Choice of method for PFCs

During electrolysis, alumina (Al2O3) is dissolved in a fluoride melt comprising about 80 weight percent cryolite (Na3AlF6). Perfluorocarbons (CF4 and C2F6 collectively referred to as PFCs) are formed from the reaction of the carbon anode with the cryolite melt during a process upset condition known as an 'anode effect'. An anode effect occurs when the concentration of alumina in the electrolyte is too low to support the standard anode reaction.

Box 4.2

Anode effect description

An anode effect is a process upset condition where an insufficient amount of alumina is dissolved in the electrolyte, causing voltage to be elevated above the normal operating range, resulting in the emission of PFC-containing gases.

Both Tier 2 and Tier 3 for PFCs are based on plant-specific process data for anode effects, which are regularly collected. In choosing a method for PFCs, it should be noted that the uncertainty associated with higher tier methodologies is significantly lower than that for Tier 1, and therefore Tier 2 and Tier 3 are strongly recommended if this is a key category. Depending on the production technology type, the uncertainty of the methods for PFCs ranges from several hundred percent for the Tier 1 method to less than twenty percent for the Tier 3 method. The Tier 3 methodology for PFC inventory should be utilized with slope or overvoltage coefficients calculated from measurement data obtained using good measurement practices (U.S. EPA and IAI, 2003). Communication with primary aluminium producers will determine the availability of process data, which, in turn dictates the method used to calculate emissions. Plants routinely measure anode effect performance as anode effect minutes per cell-day or anode effect overvoltage. PFC emissions are directly related to anode effect performance via a coefficient, either the slope coefficient or the overvoltage coefficient.

The decision tree shown in Figure 4.12 describes good practice in choosing the PFC inventory methodology appropriate for national circumstances. For high performing facilities that emit very small amounts of PFCs, the Tier 3 method will likely not provide a significant improvement in the overall facility GHG inventory in comparison with the Tier 2 Method.8 Consequently, it is good practice to identify these facilities prior to selecting methods in the interest of prioritising resources. The parameters that identify these high performing facilities depend on the type of process data collected by the facility. High performing facilities are those that operate with less than 0.2 anode effect minutes per cell day when anode effect minutes are measured. When overvoltage is recorded, high performing facilities operate with less than 1.4 mV overvoltage. In addition, for these high performing facilities accurate measurement of the Tier 3 PFC coefficient is difficult because the very low frequency of anode effects requires an extended time to obtain statistically robust results. The status of a facility as a high performing facility should be assessed annually because economic factors, such as the restarts of production lines after a period of inactivity, or, process factors, such as periods of power curtailments might cause temporary increases in anode effect frequency. In addition, over time, facilities that might not at first meet the requirements for high performers may become high performing facilities through implementation of new technology or improved work practices. Note that in all cases, applying different Tiers for different years will require careful implementation to ensure time series consistency.

For all other facilities, the Tier 3 approach is preferred because plant-specific coefficients will lead to estimates that are more accurate. If no PFC measurements have been made to establish a plant-specific coefficient, the Tier 2 Method can be used until measurements have been made and Tier 3 coefficients are established. Countries can use a combination of Tier 2 and Tier 3 depending on the type of data available from individual facilities.

Tier 1 method: Use of technology based default emission factors

The Tier 1 method uses technology-based default emission factors for the four main production technology types (CWPB, SWPB, VSS and HSS). PFC emissions can be calculated according to Equation 4.25. The level of uncertainty in the Tier 1 method is much greater because individual facility anode effect performance, which is the key determinant of anode effects and thus PFC emissions, are not directly taken into account. Tier 1 can be consistent with good practice only when PFCs from primary aluminium is not a key category and when pertinent process data are not available from operating facilities.

8 The levels for the process parameters that define high performing facilities for PFC emissions are the combined result of the magnitude of, and, the uncertainty in the Tier 2 coefficient. The levels are calculated by using the positive and negative extremes of the 95% confidence limits for the Tier 2 coefficient as a proxy for the range of likely values for Tier 3 coefficients for these facilities. The potential difference is then assessed on the overall greenhouse gas emissions from a production facility considering both PFC and CO2 emissions. When facilities operate at or below the anode effect process parameter levels noted here for high performing facilities, the impact of moving from the Tier 2 method for PFCs to the Tier 3 method would not result in a change greater than 5% in overall GWP weighted GHG emissions. PFC emissions from high performing facilities account for less than 3% of global PFC emissions based on IAI 2004 anode effect survey data.

