Numerical Simulation Of Drying Behavior And Mass And Heat Transfer Of Lignite By Hot Air Fluidized Bed

Due to high moisture, low calorific value, easy weathering and self- ignition, the application of lignite from Inner Mongolia east region has been limited, so the drying for lignite upgrading is particularly urgent. C urrently, fluidized bed drying technology has been applied to lignite drying, but most studies is only limited to traditional experimental stage, and strong nonlinearity in gas-solid two-phase and complexity of heat and mass transfer make experimental process restrained.With the advent of CFD theory, it has been use to obtain data which is difficult to be measured in the experiment process, and it can provide a more accurate and thorough theoretical basis for the study of drying the fluidized bed drying by combining with the experiment. Based on the previous experiments which have been conducted on fluidized bed of hot air drying,this article uses Fluent software and Euler two- fluid model to do 2D numerical simulation for lignite drying process. The effect of drying conditions on fluidization characteristics, drying characteristics and heat and mass transfer has been studied.Flow characteristics simulation results showed that the bubble growth, coalescence and rupture process were main factors affecting the flow of the process; the critical flow velocity could be determined from the pressure drop curve, and it increased with the increase of particle size. but initial bed height had no effect on critical flow velocity. The simulation results agreed well with the experimental result.Lignite drying characteristics of simulation and experimental results were consistent, Reducing grain size, increasing inlet air temperature could make drying rate increase, heat and mass transfer enhanced, the influence of air temperature on the drying rate was the largest. With the decreasing of grain moisture content in the later stage, drying rate slowed and heat transfer coefficient reduced. During the drying process granular temperature was rising and the final temperature tended to hot air temperature; humidity distribution can well showed the removal of water in fluidized bed.Through user-defined functions added microwaves into the fluidized bed drying process, the drying effect got better with microwave strengthen than pure hot air drying. The influence of microwave power on the drying characteristic was very significant, increasing of microwave power could accelerate drying rate and enhance heat and mass transfer significantly, but the effect of microwave weakened with moisture reduction in the later stage. The rate of lignite particles temperature rose accelerated because of microwave effect, the temperature of particles was much higher than the hot air temperature at the end of drying.