Methodological issues

CHOICE OF METHOD

It is good practice to choose the method using the decision tree shown in Figure 3.17. If the Category 2B9 Fluorochemical Production is identified as key and this subcategory is judged to be significant, inventory compilers should consider whether or not emissions are dominated by the production of a sub-set of chemicals, and focus more sophisticated data collection efforts on production of these chemicals. The number of major producers of these fluorinated greenhouse gases is quite small: in the case of SF6, there are globally about 6 companies with about 10 production facilities world-wide (Preisegger, 1999). The number of smaller producers may grow in the near future, particularly in developing economies. However, a survey of national producers should not be difficult to compile.

Tier 1

In the Tier 1 methodology, a default emission factor, or a similar number derived for the particular country's circumstances, can be used to estimate national production-related emissions of individual HFCs, PFCs, SF6 and other fluorinated greenhouse gases.

Equation 3.41 Tier 1 calculation of production-related emissions

Where:

Ek = production-related emissions of fluorinated greenhouse gas k, kg

EFdefault, k = default emission factor, kg/kg

Pk = total production of fluorinated greenhouse gas k, kg

Problems of confidentiality arising from reporting specific component data can be circumvented by providing a single number for total national emissions of each HFC, PFC and SF6. This may be facilitated if data are collected by a third party and reported only as this total.

Tier 2

The method based on process efficiencies, which works for HFC-23 emissions from HCFC-22 plants, is of less value for other types of plants. This is due in part to the lower inefficiency expected from these other by-product emissions; the uncertainty in measurement of efficiencies is likely to be much greater than the by-product emission factor. Furthermore, a range of by-products may be responsible for process inefficiency (unlike the case for HCFC-22 where one by-product predominates). However, production efficiency data should exist for each process and, in the absence of a more rigorous estimate, the quantity of emissions estimated from process inefficiencies may be used in a qualitative decision as to whether or not these emissions are a significant subcategory under a key category (in which case, Tier 3 methodology is specified).

Tier 3

The Tier 3 methodology is potentially the most accurate estimate and is the sum of factory specific emissions of each by-product fluorinated greenhouse gas determined using standard methods to estimate the composition and flowrate of gas streams actually vented to atmosphere after any abatement technology. In this case:

Equation 3.42

Tier 3 direct calculation of production-related emissions

= Z Z \pijk • fijk [ it means the quantity should be summed over time.]

Where:

Ek = total production-related emissions of fluorinated greenhouse gas k: the sum over all i plants, over all j streams in each plant of the emitted mass flows f and concentrations C integrated over time t.

or, where proxy methodology is used, for example where the emission rate of the by-product is normalised to a more easily (or accurately) measurable parameter, such as feedstock flow rate, as described in Equation 3.35 in Section 3.10.1:

Equation 3.43

Tier 3 proxy calculation of production-related emissions

Ek = ZZ itEijk [ Jt means the quantity should be summed over time.]

Where:

Ek = total production-related emissions of fluorinated greenhouse gas k: Ejk = the emissions of fluorinated greenhouse gas k from each plant and stream determined by the proxy methods, described in Equations 3.38 and 3.39 in Section 3.10.1

Note that, generally, flows are measured volumetrically and should be converted into mass flow (kg/hour) based on the ideal gas law, temperature, pressure and composition, similarly concentration should be converted into compatible units (e.g., kg/kg).

In this case, the flowrates, concentrations and duration should be calculated separately for the periods when the abatement technology is or is not operating and only those that lead to actual emissions should be summed and reported.

CHOICE OF EMISSION FACTORS

Tier 3 relies on measurements of the quantities of individual materials that are released into the atmosphere and neither Tier 2 nor Tier 3 relies on emission factors. For Tier 1, in the absence of abatement measures, a default emission factor of 0.5 percent of production, not counting losses in transport and transfer of materials, is suggested for HFCs and PFCs, based on data supplied to AFEAS (2004). There is a wide range of substances that may potentially be released. However, the AFEAS data showed that the components that were lost during production of a particular fluorochemical had, in general, radiative forcing properties similar to those of the desired fluorochemical. Consequently, for sources that are not significant subcategories under key category, fugitive and by-product emissions are the same and are included in the 0.5 percent emission factor.

In the case of SF6, based on German experience, a default emission factor of 0.2 percent of the total quantity of SF6 produced is suggested for those countries in which the predominant end use does not require highly purified SF6 gas (e.g., electrical equipment, insulated windows) (Preisegger, 1999). Based on experience in Japan, in countries where the major uses require highly purified SF6 gas (e.g., semiconductor manufacturing), the default value should be 8 percent because of handling losses during disposal of residual gas (i.e., the 'heel' that is not used or recycled) in returned cylinders (Suizu, 1999). If national data are available, these should be used, particularly for other materials not specifically listed here.

The default emission factors are based on situations where no abatement measures are employed. If the quantity of gas emitted to the atmosphere is reduced by, for example, thermal oxidation of the vent stream, the quantity emitted should be adjusted to account for the destruction efficiency of the oxidiser and the length of time that it is in service. Based on the experience in the destruction of HFC-23, a default destruction efficiency of 100 percent is suggested but the on-line time of the destruction process will have a greater effect on emissions and should be recorded.

Figure 3.17 Decision tree for emissions of fluorinated greenhouse gases from production processes, applicable to both fugitive and by-product emissions

Figure 3.17 Decision tree for emissions of fluorinated greenhouse gases from production processes, applicable to both fugitive and by-product emissions

Policy Factors

Note:

1. 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.

2. Tier 2 methodology may be used at this point to establish whether or not this is a key category but, as explained in Section 3.10.2.2, this is the only use for Tier 2.

3. Data may be collected as a country study by a third party in order to preserve confidentiality.

Note:

1. 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.

2. Tier 2 methodology may be used at this point to establish whether or not this is a key category but, as explained in Section 3.10.2.2, this is the only use for Tier 2.

3. Data may be collected as a country study by a third party in order to preserve confidentiality.

CHOICE OF ACTIVITY DATA

Again, activity data has no role in the Tiers 3 and 2 estimates, which are based on measurements. For Tier 1, the activity is the annual mass of the desired fluorochemical that is produced.

Recycling

Recycling of used gas may be done by the producers of new gas or by other recycling firms. Emissions may occur during handling and purification of old gas and handling of recycled gas. Specific emission factors are not available. Thus, good practice is to use the same default factor as for new production.

COMPLETENESS

For some inventory compilers, identifying smaller producers and, in particular, recycling firms may be a difficult task. However, initial estimates based on the national mass balance of these fluorinated greenhouse gases should identify if production related emissions from such entities provide a sizeable contribution to total national emissions.

DEVELOPING A CONSISTENT TIME SERIES

Both by-product and fugitive emissions of fluorocompounds from production processes should be estimated using the same method for the entire time series and appropriate emission factors. If data for any years in the time series are unavailable for the Tier 3 method, these gaps should be filled according to the guidance provided in Volume 1, Chapter 5.

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