Application of supercritical CO2 fluid extraction technology in dealkalization and purification of high-alkali coal: Extraction process and mechanism

Abstract

The Zhundong region in Xinjiang possesses substantial reserves of high-alkali coal, and its efficient and clean utilization is urgent. Supercritical CO₂ extraction is a promising green technology for low-rank coal upgrading. In this paper, the removal effects of alkali and alkaline earth metals and ash in high-alkali coal under different supercritical CO₂ fluid extraction system were investigated. The evolution law of the microstructural characteristics during the extraction process was analyzed. The removal mechanism of the synergistic extraction system combining supercritical CO₂ fluid and citric acid was elucidated. It is found that the removal of alkali and alkaline earth metals in supercritical CO₂ fluid-citric acid system is the best. The removal efficiency of Na, Ca, K and Mg reaches 82.23 %, 33.13 %, 63.76 % and 58.77 %, respectively, at which time the ash content is only 2.50 %. Metal cations present in the form of carboxylates and phenolates are displaced by H⁺ to form stable structures, that is, the protonation of carboxylates and phenolates, which increases the number of oxygen-containing functional groups. The methyl and methylene are extracted, and the aliphatic and aromatic compounds are decreased. The surface of coal becomes more rough and messy. The specific surface area increases, and the pore volume ratio of micropores gradually decreases, the pore volume ratio of mesopores and macropores gradually increases, and the roughness and complexity of pore surface decrease. The synergistic extraction of supercritical CO₂ fluid with additives is poised to play a pivotal role in the processing and quality upgrading of low-rank coals.