Study predicts growth of HFO emissions and TFA in China

September 07, 2023

In a new study Chinese researchers have projected, for the first time, China’s annual and cumulative emissions of HFOs, HFO-1234yf and trifluoroacetic acid (TFA).

HFO-1234yf, the most commonly used HFO, oxidizes almost entirely to TFA in the atmosphere by reacting with OH radicals, with the TFA then descending in rainfall and infiltrating the environment. HFO-1234yf and TFA are both considered to belong to the category of PFAS (per and polyfluoroalkyl substances), or “forever chemicals” that don’t break down in the environment, according to the Organization for Economic Co-operation and Development (OECD).

The study – “Projections of National-Gridded Emissions of Hydrofluoroolefins (HFOs) in China” – was published in the May 26, 2023, issue of Environmental Science & Technology. It was cited by Harvard University Ph.D student Heidi Pickard in her discussion of increasing TFA burdens on the environment at the ATMOsphere America 2023 conference in June, organized by ATMOsphere (publisher of R744.com).

According to the study, China’s annual HFO emissions are estimated to increase from 1.7kt (with a range of 1.3–2.3kt) to 148.8kt (111.4–185.4kt) between 2024 and 2060 with cumulative emissions of 2.8Gt (2.0–3.5Gt). High HFO emissions would be distributed mainly in the North China Plain and the eastern and coastal areas.

HFO-1234yf contributes most HFO emissions with cumulative emissions of 1.7Gt during 2024–2060, while the cumulative amount of TFA deposition from HFO-1234yf emissions will reach 0.4–1.0Gt the study says. Cumulative reduced HFC emissions are evaluated to be 5.4Gt CO2e.

The use and environmental impacts of HFOs – particularly HFO-1234yf and HFO-1234ze(E) – are of great concern due to the “rapid degradation of HFOs to produce persistent and phytotoxic [TFA],” says the study.

“TFA is a ubiquitous and highly persistent contaminant in the environment, with rapidly increasing concentrations in water bodies and in precipitation up to concentrations in the [μg/l] range over the past two decades,” adds the study, noting that it can result in the “contamination of water resources (e.g., drink[ing] water and groundwater)” and is found to be difficult to remove by water treatment. Other studies point to a significant increase in TFA concentrations in water bodies in the range of 3–17 times over the past 20 years.

This study calls for the periodic monitoring of TFA concentration, particularly in some sensitive ecosystems and terminal water bodies that are more susceptible to the effects of TFA, as well as drinking water for human health.

Long-term projected HFO emission inventories “can provide scientific support for evaluating the environmental risks of HFOs and developing HFC phaseout pathways for addressing climate change,” the study says.

In particular, the long-term impact of HFO-1234yf emissions on TFA deposition needs to be identified “with atmospheric transport models and hydrological models, especially for developed and industrial regions,” says the study.

In addition, the study advises that the contribution of other HFOs such as HFO-1234ze to TFA deposition and other environmental impacts needs to be further confirmed; this requires more understanding of the reaction mechanism of trifluoroacetaldehyde, (CF3CHO), the main degradation product of HFO-1234ze. CF3CHO, the study says, can be partially degraded to TFA and is mainly removed by photolysis (decomposition by light). Moreover, researchers differ on the yield of HFC-23, which has a GWP of 12,690, by photolysis of CF3CHO.

MAC is the biggest consumer

This study covers three sectors: mobile air-conditioning (MAC), commercial air-conditioning (CAC) and polyurethane foam (PU). It looks at six HFCs (HFC-134a, HFC-125, HFC-32, HFC-245fa, HFC-152a and HFC-365mfc), and four HFOs (HFO-1234yf, HFO-1234ze(E), HCFO-1233zd(E) and HFO-1336mzz(Z)).

