| Recently,with the continuous consumption of high quality coal,its reserves decreases.Lignite has been attracting people's attention.Lignite resources in China are abundant.However,its high moisture content results in the lower calorific value,lower combustion efficiency,and higher transportation costs.Therefore,lignite should be upgraded before its large scale utilization.Hydrothermal dewatering?HTD?is a promising upgrading technology,which could effectively remove moisture in liquid form and thus could save the latent heat of evaporation.Thus,in this thesis,lignite would be treated using hydrothermal upgrading technology to explore the effect of hydrothermal upgrading on product distribution and physico-chemical structure and to analyze moisture re-adsorption behavior of upgraded coal.In this thesis,two lignite samples from Inner Mongolia and Yunnan province were selected and hydrothermally upgraded under different temperature,initial pressure and atmosphere.Firstly,the effects of different upgrading conditions on the solid,liquid,and gaseous phases were investigated.Mass distribution of these three phases and migration of C,H,O,N,and S during HTD process were analyzed.Secondly,the evolution of oxygencontaining functional groups and pore structure with different upgrading conditions were studied.And then the main effects of upgrading conditions on these physic-chemical characteristics were analyzed.Thirdly,moisture re-adsorption behavior of upgraded lignite with different dewatering degrees was researched and the mechanism of adsorbing water and influencing factors were explored.The main results are shown as follows.?1?The effects of temperature on solid,liquid,and gaseous phases were higher than those of initial pressure and atmosphere.Increasing the upgrading temperature could decrease the yield of upgraded lignite,the content of inherent moisture,volatile matter,the ratios of O/C and H/C,resulting in the increasing of coal rank.?2?Some gases,e.g.CO2 and CO,would release from lignite during HTD process and their amount increase with the rise of temperature.Some minerals and organics would be obviously decomposed and leached into liquid phase.Therefore,C and O in lignite were transferred in liquid and gas phases.H and S in lignite were mainly transferred in liquid phase,whereas N was mainly released into gas phase.?3?The pore volume and specific surface area of Inner Mongolia lignite increased first and then decreased with upgrading temperature,while those of Yunnan lignite decreased,which were caused by the development and collapse of meso-and macro-pores.?4?Carboxyl and phenolic hydroxyl groups began to greatly decompose when temperature was higher than 200 °C and 230 °C respectively during HTD process and their removal percentages increased with the rise of upgrading temperature.Carboxyl and phenolic hydroxyl could be removed more than 60%.The initial pressure had a positive effect on the removal of oxygen-containing functional groups in coal samples.The removal percentages of carboxyl and phenolic hydroxyl groups increased by about 15% with increasing initial pressure of N2,while content of carboxyl group increased under CO2.?5?The effects of hydrothermal upgrading conditions on the equilibrium moisture content?EMC?of upgraded lignite were significant.Under the same relative humidity,EMC of Inner Mongolia lignite increased first and then decreased with the increase of temperature,and decreased with the increase of initial pressure,while EMC of Yunnan lignite decreased with the increase of temperature.These can be ascribed to the changes of oxygen functional groups and pore structure in lignite.?6?At low relative humidity,the moisture re-adsorption was mainly monolayer adsorption,and the main controlling factor was oxygen-containing functional groups.At high relative humidity,the moisture re-adsorption was mainly multilayer adsorption and capillary condensation,and the main controlling factor was pore structure. |
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