About the Biodiesel Fuel Quality Standard

The following is an explanation of the parameters used to define quality biodiesel.
Download the Australian Biodiesel Fuel Quality Standard here

When combusted, sulphur becomes sulphur dioxide, and causes acid rain. It also reduces the lifetime of oxidation converters. The sulphur found in biodiesel comes from the feedstock, with higher concentrations found in used cooking oil as compared with new vegetable oil or animal fats. Biodiesel compared with fossil diesel is an ultra-low sulphur fuel, and does not have the same lubrication problems as low sulphur diesel due to the other good lubricating properties of biodiesel. Sulphur is not usually an issue for biodiesel manufactures but can be removed if it is an issue.

Density relates to the weight and energy content of fuel, where high densities indicate more thermal energy for the same amount of fuel and therefore better fuel economy. Although biodiesel is more dense than diesel, its energy content is lower so fuel economy overall is reduced. Density depends mostly on the feedstock used, with tallow giving a greater density than vegetable oils. Lower density can also be caused by contaminants such as methanol and ethanol.

Distillation T90
Distillation gives an indication of the presence of components that have different boiling points and other contaminants. Biodiesel is composed of relatively few compounds compared with diesel, most of which are C16 to C18 carbon chain length alkyl esters. These boil at roughly the same temperature, so the distillation temperature is more representative of a boiling point. This boiling point does not vary widely between biodiesel made of different feedstocks, as the composition of naturally occurring oils and fats are similar.

The presence of high boiling point components influences solid deposit formation. Boiling point ranges also indicate the flash point and cold flow properties of the fuel. These have safety and storage implications, plants do occasionally have issue with this and it can be solved.

Sulphated ash
Ash is formed from abrasive solids, soluble metallic soaps and un-removed catalysts remaining in the biodiesel. Ash deposits in engines can cause clogging or filter plugging. Any abrasive solids or un-removed catalysts can also contribute to wear in the injector, fuel pump, piston and ring. High ash content can easily be solved.

Viscosity affects the flow of fuels through pipes, injection nozzles, filters and orifices and the temperature range for proper operation of fuel in burners. High viscosities can cause injector spray pattern problems that lead to excessive coking and oil dilution. Minimum viscosity limits are applied to prevent fuel from causing wear in the fuel injection system, which results in loss of power. Proper viscosity provides adequate lubrication and pumping characteristics to fuel system components. Biodiesel has 6 times the viscosity of fossil diesel, which is why biodiesel is used as a 5% additive to low sulphur diesel. Viscosity issues do arise and can be solved.

The flash point is the lowest temperature at which contact with a flame causes the vapour of the fuel to ignite. Flash points for biodiesel are approximately two to three times those of diesel. The flash point of methyl ester fuels is lower than that of ethyl esters. We can easily solve this problem.

Carbon residue
Impurities such as glycerides, free fatty acids, soaps, remaining catalyst, etc. form carbon deposits in an engine. These increase wear, reduce engine life, and reduce fuel efficiency. Particulate emissions from these are dangerous to the environment and to human health. Using canola as a feedstock creates greater particulate emissions than tallow. Easily solved.

Water and sediment
his parameter measures the amount of free water and solid debris in the fuel. Free water is generally minimised by ‘housekeeping’ issues, such as drying the biodiesel, draining water from storage tanks, ensuring no rainwater can enter through seals or valves and by not drawing from the bottom of the tank. Sediment can be carried in the water or collected from dust or the precipitation of fuel components.

Free water in biodiesel can reduce storage ability or lead to the separation of water when blending with diesel. Water can cause corrosion of engine fuel system components. This may take the form of rust, or acid corrosion, or pitting on the heads. The presence of water in fuel may also encourage microbial growth in the bottom of the storage tank, which can cause filter plugging and corrode storage tanks. Sediment causes deposits on engine parts and consequently reduces engine life. Water can be removed from biodiesel in a number of ways.

Ester content
Esters are the main component of biodiesel and the parameter giving biodiesel similar properties to diesel. The total ester content is a measure of the completeness of the transesterification reaction. A higher conversion of feedstock oils to ester gives better engine performance, as un-reacted feedstock oils lead to carbon deposits on fuel injector tips.

The ester content of biodiesel can vary widely depending on:

  • Molar ratios of glycerides to alcohol;
  • Type of catalyst(s) used;
  • Reaction temperature;
  • Reaction time;
  • Water content; and
  • Free fatty acid content of feedstock oils (which inhibit the desired reaction).

