Research On High Temperature Pyrolysis Properties And Fire Extinguishing Effect Of HFO-1336mzz(Z)

The Kigali Amendment of the Montreal Protocol reached in 2016 restricts the use of a class of hydrofluorocarbons(HFCs)with the greenhouse effect,including heptafluoropropane(HFC-227ea),hexafluoropropylene(HFC-236fa),and pentafluoroethane(HFC-125).It is crucial to develop new environmentally friendly extinguishing agents.Compared with HFCs,hydrofluoroolefins(HFOs)have lower global warming potential(GWP)and contain unsaturated double bonds that reduce atmosphere lifetime,which is a potential type of environmentally friendly extinguishing agent worth investigating.In this paper,cis-1,1,1,4,4,4-hexafluoro-2-butene(HFO1336mzz(Z))was investigated for its thermal stability,fire extinguishing performance,fire extinguishing mechanism,and synergistic fire extinguishing effect with other environmentally friendly extinguishing agents using experimental and theoretical calculation methods.The main research contents are as follows.1.The thermal decomposition properties of HFO-1336mzz(Z)were investigated by high-temperature pyrolysis experiments.The initial pyrolysis temperature of HFO-1336mzz(Z)and the pyrolysis products at different temperatures were obtained under two pyrolysis atmospheres of nitrogen and air.Special attention was paid to the generation of HF,a toxic by-product of HFO-1336mzz(Z)at different temperatures.The effect of oxygen in air on the pyrolysis of HFO-1336mzz(Z)was investigated for the first time by means of pyrolysis experiments,and the experimental results showed that oxygen promotes the pyrolysis of HFO-1336mzz(Z).Under the air atmosphere,the oxidation of double bonds into oxygen-containing rings was detected in the pyrolysis products at 520℃.The initial pyrolysis temperature of HFO1336mzz(Z)decreased and the production of HF increased,forming more complex oxygen-containing cyclic compounds.2.Based on Density Functional Theory(DFT)calculations and ReaxFF(Reactive Force Field)molecular dynamics simulation method,the mechanism of high-temperature pyrolysis of HFO-1336mzz(Z)was further studied in detail.Firstly,the bond dissociation energy of HFO-1336mzz(Z)molecule was calculated based on density functional theory,in which the carbon-carbon bond is most easily broken to release CF3 radicals,and on this basis,the energy barriers of collision of HFO-1336mzz(Z)with CF3,H,OH and F radicals were calculated respectively.Combined with the pyrolysis products obtained from the thermal decomposition experiments under nitrogen atmosphere,three reaction pathways for HF production were revealed,among which the energy barrier for the collision of H radicals with HFO1336mzz(Z)to produce HF radicals was only 4.7 kJ·mol-1,which was the main pathway for the production of HF.The formation process of three cyclic compounds in the products of pyrolysis experiments was speculated.HFO-1336mzz(Z)could further polymerize to form cyclic products through the intermediate product IM3 formed by bond breaking during the pyrolysis process.Similarly,the energy barrier required for oxygen molecules to attack the C=C in the HFO-1336mzz(Z)molecule was calculated to be 127.1 kJ·mol-1,which reveals the oxidation process of HFO-1336mzz(Z)molecule from a microscopic perspective by combining the oxidation products obtained from the thermal decomposition experiments under air atmosphere.Based on the ReaxFF molecular dynamics simulation method,the pyrolysis of the HFO-1336mzz(Z)system in a certain simulation volume was studied by simulating the temperature,focusing on the variation of the number of radicals generated by the pyrolysis with the simulation time.In the presence of oxygen molecules,the activation energy(Ea)of HFO-1336mzz(Z)pyrolysis was calculated to be about 191.9 kJ/mol,which is about 100 kJ/mol lower than that of HFO-1336mzz(Z)pyrolysis under nitrogen atmosphere.3.The fire extinguishing performance of HFO-1336mzz(Z)was tested by cup burner experiment and fire extinguishing experiment in restricted space,and its fire extinguishing mechanism was analyzed.The critical extinguishing concentration of HFO-1336mzz(Z)was measured for the first time as 7.20%(vol%).The effect of charging pressure and fire source location on the flame suppression effect of HFO1336mzz(Z)was studied in the restricted space fire extinguishing experiment.The time of HFO-1336mzz(Z)extinguishing the flame was within 10 s,and the toxicity of HF content was lower than the acute toxicity concentration.The physical and chemical fire extinguishing mechanism of HFO-1336mzz(Z)was summarized by combining the experimental results with theoretical studies.Its high potential heat of evaporation would form a gaseous mixture with high heat capacity with air,achieve the physical fire extinguishing effect of cooling fire when acting with flame,and produce fluorine radicals to inhibit chain reaction under the action of high temperature to play a chemical fire extinguishing effect.4.The synergistic fire extinguishing effect between HFO-1336mzz(Z)and perfluorotriethylamine(N(CF2CF3)3),2-bromo-3,3,3-trifluoropropene(2-BTP)and perfluoroglycone(Novec 1230)were studied.The addition of 5%molar concentration of N(CF2CF3)3 to HFO-1336mzz(Z)could effectively inhibit the combustion enhancement phenomenon of HFO-1336mzz(Z),and HFO-1336mzz(Z)and N(CF2CF3)3 have positive synergistic fire extinguishing effect.Theoretical analysis obtained that N(CF2CF3)3 could promote the pyrolysis of HFO-1336mzz(Z)and release more fluorine-containing radicals.When the molar concentration of 2-BTP was increased to 20%,the critical extinguishing concentration was reduced to 5.21%.The lower C-Br bond energy in the 2-BTP molecule could easily release Br radicals and CF3 radicals released.Adding Novec 1230 extinguishing agent to HFO-1336mzz(Z),the change of critical extinguishing concentration of the mixed extinguishing agent is little,and there is almost no synergistic extinguishing effect between HFO-1336mzz(Z)and Novec 1230.On the basis of the above small-scale fire extinguishing experiments,medium-scale fire extinguishing experiments were conducted with HFO-1336mzz(Z)mixed with N(CF2CF3)3 and 2-BTP,and these two mixed extinguishing agents could effectively extinguish the oil pool fire.The medium-scale fire extinguishing experiments are consistent with the theoretical analysis results,N(CF2CF3)3 could effectively reduce the content of the toxic product HF generated,while 2-BTP could effectively reduce the extinguishing concentration of the mixed extinguishing agent.