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.
In optimization of a utility boiler furnace operation, special attention is given to the flame geometry and position. As an illustration of possibilities for application of mathematical prediction and numerical experiment in efficient optimization of the flame, the paper presents selected results of simulations of processes in pulverized coal tangentially fired furnace of Kostolac-B 350 MW electric boiler unit. To analyze the furnace working under different conditions, a differential 3D mathematical model of two-phase turbulent reactive flow with heat and mass transfer and corresponding computer code have been developed. Using the model and the code, previously carefully verified and validated against field measurements, an extensive numerical study has been performed to investigate the dependence of the furnace flame characteristics on different operating conditions, including distribution of the coal, air flow rates, and particle size classes over the burner tiers, as well as the quality and grinding fineness of coal and the operation scheme of the coal mills. The numerical predictions of the flame characteristics enable a specific tool for optimization of the boiler unit with respect to efficiency and ecology.
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