Biodiesel is a clean-burning fuel produced from grease, vegetable oils, or animal fats and is defined as a mono-alkyl fatty acid ester (preferentially methyl or ethyl ester), it is a prominent alternative fuel which can be used in compression-ignition (diesel) engines. Biodiesel comes from renewable sources as it is plant based and not petroleum based, so it is biodegradable and produces less toxic emissions i.e. relative to conventional diesel
Biodiesel is produced by transesterification of oils with low molecular weight alcohols or by the esterification of fatty acids.
Biodiesel is produced by transesterification of oils with low molecular weight alcohols or by the esterification of fatty acids. Transesterification reaction relates to the transformation of triglycerides into fatty acid alkyl esters in the presence of an alcohol such as methanol or ethanol, and a catalyst such as an alkali or acid, with glycerol being obtained as a by-product. Biodiesel is produced today primarily by alkali-catalysed transesterification with methanol, which has a characteristic short reaction time, however many other catalytic routes are also available.
The main purpose of this reaction is basically to reduce the viscosity of oil and fats and increase the volatility so that they can be directly used in a conventional diesel engine without any modification. A catalyst, alkaline or acidic, is generally employed to increase the reaction rate and yield. Alkali-catalysed transesterification is much faster than acid-catalysed transesterification (For oil samples with FFA below 2.0%). For an alkali-catalysed transesterification, the glycerides and alcohol must be substantially anhydrous because water makes the reaction partially change to saponification, which produces soap. Acid-Catalysed Reaction is used generally where the feedstock has high acid value (FFA > 1%), where alkaline catalysis is ineffective. Strong liquid acid catalysts are less sensitive to FFAs and can simultaneously conduct esterification and transesterification. The homogeneous acids used for biodiesel production are mainly concentrated sulfuric acid, phosphoric acid, and other inorganic acids.
Although short chain alcohols, such as methanol or ethanol, can be used for the transesterification reaction, methanol is most widely used because of its low price, short carbon chain, strong polarity, and fast reaction rate. The transesterification reaction can occur with the use of acid, alkali, or enzyme as catalysts, or in a supercritical fluid system without catalyst.
Conventionally Transesterification of plant oils for production of biodiesel is base-induced, using either potassium or sodium hydroxide. The latter is often preferred because it is cheaper and more easily available. In a non-aqueous reaction system the base reacts with methanol to form methoxide, CH3O–. Methoxide attacks the ester.
The reaction produces two phases. Glycerine in the heavier phase is separated and may be purified for sale to pharmaceutical and cosmetic industries. The excess alcohol in the alcohol/ester mixture at the lighter phase is separated and recycled. The esters are purified through water washing, evaporation, and vacuum drying.
Esterification has also been carried out using ion-exchange resins such as Amberlyst-15 and Nafion with good results. In addition, various zeolites with different Si/Al ratios are also being explored, now even though zeolites provide the possibility to choose among different pore structures and surface hydrophobicity, according to the substrate’s size and polarity. Esterification does not require water-free conditions, an important characteristic given that water is a reaction by-product. This may be a preferred route for production in the future.
The yield of biodiesel from the transesterification process can vary from 70 to 90% depending on the type of feedstocks, reagents, and operational conditions including catalysts, the ratio of alcohol to oil, temperature, reaction time, and the content of water and free acids.
The present cost of biodiesel produced from vegetable oils is around USD 0.8 to USD 1.0 per litre, still a little higher than petroleum diesel.
The high production cost is mainly due to the raw materials, which account for 60–75% of the total cost. The cost of biodiesel can be lowered by increasing feedstock yields, developing novel technologies, and increasing economic return on glycerine production by finding other uses for this by-product.
EU dominates world production, Its continuous production growth though can only be partly attributed to its extensions in the number of its Member States since the core EU biodiesel production centers are Germany and France; followed by Spain, Italy, and Poland. International biodiesel markets have grown tremendously over the past decade, while practically no biodiesel was traded 15 years ago, it was estimated to be a USD 17.26 billion industry in 2016 & with a CAGR of 5.1% it is expected to grow to USD 22.13 billion by 2021. Today’s market, though volatile and policy dependent, has become much more transparent in comparison to its early stages. The EU has clearly been and will most likely remain the key production and consumption region for biodiesel until 2020. Many countries though have followed suit and implemented national blending targets for biodiesel, thus stimulating domestic production and consumption. Partly, such production has been targeted for export to the EU. These trade streams are likely to grow in the future. Economic margins under existing EU policy schemes (predominantly blending mandates) will remain low and comparative cost advantages will have to be used.