1.3- BIODIESEL

1.3.1- COMPOSITION

Biodiesel is a term which involves any ester obtained from biological sources, generally vegetal oils, which has been chemically combined with simple alcohols (transesterified) in order to make it able to work as a liquid combustible in Diesel engines. The “oil” term comprises a wide list of diverse lipids (triglycerides) from any origin. It is non-toxic and, like all the other biofuels, biodegradable and hygroscopic.

1.3.2- HISTORY AND RELEVANCE

The transesterification process, used to obtain glycerine and esters (the current biodiesel) from lipids and alcohol, was discovered in 1853 by scientists J. Patrick and E. Duffy. By that time, only glycerine was commercialised, and esters were considered a sub-product. Forty years later, Rudolf Diesel completed and run, by the first time, his compression-ignition engine with mere pure peanut oil (not transesterified). It has not been until the 1980’s when biodiesel has been reconsidered as a viable alternative to petroleum as a transport energetic source.

Currently, it is in Europe where biodiesels have their greatest relevance: unlike other regions in the world, the Diesel-cycle engine has a great role in automotion, owing to that half of the current sold cars run in it. The leading biodiesel investing countries are Austria, France, Germany and the Czech Republic.

1.3.3- PROPRIETIES AND CHARACTERISTICS IN ENGINE. APPLICATIONS

Pure oil is also capable to be used as a fuel, but it has relevant problems like its higher solidification point, a superior viscosity and a great quantity of impureness that cause sedimentation and obstructs injection systems. That’s the reason why oil is transesterified in order to optimize its characteristics.

Biodiesel can be used alone or in blends with gasoil (those mixtures are called “B” plus the ester percentage). The engine operability doesn’t suffer important variations, with the only differences of a quite lower energetic value of the biodiesel against gasoil’s (42 MJ/L and 43 MJ/L, respectively), that increases the consume, and the necessity of replacing bronze, copper, lead, zinc or rubber parts for other materials. However, blended in low percentages, i.e. B5 and B20, it can be used in standard Diesel-cycle engines.

1.3.4- OBTAINING

Currently, the industrial obtaining of the biodiesel consists in grinding and extracting the fat of vegetable matter from agricultural crops, as well as from animal feedstock.
Once having the oil feedstock, it is necessary to chemically treat it through a transesterification process. That method requires the reaction of an alcohol with the oil (triglyceride) which generates another ester (biodiesel) plus glycerine as a residual by-product. In this case, the used alcohol is supposed to be methanol.

It is convenient to consider that biodiesel receives different scientific names depending on the reactive used during its treatment. While in Europe and the US, where methanol is the most used compound, it is called fatty acid methyl ester (FAME), in Brazil, where ethanol is used instead, it is named fatty acid ethyl ester (FAEE).

Schematic map of biodiesel production via transesterification. Source: CAMPS, Manuel & MARCOS, Francisco. Los biocombustibles. Madrid : Ed. Mundi-prensa; Colección energías renovables, 2002.

Before starting the process, it is necessary to mix the catalyst (commonly potassium hydroxide) with the methanol, a blending that will emit heat that will be useful for pre-warm the reactive components. Once joined the oil with an alcohol-catalyst blend excess inside the reactor, the temperature must be of 50 ºC to start the process.

In order to increase its speed, it would be convenient to install an electric agitator to keep the components mixed. In those conditions, a conversion of the 85% will be realised in approximately 10 minutes, but the process will become slower from this point. The transesterification will be considered completed when a 98% of the transformation is achieved.

This is the graphic representation of the reaction:

Triglyceride (oil) + methanol (catalyst: KOH) Glycerine + Methyl ester

Source: CAMPS, Manuel & MARCOS, Francisco. op. cit.

Once finished this process, the glycerine and the methanol surplus must be removed from the mixture. Glycerine can be extracted via centrifuge separation, which divides the original blend in two phases with different density. After that, methanol can be easily evaporated because of its high volatility, so the resultant phases might be formed by those compounds in the following percentages:

Glyceric phase (heaviest):
• Glycerine: 80,9%
• Impurities: 12,4%
• Potassium hydroxide: 6,7%

Ester phase (lightest):
• Ester (functional biodiesel): 99%
• Oil (non-reacted fraction):1%

Focusing now the attention on the origin of the fats, biodiesel, in the same way as bioethanol, can be distinguished as a first, second or third generation biofuel depending on the obtaining of the prime matter:

· FIRST GENERATION PRIME MATTER

Extracting oils from alimentary crops is the current obtaining process of the biodiesel prime matter. It has the problematic of a low competitiveness versus the present petroleum market, and the use of products normally dedicated to human alimentation.

The msot used crops are rapeseed, sunflower, soybeans and coconut. Biodiesel from animal fat is little common due to the inferior easiness of its obtaining in comparison with great-scale crops; however, it's technically feasible.

· SECOND GENERATION PRIME MATTER

A much more efficient way to produce biodiesel is to use non-alimentary plants with a higher production per space ratio. An example of a high-efficient crop is the Jatropha curcas cultivation, a vegetable which can be planted in desert zones and whose seeds contain oil. It doesn't require the use of pesticides because of its natural resistance, and survives in adverse environments. Only a few countries have invested in those alternative crops.

· THIRD GENERATION PRIME MATTER

The biofuel market has a deep reliance in the improvement of the algae oil-obtaining technologies. Because of their fast growing, their high energy per space ratio, their notorious CO2 absorbing capacity and their non-competence with the alimentary market, the algae cultivation is considered as the most promising future biodiesel source. Anyway, those technologies aren't still properly deveolped or applied in the global energetic market.