**Biodiesel**

Biodiesel refers to the esters obtained by transesterification of triglycerides found in oils and fats. Biodiesel can be produced from different triglyceride sources such as vegetable oils (that can be edible, non-edible or waste oils [14]), animal fats (mostly edible fats or waste fats) and microalgae oil. The crops identified for biodiesel production are corn, sunflower, palm, olive, canola, soybean, rape and peanut oils. The main transesterification reaction for biofuel production is as follows (**Figure 24**):

This conversion goes in three steps where triglycerides react with methanol to give diglylcerides. These react with another methanol to monoglycerides. Finally monoglycerides react with a third methanol to five long chain esters (also called fatty acid methyl esters - FAME) and glycerol. European EN 590 specifications require gas oil fuel to contain a maximum of 7%vol biodiesel which must be compliant with the standard EN 14214 while US specifications ASTM D975 allows mixing commercial diesel oil with 5%vol biodiesel that meets the requirements of ASTM D6751.

Long chain esters (where the triglyceride R chain is from C8 to C20) are used as a blending component for gas oil fuels since the combustion ignition delay is generally long enough to be used for this application. In fact, esters display a paraffin-like reaction chemistry as their functional group provides an attractive site for H atom abstraction. These radicals are stabilized by resonance which slows down the reaction to yield an unsaturated ester which is resonance stabilized and thus has poor low temperature reactivity as follows:

Another reaction path is an elimination reaction involving a six-membered transition state producing olefins, small esters or acids. However, this is the preferred route only when the O-alkyl side is long. This clearly not the case for FAME as the O-alkyl side is the shortest possible (a methyl) (**Figures 25** and **26**).

Thus, the slow H atom abstraction is the predominant route for FAME which increase their ignition delay and consequently increase the cetane number. Note that branching effect reduces the activation energy needed for the six-membered transition state and thus straight chain and saturated R chains promote higher cetane numbers.

The main quality concerns for biodiesel are cetane number, cold properties and stability. As discussed, unbranched and saturated alkyl chains promote

#### **Figure 24.**

*Transesterification reaction.*

**Figure 25.** *Stabilized resonance and poor low temperature reactivity.*

*Quality and Trends of Automotive Fuels DOI: http://dx.doi.org/10.5772/intechopen.94167*

**Figure 26.**

*Reaction paths for producing olefins, small esters or acids.*

#### **Figure 27.**

*Each feedstock is set apart from the others because it is made of different proportions of saturated, monounsaturated, and polyunsaturated fatty acids.*

cetane number. Saturation also promotes stability as olefinic sites promote oxidation reactions. However, saturation also promotes high melting points and thus leads to high cold properties. The middle ground is to have a monounsaturated FAME such as rape seed oil methyl ester which is composed of 63.9mass% C18:1 (alkyl group is 18 carbon atoms long having one double bond). However, as shown below (**Figure 27**):

Advantages of biodiesel are cleaner burning of engines, lower emissions, better lubrication of fuel injection pumps, safe due to high flash points, non-toxic and low volatility, spills are biodegradable, mixes well with mineral diesel and needs little engine adaptations [15]. However, disadvantages are cost, restricted shelf life, season sensitive, hygroscopic, housekeeping critical, filterability issues and could be foodstuff competitive.

Hydrotreating of oils and fats is a novel process for producing renewable paraffinic diesel, abbreviated HVO. In production, hydrogen is used to remove oxygen from the triglyceride vegetable oil or animal fat molecules. Hydrogen needed for the HVO process is today made from natural gas, but it could also be made from biogas or other renewable sources. When comparing HVO and biodiesel (FAME) production processes it can be concluded that both need about the same amount fossil feed i.e. hydrogen for HVO and methanol for FAME. However, HVO production is even more expensive than biodiesel.

According to research the latest twenty years the maximum allowable limit of biodiesel in gasoil fuels is 20% v/v named B20 without engine or other issues.
