Solvent extraction of a bituminous coal using a sweet sorghum bagasse derived solvent

Abstract

The aim of the first part of this study was to investigate the extractability of a South African bituminous (Waterberg) coal using phenol as a solvent at 300 °C, 320 °C, 340 °C, and 360 °C and a solvent/coal ratio of 10:1. Secondly, investigate the feasibility of converting biomass into bio-oil via hydrothermal liquefaction and then extract a phenol-rich product from the bio-oil. Lastly, to use a model biomass-derived phenolic mixture to extract or dissolve valuable light products from coal and compare the results to that when phenol is used as a solvent. All of the reactions were conducted inside a high-pressure stainless-steel autoclave system (950 mL capacity, 90 mm diameter and 150 mm height) under inert atmosphere (nitrogen gas). Subsequently, the extraction yields and conversions were obtained. The products were subjected to chemical and physical analyses. The effectiveness of phenol for coal solvent extraction was investigated in this study. In general, coal solvent extraction using phenol increases as temperature increases, i.e. a low coal conversion of 12% was observed at 300 °C increasing to 50% at 360 °C in this study. From the proximate results, the decrease in the volatile matter of the unreacted coal residues (THFIs) from 44.7 wt. % to 28.5 wt. % daf corresponded with an increase in coal conversion upon an increase in temperature. This decrease in the volatile matter may be an indication of increased depolymerization reactions at higher temperatures. ATR-FTIR results showed that the extracted products had similar spectra to that of the raw coal in terms of similar functional groups. From the ultimate analysis results, the nitrogen and sulphur values of the hexane-soluble products were much lower than those of the coal, suggesting that the formation of oils during phenol extraction has been accompanied by a reduction in heteroatomic content. The oil yield after coal extraction increased from 3% up to 27% with an increase in temperature between 300 °C and 360 °C. The hexane-soluble products contained compounds with a wide range of boiling points (208–615 °C) as determined by SimDis analysis. The largest proportion of the boiling constituents were 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. %). Sweet sorghum bagasse (SSB) was treated with NaOH concentrations of 0.5, 1.0, 3.0 and 6.0 M. The experiments were conducted in a temperature range of 260–320 °C inside an autoclave in using a N2 atmosphere. It was observed that the bio-oil yield increased with increasing temperature for 0.5–3.0 M NaOH concentrations (5.7–53.2 wt. %). However, the bio-oil yield reduced from 47 to 39 wt. % with increasing temperatures using a concentration of 6.0 M NaOH. The bio-oil yield reached its maximum of 53.2 wt. % at a concentration of 3.0 M NaOH and a liquefaction temperature of 320 °C. This bio-oil yield at 320 °C with 3.0 M NaOH coincides with the highest yield of 40 wt. % total phenols extracted from the bio-oil. The highest variation of phenolic compounds was observed at a temperature of 280 °C and a NaOH concentration of 3.0 M and the compounds were phenol (13.8 wt. % of total liquid products), p-cresol (6.8 wt. %), 4-ethylphenol (10.0 wt. %), 4-isopropylphenol (0.9 wt. %), 2-propylphenol (0.7 wt. %), and 4-ethylguaicol (1.5 wt. %). The model biomass-derived phenolic mixture, was formulated based on the biomass liquefaction product results, was investigated for its solvent ability for extraction of a South African bituminous coal at temperatures of 300–360 °C. As the temperature increases, the yield of the residues (THFIs) decreases, while the conversion increases. The coal conversions of 14 wt. %, 20 wt. %, 32 wt. % and 37 wt. % were obtained using a model biomass-derived phenolic mixture at 300 °C, 320 °C, 340 °C and 360 °C. The oil yield obtained using a model mixture increased from 1 wt. % up to 17 wt. % (300–360 °C). From the SimDis analysis, the boiling constituents of hexane-soluble fractions (300–360 °C) included distillate fuel oil, light vacuum gas oil, heavy vacuum gas oil and heavy vacuum gas oil fractions which were 14 wt. %, 50 wt. %, and 22 wt. respectively. The phenol and a model biomass-derived phenolic mixture had similar coal conversions between 300 °C and 320 °C (13–20 wt. %). However, coal solvent extraction using phenol resulted in nearly 13 wt. % more than that of a model biomass-derived phenolic mixture at temperatures higher than 320 °C. The overall conclusion made from this investigation is that the effectivity of phenol-based solvents is characterized significantly by their ability to dissolve or penetrate the coal structure. The results obtained from this study demonstrate that the use of a model biomass-derived phenolic mixture has the potential to solubilize coal. Together with further development, using biomass as a source of mixed phenols could be used to facilitate the production of useful liquids from solvent extraction of South African coals