For several years the injection of steam in the combustion chamber has represented a common way to improve the performance of gas turbine power plants, increasing both the power output and the efficiency and reducing, at the same time, NOx emissions. Starting from the first GE STIG (STeam Injected Gas turbine) cycles, several types of gas turbine cycles with steam or water injection have been proposed. Among them, the most interesting results were obtained with the Cheng and the HAT (Humid Air Turbine) cycles. In particular, the HAT cycle, which is an intercooled gas turbine cycle, having an air-water mixing evaporator before the combustion chamber, and a recovering system for exhaust gases, has been identified as a promising way to generate electric power at high efficiency, low cost and with a simple system compared to combined cycles (Stecco, et al., 1993a-b, Gallo, et al., 1995). However, the relevant water consumption, about 1210-2420 m3/day for a 100 MW unit, continues to represent a significant drawback to the diffusion of the HAT cycle as well as of other steam injected cycles. In fact, so high a water consumption means high operational costs for water treatment, and eventual legislative restrictions limiting the use of water, not to mention the environmental impact of the depletion of water resources. The aim of this paper is to estimate the possibility of water and heat recovery from the exhaust gases (Bombarda, 1995, Bettagli and Facchini, 1994), and particularly the effect on the whole cycle, by adopting a surface exchanger before the stack. The results show the possibility to recover a large fraction of injected water, and also a significant amount of exhaust heat. The paper takes also into account all the thermodynamic processes in each cycle components with respect to the possibility to recover water from the exhaust gases. Since the presence of water in the exhaust gases is also caused by the inlet air humidity and the combustion process, the relative amounts of water and their contributions to the overall heat recovery are calculated.
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