Kinetic Analysis And Resolving Of Main Factors Influencing Moisture Re-adsorption Of Upgraded Lignite

As the continuous increase of primary energy consumption, the reserve ofhigh quality coal has dropped significantly. Therefore, exploiting low rank coalsespecially the lignite in a large scale, is of great significance to relieve the stressof resource shortage and environmental pollution. The high moisture content inlignite results in high transportation and low conversion efficiency, and then thelimitation of its utilization. In order to achieve the maximum energy efficiency,upgrading of lignite is necessary before its large-scale utilization. However, theporous physical structure and abundant oxygen-containing functional groups ofupgraded lignite lead to its high surface activity and moisture re-adsorptiontendency. Therefore, it is necessary to explore the internal influence mechanismof pore structure and oxygen-containing functional groups on moisturere-adsorption of upgraded lignite. And then keep the surface of lignite relativelystable during its storage, which would theoretically guide the upgrading oflignite.In this thesis, XM lignite was selected and treated with thermal upgradingprocess. The characteristics of pore structure, amount of oxygen-containingfunctional groups and moisture re-adsorption behavior of the upgraded lignitewere investigated. The influence of pore structure and oxygen-containingfunctional groups on moisture re-adsorption amount of lignite was analysed.Moisture adsorption isotherms and kinetics of upgraded lignite were also studied.Moisture adsorption mechanism of upgraded lignite was explored preliminarily. The main results obtained are shown as follows:(1) The changes of pore structure of lignite during heat treatment aremainly the shrinkage and collapse of macro-and mesoporous. The shrinkageand collapse of pore structure can lead to the decrease of hydrophilicity ofupgraded lignite. The water-holding capacity of dried coal from pore structuredecreases as the drying temperature increases. And the contribution toequilibrium moisture content from pore structure is relatively high when theupgrading temperature is over280oC.(2) By the method of chemical aqueous titration, amounts of the fourkinds of oxygen-containing functional groups in raw lignite and dewatered coalare examined quantitatively and the sequence of their amount is: phenolichydroxyl group> carbonyl group> carboxyl group> methoxyl group among thewhole drying process. The stabilities of these oxygen-containing functionalgroups are different in the process of heating lignite. Methoxyl and carboxylgroups firstly begin to be decomposed at about160oC, carbonyl groups arestable up to200oC and phenolic hydroxyl groups start to breakdown graduallyat above280oC. The hydrophilicity order of oxygen-containing functionalgroups is: carboxyl group> phenolic hydroxyl group> carbonyl group>methoxyl group. In addition, the hydrophilicity of the same kind ofoxygen-containing functional groups in dewatered coal obtained at differenttemperature is different, decreasing dramatically for carboxyl groups andsmoothly for methoxyl and carbonyl groups with the increase of dryingtemperature. And the contribution of oxygen-containing functional groups isrelatively high when the upgrading temperature is lower than280oC.(3) The adsorption isotherms of water vapor on coal samples can beconsidered as typeⅡin the BET classification. The results show that themodified BET model is suitable for the isothermal adsorption process. Themonolayer adsorption capacity and the primary adsorption heat calculated fromthe modified BET model are related to the content of the oxygen-containing functional groups. The value of the second sites adsorption heat is close to theheat of water liquefaction for the same temperature.(4) During the moisture re-adsorption process, the diffusion coefficient ofmoisture in coal particles is significantly influenced by the adsorptiontemperature. The relative humidity has no obvious effect on diffusion coefficient.There is a positive correlation between the diffusion coefficient and mostprobable pore size of lignite. Rate constants of water vapor adsorption onupgraded lignite increase with the increase of adsorption temperature anddecrease of relatively humidity. Moreover, the intensification of drying extentcan reduce the rate constants.(5) For the relative humidity of0.75, the activation energy of water vaporon DCT120t60and DCT240t60is45.51kJ/mol and18.85kJ/mol, respectively.