Study On The Effect Of Changes In Physical And Chemical Properties Of Upgraded Low Rank Coal On Its Grindability

Based on the current energy situation in our country,it is necessary to use lowgrade coal in the form of staged conversion.Pyrolysis has the characteristics of simple equipment,wide adaptability of coal types and rich in products,thus it is the most suitable technical route for the staged conversion of low rank coal in China.However,in its promotion,it faces the problem of large-scale utilization of semi-coke.The most feasible way to use it is direct combustion,which is faced with a process of grinding the carbonized material to suit the particle size required for the pulverized coal furnace.The crushing process of the material is directly affected by its own grindability.Therefore,the characteristics of the grindability of low rank coal during upgrading,especially pyrolysis,were investigated.In this paper,by using various detection and analysis methods,the mechanism of the change of the physical and chemical properties of low rank coal on its grindability is studied.Firstly,the mechanism of the effect of dehydration on the grindability was investigated.A high-water-containing lignite was selected and dewatering experiments were carried out at different dehydration rates.It was found that the greater the dehydration rate,the more significant the improvement of the grindability.Through the analysis of its microstructure,it is pointed out that the dehydration process damages the pore structure of coal,which is the mechanism of improving the grindability.Further analysis of the pore structure by using the fractal tool revealed that both the proportion of large pores and the fractal dimension of the pores all have a good linear correlation with the grindability of the dehydrated coal sample.Before studying the effect of pyrolysis temperature on the grindability of low-rank coal,it is necessary to understand the overall pyrolysis process of low-rank coal during through the kinetics analysis.The double Gaussian distributed activation energy model(2G-DAEM)has a good fitting effect on the pyrolysis process of various low rank coals.The chemical structure analysis of all coal samples shows that the parameters of the kinetic model have certain practical meaning.Finally,the whole process of low-rank coal pyrolysis is described and segmented by the distribution of activation energy.Through the comparative experiments of lignite and sub-bituminous coal,the influence of plastic stage on the carbonized material was studied.It is showed that for lignite without plasticity stage,its grindability changes little from the low temperature dehydration to the end of primary pyrolysis stage.For the sub-bituminous coal with plastic phase,its grindability will decrease significantly at the end of the plastic stage.This is due to the fact that the coal matrix strength of the sub-bituminous coal becomes large after the plastic stage.In the wide temperature range of 450-1200 °C,sub-bituminous coals undergoes four stages of rapid release of volatiles,plastic phase,strong condensation of aromatic nucleus and high temperature carbonization.The grindability of their carbonized material is first rising after the volatile release,and then continuously decreasing in the remaining three stages.Besides that,the change in the crystallite structural stacking heights(Lc)in the coal matrix can indicate the transition of these four stages.In the wide temperature range,the grindability of lignite will not change much after the dehydration until the end of primary pyrolysis.Then at higher temperatures,the strength of the its coal matrix becomes larger due to the polycondensation of the aromatic nucleus,and the grindability of the lignite would exhibit a certain degree of decline.Based on the background of mixed combustion of semi-coke and low-rank coal,The pulverization characteristics of raw coal and semi-coke mixed grinding were studied.The experiments selected the semi-coke produced at a typical pyrolysis temperature and mixed it with the raw coal in different proportions.Using fractal geometry and other tools to analyze the particle size distribution,it is showed that because the semi-coke has undergone the polycondensation of aromatic nucleus,the strength of its coal matrix is greater.When the output energy of the pulverizing equipment is sufficient,the interpracticle separation in semi-coke can be extended to a finer particle size range.Therefore,it could be broken to a smaller particle size than the raw low-rank coal.