NO emission characteristics of low-rank pulverized bituminous coal in the primary combustion zone of a drop-tube furnace

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 ≥ 1) and low-SR combustion conditions (SR < 1). Along the length of the furnace, concentrations of flue gas species, such as NO, HCN, NH₃, CH₄, CO, CO₂, and O₂, were carefully measured during pyrolysis and different SR combustion conditions. The results for high-SR combustion conditions show that high concentrations of O₂ 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 CH₄, C₂H ₆, and NH₃, in volatile matter were mostly oxidized and generated little effect on NO elimination. Results from low-SR combustion conditions show that there is insufficient O₂ 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 N₂. For lower SR conditions, the total fixed nitrogen (TFN), which includes NO, NH ₃, and HCN, can be reduced to N₂ by homogeneous reactions, so that the remaining N in char becomes the major contributor of NO emissions in the burnout zone.