MAC will be the major consumer of HFO-1234yf, accounting for 65% of the total HFO-1234yf emissions during 2025–2060, according to the study. For the MAC and CAC sectors, HFC and HFO emissions mainly occur during initial charge, operation, servicing and disposal of equipment, with operation and disposal accounting for 50% and 26%, respectively, of the cumulative HFOs emissions in 2025–2060. Like other studies, this study assumed no refrigerant recycling for scrap equipment in the disposal process.

The study accepts that production of TFA from HFO-1234yf “increases linearly with the increment of HFO-1234yf emissions.” Therefore, the relationship between HFO-1234yf emissions and the increment of TFA deposition in China was adopted to evaluate the ecological impact of the future use of HFO-1234yf.

However, the study did refer to other studies that noted the importance of additional sources of TFA, such as the manufacture and use of TFA and the degradation of compounds containing trifluoromethyl groups (such as pesticides, pharmaceuticals and fluoropolymers).

According to the study, HFO-1234yf and HFO-1234ze(E) will be the primarily used HFOs for the MAC and CAC sectors with consumption from 8.2 to 76.6 kt and 6.2 to 39.5 kt, respectively, during 2025–2045. Cumulative consumption of HFOs during 2025–2060 will be 3,653.7 kt, with 55% and 28% for HFO-1234yf and HFO-1234ze(E), respectively.

The projected emissions of HFOs will be on an increasing trend from 2.8 to 148.8 kt in China during 2025–2060. The emissions of HFO-1234yf, which will increase from 1.5 kt in 2025 to 79.0 kt in 2060, are believed to contribute the most to the total HFO emissions. The HFO-1234yf emissions from MAC alone were projected to be 28.8kt in 2040.

The cumulative emissions of HFOs from 2025 to 2060 will reach 2.8 Gt, with HFO-1234yf and HFO-1234ze(E) accounting for 61% (1.7Gt) and 25% (0.7Gt), respectively. Notably, banks of HFOs will be much larger than the total emissions for HFOs, “indicating that the potential HFOs which are not immediately emitted cannot be ignored in the future,” says the study.

Under the Kigali Amendment scenario that calls for the decrease of production and import of HFCs, continuous HFO-1234yf emissions in China during 2024–2060 will result in an increase in the increase of the annual mean TFA deposition rate from 0.007–0.02 to 0.9–1.8 kg/km2/year, and the annual mean TFA rainwater concentrations will increase from 0.01–0.02 to 1.2–2.4 μg/l, says the study. The cumulative amount of TFA deposition from HFO-1234yf emissions in China will reach 0.4–1.0 Gt during 2024−2060.

The study notes that the increase of annual mean TFA rainwater concentration by HFO-1234yf emissions will be well below the no observed effect concentration (NOEC) for aquatic species of 2,500 μg/l and the predicted no effect concentration (PNEC) of 560 μg/l in fresh waters. However, the increase of TFA in areas with high HFO-1234yf emissions “should be much higher than the mean levels,” and the accumulation of TFA in the water bodies “is unknown, as TFA will persist in aquatic environments,” the study says.

The study also pointed out that TFA has been detected in concentrations up to 38 μg/l in precipitation and 140μg/l in a river. Rapidly increasing TFA concentrations in drinking water and groundwater have also been observed, and TFA was detected at a median value of 8.46 ng/ml in the serum of Chinese adults. “The risk of potentially irreversible harm that may result from unregulated use of TFA and its precursors will be of continuous concern in the future,” it says.

The chemical industry addressed the environmental deposition of TFA in an October 2021 study funded by the Global Forum for Advanced Climate Technologies (globalFACT), which represents f-gas producers Chemours, Honeywell, Arkema and Koura (and equipment manufacturer Daikin). The study concludes that “with the current knowledge of the effects of TFA on humans and ecosystems, the projected emissions through 2040 would not be detrimental.” But the study also acknowledges that “the major uncertainty in the knowledge of the TFA concentrations and their spatial distributions is due to uncertainties in the future projected emissions.”