ABSTRACT: In situ burning has proven to be an effective response for oil spills in Arctic conditions. However the incomplete combustion of oil, due to an incomplete chemical reaction between the fuel and oxygen, lead to the production of solid and gaseous products, such as particulate matter (soot), polycyclic aromatic hydrocarbons (PAHs) and volatile organic compounds (VOCs). Several studies have suggested a link between the presence of these compounds and several diseases; of high concern are the particles smaller than 10 μm (PM10) that can penetrate deep in the lungs, hence the importance to understand the key parameters affecting particulate production and size. The aim of this study was to evaluate the influence of oil viscosity, pool size diameter and the presence of ice on the production of soot and size of the particles emitted. Two oils were used: DUC and REBCO, with a viscosity of 5 cP and 16 cP respectively. The scale effect was studied on two pools with diameters of 15 and 26 cm. The analysis was divided in three phases: a first outdoor phase performed in Greenland, where in situ burning experiments were performed using both oils and pools, in fresh and salt water and with the presence of ice. The smoke was collected on 2 μm pore size filters and later analysed. The same experiments were later performed in indoor conditions and the concentration of carbon monoxide was measured using optical analysis. In the last phase, the soot collected on the filters was observed with the help of the scanning electron microscope (SEM). A positive correlation between the pool size and the amount of soot produced was observed, connected with a peak in carbon monoxide production during the vigorous burning phase, while viscosities has shown different behaviour depending on the pool size. The particles size seems to be independent of both the viscosity and the pool diameter. However the presence of PM10 was observed during combustion in ice affected water.

1. Soot production and particle size distribution from in situ burning of crude
oils
Gianluca Lubelli, S130970† and Vasos Vasou, S131031
† MSc Petroleum Engineering, Technical University of Denmark
ABSTRACT: In situ burning has proven to be an effective response for oil spills in Arctic
conditions. However the incomplete combustion of oil, due to an incomplete chemical reaction
between the fuel and oxygen, lead to the production of solid and gaseous products, such as
particulate matter (soot), polycyclic aromatic hydrocarbons (PAHs) and volatile organic compounds
(VOCs). Several studies have suggested a link between the presence of these compounds and
several diseases; of high concern are the particles smaller than 10 μm (PM10) that can penetrate
deep in the lungs, hence the importance to understand the key parameters affecting particulate
production and size. The aim of this study was to evaluate the influence of oil viscosity, pool size
diameter and the presence of ice on the production of soot and size of the particles emitted. Two
oils were used: DUC and REBCO, with a viscosity of 5 cP and 16 cP respectively. The scale effect
was studied on two pools with diameters of 15 and 26 cm. The analysis was divided in three phases:
a first outdoor phase performed in Greenland, where in situ burning experiments were performed
using both oils and pools, in fresh and salt water and with the presence of ice. The smoke was
collected on 2 μm pore size filters and later analysed. The same experiments were later performed
in indoor conditions and the concentration of carbon monoxide was measured using optical
analysis. In the last phase, the soot collected on the filters was observed with the help of the
scanning electron microscope (SEM). A positive correlation between the pool size and the amount
of soot produced was observed, connected with a peak in carbon monoxide production during the
vigorous burning phase, while viscosities has shown different behaviour depending on the pool size.
The particles size seems to be independent of both the viscosity and the pool diameter. However,
the presence of PM10 was observed during combustion in ice affected water.
KEYWORDS: Particulate, spill remediation, PM10, viscosity, pool diameter.
1 INTRODUCTION
1.1 Background
As the energy demand is increasing due to population growth and rising of new emerging
economies, less accessible and higher risking petroleum resources are gathering the attention of the
industrial world. The pursuit of new hydrocarbon resources is recently concentrated in some Arctic
regions and the increase production in those areas is increasing the probability of an oil spill
occurrence, from offshore platforms, subsea pipelines, storage tanks and shipping activities (WWF,
2007).
Oil spills are a seerious concern globally. Several, unfortunate, events have revealed the wide
impact that oil has on the environment and the human health (Buist et al., 2013).
The lower temperatures usually associated with the Arctic decrease the oil evaporation and
microbial degradation, so the fluid will persist for longer times in the accident zone. Due to the
extreme conditions in the region, the wildlife has adapted itself to have a relative long life span and

