State of the art on the flammability of HFOs

Hydrofluoroolefins (HFOs) have been attracting increasing attention over the recent decade, and several of them are considered as candidates for next-generation refrigerants. HFOs are unsaturated organic compounds composed of carbon, hydrogen and fluorine. They have environmental benefits since they exhibit zero ozone depletion potential (ODP) and extremely low global warming potentials (GWPs), due to their molecular characteristics of being chlorine-free and of short atmospheric lifetime. However, many of them have a certain degree of flammability.

A recent review article ⁽¹⁾ published in the International Journal of Refrigeration presents a state of the art on the flammability of HFO refrigerants. The Chinese authors have reviewed the flammability characteristics of 48 HFO refrigerants and mixtures under a variety of ignition conditions. Among them are R1234yf, R1234ze(E), R1234ze(Z), R1243zf, R1123, R1336mzz(E), and their mixtures with R32, R134a, R152a, R161, R290 (propane), DME (dimethyl ether), R13I1 (trifluoroiodomethane) and R744 (CO₂). The flammability characteristics presented include in particular the lower flammability limit, upper flammability limit, burning velocity, minimum ignition energy, hot surface ignition temperature, autoignition temperature, thermal decomposition temperature, heat of combustion and products of combustion.

The authors conclude that the flammability of fluorinated hydrocarbons is determined by not only their constituent elements, but also their molecular configurations. Therefore, they propose a new correlation equation to roughly indicate the burning velocity and heat of combustion of fluorinated hydrocarbons once their basic molecular characteristics are known.

⁽¹⁾ X. Wu, C. Dang, S. Xu, E. Hihara ; State of the art on the flammability of hydrofluoroolefin (HFO) refrigerants. Available in FRIDOC database (free for IIR members).