Determining the optimum coal concentration in a general tangential-fired furnace with rich-lean burners: From a bench-scale to a pilot-scale study

摘要

The mass ratio of pulverized coal and air (coal concentration, kg/kg) in fuel-rich streams is important in the design and operation of rich-lean burners, in which the optimum coal concentration (C-opt) that corresponds to the best combustion situation should be achieved. This study aims to establish a practical identification method to evaluate the C-opt of the different ranks of coal in rich-lean burners. A wide range of tests were conducted in a bench-scale down-fired furnace and a pilot-scale tangential-fired furnace with rich-lean burners. Temperature distribution, unburned carbon in ash, and NOx emissions were measured, and the effects of coal quality aside from burner type and burner layout method were considered. Results show that the optimum coal concentration corresponds to the highest furnace temperature for each group of tests both in the bench-scale and pilot-scale furnaces. C-opt is significantly affected by coal quality even if a change from the use of a corner-tangential to a wall-tangential furnace lowers C-opt; however, the effect of a vertical rich-lean burner or a horizontal rich-lean burner on C-opt is negligible. The value of C-opt mainly decreases from 1.14 to 0.67 with a decrease in the volatile content from anthracite scale (<0.1) to lignite scale (>0.35). The empirical formula of C-opt = 1.19-0.15V(daf)Q(net)(0.7)/100M(ad)(0.1) is obtained to evaluate the optimum coal concentration of a general pulverized coal flame, and another formula, C-opt = 1.18-0.17V(daf)Q(net)(0.7)/100M(ad)(0.1) is especially derived for a tangential-fired furnace with a rich-lean burner. The optimum value obtained is also critical to NOx emissions because when the coal concentration surpasses the value of C-opt, NOx emissions can be much more efficiently controlled through reduction of air. The findings of this study can provide practical guidance for the design and operation of rich-lean burners to achieve high combustion efficiency and low NOx emissions. (C) 2014 Elsevier Ltd. All rights reserved.