Biodiesel is a mixture of fatty acid alkyl esters (FAAE) (mainly methyl esters) produced from lipids via transesterification (Fig. 5.2) of the acylglycerides or esterification (Fig. 5.3) of fatty acids.

Theoretically 1 mol triglycerides is reacting with 3 moles alcohol producing 3 moles esters and 1 mol glycerol.

Methanol is the major alcohol used because of the lower price but other alcohols can also be used such as ethanol, isopropanol and butanol. Although the latter alcohols can give better fuel properties, they are not used on an industrial scale due to their higher price and processing problems.

Biodiesel can be produced from a variety of feedstocks including edible vegetable oils (soybean, rapeseed, palm, sunflower, palm kernel and coconut), animal fats, non­edible oils (jatropha, camelina, rice bran, pongomia, thelvetia, etc.) and side-streams from refining (soapstock, acidulated soapstock and deodorizer distillates). A future feedstock will be algae growing either in open fields or closed reactors. The yield of oil/ha is estimated to be at least ten times higher and can be produced at any place.

The cost for the production of biodiesel consists of 85% of the feedstocks. A process model to estimate biodiesel production cost has been developed. This flexible model can be modified to calculate the effects on capital and production costs of changes in feedstocks costs, changes in the type of feedstocks employed, in the value of the glycerol co-product and change in process chemistry and technology (Haas et al., 2006).

According to the feedstock and technology used, a distinction is made between biodiesel from first and second generation (Table 5.1). Biodiesel of the first generation is considered as FAAE produced by the traditional alkaline catalyzed transesterification reaction from refined edible vegetable oils and animal fats. Biodiesel from the second generation is the production of fatty acid methyl esters (FAME) or other esters from resources other than edible oils and in most cases using alternative technologies. It is obvious that these resources are not in competition with food/feed production and can be considered as more sustainable and more ethical.

Biofuels from the third generation produced from lipid resources are oils and fats generating power and heat (CHP, couple heat and power) in stationary diesel engines and green diesel produced by hydrotreating of oils and fats producing linear alkanes, propane, CO, CO2 and water.

Why are FAMEs suitable as diesel fuel? Conventional diesel fuel for transportation (DF2) is a product obtained by cracking of petroleum and consist mainly of long chain unbranched alkanes (C14-C24) with a boiling range of 180— 240°C, cetane number (CN) of 40-50 and heat of combustion of 45 000 kJ/kg. Biodiesel has a similar chemical structure (except for the presence of the ester function) of long chain (C12-C22) with a higher boiling range (250-450°C), CN between 40 and 80 and heat of combustion of 40 000 kJ/kg. Due to the similarity in structure, CN and energy value fatty acid esters are readily replacing diesel.

Biodiesel is miscible with petrodiesel in all concentrations, namely blends B5, B20, etc. which corresponds to the percentage of biodiesel in diesel. Blend up to 20% can be used without modification of the engines. Higher blending will require modifications due to the solvent properties of the esters which are affecting the rubber tubings and fittings.

In a comparison between biodiesel and diesel the following observations can be made:

1. The CN for the biodiesel from soybean and rapeseed oil is slightly lower. The CN of esters correlates well with the boiling points. The CN from palm oil and animal fats are higher.

2. The heat of combustion is 13% lower than for DF2. However, due to the higher density there is eight per cent difference expressed in volume.

3. The viscosity of biodiesel is two times higher.

4. Biodiesel has higher cloud point (CP) and cold filter plugging point (CFPP).

5. Biodiesel is an oxygenated fuel which results in a cleaner burning.

6. Biodiesel has a higher lubricity which is advantageous in low sulfur content.

7. Biodiesel has a higher flash point.

8. Biodiesel does not contain sulfur.

9. Biodiesel has lower fine particulate matter, lower polyaromatic hydrocarbons and SO2 emissions.

10. Biodiesel has higher NOx emissions.

Biodiesel can only be commercialized and sold as biodiesel on condition when it complies with biodiesel standards EN14214:2009 (EN) or ASTM D 6751 (USA). The European specifications are summarized in Table 5.2.

The most important parameters concern the ester content (minimum 96.5%) and the acid value (maximum 0.5 mg KOH/g). The ester content is influenced by the quality of the technology and processing but also by the composition of the used feedstock. The unsaponifiable fraction (sterols, tocopherols, hydrocarbons, etc.) present in the vegetable oils in a range of one to two per cent stays in the biodiesel will decrease the theoretical ester content already to 98-99%. Other important parameters are sulfur, phosphorous, alkali metals, total contamination and non-reacted acylglycerols. The CP is region dependent while there is a difference in the standard for oxidative stability between the EU and the USA (rapeseed biodiesel has a higher oxidative stability than soy biodiesel). Some of these parameters are more difficult to achieve when alternative feedstocks are used. Usage of additional pre-refining and/or post-treatment is required to guarantee the compliance with the biodiesel standards.