Char nitrogen conversion: implications to emissions from coal-fired utility boilers

The contribution of nitrogen present in the char on the production of nitro gen oxides during char combustion was analyzed, A literature review summari zes the current understanding of the mechanisms that account for the format ion of NO and N2O from the nitrogen present in char. The review focused on: (1) the functionalities in which nitrogen is present in the coal and how t hey evolve during coal devolatilization; (2) the mechanism of nitrogen rele ase from the char to the homogeneous phase and its further oxidation to NO; and (3) the reduction of NO on the surface of the char. The critical analy sis of these three issues allowed identification of uncertainties and well- founded conclusions observed in the literature for this system. The existing models for the production of nitrogen oxides from char-N were also reviewed. A critical analysis of the assumptions made in these models and how they affect the final predictions is presented, Finally, a simplifi ed version of these models was used to perform a parametric analysis evalua ting the impact of several parameters on the total conversion of char-N to NO. These parameters include: (1) the rate of NO reduction on the char surf ace; (2) the rate of carbon oxidation; and (3) early vs, late nitrogen rele ase during the char oxidation process. The results underscore the importanc e of the reaction of NO reduction on the char surface to the final conversi on of char-N to NO. (C) 2000 Elsevier Science Ltd, All rights reserved.