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Combustion of C1 and C2 PFAS: Kinetic Modeling and Experiments, February 2022
Citation:
Krug, J., P. Lemieux, C. Lee, J. Ryan, P. Kariher, E. Shields, L. Wickersham, M. Denison, K. Davis, D. Swensen, R. Burnette, J. Wendt, AND W. Linak. Combustion of C1 and C2 PFAS: Kinetic Modeling and Experiments, February 2022. JOURNAL OF THE AIR & WASTE MANAGEMENT ASSOCIATION. Air & Waste Management Association, Pittsburgh, PA, 72(3):256-270, (2022). https://doi.org/10.1080/10962247.2021.2021317
Impact/Purpose:
This paper reflects work performed on the Rainbow Furnace in EPA’s RTP facilities, where model PFAS refrigerant compounds (CF4, C2F6, and CHF3) were fed into the furnace through the flame as well as through several post-flame positions at various firing rates, temperatures and residence times. Real-time measurements, taken with Fourier Transform Infrared (FTIR) are reported, and are compared to results from a kinetic incinerator model (the Configured Fireside Simulator, CFS) originally developed by Reaction Engineering International for the HSRP, with PFAS-specific additions funded from both SHC5.4.4 and the PITT. The paper’s first author is Jonathan Krug (CEMM, AMCD), other authors include several researchers from CEMM, Paul Lemieux (CESER, HSSMD), one Jacobs contractor, the model’s developers at Reaction Engineering International, and University of Utah Presidential Professor of Chemical Engineering Jost Wendt. This work represents a unique application of applied combustion science, where experiments were performed using model compounds in a real combustion system, real-time measurements were performed of the injected compound and trace products of incomplete combustion at operationally relevant concentrations, and the results were successfully compared to model predictions of those same trace gas-phase constituents. This represents a significant potential enhancement in available tools to support effective management of PFAS-containing waste. The paper was internally reviewed by 2 ORD researchers and will be sent to the Journal of Air & Waste Management for external peer review. This version of the paper addresses all ORD reviewer comments. Although the CFS software has been previously presented at various venues as an HSRP product, this is the first time that research using the model on PFAS compounds and comparing model predictions with experimental PFAS measurements on a pilot-scale combustor will be released outside of EPA.
Description:
A computational fluid dynamic (CFD) combustion model, originally developed for the Department of Defense (DoD) to model the destruction of chemical warfare agents in demilitarization incinerators was modified to include C1-C3 fluorinated organic chemical reactions and kinetics compiled by the National Institute of Standards and Technology (NIST) and available from the literature. As part of an initial study, a simplified plug flow reactor version of this model was used to predict the destruction efficiency (DE) and formation of products of incomplete combustion (PICs) for three C1 and C2 per- and poly-fluorinated alkyl substances (PFAS) (CF4, CHF3, and C2F6) and compare predicted values to Fourier Transform Infrared spectroscopy (FTIR)-based measurements made from a small pilot-scale EPA research combustor (40-64 kW, natural gas-fired, 20% excess air). PFAS were introduced through the flame, and at selected post-flame locations along a time-temperature profile to simulate direct flame and non-flame injection, PFAS evolution from solid wastes away from flames, and examine the sensitivity of PFAS destruction on temperature and free radical flame chemistry. Results indicate that CF4 is particularly difficult to destroy with DEs ranging from ~60-95% when introduced through the flame at increasing furnace loads. Compared to CF4, CHF3 and C2F6 were easier to destroy, exhibiting DEs >99% even when introduced downstream of the flame. This is likely due to the presence of lower energy C-H and C-C bonds to initiate molecular dissociation reactions. However, these lower bond energies may also lead to the formation of CF2 and CF3 radicals at thermal conditions unable to fully de-fluorinate these species, and lead to the formation of fluorinated PICs. DEs determined by the model agreed well with the measurements for CHF3 and C2F6, but overpredicted DEs at high temperatures and underpredicted DEs at low temperatures for CF4. However, high DEs do not necessarily mean absence of PICs, with both model predictions and limited FTIR measurements indicating the presence of similar fluorinated PICs in the combustion emissions. Results from this initial study are encouraging, in that FTIR was able to provide real-time emission measurements and additional model development may improve prediction of PFAS destruction and PIC formation.
