Alternative Fuels: Questions and Answers

 

​3. WHAT ARE THE POTENTIAL BENEFITS OF ALTERNATIVE FUELS?

• What are the benefits from alternative fuels for reducing aviation GHG emissions?

  The main benefit expected from the use of alternative fuels in aviation is the reduction of GHG emissions.

   

  If, for sustainable alternative fuels, combustion emissions are neutral and can be accounted as zero emission, this does not mean that there is no GHG emissions associated to the use of sustainable alternative fuels. The full life cycle of the fuel needs to be considered as the production of the fuel itself is likely to produce GHG emissions, including CO2 and other types of gases such as NOx or methane.

   

  The figure below provides some indicative values of the potential for emissions reductions of some biofuels compared to the case of conventional jet fuel. For conventional jet fuel, emissions associated to combustion appear in red. They are not accounted for in the case of biofuels. The indicative mean values for the different biofuels show that they have a real potential for emissions reductions, in particular for those using cellulosic feedstock.

   

   

   

  Example of biofuels potential greenhouse gas savings

  The variation ranges (black lines on the graph) illustrate possible variations of the life cycle emissions depending on the actual conditions for the production of the fuel (e.g. agriculture practices, fertilizers use, co-products use). It clearly illustrates the importance of carefully controlling and optimizing these conditions to achieve the minimum emissions.   In addition, the results presented are for the case where no land use change (LUC) is induced by the cultivation of the feedstock. LUC has emerged as a critical parameter in the life cycle assessment of GHG emissions for the production of biofuels, as significant amounts of carbon may be stored in a given tract of land, both above and underground1. A change in land use will affect carbon storage not only through the removal of the vegetation, but also through the oxidation of the soil organic carbon induced by agricultural practices such as tillage. Yet, the change may have either a positive or negative impact: converting a forest into crop land will result in carbon release, while replacing annual crops by perennial crops may result in increased carbon storage in the land. Depending on local conditions, LUC emissions can dominate all other emissions associated with biofuels; a typical example is the clear cutting of a tropical forest to grow annual crops.   Fuel life cycle and GHG emissions   From the feedstock extraction or production to the final use in an engine, the fuel goes through multiple steps constituting its life cycle. At each of these steps, GHG emissions are likely to be produced. The total carbon foot print of the fuel is obtained by adding all these emissions together in a life cycle assessment (LCA) approach.   For fossil fuels, in addition to combustion, emissions are associated to crude oil extraction and refining, as well as final fuel transport and distribution. In the case of biofuels, combustion emissions can be considered as neutral (see question 2), but there are emissions associated to the cultivation, harvesting, transport and conversion of the feedstock. In particular, depending on the feedstock and agricultural practices, the cultivation of the feedstock can represent a significant part of the emissions.

  Fuel life cycle emissions

  Thus, to assess the emissions reductions from using alternative fuels, a comprehensive accounting must be done of all emissions across all steps of the fuel’s life cycle, from the field to the tank of the aircraft. There is an environmental benefit for climate change if these emissions are lower than the emissions on the full life cycle of fossil fuels, including the combustion.

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  References:

  1. Stratton & al. - Life Cycle Greenhouse Gas Emissions from Alternative Jet Fuels – PARTNER, Project 28 report, 2010.
  2. Prieur & al. – Life Cycle Analysis Report – SWAFEA European Study, 2011.

   

  Note:

  1. Carbon that may be stored in a land includes above ground carbon contained in the vegetation and underground carbon contained in the vegetation roots, as well as well as soil organic carbon consisting of humus, and charcoal comprising decomposed plant and animals residues, substances synthesised from the decomposition and living micro-organism and small animals.

   

• What are the other potential environmental benefits?

  The alternative fuels approved to date (Fischer-Tropsch fuels and Hydroprocessed Esters and Fatty Acids – HEFA) are purely paraffinic fuels, meaning that they consist of alkane molecules only. A conventional jet fuel also contains aromatics molecules, which is the reason why current alternative fuels need to be blended with conventional fuel to obtain a drop-in fuel. This ensures that the final fuel contains the minimum required level of aromatics.

   

  However this final blend tends to have a lower content in aromatics than the average of conventional fuels. This turns out to have beneficial impacts on engine emissions with regard to local air quality. Indeed, aromatics play a major role in the formation of particulate matters (PM) and thus some alternative fuels have the potential to reduce PM emissions from engines.