Biomass conversion into biofuels by non–classical methods / Yolandi Nortjé
This investigation was launched in view of two imminent needs in industry today, viz. development of an alternative fuel to replace rapidly dwindling fossil fuel resources, preferably by biomass conversion, and production of a biofuel as a new energy source by implementing clean technology complying with the requirements of green chemistry. Seed from the diesel tree (Jatropha curcas L.) and sawdust from pine (Pinus taeda L.) were selected as biomass sources since their properties, like rich oil content or diversity of constituents, met the suitability criteria for eventual conversion to biofuels. Extracts from the two selected biomass sources were derived by three different non–classical methods, viz. supercritical carbon dioxide (sc–CO2) extraction performed with a laboratory–scale supercritical extractor (LECO TFE2000), microwave–assisted extraction using a closed–vessel industrial microwave system (MARS 5) to produce superheated water, and ultrasound–supported extraction performed in n–hexane or water sonicated by a FINNSONIC soundwave emitter. The extracted material was compared to that obtained by traditional soxhlet extraction using n–hexane as solvent. One–dimensional and two–dimensional gas chromatography with time–of–flight mass spectrometric detection using a LECO Pegasus 4D GCxGC–TOFMS and different column configurations were employed to cope with the analysis of derivatised samples of the complex, component–rich botanical extracts derived by the non–classical methods adopted. The oil content of Jatropha seed (at least 55% m/m) and the solubility of Jatropha oil in sc–CO2 (nominally 3 x 10–3 g per g CO2 at 313 K and 30 MPa) were determined by utilising the dynamic and static modes of the supercritical extractor, respectively, and extrapolating the resulting yield–time graphs to infinity. These figures proved that Jatropha seed is a favourable feedstock for biofuel production, and that sc–CO2 is an efficient solvent to extract oil from seed while avoiding harsh solvents and unwanted solvent residues in agreement with green chemistry principles. C16–C18 triglycerides were detected as major constituents of Jatropha oil obtained by soxhlet and sc–CO2 extraction, whereas free fatty acids dominate in extracts by microwave and ultrasound extractions due to thermal degradation and partial hydrolysis of triglycerides at the extraction conditions concerned.A standard solution of triolein, the most abundant C18 triglyceride in Jatropha oil, was used as a reference for the identification of mixed C16–C18 triglycerides present in the oil. By comparing the mass spectrum of each oil sample to the mass spectrum of triolein, some of the triglycerides in the oil samples could be identified with a satisfactory match factor (70 % or higher). Among these were triolein (C18:1), tripalmitin (C16:0), trilinolein (C18:2) and tristearin (C18:0). The triglycerides could be converted by means of base–catalysed transesterification to a crude biodiesel containing primarily C16–C18 but also some C13–C15 fatty acid methyl esters (FAMEs), the principal building blocks of biodiesel. The crude product could be benchmarked against an SABS approved biodiesel according to the SANS1935 standard in terms of its content of these long–chain esters. sc–CO2 and superheated water were found to be equally efficient solvents next to acetone used in soxhlet extraction to retrieve material from pine sawdust samples. Extracts were shown to comprise, among others, hydrocarbons, fatty acids, terpenoids, flavonoids and phenolics. These substances were either dissolved or desorbed by the solvent, and a “bulk solubility” of pine extractables in sc–CO2 could be determined as 7 x 10–3 g per g CO2 at 358 K and 60 MPa in a similar way as for Jatropha oil. Superheated water was the only solvent capable of cleaving the polymeric cellulose and hemicellulose chains held together by lignin in wood into a series of differently structured sugar entities, resulting in a highly complex two–dimensional chromatogram. Quantitative analysis of triglycerides had to be aborted since the low volatility of these high molar mass, high boiling point compounds necessitated modification of the instrument’s inlet, despite using pseudo on–column injection and special high–temperature columns. To the contrary, qualitative analysis of extracts and converted products demonstrated the powerful identification capability of the chromatographic system used and the diversity of substances available for conversion to biofuel. The chromatographic results published in this dissertation on the two selected biomass sources have been acquired by novel combinations of separation mode (one–dimensional or two–dimensional) and column type/configuration not specifically found in the literature. The study as a whole proved that Jatropha oil is a suitable source of biomass for biodiesel production, and that even waste wood shows potential for conversion into liquid fuels. The non–classical extraction methods were found to be capable of retrieving material relevant to biofuel production from these biomass sources.
- ETD@PUK