The biogas-oxyfuel process as a carbon source for power-to-fuel synthesis: Enhancing availability while reducing separation effort

Abstract

Producing synthetic fuels via Power-to-Fuel processes requires hydrogen and a carbon source. To attain a sustainable fuel, both reactants must originate from a renewable source. For the carbon source, biogas plants offer substantial potential. Hence, this paper presents a new biogas-oxyfuel process that couples a biogas plant with Power-to-Fuel production and enables a decentralized and economical supply of biogenic carbon dioxide for the production of renewable methanol. By using the oxygen byproduct of the Power-to-Fuel synthesis in the oxyfuel combustion of a combined heat and power unit, a simple separation of the CO2 in the flue gas is made possible. To analyze the thermodynamic changes within the combustion engine when switching from regular to oxyfuel combustion, an AspenPlus model of the combined heat and power unit of a biogas plant is built up herein. Due to the higher heat capacity of the new working gas carbon dioxide in comparison to nitrogen, the ideal Otto engine cycle’s mechanical efficiency drops by 5.8 percentage point. This drop in efficiency leads to a loss in revenue for the operator of the biogas plant. Together with the additional equipment expenditures for the CO2 separation, this loss is defined as the CO2 separation costs. For a retrofit of existing biogas plants with an installed electric power of 75−1000 kW, the CO2 separation costs are determined to be 88−33 €/t. The process shown therefore offers a promising way to deliver biogenic CO2 at low cost for decentralized Power-to-Fuel systems.