Considerable research efforts focus on modeling NO x formation/destruction and predicting NO x emission so that it can be controlled. A motivation for this numerical study was to examine the efficiency of combustion modifications in the furnaces of Kostolac B 350 MWe boiler units, tangentially fired by pulverized lignite. Numerical analysis was done by an in-house developed NO x submodel, coupled with differential comprehensive combustion model, previously developed and validated. The NO x submodel focuses on homogeneous reactions of both the fuel and the thermal NO formation/destruction processes. The submodel was validated by comparison of predicted NO x emissions with available measurements at the boiler units. Selected predictions of the emission, the furnace exit gas temperature, NO concentration, gas temperature, and velocity field are given for the case-study furnace under different operating conditions. The individual or combined effects of coal and preheated air distribution over the individual burners and the burner tiers, the grinding fineness and quality of coal, and the cold air ingress were investigated. Reduced emissions of up to 20–30% can be achieved only by proper organization of the combustion process. Obtained results were verified by the boiler thermal calculations. An optimal range of the furnace exit gas temperatures was proposed, with respect to the safe operation of the steam superheater. Simulations by means of a computer code developed for the purpose, showed that the air staging using overfire air ports might provide the NO x emission reduction of up to 24% in the test-cases with relatively high emission and up to 7% of additional reduction in already optimized cases.

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A cost-effective reduction of NO x emission from utility boilers firing pulverized coal can be achieved by means of combustion modifications in the furnace. It is also essential to provide the pulverized coal diffusion flame control. Mathematical modeling is regularly used for analysis and optimization of complex turbulent reactive flows and mutually dependent processes in coal combustion furnaces. In the numerical study, predictions were performed by an in-house developed comprehensive three-dimensional differential model of flow, combustion and heat/mass transfer with submodel of the fuel-and thermal-NO formation/destruction reactions. Influence of various operating conditions in the case-study utility boiler tangentially fired furnace, such as distribution of both the fuel and the combustion air over the burners and tiers, fuel-bound nitrogen content and grinding fineness of coal were investigated individually and in combination. Mechanisms of NO formation and depletion were found to be strongly affected by flow, temperature and gas mixture components concentration fields. Proper modifications of combustion process can provide more than 30% of the NO x emission abatement, approaching the corresponding emission limits, with simultaneous control of the flame geometry and position within the furnace. This kind of complex numerical experiments provides conditions for improvements of the power plant furnaces exploitation, with respect to high efficiency, operation flexibility and low emission.

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