Methodological issues

CHOICE OF METHOD

The choice of method will depend on national circumstances. Emissions can be estimated using an output-based approach (emissions per unit of output), or an input-based approach (emissions per unit of input). However, it is good practice to use the input-based method where data are available.

Methods are classified according to the extent of plant-level data that are available. The Tier 1 method is based on default values and national statistics, and the Tier 2 method is based on complete plant-level input or output data and plant specific emission factors. If there is monitoring and direct measurement of CO2 emissions this would be equivalent to a Tier 3 method.

Tier 1 method

Natural soda ash production emits CO2 through the thermal decomposition (calcination) of the Trona (Na2CO3.NaHCO3.2H2O) to produce soda ash. According to the chemical reaction presented above, it takes 10.27 tonnes of Trona to produce 1 tonne of carbon dioxide. Hence, for natural soda ash production using Trona, emissions of carbon dioxide can be calculated from the Trona input or natural soda ash output by the following formula:

Equation 3.14

CO2 emissions from natural soda ash production - Tier 1

Where:

ECO2 = emissions of CO2, tonnes

AD = quantity of Trona used or soda ash produced, tonnes of Trona used or tonnes natural soda ash produced

EF = emission factor per unit of Trona input or natural soda ash output, tonnes CO2/tonne of Trona or tonnes CO2/tonne natural soda ash produced: EFTrona = 0.097 tonnes CO2/tonne of Trona, EFSoda Ash = 0.138 tonnes CO2/tonnes natural soda ash produced.

It is good practice to assess the available national statistics for completeness. The choice of good practice methods depends on national circumstances, as shown in Figure 3.7: Decision Tree for Estimation of CO2 Emissions from Natural Soda Ash Production. If no data are available for the purity of the Trona input, it is good practice to assume it is 90 percent and adjust the emission factor shown in Equation 3.14.

Tier 2 method

To use the Tier 2 method, it is necessary to gather complete data on Trona consumption or natural soda ash production for each of the plants within the country along with plant-specific emission factors for the Trona input or soda ash output. The CO2 emissions for each plant can be calculated using either variation of Equation 3.14. For plants where plant-specific emission factors are not available, the default emission factors provided in Equation 3.14 can be used. Total CO2 emissions are the sum of the emissions from all plants.

Tier 3 method

The Tier 3 method uses plant-level CO2 emissions data obtained from direct measurement. Total emissions are the sum of emissions from all plants.

Figure 3.7 Decision tree for estimation of CO2 emissions from natural soda ash production

Figure 3.7 Decision tree for estimation of CO2 emissions from natural soda ash production

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.

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.

CHOICE OF EMISSION FACTORS Tier 1 method

The Tier method uses the default emission factors presented in Equation 3.14. The default emission factors are derived from the stoichiometric ratio between soda ash produced and purified sodium sesquicarbonate obtained from Trona. They are based on the main natural production process that is used at present, where soda ash is produced by calcination of purified sodium sesquicarbonate.

Tier 2 method

The Tier 2 method requires plant-level emission factors per unit of Trona input or per unit of natural soda ash output. Plant-level emission factors should reflect the fractional purities of the Trona input and natural soda ash output and it is good practice to ensure that these are taken into account in the derivation of plant-level emission factors.

CHOICE OF ACTIVITY DATA

It is good practice to compile activity data at a level of detail that allows the use of the Tier 2 method. When applying the methods it is essential that a clear distinction is made between the products to avoid multiplying the incorrect emission factor by activity data.

Tier 1 method

The Tier 1 method requires data on national consumption of Trona or national production of natural soda ash. If national-level activity data are not available, information on production capacity can be used with emissions estimated using a default emission factor. It is good practice to multiply the total national production capacity by a capacity utilisation factor of 80 percent ± 10 percent (i.e., a range of 70-90 percent).

Tier 2 method

Activity data should be collected at the plant-level to use the Tier 2 method. The most important data are the amount of Trona used for soda ash production and the amount of natural soda ash produced at each plant. Although soda ash production is not used in the calculation if emissions are derived from Trona input, it is good practice to collect and report these data to enable comparisons of inputs per unit of outputs over time and provide a sound basis for ensuring time series consistency.

COMPLETENESS

Completeness of the activity data (e.g., Trona utilisation) is a crucial attribute of good practice. Therefore, it is good practice to assess the available national statistics for completeness. If data are available at the plant-level, it is good practice to aggregate these data and check the result with the data available at a national level. This practice enables assessment of whether any significant soda ash producer is omitted, and ensures that all production processes within the country have been considered. If data at the plant-level are not available, it is good practice to use production capacity data along with national statistics to estimate the emissions for completeness purposes.

DEVELOPING A CONSISTENT TIME SERIES

It is good practice to calculate emissions from soda ash using the same method for every year in the time series. Where data are unavailable to support a more rigorous method for all years in the time series, good practice is to recalculate these gaps according to the guidance provided in Volume 1, Chapter 5.

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