbon fuels by pre-reforming to produce an anode feed gas for use in connection with a fuel cell sys ⁇ tem, said method comprising the following steps:
  • step (a) treatment of the hydrocarbon fuel with steam, with steam and hydrogen or with syngas, or with combinations thereof in a pre-reformer to convert the fuel to syngas and to remove at least a portion of the sulfur species from the fuel, (b) feeding the syngas obtained in step (a) to the anode inlet of a fuel cell system.
  • syngas is a gas containing mainly 3 ⁇ 4, CO, CO 2 , CH 4 , 3 ⁇ 40 and in some in ⁇ stances also N 2 .
  • Syngas is typically obtained at the outlet of a pre-reformer, or anode outlet of a fuel cell such as an SOFC or MCFC.
  • the method may optionally include the step of: (c) recircu ⁇ lating part of the anode off-gas from the fuel cell system to the inlet of the pre-reformer .
  • Another optional step is: treating the hydrocarbon fuel in one or more reforming beds after step (a) and before step (b) , above.
  • the invention further concerns a solid oxide fuel cell sys ⁇ tem supplied with fuel obtained by the above method.
  • a solid oxide fuel cell sys ⁇ tem supplied with fuel obtained by the above method is shown schematically in figure 1.
  • pre-reforming and pre-reformer are considered synonymous with “reforming” and “reformer”.
  • sulfur species removed from the hydrocarbon fuel include 3 ⁇ 4S, and COS but also organic sulfur com ⁇ pounds, including thiols, thiophenes, organic sulfides and disulfides .
  • the pre-reforming step (step a.) of the method may be car ⁇ ried out adiabatically, or with heating or with cooling. Suitable operating temperatures of the pre-reforming step lie between 250 and 950°C, preferably between 350 and
  • pre-reforming is carried out adiabatically .
  • an adiabatic pre-reformer is preferably used.
  • the terms "adia- batic” and “adiabatically”, when used in connection with the system or method of the invention, are used to describe a thermally insulated state, without input or removal of heat to/from the pre-reformer.
  • Figure 1 schematically illustrates a particular solid oxide fuel cell assembly according to the invention.
  • the solid oxide fuel cell assembly ac ⁇ cording to the invention comprises: a pre-reformer (Rl), wherein the hydrocarbon fuel is treat ⁇ ed with steam, with steam and hydrogen or with syngas, or with combinations thereof, to remove at least a portion of the sulfur species from the fuel, a solid oxide fuel cell system (FC1) in the form of a sin ⁇ gle solid oxide fuel cell or at least one solid oxide fuel cell stack, to the anode inlet of which the syngas from the pre-reformer (Rl) is fed, and optionally, a recycle loop from the anode outlet of the solid oxide fuel cell system (FC1), through which a part of the anode off-gas from the solid oxide fuel cell system is recirculated to the inlet of the pre-reformer (Rl) .
  • a pre-reformer Rl
  • FC1 solid oxide fuel cell system
  • FC1 solid oxide fuel cell system in the form of a sin ⁇ gle solid oxide fuel cell or at least one solid oxide fuel
  • syngas is used in the pre-reformer (Rl) to re ⁇ move at least a portion of the sulfur species from the fuel .
  • one or more reforming beds are ar ⁇ ranged between the pre-reformer (Rl) and the solid oxide fuel cell system (FC1) .
  • One or more heat exchangers may be present before and/or after one or more of said one or more reforming beds, so that the gas entering the reforming beds may be heated or cooled.
  • the reforming beds may be independently operated adiabatically, heated or cooled .
  • the pre-reformer (Rl) and any reforming beds present be- tween the pre-reformer (Rl) and the solid oxide fuel cell system (FC1) may be designed so that the pre-reforming cat ⁇ alyst therein may be easily replaced when necessary. This is especially important for the reforming beds which are present upstream (e.g. the first bed, or the first and se- cond beds) in the system, as these become most rapidly con ⁇ taminated with sulfur) .
  • the pre-reformed (Rl) and any reforming beds may be connected to the system via quick-release connections, allowing simple and rapid remov ⁇ al and replacement of these elements.
  • the pre-reformer (Rl) and any reforming beds may be present in the same vessel, or in separate vessels. If present in a single vessel, this vessel may be designed so that reform ⁇ ing beds can be replaced independently.
  • the syngas from the pre-reformer (Rl) is fed to the anode inlet of the solid oxide fuel cell assem ⁇ bly via a heat exchanger (E2) .
