Choice of method for CO2 emissions from primary

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ALUMINIUM PRODUCTION

During normal operations, aluminium is produced at the cathode and carbon is consumed at the anode per the electrolytic reduction reaction:

Most carbon dioxide emissions result from the electrolysis reaction of the carbon anode with alumina (Al2O3). The consumption of prebaked carbon anodes and Soderberg paste is the principal source of process related carbon dioxide emissions from primary aluminium production. Other sources of process related carbon dioxide emissions associated with Prebake anode baking account for less than 10 percent of the total non-energy related carbon dioxide emissions.

The reactions leading to carbon dioxide emissions are well understood and the emissions are very directly connected to the tonnes of aluminium produced through the fundamental electrochemical equations for alumina reduction at a carbon anode and oxidation from thermal processes. Both of these fundamental processes

2 Emissions from the combustion of fossil fuels associated with primary aluminium production, bauxite mining, bauxite ore refining, and aluminium production from recycled sources are covered in Volume 2: Energy. Also, carbon dioxide emissions associated with production of electricity from fossil fuel combustion to produce aluminium are also covered in Volume 2.

3 Including Point Feed Prebake and Bar Broken Prebake cells.

4 A 2004 IAI survey found no evidence of SF6 being emitted from primary aluminium smelting through the Hall-Heroult electrolytic production process.

producing carbon dioxide are included in process parameters routinely monitored at the production facilities, the net anode carbon consumed for Prebake facilities, or anode paste consumption for Soderberg facilities.

For the CO2 emissions calculation, production data require technology differentiation as Soderberg or Prebake.There is no need for further differentiation as to the specific type of Soderberg or Prebake technology.

The decision tree shown in Figures 4.11 describes good practice in choosing the CO2 inventory methodology appropriate for national circumstances.

Figure 4.11 Decision tree for calculation of CO2 emissions from primary aluminium production

Figure 4.11 Decision tree for calculation of CO2 emissions from primary aluminium production

Flussdiagramm Beispiel

Note:

1. See International Aluminium Institute, The Aluminium Sector Greenhouse Gas Protocol, 2005.

2. See Volume 1 Chapter 4, Methodological Choice and Identification of Key Categories (noting Section 4.1.2 on limited resources), for discussion of key categories and use of decision trees.

3. For CO2 emissions calculation, the production data requires technology differentiation as Soderberg or Prebake. There is no need for further differentiation as to the specific type of Soderberg or Prebake technology.

Note:

1. See International Aluminium Institute, The Aluminium Sector Greenhouse Gas Protocol, 2005.

2. See Volume 1 Chapter 4, Methodological Choice and Identification of Key Categories (noting Section 4.1.2 on limited resources), for discussion of key categories and use of decision trees.

3. For CO2 emissions calculation, the production data requires technology differentiation as Soderberg or Prebake. There is no need for further differentiation as to the specific type of Soderberg or Prebake technology.

Tier 1 method for CO2 emissions

The Tier 1 method for calculating CO2 emissions uses only broad cell technology characterizations (Prebake or Soderberg) as a lower order estimate of CO2 emissions from aluminium production. Given the uncertainty associated with the Tier 1 method, it is good practice to use higher tier methods if CO2 from primary aluminium is a key category.

Total CO2 emissions are calculated according to Equation 4.20.

Equation 4.20

Process CO2 emissions from anode and/or paste consumption (Tier 1 method)

Where:

ECO2 = CO2 emissions from anode and/or paste consumption, tonnes CO2

EFP = Prebake technology specific emission factor (tonnes CO2/tonne aluminium produced)

MPP = metal production from Prebake process (tonnes Al)

EFS = Soderberg technology specific emission factor (tonnes CO2/tonne aluminium produced) MPS = metal production from Soderberg process (tonnes Al)

Tier 2 or Tier 3 methods for CO2 emissions

For both the Prebake and Soderberg processes CO2 emissions are calculated using a mass balance approach that assumes that the carbon content of net anode consumption or paste consumption is ultimately emitted as CO2. The Tier 2 methods for both Prebake and Soderberg processes make use of typical industry values for impurities while the Tier 3 methods uses actual concentrations of impurities. The choice of method between the Tier 2 and Tier 3 method will depend on whether anode or paste composition data are available at the individual plant level.

