Analysis of Efficiency and Water Recovery in Steam Injected Gas Turbines

Abstract

The study presented in this paper has two objectives. The first objective is to analyse the efficiency of the steam injected gas turbine by modelling the thermodynamic cycle. This is done by adapting a calculation model for turbine blade cooling proposed by El Masri (1986). The expansion path is divided into small subintervals, to take into account the changing gas properties during the expansion. This model is then verified for four different industrial machines. The basic cycle as well as cycles with thermodynamic improvements as intercooling, heat recuperation by heat exchanger and blade cooling using steam are studied. The calculations are done for a range of pressure ratios (PR) and turbine inlet temperatures (TIT), with methane (CH4) as fuel being representative of natural gas. A comparison is made with a simple cycle gas turbine and with a combined cycle system. The maximum efficiency of the basic cycle is found to be around 49 % with current gas turbine technology. Steam blade cooling is extremely simple to implement in a steam injected gas turbine and is found to be thermodynamically very attractive, bringing the maximum efficiency to about 52 %. Secondly the water recuperation in the condenser is analysed. Due to the combustion of the fuel, water is formed. As a result, the dew point temperature of the combustion gas without steam injection can be rather high, i.e. around 45 °C. As a consequence, the amount of water corresponding to the injected steam can be recuperated by cooling the gas mixture to the original dew point temperature. Closing the cycle for water is in this case thermodynamically possible. The practical recuperation of water in the condenser is studied on a test rig with a simulated gas turbine augmented with a condenser and steam injection. This proves that complete recuperation of the injected water is technically possible. The conclusion of the study is that a steam injected gas turbine with complete water recuperation is possible and has a high efficiency.