# Characterization of the combustion products of fluorinated compounds

Carlos Murillo Rueda, Salma Attif, Jean-Pierre Bertrand, Guillaume Leroy, Benoit Tribouilloy, Guy Marlair, Olivier Gentilhomme

► To cite this version:

Carlos Murillo Rueda, Salma Attif, Jean-Pierre Bertrand, Guillaume Leroy, Benoit Tribouilloy, et al.. Characterization of the combustion products of fluorinated compounds. 4th European Symposium on Fire Safety Science (ESFSS 2024), Oct 2024, Barcelone, Spain. pp.012014, 10.1088/1742-6596/2885/1/012014. ineris-04888309

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# Characterization of the combustion products of fluorinated compounds

Carlos MURILLO RUEDAª, Salma ATTIFª, Jean-Pierre BERTRANDª, Guillaume LEROYª, Benoit TRIBOUILLOYª, Guy MARLAIRª, Olivier GENTILHOMMEª

INERIS, Parc Technologique ALATA, BP 2, F-60550, Verneuil-en-Halatte, France

Carlos.MURILLO-RUEDA@ineris.fr

**Abstract.** An experimental study is performed to characterize the halogenated gases produced by the combustion of five fluorinated liquid compounds. For this purpose, a FPA calorimeter is coupled with a FTIR spectrometer for the gas analysis. The characterization compares different ignition sources (horizontal and vertical injection to an acetone pool fire and pilot flame) by assessing the emission factors of HF, COF₂ and the unreacted compound. The experimental data allowed establishing that the pilot flame mainly generates an insufficient decomposition of the compound due to an excessive evaporation rate. On the contrary, the ignition through the pool fire promotes the conversion of the fluorinated compound due to greater interactions with combustion radicals. These interactions are affected by the position of the injection and might induce significant variations on the selectivity of the decomposition reactions.

**Keywords:**

Fluorinated compounds, carbonyl fluoride, hydrogen fluoride, FPA calorimeter, FTIR.

## 1. Introduction

Whilst the fluorine element is present in an organochemical structure, it generally presents a positive safety aspect because of increased thermal stability brought by the strong energy bond pertaining to the C-F chemical liaison. The counterpart is that once thermal stability of such molecules is overcome, toxic F-containing compounds are potentially released, notably in fire scenarios. As kind of paradox many compounds of interest have popped up in recent past due to their clear functional interest to energy transition and/or in support to the green deal, such as key components for Li-ion batteries, F-containing ionic liquids for batteries or biomass conversion, and in the meantime, some F-containing materials are suspected to cause significant environmental issues, like those entering in the Per- and polyfluoroalkyl substances (FPAS) family.

Moreover, the implementation of the Montreal protocol (and subsequently in the EU the so-called F-gas Regulation EU 2024/573) requests the substitution of compounds with high Global Warming and Ozone Depletion potentials (GWP/ODP) with products that generate considerably lower impacts on the ozone layer. This aspect has promoted the utilization of other F-containing products, such as Hydrofluoroolefins (HFOs) or fluorinated ethers (HFEs) for refrigeration and heat pump systems instead of formerly used hydrochlorofluorocarbons (HCFCs) or hydrofluorocarbons (HFCs). Besides

<figure><img src="image_2.png" /></figure>
achieving the goal of promoting products with lower GWPs/ODPs, the adoption of new alternatives for refrigerants has been submitted to the fulfilment of other typical requirements such as low toxicity, long-term stability and a limited flammability [1]. Nowadays, the limited information about emissions from emerging fluorinated compounds confirms the pertinence of further explorative studies.

The flammability of halogenated compounds is usually decreased by high halogenation levels [1]. Evidently, this characteristic is due to the differences in the reaction mechanisms between the radicals formed during combustion and the C bonds with H, F and C atoms. For this reason, certain fluorinated compounds can also be considered for fire suppression applications. Accordingly, in our work, five fluorinated commercially important species were selected to cover a wide range of H/F ratios as well as the presence of various functional groups with F atoms. Due to the low flammability of these compounds, the interaction with a combustion flame was selected for our emission analysis instead of a direct combustion through the standard ignition process.

