Experimental Study On The Promotion Of Low-temperature Oxidation Of Low-rank Coal By Water Immersion Process

The western mining area is the key area for future coal mining in China,but due to its coal quality and occurrence condition,coal spontaneous combustion is serious.In the spontaneous combustion of coal,the water-immersed coal is more more serious than the raw coal because of its unique property.However,few studies have been done on the spontaneous combustion characteristics of water-immersed coal,and the mechanism of promoting the spontaneous combustion of coal in the process of water immersion is still unknown.Therefore,from the perspective of physicochemical properties and functional groups of water-immersed coal,the influence of water immersion process on the properties of low-rank coal spontaneous combustion and its promoting mechanism were studied.The TG-DSC thermal analysis method was used to study the effects of different immersion time on the characteristic temperature and heat release of low-rank coals in Zhundong and Shendong.And the characteristic temperature and heat release of the water-immersed coal with different water content under different immersion time were also studied.The results show that there is a critical immersion time for both Zhundong and Shendong coal samples,which is 30 d for Zhundong and 50 d for Shendong.Under this immersion time,the coal samples have the strongest oxidation and spontaneous combustion ability.There is a most easily spontaneous combustion moisture content and range in coal samples with different water immersion time,and the range of the most easily spontaneous combustion moisture range is gradually increased with the increase of the immersion time.Compared with raw coal,the ratio of the most easily self-igniting moisture range of the Zhundong and Shendong coals is between 0.50 and 1.37.The above results provide a reference for the control of coal spontaneous combustion immersion time and water content,and also provide an experimental basis for the subsequent study of water immersion.Aiming at the change of chemical groups that promote coal spontaneous combustion in the process of water immersion,the functional group content and reaction rate of raw coal and the 30-days water immersion coal of the most easily self-igniting water content in the air and nitrogen atmosphere were studied by Fourier infrared spectrometer,and the differences of functional groups in the process of water immersion in low-rank coal were obtained.The results show that the water immersion process increases the content of active structures such as hydroxyl and aliphatic incoal,and promotes the reaction of hydroxyl and carbonyl functional groups in the low temperature oxidation process,which makes the maximum reaction rate of aliphatic functional groups advance,thus accelerating the spontaneous combustion oxidation process of coal.And the existence of oxygen mainly affects the hydroxyl and aliphatic structures before 100 °C.Combined with chain reaction analysis,the reaction of hydroxyl and aliphatic functional groups is the key to promote the coal spontaneous combustion in the water immersion.In terms of the influence of water immersion process on the physical structure of coal,low-temperature nitrogen adsorption method was used to study the influence of water immersion on the pore structure of low-rank coal with different immersion time and different particle size.Studies have shown that the water immersion changes the structural parameters of different types of pores in low-rank coal.After immersion,the proportion of micropore and small pore volume and specific surface area decreases,while the ratio of medium to large pore volume to specific surface area increases.After water immersion,the average pore size increases greatly,with a minimum increase of 23% and a maximum increase of 109%.The increase of pore size makes the pore connectivity better,thus promoting the coal-oxygen reaction.The effect of water immersion on the pore structure of coal samples with different particle sizes is basically the same.The difference is that the effect of water immersion on the micropores of 40~80 mesh coal samples is greater than that of 120~160 mesh coal samples.The maximum and minimum values of specific surface area,ratio of specific surface area,pore volume and pore volume ratio of Zhundong and Shendong coal samples with different particle sizes were obtained in 30 or 90 days of water immersion.And after 90 days of water immersion,the vast majority of the proportion changed gently with the immersion time increase.It can be obtained that the water immersion for 30~90 days is a key immersion time.In terms of the oxygen consumption in the low-temperature,the 30-days water immersion time of Zhundong and Shendong have the strongest oxygen consumption ability,and the oxygen consumption intensity of different immersion time coals is consistent with the results of low-temperature oxidation heat release.