Effectivity of Phenol during Solvent Extraction of a South African Bituminous Coal under Mild Conditions

Abstract

Solvent extraction of a South African bituminous coal was carried out under mild conditions (<400 °C) with a holding time of 20 min and an initial nitrogen (N2) pressure of 6 MPa. Thermal degradation of coal with phenol results in the depolymerization of the coal, yielding coal-derived liquids and insoluble residues. The effectiveness of phenol for solvent extraction of coal within the temperature range of 300–360 °C was investigated, with a focus on the quality of the coal-derived liquids. It was found that an increase in temperature from 300 °C to 360 °C resulted in an increase in the conversion and yields of both oil and gas, and a reduction in the intermediate components (preasphaltenes + asphaltenes, PAAs). The conversion and extraction yields of hexane-soluble (HS) oils were 49.5% (daf) and 26.3% (daf), respectively (daf = dry ash-free), for thermal depolymerization reactions at 360 °C. It seems that the extraction process dissolves molecular fragments of the coal, because the infrared spectra of the coal and the extraction products are similar. The SimDis results of the coal-derived liquids (300–360 °C) indicated that the samples consisted of light vacuum gas oil (23–31 wt %), distillate fuel oil (16–30 wt %), heavy vacuum gas oil (18–34 wt %), and residual oil (3–16 wt %). These results show the potential of phenol as a solvent to extract South African bituminous coal at mild temperatures for value-added liquid fuels and add to the general knowledge of the potential utilization of the Permian-aged South African bituminous coals.

Solvent Extraction of a South African Bituminous Coal using a Model Biomass-derived Phenolic Mixture

“…South African bituminous coal (Waterberg) was supplied by Exxaro Resources Limited (South Africa). 11 Sample sizes of less than 150 µm were used in this investigation. A model biomass derived-phenolic mixture was based on the prominence of phenolic species identified in a previous study.…”

“…They observed that the extraction yield increased as the ratio of phenol-water was increased, showing a maximum of 70 % at a water-phenol ratio of 4.5:0.5 and then reduced to 50-55 % for phenol. Recently, Sehume et al 11 showed that phenol has the potential to extract up to 50 wt. % (daf) of a South African bituminous coal at a temperature of 360°C.…”

“…All experiments were conducted in a stainless-steel highpressure autoclave as described by Sehume et al 11 The autoclave was charged with a weight ratio of 1:10 coal to solvent (model biomass-derived phenolic mixture). Darlage and Bailey 28 also investigated solvent effects in depolymerization of coal using coal: phenol ratio of 1:10.…”

“…The solvent extraction process and explanation of the terms used to describe the process (i.e. decomposition and depolymerization) was summarized by Sehume et al 11 In general, solvent extraction represents the treatment of coal with a solvent(s) to dissolve some components of the coal, at temperatures between 200 and 400°C. 11,12 The extensive experiments, using phenol as a solvent, for solvent extraction of coal date back to the 1920s.…”

“…decomposition and depolymerization) was summarized by Sehume et al 11 In general, solvent extraction represents the treatment of coal with a solvent(s) to dissolve some components of the coal, at temperatures between 200 and 400°C. 11,12 The extensive experiments, using phenol as a solvent, for solvent extraction of coal date back to the 1920s. 13,14,15 This approach was found to be ideal as it is performed at temperatures close to the boiling point of the solvent for higher extraction yields.…”