Thermal Sensitive Characteristics And Mechanism Of Thermal Fragmentation Of Low Rank Coal

As an abundant resource in our country, low rank coal needs to be upgraded through dehydration to large-scale use because of its coal quality features such as low degree of coalification, high water content, high volatile, low calorific value and strong reactivity. Low rank coal upgrading technology has been included in national energy science and technology latest five-year plan to research and develop low rank coal upgrading technology and process with independent intellectual property rights and wide adaptability. So in recent years, a number of research institutes have developed different types of low rank coal dewatering technology, several large demonstration projects have been built and put into operation.Due to the poor thermal stability of low rank coal, fragmentation and pulverization of coal particles result from thermal shock occur in all kinds of upgrading technology, which then lead to big resistance of dust removal system, environmental protection and safety troubles, and difficult to achieve security, stability, long cycle running at full capacity. Dust also has adverse influence to the environment. With successive shutdown of several large low rank coal upgrading projects, fragmentation and pulverization problems in drying process have become increasingly prominent, severely restricting the development and application of low rank coal upgrading technology.But in recent years, there were few reports of basic research aimed at the low rank coal fragmentation and pulverization characteristics and its mechanism in drying process. According to the above problem, low rank coal of different metamorphic degrees were dried by using thermal drying technology to explore their fragmentation characteristics under the action of heat, the influence of different factors on the thermal fragmentation behavior was investigated, and characteristic index was proposed to describe overall fragmentation degree and pulverization degree. Among them, the influence of dry degree, feeding particle size, heating rate and different type of coal on the grain-size distribution was relatively obvious, by contrast, the influence of different drying temperatures on the particle size distribution of discharge was small, most weak. A multiple particle size index representing the degree of particle breakage relative breakage rate Br was put forward, compared to single particle size index broken rate proposed by previous researchers, was relatively better to reflect the overall situation of the change of particle size distribution, and it was sentitive to the factors including temperature, degree of dry, the grain-size, different types, it also showed good correlation with other broken index. A index representing the degree of particle pulverization was proposed, named pulverization rate. The particle size distribution, pore size distribution and maceral composition of the different graded generated particles were examined, it was found that <1mm the fine powder common presented the self-similar characteristics on the overall particle size distribution in a certain scale. Particle size and pore size distribution between the powder smaller than 1mm and that greater than 1 mm difference was different, thus the pulverization rate critical particle size was choosed as 1 mm.The index system of low rank coal thermal characteristics was established, including pore structure, expansion characteristics, mechanical strength and oxygen containing functional groups in four aspects. Low-temperature nitrogen adsorption method was used to determine the drying process of low rank coal pore structure changes. It was showed that as the dry degree deepening, pore structure of lignite quickly get complicated. The beginning of the dry, the volume of moisture in coal expansed as absorption of heat, internal pressure of coal particles made pore expansion, particle pore volume become bigger. When the moisture content reduced to 15%, it entered the stage of slow down dehydration, lignite overall temperature was higher, the pore structure expansion phenomenon intensified, the pore volume increase rate also quickened significantly. And pore size distribution changes showed that macroporous percentage and microporous percentage decreased in different degree after drying process, mesoporous percentage rised more clearly,which indicated drying lead coal pore transition to mesoporous hole in the sample.Thermal mechanical analyzer was used to study the heating expansion and water shrinkage properties in drying process of low rank coal. Results showed that for the same coal, the drying temperature is lower, corresponding time period in the process is longer, and he corresponding maximum shrinkage of coal sample is smaller. The smaller the shrinkage rate was, the more moderate degree was. At higher drying temperature, water loss more in coal sample at the same time, water loss rate is faster, wet stress caused by moisture gradient is bigger, lead to increased internal stress points in the sample, the lower mechanical strength of coal samples, more acute degree of pore and fracture collapse, more obvious macro phenomenon of shrinkage process. Shrinkage property of various coal types in the drying process was different. Comparing TMA data and drying test data of various coal types, it showed two breakage characterization indexes including the relative breakage rate and pulverization rate of coal samples was correspond to TMA swelling index