1. Introduction
Even though halon fire extinguishing agents owned excellent fire extinguishing performance, they were prohibited due to the severe ozone damage[1, 2]. Aircraft cargo compartment was granted an exemption to keep using halon fire extinguishing agents in the short-run, mainly due to that the flight safety could not be ensured owing to complex operating environment and the unachievable fire extinguishing replacements for the alternatives. Nevertheless, the International Civil Aviation Organization (ICAO) had still proposed that newly produced aircraft after 2024 cannot be equipped with halon extinguishing agents, and halon fire extinguishing systems cannot be used on all aircraft after 2040[3, 4]. To develop the alternative agents with excellent fire extinguishing performance, FAA has issued Minimum Performance Standard (MPS) for halon replacements in aircraft cargo compartment[5-7]. It detailed that the replacement agents must pass the four fire test scenarios: bulk-load fire, containerized-load fire, surface-burning fire and aerosol can explosion simulation[6].
According to the Federal Aviation Administration-mandated test for cargo-bay fire suppression (abbreviated herein as the FAA-ACT), the fire extinguishing agents, when added at sub-suppressing concentration, cannot induce the overpressure phenomenon compared with the uninhibited case. Several potential halon replacement agents, like C3HF7, C6F12O, C3H2F3Br (2-BTP), had failed in the FAA-ACT[3, 4]. Relevant research analyzing the FAA-ACT reported that the overpressure in the fire extinguishing tests might be due to the higher heat releasing from the reactions of fire extinguishing agents[8-11]. It is surprised that the agent C2HF3Cl2 (R123) overcame the overpressure in the FAA-ACT[12, 13], hence, R123 is considered as the potential halon fire extinguishing agents.
The main advantage for R123 in the progress of fire extinguishing is that the heat release is lower after addition. Holmstedt et al.[14] conducted co-flow diffusion flame experiments using propane as fuel and HFCs, HCFCs and halon 1301 as inhibitors, and found that R123 was the only inhibitor other than halon 1301 that had not increased heat release rate. Takahashi et al.[13, 15-17] found that C2HF5 increased the total heat release in the flame by 158% and C2HF3Cl2 increased by only 37%. However, it is unfortunate that the R123 was classified as a controlled replacement by Montreal Treaty due to its low ozone destruction potential[18]. Herein, even though R123 may be used as the fire extinguishing agent of aircraft cargo compartment in the short term, it will eventually be banned in a long-term halon replacement. Even so, in depth exploration of molecular-structural advantages of R123 for fire extinguishing performance at the molecular level are extremely useful for further screening the fire extinguishing agents with excellent environmental performance and conforming to the requirements of fire extinguishing performance of aircraft cargo. However, the fire extinguishing performance and mechanism analysis of R123 are still unclear, dramatically hindering the development of halon replacement.
This study provides the fire extinguishing performance and mechanism analysis of R123. For comparison, the similar analysis is also performed for the analogous HFC compound C2HF5(R125). The fire suppression effectiveness is evaluated by the cup burner apparatus, and the mechanism is simulated with high precision quantum mechanics based on density functional theory (DFT). Moreover, theoretical study of the reactions of OH· and H. with R123 and R125 can unravel the fire extinguishing mechanism associated with Cl· and F·. In-depth research on R123 can provide a basis for exploring environment friendly and highly efficient halon replacement agents for aircraft cargo compartment.