Investigation Of Synergistic Inhibition Mechanism Based On Evolution Of Physicochemical Properties Of Coal

Coal spontaneous combustion is one of the main disasters in goaf during the safe mining process of coal resources.There are many significant differences in the evolution of physical and chemical properties during the oxidation of coal with different metamorphic degrees.Especially,there are great differences in physical pore structure,surface morphology and reaction properties of active groups.However,there is a lack of comprehensive research on the evolution of physicochemical properties of coal oxidation with different ranks,and the lack of forming a mechanism in the whole process to prevent coal oxidation.In particular,there is few quantitative description of the differential evolution of pore structure and surface morphology in the coal oxidation,and the different tendency and ability of coal oxidation caused by the differential evolution of active groups.The average pore size,BJH pore volume and BET specific surface area of lignite characterized by BET method change significantly with the increasing temperature,followed by bitumite and anthracite.The average roughness（Ra）and root mean square（Rq）of coal surface obtained by atomic force microscope raise with the increasing temperature,and it’s significantly differential miniaturization evolution on coal surface morphology.The pore structure and surface morphology during coal oxidation are characterized quantificationally based on FHH model and AFM power spectrum fractal model,respectively.The fractal dimensions D_FHH and D_psd raise with the increasing temperature,which indicates that there are growing number of complex micropores on the coal surface during low-temperature oxidation.Especially,the linear slope of D_FHHwith the raised temperature and the growth rate of D_psd of lignite are higher than those of bitumite and anthracite.That is to say,there is a continuous change in the pore structure and surface morphology,which also aggravates the risk of coal spontaneous combustion.Combined with the average pore size,BJH pore volume,BET specific surface area and surface roughness during coal oxidation,the distinction between pore structure and surface morphology are characterized qualitatively and quantitatively.Hereinbefore,it provides an approach for the development of inhibited materials that could cover the coal surface to reduce the abundance of pore structure and morphology.The surface functional groups and radicals during coal oxidation were measured by X-ray photoelectron spectroscopy and in-situ electron paramagnetic resonance spectroscopy,respectively.The results show that the decreasing relative content of C element and the increase of O element of lignite are significantly higher than those of bituminte and anthracite.And it’s same to the decreasing relative content of surface C-C/C-H bond during coal oxidation.The EPR profile of coal has a high temperature dependence,and the linewidth of lignite increases with temperature,while it’s opposite to that of bituminte and anthracite.Therefore,it is easy to form many active radical sites because there is a great g value during low-rank coal oxidation.The radical concentration of coal raises with increasing temperature,unlike the previous g factor’s trend,the increase of radical concentration in high-rank coal is striking.The evolution of radicals during coal low-temperature decomposition shows that there are large gap between g factor and radical concentration in comparison to coal oxidation.Where the g factor of low-temperature decomposition is lower than that during coal oxidation,while the radical concentration is higher as a whole and increases significantly with the increase of temperature.This difference shows that the type and complexity of radicals during low-temperature decomposition are lower than normal and accumulate high radical concentration.Based on the differential evolution of radical concentration during different rank coal oxidation is not in conformity with the tendency of spontaneous combustion,it is proposed that the relative growth rate R of stable radical concentration is used as an index to determine the oxidizability of coal with different metamorphic degrees.Based on the differential evolution of physical and chemical properties of different rank coal during low-temperature oxidation,a physicochemical synergistic inhibitor which can not only inhibit the expansion of pore structure and surface morphology,but also reduce the reaction of active groups was developed to prevent coal oxidation.The high water-absorbent resin and hydrogel constitute the physical inhibition part of the synergistic inhibitor;While the antioxidant Vc-SOD,which is mainly composed of ascorbic acid and supplemented by superoxide dismutase,forms the chemical inhibition part.Then,the green sodium alginate is used as the matrix to cross-link with polyacrylamide and CaCl₂ to form a composite hydrogel SA-PAM.Finally,the physicochemical synergistic inhibitor is taken as the core material and it’s wrapped in the hydrogel,the physicochemical synergistic sustained and controlled microspheres are prepared.Based on the relationship between various gaseous products,oxidizing temperature,characteristic temperature point,activation energy and heat release during the inhibited coal oxidation.The effect of CaCl₂ could continue until the initial stage of oxygen absorption,while the purpose of inhibiting the whole oxidation process could be realized by the effect of heat absorption,oxygen isolation and chain reaction interruption of the physicochemical synergistic inhibitor.At the initial oxidation stage of lignite,the effect of CaCl₂ could keep the content of methyl and hydroxyl at a high level.While before the temperature reaches to 100℃during the oxidation of bituminte and anthracite,the content of methyl and hydroxyl in CaCl₂-coal is similar to that of raw coal.But then the methyl and hydroxyl decrease sharply and CaCl₂loses its inhibition effect.The contents of methyl and hydroxyl decrease significantly during the oxidation of coal inhibited by physicochemical synergistic effect,it’s because that the increasing temperature compel hydrogel release antioxidants（Vc-SOD）to capture the ester oxygen radical（ROO·）to come into being a stable ether bond,as well as scavenging the primary hydroxyl and interrupting the chain reaction from aliphatic to form secondary hydroxyl.The coal surface covered with CaCl₂ only could inhibit the increase of carbonyl to a certain extent.However,the decrease of carbonyl content is striking due to physicochemical synergistic inhibition mechanism.And it’s same to the reduction of g value in lignite,so that the type and complexity of radicals in lignite are lower than those of CaCl₂-coal and raw coal.There is a kind of excellent physical and chemical doubling effect,so as to realize the purpose of preventing the whole process coal low-temperature oxidation.So it is of great significance to prevent spontaneous combustion of coal with different metamorphic degrees.