Ester content

Ester content is meant to be a guide as to the purity of biodiesel by way of measuring conversion of triglycerides to methyl esters, but it is hardly necessary as eight other parameters in the European and Australian standard cover purity and the other constituents of biodiesel. In my experience, Ester content determination is of little use due to the following:

  • The standards for biodiesel were developed by the Austrian government agriculture research department (BLT) in Wieselberg, Austria in the late 70’s and early 80’s. At that time BLT’s mandate was to determine if virgin rapeseed oil was a suitable fuel for diesel engines. Used Cooking Oil (UCO) is not pure rapeseed oil and even the canola oil regularly found in Australia is a modified form of rapeseed, with low erucic acid and low glucosinolate
  • The test method for testing Ester content is set by the European committee for Standardisation (CEN) and the following extract from their test method explains much of the difficulty in consistently meeting the 96/5% Ester concent as the biodiesel standard requires. (Note that Fatty Acid Methyl Esters (FAME) is the scientific name for biodiesel).

From the CEN test methodology:

Section 1: Scope
The purpose of this European Standard is to determine the ester content of fatty acid methyl esters (FAME) intended for use as pure biofuel or as a blending component for heating and diesel fuels. It also allows to determine the linoleic acid methyl ester content. It allows one to verify that the ester content of FAME is greater than 90 % (m/m) and that the linoleic acid content is between 1 % (m/m) and 15 % (m/m). This method is suitable for FAME that contain methyl esters between C14 and C24.

Section 11.2: Reproducibility
The absolute difference between two single test results, obtained using the same method on identical test material in different laboratories by different operators using different equipment, shall not be greater than:
For ester content 3,1 % (m/m)
For linoleic acid methyl ester content (L) 0,311 x L + 0,02 % (m/m)
more than once out of 20 determinations.

Difficulties with the CEN Ester test method:

To summarise, the test method:

  1. Was designed to verify that Ester content is greater than 90%
  2. Is suitable for Fatty Acid Methyl Esters (FAME) that contain Esters between C14 and C24, e.g. pure virgin rapeseed oil
  3. Ester content only allows 3.5% of the product to be comprised of oil with carbon chains outside C14 and C24
  4. Repeatability between labs is only expected to be within 3.1%

Regarding point 1:

The biodiesel industry is asked to meet a set Ester content of 96.5% using a test method that is only intended to verify an Ester content of >90%.

Regarding point 2:

In 2008 the biodiesel standard in Australia was changed to allow a greater scope, that is, a larger amount of carbon chain lengths to be included in the Ester content using a modified procedure or allowance as it was physically impossible to meet the standard when, for instance, any coconut or fish oil was present in the oil used to make biodiesel. Hence it has been adapted to allow a wider variety of fats and oils to make biodiesel.

At no time was any vehicle testing conducted to prove or disprove which fats and oils affect engine performance or longevity. The entire process has been done as a desktop analysis of assumptions. The biodiesel industry has welcomed the change as many plants would have otherwise gone out of business because it was almost impossible to pass the test but it is not a useful test to determine if biodiesel is fit for a combustion engine.

The Ester test method is not appropriate for determining quality biodiesel unless it is made using virgin rapeseed oil.

Regarding point 3:

Used Cooking Oil containing any oils with carbon chains outside C14 and C24 will never meet this specification. On top of this, a repeatability allowance of 3.1% variance between labs makes it near impossible to consistently meet the Ester content using the variety of oils contained in UCO and varying lab results.

The Australia fuel quality standard is expected to follow evolving international standards and go the way of the US biodiesel standard ASTM D6751-12 Standard Specification for Biodiesel Fuel Blend Stock (B100) for Middle Distillate Fuels, by eliminating the Ester content test from the standard altogether.