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HMRC internal manual

Biofuels and Fuel Substitutes Assurance

HM Revenue & Customs
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Biodiesel and bioblend - production, storage and delivery: biodiesel: processes used in production


When production of biodiesel is by transesterification, the raw material may need to be blended prior to process. This is dictated by the specific properties of the various vegetable oils, which may also be affected by temperature and seasonal variation.

Although the raw material is not liable to excise duty at this stage, the trader would still be expected to exercise effective stock control and maintain accurate production records to demonstrate that the materials have been put to biodiesel production.

(This content has been withheld because of exemptions in the Freedom of Information Act 2000)

(This content has been withheld because of exemptions in the Freedom of Information Act 2000) .

Transesterification as a process.

Transesterification involves the identification of the quantity of the catalyst (caustic soda) required to convert a given batch of vegetable oil (often delivered to a user specification concerning the Free Fatty Acid (FFA) content). If the FFA factor is high, the oil will not readily convert. The free fatty acids in vegetable oil change the more it is heated, e.g. for frying foods.

The quantification is established via ‘titration’ using a small known volume of oil dissolved in a known volume of alcohol (e.g. IPA). The caustic is dissolved in water and added in millilitre amounts until the pH reaches around 8/9.

The proportion of caustic soda is determined by adding millilitres to a given factor to compute the grams of catalyst required per litre of oil (specification).

The caustic is usually pre-mixed with methanol (10/15% by weight in relation to the mass of oil to be converted). The mixing of caustic soda generates considerable heat. The sodium methoxide produced is added to the pre-heated oil and vigorously mixed. As the reaction progresses, conversion and separation take place and crude glycerol sinks to the bottom of the reaction vessel. The biodiesel (methyl ester) rests on top.

The glycerol is drained off and the biodiesel may be washed with water to remove any soaps or salts and to neutralise the effect of any excess alkali present. The solution is allowed to separate before the biodiesel is filtered/centrifuged, ready for use. The purpose of the reaction is to separate the fatty acid chains (typically palmatic, stearicoleic and linolic acids).