Experiments were conducted in a drop-tube furnace (DTF) to investigate NO emission characteristics of low-rank bituminous coal burning in the primary combustion zone. The combustion tests were performed under high-stoichiometric-ratio (SR) combustion conditions (SR g 1) and low-SR combustion conditions (SR<1). Along the length of the furnace, concentrations of flue gas species, such as NO, HCN, NH 3 , CH 4 , CO, CO 2 , and O 2 , were carefully measured during pyrolysis and different SR combustion conditions. The results for high-SR combustion conditions show that high concentrations of O 2 favor the conversion of volatile N and char N into NO, with extensive NO formation taking place in the initial stages of combustion. Reducing species, such as CH 4 , C 2 H 6 , and NH 3 , in volatile matter were mostly oxidized and generated little effect on NO elimination. Results from low-SR combustion conditions show that there is insufficient O 2 for oxidization of combustibles, allowing more hydrocarbons to react with NO. For the coal tested in these experiments, the conversion ratio of fuel N to NO, R fuel N , yielded the lowest minimum values around SR = 0.9 among all such ratios because of a larger fraction of fuel N being decomposed from coal particles through pyrolysis, as well as char oxidization being reduced to N 2 . For lower SR conditions, the total fixed nitrogen (TFN), which includes NO, NH 3 , and HCN, can be reduced to N 2 by homogeneous reactions, so that the remaining N in char becomes the major contributor of NO emissions in the burnout zone.

A reaction model including NO reduction by reburning with pulverized coal and SO 2 absorption by CaO is presented. It includes the coal combustion model, which covers coal devolatilisation, combustion of volatiles, and combustion of char. The NO conversion model by reburning includes the formation of NO from fuel nitrogen and the reduction of NO by hydrocarbon, HCN, NH 3 as well as char. Sulfation of CaO is controlled by solid state diffusion through the product layer. The model is validated by comparing the predicted NO and SO 2 emissions with the measurements obtained from simultaneous NO/SO 2 removal experiments in an entrained-flow reactor reburning with pulverized coal together with calcium-based sorbent CaO. Low SR is beneficial for NO reduction, but high SO 2 reduction needs higher SR. It is suggested that the reburn fuel and Calcium-based sorbent CaO should be injected in different area to ensure high efficiencies of NO and SO 2 reduction.