Application of subcritical and supercritical fluids in coal extraction and analysis

Botswana is an African country with an estimated 200 billion tons of coal deposits. The coal from the Morupule mine is currently used to generate electricity with plans to diversify its use to produce high-value compounds. Unfortunately, coal combustion is accompanied by the emission of toxic gases such as NOx and SOx. These gases react with rainwater to form what is called acid rain, which affects plant growth. In addition, through chemical tests, it was established that the Morupule coal contains high amounts of mineral matter, some of which remains intact during combustion and forms the solid by-product called ash. The ash is usually taken to landfills, and over time some of the inorganic material starts to dissolve, especially if there is acid rain. The dissolved toxic substances are then washed away into rivers and lakes where they poison aquatic life, a food source in some communities. The reality is that coal will continue to be burned, especially in developing countries such as Botswana. As such, it is important to find ways to remove pollution-causing components in the coal before its combustion. This thesis presents research to find ways to remove sulfur compounds and mineral matter from coal by a technique called extraction. While a large number of extraction methods have been described in the literature, most of them cannot be utilised as the chemicals used are toxic to humans and to the environment.
This thesis presents research carried out to develop extraction methods for the removal of sulfur and minerals from coal using non-toxic chemicals. Various solvents such as water, ethanol, ethyl lactate and supercritical carbon dioxide have been tested in the framework of this thesis. The efficiency of the developed methods for extracting sulfur compounds and mineral matter was evaluated based on their selectivity, i.e., how much of the target compound can be removed from the coal while the rest of the coal properties remain unchanged. In order to try and minimize the chance of changing the properties of the coal, the experiments were performed at temperatures below 200°C. Two methods were developed. In the first method, the extraction of coal with a water-ethanol mixture (10/90 v/v %) at 129 °C (105 bar) for 10 minutes resulted in a total sulfur reduction from 1.9 to 0.4 (wt.%) from the coal. In the second method, supercritical CO2 was used. In order to achieve a supercritical state, CO2 is raised above its critical pressure and temperature (72 bar, 31°C). The density, and consequently the solvent power of supercritical fluids, is tuneable by varying pressure and temperature. If the pressure is increased, the density of scCO2 also increases, and the liquid penetrates into the coal much easier which increases the extraction rate of the target compounds, in this case, sulfur compounds and mineral matter.

In the application for coal extraction, scCO2 is reported to cause the coal to swell. Eventually, some bonds holding the coal molecules together break, and compounds of interest solubilize in the extraction solvent. Ethyl lactate was added to scCO2 with the idea to make scCO2 more polar to dissolve polar sulfur compounds better. The best conditions for extracting total sulfur were scCO2 (95%) mixed with ethyl lactate (5%) at 80 °C and 300 bar for 10 minutes with a total sulfur reduction from 1.9 to 0.4 (wt.%) from the coal.

Both methods showed approximately 10 wt. % reduction in mineral matter. Although the developed methods were not efficient in the extraction of mineral matter, they have shown that total sulfur can be reduced from coal on an analytical scale before combustion. With less acid rain formation due to less release of SOx, the ash will most likely remain intact until a better method is developed to remove mineral matter from coal. The solvents used are non-toxic, the extraction is done under mild temperatures and the extraction time is short. The methods have the potential to be scaled up for application in the industry because of the mild conditions used. The solvents also have low boiling points, another positive quality because the energy required to evaporate the solvent is low and the solvents might be recycled and reused.