Impact of Exhaust Gas Recirculation on NOx emissions in non-premixed combusted Gas Turbines using CRN

Abstract

The major challenges in the world today is to reduce pollutant emissions from gas turbines while still being able to retain the maximum efficiency in the gas turbine. Especially due to stringent norms on emissions, the research on gas turbines have increased to reduce NOx levels. Most of the gas turbines have been using non-premixed combustion for power generation and since the early 90s research on premixed combustion was sought after. Aircraft engines and many other CHP power plants still currently use non-premixed combustion. Exhaust gas recirculation (EGR) was one of the major part load operations in a gas turbine which was determined to improve part load efficiency and at the same time reduce NOx emission levels.

This thesis research would give an overview on how to model non-premixed combustion in a gas turbine and how different parameters effect NOx emissions. The two main reactors that were used are perfectly stirred reactor(PSR) and a plug flow reactor(PFR). The combustor volume was divided equally between perfectly stirred reactor and plug flow reactor. The half of the combustor where the mixing between the fuel and oxidizer and the combustion reactions occur are modelled by three perfectly stirred reactors which contain flame sheet reactor (FSR), post-flame PSR and PSR recirculation. The volume of FSR was determined by modelling the stoichiometric mixture fraction. This is done by determining the scalar dissipation rate which tells the rate of mixing between the fuel and oxidizer at the stoichiometric interface. The model was then validated using different technical studies on non-premixed combustion.

Parametric analysis was performed with this model using EGR as oxidizer and compared with results that were obtained when using air as oxidizer. It was seen that thermal NOx which was the most amount of NOx released during the combustion was reduced when EGR was used as an oxidizer. It was also found that the CO levels were increased when using EGR but still remained below the level when air was used as an oxidizer. This was due to insufficient residence time inside the combustor. When using EGR as a part load operation and at high pressures in gas turbine, the NOx emissions were lower. The results from the pressure analysis showed that at lower pressures the amount of NOx that was reduced was more when using EGR, but at high pressures the amount of NOx that was reduced was quite less even though it was EGR-oxydizer compositon. It was observed that the NOx and CO levels were still lower while using EGR compared to using inlet guide vanes as a part load operation(IGV). While using a fuel mixture of H2 and natural gas, it appeared that the NOx levels were higher with EGR and air as an oxidizer than just using only natural gas as fuel. At the same time CO emission levels showed that it was low for both EGR and air when fuel mixture of H2 and natural gas was used.