BENEFICIATION OF COAL USING SUPERCRITICAL WATER AND CARBON DIOXIDE EXTRACTION

• Main Author: DeCuir, Matthew, Virginia Commonwealth University CL
• Format: Text
• Language: unknown
• Published: VCU Scholars Compass 2020
• Subjects: Coal Beneficiation Sulfur extraction supercritical fluid; Catalysis and Reaction Engineering; Chemical Engineering; Transport Phenomena; Carbonic acid
• Online Access: https://scholarscompass.vcu.edu/etd/6413; https://doi.org/10.25772/CZXY-1C66; https://scholarscompass.vcu.edu/context/etd/article/7523/viewcontent/DeCuir_Matthew_PhD.pdf

This work explores the use of carbon dioxide, water, and their mixtures as solvent for the pre-combustion beneficiation of raw coal without using any toxic mineral acids in the temperature range of 200-400℃. The fluid polarity, ionic constant, and supercritical point can be adjusted by H2O/CO2 ratio and temperature. Adding carbon dioxide to hydrothermal fluid also increases the ionization by forming carbonic acid. Extractions with supercritical fluids have several benefits including enhanced mass transport, ease of separation and recycle, wide range of extractive capability and tunability, better inherent safety, and in the case of carbon dioxide and water – low cost. A semi-continuous extraction system was designed and built in which pressure, temperature and the relative flow rates of CO2 and H2O can be controlled. Coal powder is kept in a packed bed and the extraction is carried out at 143 bar pressure. Using sulfur as a model heteroatom, extractive efficiency is examined as a function of the temperature, fluid composition, fluid flow, and extraction time. Results indicated that carbon dioxide, water, and supercritical water-carbon dioxide (ScWC) all can effectively extract about 50% of total sulfur from bituminous coal in one hour. Extraction above 350℃ decreased effectiveness, and extraction above the supercritical point of pure water caused polymerization presumably due to hydrothermal carbonization. Elimination of organic sulfur may play a role in the polymerization. The carbonized coal that was obtained from extraction above 350℃ gives an interesting product that is clean, porous, and partly graphitic in nature. The material could have exciting applications to replace metallurgical coke in metal refining and anode carbon in energy storage applications. Some carbonization occurred in pure carbon dioxide around 350℃ as well. Additionally, ScWC extraction may provide necessary control to prevent organic dissolution while removing sulfur. While neither carbon dioxide nor water seemed to affect the ash ...