Fuels made from renewable, natural processes, including Biodiesel such as “FAME” (Fatty Acid Methyl Ester) or “FAEE” (Fatty Acid Ethyl Ester), Biogas, Bioethanol, and Biobutanol.
Biofuels can have different characteristics to conventional refined fuels, depending on the quality of the bio-extender they may potentially polymerize with heat, therefore testing of both bio-extender and finished blends is important. Testing helps to protect engines and fuel pumps from potential damage and deposits caused by poor stability, organic insoluble matter and water; also contamination from metals or bi products from the trans-esterification process.
Biodiesel generally have higher densities than mineral diesel (EN 590 820-845 kg/m3 at 15°C). Density increases with a decrease in chain length and with unsaturation. This can impact on fuel consumption as fuel introduced into the combustion chamber is determined volumetrically.
Viscosities of neat vegetable oils are many times higher which leads to serious problems in unmodified engines. The increase in viscosity results in poor atomization and incomplete combustion which leads to coking of injector tips. This results in engine power loss. Biodiesel still has higher viscosity than mineral diesel (3.50-5.00 mm2/s at 40°C vs 2.00-4.50 mm2/s). Viscosity decreases with unsaturation but increases markedly with contamination by mono, di or tri glycerides.
Pure rapeseed methyl ester has a flash point value of up to 170°C. This method is therefore looking at residual components within the fuel that are combustible, especially methanol which is a particular hazard due to its invisible flame.
Sulphur emissions are harmful to human health and high sulphur fuels cause greater engine wear and in particular shorten the life of the catalyst. Biodiesel derived from pure Rapeseed oil will contain virtually no Sulphur, however FAME derived from animal sources may contain significant quantities.
The Carbon Residue is the material left after evaporation and pyrolysis of a sample fuel. This is a measure of the tendency to of a fuel to produce depositis on injector tips and the combustion chamber. For FAME samples it is an indication of the amount of glycerides, free fatty acids, soaps and catalyst residues remaining within the sample
This serves as a measure of ignition quality. This is the most pronounced change from vegetable oil to the transesterified product. Fuels with low cetane numbers show an increase in emissions due to incomplete combustion. Palm Oil and Tallow derived biodiesels have the best cetane numbers.
Ash describes the amount of inorganic contaminants, such as catalyst residues, remaining within the fuel. Ash is related to engine deposits on combustion.
As FAME is hygroscopic it can pick up water in storage and as such there can be problems meeting the specification. At around 1500 ppm the solubility limit is reached and the water bottoms out. Free water promotes biological growth and the reverse reaction turning biodiesel to free fatty acids.
Total contamination is defined as the insoluble material retained after the filtration of a heated sample over a standardized 0.8m filter. FAME samples with high quantities of insoluble materials tend to cause fuel filter and injector blockages. High concentrations of soap stock are normally associated with high total contamination.
This is defined as the likelihood to cause corrosion to copper, zinc and bronze parts of an engine. A polished metallic strip is heated at 50°C for 3 hours, washed and compared to standards. Corrosion is likely to be caused by free acids or sulfur compounds. However FAME gives consistently good results in this area and is unlikely to fail due to the low sulfur content.
The Oxidative stability specification is defined as a minimum Rancimat induction period of six hours. Essentially a fuel is heated at 110°C in a constant air stream and the formation of volatile organic acids is detected. This property relates to the overall storage stability of the fuel and the higher the degree of unsaturation (double bonds) within the FAME molecules gives a decrease is oxidative stability. This can be improved with the addition of antioxidant additives.
Acid value is a measure of mineral acids and free fatty acids contained in a fuel sample. It is expressed in mg KOH required to neutralize 1g of FAME. High fuel acidity is linked with corrosion and engine deposits.
Iodine number is a measure of total unsaturation (double bonds) within the FAME product. It is expressed as the grams Iodine required to react with 100g of FAME sample. High Iodine value is related to polymerization of fuels, leading to injector fouling. It is also linked to poor storage stability.
This is measured using gas chromatography and is restricted to esters falling within the C14-C24 range. It is ultimately a test for reaction conversion. Linolenic and polyunsaturated esters are controlled as they have been shown to display a disproportionately strong effect on oxidative stability.
Methanol can be removed from FAME by washing or distillation. High methanol contents pose safety risks due to the very low flash point of methanol.
There is a limit on the mono, di, and triglycerides of no more than 0.80%, 0.20% and 0.20% respectively. Total glycerol is the sum of the bound and free glycerol and must not exceed 0.25%/ Failing to meet the spec implies low conversion to ester and deposit formation on injectors and valves.
Sodium and Potassium are limited to a combined 5ppm. These arise from the addition of catalyst, and result in high ash levels in the engine.
Calcium and Magnesium are limited to a combined 5ppm. These may arise from the addition of hard water in the washing process. Calcium and Magnesium soaps have been related to injector pump sticking.
The phosphorous limit is approx 10 ppm and normally arise from phospholipids within the starting material or from addition of phosphoric acid in the production process. High phosphorus fuels are suspected of poisoning catalysts and increasing emissions.
Cold-Filter Plugging Point is considered to be a suitable indicator of low temperature operability. It defines a temperature at which a fuel is no longer filterable within a specified time limit.