Ethanol as an Alternative Fuel in Gas Turbines: Combustion and Oxidation Kinetics

Some research is currently carried out in order to limit CO₂ emissions in power generation. Among alternative fuels to natural gas and gasoil in gas turbines, ethanol offers some advantages. However, while the studies dealing with the combustion of methanol are numerous, the research devoted to ethanol flames is rather scarce, in particular with regard to the use in gas turbines. The combustion of ethanol has been theoretically studied by means of a detailed kinetic model well validated in flame conditions. Thanks to quantum chemistry calculations, the reactions necessary to represent low temperature oxidation have been identified and incorporated in the mechanism and their rate parameters have been determined. Several key parameters, such as auto-ignition temperature (AIT), ignition delay times, laminar burning velocities of premixed flames, adiabatic flame temperatures, and formation of pollutants such as CO and NOx have been investigated in an effort to covers gas turbine applications. One has also explored conditions close to ambient in order to address the related safety aspects (leakages of ethanol). To take into account the potential presence of water in ethanol based fuels, similar studies have been performed for ethanol-water-air mixtures. At last, the data have been compared with those calculated for methane combustion. In the low pressure range, the calculated minimum ignition temperatures have been found to be very sensitive to the pressure and the equivalence ratio for lean mixtures. For pressures above 5 bar and moderately lean or rich mixtures, AITs tend to remain close to 440K. Ignition delay times have been calculated in adiabatic conditions at constant pressure. Surprisingly the addition of limited water contents has a very low influence on these results. The addition of water in the ethanol-air mixture decreases slightly the flame temperatures. In the low temperature range, water increases slightly the auto ignition delay times whereas an opposite effect is observed at high temperature. Calculated flame speed has been compared to that deduced from empirical relations found in the literature and the agreement is satisfactory. The formation of CO in pure ethanol flame was always higher than in methane flame while NO formation showed no difference between the amount calculated in ethanol flame and in methane flame. This result is consistent with the slight difference observed between the adiabatic flame temperatures for the two fuels. When increasing the water content up to 10% in ethanol, the laminar velocities become close to those calculated for methane.