Tar and gas production rules and physical property evolution characteristics during the pyrolysis of cohesive tar-rich coal
The softening and melting of cohesive tar-rich coal caused by heating may lead to wellbore blockage and formation disturbance,which is an important factor to be considered for in-situ pyrolysis.In this paper,the pyrolysis of cohesive tar-rich coal at 300-700℃ was conducted to investigate the oil and gas precipitation rules and their composition properties,and the evolution characteristics of physi-cal properties of semi-coke and its indicative significance for in-situ pyrolysis were analyzed.The results shows that the tar yield reaches the maximum value of 7.75%at 500℃ ,and the content of light components(boiling point lower than 300℃ )of the derived tar is the highest(32.2%).The tar components chang significantly at 400-500℃ ,and the content of phenols increases while that of aromatics de-creases.The softening temperature and curing temperature of the coal sample are 389 and 455℃ respectively.Consequently,the macro-porosity of semi-coke increases from less than 10%to about 50%at≥400℃ ,while the micro-pore structure is more developed only at the high temperature stage of 600-700℃ .The thermal conductivity of semi-coke decreases first and then increases with temperature,and reaches a minimum value of 0.06 W/(m·℃ )at 400℃ .In-situ pyrolysis of cohesive tar-rich coal may be confronted with potential problems such as channel blockage,volatile precipitation obstruction and slow thermal conductivity of coal seam.These problems are ex-pected to be solved by using fracturing proppant with viscosity breaking effect or applying near critical water in-situ conversion technology.
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