Exergetic Analysis of the nCO2PP Cycle with Particular Reference to the Exergy Destruction of Sewage Sludge Due to Gasification

Abstract

An exergy analysis is carried out on the negative CO2 emission gas power plant (nCO2PP), which integrates the process sections of fuel preparation, power generation and carbon capture. Processes of exergy destruction are studied with particular focus on the process in the gasification unit of the fuel preparation section, where a large amount of exergy is destroyed in various chemical reactions from sewage sludge to producer gas conversion. The largest exergy losses are observed in the wet combustion chamber and in the fuel line with the gasification process and water condensation in the gas scrubber, amounting to 126 kW, 43-45 kW and 56 kW respectively, which corresponds to efficiencies of 62%, 89% and 84% of these units, while the exergy efficiency of the power plant is 29.5%. The integration of the gasification unit with the gas scrubber is investigated, and a heat exchanger combination is considered. Ambient air changes in relative humidity and, due to increasing global greenhouse gas emissions, CO2 concentration are analysed. Insight into the theoretical operation of the power plant through exergy analysis allows energy efficiency to be increased by improving areas of highest exergy destruction. To represent real power plant operation, the analysis is based on an optimised process simulation calculated using the most accurate published equations of state, verified with experimental thermophysical property data from the literature.