Clean Diesel: Ultra Low Sulfur Diesel

March 2, 2006
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.

ULSD is coming. Many users of diesel fuel have known about this ultra low sulfur diesel for sometime. It’s the details that aren’t so well-known. Why are we doing this? What are the benefits? Are there drawbacks? What do I need to do to be ready? When will it be available? Must I use it?

To begin, diesel fuel quality in the United States is regulated at the state level, and most states have adopted the latest version of ASTM D 975 Standard Specification for Diesel Fuel as their standard, which defines diesel fuel into various grades. On-road vehicles use grades No. 1 and No. 2 and each grade is split into three sulfur levels, S5000, S500 and S15. The number refers to the maximum amount of sulfur allowed in parts per million, or ppm. The highest level, 5,000 ppm, is equivalent to 0.5 percent by mass. S5000 is commonly known as high sulfur (HS), S500 is low sulfur (LS) and the newest designation, S15, is ultra low sulfur (ULS). Currently, the most common grade is low sulfur No. 2.

Distillates and Diesel 101

Diesel fuel is just one of a group of fuels collectively known as “distillates.” Other well-known distillates are kerosene, jet fuel and fuel oil (for home heating). Each has its own ASTM specification. A refinery produces No. 1 and No. 2 fuels at various sulfur levels and it’s common for one refined product to be sold as multiple retail products, provided that the applicable ASTM specs are met. This sometimes requires special testing or the addition of fuel additives.

For example, a LS No. 1 fuel may be sold from a single tank as Jet A, K-1 kerosene, diesel and fuel oil, but this doesn’t mean that they are interchangeable. Kerosene and jet fuel require very special certification testing and some fuels require special additives before use. The best bet is if you want diesel fuel, purchase “diesel fuel.” More importantly, don’t use anything but ASTM D 3699 K-1 kerosene in your indoor space heater.

Diesel fuel is the most common distillate and LS No. 2 is the most commonly used grade of diesel. Nearly all diesel sold at retail outlets is LS No. 2 diesel, which is also referred to as “Low Sulfur No. 2,” “D2,” “LS2,” “LSD” or any other combination of LS, D and No. 2. Unless otherwise specified, “diesel” to most people means LS No. 2 diesel.

So what is No. 1 diesel? Grade No. 1 fuels are very similar, but lighter or less dense than No. 2 fuels. They have lower viscosity, lower boiling points, lower flash points, and typically have better cold flow properties. They are excellent for use in cold climates and in the winter. The drawback is they usually cost more and also have lower energy content, so there is a trade-off to using a No. 1 fuel—better cold flow, but higher costs.

In transit, bus fleets tend to use No. 1 diesel fuels because they usually have less smoke, less NVH (noise, vibration, harshness) and won’t gel as easily in the winter. This makes the bus more “rider-friendly,” especially the part about not stalling in the cold. It is also very common to “winter blend” a No. 1 diesel with a No. 2 diesel in order to get the cold flow needed without losing too much fuel economy. Typically these blends are between 10 to 30 percent No. 1 into No. 2.

Benefits of ULSD

So where does ULSD fit into this? With all this diesel fuel being combusted in engines, it leads to a lot of airborne emissions, most notably, nitrogen oxides (NOx) and particulate matter (PM). NOx contributes to ozone formation and PM is the black smoke pouring out of the older trucks going down the highway. In 1993, the U.S. Environmental Protection Agency (EPA) lowered maximum sulfur levels in diesel fuel from 5,000 ppm to 500 ppm and created high sulfur and low sulfur diesels. The diesels of today have much less smoke than they did when CB radios and the song “Convoy” were popular. The next step to lowering emissions further is even less sulfur. What’s lower than low? Ultra low of course. If you are looking for less than 15 ppm diesel fuel, make sure you ask for “ultra low,” because simply saying “low” will get you the current on-road 500-ppm diesel fuel.

ULSD alone will improve emissions by lowering PM by about 10 percent. The real benefit comes from using exhaust aftertreatment devices that ULSD will make possible. Much like lead poisons a gasoline engine’s catalytic converter, sulfur poisons the catalyst used in diesel converters. For 2007, many diesel engines will have diesel particulate filters (DPF) which will remove approximately 90 percent of the PM. In 2010, aftertreatment devices will dramatically lower NOx emissions. Removing sulfur from diesel fuel is much more of a challenge than removing lead from gasoline. With lead, refiners just stopped adding it (the challenge was finding other ways to increase octane). Sulfur, on the other hand, is present in crude oil and must therefore be removed.

Making Ultra Low Sulfur Diesel

There are several technologies for removing sulfur, but it is expected that most refiners will use a process known as hydrotreating. Removing sulfur to below 15 ppm will require more energy and shorten the lifetime of the catalyst used in the hydrotreater. This is one of the difficulties—another is getting the ULSD delivered to retail sites. The pipelines and distribution systems that ship ULSD will still be handling LS fuels and HS fuels (Jet fuel can be high sulfur No. 1). Keeping these LS and HS fuels out of ULSD is very difficult.

