Process simulation and exergy analysis of fermentative hydrogen and methane production

The biogas production process involves a variety of biochemical reactions, which complicates the assessment of thermodynamic performance and the enhancement of process performance. In this study, simulations of the anaerobic digestion processes were carried out to explore the effects of organic loading rate (OLR) and hydraulic retention time (HRT) on biogas production performance. The thermodynamic performance of a conventional single-stage anaerobic digestion and a two-stage anaerobic digestion separating the hydrogen and methane production stages was also evaluated. The results showed that when OLR increased from 1.0 kg volatile solids (VS)/(m³·d) to 3.0 kg VS/(m³·d), the biogas yield increased slightly from 268.46 L/kg VS and then decreased by 17.41% to 221.72 L/kg VS. When HRT extended from 4 d to 20 d, the corresponding biogas yield increased by 28.38% (from 230.88 L/kg VS to 296.41 L/kg VS). The exergy efficiencies of single-stage and two-stage anaerobic digestion were 34.39% and 53.13%, respectively. Compared to the digestion unit in the single-stage system, the input exergy decreased by 3.09%, while the output exergy increased by 48.78% in the two-stage system. It mainly resulted from the enhancement of the exergy efficiency of the digestion unit from 35.58% to 54.63%. In particular, the biogas slurry and residue caused significant exergy losses. It accounted for 47.08% of the total input exergy of the single-stage system and 25.84% of that of the two-stage system, respectively. The utilization of these wastes can play an essential role in the further improvement of the exergy efficiency of anaerobic digestion.