Thermal treatment of hexafluoropropylene oxide dimer acid (HFPO-DA) using a pilot-scale research combustor

The thermal decomposition of single-component aqueous solutions of hexafluoropropylene oxide dimer acid (HFPO-DA), a "GenX" process chemical, was investigated in a pilot-scale research combustor. Two solutions containing target HFPO-DA concentrations of ∼500 and 4000 mg/L were atomized at three post-flame temperatures of ∼920, 860, and 750 °C, during which the stack gases were characterized for combustion products including fluorocarbon products of incomplete combustion (PICs). Analytical techniques included Other Test Method 50 (OTM-50) and real-time analysis using Fourier transform infrared spectroscopy (FTIR) and chemical ionization mass spectrometry (CIMS). Quantum chemical calculations were performed to identify pathways for the thermal decomposition of HFPO-DA. Identified PICs included 1H-perfluoroalkanes, fluoroether E-1 (heptafluoropropyl 1,2,2,2-tetrafluoroethyl ether), and two ultra short-chain perfluorocarboxylic acids (PFCAs): trifluoroacetic acid (TFA) and perfluoropropionic acid (PFPrA). Most PIC concentrations increased with decreasing peak temperatures with ∼40-60 % of the fluorine in the HFPO-DA converted into PICs at the lowest peak temperature examined (∼750 °C). At higher peak temperatures, lower PIC concentrations were observed, suggesting that temperature is a critical parameter for mineralization. Modeling results proposed that the thermal decomposition of HFPO-DA can pass through 1,1,1,2,2,3,3-heptafluoro-3-[(1,2,2-trifluoroethenyl)oxy]propane (perfluoropropylvinyl ether, or PPVE) or fluoroether E-1 intermediate species with the PPVE route offering lower energy barriers.