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2.1 Introduction

Biodiesel [1–5] is a liquid biofuel obtained by chemical processes from vegetable oils or animal fats and an alcohol that can be used in diesel engines, alone or blended with diesel oil.

ASTM International (originally known as the American Society for Testing and Materials) defines biodiesel as a mixture of long-chain monoalkylic esters from fatty acids obtained from renewable resources, to be used in diesel engines.

Blends with diesel fuel are indicated as “Bx”, where “x” is the percentage of biodiesel in the blend. For instance, “B5” indicates a blend with 5% biodiesel and 95% diesel fuel; in consequence, B100 indicates pure biodiesel.

2.1.1 Advantages of the Use of Biodiesel

Some of the advantages of using biodiesel as a replacement for diesel fuel are [1–4]:

  • Renewable fuel, obtained from vegetable oils or animal fats.

  • Low toxicity, in comparison with diesel fuel.

  • Degrades more rapidly than diesel fuel, minimizing the environmental consequences of biofuel spills.

  • Lower emissions of contaminants: carbon monoxide, particulate matter, polycyclic aromatic hydrocarbons, aldehydes.

  • Lower health risk, due to reduced emissions of carcinogenic substances.

  • No sulfur dioxide (SO2) emissions.

  • Higher flash point (100°C minimum).

  • May be blended with diesel fuel at any proportion; both fuels may be mixed during the fuel supply to vehicles.

  • Excellent properties as a lubricant.

  • It is the only alternative fuel that can be used in a conventional diesel engine, without modifications.

  • Used cooking oils and fat residues from meat processing may be used as raw materials.

2.1.2 Disadvantages of the Use of Biodiesel

There are certain disadvantages of using biodiesel as a replacement for diesel fuel that must be taken into consideration:

  • Slightly higher fuel consumption due to the lower calorific value of biodiesel.

  • Slightly higher nitrous oxide (NOx) emissions than diesel fuel.

  • Higher freezing point than diesel fuel. This may be inconvenient in cold climates.

  • It is less stable than diesel fuel, and therefore long-term storage (more than six months) of biodiesel is not recommended.

  • May degrade plastic and natural rubber gaskets and hoses when used in pure form, in which case replacement with Teflon® components is recommended.

  • It dissolves the deposits of sediments and other contaminants from diesel fuel in storage tanks and fuel lines, which then are flushed away by the biofuel into the engine, where they can cause problems in the valves and injection systems. In consequence, the cleaning of tanks prior to filling with biodiesel is recommended.

It must be noted that these disadvantages are significantly reduced when biodiesel is used in blends with diesel fuel.

2.2 Raw Materials for Biodiesel Production

The raw materials for biodiesel production are vegetable oils, animal fats and short chain alcohols. The oils most used for worldwide biodiesel production are rapeseed (mainly in the European Union countries), soybean (Argentina and the United States of America), palm (Asian and Central American countries) and sunflower, although other oils are also used, including peanut, linseed, safflower, used vegetable oils, and also animal fats. Methanol is the most frequently used alcohol although ethanol can also be used.

Since cost is the main concern in biodiesel production and trading (mainly due to oil prices), the use of non-edible vegetable oils has been studied for several years with good results.

Besides its lower cost, another undeniable advantage of non-edible oils for biodiesel production lies in the fact that no foodstuffs are spent to produce fuel [4]. These and other reasons have led to medium- and large-scale biodiesel production trials in several countries, using non-edible oils such as castor oil, tung, cotton, jojoba and jatropha. Animal fats are also an interesting option, especially in countries with plenty of livestock resources, although it is necessary to carry out preliminary treatment since they are solid; furthermore, highly acidic grease from cattle, pork, poultry, and fish can be used.

Microalgae appear to be a very important alternative for future biodiesel production due to their very high oil yield; however, it must be taken into account that only some species are useful for biofuel production.

Although the properties of oils and fats used as raw materials may differ, the properties of biodiesel must be the same, complying with the requirements set by international standards.

