Defense Date

2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Chemical and Life Science Engineering

First Advisor

Dr. Ram B. Gupta

Second Advisor

Dr. Arif Sikder

Third Advisor

Dr. Michael Peters

Fourth Advisor

Dr. Hani-El Kaderi

Fifth Advisor

Dr. Xuejen Wen

Abstract

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 content, ScWC extraction decreased the ash content to 3.77%, a 45% reduction in ash.

The extraction process was further developed to introduce ultrasound energy to enhance mass transfer from solid coal particles to the fluid phase. At the high temperature and pressure conditions as noted above, the introduction of ultrasound was successful and tested for coal extraction. The degree of mass transfer enhancement can be controlled by the intensity of the ultrasound. Such an enhancement, opens up possibility of relaxing the requirement on the fine particle size of the coal. We found that the conversion of pyrite was nearly complete for the best extracted samples with organic sulfur mostly untouched, indicating that the mass transport even without ultrasound was fairly good.

In the base design of the extractor, the fluid entry and exit points are both on the top cover plate of the vessel. Here scWC fluid enters at the top and then leaves from the top after extraction, so it is possible that not all the coal solids are efficiently contacted with the fresh fluid coming in. To overcome this possible channeling effect, we have further enhanced the extractor design. The apparatus is designed to allow for the fluid to enter at the top, go through the pack bed of coal, and then exit from the bottom carrying extracted components. The design was tested successfully.

Rights

© The Author

Is Part Of

VCU University Archives

Is Part Of

VCU Theses and Dissertations

Date of Submission

8-6-2020

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