Table 1. Composition of the Liquid Oil
f

771

Sample Mass Fraction
# CH, CH GH
1 0.035 0.0047 | 0.0064
2 0.026 0.0042 | 0.0058
3 0.014 0.0058 { 0.0055
4 0.012 0.0026 | 0.0038
5 0.011 0.0044 | 0.0042



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Figure 2

GC Analysis of Alaskan North Slope Crude Oil

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Application of FTIR Remote Sensing Spectroscopy in Environmental Impact
Assessments of Oil Fires
Mare R. Nyden, William Grosshandler, Darren Lowe, Richard Harris and Emil Braun
Building and Fire Research Laboratory
National Institute of Standards and Technology
Technology Administration, DOC
Gaithersburg, MD 20899

Abstract

A series of laboratory measurements was conducted to determine the extent to which benzene
and other aromatic components are destroyed when crude oil is burned on the open seas. The
atmosphere above a pan containing Alaskan North Slope crude oi] was monitored with a remote sensing
FTIR spectrometer during both evaporation and buming. This data was used in conjunction with gas
chromatographic analyses of the oil obtained before ignition and after the Gre was extinguished (by
covering the pan with a marinite slab) to estimate the C,H, destruction efficiency of the oil fire. The
results of these laboratory measurements are viewed in the context of data collected during mesoscale
oil fires conducted at Little Sand Island in Mobile, Alabama.

Introduction

The possibility of using combustion to remediate oil spills on the open seas is being explored
{1-4]. Gas chromatography (GC) and other point sampling techniques have been used to monitor
pollutants released into the atmosphere during mesoscale oil fires in an effort to assess the environmen-
tal impact of this procedure. The potential to make continuous measurements along a line-of-sight could
open a new dimension to this research. The capabilities of the present generation of FTIR remote
sensing spectrometers, which include high resolution and fast scanning over extended optical paths,
would seem to be well-suited to this application.

This hypothesis was examined in a series of oil bum experiments conducted in a 230 m? square
pan located on Little Sand Island in Mobile, Alabama [4]. Spectra were measured over a 7 m path
about 30 m downwind of the fire. The progression, beginning with a spectrum of the atmosphere
before the oil was pumped (top) and ending with a spectrum taken shortly after the oil was ignited
(vottom), is displayed in Figure 1. The absorbance centered at ~ 2900 cm" is indicative of the presence
of non-aromatic hydrocarbons (NAHCs). Quantitative analyses of these spectra indicate that the
aumospheric concentrations of these compounds exceeded 50 ppm (mole fraction) while the oil was being
pumped from a barge into the pan, falling below the lower detection limits (LDL) of the instrument
shortly after ignition. This suggests that the NAHCSs present in the original crude oil were effectively
destroyed in the fire. The fate of the C,H,, and other aromatic hydrocarbons which are either present
in the original crude oil or produced as a result incomplete combustion, is of much greater concer.
These compounds are more thermally stable and are considered to be a far greater threat to the
environment than are the NAHCs. Unfortunately, C,H, was not consistenuy detected in the spectra
collected during the oil evaporation and burn tests conducted in Mobile. Presumably, this was because
the atmospheric concentrations of C,H, were below the instrument LDL. Consequently, no statement
can be made concerning the fraction of C,H, which is converted to CO, as compared to the amount
which is merely dispersed into the atmosphere by the convective flow of the fire.

The objective of this study is to shed some light on this issue by observing what happens to C,H,
in small, well-controlled oil fires. In reviewing the results of these experiments, it is important to keep
in mind that the motivation here, is not as it is in many other environmental studies, to measure trace
quantities of hazardous chemicals. Significant releases of C, H, and other aromatic compounds are
inevitable consequences of an oil spill. The central question addressed in this paper is whether less

767

Air and Waste Management Association. Optical Sensing
for Environmental Monitoring (SP89). International
Specialty Conference Proceedings. October 11-14, 1993,
Atlanta, GA, Air and Waste Management Assoc.,
Pittsburgh, PA, 767-779 pp, 1993.


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The destruction efficiency of fires is known to depend on their size [6]. A logical next step in assessing
environmental impact of oil spill combustion is to measure the destruction efficiency of benzene in oil

fires as a function of this variable.

