Clean Diesel: Ultra Low Sulfur Diesel

The US EPA 2007 clean air regulations are fast approaching. Diesel users prepare for ULSD; but first, they need a few answers, writes Richard George.

Now that we’ve brought ULSD to site storage tanks, we can look at the fuel itself and what it means to the vehicles that use it. If things were easy, the sulfur would be removed and nothing else would change. The reality is that the additional hydrotreating will affect the fuel. The hydrotreating removes impurities such as sulfur and some nitrogen. It also saturates olefinic and aromatic hydrocarbons. Removing the impurities, aromatics and olefins changes the fuel a bit. The changes will be small for the most part. Cetane may improve a little, cold properties will improve, boiling points will become lower, density will decrease and lubricity will decrease. The last two are the most worrisome for users.

Ultra Low Sulfur Diesel’s Density

Fuel density is directly related to the energy content. The lower the density, the less energy per gallon. Less energy means less power and less fuel economy. (Diesel fuels are much denser—approximately 20 percent—than gasoline, which is one of the reasons they get much better fuel economy.) It is expected that the additional hydrotreating to remove the sulfur will decrease the density of the fuel by a small amount. This decrease will likely be small and lead to less than a 1 percent drop in energy content. The drop in fuel economy should also be similar—less than 1 percent. It’s expected that refineries will take the opportunity to upgrade or reconfigure other equipment besides the hydrotreaters, which may also impact the fuel density.

There have been reports of field studies done that show a drop in fuel mileage greater than predicted. Some reports have differences of 3.5 percent, but this may be misleading. It’s not as simple as purchasing some of the ULSD that has been currently available on the market and comparing it to LS No. 2 diesel. Nearly all ULSD available until recently has been ULS No. 1. As already mentioned, diesel No. 1 is less dense than diesel No. 2. No. 1 fuels are typically about 3 percent less dense than No. 2 fuels and users should expect to see a 3 percent drop in fuel economy.

The drop in fuel economy of a field study like this is likely due to the No. 1 vs. No. 2 comparison and not the ultra low sulfur. Even a LS No. 2 to ULS No. 2 comparison can be problematic because the density differences in No. 2 fuels can vary greatly (some No. 2 fuels are much lighter than others). Since the natural variation in fuel density is greater than the expected change in density due to the additional hydrotreating, any comparison of two given fuels won’t accurately represent the entire fuel population. The only way to tell the drop in fuel economy due to ULSD is to look at the average density of the entire diesel fuel pool before and after the ULSD roll out.

ULSD’s Lubricity

Lubricity is the other big concern from a performance standpoint. As new vehicle fuel systems move to higher and higher operating pressures and tighter and tighter metal tolerances, fuel lubricity is becoming more of an issue. The lubricity of the current on-road LS No. 2 diesel may or may not be enough for these new systems. To correct this, last year the ASTM adopted a lubricity specification into the D 975 diesel standard. The requirement is a 520 micron maximum wear scar diameter on a special instrument called a high frequency reciprocating rig, or HFRR. It took the industry some time to get this accomplished, but by now, nearly all U.S. on-road diesel fuel has sufficient lubricity or is injected with lubricity improvers at the terminals while it is being loaded into the transport trucks. The hydrotreating that removes the sulfur will also result in lower lubricity with ULSD. Aftermarket additive manufacturers and the biodiesel proponents have been pushing their products as ways to correct ULSD’s poor lubricating properties. This is no longer necessary as terminals will continue to additize the fuels to meet the ASTM specification. The bottom line is that the additized ULSD will have better lubricity than the unadditized LSD we have been using for years. Lubricity is not a concern with ULSD, but this goes back to the earlier point—if you want diesel, buy diesel. If the fuel is purchased as kerosene or fuel oil, it may not have these necessary additives.

Breaking the Seal

In 1993 when diesel went from 5,000 ppm to 500 ppm maximum, there were reports of unexpected outbreaks of leaking seals on fuel pumps. The hydrotreatin needed to lower the sulfur also changes the aromatic content of the fuel. This in itself is not bad, but one thing aromatics do is swell gaskets and seals. When the amount of aromatics decreased, these seals contracted or would shrink, allowing fuel to leak. Once the seals were replaced, the pumps were fine for the new fuel. Like fuel density, aromatic content can vary from one fuel to another, so leaking fuel seals was still a concern after 500 ppm LSD was rolled out.

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