Development of an innovative two-stage fermentation process for high-calorific biogas at elevated pressure

Abstract

The two-stage high-pressure fermentation (HPF) process enables the production of methane at high operating pressure. Pressure significantly reduces the energy needed for injecting the produced biogas into the gas grid by 45–60%. It also allows for incorporating large parts of the necessary biogas upgrading process into the synthesis step. As a result, the two-stage HPF process provides pressurized biogas with methane volume fraction ranging from 0.75 to 0.94. The pressure is not generated by energy intensive gas compression, but in-situ by microbial gas production. In comparison to conventional biomethane production, the overall costs could be reduced up to 20%. HPF is most beneficial when its operating pressure is adapted to that of the gas grid.The article presents briefly the development of the two-stage HPF beginning with tests in batch reactors, followed by experiments on gas solubility, and proof-of-concept in continuously operated methanogenesis reactors (MR) up to 9 bar. It also represents the effect of incorporating microfiltration (MF) of the feed stream, on improving the biogas quality and process stability of a continuously operated lab scale HPF process. By linking the MF with the HPF, methane volume fraction in the MR increases from 0.86 to 0.94 at 25 bar. Finally, the simulation and experimental results show good agreement with each other thereby making them a good basis for further optimization of the HPF process.