Acknowledgements

The large scale fire tests conducted in Mobile, Alabama were funded by the Minerals

Management Service, Department of the Interior. One of us (MRN) would like to thank the entire
Large Fire Research Group at NIST for providing a friendly and understanding environment while the
field measurements were being made in Mobile. The authors would also like to acknowledge assistance
from David Mattingly in conducting the laboratory oil fire experiments.

References

1.

Evans, D.D, Mulholland, G., Gross, D., Baum, H.R. and Saito, K.; “Environmental Effects
of Oil Spill Combustion," NISTR 88-3822, National Institute of Standards and Technology;
Gaithersburg, 1987.

Evans, D.D, Walton, W.D., Baum, H.R. Lawson, J.R., Rehm, R., Harris, R., Ghoniem, A.
and Holland, J.; "Measurement of Large Scale Oil Spill Bums," in “Proceedings of the
Thirteenth Arctic and Marine Oil Spill Program Technical Seminar,” Edmonton, 1990.

Evans, D.D, Walton, W.D., Baum, H.R. Notarianni, K.A., Lawson, J.R., Tang, H.C., Keydel,
K.R., Rehm, R., Madrzykowski, D. Zile, R.H., Koseki, H. and Tennyson, E.J.; “In-Situ
Burning of Oil Spills: Mesoscale Experiments,” in “Proceedings of the Fifteenth Arctic and
Marine Oil Spill Program Technical Seminar,” Edmonton, 1992.

Walton, W.D., Evans, D.D., McGrattan, K.B., Baum, H.R. Twilley, W. H., Madrzykowsk,
Putorti, A.D., Rehm, R., Koseki, H. and Tennyson, E.J.; “In-Situ Burning of Oil Spills:
Mesoscale Experiments and Analysis,” in "Proceedings of the Sixteenth Arctic and Marine Oil
Spill Program Technical Seminar," Calgary, 1993.

Hanst, P.L. and Hanst, S.T.; “Quantitative Reference Spectra for Gas Analysis,” Infrared
Analysis, Inc.; Anaheim.

Mulholland, G.W., Henzel, V. and Babrauskas, V.; “The Effects of Scale on Smoke
Emissiom,” in “Proceedings of the Second Symposium on Fire Safety Science, 1988.

770


the major source of error in these analyses. The syringe used to make the 0.5 yL injections was only
calibrated to the nearest 0.1 yL so that the sample volume could not be determined to an accuracy better
than + 0.05 nL, or + 10% of the reported concentration.

Results

The spectrum of the atmosphere above the pan prior to ignition is compared to a C,H, reference
spectrum (5] over the region corresponding to the stretching of aromatic C-H bonds in Figure 4. The
integrated absorbance of this band corresponds to a partial pressure of about 150 + 38 ppm (mole
fraction) over the 10 cm path defined by the diameter of the pan. The intensity of this band, as well
as the intensity of the adjacent band due to nonaromatic C-H stretching, falls below detectable limits
during the period of time that the oil is burning (Figure 5). This indicates that these components are
destroyed, rather than merely dispersed into the atmosphere, by the fire. Indeed, there appears to be
almost complete conversion to CO, at the level of sensitivity of the measurement (~ 10 ppm). The peaks
corresponding to CO, CH, and C,H,, which are prominent in the spectrum of the gases evolved during
pyrolysis of the oil in N, (Figure 6), are not apparent in the spectrum of the atmosphere above the
buming oil (top of Figure 5). The C,H, destruction efficiency over all 5 bums was

pe = FM PO ny, @)
XM,

where X is the mass fraction of the component under consideration (Table I) and M is the total mass
of oil (Figure 3) before the first ignition (i) and after the last extinction (f). In contrast, the DEs for
C,H,, and C,H, were only 0.22 and 0.46, respectively. The anomalously high value obtained for C,H,
may be an indication that the added volume was never fully assimilated into the oil matrix.

The progression from the initial spectrum (top), taken before ignition, to the final spectrum
(bottom), taken after the last extinction (Figure 7), indicate that the C,H, and NAHC emisssions from
the oil (at room temperature) were reduced below the instrument LDL after the oil had been bumed for
2000 s. The GC data presented in Table I, however, indicate that the bumed oil still contained
significant amounts of these compounds. A further illustration of this effect is evident in Figure 8
which is a plot of the mole fraction of C,H, in the vapor above the pan as a function of the mass
fraction of C,H, remaining in the liquid. The progression from the initial to the final evaporation is
from right to left. The rapid fall-off in the slope of this curve after a mass fraction of 0.015 indicates
that it is becoming increasingly difficult for the C,H, molecules to escape the liquid. A likely
explanation is that the increased viscosity which results from burning the more volatile fraction
suppresses the further evaporation of the remaining low molecular weight compounds.

