Modelling NOx formation in coal particle combustion at high temperature: an investigation of the devolatilisation kinetic factors

Coal combustion computational fluid dynamic (CFD) models are a powerful pre dictive tool in combustion research. In existing coal combustion CFD models , the process is described by three kinetic rates: coal devolatilisation, v olatile combustion and char combustion. A general, representative devolatil isation rate for coal is a matter of some contention. and measured rates de pend upon the type of experimental system employed in their determination. Thus the reported rates vary considerably, causing difficulties in the choi ce of rate expression for CFD modelling applications. In this investigation , a laminar flow CFD model of a drop-tube furnace was used to assess the in fluence of global devolatilisation rates on overall combustion behaviour, a nd in particular, NOx emissions. The rates chosen include some of the commo n expressions employed by researchers in the field. Analysis, and compariso n of the modelling results with those of the experimental, indicated that a single-step devolatilisation rate can give satisfactory profiles. This rat e can be calculated from the tar release rate using a network model such as FG-DVC (functional group, depolymerisation, vaporisation and cross-linking ), together with the nitrogen partitioning between gas and char during pyro lysis. The use of these single-step models result in good predictions of NO x, and the inclusion of soot/NOx interactions can improve the model signifi cantly to give an excellent agreement with experimental results. (C) 1999 E lsevier Science Ltd. All rights reserved.