| The pyrolysis is also called carbonization or heat decomposition, it can produce gas coal, tar and semi-coke or coke, which are lustration or modified fuel, and can substitute petroleum and natural gas, and are also good chemical raw material. This method that the pyrolysis produce lustration and modified fuel both reduces environment pollution by burning coal and makes the most of the better economic value compounds in coal, it is important that it can protect environment as well as reasonably use coal resource and save energy sources. The pyrolysis is the key step of coal transformation, it must occur during gasification, liquefaction, coking and burning of coal. The pyrolysis is the first step of Underground coal gasification, modified low rank coal, in-situ pyrolysis of coal and spontaneous combustion of coal which are the key technologies to the environmental coal, so the research findings of the pyrolysis have effect on those key technologies directly. The pyrolysis occurs on the surface of coal matter, which is related to pores and cracks in coal, so pore structure and crack distribution can influence the activity and reactivity of pyrolysis. Overall consideration, the research on the evolution of pore and crack in pyrolysis is the basic research on both in-situ pyrolysis of coal and hot processing of coal such as Underground coal gasification, liquefaction, modified low rank coal, it is important to sustainable development of coal industry in our country.In this article, the pyrolysis characteristics of many ranks of coal at home are studied. The three-dimensional mecro-structure is rebuild by scanning coal samples with advancedμCT225KvFCB. The effect of temperature on pore and crack is analyzed. Main contents and conclusions are as follows:1. UsingμCT225KvFCB system and high temperature atmosphere furnace made by oneself, scanning F 1mm coal samples, studying the pore structure characteristic under different temperatures, we can draw conclusions as follows:The porosity and percolation probability of lignite and gas coal increase monotonously with temperature, specific surface area first increases then decreases, the specific surface area of lignite has peak value at 400℃, the specific surface area of gas coal has peak value at 200℃.200℃is the mutation temperature of pore structure of lignite and gas coal. The specific surface area of lignite is minimum in all coal samples, the porosity of gas coal is maximum above 200℃.The porosity, specific surface area and percolation probability of coking coal, lean coal andⅡanthracite first increase and then decrease.500℃is the peak temperature of coking coal andⅡanthracite, the specific surface area of coking coal is the largest in all coal samples. The evolvement of the pore structure of lean coal is gentle,400℃is its peak temperature.2. On the basis of already research, creating aperture fractal dimension, pore volume fractal dimension and surface area fractal dimension, analyzing the pore fractal dimensions of five ranks of coal under different temperatures, we can draw conclusions as follows:In five ranks of coal, the aperture fractal dimension of lignite is minimum, which of gas coal is bigger, which ofⅡanthracite is almost immutable. From room temperature to 300℃, the aperture of kinds of coal is in sequence:gas coal >Ⅱanthracite> coking coal> lean coal> lignite. The pore volum fractal dimensions of gas coal, coking coal and lean coal are almost constant value 3 with temperature, increment of the pore volume fractal dimension ofⅡanthracite is maximum, that of lignite second. The pore volume fractal dimensions ofⅡanthracite and lignite are close to 3 and almost immutable with temperature. At 200℃or above, the pore volume fractal dimensions of five ranks of coas are bigger than 2.96. The surface area fractal dimensions of lignite and gas coal increase with temperature, those of coking coal andⅡanthracite first increase thenⅡdecrease, and have max value at 500℃. The surface area fractal dimension of lean coal is almost immutable with temperature.3. WithμCT225KvFCB system and high temperature atmosphere furnace made by oneself, scanning F 7mm coal samples, studying the evolvement of crack emergence and expansion with temperature, we can draw conclusions as follows:the cracks emerge in soft coal matter mostly, or in boundary of soft and hard coal matter, the crack arrest is caught by hard grain and hard strip. The crack arrest can go through hard grain and hard strip to go on expanding at higher temperature.The cracks of lignite, gas coal, coking coal, poor coal andⅡanthracite produce suddenly. At 200℃, lignite produces large numbers of echelon and net cracks suddenly, gas coal produces large numbers of net cracks suddenly,Ⅱanthracite suddenly produces large numbers of polycondensation cracks and cracks perpendicular to seam. At 300℃, coking coal suddenly produces many honeycombed cracks, poor coal suddenly produces numbers of rotten grain cracks and polycondensation cracks and echelon cracks.The emergence and expansion of lean coal gradually take place with temperature, dehydrated cracks arise at 100℃, cracks parallel to seam arise at 200℃, and more honeycombed cracks arise at 300℃, like-pressing cracks arise at 500℃.Few cracks ofⅢanthracite arise and they expand slowly with temperature.4. We establish the crack fractal dimension expressions on the basis of crack quantity-scale, and doing crack fractal research on seven ranks of coal spoken of earlier, and drawing conclusions as follows:The fractal dimension ofⅡanthracite decreases monotonically with temperature, which ofⅢanthracite increases monotonically with temperature, that of others coal first increases then decreases. To lignite and coking coal, 300℃is peak temperature at which their crack fractal dimensions reach maximum,200℃is peak temperature of gas coal,400℃is peak temperature of lean coal and poor coal. From room temperature to peak temperature, crack mainly arise and expand, from peak temperature to 600℃, crack mainly expand, overlap and connect. 5. Through comprehensive analyzing the evolvement of the pore and crack of above seven kinds of coal with temperature, we draw some conclusions:(1) The change of temperature has great effect on the pore and crack structure of lignite, gas coal, coking coal andⅡanthracite, at 200℃(to coking coal at 300℃), porosity reach 30%, percolation probability reach 25%, many cracks suddenly arise, the increment of crack fractal dimension is maximum, thus theirs permeability can be improved at low temperature to these four kinds of coal. (2) The change of temperature has great effect on crack of lean coal and poor coal, in which many kinds of cracks produce. At 400℃, crack fractal dimension reaches maximum, crack full develops. (3) The change of temperature has little effect on the crack ofⅢanthracite, few cracks produce in pyrolysis. (4) From room temperature to 300℃, thermal cracking has primary role on production of pores and cracks. From 300℃to 600℃, pyrolysis has primary role on the expansion of pores and cracks.
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