Synthetic biofuels are liquid bio-hydrocarbons that are functionally equivalent to petroleum fuels and are fully compatible with existing petroleum products and transport infrastructure. Renewable hydrocarbon biofuels (also called green or synthetic biofuels) are fuels produced from biomass sources through a variety of biological, thermal, and chemical processes. These products are chemically identical to petrol, diesel, or kerosine. It meets ASTM D975 specification for petroleum diesel. These are liquid biofuels with low or no oxygen. Synthetic fuels are alternatives to existing liquid fuels without any significant modification in engines and infrastructures.
Currently thermo-chemical technology such as pyrolysis based on more widely available biomass feed stocks will provide much of the long-term supply of Synthetic biofuels in the future. Oils and animal fats can be processed into biodiesel. Synthetic biofuels use trapped vegetable oil as raw material.
Thermo-chemical pathways: Thermo-chemical technologies, based on more widely available and supposedly cheaper feedstocks such as trapped vegetable oils, will provide the long-term supply of Synthetic biofuels. These raw materials are

• Oil trapped in Bleaching Clay during processing of Vegetable oil, which improves colour of oil.

• Filter mud in Cane Sugar Industry: Cane contains 0.1% wax, part of it enters into juice stream, when cane is crushed, alongwith mud. It is filtered in Rotary Vacuum Filter and sold off. A 2,500 metric tons of cane crushing plant per day produces 2 tons of wax per day, mixed with mud. There are around 545 sugar mills in India, which can supply this important raw material during their crushing season.

Significant volumes of Synthetic biofuels that are currently produced are renewable diesel based on the vegetable oil which involves the upgrading of vegetable oils via hydrotreatment.

Conventional biofuels such as bioethanol and biodiesel (fatty acid methyl esters - FAME) have distinct chemical functional groups and can be accurately described based on their chemical composition alone. In contrast, Synthetic biofuels generally consist of a mixture of many different types of hydrocarbons, the properties of which, just like petroleum fuels, are typically characterized by the functional characteristics of the mixture, such as distillation profile, viscosity, acidity, etc. Conventional biofuels such as bioethanol and biodiesel have various limitations:

• Conventional biofuels are chemically and functionally different from petroleum-derived fuels and thus cannot make full use of the existing petroleum processing and distribution infrastructure. Unless modified engines are used, these biofuels can only be used in relatively low blends (ethanol maximum 15% and biodiesel 20%).

• Conventional biofuels are unsuitable for use in sectors such as aviation and long distance trucking as they do not meet the density requirements or other specifications of such fuels.

• Currently, conventional biofuels predominantly use “food-based” feedstocks such as corn, sugarcane and vegetable oils. Food security concerns and questions of sustainability will continue to encourage the production of biofuels from trapped oils which abundant and not in competition with food. This requires different types of technologies.

Hydro-processed Esters and Fatty Acids (HEFA) is the term used to describe advanced oil based Synthetic biofuels that are produced by hydro-treating lipids derived from vegetable oils, algae and animal fats. To distinguish Synthetic biofuels from “FAME biodiesel”, the terms “green diesel” or “renewable diesel” are often used. Fats are the simplest to convert to Synthetic biofuels because as they have a lower oxygen content and their chemistry is closer to a hydrocarbon than sugars.
Several chemical reactions take place during Synthetic biofuels production, with the carbon-carbon double bonds present in the triglyceride (TG) saturated first. This is followed by the removal of the glycerine part within the TG leaving 3 free fatty acids per TG molecule. Finally, the fatty acids are deoxygenated.

The process of hydrotreating vegetable oils can result in the production of high quality Synthetic biofuels fuels that exceed the specifications of petroleum-based transportation fuels. Synthetically derived diesel have essentially no sulfur content whereas their petroleum counterparts can contain up to 3000 ppm of sulfur. Other improved characteristics of Synthetic biofuels include higher energy density, lower aromatics content and, for diesel Synthetic biofuels, higher cetane number. The absence of aromatics in renewable Synthetic biofuels is generally viewed as an advantage from an air pollution perspective, as phenolic compounds are associated with emissions of polyaromatic hydrocarbon (PAH) pollutants.