Influence of Density Separation of Selected South African Coal Fines on the Products Obtained during Liquefaction Using Tetralin as a Solvent

Abstract

The liquefaction of two South African discarded coal fines and their density separated (float) fractions were carried out under moderate conditions in a laboratory autoclave. Liquefaction tests were carried out on both discarded coal fines and their float fraction samples at temperatures ranging between 380 and 420 °C using tetralin as a solvent and an initial nitrogen gas pressure of 3 MPa. Oil samples produced from the liquefaction of coal fines discards and their float fractions were further characterized using gas chromatography–mass spectrometry (GC–MS) and nuclear magnetic resonance. The liquefaction test results showed that carbon conversions and oil yields were higher for the float fractions when compared to the discarded coal fines samples. The oil yields ranged from 24.4–37.2% (dry ash free (daf)) for the oil produced from the float fractions of the coal fines and 19.1–29.3% for oils produced from the coal fines (daf). The oil and the gas yields of the float fraction samples were higher when compared to those of the coal fines discards. The higher carbon conversion and higher oil yields obtained from the float fractions demonstrated that density separation of the coal fines discards improves the overall carbon conversion and liquefaction yields. Coal fines discards and float fractions achieved 8 wt % (daf) and 10 wt % (daf) PAA yields at 420 °C, respectively, using tetralin as a solvent during liquefaction. The Waterberg and Highveld coal float fractions had carbon conversions of 50.7 wt % (daf) and 52.7 wt % (daf), respectively, in comparison to <42 wt % (daf) carbon conversion of the original discarded coal fines. GC–MS results for the oil samples derived from coal fines discards and their float fractions showed that these oils contained 72 and 81 wt % naphthalene, respectively. Density separation of discarded coal fines is beneficial to produce a float fraction that may be used as feedstock for direct coal liquefaction. The analytical results indicate that density separation of the coal fines may be used to reduce high costs and volumes of discarded coal fines associated with the disposal and handling of discarded coal fines. Environmental problems will also be addressed if discarded coal fines are utilized as feedstock for industrial processes.

Pyrolysis of Tetralin Liquefaction Derived Residues from Lighter Density Fractions of Waste Coals Taken from Waste Coal Disposal Sites in South Africa

“…The liquefaction-derived coal residues properties results obtained from waste coals and their <1.5 g/cm 3 lighter density fractions are similar to those of the feedstocks to other coal thermochemical processes . Discarding coal residues from liquefaction processes may have negative environmental impacts, including health risks and generation of polluting gases.…”

“…The Highveld float (<1.5 g/cm 3 ) (HF) and Waterberg float (<1.5 g/cm 3 ) (WF) fractions were produced via float–sink experiments of Highveld and Waterberg waste coals as described earlier . The HF and WF samples stand for the lighter density fractions.…”

“…About 0.2 μL of the solution (tar and solvent) was inserted into the GC with a split ratio of 100:1. The procedure for the GC–MS analysis of the tar samples is reported in the literature …”

“…The proximate analysis of the lighter density fractions, their liquefaction residues, and subsequent chars generated from the pyrolysis tests using standard procedures was followed to obtained proximate results. The percentage inherent moisture content, ash yield, and volatile matter content of all the samples were evaluated using the ISO standard methods reported in our previous study . The fixed carbon content is calculated by difference.…”

“…Currently, the information with regards to the pyrolysis of coal residues produced from the liquefaction of density-separated fractions (<1.5 g/cm 3 floats) is limited. Previous studies on pyrolysis of coal and characterization of the pyrolytic products focused on the use of bituminous coal from South Africa as a feedstock. , Characterization of the pyrolytic products generated from the residues after liquefaction of South African lighter density fractions from waste coals has not yet been reported in the literature. , The utilization of the residues produced after liquefaction of lighter density fractions may assist in decreasing the amounts of this waste material.…”