Biodiesel is produced chemically through the transesterification of vegetable oils with methanol. The general acronym for all methyl ester based on vegetable and animal oil is FAME (fatty acid methyl ester according to DIN EN 14214).

In times of increasing fuel prices, more and more private and commercial diesel vehicle owners are switching to biodiesel.

Biodiesel can be used in its pure form – known as B100 – in suitable engines or as a blend with mineral diesel in any mixing ratio (e.g. B7 with 7% FAME).

A major disadvantage of biodiesel is poor aging resistance owing to the easy oxidizability of unsaturated fatty acids by atmospheric oxygen (such as all natural vegetable oils). The consequences of oxidation are, on one hand, a breakdown of the biodiesel into short-chained fatty acids, and the formation of indissoluble polymers (gums) on the other. This may result in engine damage. The oxidation of oils and fats by atmospheric oxygen is known as rancidity. Rancid oils and fats are unfit for use.

The fatty acid methyl ester is attacked by radicals (if possible directly next to the double bond) owing to heat, light and other stress factors. The radicals react quickly with the atmospheric oxygen and peroxy radicals are formed. Now the autocatalytic oxidation process (also known as auto-oxidation) can begin. The peroxy radicals are extremely reactive and abstract weakly bonded neighboring electrons, creating new radicals. The radical concentration multiplies exponentially within a short space of time thanks to this mechanism (simplified illustration Fig. 1).

The stability of biodiesel can be significantly improved with anti-oxidant additives. These are expensive and should therefore be used as sparingly as possible. ACL Instruments’ products enable precise and extremely sensitive characterization of the stability of biodiesel and the stabilizing effects of additives.

leaflet Biodiesel FAME
Stability of esterified canola oil (biodiesel) against oxidation. The experiments were performed under synthetic air and isothermal temperature conditions: heating from 25°C to 100, 90 and 80°C respectively under nitrogen (gasflow = 30 mlmin-1). After 60 min,  the gas was switched to the oxidising atmosphere (flow = 30 mlmin-1). Data evaluation: OIT (oxidation induction time = time (h) from the gas switch until the onset of the oxidation of the sample).
Arrhenius-plot depicting the OIT of esterified canola oil (biodiesel): the experiments were performed in synthetic air (gasflow = 30 mlmin-1) at isothermal temperature conditions (100, 90 and 80°C).
Arrhenius plot

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