Experimental and numerical investigation of the combustion characteristics and NO emission behaviour during the co-combustion of biomass and coal

Abstract

As a promising technology for pollutant reduction, biomass-coal co-combustion has attracted worldwide attention. In this study, experiments combined with simulations were conducted to explore the combustion characteristics and NO emission behaviour, as well as their interrelations during the co-combustion process of biomass and coal. The experimental results reveal an obvious decrease in the NO concentration with an increased biomass blending ratio by weight. A maximum NO reduction of 24% was found when 50% biomass was blended with coal, which mainly occurred in the stage of stable char combustion and was attributed to non-catalytic effect related to free radicals. Moreover, the higher biomass blending ratio led to a more violent combustion reaction during the devolatilization stage, while the reaction remained constant during the char combustion stage. As the temperature increased from 800 °C to 1000 °C, the fuel-N/NO conversion ratio increased for the blend with 30% biomass and 70% coal, which resulted from the combined contributions of devolatilization and char combustion. Although the rise in temperature also led to an increase in the combustion rates of the fuel samples, the effect of the temperature on NO emission turned out to be more sensitive than that on the combustion rate. Additionally, when the oxygen concentration increased from 6 vol% to 21 vol%, the NO conversion ratio decreased, which is attributed to the promotion effect of the higher O2 concentration on homogeneous NO reduction. The simulation data were in good agreement with the experimental results, and demonstrated that there were six elementary reactions dominating the NO transformation. Moreover, homogeneous NO reduction plays a vital role in decreasing NOx emissions.