Experiments And Numerical Simulations Of Ignition And Combustion Characteristics Of Single Coal Particles

The ignition and combustion characteristics of coal are the theoretical basis for its efficient and clean utilization in the development of the green industry.The investigations of the combustion behavior of single pulverized coal particles,as the simplest form of combustion,are critical to understanding the mechanisms of ignition and combustion.Single-particle ignition and combustion characteristics and mechanisms of five domestic coals were studied using experimental and numerical methods.This work mainly focuses on exploring the combustion characteristics of pulverized coal（PC）particles at different stages under the coupling effect of multiple factors.The ignition and combustion behavior of PC particles（75-250μm）in the temperature range of 1100-1300K were observed at a specially-designed visual drop tube furnace.The effect of coal type,ambient temperature,particle size,and moisture content on the ignition and combustion characteristics of PC particles were intensively analyzed.The results indicated that homogeneous ignition of dry PC increased with the increase of temperature and particle size.In wet particles,the possibility of homogeneous ignition of lignite particles for the particles with a size of less than 150μm was increased.The speed of burnout and degree of fragmentation intensified with the increase in ambient temperature and moisture content,while the fragmentation of PC particles and homogeneous ignition of wet lignite particles were likely hindered by increasing the coal particle size to200-250μm.The tracks of particle motion of bituminous coal seemed to be the most turbulent at 1100K.With the comparison of the characteristics of ignition and combustion of PC under the effect of coupling factors,the results indicated that there were multiple ignition modes of PC particles in the transition zone between homogeneous and heterogeneous ignition,and quantitative analysis of the proportions of various ignition modes was also established.Meanwhile,the functional group structure of PC was also characterized with Fourier transform infrared,based on which the relationship between molecular microstructure and macroscopic ignition and combustion behavior was established.The results indicated a clear relationship between the yield and composition of volatile matter and the amount and type of functional groups in coal.The A（CH₂）/A（CH₃）value was closely related to the yield of tar,while a tail was likely to be produced by the bituminous coal with a tar yield of over 20%.The condensation degree of aromatic hydrocarbons can be regarded as an important parameter for the evaluation of the fragmentation of PC particles during the process of combustion.One-dimensional transient ignition and combustion model of a single PC particle was established.The modeling prediction results from the chemical percolation devolatilization model were embedded in the ignition model by an iterative method,which improved the prediction accuracy of volatile yield and composition during the pyrolysis process.Meanwhile,a radiation cloud was proposed to evaluate the effect of volatile matter combustion on the heating process of PC particle.The modeling studies were used to quantitatively study the ignition and combustion characteristic parameters of PC,which comprehensively analyzed the influencing mechanism of various factors on ignition and combustion characteristics of PC.The results showed that volatile matter release was much faster at higher temperatures and moisture content,which increased the fragmentation of PC particles during combustion,increasing the probability of homogeneous ignition.About 35-45%of the heat produced by the combustion of volatile matter would be transferred to residual char particle.The ignition mode of the particle was decided by a competition between heterogeneous ignition and homogeneous ignition delay time,which was also related to the heating rate of particle,the release rate and composition of volatile matter,and simultaneous release of moisture and volatile matter.However,about 50%of the released volatile matter from lignite particles is non-combustible,which significantly decreased the possibility of homogeneous ignition occurrence.