Homogeneous ignition and volatile flame structure of single bituminous coal and walnut shell particles: Effects of particle size and gas atmosphere

Homogeneous ignition and the evolution of near-spherical volatile flames from coal and biomass particles are investigated under laminar flow conditions experimentally with a collaborative endeavor of numerical simulations. Specifically, high-volatile bituminous (hvb) coal and walnut shell biomass particles are combusted in N2/O2 and CO2/O2 atmospheres with an ambient temperature of approximately 1800 K. Simultaneous laser-induced fluorescence of OH radicals (OH-LIF), diffuse back-illumination, and Mie scattering measurements are employed to study early-stage volatile combustion of two fuels. Particle size, morphology, dynamics, homogeneous ignition delay times, and volatile flame structures are evaluated by implementing advanced data processing approaches. Detailed numerical simulations are performed to encompass hard-to-measure information such as the bulk rates of particle heating and volatile release rate. Experimental results indicate that hvb coal and walnut shell particles of the same size exhibit slight difference in ignition. This is explained by the fact of early release of a relative large amount of moisture from biomass observed in detailed simulations. Moreover, ignition is delayed as particle size increases, however, only marginal difference was observed when replacing N2 with CO2. Increasing particle diameter also weakens the flame, which is visualized by the OH-LIF for the first time. Although the flame structure remains similar for two fuels (up to 8 ms after ignition), its intensity is further suppressed by the higher heat capacity (ρcp) of CO2. The dimensionless flame stand-off distance increases with particle diameter. After the ignition, a continuously rising flame distance is observed for small particles, however, it shows a decrease-and-increase behavior of large particles. This implies a dynamic process: volatile out-gassing accelerates at increasing temperature after ignition, pushing the gas flame away from the particle. The volatile release rate plays a critical role in shaping flame structure and its evolution, in contrast to the relatively minor influence of inert gas composition and the chemical composition of fuels released.