The rise in gas turbine combustion chamber temperatures requires optimal choices to be made with regard not only to performance parameters but also with a view to resolving pollutant emission problems. For this reason, the authors have set up a gas turbine cycle model which performs an accurate analysis of several processes, in terms of operating fluid chemical and thermodynamic properties. The model also enables prediction of NOx formation based upon chemical kinetics and is able to relate the amount of pollutants to a number of operating parameters (e.g. cycle pressure ratio, fuel to air equivalence ratio, residence time in combustion chamber, etc.). It can also predict the effect of the most usual NOx reduction systems, such as water or steam injection.
A comparison of several possible choices for the gas and combined cycles is then presented, in terms of thermodynamic performance (e.g. first and second law analysis) and nitric and carbon dioxide emissions. In order to find the best compromise between performance improvement and limitation of pollutant emission, enhanced gas cycles are also considered, such as STIG or intecooled-reheat cycles.
Examples also refer to medium or low BTU gases, obtained from coal gasification, in order to show not only the possible advantages in terms of thermal NOx reduction, but also the significant amounts of “fuel NOx“ which can arise from ammonia contained in the fuel.
