Numerical Simulation Of Lignite Drying Kinetics Based On The Theory Of Heat And Mass Transfer In Porous Media

Currently,evaporation drying with hot air or hot steam is still the most popular drying method.A deep study of drying kinetics can help people to better understand the lignite drying process and its drying behavior,and then the process can be controlled to achive the goal of energy saving in the industrial applications.In this thesis,on the basis of the theory of heat and mass transfer in porous media and the physical properties of lignite including its water forms and thermal physical properties,lignite drying kinetics model was investigated.After that,the distribution and evolution of heat and mass in lignite and its drying behavior were studied.Different types of water in porous lignite were studied.Based on the different congelation characteristics,the water in lignite can be classified into three forms,namely free water,bound water and non-freezable water.For XLT and SL lignites,bound water water contents were 0.179 and 0.193g/g dry coal,respectively,and non-freezable water contents were 0.330 and 0.267g/g dry coal,respectively.During drying,the free water was evaporated,and then the bound water began to be removed.From the study of water desorption on lignite,lignite was one of the hygroscopic porous media,and the relationship between water and lignite can be described by Henderson model.The water in lignite can also be classified as monolayer water,multilayer water and capillary water.For XLT and SL lignite,the proportions of monolayer water in non-freezable water were 22.8%and 28.2%,respectively.The thermal physical properties were studied by combining LFA and C80measurement methods.The effects from temperature,water content and pore structure were also investigated.Over the temperature range of 30-250^oC,the thermal diffusivities of lignite were in the range of 0.12-0.23 mm²/s,and over the temperature range of 30-180^oC,the thermal capacities of lignite were in the range of 1.10-1.60J/K/g.The effects of temperature on thermal properties can be neglected,and water content played an important role in the changes of the thermal capacities and thermal conductivities of lignite.The mathematical expressions of both thermal capacities and thermal conductivities were obtained.On the basis of heat and mass transfer in porous media,the drying kinetic model of lignite particle was derivated,and both of the water forms in lignite and its thermal physical properties were considered.To obtain the drying kinetic model with explicit physical meaning,the derivation was started from the heat and mass transfer equations in pore scale,and then by using the volume average theory,the above equations were changed and applied to the particle scale.The control equations in model were discreted by finite volume method,and then the model was converted into the solution of a series of non-linear equations,a mathematical problem.The Matlab was used to solve the euqations.The numerical results of the model were close to the experimental results,and the major differences were due to the developing fracture in the surface of particle.By using the drying kinetic model of lignite,the evolutions of temperature gradient,moisture gradient and gas pressure gradient were obtained.The effects of temperature and particle size on drying rates were studied,and it was concluded that the higher temperature causing higher drying rates was mainly affected by temperature gradient and vapor pressure gradient not water gradient.In addition,the smaller particle size can decrease the transfer time of vapor in particle and extend the first falling-rate periods of drying and promote the lignite drying.Drying behavior and dynamic process of lignite particle swarm were studied.A non-isothermal drying method was applied to control the heating rate of lignite and study its drying characteristics during the heating process.To avoid loss of organic matter caused by lignite pyrolysis,the drying temperature of lignite should be within 200^oC.For the whole process of heating up,the drying process of lignite involves the removal of water having strong and weak interaction with coal.At the beginning of drying,lignite was in the stage of the weak interaction water removal.The intensity of input external energy was the main controlling factor of drying rate,namely the control stage of the heat transfer process.The strong interaction water begins to be removed in the later period of drying.The higher external energy input was also beneficial to the removal of this part of water,but the particle size became the main control factor,namely the mass transfer process control stage.The study of lignite isothermal drying was carried out by thin layer drying method through the hot air drying wind tunnel test platform.With the increase of temperature,the difference of drying time caused by particle size was gradually weakened,and temperature became the main influencing factor of drying process.The modified Page model?b?described lignite isothermal thin layer drying process very well.The effective moisture diffusivity of lignite at the temperature of 80-170 ^oC was in the range of 1.103×10^-9-8.937×10^-9 m²/s,and increased with the increase of temperature.On the basis of the study on the drying kinetics of lignite single particle,the lignite bed was divided into a series of small thin elements along the height direction.The coupling of heat and mass tranfer between environmental parameters of gas flow and particle swarm was considered.The drying kinetic model of lignite particles was constructed and the heat and humidity distribution was also obtained.In summary,in this thesis,based on the theory of heat and mass transfer in porous media,the drying kinetics models of lignite were built by introducing the lignite physical properties including the water forms and thermal physical properties.The simulation of drying process can help people to deeply understand the process of heat and mass transfer in drying and provide a theoretical basis for the subsequent efficient dehydration process.