Ecotoxicity and environmental fate of diesel and diesel blends produced by Sasol's Fischer-Tropsch processes using natural gas and coal as feedstock as well as biodiesel and biodiesel blends

[摘要] ENGLISH ABSTRACT: World crude oil demand and production is set to increase in the long term and is projected to increase from 82 barrels per day in 2007 to an estimated 104 million barrels per day in 2030 according to the International Energy Agency. The environmental challenges posed by the current and projected increased future fuel use, with specific reference to air, aquatic and terrestrial impact, are driving producers and legislators to change fuel specifications and consequently fuel properties to be less harmful to the environment. Traditionally transportation fuels are produced through crude oil refining but in South Africa more than one third of the liquid fuels are produced synthetically through catalytic conversion of gassified coal via the Fischer-Tropsch process by Sasol. Diesel from syncrude is referred to as synthetic diesel and the refiner must blend various hydrocarbon streams, effectively tailoring the diesel to its final composition. Biodiesel from renewable sources like vegetable oils is considered environmentally more acceptable than petrodiesel because of its high biodegradability in the environment, lower sulphur and aromatic hydrocarbon content as well as lowered particulate content in the exhaust emissions. The present research was aimed at evaluating whether the composition of diesels derived from different feed stocks, that included coal, natural gas, crude oil and soybean oil, would influence its biodegradability and ecotoxicity. Acute aquatic tests that included freshwater fish, crustaceans, algae and marine bacteria were used to determine the acute toxicity of diesels. In addition, quantitative structure activity relationship models were used to estimate the biodegradation and ecotoxicity properties of the diesels in an attempt to develop a cost effective tool to determine those properties. The results indicated that the 2-D GC technique quantitatively and qualitatively identified the hydrocarbon constituents in the diesels. The relevance of using the 2-D GC technique was in identifying and quantifying the hydrocarbon breakdown products and being used in a mass balance to confirm the potential biological breakdown processes of the materials used in the present study. The differences in theoretical oxygen demand (ThOD) of the different experimental diesel blends using various blending materials and biodiesel, emphasised and confirmed the importance of calculating the ThOD for the respective blending materials when measuring the biodegradation rates. Furthermore, the biodegradation hierarchy of Pitter and Chudoba (1990) in order of decreasing biodegradability: alkanes > branched alkanes > cyclo-alkanes > aromatic hydrocarbons, could be expanded to include FAME: FAME > alkanes > branched alkanes > cyclo-alkanes > aromatic hydrocarbons. The biochemical pathways identified for the biodegradation of all the diesels was enzyme-enhanced β-oxidation. The present research also indicated that biodiesel addition to crude-derived diesels to increase the density to within the current required specifications for diesels cannot be a reality in SA because of the underdeveloped biodiesel industry. To increase the density by using biodiesel to within the specification for GTL diesel, more than 27% biodiesel would be required, which is currently is not achievable from an economic perspective as well as governmental national strategy perspective. The addition of biodiesel as lubricity enhancer seems more plausible, because less than 5% would be required for petrodiesels. The results on the ecotoxicity of the diesels and diesel blends demonstrated a general lack of acute toxic effect, especially for the fish and crustaceans used during the present study. Although algal and bacterial tests showed an effect at most of the WAF loading rates, none were high enough to enable the calculation of a median effect loading rate (EL50). QSAR‟s, like EPI Suite, together with prediction models, like the Fisk Ecotoxicity Estimation Model, can be used to screen for ecotoxicity and biodegradability of hydrocarbons found in Petrodiesels. It was less applicable for the prediction of biodiesel constituents. The use of different cut-off values for the constituents of biodiesel could be developed in future research. The use of this combination enabled the present research into the potential toxicity of hydrocarbon mixtures to be conducted, especially since tests on individual constituents are impractical. QSAR‟s may provide a relatively cost-effective way to screen for potential environmental acceptability of such mixtures. The contributors to the toxicity of mixtures of hydrocarbons found in diesels were evaluated and it appears that paraffins contribute more to the overall toxicity than previously thought and aromatics less. By putting well-defined policies and incentives in place, a robust biodiesel industry could be created that will enable SA to contribute to the mitigation of the threat of climate change, to become less dependent on foreign oil and to develop rural agriculture. The key to energy security is not one solution to South Africa‟s energy needs, but a multifaceted approach to the complex subject of sustainable energy security. The end of the hydrocarbon era of energy is not in sight, at least for the near future, but soon even hydrocarbon energy in the form of coal and crude oil will have to be re-evaluated as SA‟s major energy resource for economic and energy security. In SA the potential of developing natural gas resources through fracking, nuclear, solar, wind, biological and even wastes to energy processes as well as better energy efficiency, in a balanced and diverse energy portfolio, could pave the way toward energy security in the long run.

[发布机构] Stellenbosch University