Industrial Processes (GPGAUM-Corr.2001.01, 15 June 2001)

Chapter 3

Tier 3b Method – Smelter-specific relationship between emissions andoperating parameters based on field measurements
This method uses periodic measurements to establish a smelter-specific relationship between operating
parameters (i.e. frequency and duration of anode effects or Anode Effect Overvoltage) and emissions of CF4 andC2F6. Once established, the relationship can be used along with process data collected on an on-going basis, toestimate emissions factors over time. These emission factors are multiplied by smelter-specific production(tonnes) to estimate smelter emissions. Emissions estimates will be aggregated across smelters to estimate
national emissions.
The following estimation relationships can be used:
Slope Method: This method uses a linear least squares relationship between anode effect (AE) minutes per
cellday17 and emissions, expressed as an emission factor (EF):EQUATION 3.10
EF (kg CF4 or C2F6 per tonne of Al) = Slope • AE min / cellday

To develop an accurate estimate of the slope, simultaneous measurements of emissions and collection of anodeeffect data over an appropriate period of time are required. The Slope Method is a variant of the Tabereauxapproach described in the IPCC Guidelines:
BOX 3.3
TABEREAUX APPROACH

Slope = 1.698 • (p / CE)

andAE min / cellday = AEF • AEDWhere:
p = Average fraction of CF4 in the cell gas during anode effects for the CF4 slope
orAverage fraction of C2F6 in the cell gas during anode effects for the C2F6 slope
CE = Current Efficiency for the aluminium production process, expressed
as a fraction rather than as a percentage
AEF = Number of anode effects per cellday
AED = Anode effect duration in minutes

Pechiney Overvoltage Method: This method uses the Anode Effect Overvoltage as the relevant processparameter. The Anode Effect Overvoltage is the extra cell voltage, above 8V, caused by anode effects, when
averaged over a 24-hour period (mV/day). The correlation formula was derived from measurements of PFC
generation at smelters with Pechiney technology, expressed as an emission factor (EF):EQUATION 3.11
EF (kg CF4 or C2F6 per tonne of Al) = Over-Voltage Coefficient • AEO / CE
Where:AEO=

Anode effect over-voltage in mV/cellday

CE=

Aluminium production process current efficiency expressed in percent

17 The ‘cellday’ term really means ‘the number of cells operating multiplied by the number of days of operation’. At asmelter this would more usually be calculated (for a certain period of time, e.g. a month or a year) using ‘the average numberof cells operating across the smelter over a certain period of days multiplied by the number of days in the period’.

3.42

IPCC Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories

Chapter 3

Industrial Processes (GPGAUM-Corr.2001.01, 15 June 2001)

Tier 2 Method – smelter-specific relationship between emissions & operating
parameters based on default technology-based slope and over-voltage
coefficients
If measurement data are not available to determine smelter-specific Slope or Overvoltage coefficients, default
coefficients may be used together with smelter-specific operating parameters. Good practice default coefficientsare listed in Table 3.9, Default Coefficients for the Calculation of PFC Emissions from Aluminium Production(Tier 2 Methods).

Tier 1 Method – Production-based emission factors
The simplest estimation method is to multiply default emission factors by aluminium production. When the only
smelter-specific activity data available are metal production statistics, it is good practice to use default emissionfactors (see Choice of Emission Factors).
Default slope coefficients (Tier 2 method) and emission factors (Tier 1 method) were developed using available
data from International Primary Aluminum Institute (IPAI) surveys and other field measurement data (Bouzat etal., 1996, Leber et al., 1998, Marks, 1998, Roberts et al., 1994a and 1994b, Kimmerle et al., 1998, Marks et al.,2000). The limited information available for some data required expert judgement regarding the suitability ofsome measurement sets. As an example, the Tier 1 Method Horizontal Stud Søderberg (HSS) default emission
factors were calculated using 1991 data, rather than 1990 data.
When possible, the consistency of available measurement data surveyed over different time periods and atdifferent smelters should be used to confirm a significant degree of confidence about the magnitude and trend of
the emission factors and coefficients.

CHOICE OF EMISSION FACTORS
Tier 3b Method
For this method, it is good practice to determine the coefficients of the models by using smelter-specificmeasurements. The smelter-specific coefficients should be based on comprehensive measurements of CF4 andC2F6 emissions with simultaneous collection of process data. This means that emission factors should reflect thespecific conditions of a plant and the technologies involved. Emission factors are to be measured over a periodof time that reflects the variability of the process and accounts for both emissions captured by the fumecollection system and fugitive emissions (if this sub-source category is significant, compared with emissions
captured by the fume control system). Box 3.2, Direct Measurement Techniques, gives guidance on some
aspects of direct measurement techniques. It is good practice to follow these approaches in implementing asampling and measurement program.18

Tier 2 Method
If smelter-specific measurements are unavailable, default coefficients may be used. Default coefficients areprovided by technology type in Table 3.9, Default Coefficients for the Calculation of PFC Emissions from
Aluminium Production (Tier 2 Methods).19 The default coefficients must be applied by technology type withineach smelter. If more than one technology type is being used at a smelter, the appropriate default coefficients
must be applied separately for each technology segment.

