Research on the Fire Extinguishing Mechanism and Synergistic Fire Extinguishing Effect of HFO-1336mzz(Z)

Abstract

The Kigali Amendment of the Montreal Protocol agreed upon in 2016 limits hydrofluorocarbons’ use (HFCs) having a greenhouse effect. The amendment comprises three extinguishing agents: heptafluoropropane (HFC-227ea), hexafluoropropane (HFC-236fa), and pentafluoroethane (HFC-125). It is critical to developing novel environmentally friendly extinguishing agents. Hydrofluoroolefins (HFOs) have lower global warming potential (GWP) than HFCs and contain unsaturated double bonds reducing the atmosphere lifetime, a potential environmentally friendly extinguishing agents worth investigating. In this paper, cis-1,1,1,4,4,4-hexafluoro-2-butene (HFO-1336mzz(Z)) along with other environmentally friendly extinguishing agents were investigated for its thermal stability, fire extinguishing performance, fire extinguishing mechanism, and synergistic fire extinguishing effect, using experimental and theoretical calculation methods.

The main research contents follow.
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. The pyrolysis process of HFO-1336mzz(Z) under a nitrogen atmosphere can be divided into three stages. In the first stage, mainly tautomerization occurs to produce HFO-1336mzz(E). In the second stage, the degree of thermal decomposition increases and yields a large amount of hydrogen fluoride (HF) gas and simultaneously three cyclic compounds. In the third stage, the cyclic compounds were pyrolyzed further to form carbon black. Special attention was given to the HF generation, a toxic by-product of the HFO-1336mzz(Z) at varying temperatures. The effect of oxygen in air on the HFO-1336mzz(Z) pyrolysis was examined for the first time via pyrolysis experiments, and the experimental results revealed that oxygen triggered the HFO-1336mzz(Z)’s pyrolysis. Under the air, the oxidation of double bonds into oxygen-containing rings was identified in the pyrolysis products at 520 °C. The HFO-1336mzz(Z)’s initial pyrolysis temperature decreased, and the HF production increased, resulting in more complex oxygen-containing cyclic compounds.

2. The high-temperature pyrolysis of the HFO-1336mzz(Z) mechanism was explored in detail, depending on Density Functional Theory (DFT) calculations and ReaxFF (Reactive Force Field) molecular dynamics simulation method. Firstly, the HFO-1336mzz(Z) molecule’s bond energy was determined based on density functional theory, where the carbon-carbon single bond was most readily broken to release CF₃ radicals. Thus, the energy barriers of HFO-1336mzz(Z) collision with CF₃, H, OH, and F radicals were determined. Three reaction pathways for HF production were disclosed, combined with the pyrolysis products from the thermal decomposition experiments under a nitrogen atmosphere. The energy barrier for the collision of H radicals with HFO-1336mzz(Z) to yield HF radicals was only 4.7 kJ/mol, the main path for HF production. The formation process of three cyclic compounds in the pyrolysis products was contemplated as HFO-1336mzz(Z) could polymerize to produce cyclic products through the intermediate product IM3, resulting from bond breaking during the pyrolysis process. Likewise, the energy barrier demanded oxygen molecules to attack the carbon-carbon double bond in the HFO-1336mzz(Z) molecule was 127.1 kJ/mol, revealing the oxidation process of the HFO-1336mzz(Z) molecule from the molecular perspective by combining the oxidation products from the thermal decomposition experiments under an air atmosphere. The pyrolysis of the HFO-1336mzz(Z) system in a specific simulation volume was examined by simulating the temperature depending on the ReaxFF molecular dynamics simulation method. It centered on the variation of the number of radicals produced by the pyrolysis with the simulation time. In the oxygen present, the activation energy (Ea) of HFO-1336mzz(Z) pyrolysis was about 191.9 kJ/mol, approximately 100 kJ/mol less than that of HFO-1336mzz(Z) pyrolysis under nitrogen atmosphere.

3. The fire extinguishing performance of HFO-1336mzz(Z) was verified with a cup burner and physical fire experiment in a restricted space, and its fire extinguishing mechanism was researched. 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 of HFO-1336mzz(Z) was explored 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 and specific heat will yield a gaseous mixture having a high heat capacity with air, attain the physical fire extinguishing effect of cooling fire while acting with flame, and generate fluorine radicals to stop the chain reaction under the action of high temperature, showing a chemical fire extinguishing effect.

4. The synergistic fire extinguishing effect between HFO-1336mzz(Z) and perfluorotriethylamine (N(CF2CF3)3) mixture was examined. The addition of a 5% molar concentration of N(CF₂CF₃)₃ to HFO-1336mzz(Z) can efficiently subdue the combustion intensification phenomenon of HFO-1336mzz(Z), and HFO-1336mzz(Z) and N(CF₂CF₃)₃ forming positive synergistic fire extinguishing effect. The theoretical analysis revealed that N(CF₂CF₃)₃ could stimulate the pyrolysis of HFO-1336mzz(Z) and release more fluorine-containing radicals.

Date of Award	28 Nov 2022
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Heping Zhang (External Supervisor) & Siu Ming LO (Supervisor)