The manufacture of the halons widely used in fire extinguishing systems was banned in 1994 due to their deleterious effect on stratospheric ozone. Since the late 1980s there have been ongoing research efforts to identify replacement agents having comparable properties. This search has proven difficult and continues today with a large directed effort known as the Next Generation Fire Suppression Technology Program (NGP). As part of the NGP, the National Institute of Standards and Technology is investigating whether highly effective thermal agents are feasible. Thermal agents are defined as those that obtain their effectiveness solely by heat extraction and dilution. Excluded from investigation are species that directly or indirectly disrupt the combustion chemistry such as halons, which derive much of their effectiveness by the release of bromine atoms that catalytically remove hydrogen atoms in the flame zone. A great deal is known about the effects of thermal agents on flames. The paper by Sheinson et al. provides a good introduction. A number of endothermic physical processes can extract heat from a gaseous flame zone, thus lowering the temperature and ultimately leading to flame extinguishment. These include simple heating (i.e., heat capacity) of an agent, phase changes such as vaporization of a liquid or sublimation of a solid, endothermic molecular decomposition (which is classified as a physical process as long as the initial agent and its products do not participate in the combustion chemistry), and simple dilution, which can modify flame temperatures by spreading the heat release over larger volumes and by affecting three-body flame reactions. The flame temperature is also expected to be a function of the thermal diffusivity of an agent. During the past two decades the understanding of the chemical kinetics involved in combustion has reached the point where realistic detailed mechanisms involving large number of reactants and reactions can be written for simple combustion systems, and mathematical techniques have been developed for simultaneously solving the large number of differential equations that result. While still involving significant approximations, such modeling has advanced to the point where it can be used to gain useful insights into the behavior of practically relevant flames. This paper describes the results of a detailed chemical kinetic modeling investigation of laminar opposed-flow methane/air diffusion flame designed to provide an improved understanding of the extinguishment of fires by thermal agents. A particular focus was to test the hypothesis that the effectiveness of a thermal agent depends on the location of heat absorption relative to the flame zone. An internal report has been prepared, which summarizes the kinetic modeling in detail and also includes the results of an extensive database search of potential thermal agents and modeling results for the effectiveness of thermal agents in cooling liquid surfaces.
Citation: Special Publication (NIST SP) - 984-4
NIST Pub Series: Special Publication (NIST SP)
Pub Type: NIST Pubs
halon alternatives, diffusion flames, extinguishment, combustion, diluent gases, diluents, fire extinguishing agents, fire suppression, laminar flames, reaction kinetics, temperature effects