maximum shrinkage, strong thermal expansion of coal was easy being fragment and pulverization in the process of drying. With crushing strength and as evaluation index, mechanical strength changes of particle in the process were studied. Results showed that the crushing strength of a coal particle in certain size present a trend of decline with temperature increasing, and the decrease rate is also increasing. As the dry degree deepening, it present linear downward trend. Coal particle crushing strength showed a trend of power function decrease with the increase of particle size. The larger the particle size, the smaller the crushing strength after drying process, the bigger the corresponding relative breakage rate, the greater the whole broken degree.The method for determination of the thermal sensitivity was put forward to systematically investigate the influence of different metamorphic degree of coal, rock phase composition and other internal factors on the thermal sensitivity. The study found that thermal sensitivity of various metamorphic degrees of coal was different. With the increase of maximum vitrinite reflectance, thermal sensitivity of low rank coal index decreases. With the increase of total moisture and volatile matter content, thermal sensitivity presents exponential increase. The relationship between ash content and thermal sensitivity correlation was poor. Regularity was not obvious. Low rank coal thermal sensitivity roughly presented positive correlation with raw coal pore volume.In three kinds of Zhahanaoer long-flame coal lithotype, bright coal was rich in vitrinite, inertinite content of culm coal was in maximum, and mineral composition proportion of durain was the largest. Thermal sensitivity of three kinds of lithotype existed certain differences which bright coal had minimum thermal sensitivity, durain thermal sensitivity was highest, and culm coal in the middle. Durain due to rich in inertinite group, occur more easily broken in the fine particles generation of dry process. Mineral content of durain coal is higher, thermal stress caused by temperature gradient in drying process and caused by uneven expansion degree of stress and hydro stress caused by uneven expansion degree between minerals and soft coal matrix resulted in evolution of crack, its thermal sensitivity was the largest of the three. In terms of the macromolecular structure of coal, along with the increasing degree of aromatization of the coal molecules, its thermal sensitivity showed a trend of logarithmic decrease. Lipid parameters of carbon in coal is higher, the higher the thermal sensitivity, both as a positive correlation. The higher oxygen containing functional groups parameters, the higher the thermal sensitivity.Comparative study of the influence of thermal and mechanical force in different loading ways on low rank coal breakage features in the process of drying was done. Results showed that the deepest degree of fractured discharging in the mechanical strength and thermal coupling loading mode, pulverization rate in coupling loading mode is greater than the sum of two forces solo loading way, thermal- mechanical coupling loading mode produced a synergistic effect. And dust particle size distribution and particle diameter distribution of samples were analyzed by using three kinds of load mode to investigate the crushing mechanism of thermal and mechanical force. Results showed that when mechanical force load alone, particle breakage degree is the lightest, the distribution of coal particles by crushing is similar with common grinding equipment products, when the thermal force load alone or two of them load couple, dust distribution under mechanical loading ways showed bimodal distribution. Mechanical force separate loading condition effect on the pore diameter of coal particles change was very small, almost no change, the total volume and the proportion of different pores of the total pore volume was relatively unchanged, and under the condition of thermal load separately, lignite particle internal pore structure had swung, different pores proportion of the total pore volume and great changes had taken place, the proportion of macroporous decreased, and the pore ratio increased, while in the process of thermal- mechanical coupling loading and pore structure of coal particles increased further, but the pores in the proportion of total pore volume was still close to the thermal load. Two conclusions was consistent, all instructions of the fragmentation mechanism of heat and the external mechanical forces exist certain differences, and coupling loading was in accordance with the mechanism of thermal loading way alone. Mechanical force only exacerbated the thermal action of macro performance.By establishing the model of coal particles in drying process, distribution of thermal stress caused by temperature gradient and hydro stress caused by moisture gradient were showed to study fragmentation mechanism of coal particles under the action of pure heat. Results showed that the value of internal hydro stress caused by moisture gradient was greater than that of thermal stress caused by temperature gradient. During the drying process particles hydro stress plays a main role for the particle fragmentation. And in the surface area of particles, hydro stress performance as large tensile stress. The ultimate tensile strength of coal particles is an order of magnitude lower than the limit compressive strength, so particle surface occurs fragmentation first, peeling out of the tiny particles.