| Spontaneous combustion of lignite frequently happens during the process of mining,transportation and storage.It not only comsumes large amounts of lignite resource,but also threats the safety of workers and facilities.In order to ultilize the lignite resource extensively,it is important to preserve lignite appropriately and keep it away from spontaneous combustion.In this study,spontaneous combustion of lignite was investeged.Effect of upgrading treatment and inhibitors on spontaneous combustion were investegeted by experimental researches.In addition,the structure characters of active sites and the corresponding mechanism of spontaneous combustion were also studied using density functional theory(DFT).Characters of Ximeng lignite developed towards high-rank coals after hydrothermal dewatering treatment(HDT).As the upgrading temperature increased,content of volatile decreased and the fixed carbon content increased;oxygen-containing groups and aliphatic hydrocarbons decreased,while aromatic carbon relatively increased.The influence of HTD on pore structures concentrated in mesopores.As upgrading temperature increased,the specific surface area and total pore volume initially increased and then decreased gradually.Spontaneous combustion propensity(SCP)of HTD-treated linigte depended on upgrading temperature.When the upgrading temperature was lower than 260℃,SCP of lignite increased because pore structures developed significantly.However,when the upgrading temperature exceeded 260 ℃,changes in chemical composition and functional groups standed out and became the leading factor instead of pore structure,resulting in a decrease in the SCP of lignite.A short-term dehydration was applied to Ximeng lignite with microwave irradiation.When the irradiation time was less than 1.5min,moisture content significantly decreased and pore sturcutres developed,leading to an increase in SCP of lignite.While,when the irradiation time exceeded 1.5min,SCP of lignite was reduced because the coal quality of lignite changed significantly and that pore structures were reduced due to pyrolysis of coal matrix.There is a critical irradiation time of microwave irradiation treatment,beyond which moisture could be efficiently removed without increasing the SCP of lignite.Therefore,it is suggested to appropriately prolong the irradiation time for the sake of safe storage of upgraded lignite.A series of molecular models of lignite were constructed,based on which the reactivity of active sites during coal oxidaiton as well as the corresponding reaction mechanism were investigated.Coal oxidation went on through a sequence of free-radical chain reactions,which occurred on-CH2-/-CH3 at coal surface.When being adjacent to benzene rings or oxygen-containing functional groups(hydroxyl,carbonyl,ether,ect.),bond strength of C-H was reduced due to induction or conjugation effects.A significant linear relationship was found between the bond dissociation energy(BDE)of C-H in alkylaromatics and its reation activation energy(△E)for oxidation.According to the linear equation,△E of C-H in alkylaromatics could be predicted on account of BDE.Based on the value of △E,the reactivity of functional groups in coal for oxidation could be ranked in the following order:-CH2-/-CH3 at the β or y sites in the alkyl side chain;-CH2-located in the a position of benzene rings or ether bond and alcohol hydroxyl;-CH2-/-CH-simultaneously attached to multiple benzene rings or oxygen-congtaining functional groups,which mainly corresponds to the bridge bonds and aldehyde groups in coal.Conversion of ROO· to ROOH is mainly carried out by intra-molecule hydrogen transfer with a six-membered ring transition state or inter-molecule hydrogen transfer between adjacent side chains.H2O is generated by the exothermal reaction between HO*and the active sites on coal surface,and the activation energy of reaction is 20.11-23.07kJ/mol.CO(CO2)released during coal oxidation is generated from two reaction channels:concerted decarboxlation/decarbonylation or stepwise reaction.Activation energy required for stepwise reaction is much lower,thus most of the CO/CO2 is generated through it.C2H4/C2H6 is not resulted from the pyrolysis of alkylaromatics in coal but the decomposition of RO·,which is the intermediate production of spontaneous combustion.According to the results of DFT study,an improved reaction mechanism of spontaneous combustion was suggested.Experimental research and DFT study were applied to investigate the role of H2O in lignite oxidation.Experimental results showed that H2O would inhibit the oxygen consumption reaction during lignite oxidation and promoted the reactions to generate more CO2.H2O exerts its effect on coal oxidation through two patterns:1,changing the reaction rate through kinetics solvent effect without transforming the structure of transition state;2,forming a multy-membered transition state with reaction centre atoms and acting as a agent for proton transfer.The DFT sdudy results showed that H2O had a suppression effect on reaction of O2 with oxygen-containing functional groups.The promotion effect of H2O on coal oxidation could be explained as follows:eccelerating the rates of free-radical reactions through reducing the activation energy of reactions such as hydrogen-transfer and hyperoxide composition;promoting the regeneration of new active sites by catalyzing the decomposition of oxygen-containg functional groups.Na3PO4,(NaPO3)6,Zn3(PO4)2,BHT and sodium lignosulfonate were mixed with lignite by direct-mixing and solution-mixing.Afterwards,their inhibiting effects on spontaneous combustion of lignite were evaluated.Zn3(PO4)2 showed little inhibiting effect on spontaneous combustion as it only acted as inert minerals in lignite.BHT、(NaPO3)6 and sodium lignosulfonate showed a significant surpression effect on spontaneous combustion of lignite.BHT was only efficient at low-temperature.when heated to high-tempeture,it would accelarate the oxidation of lignite because of the rapid oxidation of BHT itself.(NaPO3)6 and sodium lignosulfonate were massively asorbed on lignite surface.They decreased the spontaneous combustion propensity of lignite mainly by decreasing the pore structions and agglomerating coal particles.Na3PO4 could accelerate the production of CO2 druging coal oxidation.When mixed with lignite by direct-mixing,Na3PO4 could increase the SCP of lignite.Howerever,if mixed with lignite by solvent-mixing,Na3PO4 could slightly inhibit the spontaneous combustion of lignite by decreasing the pore structures of lignite. |
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