18 Other methods may incorporate an explicit factor representing a contribution from newly started cells. The smelter-specific slope coefficients developed under Tier 3b will incorporate these emissions.
19 Current measurement programs are improving the quantity and quality of available data. These data should be availableby early 2000, and may supersede the values provided in Table 3.9.

IPCC Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories

3.43

Industrial Processes (GPGAUM-Corr.2001.01, 15 June 2001)

Chapter 3

TABLE 3.9
DEFAULT COEFFICIENTS FOR THE CALCULATION OF PFC EMISSIONS FROM ALUMINIUM PRODUCTION(TIER 2 METHODS)
Technologya

CWPB
SWPB

Overvoltage coefficientb

Slopeb,d [(kg PFC/tAl) / (AE-Minutes/cellday)]
CF4

Uncertainty

C2F6

0.14±0.009

0.29

±0.02
±0.02

g

VSS0.068

HSS0.18 f[(kg PFC/tAl ) / (mV/cellday)]

Uncertainty

CF4C2F6

0.018±0.004

1.9NA0.029

c±0.011.9NA0.003

g±0.001

See note e–––0.018

aCentre Worked Prebaked (CWPB), Side Worked Prebaked (SWPB), Vertical Stud Søderberg (VSS), Horizontal Stud Søderberg (HSS).
b
Source: IPAI, EPA field measurements, and other company measurement data.
c
There is inadequate data for establishing a slope coefficient for C2F6 emissions from SWPB cells based on measurement data; therefore adefault of one-tenth of the CF4 coefficient is good practice, consistent with the IPCC Guidelines.
dEmbedded in each Slope coefficient is an assumed emissions collection efficiency as follows: CWPB 95%, SWPB 90%, VSS 85%, HSS90%. These collection efficiencies have been assumed based on expert opinion. While collection efficiency for HSS cells may vary, thecompany measurement data used for calculation of these coefficients are consistent with a collection efficiency of at least 90%.
eOvervoltage coefficients are not relevant to VSS and HSS technologies.
fThe HSS Slope coefficients are based on 1991 IPAI survey data.
gFurther work on emission measurement and uncertainty analysis should be pursued for VSS. These default coefficients are based on asmall number of data, and it is expected that the uncertainty might be higher than for other coefficients (Bjerke, 1999a, and Bjerke et al.,1999b).
NA = not available.

Tier 1 Method
The simplest method is to multiply default emission factors by aluminium production. Default emission factors
by technology-type are available in the IPCC Guidelines. It is good practice to base these factors on recently
updated measurements, and revised default emission factors and associated uncertainty ranges are presented inTable 3.10, Default Emission Factors and Uncertainty Ranges for the Calculation of PFC Emissions fromAluminium Production (by Technology Type), below. As the Tier 1 method is the most uncertain of the three
approaches, it is good practice to use default emission factors as a method of last resort, when only metal
production statistics are available.
TABLE 3.10
DEFAULT EMISSION FACTORS AND UNCERTAINTY RANGES FOR THE CALCULATION OF PFC EMISSIONSFROM ALUMINIUM PRODUCTION (BY TECHNOLOGY TYPE)
Technology

CF4

C2F6kg/tonne Ale

Uncertainty Rangea

kg/tonne Ale

Uncertainty Rangea

CWPB

0.31

0.0003-1.3

0.04

0.00004-0.2

SWPB

1.7

0.8-3.8

0.17 b

0.08-0.4

0.4-1.1c

0.04-0.1

d0.00006-0.13

c

VSS

0.61

HSS

d

0.6

0.0006-1.4

0.061
0.06

a

Uncertainty was estimated by the IPCC Washington expert meeting group to a 95% confidence interval on the basis of the variance ofanode effect minute data from IPAI Survey Data for 1990 (or 1991 for HSS) for each technology type.
There are inadequate data for establishing an emission factor for C2F6 emissions from SWPB cells based on measurement data; thereforea default of one-tenth of the CF4 coefficient is good practice, consistent with the IPCC Guidelines.
cThe VSS default emission factors are based on IPAI, EPA field measurements, and other 1990 company measurement data. These default
factors are based on a small number of data, and it is expected that the uncertainty might be higher than for other factors (Bjerke, 1999a,and Bjerke et al., 1999b).
dThe HSS default emission factors are based on 1991 IPAI survey data.
beSource: IPAI, EPA field measurements, and other 1990 company measurement data, except for HSS that is based on 1991 data (Bjerke,1999a, and Bjerke et al., 1999b).

It is good practice to apply the default emission factors that are based on 1990 (or 1991 for HSS) median anodeeffect frequency and duration data, for all years for which there are no process (anode effect) data unless it canbe demonstrated otherwise.

3.44

IPCC Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories