Fundamental Study On Drying And Upgrading Of Lignite By Microwave Irradiation Treatment

In order to utilize the lignite resources cleanly,efficiently,and at a reduced cost,it is necessary to dehydrate and modify lignite prior to its deep processing and further utilization.Microwave drying technology offers some unique features,i.e.,selective,volumetric and instantaneous heating,which makes it being a promising drying approach.Therefore,the moisture within the lignite can be removed quickly and efficiently and the drying time can be shortened significantly under microwave irradiation,which is beneficial for the large-scale industrial application.Effect of microwave drying on the thin-layer drying characteristics,grindablity,pore structures and slurry ability of Ximeng lignite(XL)were studied in this thesis.Moreover,experimental research and numerical simulation were applied to investigate the heating uniformity of lignite exposed to microwaves that can lead to the poor end quality and safety concerns.The controlling mechanism in the hot spot effect of lignite during microwave processing was discussed and some suggestions were proposed for improving the heating uniformity of lignite under microwave irradiation.During the microwave drying process,the mechanism of moisture migration within lignite differed at different drying periods.Most moisture was quickily removed from the lignite in the form of pressure-driven jet flows during the primary drying period,therefore,the drying rate was controlled by the steam jet pressure.However,the residual moisture migrated from the inside of the lignite to the surrounding by thermal diffusion during the later stage of drying,and the drying rate was mainly affected by the temperature.The increases in the microwave power level,particle size,and initial sample mass were beneficial for the generation of high-pressure jet flows,which improved the maximum drying rate.While the final temperature of lignite increased with the increase in the microwave power level and initial sample mass,or with the decrease in the particle size,which helped to enhance the effective diffusion coefficient of moisture.Drying treatment for a relative long period could improve the grindability of XL and increase the mass fraction of finely ground product.The controlled mechanisms that improved the grindability of XL treated by different drying methods included the removal moisture and the physical structure damage induced by thermal stress or steam jet flows.In comparison to other drying methods,microwave drying had the most remarkable effects on the grindability and mass fraction of the finely ground product of XL because of the rapid removal of moisture and serious physical structure damage induced by steam jet flows.The economy evaluation at lab scale indicated that the treatment of XL with high moisture and coarse particle size by microwave drying for a short period could achieve energy saving after a long period of the grinding process,which had some potential in industrial application.The BET surface area of microwave-treated XL differently decreased,whereas its average pore diameter and total pore volume differently increased.The effects of macropores on the BET surface area and total pore volume of XL were significantly increased after the microwave drying treatment,while the effects of meso-and micro-pores accordingly decreased.The effects of microwave non-isothermal drying on the microscopic pore structures of XL were much more remarkable than those of microwave isothermal drying.However,the dominant mechanisms that controlled the development of pore structures during microwave drying were similar.These mechanisms included the quick removal of moisture,thermal stress induced by the non-uniform heating,and thermal decomposition of organic macromolecular structures under a high temperature.Two fractal dimensions,D1 and D2,at relative pressures of 0 to 0.5 and 0.5 to 1,respectively,were calculated with the fractal Frenkel-Halsey-Hill model.The value of D1 is mainly affected by the influence of meso-and macro-pores with an average pore size range of 10 nm to 60 nm on the specific surface area;therefore,D1 can be utilized to quantitatively describe the surface roughness of these meso-and macro-pores in coal.Meanwhile,the value of D2 is mainly related to the effects of fine mesopores with an average pore size range of 2 nm to 10 nm on the total pore volume;therefore,D2 can be utilized to quantitatively describe the volumetric roughness of these mesopores in coal.Experimental research and numerical simulation were applied to investigate the heating uniformity of lignite exposed to microwaves for the first time.Experimental results indicated that there were significant differences in the temperatures in different parts of the lignite layer and the drying conditions had important effects on the temperature variations in different parts.When the lignite layer with low moisture and fine particle size was placed in the strong electromagnetic field region of the oven cavity,obvious hot-spot effect would appear in the lignite layer under high-power microwave irradiation.The numerical simulation results shown that the hotspot effect appeared in the lignite particle under microwave irradiation obviously affected by the operating conditions of microwave oven and the physical structure of lignite particle.The focusing effect appeared in the large lignite particle and the edge/corner effect appeared in the square particle were got verified during microwave drying process with the simulation results.The verification experiment revealed that the temperature variation of lignite particle at the fast-rate drying period could be properly predicted with the mathematical model under microwave irradiation.Although the moisture migration and accumulation within lignite could affect the temperature distribution of the lignite,the high temperature region within lignite was mainly controlled by the electromagnetic field intensity.Namely,the high temperature region within lignite located at the strong electromagnetic field area.Therefore,in order to improve the heating uniformity of lignite and to reduce the hotspot effect during microwave drying processing,measures were needed to ensure the uniform absorption of microwave energy within lignite and to prevent the high temperature region within lignite from overheating.With the increase in the irradiation time and microwave power level,the final temperature of XL increased,resulting in the improvements in coal properties,hydrophilic oxygen-functional groups,surface characteristics,particle size distributions,and pore characteristics of XL.Therefore,the slurry ability of microwave-treated XL was improved.However,the unit energy cost for lignite dehydration under microwave irradiation increased with increasing irradiation time and microwave power level,it is necessary to reasonably operate the irradiation time and microwave power level to balance the energy cost and end quality in industrial application.