A Methodology for Assessing Environmental Benefits from LNG Transition: The Case of Regione Sardegna

This paragraph is devoted to illustrating the methodology followed to assess the environmental benefits of adopting LNG as fuel as a replacement for traditional fossil derivatives, such as diesel, LPG, coal, etc.

2.1 Final energy consumption

The starting point for such an analysis is to collect data on the final energy consumption for the area under analysis. This information is usually made available by energy resource managers, public authorities, or local authorities. These data are essential to be able to define the current situation in terms of final energy consumption: they are necessary to provide all the information required to characterize the energy needs profiles of the sectors analyzed.

The more detailed the analysis, the more disaggregated and sector-specific the data must be. In particular, it is necessary to know the mix of fuels that are used to meet the sector-specific primary energy demand. This is very important to be able to define in advance what data are necessary to continue the analysis (characteristics of fuels, pollutants, emission factors).

For this analysis, the final energy consumption considered, for the Sardinia Region, has been shown in Table 1 the reference year is 2019.

The data given in Table 1 represent the current scenario in terms of emission levels used for the pollutants considered. These data are used as a benchmark to assess the possible environmental benefits related to the transition to the more widespread use of LNG as fuel.

Table 1. Sardinia Region final energy consumption for the year 2019

2.2 Emission factors

The second key element for the correct methodological approach of this analysis is the identification of emission factors. In general, emission factors are considered as coefficients which, multiplied by a value expressing either the amount of energy or the amount of fuel requested, make it possible to obtain an estimate of the pollutant released into the environment as a result of the process under analysis.

From this it can be deduced that the emission factors are very specific, they must therefore be chosen carefully:

• The emission factors depend on the pollutant analyzed;
• The process or activities emitting the pollutant influence the emission factor (e.g. combustion, metallurgical production);
• The technology used in the process/transformation that produces the pollutant must be considered (e.g. for combustion, a gas turbine or a boiler);

Such a detailed approach is not always applicable, especially in the case of large-scale analysis. The emission factors used in this analysis were taken from the "EMEP/EEA air pollutant emission inventory guidebook 2019" [9] which is connected to an extensive database containing emission factors for many sectors (civil, industrial, mining, chemicals, transport, etc.) as well as for many pollutants (carbon monoxide, Sulphur and nitrogen oxides, VOC, heavy metals, particulate matter, and many others). The database also provides different levels of detail and increasing specificity for the determination of emission factors, called TIER 1, TIER 2, TIER3.

In this paper, since the scale of the analysis is as big as an entire Italian region, the emission factors have been chosen among TIER 1, which are determined by considering a production technology and abatement system typical for the analyzed sector. The TIER 1 approach groups fuels into macro-categories that allow easier aggregation of emission factors.

It is important to note that, for carbon dioxide, the EMEP/EEA Handbook does not provide emission factor values. For this reason, it was also necessary to consider a secondary source which provided the missing data [10].

As far as the marine sector is concerned, it was necessary to further integrate the information available on the EMEP/EEA handbook, as the document did not contain emission factors for carbon dioxide, as well as for LNG-powered ships. For CO₂, considering marine fuel (both marine diesel and gas oil) the reference emission factor values are extracted from the 2014 International Marine Organization Directive [11]. Missing information on emission factors for LNG-powered ships has been taken from the literature [12].  Considering MDO (marine diesel oil) and MGO (marine gas oil), the emission factors have been calculated as a percentage of the correspondent pollutant in case of fossil fuel-powered ships [12]. The calculations for carbon monoxide emissions have not been considered, for LNG-powered ships. This is because the emissions for this pollutant are closely dependent on the type of engine: to consider a ship that is natively fed by LNG is very different from considering a ship whose engine has been converted from combustible oils to the use of natural gas.

2.2.1 Emissions Calculation – Current situation

At this point it is possible to carry out the emissions assessment, performing the calculations according to the sector in question, the fuel used, and the pollutant analyzed. The total annual mass for each pollutant can be summarized by Eq. (1):

$M_{\text {pollutant,year }}=E F_{\text {pollutant }} \cdot C_{\text {energy.year }}$          (1)

where:

• $M_{\text {pollutant,year }}$  indicates the amount of pollutant released into the atmosphere in a year [ton], [Mton];
• $E F_{\text {pollutant }}$  is the emission factor for the pollutant considered [g/GJ], [g/kWh];
• $C_{\text {energy,year }}$ is the annual energy consumption from the fuel concerned [GJ], [kWh].

The emission factor can be expressed as a ratio of masses (e.g.  kg/ton_fuel): in this case, it must be multiplied by the total fuel mass used yearly, thus the fuel specific Lower Calorific Value (LCV) must be considered.

For the sake of brevity, Table 2 gives an example of the calculations performed for the residential and tertiary sectors, using the emission factors presented in Appendix A. Energy consumption refers to the values presented in Table 1.

It is important to point out that the "Large-scale thermoelectric production" sector and the "Terrestrial Transport" sector are not analyzed on this paper, but they will be in a future paper.

In Table 3 are listed the emissions globally emitted, broken down by type of pollutant, during the year 2019 for the Sardinia Region.

Table 2. Example of calculation of total annual emissions, industrial sector

Table 3. Total emissions Sardinia Region for the year 2019