Reaction behaviors in pulverized coal MILD-oxy combustion affected by physical and chemical effects of diluents

Abstract

In pulverized coal MILD-oxy combustion process, the oxidizer diluents are switched from N2 to CO2 and H2O. Hence, the remarkable discrepancies in combustion characteristics directly originate from the differences in physical and chemical properties of diluents, namely, the physical and chemical effects. This work utilizes the fictious materials, i.e., FCO2 and FH2O, which have the same physical properties with the real ones but are completely non-reactive, to isolate these two dilution effects. Their individual and combined impacts on the global reaction behaviors are numerically evaluated. Results reveal that the physical effect of CO2 and H2O plays an important role in enhancing the internal flue gas recirculation. Compared with H2O-dilution, CO2-dilution better inhibits the temperature rise and promotes the in-furnace thermal uniformity, which predominantly originates from its physical effect. Hence, the MILD reaction regime with high temperature and strong dilution levels occur in a wider region. The homogeneous diffusion/kinetics-controlled regime (0.2 < Dat < 2) is more susceptible to the chemical effect of CO2 and H2O. The kinetics-controlled regime of heterogeneous reaction is facilitated under CO2- and H2O-dilutions, where the physical effect is dominant in the former case while the physical and chemical effects act jointly in the latter case. The chemical effect of CO2 and H2O is responsible for greatly strengthening gasification reaction by direct participating in the reaction process, which in turn slows down the burnout of char. Importantly, H2O-dilution shows greater potential to lower NO emission level than CO2-dilution. The physical effect is revealed to play a dominant role in reducing the global NO formation rate, while the chemical effect enhances the reburning of NO in a wide lean-O2 reduction region. The findings of this work are expected to help gain a more comprehensive understanding in MILD-oxy combustion technology.