Abstract
Coal preheating method has demonstrated great potential to reduce NOx emissions. Interestingly, whether this method is suitable for all types of burner remains to be elucidated. Four types of burner with different air staging schemes were studied in the present experiments. With coal preheating method, numerous nitrogen (N) intermedias (HCN), soot and hydrocarbons (ΣCiHj) were yielded in the preheating chamber. However, in oxygen-rich conditions the N-intermedias were easily converted to NO, and the most soot and hydrocarbons (ΣCiHj) were oxidized before reducing the NO to N2. The type of burner and the way of air staging had a significant effect on NO emissions. Without air staging NO emissions even increased using a bad burner, and the NO emissions were almost unchanged as the speed of outer secondary air increased from 23 m/s to 118 m/s. Three fitting formulas were summarized based on a large number of experimental data. The close-coupled over fire air played a more important role when the speed of outer secondary air was lower or the volatile content of coal was lower. The residence time in the reducing zone was also a key to reduce NO emissions.
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