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NTM TN 13 Fossil diesel fuel baselines

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Summary

In order to ensure accurate and up to date greenhouse gas (GHG) emission baseline factors for the combustion of fossil diesel used  in compression-ignition engines, NTM has reviewed various data sources where the emissions of diesel as transport fuel have been estimated from a life cycle perspective. The scope included emissions of GHGs from well to tank (wtt) and tank to wheel (ttw). The ttw combustion emissions are based on the latest European emission standard, Euro 6, which was used as a global proxy. The result of the review resulted in:

  • NTM – Diesel B0 – Swe confirmed existing ttw and wtt data
  • NTM – Diesel B0 Europe confirmed ttw data but updated wtt data

Throughout the recent life cycle assessment (LCA) studies, the span of the reported wtt GHG emission factors increases substantially from low to high. In a study from 2011, based on refineries in Sweden, the wtt-result was 6,7 g CO2 eq./MJ diesel. In the latest JRC-study, (Edwards et.al. version 5, 2020) the wtt-result was 18,9 g CO2 eq./MJ diesel, for European diesel. The latter LCA is based on a system expansion approach i.e., assuming marginal diesel production, thus resulting in a significant higher CO2 eq. emission factor per MJ fuel. These refineries can be considered as worst-case scenarios.

In order to provide a more representative value NTM have decided to adjust the previous international diesel baseline assumption of 9,5 g CO2 eq./MJ diesel which was according to the LCA study by Ahlvik & Eriksson, by making a simple arithmetic average between the lowest reported value of 6,7 g CO2 eq/MJ diesel and the high 18,9 g CO2 eq/MJ diesel resulting in  12,8 g CO2 eq./MJ diesel rounded to an emission factor of 13 g CO2 eq./MJ diesel. This value is in line with values reported to life cycle inventory databases.

Real performance is likely to deviate significantly in different markets. Hence more accurate calculations require operational and verified data from the fuel supplier. For Diesel B0 – Swe in the Nordic market NTM will keep the value of 6,7 gCO2 eq./MJ diesel as this reflects existing refineries based on previous studies of specific production processes. Below is an assessment of new NTM-data and benchmarking data from the Greet model in the US and data presented by EN 16 258.

Fossil diesel is often used as a benchmark in climate impact calculations when, for example, the environmental benefits of bio-based fuels are demonstrated. However, the WTW emission factors from fossil diesel that are used as baseline can differ depending on the data source, calculation method adopted and underlying assumptions.

Here, a compilation of data for fossil diesel from different data sources is presented and key differences are briefly discussed. Results and emission factors are presented in terms of gr CO2 eq. per MJ fuel. The system boundaries are set to a Swedish and EU perspective. For additional information and emission factors see Additional picture by the end of this document.

* ttw based on Euro 6 engine, GWP according to IPPC Fifth Assessment Report (AR5)

Tank to wheel (TTW)

GHG ttw emissions are commonly calculated on the basic assumption of full oxidation of the carbon content. Moreover, a general assumption based on various diesel configurations is that the carbon content is on average 86% on a weight basis. For the light MK1 diesel the assumption is 85% due to lower density and its specific configuration. The general density of 0.832 kg/litre is used for all fuels even though there are differences from different sources and production processes.

* Assuming full oxidation

** CH4 and N2O emissions based on Euro VI engine where and CH4 GWP = 28 and N2O GWP 265 is used (IPPC (AR5))

Well to tank (WTT)

The wtt-data can vary significantly among different studies. Since fuel production processes are likely to improve, WTT emissions factors can be expected to decrease over time. This is however not visible in the set of data presented here. In order to provide a representative value, NTM have decided to adjust the previous international diesel assumption of 9,5 g CO2 eq./MJ diesel which was based on  the LCA study Ahlvik & Eriksson, by making a simple arithmetic average between the low value of 6,7 g CO2 eq./MJ reported for Sweden and the high 18,9 g CO2 eq./MJ  reported for EU. The result is an emission factor of 12,8 g CO2 eq./MJ diesel rounded to 13 g CO2 eq./MJ diesel. This value is in line with values reported in generic life cycle inventory databases.

Other data sources

The Renewable Energy directive (RED II)and the Swedish Energy agency states GHG wtw emission factors for diesel fuels as listed below. Assuming the same ttw data for these fuels the wtt data ends up even higher than the highest number in table above.  For the time being NTM has no explanation for this deviation but will further analyse the issue.

 

NTM general methodology and scope

NTM provides emission and energy data for all modes of traffic through the tool NTMCalc and the web page www.transportmeasures.org. Data are based on various fossil and biogenic sourced fuels. Electricity data is based on fossil and biogenic sources as well as electric power from hydro,  wind, photovoltaic and nuclear. In order to determine the gains from the use of various new fuels and electricity,  there is a need to present credible baseline data for existing solutions being replaced. Comparisons are not easy, as system boundaries differ between the solutions compared i.e., sometimes resulting in arbitrary results. For the time being NTM aims to include a well-to-wheel perspective according to dotted blue figure below.

NTM present system boundary

The environmental effects from the production and use of fuels and electricity can be assessed through the Life Cycle Assessment (LCA) methodology. There are two methodological approaches in LCA namely the consequential (CLCA) and attributional (ALCA) approach. These two approaches vary in scope while the choice between ALCA and CLCA guides other methodological decisions, such as the choice of input data and the modelling of processes with multiple products (Ekvall, 2019). CLCA, also called “change oriented” uses marginal data aiming to “describe how the environmental exchanges of the system can be expected to change as a result of actions taken in the system” (Rebitzer et al., 2004). The second approach (attributional or accounting) “describe a product system and its environmental exchanges” by using average data, and data that “reflect the actual physical flows to and from the studied system” (Finnveden et al., 2009).

In multioutput processes i.e., process that result in more than one product (than the product under assessment), the attributional approach allocates the emissions among the different outputs based on physical or other relationships while in the consequential approach the system is often expanded to include additional systems or processes (second order effects). NTM uses the attributional principle when emission factors are estimated and reported.

References

Ahlvik, P., Eriksson, L. (2011) Well to tank assessment – diesel fuel MK1 and EN 590, Report 127057, rev. 2, Ecotraffic.

Eriksson M., Ahlgren, S. (2013) LCAs for petrol and diesel – a literature review. Report 2013:058. SLU, Swedish University of Agricultural Science.

European Commission. (2018). DIRECTIVE (EU) 2018/2001 OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL of 11 December 2018 on the promotion of the use of energy from renewable sources. Official Journal of the European Union. L 328/82

Gode J, et. Al., (2011) Miljöfaktaboken 2011. Estimated emission factors for fuels, electricity, heat and transport in Sweden. VÄRMEFORSK Service AB. Available at https://energiforskmedia.blob.core.windows.net/media/17907/miljoefaktaboken-2011-vaermeforskrapport-1183.pdf

Network for Transport Measures: www.transportmeasures.org

JRC: https://ec.europa.eu/jrc/en

Swedish Energy Agency: https://www.energimyndigheten.se/en/

GREET model published 2018 by Argonne National Laboratory: https://greet.es.anl.gov/