Furthermore, the reaction mechanisms of fluorinated compounds have been previously discussed and modelled for molecules with 1 or 2 carbon atoms [2]. However, the detailed analysis of more complex molecules is restricted by the vast variety of intermediate radicals formed in the combustion zones. Nonetheless, the description of the chemistry of C₁ and C₂ structures can be considered along with certain qualitative trends observed with other agents to explain by analogy the chemistry of high-complexity molecules [2]. For this purpose, this study presents an experimental description of the main species formed by the thermal decomposition of a group of fluorinated molecules. In this analysis, the vapours and fumes generated with different combustion flames are characterised to estimate the emission rate of the evaporated compound as well as those of products such as hydrogen fluoride (HF) and carbonyl fluoride (COF₂). The comparison of these emission rates is considered to evaluate the influence of different parameters on the fluor selectivity in the combustion products.

## 2. Experimental protocol

### 2.1. Experimental setup

Fig. 1 describes the experimental setup considered for this study. The controlled combustion of the fluorinated compounds is carried out within a customized version of the Fire Propagation Apparatus (FPA) [3], [4]. As shown in Fig. 1, this fire calorimeter is composed of a Petri dish for the pool fire formed by the sample (section 4) and two lines for hot gases. Firstly, a combustion line ensures the combustion under controlled ventilation regimes through the adjustment and distribution of the incoming airflow (sections 1-2). Then, the combustion region is delimited by a quartz tube installed for the development of the flame (section 6). The hot gases and vapours are collected by a hood, which is also fed by a second air supply (section 7) to provide the required dilution for the gas analysis. Secondly, an exhaust gases line collects the gases, vapours and soot leaving from the top of the quartz tube and sustains a regulated temperature of 180°C to avoid any condensation before the gas analysis.

<figure><img src="image_3.png" /><figcaption>Fig. 1. Experimental setup</figcaption></figure>
Due to the significant ignition resistance of highly fluorinated chemicals, this experimental analysis considered the three ignition sources shown in Fig. 2 to analyse the influence of the interaction of the halogenated fluid with flames capable of generating different combustion conditions. For this purpose, the concentration profiles were compared for cases in which the vapours are exposed to a pilot flame or were dispersed in air and injected into the flame. In the first analysis (Fig. 2a), the air/liquid mixture is formed by the vapours and the air within the Quartz tube whereas the other cases consider a preliminary dispersion in air followed by the injection into a pool fire.

Moreover, these ignition sources were supported by a set of heat radiators (section 5), which provided a calibrated thermal load in terms of external heat flux (5-10 kW/m²) [4] to preheat the fluorinated fluid without generating an excessive mass loss in the Petri dish due to liquids evaporation.

<figure><img src="image_4.png" /><figcaption>Fig. 2. Ignition sources</figcaption></figure>

The gas collection system was connected to a Fourier Transform Infrared (FTIR) spectrometer, which continuously measures the concentrations of different chemical compounds. For this purpose, a preliminary calibration procedure allows establishing the specific IR signature of each compound as well as the specific intensity/concentration profiles. Then, the spectrum of the collected gases was contrasted with the calibration curves to characterize not only the nature of the present compounds but also the transient behaviour of the concentration profiles.

## 2.2. Fluorinated liquids

The fluorinated compounds listed in Table 1 were analysed in the FPA calorimeter. These fluids were considered in this study because they have different halogenation levels, which might lead to distinct behaviours during combustion. For instance, the high H/F ratios promote the hydrocarbon character of flammable mixtures of fluorinated refrigerants and air [5]. Besides the halogenation levels, the molecular structures of these compounds also differ due to the presence of other functional groups. For instance, compounds such as C₄H₂F₆ (fluorinated ketone) and C₆H₁₂O (fluorinated olefine) have carbon atoms with double bonds that promote the formation of intermediate radicals during the combustion.

<table><caption>Table 1. Fluorinated compounds</caption><thead><tr><td>Chemical formula</td><td>CAS number</td><td>Main use</td><td>Molecular weight (g/mol)</td><td>H/F ratio</td></tr></thead><tbody><tr><td>C₆F₁₂O</td><td>756-13-8</td><td>Cleaning agent for electric devices</td><td>316.0</td><td>0,00</td></tr><tr><td>C₉H₅F₁₅O</td><td>297730-93-9</td><td>Cleaning agent for medical devices</td><td>414.0</td><td>0,33</td></tr><tr><td>C₄H₂F₆</td><td>692-49-9</td><td>Foam-blowing agent & heat exchange fluid</td><td>164.0</td><td>0,33</td></tr><tr><td>C₆H₅F₉O</td><td>163702-06-5</td><td>Solvent, heat exchange fluid & hydraulic systems</td><td>264.0</td><td>0,55</td></tr><tr><td>C₄H₅F₅</td><td>406-58-6</td><td>Foam-blowing agent, solvent & heat exchange fluid</td><td>148.0</td><td>1,00</td></tr></tbody></table>

