Carrier-phase DNS of detailed NO_x formation in early-stage pulverized coal combustion with fuel-bound nitrogen

Carrier-phase direct numerical simulation of detailed NO_x formation in pulverized coal flames (PCC) with fuel-bound nitrogen is conducted in a 3D temporally evolving mixing layer setup where Lagrangian particles (Colombian bituminous coal) in an air stream (upper half of the domain) mix with the products of lean volatile/air combustion in the lower stream. The release of fuel-N is represented by ammonia, hydrogen cyanide, and a lumped nitrogenated tar (pyridine). Devolatilization is modeled by fitting a 2-step pyrolysis approach to the detailed heterogeneous PoliMi kinetics. A comprehensive homogeneous mechanism including all standard pathways of NO_x and pyridine oxidation is adopted. Results show a partition of NO in two distinct branches of scatter plots of NO mass fraction vs. volatile mixture fraction after flame establishment, corresponding to NO in the lower stream flame region and hot spots near the upper stream. The contribution of NO₂, prompt, and thermal mechanisms to total NO_x is limited in the early stages of PCC. The main source of NO is fuel-N, with NH being the most important precursor. Pyridine plays an important role for NO production in the upper stream through CN formed from CHCHCN. CN and ammonia oxidation have the highest contribution to NH production. Regarding NO destruction, NO reactions with HCCO, CH_i and C through the reburn process constitute the largest share. NO conversion to N₂O by NH followed by conversion of N₂O to N₂ and NO+N→N₂+O are the two most important pathways directly reducing NO to N₂.