Renewable diesel (RD) is a fuel derived from biological sources that is chemically processed to be nearly indistinguishable from conventional petroleum diesel. This fuel is a hydrocarbon, meaning its molecular structure consists only of hydrogen and carbon atoms, making it a “drop-in” replacement for fossil fuels. Renewable diesel meets the same stringent technical specifications as petroleum diesel, specifically the ASTM D975 standard in the United States. This chemical identity allows it to be used in existing diesel engines and fueling infrastructure without any modification. Its high energy density ensures equivalent engine performance, positioning it as a significant solution for reducing lifecycle greenhouse gas emissions.
Essential Feedstocks
The creation of renewable diesel begins with lipid-rich biological materials, known as triglycerides, which are the fatty components found in various oils and fats. These triglycerides form the raw input for the production process. The industry relies heavily on a diverse range of Fats, Oils, and Greases (FOGs) to maintain a sustainable supply chain. Common feedstocks include used cooking oil (UCO) and inedible animal fats, such as beef tallow and poultry grease, left over from meat processing operations. These waste streams offer a high-lipid content and help reduce the carbon intensity score of the final fuel. Dedicated oilseed crops, like soybean and canola oil, are also widely used, though their use is often balanced against food security concerns. Specialized, non-food crops like camelina or microalgae are also being explored for their potential.
The Hydrotreating Chemical Reaction
Hydrotreating Process
The core of renewable diesel production is the hydrotreating process, often called Hydroprocessed Esters and Fatty Acids (HEFA). This process is adapted from the technology used in petroleum refineries to remove contaminants from crude oil. The objective is to convert the large, oxygen-containing triglyceride molecules into straight-chain paraffinic hydrocarbons. The feedstock is mixed with a large volume of hydrogen gas and subjected to high pressure, typically ranging from 50 to 100 bar, and high temperatures, often between 300°C and 400°C. This reaction occurs over a fixed-bed catalyst, commonly made of nickel or cobalt-molybdenum compounds.
Chemical Transformation
The primary chemical action is hydrogenation, where hydrogen saturates the double bonds in the fatty acid chains, making the molecules more stable. Simultaneously, deoxygenation takes place, which is the removal of oxygen from the fatty acid structure. This is accomplished either through hydrodeoxygenation (removing oxygen as water) or decarboxylation/decarbonylation (removing it as carbon dioxide or carbon monoxide and water). Removing oxygen transforms the biological oil into a pure hydrocarbon, identical to a petroleum molecule. This severe treatment also efficiently removes other undesirable contaminants, such as sulfur and nitrogen.
Refining and Product Separation
Fractionation
Following the hydrotreating reactor, the resulting hydrocarbon stream is a complex mixture of molecules that must be separated and purified to meet final fuel standards. This crude product contains not only the renewable diesel fraction but also lighter and heavier hydrocarbons. The first major step is fractionation, which separates the mixture into distinct fuel cuts based on their boiling points. This separation isolates renewable diesel from other valuable co-products, such as renewable naphtha and gasoline at the lighter end, and renewable propane and renewable jet fuel (Sustainable Aviation Fuel, or SAF) at the heavier end.
Isomerization for Cold Flow
Once the diesel fraction is isolated, it often undergoes further treatment to ensure it performs well in all conditions. A significant purification step is required to improve the fuel’s cold-flow properties. The straight-chain paraffinic hydrocarbons tend to solidify or “wax” at higher temperatures than petroleum diesel, which is problematic in cold climates. To address this, the fuel undergoes isomerization, a catalytic process that rearranges some of the straight chains into branched chains, effectively lowering the cloud point.
Key Differences from Biodiesel
Chemical Structure
A common point of confusion is the distinction between renewable diesel and biodiesel, which are fundamentally different fuels despite sharing similar biological feedstocks. The difference stems entirely from their respective production processes and resulting chemical structures. Renewable diesel is a pure hydrocarbon, lacking any oxygen atoms in its structure, which is a direct outcome of the severe hydrotreating process. In contrast, biodiesel is produced through transesterification, a much milder chemical process that results in a Fatty Acid Methyl Ester (FAME). This ester structure means biodiesel contains approximately 10% oxygen by weight.
Performance and Logistics
This small molecular difference has major implications for fuel performance and logistics. Because renewable diesel is a pure hydrocarbon, it can be used at 100% concentration in any diesel engine without blending limits and can be transported in existing petroleum pipelines. The presence of oxygen in FAME biodiesel makes it less stable over time, more prone to absorbing water, and significantly raises its gelling temperature, limiting its use in cold weather. Biodiesel is therefore typically restricted to lower-percentage blends, such as B5 or B20, with petroleum diesel.