Pressurised anaerobic digestion for reducing the costs of biogas upgrading

Abstract

The overall purpose of this study is to investigate the potential for producing higher energy biogas at elevated fermentation pressures. Upgrading of biogas is often carried out to increase its methane (energy) content by removing carbon dioxide. Upgrading is used, for example, to give methane of sufficient purity that it can be injected directly into the gas supply grid. In this research, freshwater algae are used as the feedstock for anaerobic digestion (AD) to produce biogas as a source of renewable energy. Although this has been the subject of extensive research over the past few decades, the main reason why AD has not been more widely commercialised is because it can have poor economic viability. In this paper, we used two similar bioreactors of capacity 1.5 L to generate biogas at different pressures. The methane concentration of the biogas increases to at least 70.0% for a headspace pressure greater than 4 bara compared to 57.5% or less when the pressure is less than 1.6 bara. The higher pressure operation therefore reduces the amount of upgrading required leading to a reduction in the cost of this step. Another interesting finding of this study is that the solubility of biogas in the digestate is estimated to be only 3.7% (best fit value) of its solubility in pure water, which is much lower than the values previously reported in the literature.