Study On Water Loss Characteristics Of Lignite During Low Temperature Drying Process

China has abundant reserves of lignite,but during the production process,it is easy to cause coal spontaneous combustion fires.In practice,it has been found that with the loss of water,water containing pores gradually change into non water containing pores,providing more places for oxygen transportation and adsorption,intensifying the coal oxygen composite reaction,and thereby affecting the process of coal spontaneous combustion.Therefore,conducting research on the water migration characteristics of Naoerhao lignite during low-temperature drying process is beneficial for the early prevention and control of coal spontaneous combustion.This thesis conducts experimental research on the moisture occurrence and migration characteristics of coal samples with different drying degrees under nitrogen/dry air atmosphere.A kinetic model for vacuum isothermal drying of lignite is established,and the apparent activation energy of non-isothermal drying moisture removal is studied to reveal the water loss characteristics in the early stage of coal oxygen composite reaction and promote the safe utilization of lignite.The main content and conclusions of the thesis are as follows:1)The water occurrence and migration characteristics of coal samples were investigated by low-field nuclear magnetic resonance experiments.The results show that coal samples with different drying degrees include unfrozen water in micropores,microcapillary adsorption water in small pores,macrocapillary adsorption water in mesopores and free water in macropores/fissures.The process of water removal is divided into three stages:free water evaporation removal-capillary adsorption water diffusion to the fracture evaporation removal-freeze water removal.Under the same gas atmosphere,the longer the drying time,the higher the temperature,the more thorough the moisture removal.Under the condition of dry air atmosphere,drying at 90℃ for 360 min,the moisture content decreased by 94.1%.Compared with dry air atmosphere,the moisture migration of coal samples in nitrogen atmosphere is faster,which is more conducive to the removal of macro capillary adsorbed water.When the drying time is more than 180 min and the drying temperature is 70℃ and 90℃,the nitrogen atmosphere is more conducive to the migration and removal of micro capillary adsorbed water.2)The change of pore structure of dry coal samples was studied by N2 adsorption experiment.The results show that the adsorption isotherms of coal samples with different drying degrees belong to the second type.In nitrogen atmosphere,the specific surface area decreased with the increase of drying time at 50℃.Under nitrogen atmosphere,the average pore size of coal samples dried at 50℃ for 180 min increased by 0.65 nm.The micropores of coal samples dried at 70℃ and 90℃ for 180 min increased,the pore size of macropores decreased,and the number of macropores decreased.The reduction of pore size is beneficial to hinder the transport of oxygen,thereby inhibiting the process of coaloxygen composite reaction.3)The effects of temperature and particle size on the drying characteristics of Naoerhao lignite were investigated by vacuum drying experiments.The non-isothermal drying kinetics of lignite was analyzed by thermogravimetric experiments with different heating rates.The results showed that the lignite dehydration process was divided into quasi-steady drying stage and deceleration drying stage.The higher the temperature,the faster the water evaporation rate.The larger the particle size,the smaller the water evaporation rate.The Page model is suitable for isothermal drying of Naoerhao lignite.The average activation energy of multi-heating rate of lignite is 65.702 kJ·mol-1 by FWO method.With the increase of temperature,the apparent activation energy decreases first and then increases.When the moisture removal is 80%,the apparent activation energy is the lowest,which is 41.092 kJ·mol-1.Figure[45]Table[34]Reference[109]...