Feasibility of annual wet anaerobic digestion temperature-controlled by solar energy in cold areas

Abstract

• Propose a novel anaerobic digestion system heated by solar energy. • The pilot experiments are carried out to prove the feasibility of the system. • Proved the operate feasibility of solar heated AD system in the cold area. • Show the annual performance and control strategy of solar heated AD system. In cold rural areas of China, variation in ambient temperature leads to higher heat loss of anaerobic digester, low digestion efficiency, and reduction of biogas production. Solar energy is publicly regarded as economical and suitable energy for steady biogas production. However, the high initial investment; and unstable energy supply of the solar-assisted system was the higher challenge of the steady biogas production system. This work aimed to develop a solar water heated anaerobic digestion system with efficient temperature control and present the feasibility of the annual biogas production system temperature-controlled by solar energy in the pilot study. Energy balance was analyzed theoretically between the evacuated tube solar water collector and the anaerobic digester at first. Then a biogas production system was developed in Lanzhou City, China and it mainly consists of a 3m3 insulated anaerobic digester and solar collector of 30 Φ58×1800mm evacuated tubes. Annual batch pilot experiments have been carried out to test the feasibility of the steady biogas production system temperature-controlled by solar energy in cold and arid areas. The result shows that the temperature-controlled solar energy anaerobic system can start successfully on the 10th, 8th, and 13th during the reaction period and operate smoothly during the whole year in different seasons. The slurry temperature of the three batches was all maintained at 26±1.1℃,37±1.3℃, and 52±1.1℃, while the temperature difference of solar water was 19.9℃, 15.5℃, and 30.5℃ under 3-4 days snowy or cloudy weather. The cumulative methane production at three temperatures was 0.562,0.632 and 0.846 m 3 /kg VS. The system runs steadily during the whole year by psychrophilic and mesophilic anaerobic (26℃ and 37℃) digestion in winter and spring and by thermophilic (52℃) anaerobic digestion in summer and autumn, which implies a high efficient operation strategy for agricultural and animal husbandry wastes treatment in cold and arid areas. These theoretical and experimental results provide a scientific basis and engineering reference for the application of biogas production temperature-controlled by solar energy and have important value and significance for the efficient and low-cost anaerobic digestion treatment of agricultural and animal husbandry wastes in cold and arid areas.