Citation:

Krug, Jonathan D., P. Lemieux, C. Lee, J. Ryan, P. Kariher, E. Shields, L. Wickersham, M. Denison, K. Davis, D. Swensen, R. Burnette, J. Wendt, AND Bill Linak. Combustion of C1 and C2 PFAS: Kinetic Modeling and Experiments, February 25, 2021. Michigan PFAS Action Team (MPART) Meeting, NA, Virtual, February 25, 2021.

Impact/Purpose:

The presentation “Combustion of C1 and C2 PFAS: Kinetic Modeling and Experiments,” will be delivered virtually at the “Michigan PFAS Action Team (MPART)” meeting on February 25, 2021. The quarterly meeting with MPART has been coordinated by EPA/ORD to exchange information with the State of Michigan on different PFAS issues/topics including treatment technology. The presentation reflects a preliminary block of work performed on the Rainbow Furnace in the EPA’s RTP facilities, where model PFAS compounds (CF4, C2F6, and CHF3) were fed into the furnace through the flame as well as through several locations downstream of the flame at various temperatures and residence times. Real-time measurements, taken with a Fourier Transform Infrared (FTIR) instrument are reported, and are compared to results from a computer incinerator simulation (the Configured Fireside Simulator, CFS) developed by Reaction Engineering International. At this point experimental results are preliminary, 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 work represents a significant potential enhancement in available tools to support effective management of PFAS-containing waste.

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 limited Fourier Transform Infrared (FTIR)-based measurements made from a small pilot-scale EPA research combustor (40-45 kW, natural gas, 20% excess air). PFAS were introduced through the flame, and at selected post flame locations along a time-temperature profile to simulate 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 maximum DE of ~60% when introduced through the flame at these conditions. 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. FTIR-measured hydrofluoric acid (HF) concentrations did not agree with modeled HF concentrations, suggesting significant adsorption, reaction, or loss to refractory wall surfaces or measurement interference issues. 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.

Record Details: