Combustion behaviors of pulverized coal MILD-oxy combustion under different steam injection parameters

Abstract

MILD-oxy combustion under the wet flue gas recirculation mode is of potential in achieving a synergetic control of C/N emissions, whereas its practical realization is restricted by how to efficiently form an in-furnace H2O-rich atmosphere under MILD regime. Towards this issue, this work numerically evaluates the feasibility of applying the direct steam injection method by arranging the individual steam nozzles with different diameters (Dste) on both sides of the primary stream nozzle (i.e., the inner- and outer-steam injections). The effects of incident position and velocity on combustion behaviors are studied. Results show that compared with the advanced confluence of primary and secondary streams in the inner injection cases, outer-steam injection delays the confluence of inlet jets to enhance the in-furnace dilution level. Inner-steam injection is better in inhibiting temperature rise and fluctuation, which facilitates a more uniform distribution of temperature. MILD combustion with diffusion/kinetics-controlled regime is better maintained in the inner injection cases under a small Damkӧhler number. The share of gasification reaction on char consumption in the inner injection cases is higher than that in the outer injection cases, and the burnout time of char is also increased. The increased Dste further enhances the contribution of gasification reaction and prolongs char burnout. Inner-steam injection inhibits the oxidation of fuel-N and enhances NO-reburning in a large low-O2 region as Dste increases, which leads to the lower NO emission concentration. Meanwhile, interaction between NO and fuel-N intermediates is found to play an inevitable role in reducing the overall NO emission.