Energy & Fuels, Vol.28, No.9, 6077-6088, 2014
Effects of Equivalence Ratio and Turbulent Velocity Fluctuation on Early Stages of Pulverized Coal Combustion Following Localized Ignition: A Direct Numerical Simulation Analysis
This study utilized three-dimensional direct numerical simulations (DNS) in order to analyze the effects of turbulent velocity fluctuation, equivalence ratio based on volatile primary fuel in the particulate phase, and the concentration of primary volatile fuel in the background gas on early stages of combustion following successful localized ignition of turbulent pulverized monodispersed coal particle-laden mixtures. For this analysis, coal particles have been treated as point sources and tracked in a Lagrangian manner. It has been found that combustion takes place both in premixed and nonpremixed modes but the extent of premixed (nonpremixed) combustion is stronger (weaker) in turbulent cases than in quiescent laminar cases. The cases with high values of particle equivalence ratio Phi(p) (defined based on total amount of primary volatile fuel available in the particulate phase) have been found to be more susceptible to flame extinction than the cases considered here with small values of Phi(p). The presence of primary volatile fuel in the background gas is detrimental to the self-sustained combustion for cases with Phi(p) >= 2.0, whereas an increase in primary volatile fuel in the background gas acts to decrease the extent of burning for cases with small values of Phi(p) (e.g. Phi(p) = 0.25 and 0.5 cases) within the parameter range considered here(i.e., 0.25 <= Phi(p) <= 3). Turbulent mixing helps to mix the devolatilized fuel with the surrounding air; thus, an increase in the extent of burning has been observed for small values of turbulent velocity fluctuation, in comparison with the corresponding quiescent laminar mixtures. However, heat transfer from the hot gas kernel overcomes the chemical heat release for high values of turbulent velocity fluctuation, which eventually leads to a failure to obtain self-sustained combustion that is unassisted by external heat addition.