2.2.1 Typical Oil Crops Useful for Biodiesel Production

The main characteristics of typical oil crops that have been found useful for biodiesel production are summarized in the following paragraphs [6–10].

2.2.1.1 Rapeseed and Canola

Rapeseed adapts well to low fertility soils, but with high sulfur content. With a high oil yield (40–50%), it may be grown as a winter-cover crop, allows double cultivation and crop rotation.

It is the most important raw material for biodiesel production in the European Community. However, there were technological limitations for sowing and harvesting in some Central and South American countries, mainly due to the lack of adequate information about fertilization, seed handling, and storage (the seeds are very small and require specialized agricultural machinery). Moreover, low prices in comparison to wheat (its main competitor for crop rotation) and low production per unit area have limited its use.

Rapeseed flour has high nutritional value, in comparison to soybean; it is used as a protein supplement in cattle rations.

Sometimes canola and rapeseed are considered to be synonymous; canola (Canadian oil low acid) is the result of the genetic modification of rapeseed in the past 40 years, in Canada, to reduce the content of erucic acid and glucosinolates in rapeseed oil, which causes inconvenience when used in animal and human consumption.

Canola oil is highly appreciated due to its high quality, and with olive oil, it is considered as one of the best for cooking as it helps to reduce blood cholesterol levels.

2.2.1.2 Soybean

It is a legume originating in East Asia. Depending on environmental conditions and genetic varieties, the plants show wide variations in height. Leading soybean producing countries are the United States, Brazil, Argentina, China, and India.

Biodiesel production form soybean yields other valuable sub-products in addition to glycerin: soybean meal and pellets (used as food for livestock) and flour (which have a high content of lecithin, a protein). Grain yield varies between 2,000 and 4,000 kg/hectare. Since the seeds are very rich in protein, oil content is around 18%.

2.2.1.3 Oil Palm

Oil palm [11] is a tropical plant that reaches a height of 20–25 m with a life cycle of about 25 years. Full production is reached 8 years after planting.

Two kinds of oil are obtained from the fruit: palm oil proper, from the pulp, and palm kernel oil, from the nut of the fruit (after oil extraction, palm kernel cake is used as livestock food). Several high oil-yield varieties have been developed. Indonesia and Malaysia are the leading producers.

International demand for palm oil has increased steadily during the past years, the oil being used for cooking, and as a raw material for margarine production and as an additive for butter and bakery products.

It is important to remark that pure palm oil is semisolid at room temperature (20–22°C), and in many applications is mixed with other vegetable oils, sometimes partially hydrogenated.

2.2.1.4 Sunflower

Sunflower “seeds” are really a fruit, the inedible wall (husk) surrounding the seed that is in the kernel.

The great importance of sunflower lies in the excellent quality of the edible oil extracted from its seeds. It is highly regarded from the point of view of nutritional quality, taste and flavor. Moreover, after oil extraction, the remaining cake is used as a livestock feed. It must be noted that sunflower oil has a very low content of linoleic acid, and therefore it may be stored for long periods.

Sunflower adapts well to adverse environmental conditions and does not require specialized agricultural equipment and can be used for crop rotation with soybean and corn. Oil yield of current hybrids is in the range 48–52%.

2.2.1.5 Peanut

The quality of peanut is strongly affected by weather conditions during the harvest. Peanuts are mainly used for human consumption, in the manufacture of peanut butter, and as an ingredient for confectionery and other processed foods. Peanuts of lower quality (including the rejects from the confectionery industry) are used for oil production, which has a steady demand in the international market. Peanut oil is used in blends for cooking and as a flavoring agent in the confectionery industry.

The flour left over, following oil extraction, is of high quality with high protein content; in pellet form, it is used as a livestock feed.

