The influence of air distribution on gas-fired coal preheating method for NO emissions reduction

A gas-fired coal preheating (GFCP) method was investigated on a 35 kW drop tube furnace with different air distributions, and three coals of various rank were used in the experiments. The results of this investigation show the GFCP provides a flexible means to control NOx emissions for various coals, which can be used with traditional air staging like close-coupled over fire air (CCOFA), separated over fire air (SOFA) as well as MILD. The principle is that GFCP promotes substantial nitrogen intermediates (HCN and NH3) and soot to be generated in the preheating chamber. With proper air distribution, the former convert into N2 and the later function as reductant to destroy newly formed NO in the subsequent furnace. The global denitrification effect is more remarkable for coals of lower rank. While a primary air ratio at intermediate level may obtain the lowest NO emissions for combustion without GFCP, the NO emissions are essentially independent of variations in primary air ratio when GFCP is adopted. Our experiments also find that GFCP reduces fly-ash level by extending residence time for particles traversing high temperature zones.