Exergy-based economic, environmental, and sustainability analysis of two different biogas-fueled trigeneration proposals for a wastewater treatment plant (WWTP)

Abstract

The objective of this study is to assess the performance of two CCHP systems in wastewater treatment plants (WWTPs): one powered by a micro gas turbine (AD-CCHP-MGT) and the other by an internal combustion engine (AD-CCHP-ICE). The anaerobic digestion (AD) unit of the WWTP provides biogas, which is analyzed through experimental digestion tests. Both the biogas fueled systems use the waste heat for various applications including feedstock pretreatment. The AD condition is thermophilic process (55 °C) with thermal pretreatment at 90 °C for 0.5 h. The results indicate that the self-sufficiency of power for AD-CCHP-ICE system is 43.41 %, whereas it is 30.90 % for AD-CCHP-MGT system. Furthermore, AD-CCHP-ICE system provides low-cost energy products and more electric power output, resulting in a shorter payback period. Additionally, excluding the AD investment cost, the payback period of systems are almost halved. The exergy-based environmental and sustainability analyses indicate that the specific CO₂ emission rate of AD-CCHP-ICE is lower than that of AD-CCHP-MGT. With an environmental improvement index 1.5 times higher, the AD-CCHP-ICE is more environmentally friendly. Moreover, AD-CCHP-ICE demonstrates a larger sustainability index than AD-CCHP-MGT, as the exergy destruction and loss rates differ between the systems, following the same trend.