Heat&Mass Transfer Mechanism Of Microwave Enhanced Drying Lignite In A Fluidized Bed

The verified reserves of lignite had almost occupied 13% of coal reserves of the whole country. Lignite has some disadvantages on high risk of spontaneous combustion, lower calorific value, and high inherent moisture content. Therefore, drying of lignite is the first and essential step for their transportation and storage. Moreover, dewatering of lignite, in other words, increasing their energy density, is needed from the viewpoint of their utilization including coal water slurry, gasification, liquefaction and combustion. In this paper, flow characteristics and drying characteristics of microwave drying lignite in a fluidized bed were investigated. With the impending energy crisis and emission reduction, it will be increasingly necessary to dewater of microwave drying lignite in a fluidized bed dryer. As the large-scale utilization of lignite, the microwave-assisted fluidized bed dryer has good market demand and industrial development prospect in the future.The relationship between the superficial gas velocity and the bed pressure drop was obtained to determine the minimum fluidization velocity of lignite. Experiment results showed that the effects of bed height, air temperature, air velocity and particle diameter on the fluidized bed pressure drop were investigated. The minimum fluidization velocity decreased with an increase in air temperature, and increased with an increase in particle diameter. The Euler-Euler two-fluid model was performed to investigate the fluidized bed. The Syamlal-O’ Brien drag force model and the finite volume method of discrete control equation were studied. The simulated results were fairly in good agreement with the experimental data.The microwave drying characteristics of lignite in a fluidized bed were investigated. Experiment results showed that the drying rate increased with an increase in microwave power, air temperature and air velocity. The drying rate has been less affected by the particle size of lignite. The lignite drying model of different drying conditions was studied. Compared with the fluidized bed drying and microwave drying, the drying rate was significantly improved in microwave strengthen fluidized bed drying. The constant speed drying stage was not obvious in microwave strengthen fluidized bed drying. The dry process was mainly composed of dry preheating drying stage and falling rate drying stages.Based on multiple linear regression method and the least square method, drying model equation of microwave drying in a fluidized bed was built on the particle size microwave power, drying time, air temperature and air velocity. The balance moisture value of lignite was fitting by non-linear curve using different porous material balance moisture experience model. The equilibrium moisture formula calculation equation of lignite was built on the modified GAB model. The relationship between the effective moisture diffusion coefficients with temperature was found based on Fick’s second diffusion law under different particle sizes.The macromolecular basic skeleton structure of lignite was unchanged before and after microwave drying in a fluidized bed. Some changes of the absorption peak intensity of lignite functional groups have taken place. The mole ratio of carbon-carbon bonds or carbon hydrogen bonds was more than raw lignite. The mole ratio of carbon oxygen single bond(including phenolic carbon or ether carbon), carbonyl group and carboxyl group was reduced. The moisture removal rate and crack rate of lignite increased with an increase in air temperature and air velocity. Specific surface area of dried lignite was first decreased then increased with the increase of drying time and microwave power. Total pore volume and average pore diameter was first increased then decreased. Specific surface area was decreased with the increase of air temperature and air velocity. Total pore volume and average pore diameter was decreased.The drying curves of lignite in a fluidized bed were simulated using Fluent software. The simulation results could reasonably fit the experimental data well. The heat and mass transfer equations of microwave drying lignite were dispersed. It is found that the drying rate of lignite was increased with the increase of microwave power. The simulation prediction results could reasonably fit the experimental data well. As the drying process, the experimental value of lignite moisture content was slightly below the prediction value. The Fluent software was used to simulate the heat and mass transfer process of microwave drying lignite in a fluidized bed. The heat transfer coefficients, particle volume fraction and Nusselt number of lignite microwave drying in a fluidized bed were studied from the simulations. The results showed that, the heat transfer coefficients increased with the increasing of microwave power, air temperature and air velocity, while decreased with the increasing of drying time.The effects caused by drying time, microwave power, air temperature and air velocity on the particle sizes distribution and fragmentation rate were studied. The particle size distribution and fragmentation rate of lignite increased with an increase in drying time. The fragmentation rate depended on microwave power, air temperature, air velocity, and final moisture content. The internal stress of lignite increased with the increasing of microwave power, air temperature and air velocity in the drying process. On the other hand, the thermal stress heightened the fragmentation rate in the later stage of drying process. Therefore small particles caused by particle-particle collisions in the fluidization process. Fluidized bed drying technology and microwave drying technology have the high energy consumption. The single microwave drying energy consumption was about 1.7 times the microwave drying in a fluidized bed. Multi-energy field coupling technology for further research was suggested in order to reduce energy consumption of drying lignite process.