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.

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.

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