Numerical Study on NO Formation in a Pulverized Coal-fired Furnace using Oxy-fuel Combustion

Computational fluid dynamics (CFD) approach was employed to numerically investigate NO formation in a 600 MW wall fired pulverized coal-fired furnace retrofitted for oxy-coal combustion, aimed at the impacts of flue gas reycle ratio, O-2 staging and recycled NO with the recycled flue gas (RFG) on NO formation and emission. An in-house CFD research code for conventional air combustion was developed and extended to simulate O-2/RFG combustion with specific considerations of the change of gas properties and its impact on coal particle combustion processes. The extended De Soete mechanisms including NO reburning mechanism were applied to describe transformations of fuel nitrogen. It was shown that CFD simulation represented the significant reduction of NO formation during O-2/RFG combustion compared to that during air combustion. The in-burner and particularly the in-furnace O-2 staging were confirmed still to play very important roles in NO formation control. Changing the recycle ratio had significant impact on the combustion performance and consequently on NO formation and emission. With the combustion performance ensured, decreasing the flue gas recycle ratio or increasing the inlet O-2 concentration of combustion gas led to reduction of NO formation and emission. Although NO formation and emission was found to increase with increasing the inlet NO concentration of combution gas, CFD simulation indicated that similar to 74% of the inlet NO was reduced in the furnace, consistent with the experimental data reported in the literature. This demonstrated the significant contribution of reburning mechanism to the reduction of the recycled NO in the furnace.