Fuels for Fuel Cells | Fuel Types | Processing Required for Fuels
Processing Required for Fuels
Fuel processing or reforming depends on both the raw fuel and the fuel cell technology. The fuel cell technology determines what constituents are desirable and acceptable in the processed fuel.
For example, fuel sent to a PAFC needs to be H2-rich and have less than 5% CO, while both the MCFC and SOFC fuel cells are capable of utilizing CO. PEMFCs require a pure hydrogen stream with less than 10 ppm CO. In addition, SOFCs and internal reforming MCFCs are also capable of utilizing methane (CH4) within the cell whereas PAFCs are not. Contamination limits are also fuel cell technology specific and therefore help to determine the specific cleanup processes that are required.
Since the components and design of a fuel processing subsection depend on the raw fuel type, the following discussion is organized by the raw fuel being processed.
Hydrogen Processing
When hydrogen is supplied directly to the fuel cell, the fuel processing section becomes a simple fuel delivery system.
Natural Gas Processing
Natural gas is usually converted to H2 and CO in a steam reforming reactor. Steam reforming reactors yield the highest percentage of hydrogen. In addition to natural gas, steam reformers can be used on light hydrocarbons such as butane and propane. In fact, with a special catalyst, steam reformers can also reform naphtha. Steam reforming reactions are highly endothermic and need a significant heat source. Often the residual fuel exiting the fuel cell is burned to supply this requirement. Fuels are typically reformed at temperatures of 760 to 980oC. Partial oxidation reformers can also be used for converting gaseous fuels, but do not produce as much hydrogen as steam reformers.
Natural gas has sulfur containing odorants (mercaptans, disulfides, or commercial odorants) for leak detection. Since neither fuel cells nor reformer catalysts are sulfur tolerant, the sulfur must be removed. This is usually accomplished with a fixed or packed bed of zinc oxide or the possible use of a hydro-desulfurizer, if required.
Liquid Fuel Processing
Liquid fuels such as distillate, naphtha, diesel, and heavy fuel oil can be reformed in partial oxidation, autothermal and preferential oxidation reformers. All commercial partial oxidation reactors employ non-catalytic partial oxidation of the feed stream by oxygen in the presence of steam with flame temperatures of approximately 1300 to 1500oC.
Partial oxidation, autothermal reformation and preferential oxidation fuel processing techniques use some of the energy contained in the fuel to convert these hydrocarbons to H2 and CO. For example, the overall partial oxidation reaction for pentane is exothermic and largely independent of pressure. The process is usually performed at elevated pressure in order to yield smaller equipment.
Coal Processing
Numerous coal gasification systems are available today. The most common systems are moving-bed or fixed-bed reactors, fluidized-bed reactors, and entrained-bed reactors, all of which use steam, and air or oxygen to partially oxidize coal into a gaseous product.
Heat required for gasification is essentially supplied by the partial oxidation of the coal. Overall, the gasification reactions are exothermic; so waste heat boilers are often utilized at the gasifier effluent. The temperature, and therefore composition, of the product gas is dependent upon the amount of oxidant and steam, and the design of the reactor that each gasification process utilizes.
Gas clean-up
Gasifiers typically produce contaminants, which need to be removed before entering the fuel cell anode. These contaminants include: H2S, COS, NH3, HCN, particulates, tars, oils and phenols. The contaminant levels are dependent upon both the fuel composition and the gasifier employed. Gas clean-up equipment that efficiently and reliably removes contaminants to the specifications required by fuel cells is yet to be demonstrated.
Other Solid Fuel Processing
Solid fuels other than coal can also be utilized in fuel cell systems. For example, biomass and RDF (Refuse-Derived-Fuels) can be integrated into a fuel cell system as long as the gas product is processed to meet the requirements of the fuel cell. The resulting systems would be very similar to the coal gas system with appropriate gasifying and cleanup systems.
Fuel Reformation Technologies
Steam Reformation
- Fuel reacts with water over a catalyst
- Requires heat input
Partial oxidation
- Fuel reacts with air (w/o catalyst)
- Typically produces heat
Auto-thermal
- Combination of above (fuel, water, air)
- No net heat required or produced
Other – e.g., Cyclic
- Oxidation states of metals
- Alternatively pass fuel and oxidant over parallel beds
- Calcium oxide, calcium carbonate for carbon absorption
Energy Storage
Power storage is another aspect of power production for which fuel cells are well suited. Since the reverse hydrolysis process employed by fuel cells is significantly easier to operate backwards than most other power cycles, slightly modified fuel cells can readily be fed water and electricity and run in reverse to produce hydrogen and oxygen. These gases can then be stored for future use in power production. Such a system could be used in conjunction with solar cells to store energy during the day and produce power at night, or with a conventional power plant to store energy during off peak hours and help meet load requirements during periods of high electricity demand.