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

Albertus, Randal Marius Colin (2012-12)

Thesis (PhD)--Stellenbosch University, 2012.

Thesis

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.

AFRIKAANSE OPSOMMING: Ru-olie aanvraag en produksie wêreldwyd is besig om toe te neem en die Internasionale Energie Agentskap projekteer dat wêreld ru-olie verbruik sal toeneem van 82 vate per dag in 2007 tot „n beraamde 104 vate per dag in 2030. Die omgewings uitdagings wat huidige en toekomstige toename in brandstof verbruik, spesifiek die impak op lug gehalte, water- en grond, mag hê, is dryfvere vir produseerders en reguleerders om brandstof spesifikasies te verander om minder omgewings impak te veroorsaak. Brandstof vir vervoer doeleindes word oor die algemeen van ru-olie gemaak, maar in Suid Afrika word ongeveer „n derde van die vloeibare brandtof gemaak deur middel van gekatiliseerde omskakeling van vergasde steenkool via die Fischer-Tropsch proses by Sasol. Diesel wat uit sintetiese ru-olie gemaak is, is sinteties en die raffineerder moet verskillende koolwaterstof strome meng om „n finale produk te lewer. Biodiesel wat uit hernubare hulpbronne soos plant-olies en diervet gemaak word, kan oorweeg word vir die vervaardiging van meer omgewings aanvaarbare brandstof met laer swael en aromatiese koolwaterstof inhoud en ook minder partikel inhoud in die uitlaatgas. Die huidige navorsing het beoog om te evalueer of die samestelling van diesels wat vervaardig is uit verskillende hulpbronne, wat steenkool, aardgas, ru-olie en sojaboon olie ingesluit het, die biodegradeerbaarheid en ekotoksisiteit kan beïnvloed. Akute akwatiese toetse wat varswater vis, krustaseë, alge en marine bakterieë ingesluit het, was aangewend om die akute toksisiteit van die diesels te bepaal. Kwantitatiewe struktuur aktiwiteit verwantskaps modelle is ook gebruik om die biodegradeerbaarheid en ekotoksisiteits eienskappe van die diesels te beraam om vas te stel of 'n bekostigbare alternatief beskikbaar is om daardie eienskappe te bepaal. Die resultate het aangedui dat die 2D GC tegniek kwantitatief en kwalitatief gebruik kan word om die koolwaterstowwe in die diesels te identifiseer. Die benutting van die 2D GC tegnieke is egter om die koolwaterstof afbraak produkte te identifiseer en ook om die massa balans gedurende die biodegradering te bevestig. Die verskil in teoretiese suurstof aanvraag van die verskillende diesels het die belangrikheid daarvan blemtoon en bevestig om die teoretiese suurstof aanvraag korrek te bereken en sodoende die biodegradasie korrek te bepaal. Verder kan die biodegradasie hierargie van Pitter en Chudoba (1990) volgens afnemende biodegradasie: alkane > vertakte alkane > siklo-alkane > aromatiese koolwaterstowwe, uitgebrei word om vetsuur-metielesters in te sluit: vetsuur-metielesters > alkane > vertakte alkane > siklo-alkane > aromatiese koolwaterstowwe. Die biochemiese roetes wat geïdentifiseer is vir die biodegradasie van die diesels, was ensiem-verbeterde β-oksidasie. Die huidige navorsing het ook aangedui dat biodiesel toevoeging tot ru-olie vervaardigde diesel om die digtheid te verhoog to binne huidige spesifikasies is nog nie lewensvatbaar in Suid Afrika nie as gevolg van die onderontwikkelde biodiesel industrie. Om die digtheid te verhoog met biodiesel tot binne spesifikasie verg meer as 27% biodiesel en is huidiglik nie haalbaar vanuit 'n ekonomiese persketief en ook nie vanuit 'n regerings nasionale strategie perspektief nie. Die toevoeging van biodiesel as lubrisiteits vervetering blyk meer van toepassing te wees aangesien minder as 5% biodiesel toevoeging benodig sou wees. Die resultate van die ekotoksisiteits toetse het 'n algemene gebrek aan akute toksisiteits effek aangedui, veral vir vis en skaaldiere wat in die huidige studie gebruik is. Howel alge en bakteriële toetse daarop gedui het dat 'n toksiese effek wel aanwesig was, was dit gering en kon die median effektiewe ladings koers (EL50) nie bepaal word nie. QSARs, soos Epi Suite, tesame met voospellings modelle, soos die Fisk Ecotoxicity Estimation Model, kan gebruik word om ekotoksisiteit en biodegradeerbaarheid van koolwaterstowwe in petrodiesels te beraam, alhoewel dit minder van toepassing was op biodiesel. Die gebruik van ander afsny waardes spesifiek vir biodiesel kan oorweeg word in toekomstige navorsing. Die molecules wat bygedra het tot die toksisiteit van die koolwaterstof mengsels was geëvalueeren daar is gevind dat die paraffiniese molekules meer begedra het tot die totale toksisiteit en die aromate minder. Deur goed gedefinieerde beleid en aansporings meganismes inplek te sit, kan 'n biodiesel industrie in SA geskep word wat SA sal help om by te dra tot die bekamping van klimaats vendering en sodoende minder afhanklik te wees van buitelandse olie en ook landbou in SA te bevorder. Die sluetel tot energie sekuriteit is nie een oplossing vir SA se energie aanvraag nie, maar eerder 'n veelsydige benadering tot die komplekse onderwerp van volhoubare energie sekuriteit. Die einde van koolwaterstof energie is nog nie in sig nie, ten miste nie in die nabye toekoms nie, maar binnekort sal selfs koolwaterstof energie in die vorm van steenkool en ru-olie heroorweeg moet word as SA se hoof energie hulpbronne vir ekonomiese en energie sekuriteit. In SA moet die potensiaal van natuurlike gas ontginning deur middel van hidrauliese breking, kernkrag, wind energie, biologiese energie en selfs afval tot energie prosesse bestudeer word, so-ook beter energie doeltreffendheid om sodoende 'n gebalansweerde energie portefuelje te skep wat die weg sal baan na energie sekuriteit op die lang termyn.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/71807
This item appears in the following collections: