Exhaust Gas Recirculation (EGR) analysis of a swirl-stabilized pulverized coal flame with focus on NOx release using FPV-LES

Highly-resolved Large Eddy Simulations (LES) are performed to investigate the combustion characteristics and the NOx-formation in the swirl-induced recirculation zones of the Brigham Young University (BYU) Burner Flow Reactor (BFR). The simulations are performed using the in-house LES tool PsiPhi, utilizing a flamelet/progress variable (FPV) approach to model the reactive multiphase flow. Four dimensions are used to parameterize the thermochemical quantities consisting of two mixture fractions for volatiles and char burnout, the total enthalpy and a progress variable, defined by a linear combination of key product species mass fractions. A reduced CRECK mechanism with 120 species and 1551 reactions is used, including all NOx-formation mechanisms (prompt-, fuel-, thermal-, and pathway via N2O). Devolatilization, char burnout and radiation effects are considered in the LES. Radial profiles of major gas species are compared with experimental data, leading to an overall good agreement. Two variants to determine NO species were investigated: (1) the direct extraction of NO from the flamelet table and (2) the solution of an additional transport equation for NO with a modified NO source term, split into a formation and a rescaled consumption part. The solution of an additional transport equation for NO is clearly superior and necessary for pulverized coal simulations, showing a much improved prediction of forward- and backward reactions of NO inside the furnace, while the direct extraction of NO from the flamelet table greatly overpredicts its formation.