2.2.1.6 Flax

Flax [12] is a plant of temperate climates, with blue flowers. Linen is made with the threads from the stem of the plant and the oil from the seeds is called linseed oil, used in paint manufacture. Flax seeds have nutritional value for human consumption since they are a source of polyunsaturated fatty acids necessary for human health. Moreover, the cake left over, following oil extraction, is used as a livestock feed.

The plant adapts well to a wide range of temperature and humidity; however, high temperatures and plentiful rain do not favor high yields of seed and fiber.

Flax seeds contain between 30 and 48% of oil, and protein content is between 20 and 30%. It is important to remark that linseed oil is rich in polyunsaturated fatty acids, linolenic acid being from 40 to 68% of the total.

2.2.1.7 Safflower

Safflower adapts well to dry environments. Although the grain yield per hectare is low, the oil content of the seed is high, from 30 to 40%. Therefore, it has economic potential for arid regions. Currently, safflower is used in oil and flour production and as bird feed.

There are two varieties, one rich in mono-unsaturated fatty acids (oleic acid) and the other with a high percentage of polyunsaturated fatty acids (linoleic acid). Both varieties have a low content of saturated fatty acids.

The oil from safflower is of high quality and low in cholesterol content. Other than being used for human consumption, it is used in the manufacture of paints and other coating compounds, lacquers and soaps.

It is important to note that safflower oil is extracted by means of hydraulic presses, without the use of solvents, and refined by conventional methods, without anti-oxidant additives.

The flour from safflower is rich in fiber and contains about 24% proteins. It is used as a protein supplement for livestock feed.

2.2.1.8 Castor Seed

The castor oil plant grows in tropical climates, with temperatures in the range 20–30°C; it cannot endure frost. It is important to note that once the seeds start germinating, the temperature must not fall below 12°C. The plant needs a warm and humid period in its vegetative phase and a dry season for ripening and harvesting. It requires plenty of sunlight and adapts well to several varieties of soils. The total rainfall during the growth cycle must be in the range 700–1,400 mm; although it is resistant to drought, the castor oil plant needs at least 5 months of rain during the year.

Castor oil is a triglyceride, ricinolenic acid being the main constituent (about 90%). The oil is non-edible and toxic owing to the presence of 1–5% of ricin, a toxic protein that can be removed by cold pressing and filtering. The presence of hydroxyl groups in its molecules makes it unusually polar as compared to other vegetable oils.

2.2.1.9 Tung

Tung [12] is a tree that adapts well to tropical and sub-tropical climates. The optimum temperature for tung is between 18 and 26°C, with low yearly rainfall.

During the harvest season, the dry nuts fall off from the tung tree and are collected from the ground. Nut production starts 3 years after the planting.

The oil from tung nuts is non-edible and used in the manufacture of paints and varnishes, especially for marine use.

2.2.1.10 Cotton

Among non-foodstuffs, cotton is the most widely traded commodity. It is produced in more than 80 countries and distributed worldwide. After the harvest, it may be traded as raw cotton, fiber or seeds. In cotton mills, fiber and seeds are separated from raw cotton.

Cotton fiber is processed to produce fabric and thread, for use in the textile industry. In addition, cotton oil and flour are obtained from the seed; the latter is rich in protein and is used in livestock feed and after further processing, for human consumption.

2.2.1.11 Jojoba

Although jojoba can survive extreme drought, it requires irrigation to achieve an economically viable yield.

Jojoba needs a warm climate, but a cold spell is necessary for the flowers to mature. Rainfall must be very low during the harvest season (summer). The plant reaches its full productivity 10 years after planting.

The oil from jojoba is mainly used in the cosmetics industry; therefore, its market is quickly saturated.

2.2.1.12 Jatropha

Jatropha is a shrub that adapts well to arid environments. Jatropha curcas is the most known variety; it requires little water or additional care; therefore, it is adequate for warm regions with little fertility. Productivity may be reduced by irregular rainfall or strong winds during the flowering season. Yield depends on climate, soil, rainfall and treatment during sowing and harvesting. Jatropha plants become productive after 3 or 4 years, and their lifespan is about 50 years.

