Structural Drying Shrinkage Process Of Lignite Drying And Damage Mechanism Of Internal High-pressure Water Vapor To Structure

The high water content of lignite limits its large-scale application.However,drying is an important way to improve the combustion energy efficiency of lignite.Due to the poor thermal stability of lignite,crushing and pulverization occur during drying,which seriously threatens the safety and continuity of process production and restricts the development of lignite drying technology.As a typical porous liquid-solid structure,the gas-liquid migration of lignite in the drying process leads to the shrinkage.The low permeability forces the internal water to produce high-pressure water vapor at high temperature.Under the combined action of structural shrinkage and high-pressure water vapor,lignite is damaged.By studying the shrinkage and internal high-pressure water vapor evolution of lignite caused by gas-liquid migration,it is help to understand the fragmentation mechanism of lignite during drying and realize the accurate control and prevention of crushing behavior in the drying process.Zhaotong lignite,the representative of China’s young lignite,was taken as the research object.In this thesis,the coal quality and structural characteristics were analyzed first.Taking the drying shrinkage process and destructive of internal high-pressure water vapor as the breakthrough point,the gas-liquid migration in the drying process of Zhaotong lignite was studied by NMR technology,and the shrinkage behavior induced by water migration and the migration of high-pressure water vapor in the particles were determined.The crack development and particle strength were taken as the main indicators to characterize the structural damage of lignite,and the damage development was organically related to the drying shrinkage process and the destructive of internal high-pressure water vapor,so as to realize the systematic study on the meso and macro structural damage of lignite particles during drying.Finally,the structural damage mechanism of lignite drying process based on the influence of drying shrinkage process and high-pressure water vapor was constructed.The gas-liquid migration during drying was studied by NMR technology,which laid a foundation for exploring the shrinkage behavior induced by water migration and the migration of internal high-pressure water vapor.In the rapid drying stage for one-sided lignite drying at room temperature,free water was quickly removed in the form of liquid.Then the drying entered the slow drying stage,in which the removal of pore water mainly occurred.The water transfer coefficient decreased first and then increased with the decrease of moisture content.In the initial stage of drying,water moved rapidly in macropores in the form of liquid.When the moisture content decreased to about 0.95g_moisture/g_{dry coal},the moisture was transmitted in the form of liquid and water vapor.When the moisture content was less than 0.2 g_moisture/g_{dry coal},only water vapor transmission occurred.The one-sided heating drying process of lignite can also be divided into two stages.The water was quickly removed in the form of liquid in the rapid drying stage,and the rapid drying stage was controlled by the drying temperature.When the local moisture content was less than 0.2 g_moisture/g_{dry coal},the water only migrated in the form of water vapor.Due to the pressure at the drying front controlled the rate of water vapor migration,the higher the drying temperature is,the greater the water diffusion coefficient is.The shrinkage characteristics of Zhaotong lignite during drying were studied,and the development of surface crack under the influence of drying shrinkage process was discussed.The water removal and structural shrinkage of lignite two-dimensional structure occurred almost simultaneously,and the two met the following relationship:P=-6.922+142.488X-63.984X²+10.430X³.Different types of moisture removal produced different shrinkage behaviors.The shrinkage decreased slowly with the removal of free water in the rapid drying stage.After the moisture content was reduced to 0.95 g_moisture/g_{dry coal},the drying entered the slow drying stage,in which the shrinkage decreased rapidly due to the removal of pore water.The cracks on the surface of lignite particles experienced a process of"rapid development–stability-shrinkage/closure"under the influence of drying shrinkage process alone,and these long and wide cracks traversed the particle surface.The migration of internal high-pressure water vapor was studied by high-temperature NMR system,and the characteristics of surface cracks under the influence of internal high-pressure water vapor were analyzed.In the rapid drying stage,the heat transferred by the hot airflow was mainly used for heating lignite samples uniformly.After entering the slow drying stage,the internal temperature gradient of lignite increased.When the lignite sample was dried at 170℃for 19.5 h,the drying front moved to a position 24.1mm from the bottom,the temperature increased rapidly from 66.0℃to 117.7℃,and the pressure increased significantly from 26.1 k Pa to 184.0 k Pa.During high-temperature drying process,the water vapor pressure at the drying front was higher,and the water vapor transmission rate was faster.Additionally,due to the faster transmission rate of the drying front,the high-pressure expansion stress caused more serious damage to the lignite structure.Under the action of high-pressure water vapor,fine side cracks evenly distributed on the surface of the two-dimensional lignite plate were formed.With increasing drying temperature,the effect of high-pressure water vapor was significantly enhanced,and a dense side crack network was formed.The structural damage of lignite particles during hot air drying was systematically studied,and the lignite drying damage mechanism based on the influence of drying shrinkage process and high-pressure water vapor was constructed.The relationship between surface crack rate,moisture content and drying temperature was as follows:CR=(-56.30+0.73T′-3.81×10^-3T′²)X+(76.63-0.91T′+4.83×10^-3T′²).The crushing strength of lignite particles decreased with the decrease of moisture content,which met the following relationship with moisture content:CS=37.48+113.26X-71.78X²+15.47X³.The drying shrinkage characteristic was only related to moisture content of lignite,and the water vapor pressure at the drying front increased exponentially with increasing drying temperature.The drying shrinkage effect controlled the damage of lignite when lignite was dried at low temperature.When the drying temperature increased,the effect of high-pressure steam increased significantly.The long crack formed by the drying shrinkage effect and the side crack formed by the action of high-pressure water vapor formed a three-dimensional crack network within the lignite particles.The higher the drying temperature,the wider the long crack,the denser the side crack,and the denser the three-dimensional crack network formed.At this time,the damage to the lignite particles was more serious.Guided by the drying and damage characteristics of lignite particles,the prevention strategy of drying damage for lignite particles was established.The phased drying operation was adopted to improve the drying rate and prevent particle breakage as much as possible,so as to ensure the efficient and safe operation of the drying process.In summary,based on the gas-liquid migration in the drying process,the structural damage of lignite was discussed,and the drying damage mechanism of lignite based on the combined action of drying shrinkage effect and high-pressure water vapor was constructed,so as to better understand the inducement and evolution of lignite drying damage.This is helpful to realize the prevention and control of fragmentation behavior in the drying process,and promote the development and application of lignite drying and quality improvement technology in China.This thesis contains 106 pictures and 12 forms,and 222 pieces of refrences.