Equation 4.25 PFC emissions (Tier 1 method)

Where:

ECF4 = emissions of CF4 from aluminium production, kg CF4

EC2f6 = emissions of C2F6 from aluminium production, kg C2F6

EFCF4 l = default emission factor by cell technology type i for CF4, kg CF4/tonne Al

EFC2F6 l = default emission factor by cell technology type i for C2F6, kg C2F6/tonne Al

MPj = metal production by cell technology type i, tonnes Al

Tier 2 and Tier 3 methods: based on anode effect performance

There are two different equations for estimating individual plant CF4 emissions, which are both based on the relationship between anode effect and performance. These are the slope and overvoltage coefficient equations. Both types of coefficients are based on direct measurements of PFCs. Tier 2 makes use of an average coefficient from measurements at numerous facilities while Tier 3 is based on measurements at the individual facility. Because the process mechanisms that produce PFC emissions are similar for CF4 and C2F6, the two gases should be considered together when estimating PFC emissions. C2F6 emissions are calculated in all the methods described herein as a fraction of CF4 emissions.

With an established relationship between anode effect process data and PFC emissions, process data collected on an on-going basis can be used to calculate PFC emissions in lieu of direct measurement of PFCs. The choice between the two estimation relationships depends on the process control technology in use. Equation 4.26 should be used when anode effect minutes per cell day are recorded and Equation 4.27 should be used when overvoltage data are recorded.

Slope Coefficient: The slope coefficient represents the kg of CF4 per tonne of aluminium produced, divided by anode effect minutes per cell-day9. Since PFC emissions are measured per tonne of aluminium produced, it includes the effects of cell amperage and current efficiency, the two main factors determining the amount of aluminium produced in the cell. Equation 4.26 describes the slope method for both CF4 and C2F6.

Equation 4.26

PFC EMISSIONS BY SLOPE METHOD (TIER 2 AND TIER 3 METHODS)

Where:

ECF4 = emissions of CF4 from aluminium production, kg CF4

EC2F6 = emissions of C2F6 from aluminium production, kg C2F6

SCF4 = slope coefficient for CF4, (kg CF4/tonne Al)/(AE-Mins/cell-day)

AEM = anode effect minutes per cell-day, AE-Mins/cell-day

MP = metal production, tonnes Al

FC2F6/CF4 = weight fraction of C2F6/CF4, kg C2F6/kg CF4

9 The term 'cell-day' refers to the number of cells operating multiplied by the number of days of operation.

Overvoltage Coefficient: Some process control systems characterize anode effects by calculating an Anode Effect Overvoltage10 (AEO) statistic. AEO is defined as the extra cell voltage above the target operating voltage, and this parameter has been shown to be a good predictor of PFC emissions when recorded by the process control system. The AEO process control technology is in use at many modern smelters. AEO is calculated by summing the product of time and voltage above the target operating voltage and dividing this figure by the time over which data were collected.

Pfc Product Statistic

Where:

ECF4 = emissions of CF4 from aluminium production, kg CF4 EC2F6 = emissions of C2F6 from aluminium production, kg C2F6 OVC = Overvoltage coefficient for CF4, (kg CF4/tonne Al)/mV AEO = anode effect overvoltage, mV

CE = aluminium production process current efficiency expressed, percent (e.g., 95 percent) MP = metal production, tonnes Al F C2F6/CF4 = weight fraction of C2F6/CF4, kg C2F6/kg CF4

10 Computer control systems report either 'positive' or 'algebraic' overvoltage depending on the version of software used. Use of the expression 'overvoltage' should not be confused with the classical electrochemical terminology, which usually means the extra voltage needed for an electrochemical reaction to occur.

Figure 4.12 Decision tree for calculation of PFC emissions from primary aluminium production

Figure 4.12 Decision tree for calculation of PFC emissions from primary aluminium production

Policy Factors

Note:

1. High performing facilities emit so little PFCs that no significant improvement can be expected in the overall facility GHG inventory by using the Tier 3 method rather than the Tier 2 method. High performing facilities are defined, based on what process data are collected, as those that operate with less than 0.2 anode effect minutes per cell day, or, less than 1.4 mV overvoltage. In such facilities the improvement in accuracy in facility GHG inventory is less than 5% when moving from Tier 2 to Tier 3 methods for PFCs.

2. Good practices for obtaining facility specific PFC equation coefficients are detailed in the IAI GHG Protocol (IAI, 2005).

3. In this case, Tier 2 method should be used until site-specific Tier 3 coefficients become available and the Tier 3 method employed unless PFC emissions become immaterial, in which case facilities can choose to use either the Tier 2 or Tier 3 method.

4. 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.

5. For key categories, it is good practice to collect anode effect process data and production activity data at the individual production facility level.

6. Primary aluminium facilities regularly record activity data including metal production and anode effect process data facilitating, at a minimum, Tier 2 calculation method. Errors of magnitude of x10 can result from use of Tier 1 methods for PFCs.

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