Analysis of Ionic Liquid Pretreated Low-Rank Coal via Equilibrium-Based Gasification Model

The utilization of vast coal deposits around the globe as an energy source is significantly limited in scope owing to environmental risks. In this context, ionic liquids (ILs) provide a novel solution as eco-friendly pretreatment agents. This work examined the effects of IL pretreatment on the gasification of low-grade coal using a simulation model in Aspen Plus®. Four ILs, 1-butyl-3-methylimidazolium chloride [Bmim][Cl], 1-ethyl-3-methylimidazolium chloride [Emim][Cl], trihexyltetradecylphosphonium chloride [P66614] [Cl], and tributylmethylammonium chloride [N1444] [Cl], were employed in this study. A comparative and sensitivity analysis was conducted on untreated and IL-pretreated coal samples using an equilibrium-based steam-O2 gasification model designed in Aspen Plus®. The model was validated and utilized to vary the oxygen/fuel (O2/F) ratio, steam/fuel (S/F) ratio, and temperature within the specified ranges (700-950°C, 0.1-0.6, and 2-4, respectively). The results showed that all the ILs increased CO and H2, and decreased CO2 and CH4, in the syngas produced by coal gasification. The biggest change was affected by [P66614] [Cl], which increased CO and H2 from 12 mol% and 42 mol % to 16 mol% and 50 mol %, respectively, and reduced CO2 and CH4 from 35 mol% and 3.5 mol% to 29 mol% and 2 mol%, respectively. The lower heating value (LHV) of syngas was increased from 7.37 MJ/Nm³ for untreated coal to 7.61, 7.56, 8.1, and 7.56 MJ/Nm³ for coal treated by [Bmim][Cl], [Emim][Cl], [P66614] [Cl], and [N1444] [Cl], respectively. As opposed to raw coal, [Bmim][Cl] and [P66614] [Cl] generated more CGE, ENE, and EXE. The highest CGE, ENE, and EXE of 19, 13.3, and 38.3% were achieved by [P66614] [Cl] treated coal. Syngas having higher H2/CO ratio and LHV were produced at low temperatures, low O2/F ratio, and high S/F ratio. The results of this investigation demonstrated that ILs composed of ammonium and phosphonium might be useful coal pretreatment agents in thermochemical conversion processes.