Biodiesel is not a fossil fuel. It is a renewable fuel made from plant oils, animal fats, and recycled cooking grease. While it burns in the same diesel engines and serves the same purpose as petroleum diesel, the two fuels have fundamentally different origins. Petroleum diesel comes from crude oil that formed underground over millions of years. Biodiesel comes from recently grown biological sources.
What Makes Biodiesel Different From Fossil Fuel
The distinction comes down to chemistry and origin. Petroleum diesel is a mixture of hydrocarbons pulled from ancient deposits deep in the earth. It contains aromatic compounds and sulfur. Biodiesel, by contrast, is composed of methyl esters with oxygen embedded in the hydrocarbon chain. It contains no aromatics or sulfur. That oxygen content actually improves combustion and reduces soot formation, which is one reason biodiesel burns cleaner.
The raw ingredients tell the story most clearly. Biodiesel feedstocks include soybean oil, canola oil, animal tallow, corn oil, and used cooking oil (often called yellow grease in the industry). Vegetable oils alone accounted for at least 70% of total biodiesel feedstock use each year from 2011 through 2019, reaching nearly 80% in 2020. In recent years, waste fats and oils have gained ground as a feedstock, while soybean oil’s share has dropped.
How Biodiesel Is Made
The production process is called transesterification. Triglycerides from oils or fats react with an alcohol, usually methanol, in the presence of a catalyst. The reaction breaks the triglyceride molecule apart in stages, converting it first to diglycerides, then to monoglycerides, producing one unit of biodiesel at each stage. The byproduct is glycerol, which gets separated out. A second process called esterification handles free fatty acids in the feedstock, reacting them with alcohol to produce biodiesel and water.
This is an industrial chemical process, but the key point is that every carbon atom in the final fuel traces back to a plant or animal that was alive recently, not to a geological deposit millions of years old.
The Carbon Cycle Difference
This is the core reason biodiesel is classified as renewable rather than fossil. The carbon in biodiesel started as carbon dioxide in the atmosphere. Plants absorbed that CO2 through photosynthesis, converted it into oils and carbohydrates, and those oils were then processed into fuel. When biodiesel burns, it releases that same carbon back into the atmosphere, completing a relatively short cycle.
Fossil fuels work differently. The carbon in petroleum diesel was locked underground for millions of years, effectively removed from the atmosphere. Burning it adds brand new carbon to the atmosphere that wasn’t part of the current cycle. That’s why fossil fuels cause a net increase in atmospheric CO2, while biofuels are recycling carbon that was already circulating above ground.
This difference shows up in the numbers. Life cycle analysis by Argonne National Laboratory found that pure biodiesel (B100) produces 74% lower greenhouse gas emissions than petroleum diesel when you account for the entire supply chain, from growing the feedstock through burning the fuel.
How It Performs Compared to Petroleum Diesel
Biodiesel carries slightly less energy per gallon than petroleum diesel: 119,550 BTU per gallon versus 128,488 BTU. That’s roughly a 7% reduction, which means you’ll use a bit more fuel to cover the same distance on pure biodiesel. At a B20 blend (20% biodiesel, 80% petroleum diesel), the energy content is 126,700 BTU per gallon, close enough that most drivers won’t notice a difference.
Biodiesel actually has a higher cetane number than petroleum diesel. Cetane measures how easily a diesel fuel ignites: biodiesel ranges from 45 to 65, compared to 40 to 55 for low-sulfur petroleum diesel. Higher cetane generally means smoother, more complete combustion.
Blending and Engine Compatibility
Most biodiesel on the road today isn’t used in pure form. The most common blends are B5 (up to 5% biodiesel) and B20 (6% to 20% biodiesel). B5 is approved for safe use in any diesel engine designed to run on petroleum diesel, no modifications needed. B20 is widely supported by engine manufacturers and offers a practical balance of cost, emissions reduction, and cold-weather reliability.
Pure biodiesel, B100, is a different story. It requires special handling, can gel in cold temperatures, and may need equipment modifications like biodiesel-compatible hoses and gaskets. Some engines built since 1994 can handle it with the right materials, but B100 is primarily used as a blending ingredient rather than a standalone transportation fuel.
Regulatory Status in the U.S.
The U.S. Environmental Protection Agency classifies biodiesel as a biomass-based diesel under the Renewable Fuel Standard, not as a fossil fuel. The program sets mandatory volume targets that fuel producers must meet. For 2025, the EPA requires 3.35 billion gallons of biomass-based diesel to be blended into the national fuel supply, up from 2.82 billion gallons in 2023. These mandates exist specifically because biodiesel is a renewable alternative to petroleum.
Biodiesel must also meet quality standards under ASTM D6751 for pure B100 and ASTM D7467 for B6 to B20 blends. These specifications cover properties like flash point (minimum 93°C), cetane number (minimum 45), and limits on contaminants like sulfur, phosphorus, and residual glycerin. Meeting these standards ensures biodiesel performs reliably in engines designed for petroleum diesel, even though the two fuels come from entirely different sources.