Introduction
”Halon” 1301 (CF3Br) and ”Halon” 1211
(CF2ClBr) are widely used high-performance
fire-extinguishers. However, Halons are ozone-depleting substances
(ODSs) and synthetic greenhouse gases (GHGs), which caused serious
damage to the ozone layer and had a significant impact on the ecological
environment. Therefore, the Montreal Protocol (1987) and subsequent
amendments have prohibited the production and usage of Halons and
Chlorofluorocarbons (CFCs). Besides, the European Union is requested to
accomplish the replacement of Halon 1301 and 1211 in newly constructed
aircraft by 2018 and in existing ones by 2040. Therefore, the
exploration for low ozone depletion potential (ODP) and pronounced
fire-extinguishing performance is of crucial
significance1-3.
Among the product candidates for Halon substitutes, Hydrofluorocarbons
(HFCs), 2-bromo-3, 3, 3-trifluoropropane (2-BTP) and
perfluoro2-methyl-3-pentone (PFMP) products have been well studied and
were considered as potential alternative to clean fire-extinguishing
agents. For example, HFC-227ea is a colorless, odorless, low-toxic and
non-conductive clean gas. It has good thermal and chemical
stability4 and can reliably extinguish fires with a
low concentration (6.3vol%). PFMP5 was found to be a
Halon substitute owing to its negligible global warming potential (GWP),
zero ODP, and promising fire-extinguishing performance. And 2-BTP was
considered as the potential replacement6, because it
has an atmospheric lifetime of days to weeks and can be removed from the
atmosphere by OH· radicals in the photolysis
reaction7. Despite the obvious advantages of the above
alternatives, HFCs, 2-BTP, and PFMP also have drawbacks to a certain
extent that limit their use in some critical applications. For example,
HFCs have long atmospheric life and high GWP, which do not meet the
requirements of environmental protection. 2-BTP contains Br, which is
too toxic and can cause harm to the human body. And the boiling point of
PEMP is too high, hard to vaporize, which has impact on fire efficiency.
Therefore, the above fire-extinguishing agents will be phased out in the
future8-10.
Considering the drawbacks of these Halon substitutes, we have attempted
to assess the possibility of using the environmentally friendly
Iodinated alkanes as the Halon replacements and found that
CF3I has a FEC of 3.0vol% on n-heptane−air flame in a
cup burner, which is similar to that of Halon 1301. Besides,
CF3I is a new type of environmentally friendly
(GWP<5, ODP=0.008) alternative refrigerant and insulating
gases with low toxicity, and CF3I also has good
compatibility with the material. At present, the reaction kinetics of
CF3I and OH· have been investigated by using Ab initio
calculation in a published paper11 which analyzed the
formation enthalpy of the main reaction product HOI. Therefore, a
further study on the initial pyrolysis reaction mechanism of
CF3I is essential to evaluate the fire-extinguishing
performance of this potential Halon substitute.
In this paper, the thermal decomposition mechanism of
CF3I has been simulated by DFT with high precision
quantum mechanics, and the subsequent reaction channels between
CF3I and hydroxyl and hydrogen radicals have been
calculated, which reveals the fire-extinguishing mechanism of this
halogenated hydrocarbon containing iodine. Furthermore, the FEC of
methane-air flame was measured to compare the fire-extinguishing
performance of CF3I with HFCs. The results show that the
fire-extinguishing performance of CF3I is better, even
equal to that of Halon 1301. Therefore, we believe that
CF3I has the potential to be used as a substitute of
Halon for clean fire-extinguishing agents.