Green two-step oxidation for efficient utilization of Huolinhe lignite under the vision of carbon neutrality

Abstract

To realize carbon neutrality in energy transition process, a two-step oxidation of Huolinhe lignite (HL) with less CO2 emissions is proposed, including pretreatment with H2O2 (PT) or pretreatment with Fenton’s reagent (PTF) and further oxidation in aqueous sodium hypochlorite (ASHC). The results indicate that the insoluble macromolecular moieties in HL are mainly oxidized into alkanedioic acids (ADAs) and non-substituted benzenecarboxylic acids (NSBCAs) and the yield of soluble portion is elevated by PTF from 60.56 % to 74.99 %. The increase in the ADA carbon number indicates that PT and PTF could enhance the oxidation ability of ASHC to break bridged linkages with longer chains. As for NSBCAs, the carboxylic group number (CGN) ranges from 2 to 5 in E1 and 2 to 6 in E2 and E3, respectively. The absence of mellitic acid in E1 states the limited oxidation capacity of ASHC to degrade highly condensed aromatic rings, which could be improved by PT and PTF. Moreover, the highest yield of NSBCAs is achieved at CGN = 2 in E2 and CGN = 4 in E3, implying that PTF is more effective to destruct highly condensed aromatic species in HL. According to the characterizations of coal samples, PT could increase the volatile matter and oxygen contents of HL and create new micropores to corrode HL viaOH attack, which would be enhanced by the introduction of Fe2+ during PTF to break weak covalent bonds, resulting in the preliminary degradation of macromolecular networks in HL. Then, cleavage of strong bonds undergoes easily via the attack of O2− generated from ASHC, leading to the deep decomposition of macromolecular moieties in HL.