Opposite influences of flame suppressants on ignition of combustible mixtures behind shock waves

A number of halogenated hydrocarbons have been widely implemented as fire extinguishers. Their operation of combustion suppression is majorly linked to oxygen replacement as well as considerable endothermicity of their vaporization and pyrolysis that significantly reduces the temperature at the heat source. They also act as chemical inhibitors of the combustion chain reactions.

However, a number of recent research works have indicated that fluorinated hydrocarbons assumed as flame inhibitors can accelerate ignition and may even become combustible themselves. In particular, experimental analysis of haloalkene composed of mixtures of an array of parameters may initiate the development of models of pyrolysis and oxidation of haloalkenes and can affect development of combustion.

The development of fire extinguishing agents has been focused on ecological safety. Halon-104, which has been widely applied as a fire extinguishing agent, was rejected owing to environmental concerns. Therefore, understanding the promotion as well as inhibition mechanisms of Freon-114B2 and Halon-104 could be effective owing to their efficient influence of chain reactions of combustion. Researchers led by Professor Alexander Eremin from the Joint Institute for High Temperature in Russia investigated the effect of admixtures of Freon-23, Freon-114B2, and Halon-104 on the ignition of stoichiometric mixtures of hydrogen-oxygen, acetylene-oxygen, and methane-oxygen behind shock waves. Their work is now published in Combustion and Flame.

The authors conducted the experiments behind shock waves in a shock tube of a stainless steel standard design. The adopted shock tube was equipped with a number of pressure gauges to determine the incident shock wave velocity. Implementing a common iteration approach, the authors were capable of deriving the specific values of pressure and temperature of a shock-heated flow behind the reflected shock.

An ignition mixture of methane and oxygen was applied in the initial stage of the experiments. This was done in a wide range of pressures and temperatures behind the reflected shock wave. The researchers modelled a 20-reaction scheme for hydrogen combustion, the ignition development of hydrogen-oxygen mixtures. They further investigated, as a reference, the stoichiometric mixtures of acetylene and oxygen at 2.2-2.7 bar pressure.

The team observed an inhibiting effect for all samples of halogenated hydrocarbons in acetylene and hydrogen. On the other hand, combustion development in methane-oxygen mixture was observed to accelerate in the presence of all the admixtures adopted for the study. Through the kinematic evaluation, the researchers realized that promoting species generated in the methane-oxygen ignition by admixture pyrolysis were atomic bromine or chlorine and CF2. These generate active radicals initiate chain reactions of ignition.

The outcomes of the study indicate that chemical stability of molecules is necessary for chemical suppression of subsequent ignition under particular conditions. At low temperatures, the suppressants were observed to participate only in a few chain termination reactions and inhibited ignition. At higher temperatures, a reversal effect of shock waves induced combustion development by the released active radicals.

Freon 114B2 inhibiting effect was notable on the ignition of hydrogen-oxygen mixtures behind the shock waves. The outcomes of this study are helpful in enhancing the safety in nuclear power plants and other industrial applications that use hydrogen. Detailed analysis as well as necessary precautions are important in the adequate design of fire extinguishing systems that implement chemically active admixtures.