  • the part of the anode off-gas from the solid oxide fuel cell system (FC1) may be recirculated to the in ⁇ let of the pre-reformer (Rl) via a heat exchanger and a re ⁇ cycle blower (Bl) .
  • This heat exchanger may be the same heat exchanger (E2) as between the pre-reformer (Rl) and the solid oxide fuel cell, or may be different.
  • the pre-reformer (Rl) is an adiabatic pre- reformer .
  • the pre-reformer (Rl) is also used as a sulfur trap in the concept of the present invention.
  • the anode off-gas (2006) from the fuel cell (FC1) can be used directly in a recycle loop (2008) for mixing with the incoming fuel (2000) .
  • a suf ⁇ ficient amount of 3 ⁇ 4 can be provided.
  • a simulation model for a diesel based solid oxide fuel cell system has shown that an anode recycle ratio around 50% will be sufficient in order to achieve appropriate 3 ⁇ 4/C ratios. As mentioned above, this concept will neither require a HDS nor a ZnO bed. Thus there are benefits in terms of improved simplici- ty of the fuel processing concept, system efficiency and costs .
  • hydrocarbon fuels can be directly used within the fuel cell system without any deep desulfurization processes.
  • Hydro ⁇ carbon fuels like diesel, gasoline or jet fuel can be desulfurized with a manageable effort down to a sulfur lev- el similar to that of ultra-low sulfur diesel (ULSD) , i.e. approximately 10 ppm by weight.
  • ULSD ultra-low sulfur diesel
  • the ability to convert hy ⁇ drocarbon fuels with a sulfur content of 10 ppm by weight within an pre-reformer is thus a prerequisite to avoid any deep desulfurization technologies, thereby keeping the sys ⁇ tem simple and efficient.
  • a reformate composition with around 32% hydrogen on dry basis was demonstrated without any traces of higher hydrocar- bons for more than 400 hours.

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Abstract

A method for the processing of hydrocarbon fuels by pre-reforming to produce an anode feed gas for use in connection with a fuel cell system comprises the steps of treating the hydrocarbon fuel with steam, with steam and hydrogen or with syngas, or with combinations thereof in a pre-reformer (R1) to convert the fuel to syngas and removing at least a portion of the sulfur species from the fuel, feeding the syngas to the anode inlet of a fuel cell system (FC1) and optionally recirculating a split of the anode off-gas from the fuel cell system, to the inlet of the pre-reformer (R1), suitably via a heat exchanger (E2) and a recycle blower (B1).

Description

Pre-reforming of sulfur-containing fuels to produce syngas for use in fuel cell systems
The present invention concerns pre-reforming of sulfur- containing fuels, such as hydrocarbon fuels, to produce syngas for use in fuel cell systems. More specifically, the invention concerns a method for the processing of hydrocarbon fuels by pre-reforming to produce an anode feed gas for use in connection with a fuel cell system. The invention also concerns a solid oxide fuel cell assembly supplied with fuel obtained by the method.
Previously, high pressure steam reforming of diesel has successfully been tested by the applicant in combination with an upstream hydrodesulfurization (HDS)- ZnO bed. This process requires pressurized hydrogen. The presence of wa¬ ter within the HDS reactor and ZnO bed is most undesirable.
The basic idea underlying the present invention, which will be described in more detail below, consists in using a pre- reformer also as a sulfur trap. The off-gas from the solid oxide fuel cell downstream of the pre-reformer, which typically contains mainly ¾, N2, H20, CO and CO2, can be used directly in a recycle loop for mixing with the incoming hy- drocarbon fuel. To avoid carbon formation, a suitable amount of H2 is supplied. The method according to the in¬ vention requires neither a separate hydrodesulfurization unit nor a ZnO bed. The idea of mixing the anode recycle gas with incoming fuel upstream the HDS and an adiabatic reforming process is de¬ scribed in WO 2010/044772 Al . However, according to this prior art document a separate hydrodesulfurization unit is mandatory, and the described system is strictly related to a desulfurization-reformer configuration. A fuel cell system, wherein the fuel is made by adiabatic pre-reforming of a higher carbon (C2+) hydrocarbon fuel with steam, is disclosed in US 6.841.279 Bl . This patent is however silent as to how the problems with sulfur are dealt with .
