Abstract
This paper aims to investigate the impact of ionic liquid (ILs) pretreatment on low-grade coal for the thermochemical conversion process. Two imidazolium-based ILs, 1-butyl-3-methylimidazolium chloride [Bmim][Cl], and 1-ethyl-3-methylimidazolium chloride [Emim][Cl]; one ammonium-based IL, tributylmethylammonium chloride [N1444] [Cl]; and one phosphonium-based IL trihexyltetradecylphosphonium chloride [P66614] [Cl], were selected for this study. Reactivity, kinetic, and thermodynamic parameters were calculated via thermogravimetric analysis (TGA) conducted under N2 environment at a ramp rate of 20 °C/min. All four ILs increased the higher heating value (HHV) of coal. The IL treatment resulted in greater mass loss and enhanced reactivity of coal. The highest reactivity of 11.45%min−1°C−1 and pyrolysis factor (PF) of 23.28, nearly four and six times, respectively, higher than that of raw coal, were recorded for the [P66614] [Cl] treated sample. This could be attributed to the complete absorption of [P66614] [Cl] in coal and its high hydrogen bond basicity (HBB). Activation energies and pre-exponential factors were calculated using the model-fitting analysis with regression coefficient R2 in the range of 0.979–0.999. All ILs, except [P66614] [Cl], reduced the activation energy of coal, indicating degradation of coal structure. The higher activation energy in the case of [P66614] [Cl] suggests increase in thermal stability of coal. The results indicate that relatively low-cost phosphonium and ammonium-based ILs can be potential pretreatment agents for coal thermochemical conversion processes.
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