2. slow generational turnover (WWF, 2007). This implies that a potential oil spill in such area can have
long-term consequences in the subarctic costal environment, well beyond the initial projections
(Peterson et al., 2003). In addition, the harsh and variable climate conditions, the low temperature,
the remoteness, the reduced visibility and the lack of infrastructure create several challenges for oil
spill remediation (Fritt-Rasmussen, 2010).
Over the last years, numerous oil spill response techniques were developed involving mechanical
recovery, injection of dispersant and combustion of the oil itself. In situ burning (ISB) is a
countermeasure technique for an oil spill that involves the controlled ignition and burning of oil on
the surface of the water. As extensively reported in Buist et al. (2013), in situ burning has proved to
be very effective as a response technique in Arctic conditions compared to other approaches due to
(Buist, et al., 2013).
• High burn efficiency, sometimes can remove up to more than 90% of the ignited oil
• Low cost
• Simple logistic, using a simple but specialized technology (i.e. fire resistant boom, igniters)
• Versatility. The technique can be applied in multiple scenarios, including the presence of
ice.
1.2 In Situ Burning Mechanism
In order to ignite the oil slick; a vaporous mixture of fuel and oxygen has to be formed, because it is
the gas immediately above the liquid phase that burns. The minimum temperature required to ignite
the oil slick is called the flash point. The energy produced from this combustion will be transmitted
through convection and radiation to the surrounding. A part of this energy will be transmitted back
on the liquid phase, vaporizing more oil and adding more fuel to the flame. In order for this process
to be self-sustainable, the amount of energy produced has to be high enough to allow the
vaporization of new gaseous fuel from the oil slick. The minimum temperature required for this
process to happen is called fire point (Guenette, 1997). In the first steady state part of the
combustion the oil slick acts as an insulator between the water and the flame, the water underneath
the oil slick will be superheated until the oil slick will destabilize and droplets of water and oil will
be ejected in the flame, the vigorous burning phase begins. The burning rate, the flame height and
the energy produced increase (Fritt-Rasmussen, 2010).
1.3 Combustion Products
ISB can be considered as a starved combustion (Buist, et al., 2013), which means that not enough
oxygen is supplied to the flame in order to produce a complete chemical reaction between fuel and
oxygen. For this reason, byproducts from the incomplete combustion such as particulate matter
(PM), soot, polycyclic aromatic hydrocarbons (PAHs) and volatile organic compounds (VOCs) are
emitted in the atmosphere. In Lemieux et al. (2004), a quantitative analysis of combustion
emissions from crude oil and fuels have been performed. These compounds and in particular PM,
produce the smoke plume often observed during oil burning. Even though it has been measured that
the concentration of toxic elements becomes tolerable in a range of a few hundred meters from the
burning location, these compounds have been demonstrated to be toxic and harmful for human
health.
Particulate matter are small uncombusted particles mainly composed of carbon that aggregates
together from a nucleus to produce spherical particles of the size of 10-50 µm and chains of bigger

