Modelling And Experimental Studies On The Ignition Characteristics Of Single Coal Particles

The ignition characteristics of coal particles represent process of physico-chemical changes during the ignition period.They are the theoretical foundation for highly efficient and clean utilization of coal resources.As the simplest existence,the single coal particle avoids the mass and heat transfer between particles,and thereby the ignition studies on the single coal particles could provide an easy and ideal way to understand the coal ignition and combustion characteristics.An experimental system compatible to the drop tower in National Microgravity Laboratory Center(NMLC)of Chinese Science Academy(CAS)was built to study the ignition behaviors of typical Chinese bituminous coal under different O2 concentrations in the first time.In the same time,vast experiments were conducted at the normal gravity condition to explore the effects of coal type,particle size,heating rate,O2 concentration,convection intensity and atmosphere on the ignition characteristics for the particles with size in the order of mm.The differences in ignition temperature,ignition delay time and the flame shape between the microgravity and normal gravity were compared.The mechanisms of the convection effect were analyzed.Based on the experimental results,a diagram of ignition mechanisms of bituminous coal in different O2 concentration conditions under various forced convection intensity and atmosphere temperatures was provided.A transient 1D ignition model considering thermal conduction inside the coal particle was developed,which can enhance the prediction accuracy on the ignition parameters for the mm class coal particles.With such a model,the apply conditions of different ignition criteria were assessed and the influences of various factors on the ignition characteristics of single coal particles were systematically investigated.It was found that both the ignition delay time and temperature decreased with the increasing O2 concentration.The ignition delay time increased with the particle size.At the ignition moment,temperature on the particle surface initially decreased and then increased with increasing particle size while temperature at the particle center continuously decreased.The ignition delay time decreased with the increasing heating rate.At the ignition moment,temperature on the particle surface increased with increasing heating rate while temperature at the particle center decreased.The ignition temperature difference between particle surface and center became more and more obvious with increasing heating rate and particle size.A diagram of ignition mechanism for the typical bituminous coal in air condition with various particle sizes and atmosphere temperatures was provided.Accordingly,the small particles with high heating rates were in favor of heterogeneous ignition.Large particles with low heating rate were apt to ignite homogeneously.As large temperature gradient exists inside the particle,the surface temperature may sufficiently high so that the surface reacted with oxygen and then lit the volatile in hetero-homogeneous manner.The model with isothermal assumption could make inaccurate prediction on the influences of particle size and heating rate on ignition characteristics.Using the CPD pyrolysis model and Magnusson’s local extinction theory,a model was developed to assess the formation condition,quantity and flame height of the volatile jet flame at different oxygen concentrations for single coal particles.The results showed that the volatile flames were more spherical-shaped at a lower O2 concentration.Under high O2 concentration conditions,the ejection of volatile from particle was in favor of forming jet flames and volatile flames were non-uniform around the surface.The model well predicted in general the flame shapes under different O2 concentrations observed in the experiments at microgravity condition.