Enhanced liquid tar production as fuels/chemicals from Powder River Basin coal through CaO catalyzed stepwise degradation in eco-friendly supercritical CO2/ethanol

Abstract Developing eco-friendly thermal conversion technologies of coals into fuels/chemicals with minimal environmental impact is highly attractive and significant for efficient coal utilization. CaO catalyzed stepwise degradation at 350–400 °C in eco-friendly supercritical CO2/ethanol (scCO2/ethanol) was carried out to produce high-yield liquid tars as fuels/chemicals from Powder River Basin (PRB) coal. The total yield of the liquid tars increased from 53.9 wt% for non-catalytic stepwise degradation to 60.1 wt% for CaO catalyzed stepwise degradation, with an increase of 6.2 wt%. The resulting ash-free liquid tars are promising raw materials as clean liquid fuels for energy production due to their high H/C ratio (1.16–1.88) and heating values (32.84–35.84 MJ/kg). Compared with non-catalytic stepwise degradation, the energy recovery increases from 62.8% to 71.6% by CaO catalyzed stepwise degradation. More than 75% of the components in the liquid tars were volatilized and/or degraded at temperature