Today fuel quality issues can now be associated with
corrosion. Diesel and gasoline are not problems in and of themselves. A
combination of factors from the addition of biofuels to the removal of sulfur
have all attributed to at least a few unintended consequences.
Legislation has changed refining. Most of the conventional
diesel fuel being used is now ultra-low sulfur diesel (ULSD). A decade ago, it
was not uncommon to find diesel fuel with a sulfur content in excess of 500
parts per million (ppm). Today, ULSD is
no higher than 15 ppm sulfur in the U.S.
In the UK the requirement is 10 ppm.
The refining process starts with fractional distillation
which involves the separation of crude into component parts (fractions) through
boiling and condensation as seen in the simple diagram of a distillation unit.
The refining process changed in order to meet clean fuel
requirements and the wide range of feedstocks and products today. Hydrodesulphurization (HDS), oxygenated
additives and biofuels have presented challenges and opportunities for the
HDS is the process of removing sulfur from refined petroleum
products in order to reduce SO2. Previously, sulfur acted as a lubricity agent
in fuel. Now with its removal, fuel is very dry resulting in engine component
wear and failure. As a result, lubricity additives must be used in ULSD to
compensate. Biodiesel is often used to increase the lubricity of ULSD. In the
U.S. and other areas of the world, up to 5 percent biodiesel can be added to
conventional diesel without labeling or notice.
As a result, biodiesel is often blended and distributed to the end user
as conventional ULSD. It is important to note since biodiesel presents some
maintenance challenges. Biodiesel is generally more hygroscopic (water soluble)
than conventional diesel. Biodiesel molecules
are primarily fatty acid methyl esters (FAME) usually acquired from vegetable
oils. These oils love water and are mildly acidic.
Oxygenates are compounds that contain oxygen. Ethanol is a common oxygenated additive in
fuel, used to reduce CO and soot. When the EPA mandated the removal of MTBE -
an oxygenated additive - ethanol became its replacement. Unfortunately, ethanol
brought a host of other issues with it. Ethanol is both hydrophilic – easily
dissolves in water and hygroscopic – water absorbing. Water is always present in fuel at some level,
often suspended in the ethanol blended fuel until it becomes heavy enough to
drop out. Ethanol and water are also miscible.
That is, they dissolve in each other.
Ethanol is lighter than conventional gasoline so it remains suspended in
the fuel unless enough water bonds to it.
Because water is heavier than fuel, with enough volume, it will drag the
ethanol to the bottom once enough is present.
Research shows it only takes .398% water in fuel for phase
separation to occur. At saturation the water and much of the ethanol separate
from the fuel and drop to the bottom of the tank. This is phase separation. Because ethanol is used to raise octane, the
fuel above the phase layer at the bottom of the tank is now substandard. Fuel can also remain partially phase
separated. This happens when enough
water enters the fuel but has not reached saturation. During this stage, fuel can become hazy often
clogging filters and shutting down engines. It does not take much water to
cause fuel to phase separate. Less than
20 gallons of water in 5,000 gallons of fuel will result in phase separation.