Study On Kinetics And Mechanism Of Lignite Oxy-Cracking In Aqueous To Prepare Humic Acid

As a low-rank coal,lignite is characterized by high water content,high oxygen content and low calorific value.The traditional utilization pathway of lignite mainly for power generation has low efficiency and serious pollution.However,lignite is highly reactive,is rich in hydroxyl,carboxyl and other oxygen-containing functional groups and aliphatic bridging bonds,making it easy to be depolymerized.Therefore,according to the structural characteristics of lignite,the preparation of humic acid by the oxy-cracking of lignite in aqueous is an important way of clean,efficient and resource utilization of low-quality coal.In this work,Xiaolongtan lignite（XLT）was used as raw material,and the influences of reaction temperature,reaction time,coal-base ratio and initial oxygen pressure on conversion rate and product yield of XLT were investigated through liquid phase oxy-cracking.Combined with the composition analysis and structure characterizations of products,the oxy-cracking performance of XLT and its structural correlation were studied.Then,for XLT and Xilinguole lignite（XLGL）,their oxy-cracking kinetics under mild conditions were studied by establishing a model,and the corresponding kinetic parameters were obtained.Finally,using model compounds,the oxidation reactivity of some characteristic structures of lignite including aromatic rings,bridge bonds and oxygen-containing functional groups such as hydroxyl groups,and their interactions were investigated,and the oxidative depolymerization mechanism of lignite macromolecular structure was explored.The results of XLT oxy-cracking showed that XLT has high oxy-cracking reactivity,with coal-based humic acid（HA）as the target product,the optimized reaction conditions of XLT were 80℃,3 h,coal-base ratio of 3/1 and initial oxygen pressure 3MPa.Under the optimized conditions,the XLT conversion and HA yield reached 78.19%and 65.75%,respectively,and CO₂ carbon conversion was 3.08%.Meanwhile,the carbon balance yield of XLT oxy-cracking under the optimized conditions was 95.21%.Kinetic studies of oxy-cracking showed that the main reaction pathways of XLGL and XLT oxy-cracking in aqueous include the series reaction of coal→HA→WSC+Gas and parallel reaction of coal→WSC+Gas.Among them,coal→HA is mainly dominated by oxidative cracking of crosslinked bridge bonds in lignite.By fitting of the built two-component model,Ea of the series reactions of XLGL→HA→WSC+Gas was 19.64 k J/mol and 39.25 k J/mol,respectively,that of XLGL→WSC+Gas was 21.18 k J/mol.Ea of the series reactions of XLT→HA→WSC+Gas was 17.30 k J/mol and 40.75 k J/mol,respectively,and that of XLT→WSC+Gas was 24.18 k J/mol.Among them,the activation energy of the oxidative depolymerization of lignite into HA was significantly lower than that of the further oxidation of HA into WSC+Gas,which helps to inhibit the generation of CO₂.The oxy-cracking results of model compounds showed that the oxidation reactivity of aliphatic bridge bond was high,mainly generating ketones and aromatic aldehydes,aromatic carboxylic acids,etc.Among them,the oxidative polymerization reactivity of ether bond was higher than that of（CH₂）_n.Fused aromatic nuclear is easier than benzene ring.Aromatic anhydride is the main oxidation product of fused aromatics,but anthraquinone is also the main oxidation product of anthracene.The aliphatic side chains are relatively easy to be oxidized,especiallyα-H of the hydrogenated aromatic ring,but those can inhibit the open epoxidation of the substituted aromatic ring.Phenolic hydroxyl group has a significant activation effect on the aromatic epoxidation,while carboxyl groups have no obvious effect on the aromatic epoxidation,except can selectively oxidize the substituted aromatic ring.Therefore,the oxy-cracking of lignite in aqueous under mild conditions are dominated by the oxidation of bridge bonds and aliphatic side chains,which depolymerized the macromolecules of lignite into HA containing phenolic hydroxyl,carboxyl,carbonyl and other oxygen-containing functional groups,and can avoid excessive oxidation to generate WSC and CO₂.