PFAS waste disposal in the fire service – Part 2

Part 1 of this article pointed up the advantages of using cement kiln co-processing as a commercially and financially viable means for the efficient and environmentally sustainable destruction of solid and liquid PFAS waste, such as from firefighting foams. In Part 2 the kiln operating conditions required are discussed, together with the necessary licensing requirements to ensure environmentally safe control of fugitive emissions, based on regulatory criteria developed by the Queensland Department of Environment and Science.

PFAS, now mostly used in the form of fluorotelomers, are found in a wide range of products with modern PFAS firefighting foams entirely based on fluorotelomers. PFAS firefighting foam use is particularly dispersive and has resulted in widespread, permanent pollution. The wastes now being produced from the transition away from PFAS and from remediation are a particular problem for disposal and destruction. The challenge is to achieve disposal in a permanent, sustainable and cost-efficient way. And one that is applicable to PFAS as a class of compounds,¹ rather than having methods that are compound-specific.

Modern Class B AFFF foams use fluorotelomer surfactants whereas legacy AFFF foams such as Light Water™ were PFOS-based, until 3M announced in May 2000 that they were phasing out PFOS (and PFHxS) production and use; however, some countries still produce PFOS and PFHxS for firefighting foams. Moreover, PFOS is still subject to some exemptions under the Stockholm Convention allowing continued use in existing fixed extinguishing systems.

Fluorotelomer PFAS manufacture starts with the mineral fluorspar, natural crystalline calcium fluoride (CaF₂), being treated with concentrated sulfuric acid to produce hydrofluoric acid (HF). The HF is then combined with organic compounds to make a telogen perfluoroalkyl iodide, most commonly pentafluoroethyl iodide (C₂F₅I), which is reacted with the monomer tetrafluoroethylene (C₂F₄) to produce the desired longer-chain perfluoroalkyl iodide such as C8 heptadecafluorooctyl iodide (C₈F₁₇I), and the perfluoroalkyl iodide then reacted with a 2-carbon unit, ethylene CH₂=CH₂, to give the fluorotelomer derivative, as shown in Figure 1.

After the perfluoroalkyl iodide is formed and after addition of the CH₂-CH₂ group, a suitable non-fluorinated charged functional group is added to give the desired fluorosurfactant end-product, such as a perfluoroalkyl betaine or aminoxide. Fluorotelomer chains are linear and always contain an even number of carbon atoms (2,4,6…) compared to PFOS-based derivatives produced by electrofluorination that have both odd and even linear and branched carbon chains.

Both the Stockholm Convention and the Basel Convention Guidelines^2,3 describe the co-processing of various hazardous wastes in rotary cement kilns, a process that is characterised by extremely high kiln temperatures (~1,200–1,400°C) with long residence times of the order of minutes. These conditions far exceed those achieved in normal commercial domestic or industrial waste incinerators. The conditions for PFAS destruction are usually cited as temperatures of at least 1,100°C and residence times of >2 seconds.⁴ Cement kilns not only exceed these conditions but also have an excess of calcium in the feedstock that catalyses the breaking of the C-F organo-fluorine bonds at temperatures lower than 1,100°C with the fluorine rapidly captured as calcium fluoride (fluorspar), the mineral from which the fluorine was originally derived. By contrast, standard commercial incinerator temperatures are rarely high enough with the potential for PFAS pyrolysis products to reform in the gas phase, plus corrosive, toxic hydrogen fluoride (HF) is produced that must be scrubbed from the flue gases at additional and considerable cost.

The EU has defined a ‘circular economy’ as ‘…a model of production and consumption, which involves sharing, leasing, reusing, repairing, refurbishing and recycling existing materials…’.⁵ In practice, a circular economy implies reducing waste, and thus keeping environmental contamination to a minimum.

The classical concept of a circular economy is shown below (Figure 2). Cement kiln calcium-catalysed co-processing or incineration of PFAS waste by incineration represents a modified or quasi-circular economy in which any solid residual waste, i.e. calcium fluoride, is identical with the starting mineral from which fluorine was obtained, and rather than being recycled is incorporated as a harmless additive to the cement clinker. A classical circular economy would ideally separate and reuse fluorochemical from the waste stream.

