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--> 1 Introduction "Fire is both the servant of mankind and its destructive demon," states John Lyons in his illuminating book on the subject, written for the general reader.1 On ships and aircraft, where escape is dangerous or impossible, fire carries a special terror. The terror is compounded for warcraft, which carry large quantifies of highly flammable fuel and an arsenal of explosives and their propellants, and are required to operate in an environment that is hostile by design of an adversary. It is evident that the U.S. Navy must have at its disposal the most effective means of fighting fires that technology can supply and that also satisfy other necessary requirements. Halons are a class of halogenated hydrocarbons that are highly effective in suppressing combustion and that, accordingly, are widely deployed on the ships and aircraft of the U.S. Navy.* Anyone who has observed an inferno of spewing fuel oil extinguished in seconds by halon can appreciate the Navy' s strong reliance on this method of protection. Beyond efficacy, halons exhibit other properties that make them ideal fire suppressants in a variety of applications. These properties include ease of distribution in obstructed spaces, low toxicity, and storage stability. Unfortunately, like the chlorofluorocarbons, halons have been identified as agents of stratospheric ozone depletion, and their domestic manufacture was terminated in accordance with international treaty and U.S. law.2,3,4 In this study the Committee on Assessment of Fire Suppression Substitutes and Alternatives to Halon addresses various scientific and engineering aspects of the agents and methods that are being considered as substitutes for halons and halon systems. The Navy currently has a considerable investment in halon systems, and it would be highly desirable to identify alternative agents that can be substituted for halon in existing hardware and pose no environmental threat. It is a great challenge to find a material that matches the necessary properties of halon 1301 (chemical, physical, and toxicological) to produce a "drop-in" replacement. Once a candidate halon alternative has been identified, it must be tested under the following demanding set of criteria: Fire suppression effectiveness for a flooding agent, usually as measured by a cup burner, which is a laboratory-scale test that gives relative performance in terms of vapor concentration required for extinguishing a hydrocarbon flame; Capability for distribution through the protected space via pipes and nozzles in a few seconds; Toxicological test protocols designed to ensure that exposure of Navy personnel will not be harmful; Activity in destroying atmospheric ozone (as measured by the ozone depletion potential (ODP) metric); Effect on climate (as measured by the global warming potential (GWP) metric); Environmental consequences of decomposition products following release; Storage stability; and Compatibility with materials (metals, elastomers, lubricants) contacted in storage and distribution hardware. To be viable, an agent must satisfy all these tests. Although the test and modeling methodologies are well developed, it is still necessary to do full-scale testing to fully qualify an alternative. * The Navy relies mainly on halon 1301 (CF3Br) and uses smaller quantities of halon 1211 (CF2ClBr). For purposes of comparison in evaluating alternatives to halons, halon 1301 is the type referred to more frequently in this report.
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--> Research on flames and methods for extinguishing flames has been active in recent years, giving rise to a large literature. Theory, modeling, and direct observation of flames have attracted much attention. Flame reactions involve fuel (usually hydrocarbons in the context of this report) and oxygen. Flame reactions occur only at high temperatures (>800 K), and they are mediated by critical "radicals," the most important of which is the hydrogen atom. Therefore, the strategy for extinguishment is to cool the reaction mixture and to introduce chemical entities that will remove hydrogen radicals (e.g., bromine or, to a lesser extent, chlorine). Halon does both. Progress in the understanding of flames and their extinguishment has been quite encouraging, but it is still necessary to test extinguishing agents in full-scale conditions. Over the past decade, a systematic, broadly based search has been implemented for halon alternatives, covering many classes of compounds that are candidate alternatives or might shed light on chemical mechanisms involved. This is an active and well-directed community. The most obvious candidates for replacement of halons are perfluorocarbons (PFCs) and hydrofluorocarbons (HFCs). These chemicals can quench flames by cooling. In addition, hydrochlorofluorocarbons (HCFCs) are under consideration; for HCFCs it is hoped that the chlorine atom will contribute to extinguishing the flame by removal of the radical, but that the molecule will not survive long enough in the atmosphere to reach the stratosphere (where it could threaten the ozone layer). Although they are difficult to synthesize, a selection of these classes of compounds are available commercially. Toxicology is a key aspect of halon replacements. Although the Navy evacuates personnel from spaces to be flooded with halon 1301, there is always the possibility of accidental discharge, and the hazard must be carefully assessed. The toxicology of halon alternatives has been studied, and protocols for agent testing and use have been delineated. In the evaluation of candidate alternatives it is necessary to characterize the atmospheric chemistry of each compound, and this is a productive field of research. Reactions in the lower atmosphere are important in determining a compound's lifetime and the probability of its reaching the stratosphere. The chemistry of ozone depletion has been well documented. These complex issues have been integrated successfully under the concept of ozone depletion potential (ODP), a useful metric that has been adopted in the U.S. Clean Air Act. Because many of the compounds under consideration have long atmospheric lifetimes and strongly absorb infrared radiation, they are expected to contribute to global warming. Although at this time there are no restrictions based on global warming potential (GWP), the possibility of future restrictions should be factored into the selection of any alternative agent or system. It is further necessary to prove that agent decomposition products will not give rise to ecological problems. Here again research has been active. It is sobering to consider the extensive nature of the requirements—beyond flame supression capability—that a halon replacement must satisfy. It must be storable as a liquid (to conserve space) but must vaporize quickly to a gas on release (to fill an obstructed space and to act on the flame, within seconds). Of course, the ozone depletion potential must be acceptably low. In addition, the toxicology, storage stability, materials compatibility, and environmental consequences of decomposition products following release of the agent must be acceptable. Each of the requirements must be met. Currently, the Navy has a considerable supply of halon (in the "bank"), as allowed under regulation, and it is projected that the bank is sufficient to protect existing ships and aircraft until their retirement from service. New-design platforms will be protected by non-halon systems. Given the history of increasingly strict environmental regulation, however, there is concern that pressure will build to destroy the existing halon bank set aside for military uses in order to preclude its eventual release. While this possibility seems remote at the present time, it is prudent for the Navy to prepare for such an outcome by identifying environmentally acceptable alternative agents and investigating systems changes that will be required. If it becomes necessary to use halon replacements in existing platforms, it will be desirable to have identified alternative agents that can be used with minimal modification of existing hardware. Currently, there are no known alternative agents that can be substituted for halon 1301 in existing equipment without modification. There are, however, alternative agents that can be deployed in existing systems if
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--> modifications, such as increased storage capacity, are made. Thus, although there is no known "drop-in" replacement, there may be replacement agents available that can satisfy the Navy's requirements, including acceptable cost of retrofit. In addition to consideration of alternative agents for halons, other methods of fire suppression have been examined in the context of shipboard and aircraft use. New methods (e.g. inert gas generators and advanced water mist systems) appear promising for use in some situations now protected with halon. Replacement of halons in Navy applications is a demanding task. Fortunately, there is an abundance of active centers of research in relevant fields in the United States. References 1. J.W. Lyons, Fire, Scientific American Library, W.H. Freeman, New York (1985). 2. United Nations Environment Programme, Montreal Protocol on Substances That Deplete the Ozone Layer , Nairobi, Kenya (1987). 3. United Nations Environment Programme, Report on the Fourth Meeting of the Parties to the Montreal Protocol on Substances That Deplete the Ozone Layer, Copenhagen, Denmark, and Nairobi, Kenya (1992). 4. Clean Air Act, U.S. Code, Vol. 42, Title VI (1990).
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