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