Other fuel cell systems are described in the following doc¬ uments: US 2010/0178574 Al disclosing a system with partial external reforming and direct internal reforming, JP
11273703 A describing the use of an additional adiabatic reformer in the anode recycle stream before mixing with the reformate, which is supplied by the main reformer, and US 5.147.735 Bl describing a method of operating a solid elec¬ trolyte fuel cell, in which the use of a PSA (pressure swing adsorption) system is considered for the air side (the cathode) to increase the oxygen partial pressure.
Fuel cells are electrochemical systems which generate elec¬ tric current by chemically reacting a fuel gas and an oxi¬ dant gas on the electrode surfaces. Conventionally the oxi- dant gas is oxygen or air, and the fuel gas is hydrogen or a mixture of hydrogen, carbon oxides and traces of hydro¬ carbons. The specific fuel gas composition requirements de¬ pend on the type of fuel cell. Low temperature fuel cells, such as proton exchange membrane (PEM) cells and alkaline fuel cells (AFCs) , can only utilize hydrogen as fuel, and they contain precious metal catalysts that become poisoned by carbon monoxide. High temperature fuel cells, such as solid oxide fuel cells (SOFCs) and molten carbonate fuel cells (MCFCs) , do not usually contain precious metal cata¬ lysts, and can utilize hydrogen containing carbon monoxide and methane as fuel. Most fuel cell types are adversely af- fected by sulfur compounds.
Pure hydrogen is the ideal fuel for lots of fuel cell types, but it is not widely available. Moreover, storage and transportation involves large, heavy and costly means, such as compressed gas bottles. In practice fuel cells must therefore utilize commonly available and easily transported fuels including natural gas, methanol, ethanol, diesel fuel and hydrocarbon fuel. These hydrocarbons and alcohols must be reformed to a fuel gas that is suitable for the particu- lar fuel cell application. In addition, these fuels often contain sulfur that has to be removed.
The conversion of common hydrocarbon fuels, such as gasoline, into a gas rich in hydrogen for use in electricity- producing fuel cell systems has so far only been demonstrated within a few prototypes. Besides, oil resources are becoming increasingly less available and also less attrac¬ tive for environmental reasons. Consequently the hydrocar¬ bon processing industry has developed technologies for con- verting low value feedstocks to hydrogen and syngas. Common approaches include reforming, partial oxidation and auto- thermal reforming.
In order to support the widespread use of fuel cells in many areas of transportation it is necessary to develop methods of processing liquid fuels to generate hydrogen, which can be utilized in fuel cells. The present invention utilises hydrocarbon fuels, which contain both H and C in various ratios. Examples of hydrocarbon fuels include satu¬ rated hydrocarbons (e.g. methane, ethane, propane and bu¬ tane) , natural gas, biogas, gasoline, gasified coal or bio- mass, diesel, synthetic fuels, marine fuel and jet fuels.
The term "hydrocarbon fuels" also includes alcohols common¬ ly used as fuels, e.g. methanol, ethanol and butanol.
However, the fuels with the highest energy density, such as diesel or jet fuels, contain large amounts of heavy hydro¬ carbons as well as more than 0.5 wt% sulfur. Catalysts for high temperature reforming of these fuels are very suscep¬ tible to carbon formation from these higher hydrocarbon species, as well as from sulfur poisoning, and thus it is very difficult to develop a fuel processing unit that can operate directly on these fuels.
It is possible to convert the heavy hydrocarbons to methane through the process of pre-reforming, whereby the fuel is contacted with steam over a catalyst at temperatures around 300 to 600°C below the typical reforming temperatures to produce an equilibrium mix of methane, hydrogen, carbon oxides and water. In practice, traces of heavier hydrocarbons (C2+) will also be present. This lower pre-reforming tem- perature can reduce, but not completely eliminate the for¬ mation of carbon during the pre-reforming process. The output stream from the pre-reforming process can then be reformed at high temperatures without concerns for carbon formation with methane as essentially the only hydrocarbon present over well-known catalysts. Sulfur is generally removed from fuel for pre-reforming by either hydrodesulfurization (HDS) with a downstream adsorption of ¾S; this process requires ¾ which can be supplied by an external hydrogen source or via an anode recycle: diesel + ¾ → HDS → ZnO → diesel + ¾0 → pre-reforming or liquid desulfurization : diesel → liq. desulph. → diesel + ¾0 → pre-reforming .