3. size (Fingas 2010; Suo-Anttila et al., 2005). The PM10, particles smaller than 10 µm, are of big
concern since they can penetrate deeper in the lungs and they can easily be transported in the
atmosphere for a big distance. Moreover, depending on the initial oil composition, they can contain
metals or aromatic compounds that are extremely carcinogenic. Gaseous products consist mostly of
PAHs, carbon monoxide, carbon dioxides, sulphur dioxide, VOCs (volatile organic compounds),
benzene, toluene, xylene, dioxins and dibenzofurans (Fingas et al., 1993; NRT, 1995). These
compounds have shown a high toxic and carcinogenic activity, however it was measured that the
concentration of these elements downwind of the plume is too low to concern human health, while
they can become a real issue at close distance (Fritt-Rasmussen, 2010).
Formation of particulate is not well understood yet. The formation probably involve a series of
events that starts from pyrolysis and oxidative pyrolysis that bring to the formation of small
molecules that build up until they become big enough to be regarded as small particles. These
particles continue to grow through chemical reaction at their surface reaching diameters in the range
of 0.01 to 0.05 µm at which point they begin to coagulate to form bigger chains (Flangan &
Seinfield, 1988). The formation of these molecules is associated with poor combustion efficiency
due to a lack of oxygen in the flame. Indeed the oxygen plays an important role as an inhibitor for
particulate formation, due to the oxidative attack of precursor and incipient soot nuclei. Another
side effect of a starved combustion is the increase of carbon monoxide concentration. It is logical,
then, to speculate a connection between the amount of CO found in the smoke with the amount and
size of particulate produced. Indeed some authors found strong positive correlations between the
concentration of CO and the amount of soot generated in the combustion of liquid fuels (Koylu &
Faeth, 1991). From analysis of carbon monoxide in the flue gases is possible, then, to estimate the
concentration of particulate emitted.
1.4 Aim
Since combustion byproducts, such as particulate, can represent a hazard for human health, it is
important to understand the key parameters that influence the production of these uncombusted,
during in situ burning. The aim of the research was to study the effect that the pool diameter, the
viscosity and the presence of ice have on the amount and size of emitted smoke particles and CO
concentration, throughout the combustion of oil during in situ burning. This was achieved with
experiments conducted both outdoors and in control lab conditions. Two different oil types with
different viscosity and two different pool diameters were used to study the effect of viscosity and
the effect of pool diameter respectively. Furthermore, some of the experiments were conducted with
ice presence.
2 MATERIAL AND METHODS
2.1 Soot sampling
The experiments were conducted in two phases. The first phase took place in KTI (Greenland Tech
School) in Sisimiut, Greenland in August 2014. The experiments were performed outdoors. During
the experimental phase in Sisimiut two different oil types were used: DUC and REBCO. The main
characteristics and properties of DUC (AS, 2014) and REBCO (Environment, 2009) are reported in
Table 1. The effect of pool diameter was studied using two cylinders of 15 cm and 26 cm internal
diameter. The oil slick thickness was 10 mm for all experiments.

4. Table 1: Oil types with main characteristics and properties
Oil type Viscosity at 40°C
(mPa*s)
Density
(g/cm3
)
API gravity
DUC 5.1 0.853 34.3
REBCO 16 0.870 31.1
Due to the lack of a suitable laboratory in Greenland, the experiments were performed outdoors
with the use of an artificial oil test rig (Figure 1) that was constructed. The main parts of the
experimental apparatus are a metal hood with a stack on the top, a metal tank, a metal tray and two
Pyrex cylinders (15 cm and 26 cm internal diameter):
Figure 1: Outdoor oil test rig
A secure area was selected and the metal tray was levelled on the ground. The metal tank was
placed in the centre of the tray and an open bottom Pyrex cylinder was placed inside in a way that
the top of the cylinder and the tank were levelled. The metal hood was finally placed on top. Water
was then poured in the tank until it was almost full. After that, the oil was poured carefully in the
cylinder in a way that it wouldn’t overflow and mix with the water out of the cylinder. More water
was then poured into the tank until the oil slick’s surface was levelled with the top of the cylinder.
In order to collect soot particles, five filters with a pore size of 2 µm were placed in the stack at a
sufficient height to avoid the filters themselves to be burned by the flame during the boilover phase.

