Fog and Overspray Cooling for Gas Turbine Systems with Low Calorific Value Fuels

During the summer, power output and the efficiency of gas turbines deteriorate significantly. Gas turbine inlet air fog cooling is considered a simple and cost-effective method to increase power output as well as, sometimes, thermal efficiency. During fog cooling, water is atomized to micro-scaled droplets and introduced into the inlet airflow. In addition to cooling the inlet air, overspray can further enhance output power by intercooling the compressor. With continued increase of volatility of natural gas prices and concerns regarding national energy security, alternative fuels such as low calorific value (LCV) synthetic gases (syngas) derived from gasification of coal, petroleum coke, or biomass are considered as important common fuels in the future. The effect of fogging/overspray on LCV fuel fired gas turbine systems is not clear. This paper specifically investigates this issue by developing a wet compression thermodynamic model that considers additional water and LCV fuel mass flows, non-stoichiometric combustion, and the auxiliary fuel compressor power. An in-house computational program, FogGT, has been developed to study the theoretical gas turbine performance by fixing the pressure ratio and turbine inlet temperature (TIT) assuming the gas turbine has been designed or modified to take in the additional mass flow rates from overspray and LCV fuels. Two LCV fuels of approximately 8% and 15% of the NG heating values, are considered respectively. `Parametric studies have been performed to consider different ambient conditions and various overspray ratios with fuels of different low heating values. The results show, when LCV fuels are burned, the fuel compressor consumes about 10-18% of the turbine output power in comparison with 2% when NG is burned. LCV fueled GT is about 10-16% less efficient than NG fueled GT and produces 10-24% of net output power even though LCV fuels significantly increase fuel compressor power. When LCV fuels are burned, saturated fogging can achieve a net output power increases approximately 1-2%, while 2% overspray can achieve 20% net output enhancement. As the ambient temperature or relative humidity increases, the net output power decreases. Fog/overspray could either slightly increase or decrease the thermal efficiency depending on the ambient conditions. NOMENCLATURE CDT Compressor discharge temperature CIT Compressor inlet temperature CP Specific heat at constant pressure (kJ/kg-K) D.A. Dry air DBT Dry bulb temperature f′ Fuel Air ratio h Enthalpy (kJ/kg) H Enthalpy of reaction (kJ/kmol) k Polytropic index L Latent heat of water (kJ/kg) NG Natural gas m Mass flow rate(kg/s) n Stoichiometric coefficient OS Overspray P Pressure (kPa) QIn Heat added in combustion chamber (kJ/kg) R Gas constant (kJ/kg-K) RH Relative humidity s Entropy (kJ/kg-K) T Temperature (K) TD Dew point temperature TIT Turbine inlet temperature v Specific volume (m/kg) WBT Wet bulb temperature Greek γ Specific Heat Ratio ρ Density (kg/m) φ Relative Humidity (%) ω Humidity ratio, specific humidity (mv/mg) η Thermal efficiency Subscripts 1 Compressor Inlet 2 Compressor Exit 3 Turbine Inlet 4 Turbine Exit 1f Fuel in ambient condition 2f Fuel in fuel compressor discharge condition a Dry air Amb Ambient c Compressor fc Fuel compressor f Liquid water g water vapor i Index for different elements Pr Product Re Reactant s Isentropic process t Turbine th Thermal