Energy and exergy analysis of negative CO2 emission gas power plant operation using thermodynamic modelling results of the cycle

A developed cycle of negative CO2 emission gas power plant is presented. The cycle is an oxy-combustion gas turbine cycle where the gas fuel is burnt with pure oxygen, and liquid water is injected to the combustor to keep the temperature at an appropriate level for the gas turbine. The negative level of CO2 emission can be obtained by combining CO2 capture method with the usage of a gas fuel based on sewage sludge gasification. This fuel is regarded as a biomass fuel with zero emission of CO2. First, the paper presents modeling results of the basic cycle fired with syngas from sewage sludge. For comparison, the cycle is also simulated with methane as the fuel. Energy and exergy analyses of the cycle are conducted. They show the impact of the input values on the total energy and exergy efficiency and on exergy destruction in the basic cycle components. Next, the presented cycle based on the combustion in the oxygen atmosphere is developed by adding part of the oxy-combustion CO2 capture method using a direct-contact heat exchanger to condensate steam and separate CO2. The energy and exergy calculation results give a chance to compare the results of the developed cycle with the cycle when the natural gas (methane) is used as the primary gas fuel. In an exergy analysis, the initial state is specified as the input values for the gas fuel, oxygen, and water and is equal to 15° C and 1 bar. The energy and exergy balance calculations were conducted using developed thermodynamic models of the presented cycles. Calculation of the main cycle parameter as power output, heat, fuel consumption, and indicators such as the total energy efficiency, total exergy efficiency, heat rate, etc., gives the opportunity to evaluate the developed cycle and indicate components where the most significant losses can occur. The analysis shows that the process of water injection can give somewhat higher exergy destruction in the combustor compared with conventional gas turbine combustors; approximately 45% versus 35%. Furthermore, the chemical exergy of the captured CO2 represents 3.5% of the fuel chemical exergy for syngas, while 2.4% for methane. This is the thermodynamic value of the capture. Results of analysis will be helpful in the process design of the negative CO2 emission gas power plant cycle concept.