Optimization of a Biogas-Fuelled Cogeneration System in a Sewage Treatment Plant

Efficient utilization of biomass by a cogeneration system (CGS) is a promising technology for promoting sustainable energy development. Sewage treatment plants are facilities that have been continuously producing biogas by anaerobic digestion. Thus, the potential of a biogas-fuelled CGS in a sewage treatment plant is estimated to be very high. However, there have been few reports on the performance of a biogas-fuelled CGS, particularly regarding the effect of ambient temperature on its performance, and the most efficient arrangement of a biogas-fuelled CGS remains unknown. In this study, performance of a biogas-fuelled CGS was simulated under three typical ambient temperature (low, medium and high) conditions using actual data for a CGS with a micro gas turbine. In the beginning of this study, the relation of energy balance of the plant and ambient temperature was clarified. It was found that the amount of heat demand is ambient temperature-dependent but that the amount of biogas fuel produced is almost constant throughout the year. When a boiler is replaced with a biogas-fuelled CGS to utilize the biogas, under a high temperature condition, the CGS is not able to fully utilize all of the biogas produced, and therefore another pathway of biogas utilization is needed. Under a medium temperature condition, a gas storage system is needed for using biogas efficiently. However, some of the biogas still cannot be utilized efficiently. Under a low temperature condition, since ambient temperature varies greatly between summer and winter, the amount of heat demand of the plant also varies greatly throughout the year. This leads to an imbalance in biogas production and heat demand, and therefore attention must be given to energy management in this condition. The combination of other auxiliary equipment such as a boiler, heat pump and gas storage with the CGS is required in order to cover the total heat demand throughout the year. Four possible arrangements of the CGS with different auxiliary components were proposed and their performances were compared. It was found that all of the proposed CGS arrangements can sufficiently cover the total heat demand by only using biogas produced in the facility. Compared to the conventional system, all proposed CGS arrangements can reduce electrical power demand by 23∼28%, recover 74∼77% of the energy of biogas produced, and utilize almost 100% of the biogas produced. The arrangement with a heat pump is more efficient than the arrangement with a boiler. It was also found that excess biogas in summer can be used in winter by storing the biogas. Thus, a CGS arrangement that includes a gas storage system will enable efficient utilization of biogas and recovered exhaust heat.