Comparative exergy and energy assessment of a biogas plant with biological purification process: A multigeneration system

Abstract

The improved systems of biogas production usually increase the energy consumption of biogas plants. Therefore, it is very important to determine an appropriate improvement system to increase plant efficiency. For this purpose, a biogas plant with a biological self-purification system was energetically and exergetically analyzed, and its performance was compared with that of a base plant. To keep the temperature of digesters up to 310.2 K, a solar water heater was used. It was able to maintain a high level of efficiency for both plants. The energy analysis of the plants indicated that the overall energetic efficiency of both plants was very close. The exergy analysis of the plants showed that the overall exergetic efficiency of the self-purification biogas plant (76.24%) was higher than that of the base plant (66.78%). This is due to the fact that the total exergy destruction rate of the self-purification plant was lower than that of the base plant and the exergy rate of biogas output of the self-purification plant was higher than that of the base plant. The exergy analyses of both plant components showed that although the highest exergy destruction rates were attributed to the principle digester and separation unit, they showed the highest exergetic improvement potential rates. These results confirm that the digesters in biogas plants have a great potential to be improved exergetically, and the self-purification system is a suitable improvement system to increase the plant efficiency exergetically.