Thermochemical Recuperation of the Exhaust Gas Heat in Combined-Cycle Units: Thermodynamic Analysis

A scheme of a combined-cycle unit with thermochemical recuperation of flue gas heat using methane steam reforming is considered. The essence of thermochemical recuperation is the recovery of flue gas heat for the endothermic methane reforming yielding a new synthetic fuel. The results are presented of a thermodynamic analysis of the combined-cycle unit thermal cycle with a gas turbine inlet temperature of 1500 and 1600°C at a pressure in the combustion chamber from 2 to 4 MPa. The thermodynamic analysis of the thermal cycle of a combined cycle unit was performed using the Aspen HYSYS software package, and the thermodynamic analysis of methane steam reforming was performed using the Gibbs free energy minimization method. The efficiency of thermochemical recuperation of flue gas heat has been demonstrated to depend on the process conditions, such as pressure, temperature, and composition of the initial reaction mixture. Increasing the temperature and the steam to methane ratio increases the reaction enthalpy and the efficiency of methane reforming, while an increase in the pressure decreases these parameters. It has been found that the thermochemical recuperation of heat can increase the effectiveness of a conventional combined-cycle unit by 3–5%. For example, the efficiency of a combined-cycle unit is 59% without thermochemical recuperation or 64% with thermochemical recuperation at a turbine inlet temperature of 1600°С and a compression ratio of 30. The thermal and material balances of a combined-cycle unit with thermochemical heat recuperation are presented in the form of energy and material flows, according to which 45% of the exhaust gas heat is recuperated in the fuel cycle of the gas turbine unit and approximately 45% of the exhaust gas heat is consumed in the steam turbine cycle.