Mechanical properties of spontaneous combustion coal oxidation

To study the changes in the mechanical properties of spontaneous combustion coal after oxidation,the characteristic temperatures in the process of coal oxidation were obtained by temperature programmed and thermogravimetric experiments,and the coal samples were oxidized.The damage factor of coal body after oxidation was obtained by testing the wave velocity of coal samples before and after oxidation.Through uniaxial compression test,the change law of mechanical parameters of different oxidized coal was analyzed.Through the establishment of the force model of oxidized coal,the difference of the mechanical properties of different oxidized coal was analyzed and the typical working conditions were studied.Through the above experimental study,the following results were obtained:① The average longitudinal wave velocities of raw coal and coal samples after oxidation at 70,135,200 and 265 ℃ are 1 642,1 416,1 261,870 and 557 m/s,respectively.The average values of damage factors of coal with different oxidation degrees are 0.19,0.43,0.72 and 0.86,respectively.With the increase of oxidation degree,the wave velocity decreases and the damage factor increases.② The stress strain curve of raw coal shows good linear characteristics,and its compaction stage and yield stage are not obvious.With the increase of oxidation degree,the more obvious the compression stage and yield stage of the stress strain curve of oxidized coal,the more obvious the multi peak effect,the more prominent the step drop effect behind the peak,and the obvious residual strength after the peak,and the plasticity of the oxidized coal increased.③ With the increase of oxidation degree,compressive strength decreases from 16.36 MPa to 4.10 MPa,average modulus decreases from 3.779 to 0.437,secant modulus decreases from 2.05 to 0.19,initial modulus decreases from 0.609 to 0.082,Poisson’s ratio decreases from 0.37 to 0.25.Oxidation has the most obvious effect on the compressive strength of coal,and its softening coefficient decreases from 0.89 to 0.25.The concept of “the dynamic engineering strength of oxidized coal” is put forward.④ With the increase of oxidation degree,the peak strain of the coal samples and the maximum axial strain in the compaction stage gradually increase,and the ratio of the maximum axial strain to the peak strain in the compression stage becomes larger and larger.When the oxidation degree reaches 265 ℃,the ratio is nearly 50%.⑤ With the deepening of oxidation degree,the form of coal samples destruction becomes more complicated,the integrity of the coal samples becomes worse after the destruction,the broken coal and coal powder that fall off after the destruction increases,accompanied by the phenomenon of “peeling”.⑥ The oxidized coal body is divided into strong oxidation area,weak oxidation area and non oxidation area from outside to inside,and the stress model of oxidized coal body is established.The results show that the diameter of unoxidized area is 44.44,37.24,16.84 and 0.06 mm after oxidation at 70,135,200 and 265 ℃.The difference mechanism of mechanical properties of oxidized coal body is constructed,and the typical working conditions are analyzed by numerical simulation.