Study On The Dehydration Mechanism And Dynamics Of Shengli Lignite Dried By Different Energy Forms

In this paper, the characteristics of lignite were analyzed and the functional groupand pore structure of dewatering lignite were measured. Dewatering experiments forShengli lignite was carried by different methods. It revealed that contain of oxygenfunctional group and pore structure were changed. Lignite dehydration kinetics wasanalyzed. The heat and mass transfer processes were discussed.It’s obtained that thecharacteristics of thermodynamics and kinetics for lignite dried by different methodsand also the experimental apparatus of lignite dried by hot flue gas at low temperaturewas designed.The oxygen-containing functional group, pore structure and mineral occurrenceform of lignite were analyzed. It turned out that total water content and lower heatingvalue of lignite respectively were28.11%and12.68MJ/kg. FTIR and XPS resultsshowed that it contained a large number of hydrophilic group in lignite, such ashydroxyl, carboxyl, carbonyl and ether oxygen radicals. BET results indicated that theabsorption and desorption curves of lignite with different granularity were typeⅡ andit displayed an increase in hysteresis, particularly at high relative pressures. Thebiggest pore radius of capillary condensation was0.43nm by equation Kelvin. Andthere were differences between the adsorption and desorption curves <1mmand>1mm, because the pore shapes were different. The specific surface area and porevolume of lignite decreased with the increasing of granularity. Pore diameters weredistributed between10to50nm.Dewatering properties of lignite dried by different form of energy andenergy intensity were studied in Chapter3.The results showed that drying curvesdisplayed acceleration and slacken stages. When the energy intensity decreased andthe granularity of lignite reduced, the drying rate increased.But the drying rate oflignite dried at vacuum condtions was greater than the lignite dried by hot air. Withthe increase of the intensity of microwave energy, drying rate of ligniteincreased.Thermal drying lignite strength, microwave drying can reach higher dryingrate in a relatively short time. After drying by heat intensity and microwave energy,the oxygen containing functional groups of lignite surface fell off, such as-OH, C=O and-COOH. The influence of temperature on functional groups removal wasgreater than the time. The absorption and desorption curves of lignite dried bymicrowave were type Ⅲ,where as the absorption and desorption curves of lignitedried by hot air and vacuum were typeⅡ.The pore volume and specific surface area of lignite dried by vacuum and hot air decreased, but the average pore size increased.The pore volume and specific surface area of lignite dried by microwave decreasedwith the time increased. The fractal dimensions of pore structure were2.35-2.5.Dehydration dynamics of lignite dried by different methods were studied inChapter4.The results showed that Logarithmic model were fit for describing thedehydration dynamics of lignite dried by vacuum and hot air. Two Term model wassuited to the dehydration dynamics of lignite dried by microwave. And the correlationcoefficients were more than0.97.The effective moisture diffusion coefficient oflignite dried by vacuum and hot air(0.5-1mm) respectively were4.906×10-12-1.727×10-11m2/s-1and4.84×10-10-7.09×10-9m2/s-1. The moisturediffusion activation energy of lignite dried by vacuum and hot air(0.5-1mm)respectively were18.00kJ/mol and25.24kJ/mol.The effective moisture diffusioncoefficient of lignite dried by microwave increased with the increase of lignitegranularity.The effective moisture diffusion coefficients of lignite dried by differentmethods were mainly influenced by the pore structure. The differences fordehydration kinetics of dried lignite were mainly controlled dehydrationmechanism.The results showed that the driving force of water transfer in lignite driedby hot air were moisture gradient and temperature gradient. The results showed thatthe driving force of water transfer in lignite dried by vacuum were moisture gradient,temperature gradient and pressure gradient. The direction for heat and mass transfer inthe lignite dried by vacuum and hot air were opposite. While the direction for heat andmass transfer in the lignite dried by microwave were the same.Moisture migration for drying lignite was related to the dry wet interface in theparticles. Moisture migration in the capillary of particles was related to the shape ofpore. The capillary potential in the pore was2.68kJ/kg.The contractions in the lignitedue to wet stress and thermal stress during the process of lignite dehydration. Dryingstress was mainly controlled by the moisture gradient on the surface of the particles.The water potential theory was applied to the process of lignite dewatering. Particlesinternal Moisture difference model was obtained. The water content distribution atdifferent time and location were obtained.Moisture re-absorption experiment of lignite dried by different methods werecarried out in the Chapter6.The results showed that the lignite dried by microwavere-absorbed least water.The absorption and desorption curves of lignite dried byvacuum were type Ⅱ.And it displayed hysteresis at low relative pressure.The adsorption potential of dried lignite declined with the increasing of adsorption space.The heat of adsorption on WBHM was most, while FRHM was least. It was mainlyrelated to active sites and oxygen-containing functional group on the surface oflignite.The heat of adsorption of the lignite dried by different methods decreased withthe increasing of the water vapor adsorption amount.The water vapor effectivediffusion coefficient was WBHM>REHM>ZKHMThe drying system of lignite dried by hot flue gas at low temperature wasdesigned.The enthalpies of hot flue gas at different temperatures were calculated. Thewater content of exports of flue gas was3.47%.when the oxygen content of importflue gas was6%, the air leakage coefficient of the drying device was0.15. Accordingto heat balance, the total consumption quantity of heat of drying device was212.95kJ/h, and the consumption amount of the flue gas was2473kg/h.