Modeling, performance enhancement, and the NOx emission reductions of the triple-pressure reheat water-recuperated air-cooled gas turbine combined cycle power plant using interstage compressor water injection and optimization

Abstract

Interstage compressor water injection (ICWI) reduces the work of an air compressor, air temperature at its outlet, adiabatic flame temperature, and NOx emissions. When ICWI is introduced to combined cycles, it decreases the surge margin for the air compressor and increases the mass flow rates of the gas through the gas turbine (GT) and generated steam for the heat recovery steam generator and, therefore, power output. For the combined cycle with ICWI, air with two distinct characteristics (before and after ICWI) is used to cool the GT and mix with the expanded gas. In this paper, ICWI was introduced to the triple-pressure reheat air-cooled GT combined cycle. Detailed modeling was carried out for the processes of compression, ICWI, combustion, and GT expansion. Constraints were imposed on the amount of injected water and its temperature and pressure leading to a practical application of ICWI and a safe surge margin for the air compressor. The efficiency of the combined cycle with ICWI was optimized relative to the pressure of ICWI, injected water temperature, and relative humidity of air at the outlet of the water injector at different values of turbine inlet temperatures and air compression ratios. The results indicated that ICWI and optimization could reduce the initial NOx emissions by more than 90% (the NOx emissions that are produced at the adiabatic flame temperature) and enhance cycle efficiency by 1.9 to 2.7 percentage points and power output by 27 to 34 percent. The water recovery system could produce a significant amount of water that exceeds twice the injected amount at no fuel cost and could be integrated into the cycle at no additional equipment costs if CO2 capture or exhaust gas recirculation was implemented.