Diffusion And Adsorption Of H₂O-O₂ On Carbon Nanotubes Based On Physicochemical Structure Of Lignite

Coal is easy to spontaneous combustion during storage and transportation,causing safety problems and wasting resources.The combination of coal oxygen complex action and the moisture in coal are the causes of coal spontaneous combustion,and the pore size and surface active functional groups can affect the diffusion and adsorption of O₂ and H₂O.By studying the diffusion and adsorption of O₂ and H₂O by pore structure and functional groups,the difficulty of spontaneous combustion of coal is explored,which has certain guiding significance for preventing spontaneous combustion of coal.Firstly,the pore size and functional groups of raw coal were investigated.It was found that there are straight tube pore in raw coal,and there are micropore,mesopore and macropore,but they are all less than 100 nm.The coal contains functional groups such as-OH,-COOH and-CO-,the carbon-oxygen single bond containing-OH is about14.17%,and the carbon content in the form of-COOH is about 2.69%.According to the pore size and functional group data of raw coal,carbon nanotubes（CNTs）,which are closest to the structure of raw coal,are selected for further study.Secondly,the O₂ diffusion and adsorption effects were studied.The results showed that with the increase of the pore size,the O₂ diffusion coefficient increases,the surface potential of the large pore size sample is small,the O₂ molecule has a strong mobility and the diffusion coefficient is large.The adsorption amount of O₂ in CNTs with different pore diameters:macroporous>mesopore>microporous,which is because the macropores provide more active adsorption sites for O₂,O₂ is more easily adsorbed in coal,and O₂ adsorption is the largest.The adsorption amount of O₂ in CNTs-COOH is larger than that in CNTs-OH,and the presence of carboxyl groups promotes the adsorption of O₂.Then,the macroscopic kinetic parameters were analyzed.The results showed that when the pore size of the coal is small,the heat in the coal is difficult to be released through the pores,which aggravates the spontaneous combustion of the coal and the ignition temperature is low.However,there is a strong adsorption between-COOH and O₂,which promotes the spontaneous combustion of coal and the ignition point of the sample is low.In the stage of water evaporation and gas desorption,the activation energy and the pre-exponential factor decrease with the increase of the pore size of the sample.In the stage of oxygen inhalation and gain,as the pore size increases,the activation energy increases and the pre-exponential factor decreases.And the activation energy and pre-exponential factor of CNTs-COOH are small.This indicates that the sample with smaller pore size provides less adsorption sites for the adsorption of O₂,which limits the adsorption of coal and O₂,resulting in higher energy and higher activation energy,while the adsorption of-COOH and O₂ is more strong,promotes the composite reaction of coal and O₂,and the activation energy is low.Finally,the adsorption of H₂O was studied.The results showed that CNTs have a higher adsorption capacity for H₂O due to the stronger interaction between-COOH and water molecules.Based on the fitting effect of the six models,the Hendenson model selected as the isothermal adsorption model of raw coal.The GAB model was the adsorption model of CNTs-OH and CNTs-COOH,and the adsorption model equation of each sample was obtained.The adsorption heat of CNTs-OH is between 2.43 and22.46 KJ/mol,and the adsorption heat of CNTs-COOH is between 3.43 and 46.80KJ/mol.The adsorption heat of isothermal decreases with the increase of adsorption.When the adsorption amount is small,the water molecules are adsorbed on the sample in the form of single layer adsorption,and the water molecules form a strong interaction with coal,and the heat of adsorption increases accordingly.The molecular layer is mainly adsorbed,some active sites are occupied,and water molecules are adsorbed on some less active sites,and the binding energy of water molecules is reduced,and the heat of adsorption is small.In the mixed system of H₂O-O₂,due to hydrogen bonding,the adsorption of H₂O occupies more sites,which affects the combination of coal and O₂ and inhibits the spontaneous combustion of coal.