Infoscience

Journal article

Process flow model of solid oxide fuel cell system supplied with sewage biogas

A model for a 1000 kW class solid oxide fuel cell (SOFC) system running on biogas from a sewage sludge digestion plant was implemented in a process flow scheme using external steam reforming. The model stack consisted of planar anode supported cells operated at 800 degreesC displaying state-of- the-art electrochemical performance (0.15 W/cm(2) at 80% fuel utilisation). Real annual data from an existing sewage plant were used as input to the model. From the input of 43 m(3)/h biogas (63% CH4), equivalent to 269 kW (higher heating value, HHV), the SOFC stack was calculated to deliver 131 kW,l electricity (48.7%) using a steam-to-carbon ratio of 0.5. This would allow the sewage site to more than cover its own electrical needs, hence to depollute the waste stream at negative energy cost. In its current exploitation using a low efficient gas engine (130 M), the site is only approximate to50% self- sufficient. Special attention was given to the thermal balance of the stack. The stack developed heat (143 kW) could be balanced by endothermal reforming (78 kW) and by cathode excess air lambda (=3), allowing a temperature difference between stack inlet and outlet of 200 K. The case was compared to other fuel scenarios. Steam-added biogas behaves basically identically to steam-reformed methane. For partial oxidation of biogas or pure hydrogen feeding, electrical efficiency drops to under 43% while needs to be raised to 4.5 to maintain the 200 K thermal gradient over the stack.

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