Detailed modeling of hybrid reburn/SNCR processes for NO X reduction in coal-fired furnaces

Abstract

Mechanism reduction has made the detailed kinetic modeling of combustion problems much easier; it also offers potential improvement of modeling accuracy and flexibility in comparison to global mechanisms. The present work applies mechanism reduction in conjunction with the CHEMKIN library and develops an automatic reduction program code. Regarding the hybrid re-burn/selective non-catalytic reduction (SNCR) (“advanced re-burning”) conditions in coal-fired furnaces and based on a full mechanism “GADM98,” a skeletal mechanism with 39 species, 105 reactions, and further a 10-step/14-species reduced mechanism were established. The reduced mechanism was implemented into a 3D-combustion computational fluid dynamics (CFD) code. The eddy-dissipation-concept model was used to describe the influence of turbulence on the combustion chemistry. A large number of simulations for reburning and hybrid reburn/SNCR processes in a coal-fired reactor were executed; the predicted results were compared with experimental measurements. The reduced mechanism and the comprehensive modeling give quite satisfactory results over a wide range of mole ratios for β = [NH 3]/[NO] and air/fuel equivalence ratios λ 2 in the reburn zone. From the modeling results, it was found that adding ammonia premixed with reburn fuel (CH 4) effects no further reduction of NO x or even impairs the reduction efficiency compared to pure reburning, and in contrast, staged addition of ammonia downstream of the CH 4 injection in the reburn zone provokes a significant further reduction of NO x over a wide range of parameters. According to the predictions, NO x-reduction rates of 50–60% and of 70–80% can be achieved through pure reburning and hybrid reburn/SNCR approaches, respectively, at λ 2 = 0.95 and β = 1.5. Concerning the computational procedure, essential measures were taken to optimize convergence and computing time. The computing time with the present reduced mechanism is ∼2.5 times that with the traditional global mechanism for the same iteration number. Tabulation of the rate constants reduced the computing time of the reaction kinetics by ∼50%.