Other factors that affect the ester content of biodiesel to a lesser extent are:

  • Glycerol content of feedstock oils;
  • Type of alcohol used in the transesterification reaction;
  • Amount of residual catalyst; and
  • Soap content.

Ester content issues are the most common call out for Grown Fuel services, we have always fixed the issues. Read more about ester content here.

Phosphorous comes from the feedstock, and while biodiesel has a low phosphorous content, it is important to specify it as a parameter as it affects engine operability and can cause damage to catalytic converters. Can be a problem, but can also be fixed.

Acid value
The total acid number (TAN) is an indication of the presence of free fatty acids or acids formed due to oil degradation and combustion (during or following processing). Acidity can also result from improper manufacturing, through remaining catalyst or excessive neutralisation.

The acid number represents the amount of base required to neutralise the sample of biodiesel, expressed in terms of mass of KOH required per mass of sample. The total base number (TBN) or alkaline number indicates the ability of the lubricant to neutralise acid compounds generated by combustion and degradation of the oil.  High acid values may result in fuel system deposits and reduced pump and filter life. Testing has found that biodiesel increases the acidity of the lubricating engine oil. Acid number is also associated with corrosion. A bad sign, it can be fixed.

Total contamination
Impurities in biodiesel mostly come from the transesterification process. Leftover catalyst and unsaponifiable matter such as free fatty acids, fatty alcohols, hydrocarbons, sterols, triterpene alcohols, carotenoids, and vitamins – distribution depends on the feedstock oil used. Most of these contaminants are removed during the washing process. Unsaponifiable matter reduces biodiesel shelf life, increases boiling point, and creates engine deposits. Easily fixed.

Free glycerol
Some free glycerol (or glycerine) can remain in the biodiesel due to an incomplete reaction or insufficient washing. A higher content of free glycerol may cause problems during storage or in the fuel system due to separation of glycerol, or can lead to injector fouling or the formation of higher aldehyde emissions. It can also cause engine deposits and fill up your vehicle’s water trap. We can easily fix free glycerine issues.

Total glycerol
Glycerol (or glycerine) is a by-product of the transesterification reaction and is separated from the ester product for other industrial applications. Total glycerol is the sum of free glycerol and bound glycerol, where bound glycerol is the content of mono, di and triglycerides. The total glycerol content mainly depends on the processing techniques used and is one of the main parameters indicating the final quality of biodiesel. Low levels of total glycerine ensure that high conversion of the oil or fat into its mono-alkyl esters has taken place. High levels of glycerol can cause injector deposits and may adversely affect cold weather operation and filter plugging. You need to change things if this is an issue at your plant, we can fix it.

Oxidation stability
The presence of dissolved oxygen or active oxygen species greatly increases the oxidation potential of the fuel and lowers the temperature at which degradation occurs. Use ox stability additives.

Alkaline metals come from the metal catalyst used in transesterification and are linked with ash formation in the combustion engine. Furthermore, certain metals are known catalysts of ester polymerisation, amongst which cobalt, lead, manganese and copper are very strong and chromium, tin and calcium rate strongly. Rarely a problem, and we can fix it.

Methanol content
Alcohol left over from the transesterification reaction can be found in the final product in small quantities. It has a low flashpoint that can affect the overall flashpoint of the biodiesel fuel. High concentrations (>5%) of methanol will impact on cetane number and fuel lubricity and will lower the flash point. Low flashpoints are associated with safety issues both in storage and in the engine itself. Free methanol is also associated with corrosion of aluminium and zinc in the fuel injection engine. Becoming a common problem with dry wash plants, we can fix it.

Copper strip corrosion
Some sulphur compounds in fuel are actively corrosive and are known as active sulphur. Acids (such as fatty acids) present can also cause corrosion. These are measured as a rate of copper strip corrosion, which indicates what storage and handling problems may arise with copper, brass or bronze. We can fix copper strip issues.

Cetane number
The cetane number measures the readiness of a fuel to auto ignite when injected into the engine. It is also an indication of the smoothness of combustion. The cetane number of biodiesel depends on the distribution of fatty acids in the original oil or fat from which it was produced. The longer the fatty acid carbon chains and the more saturated the molecules, the higher the cetane number.

Good ignition from a high cetane number assists in easy starting, starting at low temperature, low ignition pressures, and smooth operation with lower knocking characteristics. Low cetane fuel with poor ignition qualities causes misfiring, tarnish on pistons, engine deposits, rough operation and higher knocking (thus noise level). Exhaust emissions of white smoke increased with increasing cetane numbers. Cetane is out of our hands as it is a property of the fuel, we can implement measures at your plant to raise or lower Cetane number.