Biomass diesel is any diesel fuel made from biological materials rather than petroleum. The term covers two distinct fuel types: biodiesel, produced through a chemical reaction called transesterification, and renewable diesel, produced through hydroprocessing. Both start with the same kinds of raw materials (plant oils, animal fats, used cooking grease), but they end up as chemically different fuels with different properties and different roles in the diesel fuel supply.
Biodiesel vs. Renewable Diesel
The distinction matters because these two fuels behave differently in engines. Biodiesel (technically called fatty acid methyl ester, or FAME) contains oxygen in its molecular structure, which makes it chemically different from petroleum diesel. That oxygen content means biodiesel carries about 7% less energy per gallon than conventional diesel. It works well as a blending component but has limitations when used on its own.
Renewable diesel is a hydrocarbon fuel, meaning it contains only hydrogen and carbon, just like petroleum diesel. It’s produced using the same refining technology that petroleum refineries use, including high-pressure hydrogen treatment that strips away oxygen, sulfur, and other impurities. The result is a “drop-in” fuel that meets the exact same specifications as petroleum diesel and can fully replace it without any blending.
What It’s Made From
In the United States, soybean oil is the primary feedstock for biomass diesel production. Other major sources include animal fats from meat processing plants, used cooking oil collected from restaurants, and yellow grease. Internationally, the feedstock picture looks different: rapeseed oil dominates in Europe, while palm oil and sunflower oil are widely used in other regions.
Waste-based feedstocks like used cooking oil and animal tallow have a significant advantage beyond availability. Because these materials are byproducts of other industries, they don’t carry the environmental burden of growing a dedicated crop. This distinction has real consequences for the fuel’s carbon footprint, as discussed below.
How Each Type Is Produced
Biodiesel production is relatively straightforward. Plant oils or animal fats are mixed with an alcohol (usually methanol) in the presence of a catalyst. This reaction breaks apart the fat molecules and recombines them with the alcohol, producing biodiesel and glycerin as a byproduct. The process runs at low temperatures (around 40°C) and takes about an hour, which is one reason biodiesel can be produced at smaller scales.
Renewable diesel requires industrial-scale hydrogen processing. The feedstock goes through two main stages. First, hydrotreating removes oxygen, sulfur, nitrogen, and metals by reacting the material with hydrogen under high pressure in the presence of a catalyst. Then hydrocracking breaks larger hydrocarbon molecules into smaller, lighter ones that match the properties of conventional diesel fuel. This involves four distinct chemical reactions: reduction, hydrodeoxygenation, deoxygenation, and decarboxylation. The equipment and energy demands are substantial, which is why renewable diesel is typically produced at large refineries.
Energy Content and Engine Performance
Pure biodiesel (B100) delivers about 119,550 BTU per gallon, compared to 128,488 BTU per gallon for ultra-low sulfur petroleum diesel. That roughly 7% energy gap means slightly lower fuel economy when running on pure biodiesel. A B20 blend (20% biodiesel, 80% petroleum diesel) narrows the gap to about 126,700 BTU per gallon, making the mileage difference barely noticeable in everyday driving.
Renewable diesel sits at about 123,710 BTU per gallon, closer to petroleum diesel than biodiesel is. Where renewable diesel really stands out is its cetane number, which measures how easily the fuel ignites in a diesel engine. Renewable diesel scores 70 to 85, well above petroleum diesel’s range of 40 to 55. Biodiesel falls in between at 45 to 65. A higher cetane number generally means smoother combustion, easier cold starts, and less engine knock.
Engine Compatibility and Blending
Low-level biodiesel blends up to B5 (5% biodiesel) are approved for safe use in any diesel engine designed for petroleum diesel. Blends up to B20 can generally run in current diesel engines without modifications. Pure biodiesel (B100) is a different story: it requires special handling and may need equipment changes, particularly to fuel lines and seals, since biodiesel can degrade certain rubber and plastic components over time.
Renewable diesel has no such limitations. Because it’s chemically identical to petroleum diesel, it works in any diesel engine at any concentration, with no modifications and no blending required.
Cold Weather Challenges
Biodiesel’s biggest practical weakness is cold weather performance. As temperatures drop, biodiesel begins to form waxy crystals that can clog fuel filters and lines. Three measurements define this behavior: the cloud point (when crystals first become visible), the pour point (when the fuel stops flowing), and the cold filter plugging point (when crystals block a standard test filter).
Untreated biodiesel from waste cooking oil, for example, has a cloud point around -3°C (27°F) and a pour point around -7°C (19°F). Specialized cold-flow treatments can improve these numbers significantly, pushing the cloud point down to -9°C (16°F) and the pour point to -15°C (5°F). Still, biodiesel generally performs worse in cold conditions than petroleum diesel, which is another reason it’s most commonly used as a blend component rather than a standalone fuel. Renewable diesel, being chemically equivalent to petroleum diesel, handles cold weather comparably.
Greenhouse Gas Reductions
The carbon case for biomass diesel is strong, though the numbers vary depending on the feedstock. Biodiesel and renewable diesel made from soybean, canola, or carinata oils reduce lifecycle greenhouse gas emissions by 40% to 69% compared to petroleum diesel, even after accounting for land-use changes associated with growing those crops. Without land-use change factored in, the reductions climb to 63% to 77%.
Waste-based feedstocks deliver even larger gains. Converting used cooking oil, animal tallow, or distillers corn oil into biodiesel or renewable diesel achieves 79% to 86% lower emissions than petroleum diesel. The reason is straightforward: these materials are waste products, so they don’t share the upstream emissions of farming, fertilizing, and harvesting a crop. This makes waste-derived biomass diesel one of the lowest-carbon liquid fuels available today.
Quality Standards
Pure biodiesel (B100) must meet ASTM D6751, which sets limits on sulfur content (15 parts per million maximum for the cleanest grade), water contamination (no more than 0.05% by volume), and a minimum cetane number of 45, among other requirements. Biodiesel blends from B6 to B20 fall under a separate standard, ASTM D7467. Renewable diesel meets the same ASTM D975 specification as petroleum diesel, which is what makes it a true drop-in replacement.
Beyond the base ASTM requirements, the National Biodiesel Accreditation Program offers a voluntary certification called BQ-9000. This program combines fuel quality standards with a quality management system that covers production, storage, distribution, and end use, giving buyers an additional layer of assurance about the fuel they’re purchasing.