Preparation and Characterization of the Glycerol-Embedded Hybrid Coal

The molasses embedded hybrid coal (Hybrid Coal by Korea Institute of Energy Research; HCK) was previously proposed as an attractive alternative to low-rank coals that tend to show low calorific values and high CO₂ emissions. HCK was synthesized by mixing molasses with a low-rank coal to enhance its heating value. Nevertheless, food ethics regarding molasses additives still have impeded its commercial acceptance. In this study, we propose a glycerol (nonfood)-based coal upgrading process to improve the combustion kinetics and heating value of hybrid coal in comparison to the previous HCK. The process involves drying at 105 °C followed by torrefaction process at 250 °C. During torrefaction, the glycerol additive starts to evaporate at about 180 °C and then is almost vaporized out to the coal surface. To avoid glycerol loss, we employ sulfuric acid as a torrefaction catalyst to suppress glycerol evaporation. In comparison to the molasses yield of 65% after torrefaction in the previous HCK synthesis process, glycerol-embedded torrefaction with sulfuric acid showed a 55% yield in mass. The glycerol-embedded hybrid coal shows a homogeneous combustion peak regardless of the mixing ratios, leading to a 35% reduction of unburned carbon emissions, which is one of the particular matter sources of combustion flue gas. Furthermore, the proposed synthesis process increases the net caloric value to 80% and lowers the water uptake to 84%, even without a higher SO₂ emission, in comparison with raw coal.