We’re now more than a decade out from the big change in diesel fuel specifications of 2006 – the lowering of the on-road sulfur limit from 500 ppm to 15 ppm. That gave us the ultra low sulfur diesel fuel that we see as the market standard today. As with many of these kind of changes, the market predicted big problems and consequences with the fuel. With ten years of hindsight in our pockets, let’s compare what actually happened vs. what they feared would happen when the change was announced.
The Concern: The concerns were legitimate at first. Removing the sulfur destroys some of the coordinating compounds that contributed to the diesel fuel having sufficient lubricity to protect injectors and fuel pumps. You could see the problem simply by running HFRR lubricity tests on the ULSD fuel. They failed. The industry was concerned for good reason.
The Reality: The problem didn’t appear nearly as often as they feared because the industry reacted in a couple of key ways. The first reaction was the creation of better lubricity additive chemistries, added to the fuel after refining. Then came the RFS (Renewable Fuels Standard) and the influx of biodiesel into the nation’s fuel supply. Just 2-3% addition of biodiesel raises the ULSD’s lubricity score back above what it was before they took the sulfur out. And just about any on-road diesel fuel that you buy today has up to 5% biodiesel already in it. So problem solved.
The Concern: Hydrotreating of diesel fuel at the refinery level was essential to removing the sulfur, enabling refineries to comply with the law and produce diesel fuel with a maximum sulfur level of 15 ppm. Sulfur was long considered to be an inhibiting factor for microbes in fuel; some described the situation as sulfur being a “natural biocide”. Take out the sulfur and you’d have a fuel with little or no resistance to its natural microbial enemies. This would lead to big problems in the industry, especially for users who stored fuel for long-term or emergency backup use.
The Reality: This problem did appear, and to a greater extent than they first thought, though not for the reasons one might think. The idea that sulfur is a natural biocide isn’t technically correct. Sulfur is a macronutrient that organisms need to live. Microbes may not like to be around excessive amount of sulfur, but it doesn’t kill microbes by its mere presence. That is what would fit the definition of a biocide, and sulfur is not that. One could argue that sulfur was a biostat, which is something that inhibits growth but doesn’t overtly kill living organisms.
How did this relate to the question at hand? What they found in the ten years following 2016 was that ULSD fuels were highly susceptible to developing severe microbial problems. But it wasn’t because of the reduced sulfur levels, as first thought. Instead, the blame lay in the coordinating regulations that not only limited sulfur content but also aromatic content.
The simple explanation would go like this. Both gasoline and diesel fuels are made up of molecules that can be divided into two classes – aromatics and aliphatics. There have always been limits on the percentage of aromatic molecules you could have in a given fuel, because aromatic molecules when burned contribute to air pollution. And when the new fuel regulations came out that capped sulfur content at 15 ppm, lower limits on aromatic content were also put into place at the same time. The problem is that if you have fewer aromatics, you have more aliphatic molecules in their place. And microbes much prefer to munch on aliphatic fuel molecule for their food sources.
So the removal of 97% of the diesel fuel sulfur did indeed create fuel that had tremendous microbe problems. If you have stored ULSD fuel that you haven’t checked in a while, it will be worth your while to examine what’s going on inside your tank. Today’s ultra-low sulfur diesel fuels need to have a lot more attention paid to them than the fuels from the past.