Microwave Drying Characteristics Of Shengli Lignite And Its Moisture Re-adsorption Behaviour

Coal is an important primary energy in China.With the gradual reduction of the storage of high-quality coal,the exploitation and utilization of lignite has attracted more and more attention.However,the properties of high moisture content and re-adsorbing water tendency after drying greatly limit its large-scale utilization.Therefore,it is of great significance to efficiently dry lignite and restrain the moisture re-adsorption of the upgraded lignite.Microwave drying is a new drying technology,the heat transfer and mass transfer are in the same direction in materials in the drying process.It has some unique advantages such as high drying efficiency and high selectivity,which makes it used widely in many fields.It is also a hot research to use microwave to dry lignite and restrain the moisture re-adsorption of the upgraded lignite.The mechanism of the effect of microwave on moisture and the effect factors are important for gaining the effective and reasonable technological parameters of upgrading in the large-scale upgrading and utilization of lignite.But there is no systematic and substantive explanation of these in the related researchs,it is necessary to have the further study.In this thesis,Shengli lignite was selected as the study object.First,the effects of microwave power,irradiation time,particle size,sample mass,drying atmosphere and gas flow on the microwave drying characteristics of lignite were investigated.Secondly,The microwave drying kinetics of the coal samples with full water and those only with the molecular water were analyzed and compared.Finally,the impacts of microwave drying on the physical and chemical structure of lignite and the relationship between them and the moisture re-adsorption behavior of the upgraded lignite were analyzed.The main research results obtained are shown as following.（1）In the microwave drying process of the lignite,the drying rate increased with the particle size increasing and decreased with the sample mass increasing,and increased firstly and then decreased with the gas flow increasing in nitrogen atmosphere in the microwave drying processes.The drying rates in differert atmosphere were: carbon dioxide>nitrogen>air.From operating conditions aspects,for the microwave drying of lignite,it is suggested that microwave power lower than “saturation power”,suitable particle size,suitable sample mass and gas flow should be elected and the drying process should be conducted in the atmosphere containing nitrogen.（2）The first-order reaction kinetic equation and Two-term exponential model could describe the constant power microwave drying processes of MWC and RC respectively.The effective diffusion coefficient and drying rate constant of RC increased with the increase of particle size,and decreased with the increase of the sample mass.In order to reduce the energy consumption,for the microwave drying of lignite,the free water and capillary water could be removed firstly under microwave power higher than 550 W,and then the molecular water should be removed under the microwave power in the range of 500⁵50 W.（3）The activation energy for removing the molecular water during microwave drying process is 28.59 kJ·mol^-1,and that for dewatering the full moisture from RC is 24.25 kJ·mol^-1.Compared with the conventional hot air drying,the average activation energy of dewatering the full moisture in the microwave drying process is smaller,which may be caused by the nonthermal effect of microwave.（4）Microwave treatment could sharply decrease the pore volume and specific surface area,and efficiently decomposed the oxygen-containing functional groups of lignite and effectively restrain the upgraded lignite re-adsorbing moisture.The pore volume of the upgraded lignite decreased firstly and then increased with the increase of microwave power and drying time.The pore volume and specific surface area of the upgraded lignite were smallest when the microwave power was 600 W and the drying time was 15 min.For lower relative humidity,the oxygen-containing functional groups and specific surface area were the main factors determining the equilibrium water quantity,while for higher relative humidity,the pore volume was the main factor determining the equilibrium water quantity.