- WHY INTRODUCE ALTERNATIVE FUELS IN AVIATION?
Limiting or reducing aviation greenhouse gas (GHG) emissions is a key objective of ICAO’s environmental protection activities. In this regard, in 2010, at the 37th Session of the ICAO Assembly, ICAO’s Member States adopted the global aspirational goal to stabilize the international civil aviation GHG emissions at their level of 2020.
The trends assessment performed by the ICAO Committee on Aviation Environmental Protection (CAEP) forecasts that, even with the anticipated gain in efficiency from technological and operational measures, aviation CO2 emissions will increase in the next decades due to a continuous growth in air traffic (figure below).
CAEP environmental trends assessment to 2040
Therefore additional measures must be taken into consideration in order to achieve a carbon neutral growth from 2020, including the use of sustainable alternative fuels that have a reduced carbon foot print compared to conventional jet fuel.
Emissions reductions accrued from the use of sustainable alternative fuels are not as a result of decreased fuel consumption, but through a reduction of the emissions generated by the use of the fuel itself.
- What are sustainable alternative jet fuels ?
Sustainable alternative fuels for aviation are fuels that have a potential to be sustainably produced and to generate lower carbon emissions than conventional kerosene on a life cycle basis.
Aviation’s focus is on “drop-in”fuels that do not require a change of aircraft and infrastructure, which would induce major logistical, safety and cost issues.
What is a drop-in fuel?
How can a drop-in fuel reduce GHG emissions?
How can sustainable alternative fuels be produced? From which feedstock?
Could alternative fuels produced from fossil feedstock be used for aviation?
How are we sure that alternative fuels are drop-in and safe?
What is a drop-in fuel?
A drop-in fuel is a substitute for conventional jet fuel, which is fully compatible, mixable and interchangeable with conventional jet fuel. Such an alternative fuel does not require any adaptation of the aircraft and or infrastructure, and does not imply any restriction on the domain of use of the aircraft. It can be used just as conventional jet fuel and does not require any new certification of the system.
Today, drop-in fuels are synthetic fuels that are designed in such a way that their components and properties are close to those of conventional jet fuels.
Why are drop-in fuels so important?
Being a drop-in fuel is currently seen by the aviation community as a major requirement for any new fuel in aviation. A "non drop-in" fuel would need to be handled separately from conventional jet fuel. This would result in safety issuesassociated to risks of mishandling, and would require aparallel infrastructure to be built up at all airports. The cost of such large parallel infrastructure networksis generally considered as prohibitive. From an operational point of view, as no aircraft is dedicated to a specific route, the new fuel and its distribution network would have to be deployed worldwide and it would benecessary to maintain different networks until the new fuel’s production covers 100% of the needs,knowing that average aircraft lifetimes exceed thirty years. In addition, guarantees should be provided to aircraft manufacturers that the fuel will be deployed before any decision is taken to develop a dedicated aircraft.
How can a drop-in fuel reduce GHG emissions?
Drop-in fuels are synthetic fuels that are designed in such a way that their components and properties are close to those of conventional jet fuels. Hence, they are still hydrocarbons and their combustion still emits CO2 in quantities similar to those emitted by fossil jet fuel.
To understand how such fuels can generate emissions reductions, two different situations should be considered depending on the type of raw materials that are used.
A first family of alternative fuels consists of biofuels made from various kinds of biomass (crops, wood, agricultural residues, etc.). In that case, the carbon contained in the fuel comes from plants and was up-taken from the atmosphere by plants’ growth through photosynthesis.This carbon is emitted back into the atmosphere during the combustion and will return to plants in a close loop. This is not additional carbon injected into the biosphere as it would be the case for fossil fuels. Thus, in the case of biofuels, the emitted carbon can be considered as neutral and combustion emissions can be accounted as zero emissions.This is the source of emissions reductions with biofuels.
A second family of alternative fuels consists of those fuels produced from different categories of waste, such as municipal solid waste or industrial waste gases. These wastes can contain or be made of fossil carbon. In this case, the mechanism for emissions savings is not the neutrality of carbon emissions, but the multiple uses of fossil carbon. Indeed, waste is discarded end-life products from valuable goods (e.g. municipal solid wastes) or by-products with no utilization value from the manufacturing of goods (e.g. industrial waste gas from steel industry). GHG emissions of waste are primarily associated with the production of these goods. Using waste does not add emissions to the system and is thus, carbon neutral.Using a different approach that would consider the value of the fuel produced from wastes, emissions could be shared between the manufacturing of the primary goods and the fuel. Then the fuel would not be zero emissions but, globally, for the same quantity of goods produced (main goods + fuel), an emissions reduction would be achieved.
