Choice of method

There are three methodological options for estimating CO2, CH4, and N2O emissions from railways. The decision trees in Figures 3.4.1 and 3.4.2 give the criteria for choosing methodologies.

Figure 3.4.1 Decision tree for estimating CO2 emissions from railways

Figure 3.4.1 Decision tree for estimating CO2 emissions from railways

Note: See Volume 1 Chapter 4, "Methodological Choice and Key Categories" (noting section 4.1.2 on limited resources) for discussion of key categories and use of decision trees.

Note: See Volume 1 Chapter 4, "Methodological Choice and Key Categories" (noting section 4.1.2 on limited resources) for discussion of key categories and use of decision trees.

Figure 3.4.2 Decision tree for estimating CH4 and N2O emissions from railways

Figure 3.4.2 Decision tree for estimating CH4 and N2O emissions from railways

Note: See Volume 1 Chapter 4, "Methodological Choice and Key Categories" (noting section 4.1.2 on limited resources) for discussion of key categories and use of decision trees.

Note: See Volume 1 Chapter 4, "Methodological Choice and Key Categories" (noting section 4.1.2 on limited resources) for discussion of key categories and use of decision trees.

The three tiers of estimation methodologies are variations of the same fundamental equation:

Equation 3.4.1 General method for emissions from locomotives

Where:

Emissions = emissions (kg)

Fuel j = fuel type j consumed (as represented by fuel sold) in (TJ)

EF j = emission factor for fuel type j, (kg/TJ)

For Tier 1, emissions are estimated using fuel-specific default emission factors as listed in Table 3.4.1, assuming that for each fuel type the total fuel is consumed by a single locomotive type. For CO2, Tier 2 uses equation 3.4.1 again with country-specific data on the carbon content of the fuel. There is little or no advantage in going beyond Tier 2 for estimating CO2 emissions.

With respect to Tier 2 for CH4 and N2O, emissions are estimated using country-specific and fuel-specific emission factors in equation 3.4.2. The emission factors, if available, should be specific to broad locomotive technology type.

Equation 3.4.2 Tier 2 method for ch4 and n2o from locomotives

Where:

Emissions = emissions (kg)

Fuel! = fuel consumed (as represented by fuel sold) by locomotive type i, (TJ)

EFi = emission factor for locomotive type i, (kg/TJ)

i = locomotive type

Tier 3 methods , if data are available, use more detailed modelling of the usage of each type of engine and train, which will affect emissions through dependence of emission factors on load. Data needed includes the fuel consumption which can be further stratified according to typical journey (e.g. freight, intercity, regional) and kilometres travelled by type of train. This type of data may be collected for other purposes (e.g. emissions of air pollutants depending on speed and geography, or from the management of the railway).

Equation 3.4.3 is an example of a more detailed methodology (Tier 3), which is mainly based on the USEPA method for estimating off-road emissions (USEPA 2005 a & b). This uses the following basic formula to calculate emissions (in Gg):

Equation 3.4.3

Tier 3 example of a method for ch4 and n20 from locomotives

Where:

Emission

= emissions of CH4 or N2O (kg)

Ni

= number of locomotives of type i

Hi

= annual hours of use of locomotive i [h]

P1

= average rated power of locomotive i [kW]

LFi

= typical load factor of locomotive i (fraction between 0 and 1)

EFi

= average emission factor for use in locomotive i [kg/kWh]

i

= locomotive type and journey type

In this methodology, the parameters H, P, LF and EF may be subdivided, such as H into age dependent usage pattern (EEA, 2005). A number of detailed modelling tools are available for estimating locomotive emissions using Tier 3 methodologies (e.g., RAILI (VTT 2003); NONROAD (USEPA 2005a and b); COST 319 (Jorgensen & Sorenson, 1997)). Please refer to Box 3.4.1 for an example of a Tier 3 approach.

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