I. What is actually happening at Port Arthur

Fill up a diesel pickup anywhere on the upper Texas coast, from the refineries at Port Arthur to a truck stop off I-10, and there is a decent chance some of what goes in the tank started as restaurant grease. The fuel looks the same, pumps the same, and burns in the same engine. What changed is the kitchen behind it.

About 90 miles east of downtown Houston, in Jefferson County, Diamond Green Diesel runs a plant at Port Arthur that makes renewable diesel. Diamond Green Diesel is a joint venture between Valero, the refiner, and Darling Ingredients, which collects and renders fats and oils. Instead of crude, the plant feeds its hydrotreaters used cooking oil, beef tallow, and vegetable oils like soybean oil.

Physically, the plant is almost a refinery running backward. This desk has watched the upper Gulf Coast turn into one of the country's larger clusters for renewable diesel and sustainable aviation fuel, and the reason is less about new chemistry than about who pays for the carbon math. Here is what that looks like at the pump and on the dock.

II. A refinery running in reverse

Renewable diesel is made by hydrotreating, which means reacting fats and oils with hydrogen under heat and pressure to strip out oxygen and leave hydrocarbons. The output is chemically close to petroleum diesel, so it drops straight into existing pipelines, tanks, and engines. That is the key difference from biodiesel, an older fuel that is chemically different and usually blends in at smaller shares.

At Port Arthur, the plant can make on the order of 470 million gallons a year of renewable diesel, per the company's project materials, and Diamond Green Diesel's combined output across its Louisiana and Texas sites runs to roughly 1.2 billion gallons a year at last public report. The company has also added a unit at Port Arthur to make sustainable aviation fuel, a similar product refined to jet specifications. The flow sheet barely moves. The feedstock coming in the gate is what changed.

III. The math the credits make work

None of this pencils out on the fuel price alone. Renewable diesel leans on three credit programs stacked on top of each other: the federal Renewable Fuel Standard, run by the EPA, which makes refiners buy credits called RINs; the federal 45Z Clean Fuel Production Credit, which took effect at the start of 2025 and pays producers based on how low the fuel's carbon intensity is; and California's Low Carbon Fuel Standard, run by the state air board, which Gulf Coast plants can earn into by shipping fuel west.

That accounting is why a Texas plant leans on animal fat. Waste fats like used cooking oil and tallow score a lower carbon intensity than crop oils under these programs, so they earn more credit per gallon. The scoring is also contested, since it depends on assumptions about land use and collection that reasonable analysts dispute, and the numbers move whenever the EPA or the California board rewrites a rule. If you want to know why the economics hold, watch the credits, not the crude price.

IV. Where the grease comes from

Feedstock is a supply chain of its own. Darling Ingredients collects used cooking oil from restaurants and renders tallow from meatpacking, and the plant also takes distillers corn oil and soybean oil. Those materials arrive at the energy capital by truck, rail, and barge, the same way crude and products have moved through the Houston Ship Channel and the Sabine-Neches waterway for a century.

A reader can check most of this. The U.S. Energy Information Administration publishes renewable diesel capacity and the volumes of each feedstock consumed, and Valero and Darling report plant output and economics in their quarterly filings. What is harder to verify from the outside is the carbon intensity claimed for any single gallon, since that rides on program formulas rather than a number you can read off a tank gauge.

V. Worth watching this month

1. The EPA's Renewable Fuel Standard volume decisions, which set how many RIN credits the market needs, are worth watching whenever the agency proposes or finalizes them, and these actually move the economics.

2. Treasury and IRS guidance filling in the 45Z Clean Fuel Production Credit is still being worked out, and any update there changes what producers can claim.

3. California's air board periodically amends its Low Carbon Fuel Standard, and a tightening of targets would raise what out-of-state renewable diesel can earn, so it matters more than it looks.

4. The EIA's monthly and annual biofuels data releases are routine, but they are the cleanest public check on whether Gulf Coast renewable diesel output is still climbing.

5. Valero's and Darling's quarterly updates often mention Port Arthur and its sustainable aviation fuel unit, and are the place a reader will first hear if plans slip.

Questions readers ask

Is renewable diesel the same as biodiesel?

No. Renewable diesel is hydrotreated into something chemically close to petroleum diesel and drops into existing engines and pipelines, while biodiesel is a different molecule that usually blends in at smaller shares.

Will this lower the diesel price at my pump?

Not in a way you would notice directly. The economics run through federal and state credits rather than a lower sticker price, so the effect on a Houston fuel bill is indirect and small.

Is it really cleaner if the fat is shipped in?

It depends on how you count. The credit programs assign waste fats a low carbon intensity, but that scoring is contested and includes assumptions about land use and collection that analysts argue over.

Is the Port Arthur area on the ERCOT grid?

Yes, most of Texas including the upper Gulf Coast sits on the ERCOT grid, though a plant like this makes much of its own heat and hydrogen and is a large electricity customer in its own right.

Sources of record

  1. U.S. Energy Information Administration, renewable diesel capacity, Federal data on renewable diesel plant capacity and feedstock use you can check against company claims., www.eia.gov/biofuels/renewable/capacity
  2. EPA, Renewable Fuel Standard program, The government page explaining RINs and annual renewable fuel volumes., www.epa.gov/renewable-fuel-standard-program
  3. California Air Resources Board, Low Carbon Fuel Standard, The state program whose credits Gulf Coast producers earn by shipping fuel to California., ww2.arb.ca.gov/our-work/programs/low-carbon-fuel-standard
  4. IRS, Clean Fuel Production Credit (45Z), Federal guidance on the carbon-intensity-based production credit that took effect in 2025., www.irs.gov/credits-deductions/clean-fuel-production-credit
  5. Diamond Green Diesel, The operating company's own pages on its Port Arthur and Louisiana plants and capacity., www.diamondgreendiesel.com
  6. Valero, renewable diesel operations, The refiner's description of its Diamond Green Diesel joint venture and reported output., www.valero.com/renewables