The emission factors of HF and COF₂ were measured during the combustion of the liquids listed above since they are considered as the most stable fluorinated products [6]. Other compounds such as CF₄ and SiF₄ were also detected by the FTIR spectrometer but were not measured due to distinct reasons. Firstly, CF₄ can easily react with intermediate combustion radicals to form F atoms
(decomposition) [2], hence, the concentration of this compound is only significant in the flammability analysis of very fuel rich mixtures [5]. Secondly, the production of SiF₄ is attributed to a degradation of the Quartz tube generated by the reaction of a minimum amount of HF molecules with the SiO₂ of the tubular structure [7]. Furthermore, the emission factor of the unreacted compound was also determined in order to estimate the global fluor recovery of the FTIR analysis for the fluorine atoms.

### 3. Results and discussion

#### 3.1. Ignition with a pilot flame

The ignition of the fluorinated liquids with the pilot flame led to emissions mainly composed of the unreacted compound (Fig 3). In this case, the liquid is only partially burned because the production of combustion radicals is not sufficient to overcome the evaporation rates of the liquid. This result can be explained with the results obtained by Kondo et al. [8] for the analysis of R-1234ze(E). These authors established that air should be preheated and adjusted to high humidities to enhance the ignitability of the fluorinated compound. Nonetheless, this type of compounds is considered to be highly flammable because their flammability limits are similar to the values of methane [9].

In spite of the low conversion, the experimental results allow identifying an influence of the halogenation level on the selectivity. The FTIR data show that the conversion of the reactant and the emission factor of HF increase as the H/F ratio is augmented. This fact can be attributed to the enhancement by higher concentrations of H radicals. These species promote abstraction reactions (loss of an atom of F with a free H radical). Additionally, more HF is rapidly produced by single-step mechanisms such as the unimolecular decompositions, which generate a double C-C bond after the loss of an atom of H along with an atom of F [2]. On the contrary, the emissions of COF₂ do not have significant variations. In this case, the preponderance of the fluid's evaporation leads to a reduction of the multi-step decompositions required for the production of this gas.

<figure><img src="image_5.png" /><figcaption>Fig 3. Emission factors of the fluorinated compounds ignited by a pilot flame</figcaption></figure>

#### 3.2. Ignition with a pool fire of acetone

The low conversion obtained with a pilot flame was addressed by substituting this ignition source with an injection system that places a lean fluid-air mixture at the middle of an acetone pool fire. For this purpose, a lean mixture of each fluorinated compound was dispersed in air at a concentration of 6% v/v before being injected into the acetone flame. Since the flame is submitted to local variations of temperature and concentration, the mixture was injected perpendicularly to the axis of the flame (Fig. 2b) and directly at the centre of the flame (Fig. 2c).
3.2.1. *Horizontal injection.* The emission factors shown in Fig. 4 show that the conversion of the fluorinated fluid is almost complete with the pool fire as an ignition source. In this experimental analysis, the pool fire becomes a preheating source and increases the concentration of intermediate radicals along with the humidity generated by the combustion of acetone. This effect is enhanced by an increase in the H/F ratio. Conversely, the emissions of COF₂ decrease as the halogenation level is augmented. These results agree with the numerical results obtained by Babushok et al [5], who established that added H₂O molecules increase the volume fraction of HF, CO, and CO₂ in the products, and decrease the amounts of COF₂ in the products. This fact is observed due to higher concentrations of OH and H radicals in the combustion zone that promote the decomposition of this compound. For this reason, its emission rates are usually greater for high equivalence ratios and not for lean mixtures [5].

<figure><img src="image_6.png" /><figcaption>Fig. 4. Emission factors of the fluorinated compounds injected horizontally into a pool fire of acetone</figcaption></figure>

3.2.2. *Vertical injection.* The emission factors shown in Fig. 5 indicate a similar behaviour in the conversion of the fluorinated fluid when it is injected vertically into the pool fire. Nevertheless, the vertical injection zone enhances the selectivity to HF and reduces considerably the emission factor of COF₂. As discussed above, this effect is augmented by the increase of the H/F ratio due to a greater interaction probability with H, O and F atoms.

