Experimental and numerical studies have been done to examine the formation characteristics of NOx in a pilot scale combustor (0.2MW) adopting a multi-air staged burner.
Experimentally, the temperature and species characteristics were studied in the flow field of multi-staged burner flame. The gas temperature was measured with a bare-wire Pt/13% Rh-Pt thermocouple sheathed in straight length of twin-bore ceramic cladding of 5mm-o.d. The species concentration of CO, NOx and $O_2$ were measured by analyzing the sample gases taken using a water-cooled stainless steel probe through the inspection holes. Malvern system was used to measure drop size of diesel fuel. Diesel doped with pyridine $(C_5H_5N)$ was used to investigate fuel NOx formation.
Numerically, thermal an prompt NOx were calculated in the flow field of LPG flame. The mathematical models for turbulence, radiation and nitric oxide chemistry were taken into account. The radiative transfer equation was solved using the discrete ordinates method with the weighted sum of gray gases model. In the NOx chemistry model, the chemical reaction rates for thermal and prompt NOx were statistically averaged using a probability density function. The results were validated by comparison with measurements.
From the results, the formation of NOx was changed by the air supply conditions of each air staged air ratios and swirl numbers. The formation of NOx was decreased as the mixing combustion air and fuel was delayed in front of fuel nozzle. The multi-staged burner has a good capability in reducing thermal NOx resulting from the distributed heat release rate and lower flame temperature in fuel-rich and fuel-lean combustion zones. The formation of fuel NOx was also decreased as the mixing combustion air and fuel was delayed in front of fuel nozzle, because fuel N is converted to $N_2$ more than NOx in this zone. A good agreement of numerically predicted results with measurements was obtained only when the radiation was taken into consideration, except the upstream region of combustor near the burner inlet.
Using the numerical simulation developed here, a variation of thermal and prompt NOx formation was predicted by changing the excess air ratio in LPG flame. As the excess air ratio increased up to 1.9, the formation of the total as well as thermal NOx at exit increased while the prompt NOx decreased. The formation of thermal NOx was more affected by concentration of $O_2$ and $N_2$ than gas temperature in this system.
