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Allen D.
Uhler, Scott A. Stout, Kevin J. McCarthy,
Stephen Emsbo-Mattingly, Gregory S. Douglas, and Phillip W.
Beall
Introduction
The operations at modern
refineries impart certain chemical characteristics to the
petroleum products produced. Part 1 of this series provided an
overview of the variety of major refining processes and the
general influences these have on production of petroleum
products and intermediates. Part 2 of this series focused on
the blending practices used in the production of automotive
gasolines and the effects these have on ‘fingerprinting’
gasolines. Part 3 was a discussion on the refining practices
used in the production of distillate fuels. This installment
focuses on residual fuels—what they are, their
characteristics, and the implications of the compositions of
these pragmatic fuels in chemical fingerprinting
investigations.
What are
Residual Fuels?
The term ‘residual fuel’ harkens to the nature of this class
of petroleum–a suite of products blended from the residuum
(termed resid) of the refining process. Plainly stated, these
are fuels cobbled together by refiners with the materials left
over after virtually all of the higher quality hydrocarbons
have been distilled, cracked, or catalytically removed from
crude oil feedstock. In the early days of refining, resid was
a terminal waste material that proved a major disposal
headache for refiners. The development and commercialization
of residual fuels in the early 20th century provided refiners
with a commercial means to rid themselves of these low-value
refining by-products.
Much like distillate fuels, residual fuels are classified by
their end-use. The major end-uses of residual fuels are either
as fuel oils—heavy gas oil-range blends that are a
(relatively) cheap form of high-BTU content fuel capable of
being fired in a number of different, but generally simple
commercial boilers, or as fuels for a special class of heavy
diesel engines. In the very early days of refining, there were
four grades of residual fuels; today, that list has been
whittled to three dominant products:
- Fuel #4 (light and
heavy)—residual/distillate blends that meet specified
viscosity parameters. This fuel is blended for use in simple
pressure-atomizing commercial burners. Light Fuel #4 has
viscosity characteristics that allows the fuel to be pumped
and handled at relatively low temperatures, while heavy Fuel
#4 has viscosity characteristics that prohibit its pumping
and handing at cold-weather temperatures.
- Fuel #5 (light and
heavy)—residual/distillate blends that are intended for
industrial burners capable of handling higher viscosity
fuels. Light Fuel #5 is of sufficient viscosity such that it
does not require pre-heating prior to firing; Heavy Fuel #5
is intended for similar burners, but is more likely to
require pre-heating in its pumping and handling.
- Fuel #6—alternatively named
Bunker C, this residual blend is sufficiently viscous to
require pre-heating in its pumping and handling; in
addition, this fuel must be pre-heated at the burner to
induce atomization at the burner nozzle. The pre-heating
requirements of this fuel limit its utility to all but the
most specialized applications, e.g., as a marine ship fuel.
Characteristics of Residual Fuels
By design, residual fuels can be
blended using various residual streams in the refinery.
These resids are in turn cut with lower quality gas oils or
other distillates to formulate the commercial fuel product.
The choice of this distillate cutter stock is itself
variable and largely a function of availability at any given
time within the refinery. Because residual fuels are blended
from a variety of different residual materials (and in turn
produced from different types of crude oils from refinery to
refinery), the specifications for residual fuels are, by
design, remarkably liberal (Table 1). In fact, the
controlling specification on residual fuels is viscosity;
other limiting requirements—notably boiling point ranges—are
unspecified. The practical advantage of these fuel
composition requirements is that the refiner is allowed
reasonably wide latitude in deciding what goes into the
blending of a residual fuel. By extension, this creates
significant variations in the potential chemical composition
of the resulting fuel products. What is a tremendous benefit
to the refiner (latitude in residual fuel blending) creates
a unique challenge for the environmental forensic
investigator.
Residual
Fuel Characteristics Relevant to “Fingerprinting” Table 1 reveals a number of interesting, comparative
specifications between residual fuels and select distillate
fuels that have implications for the environmental forensic
investigator. These specifications, codified in ASTM D396
Standard Specifications for Fuel Oils, are used by
refineries as guidelines in the production of commercial
fuels. While unique customer requirements and/or state
regulations can alter these specifications somewhat, the
gross differences among distillate and residual fuels are
evident. As mentioned above, residual fuels—unlike
distillate fuels—do not have specific boiling point range
specifications. Whereas the forensic investigator can often
rely upon the boiling point specification and the unique
character of recondensed products to identify a distillate
fuel (e.g., diesel #2 and fuel oil #2 typically have boiling
point ranges that span the C10 to C25 carbon range in a
regular, Gaussian distribution of hydrocarbons), he or she
is faced with the fact that residual fuels blends
often have variable and surprisingly different gross
chromatographic fingerprints and chemical compositions. Bulk chemical parameters, particularly sulfur content, which
can be a valuable tool in identifying and classifying
distillate fuels, are less useful for characterizing
residual fuels. Sulfur in distillate fuels has always been a
concern due to the acidity it produces during combustion,
the detrimental effects (corrosion, wear, and deposit
build-up) this has on engine and furnace parts, and in the
latter part of the 20th century, because of high sulfur fuel
use implications for deleterious air quality impacts. As a
result, both end-users and federal and state regulators have
mandated sulfur limits for most distillate fuels. Residual
fuels do not have federal requirements for sulfur content.