Oil yield depends on the method of extraction; it is 28–32% using presses and up to 52% by solvent extraction. Since the seeds are toxic, jatropha oil is non-edible. The toxicity is due to the presence of curcasin (a globulin) and jatrophic acid (as toxic as ricin).

2.2.1.13 Avocado

Avocado is a tree between 5 and 15 m in height. The weight of the fruit is between 120 and 2.5 kg and the harvesting period varies from 5 to 15 months. The avocado fruit matures after picking and not on the tree.

Oil may be obtained from the fruit pulp and pit. It has a high nutritional value, since it contains essential fatty acids, minerals, protein and vitamins A, B6, C, D, and E. The content of saturated fatty acids in the pulp of the fruit and in the oil is low; on the contrary, it is very high in mono-unsaturated fatty acids (about 96% being oleic acid). The oil content of the fruit is in the range 12–30%.

2.2.1.14 Microalgae

Microalgae have great potential for biodiesel production, since the oil yield (in liters per hectare) could be one to two orders of magnitude higher than that of other raw materials. Oil content is usually from 20 to 50%, although in some species it can be higher than 70% [13]. However, it is important to note that not all microalgae are adequate for biodiesel production.

High levels of CO2, water, light, nutrients and mineral salts are necessary for the growth of microalgae. Production processes take place in raceway ponds and photobiological reactors [13].

Leading oil crops used in biodiesel production are indicated in Box 2.1.

Figure 2.1 presents approximate oil-yield values (in liters per hectare) for some of the crops [13] discussed in this chapter.

Fig. 2.1
figure 1

Approximate oil yields for different crops

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It is important to note that the data in Fig. 2.1 only show the oil yields of different crops. However, for the comparison of economical suitability it must be borne in mind that in addition to oil, some crops are grown for fiber or protein production. For instance, soybean has an oil content of 18% (maximum), whereas the remainder is mostly protein (usually used as livestock feed).

2.5 Biodiesel Production Process

Biodiesel is produced from vegetable oils or animal fats and an alcohol, through a transesterification reaction [1, 2, 4, 5]. This chemical reaction converts an ester (vegetable oil or animal fat) into a mixture of esters of the fatty acids that makes up the oil (or fat). Biodiesel is obtained from the purification of the mixture of fatty acid methyl esters (FAME). A catalyst is used to accelerate the reaction (Fig. 2.5). According to the catalyst used, transesterification can be basic, acidic or enzymatic, the former being the most frequently used, as indicated in Box 2.3.

Fig. 2.5
figure 5

Basic transesterification reaction with methanol

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A generic transesterification reaction is presented in Eq. (2.1); RCOOR′ indicates an ester, R″OH an alcohol, R′OH another alcohol (glycerol), RCOOR″ an ester mixture and cat a catalyst:

$$ {\text{RCOOR}}^{\prime} + {\text{R}}^{\prime\prime}{\text{OH}}\mathop \Leftrightarrow \limits^{\text{cat}} {\text{R}}^{\prime}{\text{OH}} + {\text{RCOOR}}^{\prime\prime} $$
(2.1)

When methanol is the alcohol used in the transesterification process, the product of the reaction is a mixture of methyl esters; similarly, if ethanol were used, the reaction product would be a mixture of ethyl esters. In both cases, glycerin will be the co-product of the reaction. This is shown schematically in Figs. 2.5 and 2.6.

Fig. 2.6
figure 6

Basic transesterification reaction with ethanol

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Although transesterification is the most important step in biodiesel production (since it originates the mixture of esters), additional steps are necessary to obtain a product that complies with international standards [4, 17], as shown in Box 2.4. In consequence, once the chemical reaction is completed and the two phases (mix of esters and glycerin) are separated, the mix of methyl esters must be purified to reduce the concentration of contaminants to acceptable levels. These include remnants of catalyst, water and methanol; the latter is usually mixed in excess proportion with the raw materials in order to achieve higher conversion efficiency in the transesterification reaction. In the following sections the steps of the purification process will be described in detail.

