Biodiesel and unsaturation effects

The chemical composition of biodiesel is dependent upon the length and degree of unsaturation of the fatty acid alkyl chains. With few exceptions, the carboxylic (fatty) acids are all straight-chain compounds ranging in size from 8−20 carbons. Acids may be saturated (cannot chemically add hydrogen), which means that they contain only single bonds, or unsaturated (can be hydrogenated), which means that they contain at least one double bond. For each double bond, one hydrogen molecule can be added per fatty acid.

Although for the majority of the biodiesel feedstocks the primary unsaturated acids are oleic, linoleic and linolenic ,there are some exceptions. For example, castor is rich in the mono-unsaturated ricinoleic acid (contains hydroxyl), which is responsible for the FAME‟s high viscosity and density. Fish, on the other hand, is rich in poly-unsaturated acids of longer than 18 carbon atoms chain, comprising up to 6 double bonds. Apart from castor, other feedstock that are well known to differentiate as regards their composition (and some of the corresponding biodiesel properties  are: coconut, palm, beef tallow, and lard

A very influencing parameter of biodiesel (feedstock) is its degree of unsaturation, with a usual measure being the iodine number.

There are three approaches that are mostly used to evaluate the degree of unsaturation

-        The first simply counts the saturated and unsaturated percentage weights without any distinction between mono-unsaturated and poly-unsaturated fatty acids (termed  „unweighted‟ degree of unsaturation).

-       In the second approach, all fatty acids with 3 or more double bonds are assumed to weigh equally as the ones with 2 double bonds (termed „partially weighted‟ degree of unsaturation) [. In the third and more accurate approach, each unsaturated fatty acid weighs according to the number of double bonds in its molecule („fully weighted‟ degree of unsaturation);

-         The later degree of unsaturation also corresponds to the average number of double bonds. The average chain length of each feedstock; with the clear exception of coconut, all the other oils range from 17 to 18 carbon atoms on average.

   

For those feedstocks with small amount of linoleic and linolenic acids (e.g. coconut) the „un-weighted‟ and the „partially‟ or even „fully weighted‟ degrees of unsaturation are comparable. Likewise, for those feedstocks where the majority of unsaturated fatty acids are 18:1 oleic and 18:2 linoleic (e.g. animal fats, castor, corn, cottonseed, hazelnut, jatropha, mahua, neem, palm, peanut, safflower, sunflower, the partially weighted‟ and the „fully weighted‟ degrees of unsaturation are close or even equal. The existence of fatty acids with three (e.g. linolenic) or more double bonds is reflected in the differences between the „partially‟ and the „fully weighted‟ degrees of unsaturation. The most obvious cases where the „partially‟ and the „fully weighted‟ unsaturation degrees differ by a lot are those feedstocks that are rich in linolenic (primarily linseed, and to a lesser extent rubber seed, and rapeseed/canola) or even more highly unsaturated acids (fish).

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Iodine value

The iodine number (IN, or iodine value IV) is a parameter used to determine the degree of unsaturation in a vegetable oil or animal fat. This number indicates the mass of iodine (I₂) in grams that is necessary to completely saturate, by means of a stoichiometric reaction, the molecules of 100g of a given oil. The (average) iodine values of the examined feedstocks range from 7.8 (for the most saturated ME, coconut) to 184.5 (for the most unsaturated one, linseed), with an overall average value of 98.4. There is no specification in the US for the IN, but European specifications require that biodiesels used in compression ignition engines have a (rather low) maximum value of IN of the order of 120. The idea behind this specification is that high fuel iodine values indicate propensity for polymerization resulting in deposit formation. This means that many of the investigated FAMEs have to be excluded from use in pure form in Europe, namely croton, fish, linseed, rubber seed and safflower, whereas corn, soybean and sunflower are only marginally accepted; moreover, the most popular in Europe rapeseed ME, having an average IV of 112, is actually quite close to the specification limit.

Since the composition of the parent oil/fat was found to affect the final ester‟s properties decisively, a question arises as to which is the biodiesel feedstock with the best properties, or whether it is more preferable to use saturated or unsaturated oils for the production of methyl esters. Although production cost, apart from the chemical properties, plays a pivotal role here, it is not easy to answer this question in a single and irrevocable manner. Saturated feedstocks (such as those derived from coconut, palm and tallow) excel in cetane number and oxidation stability (and usually lower NOx emissions), while exhibiting poor cold flow properties, higher kinematic viscosity, lower (but still high enough) flash point and lower heating value. 

In contrast, increasing the unsaturation decreases the kinematic viscosity, improves the cold flow properties (additives are most probably still required) and increases moderately the heating value, but also lowers the CN and deteriorates the oxidation stability. Interestingly, it is the most unsaturated feedstocks that are susceptible to rejection based on the existing specification limits. Specifically, and based on the linear best-fit curves derived, feedstocks with more than 1.47 double bonds in their molecule (corn, croton, fish, linseed, rubber seed, safflower, sunflower, soybean) have to be excluded in Europe in pure form on the grounds of IN higher than 120. Further, if the number of double bonds is higher than 1.84 (corresponds to IN>148), then the minimum European limit of CN=51 is not probably met, whereas for more than 2.15 double bonds (IN>171; linseed only) it seems that the American upper limit of T90 might not be met too.

There are two feedstocks that differentiate by a lot from the others, i.e. coconut and castor. Coconut ME is the most saturated biodiesel, and is characterized by low viscosity and high cetane number. Castor, on the other hand, although exhibiting excellent oxidation stability and at the same time has very good cold flow properties, fails to fulfill three major fuel specifications namely, cetane number, viscosity and density. Interestingly, the most popular ME worldwide, SME, should be rejected in pure form in Europe since it does not meet the (rather strict) IN, the oxidation stability and usually the CN specification. Other feedstocks that do not (or only marginally) fulfill at least two specifications in the EU or the US are the most unsaturated ones, namely linseed, fish, safflower, sunflower rubber seed and croton. In any case, the use of the more realistic smaller biodiesel ratios, e.g. up to 20% v/v in the fuel blend, tends to reduce considerably the differences between the various feedstocks, and renders practically all biodiesel blends acceptable, with the possible exception of castor.