Approaches to modelling heterogeneous char NO formation/destruction during pulverised coal combustion

Emissions of nitrogen oxides during combustion pose a major environmental problem and hence there is considerable interest in reducing the NOx levels encountered during pulverised coal combustion. The chemically bound nitrogen in the fuel is known to account for up to 80% of total NOx. Whilst the homogeneous production pathways of NOx from the nitrogen present in the coal volatiles are relatively well known, the heterogeneous conversion of the nitrogen retained in the char to NO is less well understood. As part of an ongoing project to develop an advanced coal model to describe the combustion process in pulverised coal flames this study examines the possible mechanisms involved in both nitric oxide formation and reduction at the char surface for a more accurate prediction of char-NO interactions. The use of a model to predict the partition of coal nitrogen between volatiles and char has also been explored. Various mechanisms of NOx production/destruction at the char surface are discussed and it is concluded that the best approach for computational modelling is to treat the formation and reduction pathways separately. The rate of NO formation is related to that of the char oxidation by the molar ratio of carbon to nitrogen in the char. The rate of NO reduction at the char surface is represented by R-NO = 2.6.10(4) exp(-15 900/T)A(E)P(NO) mol s(-1), where A(E) is the external area of the char (m(2) g(-1)) and P-NO is the partial pressure of NO (atm). (C) 1999 Elsevier Science Ltd. All rights reserved.