Features of ignition and combustion of a two-component gas suspension of coal particles

Physical and mathematical modeling of high-temperature heat and mass transfer and combustion kinetics of a two-fraction gas suspension of carbon particles at different gas temperatures is carried out. It was assumed that two parallel chemical reactions occur on the surface of the carbon particles: C + O₂ = CO₂ (I), 2C + O₂ = 2CO (II). Molecular-conjugate heat exchange of particles with gas was taken into account, as well as heat exchange by radiation with the walls of the reaction apparatus. As a result of the numerical experiment, the characteristics of ignition and combustion of gas-suspension particles in air at various temperatures of the surrounding gas are determined. For calculations, a two-fraction gas suspension was chosen with equal mass fractions and particle diameters, which differ by a factor of 2: d_b1 = 60 μm, d_b2 = 120 μm. The range of the initial gas temperatures studied is 1100…1500 K. As a result of the calculations carried out, it was established that as the gas temperature decreases, the coarse fraction can ignite earlier than the fine fraction. At high gas temperatures, on the contrary: the induction period of fine particles is much larger than the period of induction of coarse particles. Critical parameters of ignition of two-fraction gases are found. The main characteristics of combustion are the time and temperature of combustion of the particles. It is proved that under the conditions of a two-fraction gas suspension the burning time of the particles depends weakly on the temperature of the gas. It has also been established that in the region of low gas temperatures, the combustion temperature of the fine fraction is smaller than that of the large fraction. This is explained by the large heat removal from small particles by the molecular-convective process and by the lack of oxidizer in the combustion stage. Oxygen is consumed in the combustion of large particles, which, at low temperatures, ignite earlier. The burning times of particles of coarse and fine fractions of a gas suspension were found. Comparison of the combustion time of small and large particles led to the conclusion that under the conditions of a gas suspension with an excess of an oxidizer close to unity, the particles burn in a diffusion mode. It is shown that the extinction of particles is degenerate. After the moment of extinction, the particles are oxidized in the kinetic regime at a high gas temperature.