Because there has been a recent increase in the use of low calorific coal compared to standard coal, it is crucial to control the char flame length governing the burning life-time of coal in a coal-fired utility boiler. The main objective of this study is to develop a simplified model that can theoretically predict the flame length for burning coal in a laboratory-scale entrained laminar flow reactor (LFR) system. The char burning behavior was experimentally observed when sub-bituminous pulverized coal was fed into the LFR under burning conditions similar to those in a real boiler: a heating rate of 1000 K/s, an oxygen molar fraction of 7.7 %, and reacting flue gas temperatures ranging from 1500 to 2000 K. By using the theoretical model developed in this study, the effect of particle size on the coal flame length was exclusively addressed. In this model, the effect of particle mass was eliminated to compare with the experimental result performed under a constant mass feeding of coal. Overall, the computed results for the coal flame length were in good agreement with the experimental data, particularly when the external oxygen diffusion effect was considered in the model.
In this work, subair injection was
proposed to improve the combustibility
and NOx emission in a 500 MW tangentially fired coal boiler. The location
of injection ports was determined based on the coal particle trajectory
and its effect was investigated numerically. The flow rate of subair
was set to 0, 5, and 10% of the total combustion air. The secondary
air flow rate was decreased appropriately to ensure that the total
quantity of combustion air remained constant. The over-fire air was
not adjusted to retain the effect of an air-staged combustion. The
simulation results showed that the subair improved the combustibility
of coal particles originating from burners A and B in the lower part
of the furnace. Particles from other burners were not affected significantly.
In addition, this method achieved reduction of NOx by 6.3 and 13.2%
when the subair accounted for 5 and 10% of the combustion air, respectively.
This reduction was attributed to the decrease in the peak temperature
as a result of a wider combustion region. The proposed subair technique
improved the coal combustibility and reduced the NOx emissions successfully
in the furnace.
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