The different types of biodiesel
Biodiesel is a biofuel produced from biomass: vegetable oils, animal fats or food waste. The main raw materials used in biodiesel production include soy (SME), rapeseed (RME), sunflower (SLE), palm oil (PME) and jatropha (JME). Animal fats (FAME), food waste and used cooking oils (UCOME) are also used.
Soya-based biodiesel is widely used in the USA, while rapeseed-based biodiesel is more common in Europe and sunflower-based biodiesel in Mediterranean countries.
Biodiesel production methods
Biodiesel is produced from vegetable oils, animal fats or used cooking oils through chemical processes. The two main biodiesel production methods are transesterification and esterification. These processes are described below:
- Transesterification: Transesterification is the most commonly used process for biodiesel production. It involves reacting vegetable oil or animal fat with an alcohol, usually methanol or ethanol, in the presence of a catalyst (often an alkali, such as sodium hydroxide or potassium hydroxide). The reaction produces biodiesel (methyl or ethyl esters of fatty acids) and a by-product, glycerine. The glycerine is then separated from the biodiesel, which is purified and ready for use as a fuel.
- Esterification: Esterification is another method of biodiesel production, mainly used for feedstocks with a high free fatty acid (FFA) content, such as used cooking oils or low-grade animal fats. Esterification involves reacting FFAs with an alcohol, usually methanol, in the presence of an acid catalyst (e.g. sulfuric acid). The reaction produces fatty acid methyl esters (biodiesel) and water. The water is then removed, and the biodiesel is purified. In some cases, a combination of transesterification and esterification is used to process feedstocks with mixed triglyceride and GLA contents.
These methods enable biodiesel to be produced from a variety of feedstocks, including vegetable oils such as soybean, rapeseed, sunflower, palm or coconut, animal fats and used cooking oils.
Generations of biodiesel
Biodiesels are generally classified into different generations according to the raw material sources and technologies used in their production. Here’s an overview of the different biodiesel generations:
First-generation biodiesel
First-generation biodiesels are produced from vegetable oils and animal fats. Common feedstocks include soybean, rapeseed, sunflower, palm oil, tallow and poultry fat. The main production method for this generation of biodiesel is the transesterification of oils and fats with alcohol in the presence of a catalyst.
Second-generation biodiesel
Second-generation biodiesel uses non-food raw materials and waste products, such as used cooking oils, agricultural residues and forestry waste. Production technologies are more advanced, and include the hydrotreatment of vegetable oils and animal fats to produce renewable diesel, also known as hydrocarbon renewable diesel (HDRD) or “green diesel”.
Third-generation biodiesel
This generation of biodiesel focuses on the use of microalgae as a feedstock source. Microalgae have a much higher oil production potential than terrestrial plants and can be grown on non-agricultural land, reducing competition for arable land. However, large-scale production and commercialization of algae-based biodiesels remain a challenge due to high costs and technological hurdles.
Fourth-generation biodiesel
Fourth-generation biodiesel focuses on advanced biofuels produced from lignocellulosic biomass, such as agricultural residues, forestry waste and non-food energy crops. Production technologies include pyrolysis, gasification and fermentation, which can convert biomass into liquid hydrocarbons such as diesel and kerosene. Challenges for this generation of biodiesel include reducing costs and improving the efficiency of conversion processes.
Frequently asked questions
What is advanced biodiesel?
Advanced biodiesel refers to second- or third-generation biodiesels, biofuels produced from non-food sources, waste or residues, and more advanced and efficient conversion technologies. These biofuels aim to reduce the sustainability issues and environmental impacts associated with first-generation biofuels, which are generally produced from food crops such as soy, rapeseed or corn.
What is synthetic diesel?
Synthetic diesel is a second-generation biofuel produced from biomass or natural gas using advanced conversion processes, such as hydrotreating (HVO – Hydrotreated Vegetable Oil) or gas liquefaction (GTL – Gas to Liquid). Hydrotreating (HVO) involves treating vegetable or animal oils with hydrogen at high pressure and temperature, in the presence of a catalyst. Gas liquefaction (GTL) involves converting natural gas into liquid fuels through a series of chemical reactions, such as Fischer-Tropsch synthesis.
Biodiesel pollutants
Biodiesel is generally considered to be more environmentally friendly than traditional fossil fuels, but it can nevertheless produce certain pollutants during combustion. Here are some of the main pollutants emitted by biodiesel:
- Nitrogen oxides (NOx): NOx emissions can vary depending on the type of biodiesel used and combustion conditions. Some biodiesels may generate slightly higher NOx emissions than traditional diesel fuels, while others may produce less.
- Non-methane hydrocarbons (NMHC): Biodiesel can emit NMHC during combustion. However, these emissions are generally lower than those of traditional diesel.
- Fine particles (PM): Biodiesel generally emits fewer fine particles than conventional diesel, especially biodiesels based on animal fats and vegetable oils. This helps reduce air quality problems and risks to human health.
- Carbon monoxide (CO): Biodiesel combustion produces CO, although emissions are generally lower than for conventional diesel.
- Volatile organic compounds (VOCs): Biodiesel can emit VOCs during combustion. However, these emissions are generally lower than those of traditional diesel fuels.
- Aldehydes and ketones: Biodiesel combustion also produces aldehydes and ketones, including acetaldehyde, formaldehyde and acrolein. These compounds can affect air quality and human health.
It is important to note that pollutant emissions vary according to biodiesel type, engine technology and combustion conditions. Overall, biodiesel generally has a cleaner emissions profile than traditional diesel, although some pollutants may be emitted in slightly higher or lower quantities.