Physicochemical Properties and Slurry Ability of Lignite Upgraded by a Solvent-Extraction Approach

Abstract

Supercritical carbon dioxide (scCO2)/coal tar/N-methyl-2-pyrrolidinone (NMP) mixed solvent was used to upgrade lignite by extraction in a semicontinuous supercritical extraction device. The upgraded lignite has potential applications in coal–water slurry (CWS) gasification. The effects of the process conditions (operating temperature, pressure, solvent ratio, and particle size) on the physical and chemical properties of the upgraded lignite were studied. The Brunauer–Emmett–Teller specific surface area and the Barrett–Joyner–Halenda absorption pore volume of the upgraded coal increased to approximately twice those of the raw lignite. The BJH adsorption average pore diameter of the upgraded coal was smaller than that of raw lignite. The heavy constituents in coal tar were absorbed into the coal pore structure, and the low-molecular-weight compounds in lignite and coal tar could be extracted. The results showed that upgraded lignite with a better slurry ability was achieved by extraction with the scCO2/coal tar/NMP mixed solvent. The upgraded CWS concentration increased by 6–10 %. The flowability of the upgraded CWS improved slightly. After upgrade through the scCO2/tar/NMP process, the pseudoplastic-fluid trend of the upgraded CWS improved, and the lignite gasification reactivity improved.