Pfc Emissions from Primary Aluminium Production

The primary aluminium production process has been identified as the largest anthropogenic source of emissions of two perfluorocarbons (PFCs): tetrafluoromethane (CF4) and hexafluoroethane (C2F6). Primary aluminium is produced using the Hall-Héroult electrolytic process, where the smelting pot itself acts as the electrolysis cell during the reduction process. When the alumina ore content of the electrolytic bath falls below critical levels required for electrolysis, rapid voltage increases occur, termed “anode effects”. Anode effects cause carbon from the anode and fluorine from the dissociated molten cryolite bath to combine, producing CF4 and C2F6. The frequency and duration of anode effects depend primarily on the pot technology and operating procedures. Emissions of CF4 and C2F6, therefore, vary significantly from one aluminium smelter to the next, depending on these parameters. The Revised 1996 IPCC Guidelines for National Greenhouse Gas Inventories (IPCC Guidelines) recommend multiplying aluminium production by a CF4 emission factor (i.e., CF4 emissions per unit of aluminium production). To estimate the emission factor, different methods have been proposed based on the availability of data. The Tier 1 method uses PFC emission factors that are based on actual emission measurements. This is the most accurate method because PFC emissions vary so significantly from one smelter to the next. Where actual measurements are not feasible due to economic or time constraints, however, a Tier 2 approach can be applied in which operating parameters and an industry model are used to derive emission factors. Where necessary, the IPCC Guidelines provide default parameter values by technology type to fit the model. When relevant operating parameters are not available, a simple Tier 3 method can be used, in which aluminium production is multiplied by a default emission factor that is specific to the cell technology. The IPCC provides default emission factors for four technology types, and recommends a default rate for C2F6 emission that is 1/10 that of CF4. Emission estimates will be highly uncertain unless actual emission measurements have been made at the smelter level because emissions of CF4 and C2F6 vary so significantly from one aluminium smelter to the next, depending on cell type and anode effect parameters. Most aluminium producing countries have a relatively small number of primary aluminium facilities, so it may be possible to develop smelter-specific emission factors. The most common PFC estimation methods available to countries include: the Tabereaux Method (Modified Faraday’s Law Method), the Pechiney Over-voltage Method, and the Slope Method. These relationships have been proven to be useful predictors of PFC emission rates for particular smelter technologies and ranges of operating conditions. Default parameters are available for these relationships based on field measurements of PFC emissions. Smelters may adopt one of the above relationships and use the default parameters if it is demonstrated to be representative for their plant characteristics and operating conditions. It is recommended that smelters conduct measurements to estimate the parameters that reflect their particular operating conditions. When developing measurement programmes, smelters should ensure that the sampling scale provides a comprehensive measurement for the entire facility and is as compatible as possible with the operational data. In addition, if there is high daily variability in emission rates, enough daily samples should be taken to assure that the range of emission rates for the current operating conditions is covered. The analytical system must also be appropriate for the sampling method, stable, and free of (or corrected for) interferences. Aluminium production data are necessary to estimate PFC emissions from aluminium smelting regardless of the estimation method chosen. Countries should obtain smelter-level data on an annual basis, so that they can use smelterspecific emission factors. At a plant-level, actual production data should be obtained directly from producers. Since aluminium production data may be considered proprietary by producers, it may be necessary to ensure that it is not released to the public, or to publish it in such a way so that individual plants are not identified. It appears that PFC emissions associated with aluminium production are reported by many emitting countries. The methods, assumptions and data used to develop these estimates are not clear, transparent or well-