incidents of vessel oil spills that inadvertently caught re, the
intentional ignition of oil slicks on open water has only been seriously considered since the development
of re-resistant oil containment boom beginning in the early 1980s. The development of these booms
offered the possibility of conducting controlled burns in open water conditions. In situ burning operations
using these booms have been conducted at three spills in the last decade: a major offshore tanker spill, a
burning blowout in an inshore environment, and a pipeline spill into a river.
In situ burning of thick, fresh slicks can be initiated very quickly by igniting the oil with devices as
simple as an oil-soaked sorbent pad. In situ burning can remove oil from the water surface very efciently
and at very high rates. Removal efciencies for thick slicks can easily exceed 90%. Removal rates of
2000 m
3
/h can be achieved with a re area of only about 10 000 m
2
or a circle of about 100 m in diameter.
The use of towed re containment boom to capture, thicken and isolate a portion of a spill, followed by
ignition, is far less complex than the operations involved in mechanical recovery, transfer, storage,
treatment and disposal. If the small quantities of residue from an efcient burn require collection, the
viscous, tar-like material can be collected and stored for further treatment and disposal. There is a limited
window of opportunity for using in situ burning with the presently available technology. This window is
dened by the time it takes the oil slick to emulsify; once water contents of stable emulsions exceed about
25%, most slicks are unignitable. Research is ongoing to overcome this limitation.
Despite the strong incentives for considering in situ burning as a primary countermeasure method,
there remains some resistance to the approach. There are two major concerns: rst, the fear of causing
secondary res that threaten human life, property and natural resources; and, second, the potential
environmental and human-health effects of the by-products of burning, primarily the smoke.
The objective of this chapter is to review the science, technology, operational capabilities and limitations
and ecological consequences of in situ burning as a countermeasure for oil spills on water. The main focus
of this section is on marine oil spills in open water conditions. The use of in situ burning for spills in ice
conditions is dealt with in another chapter. Much of the content of this chapter is updated from an in-depth
review of in situ burning produced for the Marine Spill Response Corporation (MSRC) in 1994 [1].
Interested readers are encouraged to refer to the original report for fully referenced details of the summary
presented here. The MSRC report is available from the American Petroleum Institute in Washington, DC.
THE FUNDAMENTALS OF
IN SITU
BURNING
Requirements for ignition
In order to burn oil spilled on water, three elements must be present: fuel, oxygen and a source of ignition.
The oil must be heated to a temperature at which sufcient hydrocarbons are vaporized to support
combustion in the air above the slick. It is the hydrocarbon vapours above the slick that burn, not the
*Pure Appl. Chem. 71(1) (1999). An issue of special reports reviewing oil spill countermeasures. liquid itself. The temperature at which the slick produces vapours at a sufcient rate to ignite is called the
ash point. The re point is the temperature a few degrees above the ash point at which the oil is warm
enough to supply vapors at a rate sufcient to support continuous burning.
Heat transfer back to slick
Figure 1 illustrates the heat transfer processes that occur during the in situ burning of an oil slick on water.
Most heat from the burn is carried away by the rising column of combustion gases, but a small percentage
(about 1%) radiates from the ame back to the surface of the slick. This heat is partially used to vaporize
the liquid hydrocarbons which rise to mix with the air above the slick and burn; a small amount transfers
into the slick and eventually to the underlying water. Once ignited, a burning thick oil slick reaches a
steady-state where the vaporization rate sustains the combustion reaction, which radiates the necessary
heat back to the slick surface to continue the vaporization.
Flame temperatures and total heat uxes
Flame temperatures for crude oil burns on still water are about 9001200 C [2]. But the temperature at
the oil slick/water interface is never more than the boiling point of the water and is usually around
ambient temperatures. There is a steep temperature gradient across the thickness of the slick; the slick
surface is very hot (350500 C) but the oil just beneath it is near ambient temperatures. Total heat
uxes generated by an oil pool re are on the order of 100250 kW/m
2
measured both inside and at the
periphery of the re [3,4]. The higher heat ux values are associated with windy conditions which
promote better combustion.
