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.