MECHANISM OF N_2 FORMATION DURING COAL CHAR OXIDATION

摘要

In spite of many studies, the reaction mechanism of the evolution of nitrogen-containing gases during coal oxidation is not fully understood yet. One reason is that the nitrogen mass balance has not been well established in most studies. In the present study, we attempted to clarify the N_2 formation mechanism during the reaction between coal char and O_2 by paying special attention to nitrogen mass balance. The closure of the nitrogen mass balance was 100 +- 6%. Blair Athol coal char was used as a sample, and the formation of N-containing species was determined during temperature-programmed reaction as well as isothermal reactions at 700 and 850°C in a packed-bed reactor. In all the cases, N_2 was the major product, with a little amount of NO as a minor product. Under the isothermal reaction conditions, the ratio of NO/ N_2 was low in the initial stage and increased in the later stage. The effect of bed height on the product gas distribution was examined, and it was found that the shallower the bed, the higher the ratio of NO/ N_2. When the O_2/He gas mixture was switched to He, the formation of N_2 and NO became almost negligible. From these observations, we proposed the following reaction mechanism for the N_2 formation: the formation of NO as a primary oxidation product, and the reaction of NO with surface nitrogen species to form N_2. The NO capture on carbon surface to form surface nitrogen species is thought to be an important step during the course of this reaction.