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

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_optof 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 NO_xemissions 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_optis 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_optis negligible. The value of C_optmainly 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_dafQ_net^0.7/100M_ad^0.1is obtained to evaluate the optimum coal concentration of a general pulverized coal flame, and another formula, C_opt= 1.18-0.17V_dafQ_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 NO_xemissions because when the coal concentration surpasses the value of C_opt, NO_xemissions 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 NO_xemissions.