A single process upset in electrolysis — the anode effect — can multiply an aluminium smelter's embedded emissions several times over. Here's how it's actually calculated.
Last verified: 02 September 2026
Primary aluminium is produced by electrolysis — passing a large current through molten alumina dissolved in a cryolite bath. When the alumina concentration in that bath drops too low, the cell voltage spikes in an event called the anode effect. During an anode effect, the electrolysis reaction shifts and starts producing perfluorocarbon gases — mainly CF4, with smaller amounts of C2F6 — instead of the intended aluminium and oxygen.
CF4 has a Global Warming Potential of 6,630 and C2F6 has a GWP of 11,100 (AR4, per EU Reg 2023/956 Annex III Table 1). A relatively small mass of these gases converts to a large tCO2e figure — anode-effect frequency and duration matter far more to total embedded emissions than a small process detail might suggest.
Actual PFC emissions can be calculated either of two ways, both converting to CO2e using the same GWP constants:
The slope method directly uses measured anode-effect frequency and duration (minutes per cell-day) — the more directly observable of the two if a smelter tracks anode effects as an operational metric already. The overvoltage method instead measures the voltage excess during those events, useful where minute-by-minute anode-effect logging isn't available.
The production volume and anode-effect minutes below are illustrative round numbers — only the formula structure and the CF4 GWP constant (6,630) are asserted as regulatory fact. Take two smelters producing the same 1,000 tonnes of aluminium, differing only in anode-effect control:
| Well-controlled smelter | Poorly-controlled smelter | |
|---|---|---|
| Anode effect minutes/cell-day | 3 | 8 |
| CF4 produced | 300 kg (0.3 t) | 800 kg (0.8 t) |
| CO2e (× GWP 6,630) | ≈ 1,989 tCO2e | ≈ 5,304 tCO2e |
Same production volume, same aluminium output — but the poorly-controlled smelter's PFC-related embedded emissions come out roughly 2.7× higher. This is exactly why using a smelter's actual, verified anode-effect data (rather than a conservative default value) can materially change the CBAM obligation on the same tonnage of aluminium.
A process upset in aluminium electrolysis where the alumina concentration in the electrolyte drops too low, causing the cell voltage to spike and triggering the formation of perfluorocarbon gases (CF4 and C2F6) — potent greenhouse gases with GWP values in the thousands.