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• There is vastly more surface area available for catalytic reforming and shift reaction throughout the anode of a practical fuel cell than there is surface area in the three-phase- boundary for electrochemical oxidation. • Mass-transfer of CO, CH4, and even more so of higher hydrocarbons, to the three-phase boundary and through the porous anode is more than ten times slower than that of hydrogen, leading to a more significant impact of concentration polarization. Nevertheless, direct oxidation can be important under certain conditions, such as at the entrance of a cell. The degree to which an anode supports direct oxidation will then impact the degree of pre- reforming of the fuel that is required, which in turn typically impacts balance of plant complexity and cost. This is why there remains strong interest in the development of direct oxidation anodes. The H2 that can be produced from CO and CH4, along with any H2 in the fuel supply stream, is referred to as equivalent H2. The temperature and catalyst of state-of-the-art SOFCs and MCFCs provide the proper environment for the water gas shift reaction to produce H2 and CO2 from CO and H2O. If only H2 and CO are fed to the fuel cell, it is known as an external reforming (ER) cell. In an internal reforming (IR) fuel cell, the reforming reaction to produce H2 and CO2 from CH4 and H2O occurs inside the stack. In some IR fuel cells, reforming takes place on the anode (on-anode reforming) while in others a reforming catalyst is placed in proximity to the anode to promote the reaction (in-cell reforming). 2.3 Cell Energy Balance The discussion above can be used to formulate a mass and energy balance around a fuel cell to describe its electrical performance. The energy balance around the fuel cell is based on the energy absorbing/releasing processes (e.g., power produced, reactions, heat loss) that occur in the cell. As a result, the energy balance varies for the different types of cells because of the differences in reactions that occur according to cell type. In general, the cell energy balance states that the enthalpy flow of the reactants entering the cell will equal the enthalpy flow of the products leaving the cell plus the sum of three terms: (1) the net heat generated by physical and chemical processes within the cell, (2) the dc power output from the cell, and (3) the heat loss from the cell to its surroundings. Component enthalpies are readily available on a per mass basis from data tables such as JANAF (1). Product enthalpy usually includes the heat of formation in published tables. A typical energy balance determines the cell exit temperature knowing the reactant composition, the feed stream temperatures, H2 and O2 utilization, the expected power produced, and a percent heat loss. The exit constituents are calculated from the fuel cell reactions as illustrated in Example 9-3, Chapter 9. 2.4 Cell Efficiency The thermal efficiency of a fuel conversion device is defined as the amount of useful energy produced relative to the change in enthalpy, ∆H, between the product and feed streams. 2-7PDF Image | Fuel Cell Handbook (Seventh Edition)
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