Effects Of Upgrading On The Physico-chemical Structures Of Lignite And The Correlations With Its Surface Stability And Combustion Reactivity

With the rapid development of society and economy in China,the utilization of lignite with relatively rich resources has been paid more and more attention.Combustion is one of the main ways to utilize it.The high water content,low calorific value and high spontaneous combustion tendency of lignite lead to some issues such as high transportation cost and low utilization efficiency during its utilization process.Drying and upgrading could reduce the water content and increase the calorific value of lignite,which are important aspects for its effective conversion.However,the structural characteristics of lignite that are porous and rich in active functional groups make its poor surface stabilities.For lignite,it is easy to adsorb water and oxygen molecules,resulting in the quality decline and spontaneous combustion.The physico-chemical structures of lignite could change with the drying process,which directly affects its behavior of surface stability and combustion reactivity.It is the key of lignite utilization to master the essence of relationship between lignite upgrading,physico-chemical structures regulating,surface stability and combustion reactivity.This thesis aims to explore the effect of drying and upgrading on physico-chemical structures changes of lignite and the effect mechanism of its correlation with surface stability and combustion reactivity.The results are expected to provide theoretical guidance for the safe and efficient use of lignite,and to promote its large-scale utilization.Hydrothermal dewatering,fixed bed drying and microwave drying technologies are applied to upgrade lignite samples.The evolutions of surface functional groups and pore structure of lignite samples in these three processes are focused on.The change rules of water re-adsorption,spontaneous combustion tendency and combustion reactivity of upgraded samples are revealed.Yunnan lignite and Inner Mongolia lignite with different degrees of metamorphism are selected as raw materials.Upgraded samples are prepared in the hydrothermal reactor,fixed bed reactor,and microwave reactor under different conditions.The changes of composition,chemical and pore structures of upgraded samples are characterized by proximate and ultimate analyses,FT-IR,N₂ adsorption and other methods.The water re-adsorption and low-temperature oxidation experiments are conducted on different upgraded samples to explore the effects of chemical and pore structures on its surface stabilities.The combustion experiments are performed to investigate the influence of different upgrading methods on the combustion reactivity of lignite.By analyzing the combustion reactivity parameters of hydrothermally upgraded samples,the influence mechanism of the change of aromaticity and pore structure on the combustion stage is revealed.The relevant research contents and main conclusions are shown as follows.(1)The water distribution of hydrothermally upgraded lignite samples and its water re-absorption characteristics are investigated.The results show that the water removed from Yunnan lignite and Inner Mongolia lignite in the hydrothermal upgrading process mainly includes molecular water,capillary water and free water,among which the removal rate of molecular water is higher than that of capillary water and free water.Hydrothermal upgrading decomposes a large number of oxygen-containing functional groups in lignite,reduces the water adsorption sites,resulting in the decrease of molecular water content and part of capillary water and free water.Compared with Inner Mongolia lignite,more water is removed from Yunnan lignite with a lower degree of deterioration.Hydrothermal upgrading can effectively reduce the equilibrium moisture content of lignite and inhibit its water re-adsorption.The reduction of oxygen-containing functional groups in lignite during hydrothermal upgrading can decrease the first type of water adsorption capacity,and the change of pore structure can affect the second type of water adsorption capacity.The most important measures to control the water re-adsorption are firstly to decompose the hydrophilic oxygen-containing functional groups in lignite,which can reduce monolayer adsorbed water and multilayer adsorbed water,secondly to destroy the meso-pore structure,which can reduce the formation of capillary condensed water.