Page 113 - Synthetic Fuels Handbook

FUELS FROM PETROLEUM AND HEAVY OIL 99

It is found in asphalt coatings, enamels, paints, thinners, and varnishes. No. 1 fuel oil is a
light petroleum distillate (straight-run kerosene) consisting primarily of hydrocarbons in
the range C to C . No. 1 fuel oil is very similar in composition to diesel fuel; the primary
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difference is in the additives.
No. 2 fuel oil is a petroleum distillate that may be referred to as domestic or industrial.
The domestic fuel oil is usually lower boiling and a straight-run product. It is used primarily
for home heating. Industrial distillate is a cracked product or a blend of both. It is used in
smelting furnaces, ceramic kilns, and packaged boilers. No. 2 fuel oil is characterized by
hydrocarbon chain lengths in the C to C range. The composition consists of aliphatic
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hydrocarbons (straight-chain alkanes and cycloalkanes) (64 percent), unsaturated hydro-
carbons (alkenes) (1–2 percent), and aromatic hydrocarbons (including alkyl benzenes and
2-ring, 3-ring aromatics) (35 percent), but contains only low amounts of the polycyclic
aromatic hydrocarbons (<5 percent).
No. 6 fuel oil (also called Bunker C oil or residual fuel oil) is the residuum from crude
oil after naphtha-gasoline, No. 1 fuel oil, and No. 2 fuel oil have been removed. No. 6 fuel
oil can be blended directly to heavy fuel oil or made into asphalt. Residual fuel oil is more
complex in composition and impurities than distillate fuels. Limited data are available on
the composition of No. 6 fuel oil. Polycyclic aromatic hydrocarbons (including the alkyl-
ated derivatives) and metal-containing constituents are components of No. 6 fuel oil.
Stove oil, like kerosene, is always a straight-run fraction from suitable crude oils, whereas
other fuel oils are usually blends of two or more fractions, one of which is usually cracked gas
oil. The straight-run fractions available for blending into fuel oils are heavy naphtha, light and
heavy gas oils, reduced crude, and pitch. Cracked fractions such as light and heavy gas oils
from catalytic cracking, cracking coil tar, and fractionator bottoms from catalytic cracking
may also be used as blends to meet the specifications of the different fuel oils.
Since the boiling ranges, sulfur contents, and other properties of even the same fraction
vary from crude oil to crude oil and with the way the crude oil is processed, it is difficult
to specify which fractions are blended to produce specific fuel oils. In general, however,
furnace fuel oil is a blend of straight-run gas oil and cracked gas oil to produce a product
boiling in the 175 to 345°C (347–653°F) range.
Diesel fuel oil is essentially the same as furnace fuel oil, but the proportion of cracked
gas oil is usually less since the high aromatic content of the cracked gas oil reduces the
cetane value of the diesel fuel. Under the broad definition of diesel fuel, many possible
combinations of characteristics (such as volatility, ignition quality, viscosity, gravity, stabil-
ity, and other properties) exist. To characterize diesel fuels and thereby establish a frame-
work of definition and reference, various classifications are used in different countries. An
example is ASTM D975 in the United States in which grades No. 1-D and 2-D are distillate
fuels, the types most commonly used in high speed engines of the mobile type, in medium
speed stationary engines, and in railroad engines. No. 4-D covers the class of more viscous
distillates and, at times, blends of these distillates with residual fuel oils. No. 4-D fuels
are applicable for use in low- and medium-speed engines employed in services involving
sustained load and predominantly constant speed.
Cetane number is a measure of the tendency of a diesel fuel to knock in a diesel engine.
The scale is based upon the ignition characteristics of two hydrocarbons n-hexadecane
(cetane) and 2,3,4,5,6,7,8-heptamethylnonane. Cetane has a short delay period during ignition
and is assigned a cetane number of 100; heptamethylnonane has a long delay period and has
been assigned a cetane number of 15. Just as the octane number is meaningful for automo-
bile fuels, the cetane number is a means of determining the ignition quality of diesel fuels
and is equivalent to the percentage by volume of cetane in the blend with heptamethylnon-
ane, which matches the ignition quality of the test fuel (ASTM D-613).
The manufacture of fuel oils at one time largely involved using what was left after
removing desired products from crude petroleum. Now fuel oil manufacture is a complex