Though some states do have sulfur limits on residual fuels,
the more variable restrictions on residual fuel sulfur
levels (and the ways that refiners can achieve these lower
sulfur level, e.g., by dilution with low quality gas oil),
minimizes the utility of sulfur as a forensic tool in
residual fuel investigations. A good example of the challenges facing the forensic chemist
in the realm of residual fuel fingerprinting is illustrated
in Figure 1. Here, the GC/FID chromatograms for six fresh
Fuel #6 (Bunker C) marine fuels are shown. The variety in
chemical composition among these is remarkable and
exemplifies the lack of any such thing as a ‘typical’ Fuel
#6. At first blush (and certainly to the inexperienced
forensic chemist), the gross chemical differences among
residual fuels such as typified by those in Figure 1 could
be viewed as a tremendous forensic hurdle, e.g., how can one
classify or differentiate among residual fuels when they
lack the predictable chromatographic features that are
dominant among distillate fuels? The experienced forensic
chemist who understands how residual fuels are blended can,
in fact, leverage this chemical variability by using a
variety of fingerprinting characteristics to distinguish
among or correlate different residual fuel types, and thus,
different potential sources. With an understanding of the basic characteristics of
residual fuels in hand, some strategies for identifying the
presence and tracking the fate of residual fuels in the
environment can be offered.
- Understand the refining practices relevant in the blending
of residual fuels. This knowledge is important toward
developing a strategy for differentiating residual fuels
from other hydrocarbons (e.g., crude oil, distillate fuels),
and for differentiating among candidate sources of residual
fuel. See Leffler (2000) as a starting point to develop this
knowledge.
- Develop a clear understanding of the gas chromatographic
features of residual fuels. High-resolution gas
chromatography is often the first line of evidence an
investigator will use to determine the type(s) of petroleum
or hydrocarbons in an investigation. Recognize that
“off-the-shelf” gas chromatography used for measurement of
TPH (e.g., EPA Modified 8015) will usually not be sufficient
to distinguish among residual fuels and other candidate
hydrocarbon sources—high-resolution gas chromatography is
warranted in these matters (e.g., Uhler et al., 1998).
- Understand the weathering characteristics of the major
blending components (e.g., resids, cutter stocks) as well as
individual chemicals within the principal blending stocks.
Once in the environment, the features and chemical
composition of residual fuels (and other hydrocarbon
products) will alter. The investigator must acknowledge
these processes and understand how to interpret their
impacts in a forensic investigation (e.g., McCarthy et al.,
1998).
- Identify the classes of recalcitrant compounds within the
residual fuel blending stocks that can be used as
diagnostic, source-specific markers. The backbone of most
advanced forensic investigation of petroleum products relies
on the use of recalcitrant markers for identification and
differentiation of fugitive petroleum. Recognition that
residual fuels often contain both lighter distillates and
heavy resid components offers the forensic chemist a broad
spectrum of chemical compounds as potential markers (e.g.,
Stout et al., 2002).
- Develop a list of candidate marker compounds from within
the classes of recalcitrant compounds identified in the
residual fuels of interest. Based on literature and
practical evidence, select marker compounds that can be
expected to exhibit high environmental stability while
providing maximum discrimination among potential sources.
Recent work by Stout et al. (2001) exemplify one methodology
that proved successful in an environmental forensic
investigation of the source and fate of residual fuels.
Remember, residual fuels are unique; as such, refinery-,
regulatory-, geographic- or case-specific considerations may
make some of the above suggestions moot, or other points
overwhelmingly important. In the end, when developing a
strategy to identify and fingerprint residual fuels there is
no substitute for a clear understanding of the refining and
blending processes of residual fuels, and a thorough
knowledge of the chemical features that distinguish these
products from other fuels and petroleum products. As always,
experience and knowledge of the chemistry and behavior of
these fuels is the key toward the solution of forensic
investigations regarding their occurrence, sources, and fate
in the environment.
References
Leffler, W.L. (2000). Petroleum
Refining. 3rd Edition. Penwell Corporation, Tulsa,
OK. McCarthy, K.J., Uhler,
A.D., and Stout, S.A. (1998). Weathering affects petroleum ID.
Soil & Groundwater Cleanup.
Stout, S.A., Uhler, A.D.,
McCarthy, K.J. and Emsbo-Mattingly, Stephen (2002) Chemical
Fingerprinting of Hydrocarbons. In: Introduction to
Environmental Forensics, (B. Murphy and R. Morrison,
Eds.), Academic Press, New York, p. 135-260.
Stout, S.A., Uhler, A.D.,
McCarthy, K.J. (2001) A Strategy and Methodology for
Defensibly Correlating Spilled Oil to Source Candidates.
Env. Forensics 2: 87-98
Uhler, A.D., K.J. McCarthy, and
S.A. Stout. (July 1998). Get to know your petroleum types.
Soil and Groundwater Cleanup. |