(2)The low-temperature oxidation behavior and spontaneous combustion tendency of hydrothermally upgraded lignite samples are analyzed.The results show that the low temperature oxidation process of coal at 30–150?can be divided into two stages:slow oxidation and rapid oxidation.In the slow oxidation stage(30–65?),-CH₃ and-CH₂-in coal absorb oxygen to form peroxides,and further generate oxygen-containing groups such as>C=O,-CHO,and-COOH.At this stage,the released heat from coal oxidation is low and the coal temperature changes little.With the temperature rising to more than 65?,the rapid oxidation stage is reached.Oxidation reaction of-CH₃ and-CH₂-is accelerated,and the new and the original oxygen-containing groups are decomposed to produce CO and CO₂ and other gas products.When the coal temperature rises to 90?,-OH and-COOH will be dehydrated to form ester,two-COOHs can also be dehydrated to form anhydride,and Ar-OH can be oxidized to quinones.When the coal temperature rises to 120?,the oxidation reaction takes place violently and generates a large amount of heat.Active functional groups,free radicals and pore structure are important factors affecting the low-temperature oxidation of lignite.Among them,the active functional group is one of main causes.The spontaneous combustion tendency of hydrothermally upgraded samples from Inner Mongolia lignite varies with the upgrading temperature.When lignite is hydrothermally upgraded at 230?,a small amount of oxygen-containing functional groups(-COOH and Ar-OH)are decomposed,but the development of pore volume and specific surface area makes the active functional groups easier to contact with oxygen,resulting in that the tendency towards oxidation and spontaneous combustion increasing.When lignite is hydrothermally upgraded at 300?,a large number of oxygen-containing functional groups(-COOH and Ar-OH)are decomposed,and some aliphatic side chains are broken.At the same time,the pore volume and specific surface area decrease.So the low-temperature oxidation and spontaneous combustion tendency are reduced for it.(3)The physico-chemical structures and surface stabilities of fixed bed dewatered and microwave dewatered lignite samples are investigated.The results show that oxygen functional groups of 120? fixed bed dewatered sample are unchanged.For 160? fixed bed dewatered samples,there is a small amount of-COOH decomposed,and the change of other groups is small.The increase of micro-pores leads to the increase of specific surface area,while the decrease of meso-pores and macro-pores leads to the decrease of pore volume.For 500 W and700 W microwave dewatered samples,contents of-COOH and Ar-OH significantly decrease;and the meso-pores and macro-pores decrease significantly,resulting in the decrease of specific surface area and pore volume.The equilibrium moisture contents of Inner Mongolia lignite under relative humidity of 11.3%,50.0%and 95.0%are all decreased after fixed bed and microwave drying processes.The decrease in the equilibrium moisture content of the fixed bed and microwave dried samples under low relative humidity is mainly due to the reduction of oxygen-containing functional groups,and the decrease under high relative humidity is mainly due to the decrease of oxygen-containing functional groups and pore volume.Since the influence of microwave drying on the oxygen-containing functional groups and pore structure of Inner Mongolia lignite is greater than that of fixed bed drying,and the decrease in equilibrium moisture content of the microwave dried samples is more significant,microwave drying has a more effective inhibitory effect on the water re-adsorption.Fixed bed and microwave dewatered samples releases less CO₂ in the process of low-temperature oxidation,and their temperatures of critical spontaneous combustion are slightly lower than that of raw coal.(4)The combustion reactivity of different upgraded samples and its correlation with the physico-chemical structures are revealed.The results show that fixed bed drying,microwave drying and hydrothermal upgrading all improve the combustion reactivity of Inner Mongolia lignite,and the effect of hydrothermal upgrading is more significant than the other two methods.The combustion process of raw lignite and hydrothermally upgraded samples can be described as three stages:water evaporation stage,coal devolatilization stage and char combustion stage.The oxygen-containing functional groups in coal mainly affect the heat release in the water evaporation stage and devolatilization stage,and the aromaticity mainly affects the intrinsic activation energy in the char combustion stage.Hydrothermal upgrading increases the aromaticity of Inner Mongolia lignite and increases the intrinsic activation energy of the combustion stage.The change of pore structure mainly affects the behavior of gas diffusion.The large pore volumes and high specific surface areas of 230–300? hydrothermally upgraded samples reduce their diffusion activation energies,while the pore volume and specific surface area of 330? hydrothermally upgraded sample is smaller,which increases its diffusion activation energy.For hydrothermally upgraded samples,the influence of pore structure changes on the combustion process is greater than that of aromaticity changes.