Comparative analysis of rheological characteristics for chemically treated and untreated COSL with varying minerals matter at low temperature

Indian coal having ash content of 38.5 % by wt. with particle size below 106 µm underwent treatment with an aqueous solution of HF and HNO3 at ambient conditions, maintaining a coal-to-solvent ratio of 3:20. Following this treatment, the ash content decreased significantly to 0.9 % by weight. However, the calorific value of the coal decreased from 4865 Kcal/kg to 4766 Kcal/kg because of chemical attack on the carbonaceous matrix. Additionally, sulfur content decreased as iron was oxidized when it reacted with HNO3 to form sulfate compounds. To examine the impact of mineral matter available in coal, a slurry was prepared using both untreated and chemically washed coal, mixed with diesel oil (with calorific value of 44,600 kcal/kg). The research was conducted using a rotary-type Anton Parr RheolabQC at three different solid concentration levels (20 %, 30 %, and 40 %) of coal in oil at a temperature of 293 K. The results revealed that mineral content had a fundamental determinant on the viscosity of COSL. The ultra-clean COSL exhibited the highest viscosity across all concentrations, while viscosity decreased as the percentage of treated coal decreased. Furthermore, viscosity reduced with enhanced in shear rate, and all slurries exhibited shear-thinning behavior. Scanning Electron Microscopy (SEM) analysis indicated the presence of nonporous ash materials in untreated coal, which contributed to its lower viscosity.