5. The ignition of the oil slick was achieved with the use of a flame torch. After the flash point was
reached and the oil slick was burned the filters were removed and examined. The smoke was
collected during the whole burning period. The use of windscreens was necessary in order to
prevent the wind from blowing the smoke away. Finally, the filters were stored in plastic laboratory
bags and shipped back to Denmark for analysis.
All the experiments were performed in using fresh water. For the experiments, that the recreation of
arctic temperature in the water was necessary, ice was placed in the bucket covering the whole
surface. This way the reduction of the water’s temperature was achieved.
2.2 Smoke analysis
The second experimental phase took place in laboratory conditions where the concentration of CO2,
CO and O2 were additionally measured through optical measurement. As in Sisimiut, both DUC
and REBCO oils were used for the experiments. For the experiments, the effect of pool diameter
was studied using the same two cylinders as in the first phase.
The main equipment used at the laboratory was similar with the one in Greenland. The main
difference was the more controlled environment in the lab due to the absence of wind. The
experimental procedure was the same as in the first phase. The experimental setup can be seen in
Figure 2. The concentrations of CO, CO2 and O2 in the fuel gases were optically measured in the
stack.
Figure 2: Laboratory setup

6. 2.3 SEM analysis
In order to analyse the soot samples collected in Greenland, a Scanning Electron Microscope (SEM)
analysis was performed. For every experiment performed, a single filter was chosen and a small
sample of it was scratched through a carbon tape and inserted into the machine. Since it was
assumed that the concentration of particulate matter formed during the combustion was
homogeneous it could be assumed that the small sample was representative for the full amount of
particulate matter produced. The measurement was performed at low vacuum, since the samples
were not dry before the measurement and even the slightest amount of moisture could destroy the
sample.
3 RESULTS AND DISCUSSIONS
3.1 Visual comparisons
It is known that viscosity could have a positive effect on combustion efficiency and particulate
production in fuels (Lee & Hayden, u.d; Yang, et al., 2005). Moreover, other studies suggested a
positive effect of pool diameter on smoke yield and particulate size emissions (Mulholland, et al.,
1996). As it can be seen from the previous table, the two oils used for the analysis have a
comparable density, while their viscosity changes substantially with REBCO being more than three
times more viscous than DUC. As first qualitative result, a higher amount of particulate would be
expected from the REBCO oil than the DUC. In the following figures, it is possible to make a
visual comparison on the particulate collected on the filters during the outdoor experiments.
A: DUC pool size 25 cm B: REBCO pool size 25 cm

7. C: DUC pool size 15 cm D: REBCO pool size 15 cm
E: DUC 26 cm in ice affected water
Figure 3: Visual analysis of soot
As the effect of viscosity is not clearly visible, the influence of pool size diameter on particulate
emissions can be qualitatively observed without the aid of other instruments. The positive effect of
pool diameter on particulate emission can be understood by the decrease of burning efficiency by
increasing the pool diameter (Koseki & Mulholland, 1991). This negative relationship could be
explained by the fact that the oxygen needed for the combustion processes naturally flows from the
surrounding to the flame. Increasing the pool size will increase the amount of oxygen needed for a
complete combustion, so the reaction will become more “starved”. An empirical relationship that
roughly estimates the smoke yield, defined as the mass of particulate emitted on unit mass of fuel
burned, is (Fraser J., 1997)
. [1]

8. As it can be seen in figure E, ice seems to show a negative effect in the amount of particulate
produced. The physical mechanism behind this phenomena is not very clear, but could involve the
effect of water in the oil slick. Indeed presence of water lower the amount of particulate produced,
particularly in the case of medium-heavy oil (Fritt-Rasmussen, 2010).
3.2 Concentration of carbon monoxide in flue gases
From the results obtained, a positive relationship between the viscosity and the concentration of
carbon monoxide produced is found for a pool size diameter of 15 cm. As an higher concentration
of carbon monoxide is associated with an higher particulate production, the result found is in line on
our expectations; however this effect disappear when the pool diameter is increased at 26 cm. A
substantial difference can be observed during the vigorous burning phase, where REBCO shows an
increase of CO concentration of 1200% compared to DUC that shows an increase of just 400%.
When the effect of the pool diameter is analysed, the concentration of carbon monoxide is
comparable for both sizes apart for the vigorous burning phase. It can be concluded, then, that the
biggest amount of particulate is produced during the last stage of combustion.
Figure 9: CO concentration in function of time

