Net energy analysis of small-scale biogas self-supply anaerobic digestion system operated at psychrophilic to thermophilic conditions

Abstract

Anaerobic digestion is a promising technology for the treatment of high strength organic wastes, since it produces renewable energy (biogas) and valuable digestate as fertilizer. The emphasis of this paper lies in demonstrating the net energy characteristics of a small-scale biogas self-supply anaerobic digestion system under three operation modes, considering the digestion temperatures varying from the psychrophilic to thermophilic and dry matter concentrations from 4 to 10%. The system mainly includes the reactor and biogas boiler at Mode 1, which is added slurry waste heat recovery at Mode 2 and further incorporated a thermal pretreatment process at Mode 3. In addition, a numerical energy balance model and an evaluation framework are developed for net energy feature exploration and contrastive analysis between different modes. The results show that the required digestion conditions (the highest digestion temperature, the lowest dry matter concentration) to guarantee the benchmark at Mode 1, are (25 °C, 4%), (35 °C, 6%), (40 °C, 8%), and (55 °C, 10%), and become more lenient at Mode 2. The maximum net energy production occurs at approximately 25 °C or 50 °C, depending on the concentration and operation mode. Moreover, thermal pretreatment for sterilization would not reduce much net energy. Therefore, it should consider net energy production, operation mode, digestion temperature, concentration as well as other related factors for optimal operation parameter determination of anaerobic digestion system.