- In-Situ Burning

Abstract

In situ burning is recognized as a viable alternative for cleaning up oil spills on land and water. When performed under the right conditions, in situ burning can rapidly reduce the volume of spilled oil and eliminate the need to collect, store, transport, and dispose of recovered oil. In situ burning can shorten the response time to an oil spill, thus reducing the chances that the oil will spread on the water surface or penetrate further into land, thereby aiding in environmental protection. This chapter contains a compilation of information about in situ burning of oil spills and includes the scientific aspects of the burning process and its effects, and practical information about the procedures to be followed and equipment required for carrying out an in situ burn. Ignition is easy for volatile oils and is more difficult for heavier oils, which require a primer such as diesel fuel for sufficient heat. If not enough vapors are produced, the fire will either not start or will be quickly extinguished. The amount of vapors produced is dependent on the amount of heat radiated back to the oil. If the oil slick is too thin, some of this heat is conducted to the water layer below it. Oil that is completely emulsified with water can be ignited, given that sufficient heat is supplied, typically by burning it alongside unemulsified oil. Containment of the oil on water may be necessary to carry out in situ burning as the oil must be thick enough to quantitatively burn. Once burning, the heat radiated back to the slick and the insulation are usually sufficient to allow combustion down to about ½ to 1 mm of oil. The oil burn rate is a largely a function of oil type. The residue from burning oil is largely unburned oil with some lighter or more volatile products removed. When the fire ceases, unburned oil is left that is simply too thin to sustain combustion. In addition to unburned oil, weathered oil is present that has been subjected to high heat. Heavier soot particles are reprecipitated from the smoke plume into the fire and thus become part of the residue. Highly efficient burns of some types of heavy crude oil may result in oil residue that sinks after cooling in seawater. Burning oil on land or wetlands is a technique that can reduce the environmental impact of oil spills. Burning vegetation is a frequent method of maintaining certain ecosystems and these same ecosystems can be protected from the effects of oil spills using burning. The important factors relating to burning are the water level of wetlands and the moisture content of soils. Burning under the correct circumstances will not affect roots and thus restoration is rapid. Firebreaks are created to avoid spreading of fire to other locations. Safety is of prime concern. A burn plan and a safety plan must be prepared to encompass the concerns noted in this chapter. Training of personnel including a field practice, is essential to ensure safety. The emissions of burning can be of concern: Particulate matter/soot—Burns produce an abundance of particulate matter (soot). Particulate matter at ground level is a health concern close to the fire and under the plume. The particulate concentrations from in situ oil fires should be monitored in some circumstances. Polyaromatic hydrocarbons (PAHs)—Oils contain significant quantities of PAHs which are largely destroyed in combustion. The PAH concentrations in the smoke, both in the plume and the particulate precipitation at ground level, are much less than in the starting oil. VOCs (volatile organic compounds)—Many VOCs are emitted by fires, but in lesser quantity than when the oil is not burning. VOCs are not a significant concern. Organic compounds—No exotic or highly toxic compounds are generated as a result of the combustion process. Organic macromolecules are in lesser concentration in the smoke and downwind than they are in the oil itself. Dioxins and dibenzofurans have not been measured as emissions of oil fires. Carbonyls—Carbonyls such as aldehydes and ketones are created by oil fires but do not exceed health concern levels even close to fires. Gases—Combustion gases such as carbon dioxide, carbon monoxide, and sulfur dioxide are produced by oil fires but are significantly below any health concern level. Overall, emissions are now understood to the extent that emission levels and safe distances can be predicted for fires of various sizes and types.