Pilot-Scale Thermal Destruction of Per- and Polyfluoroalkyl Substances in a Legacy Aqueous Film Forming Foam.

The destruction of per- and polyfluoroalkyl substances (PFAS) is critical to ensure effective remediation of PFAS contaminated matrices. The destruction of hazardous chemicals within incinerators and other thermal treatment processes has historically been determined by calculating the destruction efficiency (DE) or the destruction and removal efficiency (DRE). While high DEs, >99.99%, are deemed acceptable for most hazardous compounds, many PFAS can be converted to other PFAS at low temperatures resulting in high DEs without full mineralization and the potential release of the remaining fluorocarbon portions to the environment. Many of these products of incomplete combustion (PICs) are greenhouse gases, most have unknown toxicity, and some can react to create new perfluorocarboxylic acids. Experiments using aqueous film forming foam (AFFF) and a pilot-scale research combustor varied the combustion environment to determine if DEs indicate PFAS mineralization. Several operating conditions above 1090 °C resulted in high DEs and few detectable fluorinated PIC emissions. However, several conditions below 1000 °C produced DEs >99.99% for the quantifiable PFAS and mg/m3 emission concentrations of several non-polar PFAS PICs. These results suggest that DE alone may not be the best indication of total PFAS destruction, and additional PIC characterization may be warranted.

Combustion of C1 and C2 PFAS: Kinetic modeling and experiments.

A combustion model, originally developed to simulate the destruction of chemical warfare agents, was modified to include C1-C3 fluorinated organic reactions and kinetics compiled by the National Institute of Standards and Technology (NIST). 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 pilot-scale EPA research combustor (40-64 kW, natural gas-fired, 20% excess air). PFAS were introduced through the flame, and at post-flame locations along a time-temperature profile allowing for simulation of direct flame and non-flame injection, and examination of 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 to 95% when introduced through the flame at increasing furnace loads. Due to the presence of lower energy C-H and C-C bonds to initiate molecular dissociation reactions, CHF3 and C2F6 were easier to destroy, exhibiting DEs >99% even when introduced post-flame. 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 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. The FTIR was able to provide real-time emission measurements and additional model development may improve prediction of PFAS destruction and PIC formation.Implications: The widespread use of PFAS for over 70 years has led to their presence in multiple environmental matrixes including human tissues. While the chemical and thermal stability of PFAS are related to their desirable properties, this stability means that PFAS are very slow to degrade naturally and potentially difficult to destroy completely through thermal treatment processes often used for organic waste destruction. In this applied combustion study, model PFAS compounds were introduced to a pilot-scale EPA research furnace. Real-time FTIR measurements were performed of the injected compound and trace products of incomplete combustion (PICs) at operationally relevant conditions, and the results were successfully compared to kinetic model predictions of those same PFAS destruction efficiencies and trace gas-phase PIC constituents. This study represents a significant potential enhancement in available tools to support effective management of PFAS-containing wastes.

Characterization of emissions from a pilot-scale combustor operating on coal blended with woody biomass.

Emissions generated from the combustion of coal have been a subject of regulation by the United States Environmental Protection Agency (U.S. EPA) and State agencies for years, as they have been associated with adverse effects on human health and the environment. Over the past several decades, regulations on these facility emissions have become more stringent and have therefore caused industry to look toward new pre- and post-combustion control technologies. In more recent years, there has been a "push" toward renewable and cleaner burning alternative fuels as replacements for traditional fossil fuels. Part of this "push" has been accomplished by States and Regions offering incentives and options for renewable portfolios, which over half of the states now have in some form. The current study investigates the potential changes in both gaseous and particulate emissions from the use of a variety of woody biomass materials as a drop-in replacement for coal as compared to use of 100% bituminous coal. Four different biomass materials are blended individually with coal at 20% and 40% by mass for testing on the U.S. EPA's Multi-Pollutant Control Research Facility, a pilot-scale coal-fired facility located in Research Triangle Park, North Carolina. Emissions are calculated based on measurements from the flue gas to characterize gaseous species (CO, CO2, NOX, SO2, other acid gases, and several organic hazardous air pollutants) as well as fine and ultrafine particulate (mass, size distribution, number count, elemental carbon, organic carbon, and black carbon) and compared among each combination of fuels and 100% bituminous coal.

Comparison of gaseous and particulate emissions from a pilot-scale combustor using three varieties of coal.

Gaseous and particulate emissions generated from the combustion of coal have been associated with adverse effects on human health and the environment, and have therefore been the subject of regulation by federal and state government agencies. Detailed emission characterizations are needed to better understand the impacts of pre- and post-combustion controls on a variety of coals found in the United States (U.S.). While the U.S. Environmental Protection Agency (EPA) requires industry reporting of emissions for criteria and several hazardous air pollutants (HAPs), many of the methods for monitoring and measuring these gaseous and particulate emissions rely on time-integrated sampling techniques. Though these emissions reports provide an overall representation of day-to-day operations, they represent well-controlled operations and do not encompass real combustion events that occur sporadically. The current study not only characterizes emissions from three coals (bituminous, sub-bituminous, and lignite), but also investigates the use of instrumentation for improved measurement and monitoring techniques that provide real-time, continuous emissions data. Testing was completed using the U.S. EPA's Multi-Pollutant Control Research Facility, a pilot-scale coal-fired combustor using industry-standard emission control technologies, in Research Triangle Park, North Carolina. Emissions were calculated based on measurements from the flue gas (pre- and post-electrostatic precipitator), to characterize gaseous species (CO, CO2, O2, NOX, SO2, other acid gases, and several organic HAPs) as well as fine and ultrafine particulate (mass, size distribution, number count, elemental carbon, organic carbon, and black carbon). Comparisons of traditional EPA methods to those made via Fourier Transfer Infrared (FTIR) Spectroscopy for CO, NOX, and SO2 are also reported.