Effect Of Halogen Composition On The Fire-extinguishing Mechanism And Performance Of Halogenated Olefin Extinguishants

The Halon extinguishing agent in the aircraft has been eliminated within a limited time due to the destruction of the ozone layer,and the ideal Halon alternative product has not been found.Therefore,the development of Halon replacements is an urgent task faced by all countries in the world.There are many types of agents that may replace Halon substances,among which environmentally friendly halogenated alkene fire extinguishing agents are a promising alternative product.There are many experimental research results surrounding this type of fire extinguishing agent,which has an important role in promoting the development of Halon alternative fire extinguishing agents.However,most of the existing studies have focused on the fire extinguishing performance and phenomenon of halogenated alkenes,and have not systematically analyzed the underlying reasons behind these"performance"and"phenomena".In order to further explore the important influence of the microstructure of halogenated olefins on its thermochemical reaction mechanism and fire extinguishing process,this paper organically combines experiments and calculation methods to explore the influence mechanism of different halogen components in the three halogenated olefin gas fire extinguishing agents on their fire extinguishing performance and mechanism.The paper analyzes the high-temperature cracking mechanism,fire-extinguishing mechanism and fire-extinguishing performance of three potential Halon alternative fire extinguishing agents:HFO-1234yf（CF₃CF=CH₂）,HCFO-1233xf（CF₃CCl=CH₂）and 2-BTP（CF₃CBr=CH₂）.The influence of different halogen components on the second carbon atom on its thermochemical process and mechanism have explored.Not only that,in order to measure the impact of the halogen composition of the fire extinguishing agent on its environmental friendliness,this paper also adopts theoretical calculations to predict and analyze the atmospheric degradation process of these three potential Halon alternative fire extinguishing agents and their pyrolysis products.The calculation results show that varied halogen atoms and thus different carbon-halogen bond distances separately endow these alkenyl halides with dissimilar chemical and geometric structures,which are believed to influence their chemical properties.According to the experimental results,HFO-1234yf,HCFO-1233xf,and 2-BTP start to decompose at 700,600,and 550°C respectively.Their thermal decomposition products are mainly CF₃H,CFH=CH₂,CF₂=CH₂,CF₃CH=CHCF₃（Z）,CF₃CH=CHCF₃（E）,CF₃C≡CH,CF₃Br,CF₃CH=CHBr,HF,HCl,HBr.Calculation studies have shown that the three potential fire extinguishing agents will generate a large number of active free radicals such as·CF₃,·Cl,·Br during the process of pyrolysis to produce the above gas products.The values of minimum extinguishing concentration of HFO-1234yf,HCFO-1233xf and 2-BTP to extinguish propane-air flames measured by cup burner experiments are 9.80vol.%,7.28vol.%,2.92vol.%,respectively,and the MEC of methane-air flames are 9.80vol.%,7.28vol.%,2.56vol.%.Its extinguishing concentration is equivalent to or even better than other fire extinguishing agents such as hydrofluoroolefins（HFOs）and hydrofluoroalkanes（HFCs）.The reason for the fire extinguishing effect of these three halogenated olefins may be due to the formation of highly reactive free radicals such as·CF₃,·Cl and·Br during the pyrolysis process.These free radicals can interact with·OH,·H,etc.in the fire field,leading to the quenching of radicals such as·OH and·H and the interruption of the combustion chain reaction.The fire extinguishing performance of these three substances increases with the increase of the halogen atomic number,because the pyrolysis of HFO-1234yf substance only generates CF₃ radicals,while HCFO-1233xf and 2-BTP substances not only generate CF₃ radicals during the pyrolysis process,but also·Cl and·Br radicals are generated respectively.In addition,in the subsequent reaction between the pyrolysis products of 2-BTP and free radicals in the fire field,·Br and·CF₃ radicals will also be generated,which may also be one of the important reasons for the best fire extinguishing performance of 2-BTP.Because these unsaturated halogenated olefins and their unsaturated pyrolysis products have C=C double bonds,they have extremely high reactivity under high temperature and oxygen-rich conditions.They can react with radicals such as·OH and·O to release a large amount of heat,resulting in HFO-1234yf,HCFO-1233xf and 2-BTP all having different degrees of combustion promotion effects in the initial stage of fire extinguishing.Among them,the combustion promotion effect of 2-BTP is the least obvious.The root cause may be that the weakest C-Br bond leads to the highest generation efficiency of Br radicals,which leads to the best combustion suppression performance of 2-BTP.In addition,compared with HFO-1234yf and HCFO-1233xf,the exothermic reaction of 2-BTP with·OH,·O and other free radicals releases the lowest energy,which may also be the root cause of the lowest combustion-promoting effect of 2-BTP.Calculation studies have shown that HFO-1234yf,HCFO-1233xf,and 2-BTP can undergo an addition reaction with the·OH radicals in the atmosphere,and the main products are·CF₃CF（OH）CH₂,·CF₃C（OH）Cl CH₂ and CF₃CBr CH₂OH.The addition reaction product can then be oxidized by O₂ in the atmosphere,and then undergo further degradation reactions with the·HO₂ radicals and NO in the air,and finally produce ketones or carboxylic acids.The negative energy barrier of each step of the reaction means that the above reactions are very easy to occur,so HFO-1234yf,HCFO-1233xf and 2-BTP have been degraded before they rise to the atmospheric ozone layer.These three substances not only have no destructive effect on the ozone layer,but also the atmospheric lifetime is relatively low,so there is no obvious greenhouse effect.