Functionalized ionic liquids for lignite dissolution and treatment

Abstract

AbstractBACKGROUNDAs a low‐rank coal, lignite (frequently referred to as ‘brown coal’) has a high latent moisture content and low energy density and thus has low value as a solid fuel, often being burned at power stations close to where it is mined. For this reason, an effective pretreatment is essential to the better utilization of low‐rank coal for energy and materials. Ionic liquids (ILs) are alternatives to conventional volatile organic solvents for pretreating coal due to their tailorable structures and physical properties, as well as their ability to partially dissolve/swell coal components. This study aimed to rationally design ionic liquids having a high hydrogen‐bond basicity, based on the acetate anion, paired with ether‐functionalized cations (especially the less‐expensive piperidinium) for lignite dissolution and treatment.RESULTSLignite treated by these functionalized ionic liquids at 105 °C led to the dissolution of 15.7–24.5 wt% lignite with a concomitant increase in coal aromaticity and rank, a reduction in coal decomposition residues, and the production of smaller and more uniform particles. The ether‐functionalized N‐(2‐ethoxyethyl)‐N‐methylpiperidinium acetate is a particularly attractive alternative to conventional imidazolium ILs due to its lower cost, high hydrogen‐bond basicity, and a strong capability for improving the coal aromaticity.CONCLUSIONOur results have demonstrated that pretreatment with functionalized ionic liquids can improve the coal aromaticity and rank, decrease coal decomposition residues, and yield smaller and more uniform particles, which enables a better utilization of low‐rank coal for fuel and materials. © 2021 Society of Chemical Industry (SCI).