Renewable Diesel Is Not a Synonym for Biodiesel

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The terms renewable diesel and biodiesel get used interchangeably, but they are not the same fuel. They are made differently, behave differently in engines, and meet different specifications. Mixing them up matters, especially if you are evaluating either as a drop-in replacement for conventional diesel.

Here is the distinction in two sentences: renewable diesel is a drop-in replacement for conventional diesel. Biodiesel is only approved for blending.

Renewable Diesel: A True Drop-In Fuel

Renewable diesel is chemically identical to petroleum-derived diesel. It meets the same ASTM D975 specification used in the US (EN 590 in Europe) and can be run at 100% concentration in existing diesel engines with no modifications required. That last point is significant. Most alternative fuels require either engine changes or blending limits to work safely. Renewable diesel does not.

The feedstocks are biological, used cooking oil, animal fats, and inedible corn oil from ethanol plants, but the finished product is indistinguishable from conventional diesel at the molecular level. The oxygen that comes with those biological feedstocks is removed during processing, leaving a clean hydrocarbon fuel.

Biodiesel: A Blend Component

Biodiesel is a different product with a different chemistry. It is made up of long-chain fatty acid esters, molecules that retain oxygen in their structure. Because of this, most existing engines cannot run on 100% biodiesel. It is typically sold in blends: B10 or B20 (10% or 20% biodiesel blended with conventional diesel) are common. It meets ASTM D6751, a separate specification from conventional diesel.

A useful analogy: if biodiesel is like ethanol blended into gasoline, renewable diesel is more like running a gasoline engine on 100% ethanol-equivalent, except the fuel is already spec-equivalent, so no engine changes are needed.

How They Are Made

The production routes are what really set these two fuels apart. Renewable diesel is made using processes that will look familiar to anyone in the downstream oil industry: hydrotreating to remove oxygen and contaminants, isomerization to optimize cold-flow properties, and distillation to separate to final product spec. These are standard refinery operations.

Biodiesel is made by transesterification, reacting vegetable oil or animal fat with an alcohol (typically methanol) using an acid or base catalyst. The result is fatty acid methyl esters (FAME), the biodiesel product, plus glycerin as a byproduct. This is a chemical process quite unlike anything in a conventional refinery.

Who Is Producing Renewable Diesel

Several major refining companies are actively investing in renewable diesel. Valero has a joint venture with Darling Ingredients called Diamond Green Diesel, with a plant operating in St. Charles, Louisiana and a large facility announced in Port Arthur, Texas. Phillips 66 has plans to convert its San Francisco refinery to renewable fuels, and Marathon has announced a similar conversion at its Martinez, California refinery, both facilities sitting close together in Northern California.

Globally, Neste is the world's largest producer of renewable diesel. Founded over 70 years ago, the Finnish company made an early bet on renewable fuels in 2005, well before a meaningful market existed. That early commitment has put them well ahead of the field.

The Bottom Line

Renewable diesel and biodiesel both use biological feedstocks and reduce the carbon footprint of transportation fuel. But they arrive at very different endpoints. Renewable diesel slots directly into the existing fuel infrastructure with no compromises. Biodiesel requires blending and has engine compatibility limits. They are not synonyms, and in a conversation about decarbonizing transportation or refinery economics, the difference matters.