Fuels in pipelines are not physically separated, and some mixing occurs at the interface. There are also plenty of places where fuels get hung up (valves for example) and then mixed into subsequent batches. Transport trucks that carry fuel from terminals to retail sites are not typically flushed between loads. All of this mixing was never much of an issue before, but ULSD is very, very sensitive to contamination. When loading a 2,500-gallon compartment on a transport for delivery, 1 gallon of HS diesel left behind from the previous delivery can contaminate the ULSD by about 2 ppm. This forces the refiners to produce the ULSD much lower than 15 ppm in order to get it to retail on specification. Most pipelines are requiring that ULSD be no higher than 8 ppm to start with, which makes the refiner’s job even harder.

For the petroleum industry, the transition to ULSD will be complicated—to say the least. Focusing on on-road diesel, refiners will have to start producing ULSD by June 1 of this year. This has required extensive modifications and capital investment, so most refiners won’t be ready much sooner than June 1. Terminals have until Sept. 1 and retail sites until Oct. 15 before they convert to ULSD. The complication is that refiners are allowed to produce 20 percent of their on-road fuel as 500 ppm LSD. Then at each point in the distribution system, up to an additional 20 percent more of a delivery batch can be downgraded to LSD. This gives an outlet for contaminated fuel, but also means that at retail, there will likely be ULSD and a significant amount of LSD. If more ULSD is contaminated than allowed, it will become “off-road” only and must be dyed red before sale. To ease the transition, the EPA will allow ULSD until the Oct. 15 deadline to be 22 ppm, then it must meet 15 ppm. After 2010, all on-road fuel must be ULSD.

Making Room for ULSD

Another important consideration is that terminals and retail sites will likely not have enough storage tanks to handle these additional grades of fuel, so in most cases they will have to choose. According to EPA surveys, most refiners will produce only ULSD, and they estimate that up to 90 percent of produced on-road diesel will be ULSD. Since all vehicles can run on ULSD, but new 2007 vehicles cannot use LSD (it will damage the diesel particulate filters), most retail sites with one tank are likely to offer only ULSD.

Converting storage tanks from LSD to ULSD does not require any special measures. Removing water bottoms is always a good idea. (In case you were wondering, all fuel contains some small amount of dissolved water when the fuel was refined. The fuel starts out warm, but as it cools, the water drops out and collects in storage tanks.) Storage tanks should not be taken much below their normal low level or there is a risk of stirring up bottom sediments. No special cleaning is required. Three tank “turns” will usually be enough to be on-spec. Retail sites that have legal requirements to sell may require more turns and testing to ensure they are at or below 15 ppm. 2007 model year vehicles that require ULSD should not be fueled until the storage tank is fully turned at or below 15 ppm.

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.

To prevent this from reoccurring, nearly all engine manufacturers have changed gaskets to materials less susceptible to changes in aromatic content, which should prevent an outbreak like in 1993, but it is still worth watching. Fleet vehicles that are dedicated to a single source of fuel may be more likely to develop problems from a fuel change than vehicles that commonly get a variety of fuel. If leaks develop, the solution is usually just a new gasket.

The Availability of ULSD

So when will this fuel be available? Some ULSD is in the market now, but don’t expect to see much of it before the second half of this year. Stations will be required to label their pumps as ULSD or LSD to prevent misfueling of the new 2007 vehicles. It’s these 2007 model year vehicles and any vehicles retrofitted with exhaust aftertreatment devices that are required to use it. Most others vehicles can use either LSD or USLD fuel. With approximately 90 percent of U.S. diesel production as ULSD, it should be available everywhere. Some regions like the Northeast may be more challenging for suppliers, since the Northeast relies heavily on pipeline shipments from the Gulf Coast. Texas to New Jersey is a long way to move ULSD without contamination. Eventually the bugs will be worked out of the distribution systems and demand will increase as newer vehicles appear on the road, so all areas should have an ample supply of ULSD.

What about customers that want to continue to use the current LSD? LSD will still be available, but it might be the harder of the fuels to find at retail. If the concern is about fuel economy, lubricity or seal swell, it may not be worthwhile to hunt for LSD. Much of the LSD in the market after the ULSD rollout might simply be “off-spec” ultra low. If ULSD is contaminated with just a little sulfur, it will be downgraded to LSD. This LSD will have ULSD-like properties and will not be the same old LSD you are using now.

So again, ULSD is coming. It is going to be a headache for producers and distributors, but hopefully not for the end users. Any problems encountered in the next year or so, whether related or not, could be blamed on ULSD, and this may send users looking for their old fuel. If all goes well, it will lead to less vehicle emissions and hopefully will shake diesel of its stinky, dirty image. This clean diesel could lead the way for more light-duty diesel cars and trucks with better fuel economy than their gasoline counterparts. Better fuel economy truly means less emissions and less dependence on crude oil. This is a big step for diesel fuel, the petroleum industry and the environment.