<figure><img src="image_7.png" /><figcaption>Fig. 5. Emission factors of the fluorinated compounds injected vertically into a pool fire of acetone
According to previous flammability studies of fluorinated compounds, a structure of two zones can
be identified in the combustion flame of a refrigerant [5], [10]. The first region constitutes the major</figcaption></figure>
consumption of the initial compound along with the formation of species such as HF, $CF_2O$ and $CF_4$. The peak concentration of the intermediate radicals is reached by the end of this zone. Subsequently, the presence of these radicals decreases rapidly in the second zone, which mainly promotes the conversion of CO to $CO_2$ as well as the consumption of $O_2$ to equilibrium levels. These secondary combustions provide an additional energy release that defines the peak of temperature in this zone [5].

The comparison between the emission factors of both injections shows that the horizontal feed defines a shorter residence time for the compound/air mixture in the second region and reduces the decomposition of $COF_2$ molecules into HF. Hence, its emission factor is considerably higher due to a minor interaction with the radicals present in the first zone of the flame. Conversely, the vertical injection feeds the air mixture in a position closer to the centre of the flame. This fact provides a better interaction with the pool of radicals, which enhances the decomposition of $COF_2$ as well as the production of HF due to a longer residence time.

## 4. Conclusions

An experimental analysis with five fluorinated compounds allowed characterizing the emission factors of HF, $COF_2$ and the unreacted fluid. In spite of being considered as low-flammability compounds, these fluorinated fluids are susceptible to thermal decomposition reactions. This phenomenon can lead to significant variations in the emission rates of the three toxic compounds when the combustion conditions are modified considerably. In this study, the experimental results showed that certain parameters such as a hot air temperature, a high concentration of H and OH radicals and a good interaction with the flame can enhance the production of HF and reduce the emission rate of $COF_2$.

## References

[1] M. O. McLinden, C. J. Seeton, et A. Pearson, « New refrigerants and system configurations for vapor-compression refrigeration », *Science*, vol. 370, n° 6518, p. 791-796, 2020.

[2] D. R. Burgess, M. R. Zachariah, W. Tsang, et P. R. Westmorelandt, « Thermochemical and chemical kinetic data for fluorinated hydrocarbons », *Prog. Energy Combust. Sci.*, vol. 21, n° 6, p. 453-529, 1995.

[3] ISO 12136:2011, « Reaction to fire tests. Measurement of material properties using a fire propagation apparatus ». International Organization for Standardization, 2011.

[4] S. Brohez, G. Marlair, et C. Delvosalle, « Fire calorimetry relying on the use of the fire propagation apparatus. Part I: early learning from use in Europe », *Fire Mater.*, vol. 30, n° 2, p. 131-149, 2006.

[5] V. I. Babushok, D. R. Burgess, M. J. Hegetschweiler, et G. T. Linteris, « Flame propagation in the mixtures of $O_2/N_2$ oxidizer with fluorinated propene refrigerants ($CH_2 CFCF_3$, $CHFCHCF_3$, $CH_2CHCF_3$) », *Combust. Sci. Technol.*, vol. 193, n° 11, p. 1949-1972, 2020.

[6] N. D. Smith et M. W. Tufts, « Technical Brief: Thermodynamic Properties of Selected HFC Refrigerants », *HVACR Res.*, vol. 2, n° 3, p. 257-261, 1996.

[7] Y. Feng, X. Jiang, et D. Chen, « The emission of fluorine gas during incineration of fluoroborate residue », *J. Hazard. Mater.*, vol. 308, p. 91-96, 2016.

[8] S. Kondo, K. Takizawa, A. Takahashi, K. Tokuhashi, J. Mizukado, et A. Sekiya, « Flammability limits of olefinic and saturated fluoro-compounds », *J. Hazard. Mater.*, vol. 171, n° 1-3, p. 613-618, nov. 2009.

[9] B. Tribouilloy, G. Binotto, F. Flécheux, A. Vignes, et G. Marlair, « Assessing genuine flammability hazard of halogenated species for their safe processing and use: Case studies », *Process Saf. Environ. Prot.*, vol. 174, p. 818-827, 2023.

[10] C. D. Needham et P. R. Westmoreland, « Combustion and flammability chemistry for the refrigerant HFO-1234yf (2,3,3,3-tetrafluoropropene) », *Combust. Flame*, vol. 184, p. 176-185, 2017.