Modeling nitric oxide formation in a drop tube furnace burning pulverized coal

Computational flow modeling has been done to study NOx formation under laminar flow conditions in drop tube furnaces (DTF) burning pulverized coal. The generation of NOx from nitrogen in the coal's volatile matter was simulated using de Soete's reactions involving HCN, NO and N-2 as the basic homogeneous gas-phase system. The effects of ammonia release and hydrocarbon reburn were then included. Three approaches to the oxidation of char-nitrogen were investigated, one simply using experimental conversion data. The simulations for a number of coals and chars were compared to three experimental studies in the literature. Even though laminar flow combustion conditions were modeled, it was possible to achieve carbon burnouts representative of those obtained experimentally. Consideration of disturbances to the flow from jets of Volatiles improved the predicted carbon burnout for fuel-lean conditions (phi < 1) in one furnace. The NOx models gave good results with bituminous and sub-bituminous coal under fuel-lean conditions at 1750 K, and reasonable results in fuel-rich conditions. At 1250 K the models failed and it is concluded that all the reaction mechanisms are inadequate at that low temperature. With higher rank coals at boiler temperatures, the full NOx chemistry involving ammonia and hydrocarbon reburn gave the best results. With lignite coals, the simple gas-phase kinetics of de Soete gave good predictions, and the inclusion of ammonia and reburning did not improve the result. For the high-temperature (1750 K) combustion of char or petroleum coke, two of the models fitted equally well to the experiments on the oxidation of char-bound nitrogen. One model involved an effectiveness factor, where char-N was converted to NO and then reduced to N-2 while diffusing into the particle. The second assumed that all char-N was converted to HCN, which was then subject to gas-phase reactions. (C) 1999 by The Combustion Institute.