2.9 Glycerin

Glycerin is the usual name of 1,2,3-propanetriol; it is also referred to as glycerol, glycerin or glycyl alcohol. Chemically an alcohol, it is a liquid of high viscosity at room temperature, odorless, transparent, colorless, of low toxicity and sweet taste. The boiling point of glycerin is high, 290°C (563 K), and its viscosity increases noticeably at low temperature, down to its freezing point, 18°C (291 K). It is a polar substance that can be mixed with water and alcohols, and is also a good solvent. Glycerin is hygroscopic and has humectant properties.

Until the last years of the nineteenth century, glycerin was a product of candle manufacturing (from animal fat) and it was used mainly in the production of nitroglycerin for explosives. Later, separation processes from soap were developed and most glycerin was obtained as a sub-product of the soap industry. Since mid-century, synthetic glycerin can also be obtained using raw materials from the petrochemical industry.

At present, glycerin is obtained as a sub-product of soap or biodiesel production, and it is purified to eliminate the contaminants, mainly partially dissolved soap or salt (for the sub-product of soap production), or catalyst and methanol (from biodiesel production).

2.9.1 Uses

Even though the main use of glycerin was traditionally in the soap industry, about the middle of the twentieth century more than 1,500 uses for glycerin had been identified. These include the manufacture, conservation, softening and moisturizing of an ample variety of products [65]. Some of the uses of glycerin are:

  • As an additive in the manufacture of soaps, to improve their properties

  • In the manufacture of nitroglycerin for the production of explosives

  • In the food industry, for the manufacture of sweets, soft drinks, and pet foods and in the conservation of canned fruit

  • Due to its moisturizing and emollient properties, in the cosmetics industry for the manufacture of creams and lotions

  • In the chemical industry, for the fabrication of urethane foams, alkydic resins and cellophane, among other uses

  • In the pharmaceutical industry, for the manufacture of ointments, creams and lotions

  • In the manufacture of certain inks

  • For the lubrication of molds.

In the last years, glycerin production has increased, due to the steady growth of biodiesel production. Several academic and industrial research groups are actively pursuing new applications for glycerin, particularly in connection with polymers and surfactants [65]. It must be noted that the uses of glycerin are in principle similar to those of other widely used poliols (glycol, sorbitol, pentaerythritol, etc.), thus opening the technological possibility of replacing these poliols in new applications. Of course, these substitutions will take place if economically viable, and will depend on the prices of glycerin and the poliols involved.

The main research objective is the production of high-value-added products using glycerin, for instance, as a substrate for protein production from single-cell organisms, as a raw material for the production of detergents and bioemulsifiers, for the production of other poliols by fermentation (such as 1,2-propanediol or 1,3-propanediol), or for the production of other biofuels (bioethanol, biogas, hydrogen).

2.10 Concluding Remarks

There are significant advantages in the use of biodiesel as a replacement of diesel fuel and in blends.

The vegetable oils used as raw materials can be obtained from different oil crops that may be grown in a wide variety of environments, some of which are not adequate for traditional agricultural production. Microalgae grown in ponds and photobiological reactors have also great potential for the production of oils for biodiesel production. Moreover, used cooking oils and fat residues from the meat processing industry may also be employed in biodiesel production.

The production process has the same stages, irrespective of the production scale, although the differences in equipment may be significant. After the treatment of the raw materials, the transesterification reaction (usually with methanol and a basic catalyst) produces a mixture of fatty acids methyl esters (FAME) with glycerin as a co-product. The mixture of methyl esters must be separated from the glycerin and purified in order to comply with the requirements set by international standards for biodiesel.

In large-scale production plants, glycerin is usually recovered and purified since it is a valuable substance, with many applications in the pharmaceutical, cosmetics and chemical industries.