Exergetic, Thermal, and Externalities Analyses of a Cogeneration Plant

Abstract

A thermodynamic study of an 88.4MW cogeneration plant located in the United States is presented in this paper. The feedstock for this actual plant is culm, the waste left from anthracite coal mining. Before combustion in circulating fluidized bed boilers, the usable carbon within the culm is separated from the indigenous rock. The rock and ash waste from the combustion process fill adjacent land previously scared by strip mining. Trees and grass are planted in these areas as part of a land reclamation program. Analyses based on the first and second laws of thermodynamics using actual operating data are first presented to acquaint the reader with the plant’s components and operation. Using emission and other relevant environmental data from the plant, an externalities study is outlined that estimates the plant’s effect on the local population. The results show that the plant’s cycle performs with a coefficient of utilization of 29% and an approximate exergetic efficiency of 34.5%. In order to increase these values, recommended improvements to the plant are noted. In addition, the externality costs associated with the estimated SO2 and NOX discharge from the culm fed plant are approximately 1mil∕kWh produced. This is considerably lower (85–95%) than those associated with a similarly sized coal fed plant. The plant’s cycle efficiencies are lower than those associated with more modern technologies; such as an integrated gas turbine combined cycle. However, given the abundant, inexpensive supply of feedstock located adjacent to the plant and the environmental benefit of removing culm banks, the plant’s existing operation is unique from an economical and environmental viewpoint.