The use of additives to improve certain fuel properties – which has been normal for fossil fuels for a long time – has also gained increasing importance concerning Biodiesel. So far predominantly flow improvers and oxidation stabilisers are utilised; but additional additives are imaginable, too, and may play a part in the production and application of Biodiesel in the future.

The utilisation of stabilisers in Biodiesel as blend component is called for more and more frequently in order to ensure sufficient stability of the final fuel; the European Standards for Diesel fuel and Biodiesel (EN 590, DIN 51628, EN 14214 as well as the French national B7 specification) even expressly advised it, with recommended doses of BHT or BHT equivalent varying between 200 mg/kg (DIN 51628) and 1.000 mg/kg (EN 590 and French B7 specification).

Apart from the mostly utilised stabiliser BHT (3,5-di-tert-butyl-4-hydroxy-toluene) there is a variety of other products some of which show a significantly better effect and are also suitable for the use in Biodiesel. However, it is of great importance for the release of those substances for the blend component Biodiesel that they do not show any unwanted interaction with the Diesel fuel, its additives or the engine oil. For a safe use – from blending to the application and combustion in the engine – negative impacts of those additives must be detected beforehand by means of suitable test methods (so-called “No-Harm-Tests”).

In cooperation with the mineral oil industry a catalogue of test methods was defined by means of which oxidation stabilisers can be tested for possible unwanted side effects. The catalogue comprises the following test criteria:

• Observance of minimum requirements, such as:

• no alteration of safety relevant Biodiesel properties (e.g. flash point, water hazard class)

• no impermissible alteration of standard parameters according to EN 14214 for Biodiesel

• submission of an explicit application description for the additive

• XUD9 Test, according to CEC F-23-1-01 (engine test – nozzle fouling)

• DGMK filtration test 663

• Check of compatibility with engine oil (derived from DGMK 531-1)

Products which pass the test and fulfil the criteria above are published in an AGQM list which is named as a reference in the German Diesel fuel standard DIN 51628. Additionally, it is determined according to a defined method (Relative Efficiency Test – RET) on the basis of four differing scenarios which doses of the tested stabilisers effectively correspond to the reference dose of 200 mg BHT/kg.

The evaluation of the tested additives refers to analysis carried out in a reference diesel fuel. Interactions with the specific additive packages of mineral oil companies were not analyzed in this context.

AGQM’s list is open – interested producers of additives can apply any time for their products to be tested. The list of the stabilisers tested already (as by Dec 31, 2008 and March 24, 2010 and the relevant test reports are available and can be obtained from AGQM.

The products of the following producers have successfully passed the No-Harm-Test (as byMarch 24, 2010)

Note: The order and the registration numbers are only for internal use and are no assessment or rating of the products.

For all of the products given above the relative efficiency in comparison with BHT was determined in four differing scenarios. In case of legitimate interest the data can be passed on with the consent to the company concerned.