Numerical Investigation on the Detailed Structure of a Coaxial Coal Jet Flame Using Large-Eddy Simulation with Elementary Reactions

A numerical simulation of a pulverized coal jet flame was performed to investigate the physics and flame structure in detail by means of large-eddy simulation with an elementary reaction mechanism. The elementary reaction mechanism consisted of 67 species and 204 reactions, and it was derived from a detailed mechanism including the oxidation of polycyclic aromatic hydrocarbons (PAHs) through a combined simplification process of the directed relation graph with error propagation and the computational singular perturbation. The simulation was validated through comparisons with an experiment as well as a simulation employing a global reaction mechanism. The results showed that the simulation could capture specific characteristics of the flame such as particle dispersion and velocity with reference to the experiment. Furthermore, the numerical simulation with the elementary reaction mechanism predicted the distributions of temperature and oxygen concentration more precisely than that with the global reaction mechanism. The flame structure was discussed in detail in terms of particle dispersion by introducing the particle number intensity, PAHs, and radical distributions. The particle cloud was broken up by turbulent flow, which evolved with the combustion of volatile matter, and the particles spread out in the radial direction. Methane was dominant among hydrocarbons released through devolatilization, while anthracene accounted for the largest amount of aromatic hydrocarbons. Reactive characteristics of the released gases were investigated. Benzene released during devolatilization was mainly consumed during combustion at the upstream, whereas anthracene had lower reactivity at the upstream and higher production rate at the downstream. A layered structure following the order of particles, PAHs, OH radicals, and peak gaseous temperature from the center axis in the radial direction was observed at the downstream.