Study On Thermal And Mass Transfer Dehydration Process And Water Reabsorption And Spontaneous Combustion Of High Moisture Coal Based On Steam Instantaneous Dynamic Dehydration Technology

With the rapid development of Chinese economy,the high quality coal resource is consumed in large quantities,and the efficient utilization of high moisture coal becomes one of the main tasks in ensuring our energy security.However,the characteristics of high moisture coal,such as low calorific value,poor thermal stability and easy spontaneous combustion,seriously restrict its efficient utilization.Therefore,dehydration and quality improvement technology becomes an essential link before the large-scale efficient utilization of high moisture coal.The steam instantaneous dynamic dehydration technology with high pressure steam as drying medium has the advantages of short drying time,high dehydration efficiency,safe and reliable drying process,etc.,which is a very promising dehydration quality improvement technology.Therefore,this thesis uses steam instantaneous dynamic dehydration technology to treat high moisture coal,explores the influence of steam instantaneous dynamic dehydration technology on the physical and chemical structure characteristics of high moisture coal,builds the heat and mass transfer dehydration process equation,and analyzes the water reabsorption and spontaneous combustion behavior of dehydrated coal.This thesis takes Ordos lignite as the research object,and conducts steam instantaneous dynamic dehydration under different steam pressure and pressure holding time.First of all,the optimal dehydration conditions of steam instantaneous dynamic dehydration technology are explored,the changes of physical and chemical structure characteristics of dehydrated coal are investigated,and the effects of steam instantaneous dynamic dehydration technology on the pore structure,oxygencontaining functional groups and contact angles of lignite are analyzed.Secondly,the material energy balance and response surface are calculated and analyzed to explain the heat and mass transfer mechanism in the steam instantaneous dynamic dehydration process.Finally,the difference of water reabsorption behavior between lignite raw coal and dehydrated coal is investigated,the change of spontaneous combustion characteristics of lignite after dehydration is explored,and the main mechanism and influencing factors of reabsorption and spontaneous combustion of dehydrated coal are studied.The main conclusions are as follows:(1)Taking dehydration rate and calorific value as indexes,the optimal conditions for the treatment of high moisture coal by steam instantaneous dynamic dehydration technology are steam pressure of 2.5 MPa,pressure holding time of 20 min and coal grade of 6-13 mm.Under such conditions,coal dehydration rate reaches 55.73% and calorific value increases by 1000 cal/g.(2)In the process of steam instantaneous dynamic dehydration,steam pressure has a significant effect on the dehydration rate of high moisture coal.The increase of pressure can make the steam penetrate deep into the pore of coal,reduce the oxygencontaining functional groups and alkyl side chains on the surface of dehydrated coal,increase the contact Angle of coal and reduce the hydrophilicity.The mechanical force generated by instantaneous pressure relief can destroy the original pore structure of coal,increase pore volume and reduce specific surface area.(3)Response surface analysis method is used to establish the quadratic orthogonal regression equation of the dehydration rate changing with the operating parameters in the process of steam instantaneous dynamic dehydration,so as to achieve the prediction of dehydration rate.By combining the heat and mass transfer equation and quadratic orthogonal regression equation of the steam instantaneous dynamic dehydration process,the prediction equation of heat and mass transfer dehydration process is established to realize the prediction of the consumption/output of each substance and the energy consumption of the dehydration process.(4)Coal sample storage environment is the main influencing factor of coal sample water reabsorption,and reducing environmental humidity and temperature is an effective means to inhibit coal sample water reabsorption;Coal grade is a secondary factor affecting water reabsorption in coal samples,and increasing coal grade can inhibit water reabsorption in coal samples to some extent.The moisture of Dehydrated coal after water reabsorption is lower than that of raw coal,that is,there is no reabsorption phenomenon.(5)The water adsorption process of coal samples before and after dehydration belongs to type II adsorption.In the low relative humidity section,the fitting effect of Freudlich model is better,and the water molecules are mainly monolayer adsorption with the oxygen-containing functional groups on coal surface.In the middle and high relative humidity segment,the Dent model has a better fitting effect,and the water molecules are mainly hydrogen bonded with the water molecules adsorbed on the coal surface,resulting in multilayer adsorption.(6)The specific surface area of dehydrated coal decreases and the alkyl side chain decreases,thus reducing the number of oxygen adsorption sites.Moreover,the moisture content of dehydrated coal is in the oxygen adsorption inhibition section of water oxygen,resulting in the maximum oxygen absorption amount and oxygen absorption rate of dehydrated coal are lower than that of raw coal.The spontaneous combustion characteristic parameters of dehydrated coal are lower than those of raw coal,which proves that steam instantaneous dynamic dehydration technology has certain inhibition effect on spontaneous combustion of coal.This thesis has 27 figures,37 tables and 93 references.