Overall, Canadian fuels tend to have low density, low viscosity, and lighter distillation characteristics than those used in the United States, and are among the worst lubricity fuels in the world. Diesel fuel No. 1, as used for much of the year in Canada, is broadly similar to the kerosene fuels that caused durability problems in military vehicles, municipal buses, and aviation equipment.
Even prior to the introduction of 500ppm-sulfur diesel fuel, Canada had reported problems with reduced equipment life. These failures were typically associated with winter grade diesel fuels, particularly when they were used in warmer conditions.
500ppm sulfur fuels have been available in Canada since the 1980s, and a maximum sulfur content of 500 ppm was mandated in 1994. Fleet testing repeatedly demonstrated catastrophic pump failure in less than 500 miles of operation on Canadian fuel. In 1997, Canada modified its low-sulfur diesel fuel specification to address the lubricity of winter fuels - those having a viscosity below 1.9 cSt at 40ºC or less and a cloud point of -30ºC or lower. A fuel supplier can "qualify" its fuel using one of several options, ranging from a field test to pump rig tests to the HFRR or BOCLE laboratory scale test. A fuel supplier must use lubricity additives if the fuel fails the selected test.
The primary sources of sulfur in diesel fuel are the sulfur-containing compounds which occur naturally in crude oil. Depending on the source, crude oil contains anywhere from fractions of a percent of sulfur, such as less than 0.05 weight percent (500 ppm) to as much as several percent. The average amount of sulfur in crude oil refined in the U.S. is about one percent. Most of the sulfur in crude oil is in the heaviest boiling fractions. Since all of the refinery blendstocks that can be used to manufacture diesel fuel come from the heavier boiling components of crude oil, they contain substantial amounts of sulfur.
On an aggregate basis, most of the highway diesel fuel volume manufactured in the U.S. comes from the straight-run product of the crude fractionation tower (called straight run). Most of the remainder, comes from the fluid catalytic cracker (FCC) conversion unit (called light cycle oil).
The remaining small fraction of diesel fuel volume comes from a coker conversion unit (called light coker gas oil), or from the hydrocracker conversion unit (called hydrocrackate).
Sulfur containing compounds in distillate can be classified according to the ease with which they are desulfurized. Sulfur contained in paraffins or aromatics with a single aromatic ring are relatively easy to desulfurize. These molecules are sufficiently flexible so that the sulfur atom is in a geometric position where it can make physical contact with the surface of the catalyst. The more difficult compounds are contained in aromatics consisting of two aromatic rings, particularly dibenzothiophenes.
Dibenzothiophene contains two benzene rings which are connected by a carbon-carbon bond and two carbon-sulfur bonds (both benzene rings are bonded to the same sulfur atom). This compound is essentially flat in nature and the carbon atoms bound to the sulfur atom hinder the approach of the sulfur atom to the catalyst surface. Despite this, today's catalysts are very effective in desulfurizing dibenzothiophenes, as long as only hydrogen is attached to the carbon atoms bound directly to the sulfur atom.
However, distillate can contain dibenzothiophenes which have methyl or ethyl groups bound to the carbon atoms which are in turn bound to the sulfur atom. These extra methyl or ethyl groups further hinder the approach of the sulfur atom to the catalyst surface.
Dibenzothiophenes with such methyl or ethyl groups are commonly referred to as being sterically hindered. An example of a dibenzothiophene with a single methyl or ethyl group next to the sulfur atom is 4-methyldibenzothiophene. An example of a dibenzothiophene with two methyl or ethyl groups next to the sulfur atom is 4,6-dimethyl dibenzothiophene. In 4,6-dimethyl dibenzothiophene, and similar compounds, the presence of a methyl group on either side of the sulfur atom makes it very difficult for the sulfur atom to react with the catalyst surface to assist the hydrogenation of the sulfur atom.
Straight run distillate (or straight run gas oil (SRGO)) contains relatively low levels of these sterically hindered compounds. LCO, contains the greatest concentration of sterically hindered compounds. Thus, LCO is generally more difficult to desulfurize than straight run distillate.1 In addition, cracked stocks, particularly LCO, have a greater tendency to form coke on the catalyst, which deactivates the catalyst and requires its replacement.
Overall, Canadian fuels tend to have low density, low viscosity, and lighter distillation characteristics than those used in the United States, and are among the worst lubricity fuels in the world. Diesel fuel No. 1, as used for much of the year in Canada, is broadly similar to the kerosene fuels that caused durability problems in military vehicles, municipal buses, and aviation equipment.
Even prior to the introduction of low-sulfur diesel fuel, Canada had reported problems with reduced equipment life. These failures were typically associated with winter grade diesel fuels, particularly when they were used in warmer conditions. Low-sulfur fuels have been available in Canada since the 1980s, and a maximum sulfur content of 500 ppm was mandated in 1994. Fleet testing repeatedly demonstrated catastrophic pump failure in less than 500 miles of operation on Canadian fuel. In 1997, Canada modified its low-sulfur diesel fuel specification to address the lubricity of winter fuels - those having a viscosity below 1.9 cSt at 40ºC or less and a cloud point of -30ºC or lower. A fuel supplier can "qualify" its fuel using one of several options, ranging from a field test to pump rig tests to the HFRR or BOCLE laboratory scale test. A fuel supplier must use lubricity additives if the fuel fails the selected test.