Conclusions

Although the results presented in this paper are of a preliminary nature, some clear trends have
emerged. The destruction efficiency of benzene is high during the actual burning, at least in small
laboratory oil fires such as the one considered in this investigation. The conversion to carbon dioxide
was complete at the level of sensitivity of the FTIR and the spectra gave no indication of the presence
of carbon monoxide or any other product of incomplete combustion. A significant fraction of the low
molecular weight components, including benzene, are retained in the residue. The further volatilization
of these compounds is apparently suppressed by viscosity effects which are more pronounced in the high
molecular weight residue than in the original crude oil. A comparison of the spectra in Figures 5 and
7 clearly shows that there is less benzene above the pan during the burning and eventual evaporation
of the residue than in the evaporation of the original oil. On this basis, it appears that combustion may
be be effective in reducing the amount of benzene released into the atmosphere during oil spills.

769


C,H, is released inta the atmosphere as a result of burning the oil than would be the case if the oil wa
allowed to evaporate on the open seas.

Experimental

A circular pan (10 cm in diameter) was placed on a load-cell and filled with 540 g of Alask.
North Slope crude oil. A GC analysis (Figure 2) of this oil indicated that it contained about 0.25% ()
mass) C,H,. Unfortunately, the corresponding partial pressure was below the instrument LDL. This
problem was solved by enriching the oil with an additional 20 g of C,H,. A spectrum of the vapor wa
collected and a sample of the liquid oil was extracted before igniting the oil. A second spectrum, this
time of the atmosphere over the burning oil, was taken and the fire was extinguished by covering th
pan with a marinite slab. This sequence, consisting of evaporation followed by burning, was repeated
until the C,H, and NAHCs could no longer be detected in the evaporation spectra. Spectra, which toot
approximately 5 minutes to acquire, were collected at each step and a sample of the liquid oil wa
extracted (for GC analyses) before each ignition. The line-of-site of the measurements, which wa
off-center and slightly above the flame, was fixed throughout the course of the experiment. About 100
& (18%) of the oil-benzene mixture were incinerated in 5 bums which lasted a total of 2000s. Th
mass-loss curve over the full 4000 s (including both evaporation and burning) is displayed in Figure 3.
The spikes, which occur at approximately 10 minute intervals, indicate when the fire was extinguished.

Absorbance spectra were obtained as signal averages over 1024 interferograms collected at 4 cm!
resolution using a Midac' monostatic remote sensing FTIR. The concentrations of C,H, were estimated
from

A, 1,

= 4s 1
x, a Q

where A, | and x denote band integrated absorbance, optical path length through the sample, and mok
fraction of the analyte (a) and reference (r). This strategy, consisting of oversampling the spectra and
estimating concentrations from band averaged (rather than line) intensities was effective in reducing th
level of random error. The precision of the reported concentrations was estimated by substituting

1
AA, = {f{5,(v) ~ 28,()Pavl? @

for A, in Eq.(1). Here S(u) denotes spectral absorbance and the integration is over the frequencies (»)
which define the band. In the case of C,H, , this procedure gave estimates ranging from about 2%
(highest concentration) to 25% (lowest nonzero concentration) of the corresponding mole fraction. Tit
contribution from systematic errors is more difficult to quantify. However, a total error of + 25%
of the reported value is consistent with previous experience in the quantitative analyses of similar
compounds.

Aliquots of the oil samples were dissolved in CH,Cl, and analyzed on a HP5730A GC equippet
with a HP 3380A integrator and a flaine ionization detector. An acceptable resolution of the lowe
molecular weight hydrocarbons was obtained using a 30 mL/min flow of He through a 3 m colum:
packed with 10% SP 2100 on 100/120 mesh Supelcoport at 50 *C. Concentrations were determined
from the ratio of the appropriate peak in the chromatogram of the sample to the corresponding peak is
the chromatogram of the reference. The uncertainty in the amount of sample injected into the GC wa

‘Certain commercial equipment, instruments, or materials are identified in this paper in
order to specify the experimental procedure. Such identification does not imply that the material
or equipment identified is necessarily the best available for the purpose.

768


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