High-Calorific Biogas Production by Selective CO2 Retention at Autogenerated Biogas Pressures up to 20 Bar

Autogenerative high pressure digestion (AHED) is a novel configuration of anaerobic digestion, in which micro-organisms produce autogenerated biogas pressures up to 90 bar with >90% CH4-content in a single step reactor. The less than 10% CO2-content was postulated to be resulting from proportionally more CO2 dissolution relative to CH4 at increasing pressure. However, at 90 bar of total pressure Henry's law also predicts dissolution of 81% of produced CH4. Therefore, in the present research we studied whether CO2 can be selectively retained in solution at moderately high pressures up to 20 bar, aiming to produce high-calorific biogas with >90% methane. Experiments were performed in an 8 L closed fed-batch pressure digester fed with acetate as the substrate. Experimental results confirmed CH4 distribution over gas and liquid phase according to Henry's law, but the CO2-content of the biogas was only 1-2%, at pH 7, that is, much lower than expected. By varying the ratio between acid neutralizing capacity (ANC) and total inorganic carbon (TICproduced) of the substrate between 0 and 1, the biogas CO2-content could be controlled independently of pressure. However, by decreasing the ANC relative to the TICproduced CO2 accumulation in the aqueous medium caused acidification to pH 5, but remarkably, acetic acid was still converted into CH4 at a rate comparable to neutral conditions.