Fuels from Petroleum
 

What is this stuff we want to replace?

Diesel fuel is derived from petroleum through a refining process.  The primary purpose of a petroleum refinery is to separate the complex mixture of hydrocarbons into usable products.  Petroleum is separated into fractions whose distinguishing feature is their different boiling points. Table 1 shows the boiling point ranges corresponding to the various commercial fuels.

Table 1.  Typical Refinery Products

From: Schmidt, G.K. and Forster, E.J., “Modern Refining for Today’s Fuels and Lubricants,” SAE Paper 861176, 1986.

As indicated in the table, kerosene, jet fuel (Jet A), and No. 1 diesel fuel are the same fraction of petroleum.  In most refineries, this fraction is straight run, that is, it is produced directly from compounds that were present in the crude petroleum.  In contrast, No. 2 diesel fuel may contain some straight run material but it also contains streams that are byproducts of the refining processes that produce gasoline.  No. 2 diesel was traditionally used as a “dumping ground” for refinery streams that could not be economically processed into higher value fuels. 

Most refineries are designed so that their primary product is gasoline, since this is the product in greatest demand in the United States.  The actual fraction of petroleum that is in the proper boiling point range for gasoline is usually quite low (20-40%) and most of this has too low of an octane number for direct use.  It is necessary to use a variety of refining processes to convert other, usually higher boiling point, fractions to gasoline and to convert low octane number material to higher octane number.  By-products of these processes that fall into the proper boiling point range, will often end up as No. 2 diesel fuel.  The potential variability in properties that this allows has been a major concern for engine manufacturers and the Environmental Protection Agency.