Ultra Low Sulfur Diesel: Great for Emissions - Here’s What's Missing

When you think of diesel engine emissions, the two elements that come to mind are soot and sulfur oxides. Both of these have negative impacts on our environment.

Sulfur is a natural part of the crude oil from which diesel fuel is refined, and is one of the key causes of acid rain pollution when the SOx gases from combustion combine with water vapor in the atmosphere. Soot is the main component of diesel engines' noxious black exhaust fumes, and is among the prime contributors to air pollution. The regulatory transition to ultra-low sulfur diesel (ULSD) use, which happened between 2006 and 2010, is aimed at lowering diesel engines' harmful exhaust emissions and improving air quality.

ULSD is produced by removing sulfur during the oil refining process, through the process of hydrotreating (more specifically, hydrodesulfurization). In this process, a heated mixture of petroleum feedstock (diesel) and hydrogen passes through a reactor with catalysts—substances that facilitate chemical reactions without being consumed by the reaction—to separate the sulfur from the hydrocarbon molecules. They can then take the H2S gas and strip the sulfur out of it for use in other areas. But after a final "treatment" to purify the diesel fuel, they've now got ULSD fuel to send out into the marketplace.

Though it’s great for emissions, here are three essential things that ULSD is missing:

Resistance to microbes

Sulfur is a natural biocide - or that's the line they used to use. In truth, there is some relationship to sulfur vs. microbes in that some kinds of microbes don't like to be around higher sulfur levels. In that sense, it can help to prevent the growth of bacteria and fungus in stored diesel fuel. Some microbes simply don't like to be around sulfur (then again, neither would we). Yet, there are plenty of kinds of microbes that do. In addition, ULSD has a lower "aromatic" content than diesel fuels of the past. Microbes prefer to consume non-aromatic molecules, so if the ULSD has fewer aromatic molecules, by definition it contains higher non-aromatic content. All of this together makes ULSD a more hospitable place for microbes to grow and contamination to develop.

It lacks lubricity

ULSD is a very dry fuel, which means that it has a very low amount of lubricating properties. Removing the sulfur contributes to this, as hydrotreating destroys some of the essential organic parts of the diesel that otherwise would provide lubrication in the fuel. This lack of lubricity can cause premature failure of fuel pumps and injectors that normally rely on diesel fuel for lubrication.

Good cold flow properties

The process that refineries use to remove sulfur from diesel not only raises the fuel cloud point (the temperature at which the paraffin in the fuel changes from a liquid to a solid wax), but also can significantly lower the aromatic content of the fuel. As a result, when the paraffin changes to solid wax, the crystals are less likely to remain suspended or dissolved in the fuel. Instead, the wax crystals tend to precipitate out (they drop out faster the colder it gets) and accumulate in the fuel and the bottom of the tank. As more crystals drop out, they stick together and get bigger. The solid wax crystals are drawn into the fuel filter as soon as the pump system is activated, quickly plugging the fuel filter and shutting down the engine.

All of these issues can be addressed through cost-effective additization. Please contact us for more information on supplementing your ULSD fuel to offset these downsides.

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