9. 3.3 SEM analysis
From the pictures obtained, complex structures formed by the agglomeration of small particulate
nuclei can be recognized. The size of these observed structures range from 100 μm to 5 µm with
different irregular shapes. A. Hamins (1993) describes these irregularities as fractal structures
characterized by an invariant symmetry with scale that is the object appears similar on a variety of
length scales. These structures are characterized by a parameter known as fractal dimension (FD)
(Hamins, 1993). A useful qualitative description of the fractal nature of these agglomerates is:
[2]
where is the number of primary particles contained in an agglomerate of radius R. The value of
FD has been found to be in the range of 1,6 to 2,6 (Hamins, 1993).
No substantial difference in agglomerate dimension could be observed for different oil type and
pool sizes, suggesting that the growing mechanism of these structures can be independent from
viscosity variation and pool diameter. However when ice is added, a reduction in agglomerate
dimension can be observed from an average of 50-30 μm to dimensions as small as 10 μm,
suggesting that other parameters have an effect on the particulate growing rate, such as water
content in the oil slick or in the smoke, on the particulate growing rate. However, further studies are
needed in order to identify these parameters.
A: DUC 15 cm pool diameter B: DUC 26 cm pool diameter

10. C: REBCO 15 cm pool diameter D: REBCO 26 cm pool diameter
E: DUC 26 cm pool diameter with ice on the surface
4 CONCLUSIONS
The experiments show a strong positive correlation between the pool diameter and the amount of
particulate produced, clearly visible even without the aid of specific instruments. This size effect
can be explained due to lower burning efficiency and a lower oxygen/fuel ratio. The analysis of
concentration of carbon monoxide show a peak during the vigorous burning phase; this peak
increase of a factor of 5 for DUC and 6 in REBCO increasing the pool size from 15 cm to 26 cm.
This effect can be explained due to the fact that during boilover an uncontrolled combustion takes
place in oxygen starved condition, bringing to an increase in the production of uncombusted
particles and carbon monoxide.

11. Since a positive correlation between the amount of carbon monoxide produced and the amount of
particulate exists, it is likely that most of the soot is produced during the vigorous burning phase.
Analysis of the filters collected from burns involving the ice show a reduction in particulate matter,
however it also shows a reduction in the size of the particulate emitted. Although a reduction in
particulate matter appears to be a positive effect induced by the ice, the smaller particulate matter
may be more detrimental to human health, as this matter will penetrate deeper into the lungs.
A positive correlation is observed between viscosity and carbon monoxide concentration for a pool
diameter of 15 cm. However, these effects seems to disappear as diameter increase to 26 cm during
the normal combustion phase, while an increase of CO production is observed in REBCO during
the vigorous burning phase compared to DUC.
From SEM analysis no correlation can be observed between viscosity and pool diameter with
particle size, observed in the range of 100 µm 30 μm. However, a reduction of particles agglomerate
even below 10 μm can be observed when ice is introduced, hinting that the water content on the oil
or in the smoke can have an influence on the agglomeration mechanism of the particulate.
In conclusion two key parameters were identified, pool size and ice, affecting the production and
the size of particulate matter produced during combustion. Viscosity seems to have a weak
influence too, however more studies are required in order to confirm this assumption.
AKNOWLEDGEMENTS
The authors thank Dr Grunde Jomaas and Laurens van Gelderen for support and coordination, Rolff
Ripke Leisted for the help in CO measurements and Ebba Cederberg Schnell for support in SEM
analysis.
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