PFAS destruction via cement kiln co-processing A Quasi-Circular Fluorine Economy

Nigel Holmes, Department of Environment and Science (DES), Queensland, and Roger Klein, Cambridge UK, and Christian Regenhard Center for Emergency Response Studies (RaCERS)

Background

The recognition of perfluoroalkyl substances (PFAS) as highly persistent organic pollutants that are toxic, dispersive, and bio-accumulative has galvanised their phase-out from a wide range of uses. Large PFAS waste streams that need to be safely and sustainably disposed of arise from industry wastes such as PFAS firefighting foam and wastes from remediation of soils and water. The exceptional temperature and chemical resistance properties of PFAS that have made them useful, particularly for firefighting, but also make them highly problematic to dispose of.

The most dispersive source of PFAS contamination is from firefighting foam use in training, maintenance and at operational incidents - apart from the indiscriminate waste disposal practices of some manufacturers. Most contamination has been from uncontained releases during training and testing, and not from the majority of firefighting incidents. Although major incidents, such as Coode Island or Buncefield, led to the release of millions of litres of PFAS-contaminated runoff, these are relatively rare. The transition from PFAS-based firefighting foams to fluorine-free foams has resulted in very large volumes of PFAS foam concentrate and wastewater needing specialist disposal.

The exceptional properties of PFAS are a result of the short covalent bond between carbon and fluorine (typically 1.35Å), plus the high electronegativity of fluorine gives a high bond dissociation energy (130 kcal/mol) making fluorocarbons highly resistant to thermal and chemical attack. This results in the PFAS carbon chain being stable at high temperatures and so tightly surrounded by fluorine atoms that the C-F bonds are well protected from chemical attack. The extreme environmental persistence of PFAS, effectively as permanent pollutants, coupled with their mobility, bioaccumulation, bio-concentration and chronic toxicity potential now drives increasing global pressure for their phase-out and safe disposal.

As part of the 2012 to 2016 development and implementation of the Queensland Firefighting Foam Policy, which has a particular focus on PFAS, the need for disposal options was recognised early on and reviewed in anticipation of the PFAS waste streams. It was found that there were very few facilities in Australia (or globally) with a demonstrated ability to handle and destroy large volumes of PFAS wastes. Amongst possible disposal options, the potential for cement kilns to destroy halogenated materials was highlighted in the Basel Convention technical guidelines, however, information on the ability of cement kilns to destroy fluorinated organics was very limited and coloured by cautions about chlorinated organics disposal such as PCBs and dioxins.

In considering rotary cement kilns for PFAS destruction several favourable process characteristics soon became evident, principally the high temperatures and long residence times that are normal in clinker production, significantly greater than the often cited 1,100°C and much longer than the 2 seconds required for PFOS destruction. This left open the issues of how and in what form the fluorine could be captured and would there be PFAS that could survive the process or reform from carbon and fluorine in the cooling gases. Given this potential for PFAS destruction in widely available existing facilities discussions were initiated with the cement industry in Queensland in 2015-16 about the feasibility of trials to investigate the method.

The how and why of PFAS

Initially the PFAS used in early products like fabric protectants and firefighting foam were perfluoroalkyl acids like C8 PFOS and PFOA produced by electrofluorination. Since about 2000 there had been a switch towards fluorotelomer compounds with most modern PFAS firefighting foams and other products based on a diversity of complex fluorotelomers. This has presented challenges for development of methods for disposal that are economic and effective across the very wide diversity of PFAS in wastes [1], which include mixtures of anionic, cationic and zwitterionic compounds, with some methods only effective for specific compounds or types and subject to interference from other components in the wastes.

Disposal methods therefore need to be effective across the range of all modern fluorotelomer surfactants as well as for legacy perfluoroalkyl PFAS in products, as well as the intermediates that result from the transformation of fluorotelomers in the environment.

Modern 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 telogen perfluoroalkyl iodides, most commonly pentafluoroethyl iodide (C2F5I) which is reacted with tetrafluoroethylene (C2F4) to produce the desired longer-chain perfluoroalkyl iodide, such as C8 heptadecafluorooctyl iodide (C8F17I); these perfluoroalkyl iodides are then used to produce fluorotelomers, as summarised in Figure 1. After the perfluoroalkyl iodide is formed, a suitable non-fluorinated functional group is added to give the desired fluorosurfactants end-product, such as a perfluoroalkyl betaine or aminoxide. The end-products are fluorotelomer chains that have diverse and complex functional groups, but all have linear perfluoroalkyl C-F chains that always contain an even number of carbon atoms (4,6,8, etc.), usually with a non-fluorinated dimethylene (-CH2-CH2-) group between the perfluoroalkyl group and the functional group. This contrasts to the PFOS related compounds produced by electrofluorination that contain both odd and even (5,6,7,8...) linear and branched fluorinated carbon chains.

Fluorotelomers are sometimes dismissively referred to as “PFAS precursors” with the misconception that they are benign. Fluorotelomers are not benign or inert, they are nonetheless PFAS as are the endpoint products. Their functional groups can confer toxicity, bioavailability and bioaccumulation characteristics greater than the more commonly referenced endpoint PFAS sulfonates and carboxylates like PFOS and PFOA. The key issue from the point of view of toxicity, persistence, disposal and destruction is that no matter what their origins and forms are, all PFAS have perfluorinated carbon moieties that are part of, or result in, persistent, bioaccumulative and toxic products highly resistant to degradation.

Figure 1. The main building blocks for fluorotelomer PFAS. The fluorotelomer perfluoroalkyl group (left) separated from the functional group (right) by the non-fluorinated -CH2-CH2- “spacer” group (circled).

