Renewable diesel is an advanced liquid transportation fuel offering a cleaner, non-petroleum alternative to traditional diesel. It functions as a complete replacement fuel that is chemically indistinguishable from its fossil-based counterpart. This fuel is produced from various biological sources, which are converted into a pure hydrocarbon. Its growing adoption is driven by the need to reduce carbon emissions from the transportation sector.
Defining Renewable Diesel
Renewable diesel is a straight-chain paraffinic hydrocarbon, identical in chemical structure to diesel derived from crude oil. This makes it a “drop-in” fuel, usable at any blend level, including 100% (R100), without modifying existing engines or infrastructure. This sets it apart from traditional biodiesel (FAME), which contains oxygen in its molecular structure.
The production process removes all oxygen, giving renewable diesel significant performance advantages over FAME. It meets the American Society for Testing and Materials (ASTM) D975 specification for petroleum diesel fuel. Its cetane number, a measure of ignition quality, is high, often ranging from 70 to 90, superior to both FAME and petroleum diesel. This cleaner composition also results in a low cloud point, meaning the fuel resists gelling in cold weather.
Primary Feedstocks Used in Production
The raw materials for renewable diesel production are primarily fats, oils, and greases (FOGs) derived from various biological sources. These feedstocks are triglycerides, molecules composed of a glycerol backbone attached to three fatty acid chains. The choice of feedstock directly impacts the fuel’s carbon intensity score, a measure of its total life-cycle greenhouse gas emissions.
The most preferred feedstocks are waste products, which boast the lowest carbon intensity scores. Major sources include used cooking oil (UCO) and yellow grease, rendered waste fat from restaurant fryers and food processing. Animal fats, such as beef tallow and pork lard, are also highly utilized as byproducts of the meat processing industry. Using these waste materials provides a dual environmental benefit by diverting waste from landfills while creating a low-carbon fuel.
Virgin vegetable oils currently make up a substantial portion of the feedstock supply. Soybean oil is a significant contributor in the United States, alongside canola oil and corn oil. While these oils are readily available, their use raises concerns about sustainability, particularly the potential for Indirect Land Use Change (ILUC). This debate centers on whether diverting food crops to fuel production may lead to clearing new land elsewhere to grow replacement crops, increasing the overall carbon footprint.
Advanced Feedstocks
Researchers are actively exploring advanced, non-food-competitive feedstocks for future expansion. Emerging sources include oils derived from microalgae, which can be cultivated in non-arable land and offer high oil yields. Inedible industrial greases and certain forms of lignocellulosic biomass are also being evaluated for their potential to provide a more sustainable, scalable supply of FOGs.
The Conversion Process: Hydrotreating
The chemical process transforming biological FOGs into renewable diesel is called hydrotreating, or hydroprocessing. This technology is similar to that used in petroleum refineries to remove contaminants from crude oil. The process involves treating the feedstock with hydrogen gas under high heat and pressure, typically around 650 to 800 degrees Fahrenheit, in the presence of a catalyst.
The initial step is hydrodeoxygenation (HDO), which removes the oxygen atoms from the triglyceride structure. The fatty acid chains react with the hydrogen, breaking the chemical bonds that hold the oxygen. This converts the oxygenated triglyceride molecules into pure, saturated hydrocarbon chains.
The primary byproducts of this deoxygenation step are water and propane, derived from the glycerol backbone. The resulting long-chain normal paraffins are then often isomerized, rearranging the straight chains into branched hydrocarbons. This isomerization improves the fuel’s cold-flow properties, ensuring the final product maintains a low cloud point and meets the D975 specification.