The Effect of Microwave Pyrolysis Temperature on Liquid Fuel Yield from Low-Rank Coal Using TiO2-Fe2(SO4)3 and Activated Carbon-Fe2(SO4)3 Catalysts

Abstract

Low-rank coal (LRC) offers substantial reserves as an alternative energy source, but its low calorific value and high moisture content limit direct combustion. This study investigated the effect of microwave-assisted pyrolysis temperature (550 °C, 600 °C, and 650 °C) and reaction time (60 and 120 minutes) on the liquid yield of sub-bituminous LRC, using a microwave pyrolysis reactor (800 W, 2.45 GHz) combined with TiO₂-Fe₂(SO₄)₃ and Activated Carbon-Fe₂(SO₄)₃ catalysts at a 2% w/w ratio. Proximate and ultimate analyses characterised the feedstock, while SEM, FTIR, and GC-MS characterised the pyrolysis liquid product. Each condition was tested once, and results were analysed descriptively rather than through replicate-based statistics. The highest liquid yield, 46.5%, was obtained at 600 °C and 120 minutes using the Activated Carbon-Fe₂(SO₄)₃ catalyst, exceeding the 40.0% achieved with TiO₂-Fe₂(SO₄)₃ under identical conditions. Yield increased from 550 °C to 600 °C due to enhanced thermal cracking, then declined at 650 °C due to secondary cracking reactions. FTIR indicated stronger aromatic and sulfur-related functional-group signals, and SEM revealed a denser, more homogeneous char morphology for the activated carbon-Fe₂(SO₄)₃ system, while GC-MS showed phenolic compounds dominating the liquid product at the optimum condition for both catalysts (59.68-78.54% of total peak area). These results demonstrate that microwave-assisted pyrolysis with Fe₂(SO₄)₃-based catalysts effectively converts low-rank coal into a phenol-rich pyrolysis liquid, with the Activated Carbon-Fe₂(SO₄)₃ catalyst providing superior performance at 600 °C and 120 minutes