The Burning Question

Dozens of large oil spills occur around the world each year—many at sea when tankers capsize, run aground or collide with other ships. Using mechanical skimmers or absorbent materials to clean up such spills is a costly and labour-intensive effort that can take months or years to complete. In cases where access to the site has been difficult or snow and ice interfered with such methods, spills have been successfully burned.

Despite the negative visual connotations of a thick black smoke plume, in-situ burning has been proven through extensive laboratory and field testing to be a fast, effective and often environmentally acceptable oil-spill countermeasure. Burns rapidly reduce the volume of spilled oil, decrease or eliminate the need to collect, store, transport and dispose of large volumes of recovered material, and shorten response time—thereby reducing the chances of a spill spreading and harming aquatic or shoreline wildlife.

Burns are often used as an oil-spill countermeasure in the Arctic, as well as on muskeg, swamps, and remote shorelines that have no vegetation. Yet concerns over atmospheric emissions, and a lack of understanding about combustion products and the combustibility of oil on water, have greatly limited their application. In an effort to address these questions, an international group of scientists and spill response specialists have carried out extensive laboratory tests and more than 45 large-scale burns over the past decade to study various aspects of diesel and crude-oil burning.

Environment Canada's Environmental Technology Centre (ETC) plays a lead role in the group, which comprises more than two dozen government agencies, oil companies and petroleum associations from Canada and the United States, including the Canadian and American coast guards, and the U.S. Minerals Management Service and Environmental Protection Agency. The focus of their efforts has been on measuring emissions to air and water. Data from numerous small burns conducted at the U.S. Coast Guard facility in Mobile, Alabama, and a major large-scale open-water burn carried out off the coast of Newfoundland have been used to develop concentration prediction equations for more than 150 compounds or emission categories. The equations are used to calculate safe distances and emission levels for various burn sizes.

Results of these tests show that levels of most substances released through the in-situ burning of crude oil are below human health limits quite close to the fire—even within 500 metres downwind of the burn. Moreover, if the oil were burned as a fuel source—as usually intended—it would generally emit higher total levels of pollutants to the atmosphere than it would in an in-situ burn. The total emissions of many substances from an in-situ burn are also lower than those released by crude oil or diesel fuel through evaporation. So the longer a spill sits unremediated, the more of these pollutants it releases to the atmosphere.

A major product of all burns is particulate matter. Both crude oil and diesel fuel produce particles when they burn; however, the levels for diesel fuel are about four times those of crude oil, which are considered safe for a typical large burn at a distance of half a kilometre downwind from the source. Concentrations of polycyclic aromatic hydrocarbons (PAHs) are found in the particulate matter, soot and residue from such burns, but overall mass concentrations are typically 92-98 per cent lower than in the original oil.

Even close to the fire, combustion gases, including carbon dioxide and carbon monoxide, are typically below exposure-level limits. For example, concentrations of carbon dioxide around a burn can be around 500 parts per million (ppm), compared to normal atmospheric levels of about 300 ppm—but they pose no danger to human health at that concentration. Volatile organic compound (VOC) emissions from burns are extensive, but typically three times lower than the levels emitted from fresh spills through evaporation. Very low concentrations of aldehydes and carbonyls are produced from crude-oil fires, but are well below health concern levels—even close to the source fire.

Analyses of soot and residue samples show that the bulk of this material is carbon, with several hundred absorbed or adsorbed chemicals also present in very low concentrations. The volume of soot produced through in-situ burning is uncertain because there are no measurement techniques to determine the total emissions for such a large area; however, estimates are from 0.2 to 2 per cent of the original volume for crude oil and about five times that for diesel fuel. The residue itself is mostly unburned oil, which is adhesive and therefore fairly easy to recover using mechanical or manual techniques.

Contrary to what many people think, most if not all oils will burn on water if the slick thickness is at least two to three millimetres. This thickness is required because sufficient heat is needed to continually vaporize the material for sustained combustion. When slicks are thinner, most of their heat is lost to the water below, and the burn cannot be sustained. Most oil pools burn at a rate of about three to four millimetres per minute, regardless of type, weathering and water content. Many oils that are left on water, through the action of wind and waves, can take up significant amounts of water through a process known as emulsification. Although oil that is completely emulsified with water cannot be ignited, some tests show that crude oil can be ignited with up to 70 per cent water content.

Mobile instruments record the type and quantity of pollutants emitted at various distances from an experimental oil-spill burn.

Burning in situ without slick containment is usually an option for only a few hours after a spill event, as oil spreads rapidly to an equilibrium thickness of just a fraction of a millimetre on the open sea. Lightweight and fire-resistant containment booms are usually required to concentrate oil slicks so they will ignite easily and continue to burn efficiently until the thickness of the oil and residues falls below two to three millimetres. These booms are typically towed slowly in a "U" shape by two boats, so that the oil will continue to collect and thicken in the apex. The oil can be ignited using a variety of unsophisticated methods, although the latest technology is a helicopter-mounted device that slings packets of burning, gelled fuel at various spots on the surface of the slick.

A trial burn conducted at the site of the 1989 Exxon Valdez spill off the coast of Alaska showed that in-situ burning can be used effectively without threatening to ignite the spill source by towing the booms through the slick until they reach capacity, and then moving the captured oil away from the main slick and igniting it. Had in-situ burning been used as the primary countermeasure in this case, scientists estimate that over 60 per cent of the spill would have been destroyed quickly—representing a considerable savings in time and effort, since the actual clean-up cost $2 billion and took two years.

The ETC has written dozens of scientific reports on the results of its in-situ burning studies over the past several years, the last of which were published this summer and synthesized into a summary document. Last year, a special handbook on the in-situ burning of oil and diesel spills was also produced to guide emergency responders. It is hoped that increased scientific and operational knowledge and better awareness of the economic and environmental benefits of in-situ burning will increase the acceptability of this oil-spill countermeasure option—not only in North America, but also in the rest of the world.

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