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8.4.7 PowerGenerationbyCombinedFuelCellandGasTurbineSystem In general, the oxidation of H2, CO, CH4, and higher hydrocarbons in fuel cells to produce power also produces reject heat. This heat arises from two sources: • the entropy decrease, ∆S, resulting from the overall oxidation reaction -- accompanying the usual decrease in the number of mols of gas, from reactants to products; and • the loss in work, or a conversion of "reversible" work from the oxidation process to heat, due to irreversible processes occurring in the operation of the cell. Heat from these two sources must be rejected from the fuel cell in order to maintain its temperature at a desired level. The heat can be removed and recovered by transferring it across a bounding surface to a heat transfer fluid, but care must be taken to maintain the cell at its desired temperature in this and adjacent regions. Alternatively, heat can be removed in one of the reactant streams passing through the cell -- most practically the air, oxidant stream. Also in the operation of a practical fuel cell, some unburned fuel must remain in the combustion products leaving the cell in order to maintain a significant generated voltage throughout the cell. In order to obtain the highest possible efficiency in electrical generation, both the thermal energy in the heat and the unburned fuel rejected from the cell must be recovered and converted into additional electrical energy. This can be accomplished by means of a heat engine cycle making use of a gas turbine operating in a regenerative Brayton or combined Brayton-Rankine cycle or a steam turbine operating in a Rankine cycle. The relative merits of these three heat engine cycles depend on their overall efficiencies and on the practical aspects of integration, operation, and cost of the power generation plant as a whole. 8.4.8 HeatandFuelRecoveryCycles Simple representations of three fuel cell based heat and fuel recovery cycles are shown in Figures 8-32, 8-33, and 8-36. Regenerative Brayton Cycle: The regenerative Brayton cycle, Figure 8-32, shows a gas turbine compressor for the air flow to the cell. The flow then passes through a countercurrent, recuperative heat exchanger to recover heat from the combustion product gases leaving the gas turbine. The air and the fuel streams then pass into the cathode and anode compartments of the fuel cell(s). The air and fuel streams leaving the cell(s) enter the combustor where they mix and the residual fuel burns. The combustion products enter the turbine, expand, and generate additional power. The turbine exhaust gases pass through the recuperative exchanger to the stack. The most significant variables characterizing the cycle are the fuel cell operating temperature range and the temperature and pressure at the gas turbine expander inlet. These variables are directly related to certain operating variables: the air/fuel ratio entering the fuel cell, the fraction of the fuel leaving the cell unburned, and the temperature difference between the combustion products and air at the high temperature end of the recuperative heat exchanger. The operating variables must be selected and controlled to allow effective operation of the fuel cell, combustor, and gas turbine. There may well be an optimal quantity of unburned fuel leaving the fuel cell, depending on the acceptable fuel cell operating temperature range and turbine inlet temperature. 8-70PDF Image | Fuel Cell Handbook (Seventh Edition)
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