considerably more prone to microbiological attack than a conventional fossil fuel, with the associated problems mentioned above. A number of studies have been performed in this area which highlight the need for further detailed analysis and research into the potential for serious microbiological contamination occurring at various stages in the biofuel supply chain.
Possible sources of water contamination aboard a vessel range from the obvious – sea water ingress or residues of tank washing operations – to the less obvious – moisture in an inert gas blanket produced from a faulty flue gas generating system, or atmospheric humidity in tanks’ ullage spaces that are not under a positive pressure of dry inert gas. Despite having relatively high flash points, FAME cargos are generally carried under a (dry) nitrogen blanket in order to avoid the potential increase in water due to absorption of moisture from tank ullage spaces.
Stability problems: FAMEs are generally more prone to issues with regard to their stability than conventional petroleum diesel. We have mentioned previously that certain FAME/diesel blends can be oxidised to form unwanted degradation products and that the addition of antioxidants may prevent the formation of these sediments. FAME can degrade under the influence of air, heat, light and water, and degradation may occur during transport, storage or even during end-use. FAME cargoes may display different levels of stability dependent upon their composition and the feedstock(s) used in their production. In general FAME with higher levels of unsaturated fatty acids, such as soybean and sunflower oil derived FAME, will be less stable than those composed of higher levels of saturated fatty acids, for example, palm oil or coconut oil derived FAMEs.
Potential shipping problems include the promotion of degradation reactions by trace metals (copper heating coils or zinc-containing tank coatings have the potential
Oil Type
to cause deterioration in quality) and thermal stability issues if the FAME cargoes are stored next to heated tanks, for example, bunker settling tanks. Issues with the promotion of instability by the presence of trace metals are worse for B100 than for lower biodiesel blends (e.g. B5, B20). Degradation reactions can form insoluble sediments and gums, which may increase the viscosity of the FAME, lead to filter blocking or potentially further decompose to other more corrosive species. The carriage of FAME under dry nitrogen blankets can also help to prevent unwanted degradation reactions caused by the material coming into contact with air.
Low temperature behaviour: The poor low temperature properties of FAME when compared to conventional diesel may give rise to issues where FAME cargoes are shipped through extremes of cold temperature. It is possible for certain FAMEs to form waxy precipitates at low temperatures which will then not re-dissolve when the product is reheated, although this would not appear to be a common problem. However, there is the potential for FAME cargoes shipped from a warm, humid climate to extremely cold conditions, if the correct measures for heating the cargo are not applied, to form unwanted waxy precipitates which may lead to specification failure or pumping problems.
As has been mentioned previously, it is generally recognised that FAMEs produced from vegetable oils with relatively high proportions of unsaturated fatty acids, for example, soybean oil, will have better cold- temperature properties than FAMEs produced from vegetable oils with high proportions of saturated fatty acids, such as palm oil. It is therefore vital that the correct heating instructions are issued and followed – an understanding of the nature of the FAME will impact upon the necessary heating instructions – this should be borne out by the appropriate research into the origin of the FAME and indicated by the results of any pre- shipment testing.
FOSFA International (The Federation of Oils, Seeds and Fats Associations Ltd) has now included FAME products in its published heating recommendations:
Temperature during voyage Min (oC) FAME from Maize/Rapeseed/Soya/Sunflower FAME from Coconut/Palm/Palm Kernel/Tallow 25 23 30 30 40 Ambient Ambient Max (o C) Temperature at discharge Min (o C) Max (o C)
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