These known technologies are encumbered with a number of drawbacks. In the hydrodesulfurization process the fuel is contacted with hydrogen at pressures around 5-20 atmos- pheres and temperatures of 350-500°C, whereby most of the sulfur is converted to components (e.g. ¾S, COS) which are suitable for easy downstream removal. The sulfur components will still need to be removed prior to sending the sulfur- free fuel to a reformer. Sulfur can also be removed by an adsorption bed, usually based on zinc oxide. While this bed will capture not only ¾S, but also sulfur-containing hydrocarbons, it will have a limited adsorption capacity for sulfur uptake. Neither of these options is ideal for a fuel processing unit. The zinc oxide bed requires frequent maintenance, and a hydrodesulfurization system will be rather energy intensive and also difficult to design for a small fuel pro¬ cessing unit. It would therefore be desirable to provide a novel method for pre-reforming of sulfur containing fuels to produce syngas for solid oxide fuel cell applications, in which method neither a hydrodesulfurization nor a zinc oxide bed will be required.
Solid oxide fuel cells (SOFCs) provide promising improve- ments with regard to efficiency and emissions. The choice of fuel processing method, e.g. catalytic partial oxida¬ tion, autothermal reforming or steam reforming, strongly affects the efficiency and power density of the system. Pre-reforming of hydrocarbon fuels is one of the most at- tractive solutions for SOFCs and MCFCs, both making it pos¬ sible to obtain high electrical system effectivities and also allowing more compact SOFC systems than hitherto pos¬ sible . It has now been found that it is possible to remove the sulfur from the fuel (particularly the heavy hydrocarbons thereof) via the reforming process instead of having to make use of an upstream HDS-ZnO process or an upstream liquid desulfurizer .
The invention therefore concerns a method for the pro¬ cessing of hydrocarbon fuels by pre-reforming to produce an anode feed gas for use in connection with a fuel cell sys¬ tem, said method comprising the following steps:
(a) treatment of the hydrocarbon fuel with steam, with steam and hydrogen or with syngas, or with combinations thereof in a pre-reformer to convert the fuel to syngas and to remove at least a portion of the sulfur species from the fuel, (b) feeding the syngas obtained in step (a) to the anode inlet of a fuel cell system.
Within the meaning of the present invention, syngas is a gas containing mainly ¾, CO, CO2, CH4, ¾0 and in some in¬ stances also N2. Syngas is typically obtained at the outlet of a pre-reformer, or anode outlet of a fuel cell such as an SOFC or MCFC. The method may optionally include the step of: (c) recircu¬ lating part of the anode off-gas from the fuel cell system to the inlet of the pre-reformer . Another optional step is: treating the hydrocarbon fuel in one or more reforming beds after step (a) and before step (b) , above.
The invention further concerns a solid oxide fuel cell sys¬ tem supplied with fuel obtained by the above method. One embodiment of this system is shown schematically in figure 1.
In the present invention, the terms "pre-reforming" and "pre-reformer" are considered synonymous with "reforming" and "reformer". Examples of "sulfur species" removed from the hydrocarbon fuel include ¾S, and COS but also organic sulfur com¬ pounds, including thiols, thiophenes, organic sulfides and disulfides . The pre-reforming step (step a.) of the method may be car¬ ried out adiabatically, or with heating or with cooling. Suitable operating temperatures of the pre-reforming step lie between 250 and 950°C, preferably between 350 and
650°C. Suitably, in the method according to the invention, pre-reforming is carried out adiabatically . Accordingly, an adiabatic pre-reformer is preferably used. The terms "adia- batic" and "adiabatically", when used in connection with the system or method of the invention, are used to describe a thermally insulated state, without input or removal of heat to/from the pre-reformer. Figure 1 schematically illustrates a particular solid oxide fuel cell assembly according to the invention.
In general terms, the solid oxide fuel cell assembly ac¬ cording to the invention comprises: a pre-reformer (Rl), wherein the hydrocarbon fuel is treat¬ ed with steam, with steam and hydrogen or with syngas, or with combinations thereof, to remove at least a portion of the sulfur species from the fuel, a solid oxide fuel cell system (FC1) in the form of a sin¬ gle solid oxide fuel cell or at least one solid oxide fuel cell stack, to the anode inlet of which the syngas from the pre-reformer (Rl) is fed, and optionally, a recycle loop from the anode outlet of the solid oxide fuel cell system (FC1), through which a part of the anode off-gas from the solid oxide fuel cell system is recirculated to the inlet of the pre-reformer (Rl) . Preferably, syngas is used in the pre-reformer (Rl) to re¬ move at least a portion of the sulfur species from the fuel . Optionally, one or more reforming beds (not shown) are ar¬ ranged between the pre-reformer (Rl) and the solid oxide fuel cell system (FC1) . One or more heat exchangers may be present before and/or after one or more of said one or more reforming beds, so that the gas entering the reforming beds may be heated or cooled. In addition, the reforming beds may be independently operated adiabatically, heated or cooled .
