Supercritical extraction of coal

Supercritical extraction of coal is removal of a select fraction of the coal by a solvent which is slightly above its critical temperature and above its critical pressure. The objective of this dissertation was to understand the mechanism of supercritical extraction, to test some promising solvents, and to explore the design implications of the findings.

Supercritical extraction of Wyodak coal was studied by passing various solvents upwards through a 15-gram sample of 12-20 mesh coal. For the high temperature experiments, the coal was heated to 375°C and 425°C in a hot fluidized sand bath. The main solvent used was toluene, while extractions with n-pentane, xylene, methanol, and water were also done. The extract was fractionated into oils, asphaltenes, and asphaltols. Supercritical extraction of coal near pyrolysis temperatures affords an opportunity to remove unstable decomposition products from the reaction environment to avoid repolymerization and pore blinding. Stronger aromatic solvents removed the decomposition products as they were formed. However, product degradation even with the strongest solvents was inevitable during the initial few minutes.

For the low temperature experiments (below 95°C), the solvent was carbon dioxide. Effects of liquid entrainers (pre-mixed with the coal), and heat-pretreatment of the coal (at 400°C for 1 hour) were also studied. The major difference between the high and low temperature extractions was that coal reactions occurred at high temperatures simultaneously with solubilization. Extraction of raw coal and heat-pretreated coals with carbon dioxide was negligible. However, extractions as high as 12% were possible when small amounts of liquid entrainers such as pyridine, toluene, and tetralin were pre-mixed with the coal. The entrainers were almost completely recovered with the extract.

The process design implications of the supercritical extractions of coal were studied using the method developed by ESCOE (Engineering Societies Commision On Energy Inc.). Preliminary design estimates showed that the following supercritical extraction processes were possible alternatives to present commercialization efforts and deserve further attention:

1. Gasification of the extraction residua;

2. Satellite plants operating in parallel with coal-burning utilities;

3. Entrainer-aided extraction.