Citation:

Krug, J., W. Roberson, P. Lemieux, J. Ryan, P. Kariher, E. Shields, S. Jackson, L. Wickersham, Bill Linak, C. Lee, P. Burnette, J. Nash, L. Virtaranta, W. Preston, M. Denison, K. Davis, D. Swensen, AND J. Wendt. Thermal Destruction of Perfluorinated Akyl Substances (PFAS) in a Pilot-Scale Combustor. 39th Internationa; Symposium on Combustion, Vancouver, British Columbia, CANADA, July 24 - 29, 2022.

Impact/Purpose:

This presentation is to inform a technical orientated audience about issues related to PFAS chemistry pertaining to disposal by incineration, and explain ORD's ongoing approach to PFAS incineration research. The intent is to inform the audience about the fundamentals of combustion science, and how these tools are being used to address questions about PFAS disposal by incineration. This presentation includes results from specific EPA studies examining the thermal destruction of PFAS refrigerants (CF4, CHF3, C2F6) and aqueous film forming foam (AFFF).

Description:

Per/polyfluorinated alkyl substances (PFAS), are a group of anthropogenic chemicals used for their hydrophobic and lipophobic properties, as well as their chemical and thermal stability. PFAS can be present in multiple waste streams, some of which, are treated by thermal destruction processes. However, as the C-F bond is one of the strongest in organic chemistry, it is unclear how effectively different thermal destruction processes can fully transform fluorocarbon chains in PFAS to produce only CO2 and HF. We present results from a combined experimental and modeling study of the thermal destruction of PFAS in a pilot-scale down-fired natural gas combustor. Experiments have been completed for three volatile PFAS (CF4, CHF3, and C2F6) and an aqueous film forming foam (AFFF) containing known concentrations of 10 PFAS, primarily perfluorooctane sulfonic acid (PFOS). Variations in combustion conditions, furnace load, stoichiometry, and PFAS injection location help evaluate the effectiveness of thermal destruction processes under different thermal environments, including those in the flame and the post-flame. For the three volatile PFAS, Fourier Transform Infrared (FTIR) spectroscopy was used to characterize destruction efficiency (DE) and a limited set of products of incomplete combustion (PICs). For AFFF, FTIR measurements are being augmented by source sampling methods including Other Test Method 45 (OTM-45), thermal desorption (TD) tubes, and stainless steel cannisters followed by either gas chromatography or liquid chromatography coupled with triple quad mass spectroscopy analyses for targeted and non-targeted PICs. Results for the volatile PFAS indicate that, in line with theory, CF4 is particularly difficult to destroy, with measured DEs ranging from ~60 to 95% even when introduced through the flame and experiencing peak bulk gas temperatures >1420 °C. Due to the presence of susceptible C-H and C-C bonds measurements show that CHF3 and C2F6 were easier to destroy, exhibiting DEs >99% even when introduced post-flame. CHF3 was especially easy to destroy most probably due to hydrogen abstraction by OH well away from the flame. However, high DEs do not preclude PICs, as thermal dissociation of C-H and C-C for CHF3 and C2F6, respectively, at low temperatures can lead to the formation of CF3 radicals unable to dissociate further, and the formation of CF4 and C2F6 PICs. Further, formation of C2F6 during the injection of CHF3 suggests that PICs larger than the parent PFAS may be formed. These experimental results are supported by a parallel modeling effort using a 3-dimensional reacting flow model based on methane-air combustion and National Institute of Standards and Technology (NIST) developed C1-C3 fluorinated organic chemical kinetics. Canister results for the AFFF experiments indicate mostly non-detects for 30 targeted volatile non-polar PFAS when the AFFF was introduced through the flame or at post-flame locations >1090 °C. However, as injection location temperatures fell below 1000 °C, concentrations of PFAS PICs (and CO) increased notably. This research was funded by the U.S. Environmental Protection Agency (EPA). The views expressed in this abstract are those of the authors and do not represent the views and policies of the U.S. EPA.

Record Details: