BIODIESEL: HISTORY, PROPERTIES, TYPES, ADVANTAGES, DISADVANTAGES - CHEMISTRY - 2025

The biodiesel fuel is a natural origin which is obtained the reacting vegetable oils or animal fats with low molecular mass alcohols. This reaction is called transesterification; that is, new fatty acid esters (also called mono alkyl esters) are formed from the original triglycerides.

In other contexts instead of using the word 'transesterification', it is said that the biomass undergoes an alcoholysis, because it is being treated with alcohols; among them and predominantly, methanol and ethanol. The use of methanol to produce this biofuel is so common that it is almost synonymous with it.

Biodiesel pump B5. Source: Pxhere.

Biodiesel is a green alternative for the use of diesel fuel, diesel or petrodiesel (highlighting even more that its composition consists of petroleum hydrocarbons). However, their properties and quality in terms of performance in diesel engines do not differ too much, so that both fuels are mixed in different proportions.

Some of these blends may be richer in biodiesel (B100, for example) or richer in petrodiesel (with just 5-20% biodiesel). In this way, diesel consumption spreads as biodiesel is introduced to the market; not without first overcoming a series of ethical, productive and economic problems.

From a simple point of view, if oil can be obtained as a liquid capable of burning and generating energy to move machines, why not an oil of natural origin? However, this alone is not enough: you must receive chemical treatment if you want to compete or keep up with fossil fuels.

When this treatment is carried out with hydrogen, one speaks of a refinement of the vegetable oil or animal fat; its degree of oxidation is low or its molecules are fragmented. Whereas in biodiesel, instead of hydrogen, alcohols (methanol, ethanol, propanol, etc.) are used.

History

Transesterification reaction

The answer to the first problem that biofuels would face was discovered in the past. Back in 1853 two scientists, E. Duffy and J. Patrick, achieved the first transesterification of a vegetable oil, even long before Rudolf Diesel started his first working engine.

In this transesterification process, the triglycerides of oils and / or fats react with alcohols, mainly methanol and ethanol, to produce methyl and ethyl esters of fatty acids, as well as glycerol as a secondary product. A basic catalyst such as KOH is used to speed up the reaction.

The most important point of the transesterification of fats is that eighty years later a Belgian scientist, named G. Chavanne, would redirect this reaction to decrease the high and counterproductive viscosity of vegetable oils.

Rudolf Diesel and his engine

The diesel engine emerged in 1890, already at the end of the 19th century, in response to the limitations of steam engines. It brought together everything you wanted from an engine: power and durability. It also worked with any type of fuel; and to the admiration of Rudolf himself and the French government, he could work with vegetable oils.

Being triglycerides sources of energy, it was logical to think that when burned they would release heat and energy capable of generating mechanical work. Diesel supported the direct use of these oils, as it welcomed the fact that farmers could process their own fuels in places far away from oil fields.

The first functional model of the diesel engine was a success at its presentation on August 10, 1893, in Augusta, Germany. Its engine ran on peanut oil, as Rudolf Diesel firmly believed that vegetable oils could rival fossil fuels; but just as they were processed in a crude way, without subsequent treatments.

This same engine that ran on peanut oil was unveiled at the World's Fair in Paris in 1900. However, it did not attract as much attention because by then oil was a much more accessible and cheaper source of fuel.

Petrodiesel

After Diesel's death in 1913, diesel oil (diesel or petrodiesel) was obtained from petroleum refining. And so the diesel engine model designed for peanut oil had to be adapted and rebuilt to work with this new fuel, which was less viscous than any other vegetable or biomass oil.

This is how petrodiesel prevailed for several decades as the cheapest alternative. It was simply not practical to sow large hectares of vegetable masses to collect their oils, which in the end, being so viscous, ended up causing problems for the engines and did not equal the same yields obtained with gasoline.

The problem with this fossil fuel was that it increased the pollution of the atmosphere, and it also depended on the economics and politics of oil activities. Given the impossibility of resorting to it, in some contexts vegetable oils were used to mobilize heavy vehicles and machinery.

Biofuel in World War II

When in the Second World War oil began to be scarce as a result of the conflict, several countries found it necessary to turn again to vegetable oils; but they had to deal with the damage of hundreds of thousands of motors due to the difference in viscosity that their design could not tolerate (and even less if they had emulsified water).

After the war, the nations once again forgot about vegetable oils and resumed the practice of burning only gasoline and petrodiesel.

Birth of biodiesel

The viscosity problem had been solved on a small scale by Belgian scientist G. Chavanne in 1937, who was granted a patent for his method of obtaining ethyl esters of fatty acids from ethanol-treated palm oil.

It can be said, therefore, that biodiesel was formally born in 1937; but its planting and mass production had to wait until 1985, carried out at an Austrian agricultural university.

By subjecting these vegetable oils to transesterification, the viscosity problem was finally solved, matching the performance of petrodiesel and even representing a green alternative above it.

Properties

The properties of biodiesel depend globally on the raw material with which it was produced. It can have colors ranging from gold to dark brown, a physical appearance that depends on the production process.

In general terms, it is a fuel with good lubricity, which reduces the noise of the engines, lengthens its life time, and requires less investment for its maintenance.

