Study On The Mechanism Of Re-absorption Of Lignite After Low-temperature Pyrolysis

As the largest part of the energy consumption structure in China,the clean and efficient utilization of coal has been paid more and more attention by the society.The lignite has many advantages such as thick coal seam,shallow burying and low mining cost,so it is suitable for largescale development and utilization.However,the higher moisture content in the lignite severely impacted its large-scale industrial application.After spreading out the lignite in natural conditions and stacking it in the open-air,the lignite will absorb the water in the air and form the moisture re-absorption phenomenon,which will affect the drying efficiency seriously.In this paper,XiMeng(XM)lignite was dewatered by low temperature pyrolysis method.The effects of pyrolysis temperature,grain diameter and relative humidity(RH)on the moisture absorption characteristics of XM lignite were studied.The results show that the yield of semi-coke decreased gradually with the increase of pyrolysis temperature,and the smaller the grain diameter,the higher the yield of semi-coke.The moisture content of upgrading lignite decreased greatly,and the low heat values increased gradually.In addition,the molar ratio of H/C and O/C decreased gradually.The rank of upgrading lignite increased gradually,the effect on the upgrading lignite is remarkable after the low temperature pyrolysis experiment.The moisture re-absorption rate of semi-coke decreased with the incased of pyrolysis temperature,which is only 8.13% under the conditions of relative RH is 50~60% and the pyrolysis temperature is 700 ?.Under the same other conditions,the grain diameter has little effect on the re-absorption of water,but the moisture re-absorption rate of semi-coke is higher at the condition of high RH.The molecular structure models of lignite and its upgrading coals were constructed based on the coal quality analysis of low temperature pyrolysis experiments,the relationship diagram of oxygencontaining functional groups and the degree of coalification,and coalification line diagram.All the molecular structure models were optimized by density functional theory(DFT).In this paper,IGM,AIM,MEP and weak interaction energy are used to analyze the weak interaction of lignite and its upgrading coals.The results show that,the IGM method can separate the intermolecular and intramolecular weak interactions,which is very beneficial for only studying the weak interaction between coal-water complexes.Two types of hydrogen bonds O-H…O and C-H…O were formed between different oxygen-containing functional groups of lignite and water molecules.The values of the former are greater than the latter.The electron density and potential energy density()values of the former are one order of magnitude bigger than the latter.For the same type of hydrogen bond,the value is always greater than the()value(absolute value),and both of them have the same order of magnitude.The hydrophilic order of different oxygen functional groups in lignite is: carboxyl > phenolic hydroxyl > carbonyl > ether bond > methoxyl.In the weak interaction of lignite and its pyrolysis upgrading coals,the maximum value of the molecular electrostatic potential(MEP)decreases gradually with the deepening of pyrolysis degree,the distribution of MEP on the molecular surface tends to average gradually.The average weak interaction energy of coal-water complexes decreases with the deepening of lignite upgrading,and the weak interaction strength of coal-water complexes are determined by the type and the quantity of weak interaction forces.