Fuel Cell System

A fuel cell power system requires the integration of many components. The fuel cell produces only dc power and utilizes only certain processed fuels. Besides the fuel cell stack, various components are incorporated in a fuel cell system. A fuel processor is required to allow operation with conventional fuels; a power conditioner is used to tie fuel cells into the ac power grid or distributed generation system; for high-temperature fuel cells, a cogeneration or bottoming cycle plant is needed to utilize rejected heat for achieving high efficiency. A schematic of a fuel cell power system with interaction among various components is shown in Fig. 9.13.

9.4.1 Fuel processor

A fuel processor converts a commercially available fuel (gas, liquid, or solid) to a fuel gas reformate suitable for the fuel cell use. Fuel processing involves the following steps:

1. Fuel cleaning—It involves cleaning and removal of harmful species (sulfur, halides, and ammonia) in the fuel. This prevents fuel proces­sor and fuel cell catalyst degradation.

2. Fuel Conversion—In this stage, a naturally available fuel (prima­rily hydrocarbons such as natural gas, petrol, diesel, ethanol, methanol, biofuels [such as produced from biomass, landfill gas, biogas from anaerobic digesters, syngas from gasification of biomass and wastes] etc.) is converted to a hydrogen-rich fuel gas reformat.

3. Downstream processing—It involves reformate gas alteration by converting carbon monoxide (CO) and water (H20) in the fuel gas reformate to hydrogen (H2) and carbon dioxide (C02) through the water gas shift reaction, selective oxidation to reduce CO to a few parts per million, or removal of water by condensing to increase the H2 concentration.

A schematic showing the different stages in the fuel-processing system is presented in Fig. 9.14. Major fuel-processing techniques are steam reforming (SR), partial oxidation (POX) (catalytic and noncatalytic), and autothermal reforming (ATR). Some other techniques such as dry reforming, direct hydrocarbon oxidation, and pyrolysis are also used. Most fuel processors use the chemical and heat energy of the fuel cell effluent to provide heat for fuel processing. This enhances system efficiency.

Steam reforming is a popular method of converting light hydrocarbons to hydrogen. In SR, heated and vaporized fuel is injected with super­heated steam (steam-to-carbon molar ratio of about 2.5:1) into a reac­tion vessel. Excess steam ensures complete reaction as well as inhibits soot formation. Although the steam reformer can operate without a cat­alyst, most commercial reformers use a nickel — or cobalt-based catalyst to enhance reaction rates at lower temperatures. Although the water gas shift reaction in the steam reformer reactor is exothermic, the com­bined SR and water gas shift reaction is endothermic. It therefore requires a high-temperature heat source (usually an adjacent high — temperature furnace that burns a small portion of the fuel or the fuel effluent from the fuel cell) to operate the reactor. SR is a slow reaction and requires a large reactor. It is suitable for pipeline gas and light dis­tillates using a fuel cell for stationary power generation but is unsuit­able for systems requiring rapid start and/or fast changes in load.

In POX, a substoichiometric amount of air or oxygen is used to par­tially combust the fuel. POX is highly exothermic, and the resulting high-temperature reaction products are quenched using superheated steam. This promotes the combined water gas shift and steam-reforming



reactions, which cools the gas. In a well-designed POX reformer with controlled preheating of the reactants, the overall reaction is exother­mic and self-sustaining. Both catalytic (870-925°C) and noncatalytic (1175-1400°C) POX reformers have been developed for hydrocarbon fuels. The advantage of POX reforming is that it does not need indi­rect heat transfer, resulting in a compact and lightweight reformer. Also, it is capable of higher reforming efficiencies than steam reformers [3, 6].

Autothermal reforming combines SR with POX reforming in the pres­ence of a catalyst that controls the reaction pathways and thereby deter­mines the relative extents of the POX and SR reactions. The SR reaction absorbs part of the heat generated by the POX reaction, limiting the maximum temperature in the reactor. This results in a slightly exother­mic process, which is self-sustaining, and high H2 concentration. The ATR fuel processor operates at a lower operating cost and lower tem­perature than the POX reformer, and is smaller, quicker starting, and quicker responding than the SR.

Most of the natural hydrocarbon fuels, such as natural gas and gasoline, contain some amount of sulfur, or sulfur-containing odorants are added to them for leak detection. As the fuel cells or reformer cat­alysts do not tolerate sulfur, it must be removed. Sulfur removal is usually achieved with the help of zinc oxide sulfur polisher, which removes the mercaptans and disulfides. A zinc oxide reactor is oper­ated at 350—400°C to minimize bed volume. However, removing sulfur — containing odorants such as thiophane requires the addition of a hydrodesulfurizer stage before the zinc oxide polisher. Hydrogen (sup­plied by recycling a small amount of the natural gas-reformed prod­uct) converts thiophane into H2S in the hydrodesulfurizer. The zinc oxide polisher easily removes H2S.

To reduce the level of CO in the reformat gas, it must be water gas shifted. The shift conversion is often performed in two or more stages when CO levels are high. A first high-temperature stage allows high reaction rates, while a low-temperature converter allows a higher con­version. Excess steam is used to enhance the CO conversion. In a PEMFC, the reformate is passed through a preferential CO catalytic oxi­dizer after being shifted in a shift reactor, as a PEMFC can tolerate a CO level of only about 50 ppm.

A fuel processor is an integrated unit consisting of one or more of the above stages, as per the requirements of a particular type of fuel cell. High-temperature fuel cells such as the SOFC and MCFC are equipped with internal fuel reforming and hence do not require a high — temperature shift, or low-temperature shift stage. The CO removal stage is not required for the SOFC, MCFC, PAFC, and circulating AFC. For the PEMFC, all the stages are required.

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