On energy, exergy, and environmental aspects of a combined gas-steam cycle for heat and power generation undergoing a process of retrofitting by steam injection

The paper presents a study of retrofitting of a combined heat and power (CHP) plant aimed to increase the electric to heating power ratio. The modification would involve diverting part of the steam to the gas turbine, instead of letting it go to the steam turbine, using injection technique into a combustion chamber. Therefore, two potential retrofitting approaches are analyzed: steam injection gas turbine (STIGT) and combined steam injection gas turbine (CSTIGT). STIGT is a classic solution where live steam is diverted before the valves of a steam turbine. In CSTIGT steam is received from the turbine extraction. Both solutions increase electric power output of the cycle (STIGT by 1.33 MWe, CSTIGT by 2.68 MWe), decrease heating power (STIGT by 52.90 MWth, CSTIGT by 31.75 MWth) and decrease CHP energy efficiency (STIGT by 32 pp., CSTIGT by 22 pp.). The exergy analyses indicate that the highest exergy losses occur in the combustion chamber (61.30 MW for the reference cycle). Steam injection increase this value by 7.34 MW in STIGT and 5.65 MW in CSTIGT. However, decreasing of combustion temperature prevent the NOx emission by more than 8 g/MWh for both solutions.