Generation of Ultra-Clean Coal from Victorian Brown Coal: Effect of Hydrothermal Treatment and Particle Size on Coal Demineralization and the Extraction Kinetic of Individual Metals

This paper addressed the influences of hydrothermal treatment and particle size on the demineralization extent of Victorian brown coal for the generation of ultra-clean coal to burn directly in a gas turbine combined cycle. Extraction kinetics of individual metals have been investigated. The results for four different sizes of two Victorian brown coal samples showed that, for brown coal that is rich in the aluminum-/silicon-bearing mineral grains, its demineralization extent was dependent on coal particle size, showing the best result for the coal size of 150–300 μm. In contrast, the ash removal efficiency of brown coal rich in organically bound metals remained unaffected by particle size, substantiating a uniform distribution of ash-forming metals on the coal surface as a weak association with the oxygen-containing functional groups. The elution of most metals followed a pseudo-second-order with a confidence interval ≥90%. The activation energy and pre-exponential factor varied significantly with element type and coal sample. Irrespective of coal sample, the extraction of sodium (Na) was achieved instantaneously upon acid attack, relative to iron (Fe) demonstrating an intraparticle diffusion controlling extraction with an activation energy less than 20 kJ/mol. The coal sample rich in mineral grains exhibited an elution behavior limited by both surface reaction and intraparticle diffusion control with respect to a variety of metals. The mineral-grain-rich coal was further treated with pyroligneous acid, citric acid, and Na-EDTA followed by pyroligneous acid at 200 °C to maximize its demineralization extent. As has been confirmed through the use of these three reagents, the overall ash content in coal has been reduced to ∼1.59, 0.95, and 1.17 dry basis (db)-wt %, respectively, as opposed to 2.49 db-wt % in the corresponding raw coal. This study has demonstrated the potential use of waste pyroligneous acid and citric acid for brown coal leaching to generate ultra-clean coal.