- Author
- Paige, H. L. | Petersson, G. A. | Marshall, P.
- Title
- Kinetic Modeling of Flame Suppression: A Report on the MLBT Halon Replacement Initiative, Including the Third Wright Laboratory Symposium on Halon Replacements. Final Report. October 1993-September 1997.
- Coporate
- Materials Directorate, WPAFB, OH Wesleyan Univ., Middletown, CT University of North Texas, Denton
- Sponsor
- Air Force Material Command, Wright-Patterson AFB, OH
- Report
- WL-TR-97-4117, November 1997, 90 p.
- Distribution
- AVAILABLE FROM National Technical Information Service (NTIS), Technology Administration, U.S. Department of Commerce, Springfield, VA 22161. Telephone: 1-800-553-6847 or 703-605-6000; Fax: 703-605-6900. Website: http://www.ntis.gov
- Keywords
- kinetics | flame suppression | halon alternatives | reaction kinetics | fluorocarbons | thermodynamics | photolysis
- Identifiers
- halofluorocarbons; computational chemistry; transition states
- Abstract
- Halon compounds such as CF3Br (halon 1301) and CF2ClBr (halon 1211) have been employed extensively in fire extinguishers because they exhibit many desirable properties: they are efficient, inexpensive, non-toxic (which is critical to their application in areas which must remain occupied after use such as aircraft cockpits), non-corrosive (and therefore do not damage electronic and other sensitive equipment) and are easily dispersed at the site of the fire. Unfortunately, these halons have an adverse environmental impact. Because of their lack of chemical and photochemical activity in the lower atmosphere, they survive to diffuse into the stratosphere where Br and Cl atoms are released by photolysis and participate in catalytic cycles that destroy ozone. The Br cycle is particularly efficient, making halons a serious threat to the protective ozone layer. The Montreal Protocol on Substances that Deplete the Ozone Layer ("Montreal Protocol"), is a treaty that was signed in 1987 and ratified by the United States Senate in 1988. The implementation of this treaty, and subsequent Amendments and Adjustments, specificalIy the Copenhagen Agreement of 1992, banned the production of halons as of 1 January 1994. Even before the halt to production, significant efforts were underway to find alternatives to halons, particularly halon 1301, which had been vital for Air Force applications from computer rooms and chemical storage facilities to engine nacelles and dry bays, and for fuel tank inerting. In the case of the aircraft applications, for which fully satisfactory replacements have not been found, "banked" halons continue to be used. The Air Force has recognized that replacements will be needed for these applications and efforts to find such replacements are continuing. Fire suppression agents may act in several ways. In a flame, fuel and oxygen react via chain reactions involving highly reactive radical intermediates such as H, OH and 0. These radicals are initially formed in an ignition step, for example by dissociation of fuel or oxygen molecules from a spark or high temperatures. The radicals may be lost through recombination reactions but the flame is sustained provided the rate of oxidation and radical production (which increase with temperature and concentration) is high enough to replenish the radical pool. Fire suppression mechanisms may be classified as physical or chemical. Physical actions include smothering i.e. separating fuel and air, and cooling and dilution, which can lower the rates of flame reactions to the point that combustion is no longer sustained. All extinguishing agents exhibit some degree of physical action, but halons are particularly effective because of their chemical action, where they interfere with the combustion chemistry, by accelerating (catalyzing) chain termination reactions. Catalytic acitvity is a desirable agent property, because small smounts of agent can remove many radicals. the detailed chemical mechanisms are, however, often incompletely understoos.