A role of hydrocarbon reaction for NO sub(x)formation and reduction in fuel-rich pulverized coal combustion

We have investigated an index for modeling a NO sub(x)reaction mechanism of pulverized coal combustion. The reaction mechanism of coal nitrogen was examined by drop-tube furnace experiments under various burning conditions. We proposed the gas phase stoichiometric ratio (SRgas) as a key index to evaluate NO sub(x)concentration in fuel-rich flames. The SRgas was defined as:[MathML equation] where, the amount of gasified fuel was defined as the amount of fuel which had been released to the gas phase by pyrolysis, oxidation and gasification reactions. When SRgas < 1.0, NO sub(x)concentration was strongly influenced by the value of SRgas. In this condition, the NO sub(x)concentration was hardly influenced by coal type, particle diameter, or reaction time. We developed a model to analyze NO sub(x)and XN(HCN, NH sub(3)) concentrations for pulverized coal/air combustion and coal/CO sub(2)/O sub(2) combustion, based on the index. NO sub(x)and XN concentrations did not reproduce the experimental results without considering reactions between hydrocarbons and NO sub(x) The hydrocarbon reaction was important for both NO sub(x)and XN, especially for air combustion. In the present model, an empirical formula was used to estimate the total concentration of hydrocarbons in coal flame. The reaction of heavy hydrocarbons which had plural aromatic rings was very important to analyze the reaction mechanism of hydrocarbons for coal combustion in detail. When burning temperature and SRgas were the same, total hydrocarbon concentration in a coal flame was larger than that of a light gaseous hydrocarbon flame. Total hydrocarbon concentration in oxy-fuel combustion was lower than that in air combustion. We verified the proposed model by experimental results obtained for a drop-tube furnace and a laboratory-scale furnace that had an installed low-NO sub(x)burner.