Experimental Study On Cracking And Improving Wettability Of Coal Through Liquid CO₂ Cyclic Cold Soaking For Dust Reduction

Coal is the most abundant fossil energy resource on the earth and an important support for global economic and social development.A large amount of dust will be produced during the underground mining of coal resources,which can not only cause coal dust explosion accidents,but also often induce pneumoconiosis,which seriously threatens the safety of coal mine production and the physical and mental health of employees.Coal seam water injection is the initiative and cure measure to reduce the concentration of mine dust,but more than 60%of China’s coal seam is difficult to inject water coal seam,its significant characteristics are low permeability,high hydrophobicity,facing water injection failure,long water injection cycle and coal body false wetting and other bottlenecks,the applicability and dust reduction effect of traditional water injection technology is greatly restricted.In view of this problem,this thesis proposes a new idea of liquid CO₂ cyclic cold-soaked coal body cracking,wetting and dust reduction,and carries out corresponding experimental research,the main work and results are as follows:Independently designed and constructed liquid CO₂ cycle low temperature cold soaking automatic control simulation test system,the system is composed of three parts:liquid CO₂ and pressure supply,low temperature closed reaction,automatic control and data acquisition,used to create a low temperature and high pressure environment to simulate the wrapping contact reaction of liquid CO₂ with the coal body unit after entering the coal seam.It realizes real-time monitoring and automatic control of parameters such as pressure,temperature and liquid level in the reaction vessel.Based on the Box-Behnken principle,the optimal parameters of the experiment were analyzed by Design-Expert software:soaking pressure 2MPa,soaking time 60min,number of cycles 3 times.The liquid CO₂ cyclic cold soak test scheme was determined,and the experimental feasibility was verified by Comsol Multiphysics numerical simulation method.The influence of liquid CO₂ cyclic cold soaking on the mechanical properties,microscopic pores and morphology of coal was revealed.The mechanical strength of the coal body is significantly weakened,and the contraction-expansion coupling effect formed by low-temperature thermal stress and water ice phase transition caused by heterogeneity aggravates the structural damage of the coal body,thereby changing the load deformation and failure mode of the coal body.The ultimate compressive strengthσ_c of the coal samples in each group decreased,the strainε_c increased,the elastic modulus E and brittleness index B₅ decreased significantly,the bonding between the coal matrix and the particles weakened,the ultimate stress that the coal body could carry decreased,the brittleness decreased,the plasticity was enhanced,and the release rate of elastic potential energy slowed down.The micromorphological characteristics of coal are significantly affected by liquid CO₂,the surface of coal tends to be loose and granular,the macroscopic fracture size expands,and the microscopic crack density increases.The pore connectivity of coal was enhanced,the small pores developed through,the proportion of medium pores increased,the effective porosity of coalwas positively correlated with the number of cycles,the maximum was about 3.72 times that before the experiment,the maximum increase in total porosity was 127.44%,the number of free fluid channels continued to increase,and the pore structure and seepage conditions of coal samples were optimized.The influence of liquid CO₂ cyclic cold soaking on the water absorption and wetting performance of coal body was clarified.The decrease rate of the contact angle of coal samples in water-saturated experiments is relatively faster,which is related to the non-polarity and acidification extraction properties of CO₂,the water absorption performance of coal samples is significantly improved,and the water absorption rate can reach up to 6.73 times before the experiment,and the increase of coal moisture strengthens the structural damage caused by freeze-thaw cycle and chemical erosion,and promotes the development and formation of loose porous structure of coal body.The aromatic structure of each group and the content of oxygen-containing functional groups in the 1000-1800cm^-1 band decreased,and the decreases of 3H and 5H decreased significantly,and the contents of C-O and C=C of water-saturated coal samples were negatively correlated with the number of cycles.The structural content of aliphatic hydrocarbons in coal varies greatly,the I₁ value of coal decreases,and the length of alkyl side chains decreases.The hydroxyl functional group of dry cycle cold soaking coal samples was greatly reduced,and the small molecule phase of-OH mainly detached from the coal structure in the form of polymer and dissolved in cold soaking solution,while the hydroxyl group of water-saturated circulating coal samples increased significantly,and the self-association probability of-OH increased.The influence of liquid CO₂ cyclic cold soaking on the dust production characteristics of coal body cutting was obtained.The total dust quality and dust production rate obtained by coal cutting samples decreased by about 74%.The particle size volume distribution curve shifts to the right as a whole,the characteristic particle sizes D10,D50 and D90 increase significantly,and the average particle size of dust generated by the coal sample interception of the cycle test is up to 71.27μm,which is2.8 times higher than that of raw coal,and the average specific surface area of dust decreases by 80.11%,effectively reducing the generation of fine particles,especially respiratory dust.The porosity and moisture content of coal body are positively correlated with the characteristic particle size,the higher the porosity and moisture content,the lower the proportion of fine particles produced by interception,and the more concentrated the dust particle size distribution,the correlation coefficients of the two are more than 80%and 70%respectively with the cumulative proportion of exhaled dust and the average specific surface area of dust.Liquid CO₂ cyclic cold soaking effectively improved the porosity and water absorption and wetting effect of coal body,resulting in a decrease in the volume of pulverized nuclei formed by its interception,difficulty in forming dense micro-cracks in the stress concentration area,and correspondingly reduced fine particles such as respirable dust,and a more significant improvement in key indicators such as characteristic particle size and proportion of respirable dust in circulating cold-soaked-saturated coal samples,the dust reduction effect is obvious.