It has an ignition point higher than 120ºC, which means that as long as the outside temperature does not exceed this, there is no risk of fire; This is not the case with diesel, which can burn even at 52ºC (very easy to achieve for a lit cigarette).

Because it is free of aromatic hydrocarbons such as benzene and toluene, it does not represent a carcinogenic risk in case of spill or prolonged exposure.

Likewise, it does not have sulfur in its composition, so it does not produce polluting gases SO ₂ or SO ₃. When mixed with diesel, it gives it a greater lubricating character than its natural sulfur compounds. In fact, sulfur is an undesirable element, and when diesel is desulfurized it loses lubrication that must be recovered with biodiesel or other additives.

Obtaining and production

Biodiesel is obtained from transesterified vegetable oils or animal fats. But, which of all of them should constitute the raw material? Ideally, the one that generates greater amounts of oil or fat from a smaller growing area; that in more appropriate terms, it would be the number of hectares that your farmland occupies.

Good biodiesel must come from a crop (grains, seeds, fruits, etc.) that produces large volumes of oil from small fields; otherwise, their crops would be required to cover entire countries and would not be economically viable.

Once the biomass has been collected, the oil must then be extracted through infinite processes; among them, for example, is the use of supercritical fluids to carry and dissolve oil. Once the oil is obtained, it is subjected to transesterification in order to reduce its viscosity.

Transesterification is achieved by mixing the oil with methanol and a base in batch reactors, either under ultrasound, supercritical fluids, mechanical stirring, etc. When methanol is used, fatty acid methyl esters (FAME, for its acronym in English: Fatty Acid Methyl Ester) are obtained.

If, on the other hand, ethanol is used, fatty acid ethyl esters (FAEE) will be obtained. It is all these esters and their oxygen atoms that characterize biodiesel.

Methanol and glycerol

Methanol is the alcohol predominantly used as a raw material in the production of biodiesel; and glycerol, on the other hand, is a by-product that could be used to support other industrial processes and therefore make biodiesel production more profitable.

Glycerol comes from the original triglyceride molecules, which is replaced by methanol to create three DMARDs.

Types of biodiesel

Different oils or fats have their own fatty acid profiles; therefore, each biodiesel has different mono-alkyl esters as a result of transesterification. Even so, since these esters hardly differ in the lengths of their carbon chains, the resulting fuels do not show large oscillations between their properties.

So there is no classification for biodiesel, but rather a different efficiency and profitability depending on the source of oil or fat that is selected for its production. However, there are biodiesel-petrodiesel mixtures, because both fuels can be mixed and are miscible with each other, providing their beneficial qualities for the engine.

Pure biodiesel is said to be B100; which is equal to 0% petrodiesel in its composition. Then there are other mixes:

- B20 (with 80% petrodiesel).

- B5 (with 95% petrodiesel).

- B2 (with 98% petrodiesel).

Cars built before 1996 could not use B100 in their engines without having to replace certain components that deteriorated due to its solvent action. However, even today there are car models that do not allow large concentrations of biodiesel in their factory warranties, so they recommend using mixtures lower than B20.

Advantage

Below is a breakdown of a series of advantages that biodiesel has over petrodiesel and that make it a green and attractive alternative:

- It is obtained from biomass, a raw material that is renewable and that is often lost as waste.

- It is biodegradable and non-toxic. Therefore, it will not pollute the soils or seas if accidentally spilled.

- Its high flash point makes it safer when storing and transporting it.

- It does not produce greenhouse gases because the CO ₂ released represents the same amount of that absorbed by plants. Thanks to this, it also complies with the Kyoto protocol.

- Encourages rural activities for planting crops from which vegetable oil is extracted.

- It can even be produced from fried oil. This point greatly favors you because recycled oil, domestic or from restaurants, instead of being disposed of and polluting groundwater, can be used to produce more green fuel.

- Represents a way to become independent in the long term from oil and its derivatives.

- Leaves less residue when burning.

- Bacterial algae are, in addition to soybeans and sunflower seeds, a promising source of inedible (and undesirable for many) biodiesel.

Drawbacks

Not everything is perfect with this fuel. Biodiesel also has limitations that must be overcome if it is to replace petroleum diesel. Some of these limitations or inconveniences of its use are:

- It has a higher solidification temperature, which means that at low temperatures it becomes a gel.

- Its solvent power can destroy the natural rubber and polyurethane foam present in cars assembled before 1990.

- It is more expensive than petrodiesel.

- Increases the prices of crops and food because they incorporate an added value when used as biodiesel raw material.

- Depending on the biomass, it may need many hectares of cultivation, which would mean the taking of ecosystems foreign to this purpose, and would therefore affect the wild fauna.

- Although it does not produce sulfur gases during its combustion, it does release higher concentrations of nitrogen oxides, NO _x.

- Large quantities of food would be used, which instead of satiating famines, would be used for the production of biodiesel.

References

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5. Scott Hess. (2019). How biodiesel works. Howstuffworks. Recovered from: auto.howstuffworks.com
6. Pacific Biodiesel. (2019). Biodiesel. Recovered from: biodiesel.com