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In this study, efficient configuration of a biogas‐fuelled cogeneration system (CGS) in a sewage treatment facility was investigated. The efficient configuration of the CGS was clarified on the basis of the relationship between exhaust heat recovery efficiency (ηehr) of the CGS and the ratio of yearly average heat demand to yearly average biogas production of the facility (Qh.d/Qb.p). The CGS was assumed to be used under Qh.d/Qb.p<ηehr,Qh.d/Qb.p≈ηehr, and Qh.d/Qb.p>ηehr conditions. It was found that although the CGS was able to cover total heat demand of the facility by only consuming biogas produced, from the point of view of energy utilization, reduction of unutilized biogas and reduction of electricity demand efficiencies, the most efficient CGS was obtained under the Qh.d/Qb.p≈ηehr condition. Under the Qh.d/Qb.p≈ηehr condition, energy utilization, reduction of unutilized biogas, and reduction of electrical demand efficiencies were 0.64, 0.99, and 0.32, respectively, whereas under the Qh.d/Qb.p<ηehr and Qh.d/Qb.p>ηehr conditions, energy utilization, reduction of unutilized biogas, and reduction of electrical demand efficiencies were in ranges of 0.56–0.64, 0.43–0.99, and 0.16–0.20, respectively. A more efficient system can be obtained if a CGS with lower ηehr such as a fuel cell is used under the Qh.d/Qb.p<ηehr condition and if a CGS with higher ηehr such as a steam turbine is used under the Qh.d/Qb.p>ηehr condition. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library (wileyonlinelibrary.com/journal/htj). DOI 10.1002/htj.20389

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.

In recent years, production of biogas by an anaerobic digestion method has been reconsidered as an alternative means for producing clean fuel. Thus, research on various aspects of biogas production by anaerobic digestion has become important for its widespread utilization. One of the important aspects is energy balance of the process. The energy balance is affected by a heat demand, whereas the heat demand is affected by ambient temperature. In the present study, in order to clarify the effect of ambient temperature on the energy balance of anaerobic digestion, typical municipal sewage sludge anaerobic digestion in an actual wastewater treatment plant was adopted as analysis model and was studied under various temperature conditions. It was found that the heat demand for heating the sludge was temperature-dependent and that 26~39% of the total amount of heat demand was heat losses from digester tanks. From the energy balance, it was found that net energy can be produced by the anaerobic digestion in any region and season in Japan, and this shown that it has a possiblity to be a new energy resource. It was also shown that, when ambient temperature became closer to the digestion temperature, more efficient anaerobic digestion can be obtained. These results were confirmed by indexes of energy balance that were defined in this study, with notable proportional relations being found between indexes and ambient temperature. Ratio of energy of heat demand to biogas produced had an inverse proportional relation with ambient temperature, whereas ratio of energy of net energy to biogas produced had a proportional relation, and ranges of these values in all of Japan were 0.13~0.44 and 0.45~0.84, respectively.

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