The Basics of Oil Spill Cleanup - Chapter 10

CHAPTER 10 In-Situ Burning Basics of In-Situ Burning · For oil to ignite on water, it must be at least 2 to 3 mm thick. Most oils must be contained to maintain this thickness. · Ignition is relatively easy. More weathered and heavier oils require a longer ignition time. · Most types of oils will burn, although emulsions may require treatment before they will burn and the water in the oil affects the burn rate. · Oils burn at a rate of about 3 to 4 mm per minute or about 5000 L per m 2 per day. · The emissions of importance from burning include respirable particulates from the smoke plume, PAHs on particulate matter, and soot. · Studies have shown that emissions from burning oil generally result in con-centrations of air contaminants that are below health concern levels 500 m downwind from the fire. In-situ burning is an oil spill cleanup technique that involves controlled burning of the oil at or near the spill site. The major advantage of this technique is its potential for removing large amounts of oil over an extensive area in less time than other techniques. Extensive research has been conducted into in-situ burning, beginning in the 1970s and continuing today. The technique has been used at actual spill sites for some time, especially in ice-covered waters where the oil is contained by the ice. It is now an accepted cleanup technique in several countries, while in others it is just becoming acceptable. The advantages and disadvantages of in-situ burning are outlined in this chapter, as well as conditions necessary for igniting and burning oil, burning efficiency and rates, and how containment is used to assist in burning the oil and to ensure that the oil burns safely. Finally, the air emissions produced by burning oil are described and the results of the many analytical studies into these emissions are summarized. The discussion in this chapter focuses primarily on burning of oil on water. Burning of oil on shorelines and land is discussed briefly in Chapters 11 and 12. ©2000 by CRC Press LLC Photo 92 A large test burn was conducted off the coast of Newfoundland in 1993. (Environ-ment Canada) Advantages Burning has some advantages over other spill cleanup techniques, the most significant of which is its capacity to rapidly remove large amounts of oil. When used at the right time, i.e., early in the spill before the oil weathers and loses its highly flammable components, and under the right conditions, in-situ burning can be very effective at rapidly eliminating large amounts of spilled oil, especially from water. This can prevent oil from spreading to other areas and contaminating shore-lines and biota. Burning oil is a final, one-step solution. When oil is recovered mechanically, it must be transported, stored, and disposed of, which requires equipment, personnel, time, and money. Often not enough of these resources is available when large spills occur. Burning generates a small amount of burn residue that can be recovered or further reduced through repeated burns. In ideal circumstances, in-situ burning requires less equipment and much less labour than other cleanup techniques. It can be applied in remote areas where other methods cannot be used because of distances and lack of infrastructure. In some circumstances, such as when oil is mixed with or on ice, it may be the only available option for dealing with an oil spill. Finally, while the efficiency of a burn varies with a number of physical factors, removal efficiencies are generally greater than those for other response methods such as skimming and the use of chemical dispersants. During a series of test burns ©2000 by CRC Press LLC conducted off the coast of Newfoundland in 1993, efficiency rates of 98 and 99% were achieved. Disadvantages The most obvious disadvantage of burning oil is concerns about toxic emissions from the large black smoke plume produced. These emissions are discussed in this chapter. The second disadvantage is that the oil will not ignite and burn unless it is thick enough. Most oils spread rapidly on water and the slick quickly becomes too thin for burning to be feasible. Fire-resistant booms are used to concentrate the oil into thicker slicks so that the oil can be burned. And finally, burning oil is sometimes not viewed as an appealing alternative to collecting the oil and processing it for reuse. Reprocessing facilities for this purpose, however, are not readily accessible in most parts of the world. Another factor that discourages reuse of oil is that recovered oil often contains too many contaminants for reuse and is incinerated instead. Photo 93 Recovered oil is burned at a spill in the U.S. Beaufort Sea. (Al Allen) Ignition and What Will Burn The first major spill incident at which burning was tried as a cleanup technique was when the Torrey Canyon lost oil off the coast of Great Britain in 1967. The military dropped bombs and incendiary devices on the spill, but the oil did not ignite. These results discouraged others from trying this technique. Only two years later, however, Dutch authorities were successful at burning test slicks both at sea and on shore. In 1970, Swedish authorities successfully burned Bunker C oil from a ship ©2000 by CRC Press LLC Photo 94 Oil in this ditch was burned to avoid damage to the surrounding land. (Environment Canada) accident in ice. It has since been found that burning is often the only viable coun-termeasure for oil spills in Arctic regions. Early studies of in-situ burning focused on ignition as being the key to successful burning of oil on water. It has since been found that ignition can be difficult, but only under certain circumstances. More recent studies have shown that slick thick-ness is actually the most important factor required for oil to burn and that almost any type of oil will burn on water or land if the slick is thick enough. Ignition may be difficult, however, at winds greater than 20 m/s (40 knots). In fact, the prime rule of in-situ burning is that oils will ignite if they are at least 2 to 3 mm thick and will continue to burn down to slicks about 1 to 2 mm thick. This thickness is required in order to insulate the oil from the water. Sufficient heat is required to vaporize material so the fire will continue to burn. In very thin slicks, most of the heat is lost to the water and vaporization/combustion is not sustained. In general, heavy oils and weathered oils take longer to ignite and require a hotter flame than lighter oils. This is also the case for oil that contains water, although oil that is completely emulsified with water may not ignite at all. While the ignit-ability of emulsions with varying water concentrations is not well understood, oil containing some emulsion can be ignited and burned. Several burns have been conducted in which some emulsion or high water content in the oil did not affect either the ignitability of the oil or the efficiency of the burn. Chemical emulsion breakers can be used to break down enough of the emulsion to allow the fire to get started. As it is suspected that fire breaks down the water-in-oil emulsion, water content may not be a problem once the fire is actually burning. ©2000 by CRC Press LLC Photo 95 A helitorch is an efficient way to light a slick. In the photo, extra fuel is being discharged before the helicopter returns to its base. (Environment Canada) Only limited work has been done on burning oil on shorelines. Because substrata are generally wet, minimum thicknesses are probably similar to those required on water, that is from 2 to 3 mm. Oil is sometimes deposited in much thinner layers than this. Burning may cause portions of the oil to penetrate further into the sedi-ments. Furthermore, burning oil on shorelines close to human settlements and other amenities may not be desirable. Most ignition devices burn long enough and generate enough heat to ignite most oils. Several igniters have been developed, ranging from simple devices made of juice cans and propellant to sophisticated helicopter-borne devices. The state of the art in ignition technology is the helitorch, a helicopter-slung device that dispenses packets of burning, gelled fuel that produce a flame of 800°C lasting for up to 6 minutes. The device was developed to start back-fires for the forestry industry. Fires at actual spills and in experiments have been ignited using much less sophisticated means. One spill in the Arctic was lighted using a roll of diesel-soaked paper. A set of experimental burns was lighted using oil-soaked sorbent. The test burn conducted at the Exxon Valdez spill was ignited using a plastic bag filled with burning gelled gasoline. Burn Efficiency and Rates Burn efficiency is measured as the percentage of starting oil removed compared to the amount of residue left. The amount of soot produced is usually ignored as it is a small amount and difficult to measure. Burn efficiency is largely a function of ©2000 by CRC Press LLC ... - tailieumienphi.vn