Experimental study on NO_x reduction from staging combustion of high volatile pulverized coals. Part 2. Fuel staging

This paper focused on NO_x reduction from high volatile pulverized coals by fuel-staged combustion, especially from low rank coal in China with strong slagging tendency, high ash, high moisture, and low calorific value. The results are very important to further enrich the database of fuel-staged combustion. The influences of the main process parameters, including reburn fuel fraction, stoichiometric ratio, residence time in reduction zone, and fuel properties, on NO_x emissions were studied experimentally using an Entrained Flow Reactor with Multiple Reaction Segment (EFRM). The present experiments verified that the comprehensive NO_x reduction index (S_z) proposed by the authors for air-staged combustion is still applicable to correlate the maximum NO_x reduction rate and coal characteristics in fuel-staged combustion, which would help to predict NO_x emissions from fuel-staged combustion more accurately. The experiments also verified that the NO_x reduction rate increases with the increase of reburn fuel fraction, with the increase of residence time in reburn zone, and with the decrease of stoichiometric ratio in reburn zone. The results also showed that these parameters have a critical range value, which is 15%–20% for reburn fuel fraction, less than 0.8 for reburn zone stoichiometric ratio, and less than 0.8 s for residence time in reburn zone, respectively. NO_x emissions drop with the decrease of the stoichiometric ratio in main combustion zone until it reaches 0.8. For high volatile coals, the effect of fuel-staged combustion on the reduction of NO_x emissions increases as the volatile content or fuel-N in the coal increases. The NO_x reduction rate increases while the comprehensive NO_x reduction index goes up. Moreover, fuel-staged combustion did not significantly reduce the burnout rate of the high volatile coal.