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In a similar study, Liebhafsky and Cairns (26) compared two arrangements of tubular, calcia- stabilized solid oxide fuel cells. In one arrangement, hydrogen and air were supplied to a single, 30-cm cell. In the other arrangement, the same cell was segmented into three, 10-cm cells which were ducted such that the same reactant streams flowed through them in series. Each arrangement had a total fuel utilization of 90 percent and each cell had the same average current density. Each cell in the series arrangement accomplished one-third of the total fuel utilization. Calculations showed that the series arrangement produced 5 percent more power than the single cell, and that further sectioning would produce greater improvements. It was concluded that the increase in irreversibility associated with changes in gas composition has nothing to do with electrode kinetics, but is rooted in the Nernst equation. 8.5.2 MCFCNetwork When designing an MCFC power system, several requirements must be met. An MCFC system must properly condition both the fuel and oxidant gas streams. Methane must be reformed into the more reactive hydrogen and carbon monoxide. Carbon deposition, which can plug gas passages in the anode gas chamber, must be prevented. To supply the flow of carbonate ions, the air oxidant must be enriched with carbon dioxide. Both oxidant and fuel feed streams must be heated to their proper inlet temperatures. Each MCFC stack must be operated within an acceptable temperature range. Excess heat generated by the MCFC stacks must be recovered and efficiently utilized. Figure 8-40 shows an MCFC network. The arrangement of stacks in series, as well as a unique recycle scheme, allows an MCFC network to meet all the requirements of an MCFC power system, while achieving high efficiency. 8.5.3 RecycleScheme In the network's recycle scheme, a portion of the spent fuel (Stream 5) and oxidant (Stream 4) is mixed and burned. The products of combustion (Stream 3) are then recycled through the cathode in order to provide the necessary carbon dioxide to the stacks. This eliminates the need for an external source of pure carbon dioxide. The cathode-cathode recycle (Stream 4) is large enough to cool the stacks, transferring excess energy to the heat recovery boilers. During the transfer of heat, enough energy is left in the oxidant recycle to heat the fresh air feed to the designated cathode inlet temperature. A second portion of the spent fuel (Stream 1) is recycled through the anode to provide enough steam to prevent carbon deposition and internally reform methane. This eliminates the need for steam to be supplied from another source. The anode-anode recycle also heats the fresh fuel feed to the designated anode inlet temperature. 8.5.4 ReactantConditioningBetweenStacksinSeries When MCFC stacks are networked in series, reactant streams can be conditioned between the stacks -- at different stages of utilization. The composition of reactant streams can be optimized between stacks by injecting a reactant stream (see Figure 8-40) or by mixing the existing reactant streams. 8-86PDF Image | Fuel Cell Handbook (Seventh Edition)
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