Study On Ignition Of Single Coal Particle Under Normal And Microgravity Condition

Ignition is of great important in practical situations such as burner and combustor design. However, one of the drawbacks of existing of researches is that on one hand almost all ignition experiments are conducted under normal gravity(1-g) which is influenced by convection, while on the other hand the datum are often used to validate theoretical analysis in which convection is often assumed negligible for granted. As a result, discrepancy and inaccuracy may be caused for the model development. In this study, experiments on the ignition of single coal particles under micro gravity (μ-g) and 1-g are simultaneously conducted. The buoyancy effect on the coal ignition is assessed. Based on the experimental data, an improved 1-D ignition model is proposed. Besides, some experiments on the influence of the forced convection on the coal ignition are conducted as well.Firstly, the experiment on ignition and combustion of single coal particle in a hot air is designed and conducted at normal gravity and microgravity, with three high volatile coals in diameter of 1.5mm and 2.0mm respectively. A non-intrusive colorimetric method was developed to derive the surface temperature of the coal particle through processing the images taken by a color CCD camera. It is found that ignition occurred homogeneously then heterogeneously atμ-g for the testing coal particles and releasing of volatile and volatile combustion overlap after ignition. Experimental results also confirmed that ignition temperature decreased with increasing volatile content and increasing particle size. However, contradict to previous studies, for a given coal and particle size, ignition temperature was about 50-80K lower atμ-g than that at 1-g.Secondly, surface oxidation parameters are calculated by TGA and then a mathematical model, considering thermal conduction inside the coal particle, was developed to simulate the ignition process atμ-g. Compared with the model neglecting conduction, ignition temperature from this improved model is closer to microgravity data. The model predictions also agreed in general the microgravity experimental on flame standoff from coal particle surface. The temperature gradient inside coal particle is significant. The temperature difference between coal surface and center increases to a maximum and then decreases before ignition and the maximum difference becomes much larger as the increasing of particle diameter. And the transient diameter from heterogeneous to homogeneous are greatly effected by inside conduction varying with different ambient temperature.Finally, the effect of gas flow on ignition temperature and mechanisms are conducted and results show that ignition mechanism can be divided into homogeneous, heterogeneous and combined ignition due to surface reaction under different gas flux and different ambient temperature. Ignition time first decreases and increases with the increasing of gas flux while the ignition temperature always increases.