Importance of slick thickness
The key oil slick parameter that determines whether or not the oil will burn is slick thickness. If the oil
is thick enough, it acts as insulation and keeps the burning slick surface at a high temperature by
reducing heat loss to the underlying water. This layer of hot oil is called the hot zone. As the slick
thins, increasingly more heat is passed through it; eventually enough heat is transferred through the slick to
allow the temperature of the surface oil to drop below its re point, at which time the burning stops.
The vigorous burning phase
At the nal stages of burning, the hot zone approaches the water surface. The temperature of the
layer of water directly beneath the slick, no longer insulated by a thick slick, increases. For slicks on
calm water with no current, the temperature of the underlying water can increase to the boiling point.
When the water begins to boil, the steam vigorously mixes the remaining oil layer and ejects oil
droplets into the ames. This results in increased burn rate, ame height, radiative output and
foaming. This is called the vigorous burning phase. This phenomenon has not been observed in
burns using a towed boom, probably because the water beneath the slick does not stay there long
enough to boil.
44
I. BUIST
et al.
1999 IUPAC,
Pure Appl. Chem.
71, 4365
Fig. 1
Key heat and mass transfer processes in in situ burning. Effect of evaporation on slick ignition
Extensive experimentation on crude and fuel oils with a variety of igniters in a range of environmental
conditions has conrmed the following rules-of-thumb for relatively calm, quiescent conditions:
X
the minimum ignitable thickness for fresh, volatile crude oil on water is about 1 mm;
X
the minimum ignitable thickness for aged, unemulsifed crude oil and diesel fuels is about 35 mm;
X
the minimum ignitable thickness for residual fuel oils, such as Bunker C or no. 6 fuel oil, is about
10 mm; and
X
once 1 m
2
of burning slick has been established, ignition can be considered accomplished.
Other factors affecting ignition
Aside from oil type, other factors that can affect the ignitability of oil slicks on water include: wind speed,
emulsication of the oil and igniter strength. Secondary factors include ambient temperature and waves.
X
The maximum wind speed for successful ignition of large burns has been determined to be 1012 m/s.
X
For weathered crude that has formed a stable water-in-oil emulsion, the upper limit for successful
ignition is about 25% water. Some crudes form meso-stable emulsions that can be easily ignited at
much higher water contents. Parafnic crudes appear to fall into this category [5].
X
If the ambient temperature is above the oils ash point, the slick will ignite rapidly and easily and the
ames will spread quickly over the slick surface; ames spread more slowly over oil slicks at subash
temperatures.
Oil burning rates
The rate at which in situ burning consumes oil is generally reported in units of thickness per unit time
(mm/min is the most commonly used unit). The removal rate for in situ oil res is a function of re size
(or diameter), slick thickness, oil type and ambient environmental conditions. For most large (
> 3 m
diameter) res of unemulsied crude oil on water, the rule-of-thumb is that the burning rate is 3.5 mm/
min. Automotive diesel and jet fuel res on water burn at a slightly higher rate of about 4 mm/min.
Factors affecting residue amounts and burn efciency
Oil removal efciency is a function of three main factors: the initial thickness of the slick; the thickness of
the residue remaining after extinction; and, the areal coverage of the ame. The general rules-of-thumb
for residue remaining after a successful burn are described below. Other, secondary factors include
environmental effects such as wind and current herding of slicks against barriers and oil weathering.
The following rules-of-thumb apply for the residue thickness at burn extinction:
X
for pools of unemulsied crude oil up to 1020 mm in thickness the residue thickness is 1 mm;
X
for thicker crude slicks the residue is thicker; for example, 35 mm for 50 mm thick oil;
X
for emulsied slicks the residue thickness can be much greater; and
X
for light and middle-distillate fuels the residue thickness is 1 mm, regardless of slick thickness.
Wind and current can herd a slick against a barrier, such as a towed boom, thus thickening the oil for
continued burning. As little as a 2-m/s wind is capable of herding oil to thicknesses that will sustain
combustion. Indeed, the phenomenon of uncontained in situ burning is based on the requirement of a
self-induced wind (drawn in by the combustion process and the rising column of hot gases), to herd and
keep an uncontained slick