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

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

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.

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.

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

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

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]

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

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

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.

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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