Calcium-catalysed cement kiln co-processing of PFAS waste is characterised by (i) very high destruction efficiency for PFAS (~ 99.999%); (ii) fugitive emissions in the flue gas well below regulatory levels (i.e. for HF, dioxins, sulfur oxides, etc); (iii) no detectable residual PFAS; (iv) very high temperatures ~1,200–1,400°C; (v) long residence times at high temperature; (vi) high energy efficiency and no additional fuel/energy use and thus no related carbon footprint; (vii) ability to deal with large charges in ~100 ton range; (viii) ready availability of facilities; (ix) ability to handle solid and liquid wastes in the raw state; and (x) low cost compared to other currently available methods with permanent capture of the fluorine as inert solid calcium fluoride minerals. The modified or quasi-circular economy represented by cement kiln co-processing is illustrated in Figure 3.

The recommended minimum operating conditions for rotary cement kiln co-processing of hazardous wastes (Basel Convention (2011)) are shown in the adjacent figure IV taken from the guidelines.³ Maintenance of kiln conditions to achieve destruction are a matter for the national Competent Authority, i.e. the local regulatory licensing authority. For the most part the stable operating conditions of cement kilns provide wide safety margins for PFAS destruction so the primary regulatory controls need to be around the process points and timings for PFAS waste introduction.

It is clear the temperatures and residence times in the main burner region exceed the minimum considered to be required for PFAS in isolation to be broken down (>1,100°C and >2 seconds). Noting that it has now been established that the presence of calcium catalyses PFAS breakdown at much lower temperatures around 800°C. Once the kiln clinker production conditions are stable liquid wastes are typically injected into the hot end (main burner) of the kiln, most effectively when incorporated with fuel such as waste oil. Solid wastes can be introduced into the calcining zone at some facilities. This is at mid-kiln for long kilns, and onto the feed shelf in the high-temperature section for pre-heater/pre-calciner kilns.

Thermal breakdown of fluorochemicals

Decomposition mechanisms for the breakdown of perfluoroalkyl substances (PFAS) at elevated temperatures are beginning to be better understood, as are the influences of the presence of calcium compounds that are available in excess in cement kilns (as crushed limestone or marble meal). Calcium has a dual action, catalysing the cleavage of the carbon-fluorine bonds at lower temperatures,^6,7 as well as progressively capturing the fluorine as inert calcium fluoride minerals throughout the process. When PFAS are destroyed by high-temperature incineration in the presence of oxygen and calcium compounds the fluorine released in the form of free-radicals or ions is captured by the calcium to form calcium fluoride, CaF₂. This calcium fluoride remains in the cement clinker as a harmless, minor additive which does not affect final cement performance.

High temperature cement kiln incineration of PFAS in the presence of calcium represents a key advance in closing the loop of the circular fluorine economy that starts as calcium fluoride mineral and finishes with calcium fluoride as a trace component of cement.

Regulation of PFAS destruction

Disposal and destruction of hazardous wastes is subject to legislative and specific regulatory provisions to manage handling, transport, storage, treatment and disposal of the wastes. PFAS wastes are of particular concern being permanent and dispersive pollutants (POPs) that are toxic, highly mobile and do not degrade in the environment. Dedicated hazardous waste incinerators are often operating at the limits of their temperature control and the ability to scrub toxic and hazardous materials from the flue gases, with process upsets and reformation of ultra-short chain C1 and C2 PFAS likely. By comparison trials of PFAS destruction in cement kilns have found that normal cement kiln operation has excess capacity to destroy PFAS and sequester fluorine with wide safety margins around destruction efficiencies, temperature, residence times and effective capture of fluorine plus significant fuel efficiencies.

Cement kiln PFAS destruction trials in 2016–17 in Queensland, Australia included fluorine inputs of up to 5kg/hr in PFAS plus runs with an additional 325kg/hr fluorine in calcined ash from aluminium smelter pot-line wastes. In view of research^6,7 that had shown that calcium catalysed PFAS destruction at temperatures lower than the 1,100°C commonly cited, the trials also included input of PFAS wastes into the calciner where temperatures range from 800°C to 1,000°C. For PFAS input to both the calciner and main kiln no PFAS were detected in emissions or cement clinker with PFAS destruction efficiencies for PFOS, PFOA and PFHxS of 99.999% or better. There was no change in the normal concentration of HF in flue gases of 0.045mg/Nm³ (1,000 times lower than the licence limit of 50mg/Nm³) despite the extra input of 325kg of fluorine in the non-PFAS calcined ash.