PFAS destruction practicalities

Both the Stockholm Convention and the Basel Convention Guidelines [2,3] describe the co-processing of various hazardous wastes in rotary cement kilns, in the well-established and common industrial process that is characterised by extremely high kiln temperatures (~1,200-1,600°C) with long residence times of the order of minutes. These conditions far exceed temperature and residence times achieved in normal commercial domestic 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 the significant advantage of an excess of calcium in the feedstock that not only catalyses the breaking of the C-F organo-fluorine bonds at temperatures much lower than 1,100C but also progressively and permanently captures the fluorine as calcium fluoride minerals (e.g., alite, pseudoalite and fluore-spurrite).

By contrast, standard commercial incinerator temperatures are rarely high enough, with the potential for ultra-short PFAS like CF4 and C2F6 to pass through, or for other PFAS to reform in the flue gases, plus the generation of corrosive, toxic hydrogen fluoride (HF) that must be scrubbed from the flue gases at additional and considerable cost.

For the most part the stable operating conditions of cement kilns with an excess of calcium provide wide safety margins for PFAS destruction and fluorine capture, so the primary regulatory controls need to be around the process points, timing and temperatures for PFAS waste introduction.

The temperatures and residence times of material in the kiln main burner region exceed the minima that are considered to be required for PFAS to be broken down. It has now been established that the presence of calcium catalyses the breakdown of PFAS at around 800C PFAS so that wastes can also be introduced earlier into the kiln during the calciner process.

In terms of control over the timing of PFAS introduction to the kiln for destruction, this needs to be once the kiln production conditions of temperature and throughput are stable (Figure 2). For liquid PFAS wastes, introduction can be by incorporation into liquid fuel such as waste oil, rather than as a waste introduced directly into the burner flame (which can cause disturbance to the flame by the water content). Introduction with the liquid fuel also gives the ability to immediately cease waste input should kiln conditions become unstable.

Solid wastes can also be introduced into the calcining zone at some facilities for disposal of wastes such as PFAS-contaminated activated carbon and resin remediation wastes.

Figure 2. Typical temperature profile in a long rotary cement kiln.

Thermal Breakdown of Fluorochemicals
Decomposition mechanisms for the breakdown of 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. Calcium has a dual action, it catalyses the cleavage of the PFAS carbon-fluorine bonds at lower temperatures than for PFAS alone [6,7]. Calcium also progressively captures the liberated fluorine free-radicals, forming inert calcium fluoride minerals progressively throughout the process (Figure 3). The captured fluorine is very strongly bound, moving through the clinker process to finally be a harmless, trace component of the final cement product.

Co-processing of PFAS and other fluorine wastes in cement kilns does not require any modification of the process. Other significant advantages are that it does not require additional fuel and has no end-point waste for disposal, and as such it represents a sustainable, permanent disposal of PFAS as inert mineral trace components of cement.

Figure 3. Cement kiln destruction of PFAS and the capture of the fluorine

PFAS destruction trials and proof-of-concept

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 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 range, stability and capacity to scrub toxic and hazardous materials from the flue gases associated with process disturbances and the possibility of carbon and fluorine reforming into ultra-short chain C1 and C2 PFAS. By comparison, trials of PFAS destruction in cement kilns have found that normal cement kiln operation has excess capacity to destroy PFAS and remove the fluorine, with wide safety margins around destruction efficiencies, temperature, and residence times, with permanent capture of fluorine plus significant fuel efficiencies.

Cement kiln PFAS destruction trials in 2016-17 in Queensland included fluorine inputs of up to 5kg/hr in PFAS plus runs with an additional 90 to 325kg/hr fluorine in wash-water and 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 commonly cited 1,100°C (from ~350°C), 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 there were no PFAS detected in emissions or in cement clinker with PFAS destruction efficiencies for PFOS, PFOA and PFHxS of 99.999% or better (Figure 4). There was no change in the normal process concentration of HF in flue gases of 0.045 mg/Nm3 (1,000 times lower than the licence limit of 50 mg/Nm3) when PFAS was introduced, or even when there was an extra input of up to 325 kg of fluorine when smelter pot line waste was also introduced.

Figure 4. Cement kiln process and advantages.

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 fluorine content of one 1,000L IBC of foam concentrate) introduced into the calciner or the main kiln, on condition that the kiln process was stable with minimum temperatures of 850°C in the calciner and 1,200°C in the main kiln.

Analyses to monitor for PFAS in flue gases are conducted annually with no detections of PFAS since then. Operationally there was no need for changes to the normal kiln processes, no extra energy input and no change to cement clinker quality with the fluorine bound up in the 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 co-processing as a viable and energy efficient method for disposing of PFAS waste streams [9,10,11]. The Basel Convention General technical guidelines on environmentally sound management of wastes now also reference cement kiln co-processing as a method for PFAS destruction [12]. The life cycle of any resource should be a closed loop of use and reuse or, if that is not practical, then at least have a disposal end point that is benign. 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.

Calcium-catalysed cement kiln co-processing for destruction of PFAS waste and the capture of the fluorine can be considered as a pseudo-circular economy (Figure 5) at least resulting in the return of fluorine to an inert, non-toxic state.

Figure 5. Cement kiln destruction of PFAS achieves a quasi-circular life cycle.

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 (2012) “Technical guidelines on the environmentally sound co-processing of hazardous wastes in cement kilns”.
[4] Basel Convention (2012) In the case of hazardous wastes with a content of more than 1 per cent halogenated organic substances (expressed as chlorine), the temperature should be raised to 1100°C for at least two seconds. Under the United States TSCA disposal of PCBs requires a temperature of 1200°C and 2 seconds retention time (at 3 per cent excess oxygen in the stack gas).
[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.
[12] UN Basel Convention (2023) “General technical guidelines on environmentally sound management of wastes“ UNEP/CHW.16/CRP.12, Geneva CoP, May 2023.