CO2 emissions for Prebake cells (CWPB and SWPB):

The CO2 emissions for the Tier 2 and the Tier 3 method for Prebake cells are calculated according to Equation 4.21. Tier 3 requires specific operating facility data for all the components in Equation 4.21, whereas Tier 2 is based on default values for some of the components. Section 4.4.2.2 below provides more details on using these parameters.

Equation 4.21

CO2 EMISSIONS FROM PREBAKED ANODE CONSUMPTION (TIER 2 AND TIER 3 METHODS)

Where:

ECO2 = CO2 emissions from prebaked anode consumption, tonnes CO2 MP = total metal production, tonnes Al

NAC = net prebaked anode consumption per tonne of aluminium, tonnes C/ tonne Al Sa = sulphur content in baked anodes, wt % Asha = ash content in baked anodes, wt %

44/12 = CO2 molecular mass: carbon atomic mass ratio, dimensionless

Equation 4.21 should be applied to each Prebake smelter in the country and the results summed to arrive at total national emissions. It is possible to use a hybrid Tier 2/3 approach if data on ash or sulphur content are not available for each smelter.

Emissions from the combustion of fossil fuels used in the production of baked anodes are covered in Volume 2: Energy. However, two other sources of CO2 emissions are associated with anode baking furnaces - the combustion of volatile matter released during the baking operation and the combustion of baking furnace packing material (coke). Equations 4.22 and 4.23 can be used for the calculation of such emissions.5

5 For additional information on the application of these equations to estimate emissions from combustion of volatile matter, see the IAI Greenhouse Gas Protocol (IAI, 2005a).

Where:

ECO2 = CO2 emissions from pitch volatiles combustion, tonnes CO2 GA = initial weight of green anodes, tonnes Hw = hydrogen content in green anodes, tonnes BA = baked anode production, tonnes WT = waste tar collected, tonnes

Equation 4.23

CO2 EMISSIONS FROM BAKE FURNACE PACKING MATERIAL (TIER 2 AND TIER 3 METHODS)

100 12

Where:

ECO2 = CO2 emissions from bake furnace packing material, tonnes CO2

PCC = packing coke consumption, tonnes/tonne BA

BA = baked anode production, tonnes

Spc = sulphur content in packing coke, wt %

Ashpc = ash content in packing coke, wt %

CO2 emissions for Soderberg cells (VSS and HSS):

The CO2 emissions for the Tier 2 and the Tier 3 method for Soderberg cells are calculated according to Equation 4.24. Tier 3 requires specific operating facility data for all the components in Equation 4.24, whereas Tier 2 is based on default values for some of the components. Section 4.4.2.2 below provides details on parameters to be used:.

Equation 4.246

CO2 EMISSIONS FROM PASTE CONSUMPTION (TIER 2 AND TIER 3 METHODS)

ECO 2

1000 100 100

100 - BC • PC • MP • Sc + Ashc - MP • CD ^ 44

100 100 J 12

Where:

ECO2 = CO2 emissions from paste consumption, tonnes CO2

MP = total metal production, tonnes Al

PC = paste consumption, tonnes/tonne Al

CSM = emissions of cyclohexane soluble matter, kg/tonne Al

BC = binder content in paste, wt %

Sp = sulphur content in pitch, wt %

6 An acceptable alternative method is to use the parameter of 'pitch coking' in lieu of deducting measured or default values for Sp, Hp, Ashp and CSM from Equation 4.24. The pitch coking value is a commonly determined parameter for many facilities with Sederberg cells and standard methodology for performing the pitch coking test is described in ASTM D2416.

Ashp = ash content in pitch, wt % Hp = hydrogen content in pitch, wt % Sc = sulphur content in calcined coke, wt % Ashc = ash content in calcined coke, wt %

CD = carbon in skimmed dust from Soderberg cells, tonnes C/tonne Al

44/12 = CO2 molecular mass : carbon atomic mass ratio, dimensionless

Equation 4.24 should be applied to each smelter in the country using the Soderberg process and the results summed to arrive at total national emissions. It is possible to use a hybrid Tier 2/3 approach if data on ash or sulphur content are not available for each smelter.

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