Net energy characteristic in biogas plants with slurry waste heat recovery

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 object of this paper is a small-scale anaerobic digestion system treating swine manure, which can operate at two modes. Mode 1 is a base anaerobic digestion unit, mainly including the reactor and biogas boiler, and based on Mode 1, Mode 2 incorporates a slurry waste heat recovery unit. In order to explore the effects of fermentation temperature and volatile solid (VS) concentration on net energy production characteristic of biogas plants, this paper built a numerical energy balance model based on the above system and the two operation modes. An evaluation framework, including energy efficiency ratio and net energy production, was proposed for contrastive analysis between different operation modes. In addition, slurry temperature distribution at ambient conditions without any heating measures and system heat load characteristic at Mode1 were also studied in this paper. It shows that at ambient conditions, the digester could operate normally for 5 months per year with the slurry temperature ranging from 20-35 ℃, and it could only last 2 hours for temperature 35 ℃. When heated by biogas boiler, the heat required to raise influent slurry temperature to working temperature (20-60 ℃) accounted for approximately 69% of the total heat load. For evaluation indicator of energy efficiency ratio, a corresponding maximum digestion temperature to guarantee the benchmark indicator (energy efficiency ratio of 6.4) was determined at Mode 1 with the VS of 6% and digestion temperature of 35 ℃. From 4% to 10% of VS, the maximum temperature at Mode 1 were 28.9, 35, 39.6 and 39.5 ℃, respectively, and became more lenient at Mode 2, they were 36.5, 46.4, 54.6 and 58 ℃, respectively. From 4% to 10% of VS, the digestion temperature for achieving the maximum net energy at Mode 1 were 30, 35, 45 and 60 ℃, respectively, while at Mode 2, they were 35, 40, 50 and 60 ℃. Furthermore, heating measures for anaerobic digestion system were beneficial to improve net energy production. At the VS of 6%，biogas production at ambient conditions was only 0.36 times as much as that at mesophilic temperature (35 ℃). When heated by biogas boiler, the ratio of the maximum net energy was 2.37 times of that at ambient temperatures; While coupled with the recovery of effluent slurry waste heat, it was 2.53 times. The largest increase of the maximum net energy of between Mode 1 and Mode 2 at specific VS was approximately 11.5%, with the corresponding energy efficiency ratio was around 53.1%. Therefore, it is obvious that feeding heating load is a key factor influencing the digestion process. Hence, for the stable fermentation system, it is of great significance to pay more attention to the heating process of the feedstock. Meanwhile, it should consider the digestion temperature, the VS concentration, the energy efficiency ratio, the net energy production, the operation mode and other related factors for optimal operation parameter determination of the anaerobic digestion system. Last but not least, it is necessary to further research the characteristics of the gas production during continuous changes of the influencing factors such as VS concentration and digestion temperature to ensure the accuracy of the optimal parameters.