Based on the PFAS destruction trial results, in 2018 the cement kiln licence was amended to allow destruction up to 5kg-F/hr of PFAS (approximately equivalent to the F-content of one 1,000L IBC of foam concentrate) introduced into the calciner or main kiln on condition that the kiln process was stable and that there are minimum temperatures of 850°C in the calciner and 1,200°C in the main kiln. Analyses to monitor for PFAS in flue gases is conducted annually with no detections of PFAS. Operationally no changes to the normal kiln processes or extra energy input were needed and there was no change to cement clinker quality with the fluorine bound up in cement as trace amounts of insoluble, inert, non-toxic calcium fluoride minerals.

Interest in effective and safe high-temperature incineration or co-processing of fluorine-containing PFAS waste using rotary cement kilns has increased substantially since we first published a brief description of the Queensland trials in 2018.8 Recent work by the US EPA has identified high-temperature cement kiln coprocessing as a viable and energy efficient method for disposing of PFAS waste streams.^9,10,11

Currently the Gladstone (Queensland) and Railton (Tasmania) cement kilns are able to process solid and liquid PFAS waste streams, respectively, in an environmentally sustainable and energy-efficient way, returning the fluorine to its original state as calcium fluoride incorporated as a harmless additive to the cement clinker end-product.

For more information, email rogeraklein@yahoo.co.uk or nigel.holmes@des.qld.gov.au

References

• • 1. Kwiatkowski, C.F., Andrews, D.Q., Birnbaum, L.S., Bruton, T.A., DeWitt, J.C., Knappe, D.R.U., Maffini, M.V., Miller, M.F., Pelch, K.E., Reade, A., Soehl, S., Trier, X., Venier, M., Wagner, C.C., Wang, Z., and Blum, A. (2020) Scientific Basis for Managing PFAS as a Chemical Class. Environ. Sci. Technol. Lett. 7, 532−543.
  • 2. UN Stockholm Convention (2008) “Guidelines on best available techniques and provisional guidance on best environmental practices relevant to Article 5 and Annex C of the Stockholm Convention on Persistent Organic Pollutants- cement kilns firing hazardous waste”.
  • 3. UN Basel Convention (2011) “Technical guidelines on the environmentally sound co-processing of hazardous wastes in cement kilns”.
  • 4. L. J. Winchell, J. J. Ross, M. J. M. Wells, X. Fonoll, J. W. Norton Jr, and K. Y. Bell (2021) “Per- and polyfluoroalkyl substances thermal destruction at water resource recovery facilities: A state of the science review” Water Environment Research 93: 826–843.
  • 5. European Commission (2020) “A new Circular Economy Action Plan for a cleaner and more competitive Europe”, COM/2020/98 final, Brussels. European Parliament News Item Updated 26 April 2022.
  • 6. Wang, F., Xinghwen, L., Xiao Yan, L., Kaimin, S. (2015) “Effectiveness and mechanisms of defluorination of perfluorinated alkyl substances by calcium compounds during waste thermal treatment”. Environ. Sci. Technol. 49 (9) 4672-4680;
  • 7. Riedel, T.P., Wallace, M.A.G., Shields, E.P.; Ryan, J.V.; Lee, C.W.; Linak, W.P. (2021) “Low temperature thermal treatment of gas-phase fluorotelomer alcohols by calcium oxide”. Chemosphere 272, 129859.
  • 8. Klein, R.A. (2018) Recycling Bubbles. Industrial Fire Magazine Q2, 24-26.
  • 9. Longendyke, G.K., Katel, S., and Wang, Y. (2022) PFAS fate and destruction mechanisms during thermal treatment: a comprehensive review. Environmental Science: Processes Impacts 24, 196-208.
  • 10. Patterson, C., and Dastgheib, S.A. (2020) “Cement Kiln and Waste to Energy Incineration of Spent Media” presentation at a joint USEPA/ORD and DOD/SERDP/ESTCP Workshop on the Thermal Treatment of PFAS, Cincinatti, Ohio, 25 February 2020. pp.15.
  • 11. Gullett, B., and Gillespie, A. (2020) US Environment Protection Agency Technical Brief. “Per- and Polyfluoroalkyl Substances (PFAS): Incineration to Manage PFAS Waste Streams” February 2020.