Study On Mass And Heat Transfer Of Coal Drying Based On Nonequilibrium Thermodynamics

Energy is an important foundation for national economic and social development.In China’s energy consumption structure,coal accounts for more than 55%year-round.In 2022,China’s raw coal production increased by 8%year-on-year to 4.45 billion tons,still playing the role of ballast to ensure China’s energy security.At present,due to the decline of coal quality in the eastern coal mining area and the concentration of new coal production capacity in the west,it greatly increases the transportation costs of coastal energy-consuming regions.In order to alleviate the contradiction between China’s coal supply and demand,the development of low-rank coal quality utilization is an essential way.At present,high efficiency separation of low-rank coal above 6mm has been achieved in dry separating.However,the low-rank coal below 6mm is easy to formation liquid bridge between particles in the dry separation process,which increases the adhesion force between particles and forms agglomerates.In the production process,it is easy to cause the blockage of separation equipment,increase the maintenance cost of equipment,reduce the production efficiency,etc.,which becomes a challenging problem for the large-scale quality utilization of low-rank coal.In this thesis,coal samples of Inner Mongolia Shengli lignite（NM,dry basis moisture content 42%）,Yunnan Zhaotong lignite（YN,dry basis moisture content 21%）,and Shanxi Huozhou anthracite（SX,dry basis moisture content 42%）with less than6mm were selected as test materials.The physical analysis and structural characterization showed that the organic element content of the YN samples was higher,with N and S contents of 1.35%and 1.57%,respectively,and more organic functional groups,etc.The NM and YN samples contain fewer organic elements and functional groups.the water contents of the YN,NM,and SX samples were 21.6%,41.90%,and42.16%,respectively,and the true densities were 1.575 g/ml,1.285 g/ml,1.697 g/ml.The dehydration weight loss temperature of each sample was in the range of 75°C-220°C,and the auto-ignition temperature was above 750°C.Therefore,the subsequent tests were conducted at 100°C-180°C as the drying temperature conditions.The thin-layer drying test showed that there were three drying periods for SX and NM samples when the temperature was less than 160°C,namely:rising drying period,constant speed drying period and reduced speed drying period.When the temperature is greater than 160°C,there are only 2 drying periods,which are the rising drying period and the reduced speed drying period.YN samples have only two drying periods at100°C-180°C,which are the rising drying period and the reduced drying period.At the same time,the effect of increasing the temperature on the drying efficiency will become less and less obvious as the temperature increases.And the drying efficiency will be reduced when the organic components are higher,and its volatilization is mainly in the reduced drying period.The larger the particle size,the greater the effective moisture diffusion coefficient and apparent activation energy,and the lower the drying efficiency.The microscopic pore structure changes drastically before and after drying.In the beginning stage the pores gradually increase and become more permeable.After the organic components evaporate,the pore structure collapses causing blockage.After that,new pore structure will be generated.Finally,the pore structure gradually stabilizes and becomes more stable.The fitting simulation results show that the NM and SX samples are suitable for two diffusion models,namely:ΜR=a₁exp（-k₁τ）+(6₂exp（-k₂τ）.The best drying models for YN’s three different particle sizes are all Hill equations,namely:ΜR=a₁exp(-k₁τ⁹))+(6₂exp(-k₂τ⁹)).The R² of the simulation fitting results is greater than 0.99,and the reliability of the fitting results is high.The calculation results of the numerical model based on nonequilibrium thermodynamics are good,and the maximum error between the simulated and experimental values is 25%,the minimum error is 10%,and the average error is 15%using the NM sample as the validation simulation.It is proved that the non-equilibrium thermodynamic mathematical calculation model used in this paper can accurately describe the low-order coal drying process.Meanwhile,the simulation results show that the effect of wind speed on the drying process is much smaller than that of temperature.The optimal drying conditions obtained in this paper are:wind speed of 1.2-1.8 m/s and drying temperature of 140°C-160°C.In the cloud diagram of temperature,moisture and heat transfer distribution,there is a convective exchange region between the particles and the external hot air.During the drying process,the influence of external forces on the drying process is negligible.Based on the pre-operation economic evaluation of the low-temperature drying system with drying temperature of 220℃and air speed of 1.8m/s,it is known that the low-temperature drying condition will reduce the heat supply cost by 8.16 million yuan per year,and the drying system will increase the revenue by 37.7 million yuan per year.As the mine continues to be mined,the production capacity of high water content low rank coal will continue to increase,and the marginal cost of the low temperature drying system will continue to decrease.Comprehensive economic and application benefits,it is feasible and necessary to carry out the design and application of low-temperature drying process.This thesis has 59 figures,19 tables,and 225 references.