A REVIEW OF NOX FORMATION AND REDUCTION-MECHANISMS IN COMBUSTION SYSTEMS, WITH PARTICULAR REFERENCE TO COAL

In recent years there have been considerable reductions in the amount of NOx emitted by burners fired by natural gas, oil or pulverised coal . This has been brought about by a number of approaches such as reduci ng the temperature and by fuel staging, both of which minimise the NOx -forming reactions in fuel-rich zones, In gaseous or light-fuel flames this reduction in NOx is considerable-80-90%-but in the combustion of pulverised coal it is only about 50-60%. Thus there is considerable i nterest in reducing NOx further, and this is the subject of a number o f collaborative research projects such as the current UK DTI-NOx proje ct. In this review the NOx-forming reactions are discussed, giving an indication of the way that they can be used for computational fluid dy namic (CFD) modelling of flames. A number of applications are consider ed, including natural-gas burners and staged oil-spray flames, where i n both cases prompt-NOx becomes dominant at low NOx levels. However, p articular attention is devoted to combustion of pulverised coal and so me of the DTI-NOx project results from laboratory experiments. An 'adv anced coal model' detailing some aspects of the work is outlined; in t his, details of devolatilisation of speciated volatiles, including HCN , are also included for a number of coals. The char left by the devola tilisation process is a porous carbonaceous material, and a model is g iven for the formation of network char and cenospheres, and the subseq uent reaction of the fuel-nitrogen compounds in the char to give NOx. The effect of coal composition, rank, and the nature of the nitrogen f unctionalities on NOx formation, and on residual carbon burn-out, are discussed. Methods of improving NOx are considered.