Research On The Oxidizing Depolymerization Of Low Rank Coal And The Alkali Dissolution Of Oxidized Residual Coal

Oxidizing depolymerization is an important approach to high-efficiency utilize ligniteby gradual conversion. In this paper, Xianfeng and Shenli lignites were used to carry outthe experiments of oxidizing depolymerization by H2O2aqueous, and subsequent alkalidissolution of oxidized residual coal. Their products at different levels were separated andcharacterized. The influences of main reaction conditions on the conversion properties oflignite were investigated. The mechanisms of gradual conversion of low rank coal, basedon the oxidizing depolymerization and alkali dissolution of oxidized residual coal, werefinally proposed.Results indicate that the oxidizing depolymerization performances of coal depend onits metamorphic grade. As a low rank coal, lignite shows high reactivity of oxidizingdepolymerization and yield of water-soluble products, but the sub-bituminous has a lowconversion and hardly yields substantial water-soluble products. The conversion of coaloxidation increases with the increasing reaction temperature, concentration of H2O2aqueous and reaction time, respectively. However, the yield of water-soluble productdecreases under high reaction conditions because these water-soluble products are easy tobeing further oxidized and decomposed. The oxidizing reaction mainly happens in thealiphatic structures to rupture the bridge bonds between aromatic nucleuses and theside-chain substituents, so that the products obtained contain more functional groupscontaining oxygen such as carboxyl and phenolic hydroxyl. Meanwhile, the acidic groupsin water-soluble oxidation product are more than those in oxidized residual coal. Theymainly consist of polycarboxylic acids and polyhydric phenols. For example, substantialmalonic acid and succinic acid predominate in the small molecular polycarboxylic acidsdetermined in the water-soluble product from the oxidation of Xianfeng lignite, andpolyhydric phenols mainly consist of phenolic compounds with a certain amount ofnaphtholic compounds. The oxidized residual coal shows a low atomic ratio of H/C andhigh content of phenolic hydroxyl. Since the network structure of macro-molecule wasdeploymerized by the rupture of the bridge bonds between aromatic unites, substantialacidic oxidation products with poor water-solubility could be further dissolved in dilutealkaline solution. So the oxidized residual coal shows higher alkali dissolutionperformance than raw coal, for example, the alkali dissolution yield and total conversion ofShengli lignite by oxidizing depolymerization and subsequent alkali dissolution couldreach58.2%and79.6%, respectively. Therefore, it is expected to achieve low rank coal gradual conversion and high value-added utilization by selective oxidizingdepolymerization combined with alkali dissolutionAbove work provided some basic researches for the further separation andpurification of oxidizing depolymerization and alkali soluble products, and has alsoimportant reference value for the studies on coal structure and multi-level utilization.