Biogas Combustion in Premixed Flames or Electrochemical Oxidation in SOFC: Exergy and Emission Comparison

In the future energy pathway, characterized by flexibility of technologies and fuels, biogas could represent an alternative to conventional natural gas in feeding multiple types of technologies, both traditional thermal machines (chemical reactions), and innovative electrochemical generators such as fuel cells (electrochemical reactions). To compare the two pathways of energy production, two criteria are considered: (a) environmental analysis (emissions) and (b) exergy analysis. The results of the environmental and exergy comparison are presented and discussed in case of two selected transformation processes: partially premixed flames (PPFs, for chemical processes) and solid oxide fuel cells (SOFCs, for electrochemical processes), for a range of operating conditions. From an environmental point of view, the PPF exhaust stream has significant traces of NO_x and C₂H₂, which are precursors of atmosphere pollution, while the SOFC exhaust stream does not contain such chemical species due to the absence of combustion. From a exergy point of view, the utilisation of the biogas in form of electrochemical oxidation in a SOFC indicates significantly higher exergetic efficiency compared to the chemical oxidation in partially premixed flames.