The Humid Air Cycle: Some Thermodynamic Considerations

The humid air cycle (Rao,1990), recently proposed, is an intercooled gas turbine cycle, having an air-water mixing evaporator before the combustion chamber, and a recovering system for exhaust gases. The solution appears to have several advantages: increase in efficiency, increase in power output, reduction of NOx. These important effects are similar to those encountered in STIG (STeam Injection Gas turbine) or CHENG (Saad and Cheng, 1992) power plants: however the particular non-isothermal vaporisation here considered enhances the efficiency increase. Considering a TIT (Turbine Inlet Temperature) at 1273 K and combustion with methane, three different plant solutions are considered, where modifications are related to water circuit, to determine the most important parameters affecting the cycle's performances. The results show the advantage of employing a dry cooling tower to lower the recirculating water temperature or the total mass flow coming from evaporator. Both cases result in a lower exhaust gases temperature. The advantages and comebacks of the three cycles are taken into consideration and discussed in detail, selecting a possible optimised plant solution. The paper emphasises also the practical feasibility of this plant