Today's #2 ultra low sulfur diesel fuels (ULSD) fall short of the desirable properties of diesel fuels from the past in several key areas. These shortcomings necessitate the use of ULSD additives to assist these fuels in both meeting legal specification and in minimizing problems in both storage and equipment.
To meet the requirements of the Clean Air Act, ULSD fuels have their sulfur contents removed to a legal maximum of 15 ppm. This is down from 500 ppm ten years and 5,000 ppm twenty years ago. No doubt this has helped the environment tremendously, keeping millions of tons of sulfur from entering the atmosphere over the years. But the resulting fuel falls short of historical performance in several key areas.
Sulfur removal through hydrotreating strips fuel compounds that enhance the fuel's traditional lubricity value. This necessitates addition of lubricity treatments at the refinery level to keep the ULSD from entering the marketplace with lubricity values below the required specification.
Sulfur removal processes react with the paraffins and other heavy wax constituents in the diesel fuel, resulting in a fuel blend that gels much more easily in cold weather due to the wax dropout and agglomeration. This creates problems for fuel professionals in colder climates, with ULSD fuels not responding as readily to cold flow improver treatments. Treatments that used to reduce cold filter plug points in diesel fuels by 25 or 30 degrees may now only have effective drops of 15 degrees.
This is an overlooked but serious shortcoming for ULSD fuels. Sulfur in traditional or low sulfur fuels acted as a natural biocide, enabling those fuels to retain some modicum of resistance to microbial growth. ULSD fuels are more more susceptible to microbial growth. Indeed, it used to be that simply controlling water collection in diesel fuels was enough to prevent most microbial colonies from establishing. This is far from the case, now. Simplly controlling accumulation of water is now not enough to confidently store ULSD without expecting microbial growth in the fuel. Therefore, it is recommended to implement regular biocide treatment of fuel tanks. It is typically much less expensive and problematic to treat the fuel with biocide in a preventive manner than it is to try and eradicate a microbial problem after it has established.