Biogas SOFC µCHP - a simple process concept with high electrical efficiency

Biogas is seen as one of the most promising fuels for the market entry of stationary fuel cells. Biogas SOFCs because of the special chemical composition of biogas . can be designed in a much simpler way than conventional natural gas SOFCs while still retain their high electrical efficiency. It was shown that a biogas driven SOFC system may be run at significantly higher electrical efficiencies than a methane or natural gas driven unit and still remain with a simple and robust process design. That is due to the great amount of carbon dioxide within the biogas. It allows the biogas to be reformed at significantly lower air-to-fuel-ratios (methane: λ min≡0,26; biogas: λ min≡0,05) without the risk of carbon formation resulting in a reformate with substantially greate r chemical energy flux Pch. The greater chemical energy flux within the reformate allows more current to be drawn from the same chemical energy flux input into the system. This leads to a greater gross electric power output at the same chemical energy flux input into the system resulting in a higher electrical efficiency. It was shown that at probable operating conditions the ideal gross electrical efficiency of a biogas-fed system may be as much as 14 percentage points greater than that of a methane-fed system (FU=0,75; methane: η e,ideal≡0,50; biogas: η e,ideal≡0,64). Experiments were run on an SOFC system laid out for both methane and biogas operation. Through the course of these experiments it was not possible to demonstrate the calculated results. It was shown th at using biogas the ratio of output and input chemical energy fluxes can be larger than for methane operation. Due to the insufficient heat flux directed at the reformer it was not possible to achieve higher electrical efficiencies with biogas. As a result a new system should be designed solitarily for biogas operation. For this system to deliver the desired performance it is necessary to couple heat sources with the drain the reformer poses. Present activities aim at coupling reformer and afterburner into one proprietary component especially designed to allow maximum heat flux from afterburner to reformer.