The pre-reformer (Rl) and any reforming beds present be- tween the pre-reformer (Rl) and the solid oxide fuel cell system (FC1) may be designed so that the pre-reforming cat¬ alyst therein may be easily replaced when necessary. This is especially important for the reforming beds which are present upstream (e.g. the first bed, or the first and se- cond beds) in the system, as these become most rapidly con¬ taminated with sulfur) . Suitably, the pre-reformed (Rl) and any reforming beds may be connected to the system via quick-release connections, allowing simple and rapid remov¬ al and replacement of these elements.
The pre-reformer (Rl) and any reforming beds may be present in the same vessel, or in separate vessels. If present in a single vessel, this vessel may be designed so that reform¬ ing beds can be replaced independently.
In the particular embodiment of the solid oxide fuel cell assembly shown, the syngas from the pre-reformer (Rl) is fed to the anode inlet of the solid oxide fuel cell assem¬ bly via a heat exchanger (E2) .
In addition, the part of the anode off-gas from the solid oxide fuel cell system (FC1) may be recirculated to the in¬ let of the pre-reformer (Rl) via a heat exchanger and a re¬ cycle blower (Bl) . This heat exchanger may be the same heat exchanger (E2) as between the pre-reformer (Rl) and the solid oxide fuel cell, or may be different.
Suitably, the pre-reformer (Rl) is an adiabatic pre- reformer .
As mentioned previously, the pre-reformer (Rl) is also used as a sulfur trap in the concept of the present invention.
Thus, the anode off-gas (2006) from the fuel cell (FC1) can be used directly in a recycle loop (2008) for mixing with the incoming fuel (2000) . To avoid carbon formation, a suf¬ ficient amount of ¾ can be provided. A simulation model for a diesel based solid oxide fuel cell system has shown that an anode recycle ratio around 50% will be sufficient in order to achieve appropriate ¾/C ratios. As mentioned above, this concept will neither require a HDS nor a ZnO bed. Thus there are benefits in terms of improved simplici- ty of the fuel processing concept, system efficiency and costs .
Using the method and the system according to the invention, hydrocarbon fuels can be directly used within the fuel cell system without any deep desulfurization processes. Hydro¬ carbon fuels like diesel, gasoline or jet fuel can be desulfurized with a manageable effort down to a sulfur lev- el similar to that of ultra-low sulfur diesel (ULSD) , i.e. approximately 10 ppm by weight. The ability to convert hy¬ drocarbon fuels with a sulfur content of 10 ppm by weight within an pre-reformer is thus a prerequisite to avoid any deep desulfurization technologies, thereby keeping the sys¬ tem simple and efficient.
The following example serves to illustrate the invention further .
Example
Various long term tests have been carried out with one of the applicant's own catalysts. The pre-reformer was operat- ed on ULSD obtained from the gas station. This example de¬ scribes a test with ultra low sulfur diesel (ULSD) and a simulated anode recycle gas, operating close to the condi¬ tions of a commercial fuel cell system. Higher hydrocarbons (HHCs) in the reformate gas were ana¬ lysed using gas chromatography with a flame ionization detector (GC-FID; Agilent GC 7890) up to C4. The average HHC contents in the reformate gas for Test no. 2 (Set-1) and Test no. 5 (Set-2) were 2 and 1 ppmv, respectively.
For sulfur analysis, gas samples from the reformer outlet were taken in sampling bags to eliminate sulfur adsorption in the long distance sampling line between the test rig and the GC . The samples were analysed for hydrogen sulphide, carbonyl sulphide and light mercaptans using a GC equipped with FPD. No sulfur was detected in the reformate gas in any tests. The ULSD slip was worked out by measuring the oil phase in the condensate. Approximate steam outlet in the reformate gas was calculated and the ULSD slip was estimated by com- paring the volumes of aqueous and organic phases. The ULSD slip was detected after more than 400 hours of operation.
A reformate composition with around 32% hydrogen on dry basis was demonstrated without any traces of higher hydrocar- bons for more than 400 hours.
Dry composition:
C02: 48%
H2: 32%
CO: 2%
CH4: 18%
These results reflect the high potential of pre-reforming for SOFC systems (both mobile and stationary) utilizing hydrocarbon fuels.

Claims

A method for the processing of hydrocarbon fuels by pre-reforming to produce an anode feed gas for use in connection with a fuel cell system, said method comprising the following steps:
(a) treatment of the hydrocarbon fuel with steam, with steam and hydrogen or with syngas, or with combinations thereof in a pre-reformer to convert the fuel to syngas and to remove at least a portion of the sulfur species from the fuel ,
(b) feeding the syngas obtained in step (a) to the anode inlet of a fuel cell system.
The method according to any one of claims 1-2, in¬ cluding the step of (c) recirculating part of the anode off-gas from the fuel cell system to the pre- reformer, preferably to the inlet thereof.
The method according to any one of claims 1-3, in¬ cluding the step of recirculating at least a part of the reformer effluent to the pre-reformer, prefera¬ bly to the inlet thereof.
The method according to any one of the preceding claims, further including the step of: treating the hydrocarbon fuel in one or more reforming beds after step (a) and before step (b) . The method according to any one of the preceding claims, wherein the pre-reformer and any reforming beds present between the pre-reformer and the solid oxide fuel cell system are designed so that the pre reforming catalyst therein may be easily replaced.
The method according to any one of the preceding claims, wherein the hydrocarbon fuel is a fuel that has not been treated by a deep desulfurization pro¬ cess.
The method according to any one of the preceding claims, wherein the fuel cell system is a medium to high temperature fuel cell system operating at ap¬ proximate temperatures above 500°C and below 950°C.
The method according to any one of the preceding claims, wherein the fuel cell system is a solid ox¬ ide fuel cell system comprising a single solid oxide fuel cell or at least one solid oxide fuel cell stack .
9. The method according to any one of the preceding claims, wherein the pre-reformer is an adiabatic pre-reformer.
A solid oxide fuel cell assembly suitable for carry ing out the method of any one of the preceding claims, said assembly comprising:
(a) a pre-reformer (Rl), wherein the hydrocarbon fuel is treated with steam, with steam and hy drogen or with syngas, or with combinations thereof, to remove at least a portion of the sulfur species from the fuel, (b) a solid oxide fuel cell system (FC1) in the form of a single solid oxide fuel cell or at least one solid oxide fuel cell stack, to the anode inlet of which the syngas from the pre- reformer (Rl) is fed.
11. The solid oxide fuel cell assembly according to
claim 10, further comprising a recycle loop from the anode outlet of the solid oxide fuel cell system (FC1), through which a part of the anode off-gas from the solid oxide fuel cell system is recirculat¬ ed to the pre-reformer (Rl), preferably the inlet thereof .
12. The solid oxide fuel cell assembly according to any one of claims 10-11, wherein the hydrocarbon fuel is treated with syngas in the pre-reformer (Rl) to re¬ move at least a portion of the sulfur species from the fuel. 13. The solid oxide fuel cell assembly according to any one of claims 10-12, wherein the syngas from the pre-reformer (Rl) is fed to the anode inlet of the solid oxide fuel cell assembly via a heat exchanger (E2) .
14. The solid oxide fuel cell assembly according to any one of claims 10-13, wherein said part of the anode off-gas from the solid oxide fuel cell system is re circulated to the pre-reformer (Rl), preferably the inlet thereof, via a heat exchanger (E2) and a recy cle blower (Bl ) .
The solid oxide fuel cell system according to any one of claims 10-14, wherein the pre-reformer (Rl) is an adiabatic pre-reformer.
The solid oxide fuel cell system according to any one of claims 10-15, further comprising one or more reforming beds arranged between the pre-reformer (Rl) and the solid oxide fuel cell system (FC1) . 17. The solid oxide fuel cell system according to any one of claims 10-16, wherein pre-reformer (Rl) and any reforming beds present between the pre-reformer (Rl) and the solid oxide fuel cell system (FC1) are designed so that the pre-reforming catalyst therein may be easily replaced.
PCT/EP2013/059356 2012-05-29 2013-05-06 Pre-reforming of sulfur-containing fuels to produce syngas for use in fuel cell systems WO2013178430A1 (en)

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