How can alternative fuels be produced? From which feedstock?
Alternative jet fuels can be produced from a variety of feedstock including renewable biomass, waste or fossil feedstock, such as coal and natural gas. The focus here is on sustainable alternative fuels that have the potential to reduce GHG emissions. Thus, alternative fuels from fossil feedstock are not included as they are not likely to generate emissions reductions.
The figure bellow presents a simplified view of pathways for the production of sustainable alternative fuels. Only the routes that have already been approved or that are currently being submitted for approval to ASTM are represented.
Simplified view of pathways to sustainable fuels
There are mainly three families of bio-feedstock that can be used to produce alternative fuel jet fuels: the family of oils and fats, or triglicerides, the family of sugars, and the family of lignocellulosic feedstock.
Trigliceridescurrently come largely from oil crops, animals fats and used cooking oil. Production from micro-algae is an additional promising pathway that is currently inthe research and development stage. Triglicerides contain oxygen that need to be removed to produce jet fuel components which are pure hydrocarbons. Different processes are proposed for this, in particular hydroprocessing, one of the two processes already approved.
Sugars come from sugar crops and cereals starch. Theyare mainly associated to fermentation routes that generally produce alcohols,which are further upgraded into hydrocarbons. This is the “alcohol-to-jet” route. Advanced fermentation has also been developed that directly produces hydrocarbons which can be upgraded in jet fuel components. It should also be noted that fermentation has been developed from industrial waste gas as well. In that case, it is carbon monoxide that is used. Cultivation of algae is also a way to use waste gas to produce feedstock: CO2 is indeed needed to grow algae.
Lignocellulose is found in the wall of plants’ cells and in wood, and come from various energy crops, as well as from agriculture or forest residues and from macro-algae. Lignocellulose can be directly converted into hydrocarbons using thermochemical processes such as Fischer-Trospch, pyrolysis or catalytic cracking. The Fischer-Tropsch process can also be used to convert municipal solid wastes.
But lignocellulose can also be transformed into sugar and can thus be used for the aforementioned fermentation routes. In a similar way, sugars can be transformed into oil by yeast or micro-algae and thus further processed into jet fuel through deoxygenation.
Thus, there are a large number of processes under development that allow for processingof almost all kinds of feedstock into aviation fuel components, which offersflexibility for regional adaptation and optimization.
Additional routes are also being studied to produce alternative fuels directly from CO2, including CO2 captured from the atmosphere, without using biomass. Conversion then uses renewable energy to break down CO2 into CO and O2, and water into H2 and O2, and then recombines CO and H2 in liquid hydrocarbon using the Fischer-Tropsch synthesis. These processes (e.g. solar fuels) are currently in the research stage.
Most of the various pathways do not directlyproduce a drop-in jet fuel. They produce components that need to be blended with Jet A-1 to obtain the final drop-in fuel. It should also be noted that although they are not the processes currently deployed for road-transportation, these processes co-produce fuels that can be used for road transportation.
Could alternative fuels produced from fossil feedstock be used for aviation ?
Alternative jet fuels can be produced from fossil feedstock. Examples are the Coal-to-Liquid (CTL) and Gas-to-Liquid (GTL) made from coal and natural gas using the Fischer-Tropsch pathway.
These fuels are approved for use in aviation as part of the general approval of Fischer-Tropsch fuels (Fischer-Tropsch process is feedstock agnostic and CTL or GTL are similar to biomass-to-liquid,BTL). In addition, commercial production already exists. GTL produced by Shell in Qatar is available at Doha airport and CTL is supplied to Johannesburg airport in South Africa by Sasol.
In this case, the carbon contained in the fuel is fossil carbon and the conversion process adds to the emissions. As a result, these fuels create larger CO2 emissions than conventional jet made from crude-oil. Carbon sequestration can be used to reduce global emissions but, even with the most aggressive existing sequestration technologies, there is currently little evidence that emissions reductions could be achievedas compared to current petroleum-based fuels. As these fuels are also not using renewable feedstock, they are not considered sustainable alternative fuels.
How are we sure that alternative fuels are drop-in and safe?
Jet fuels must meetspecific requirements corresponding to their severe constraints of use in an aircraft. For example, the fuel should not freeze at temperatures down to -47 C to ensure that it is still liquid at high altitudes of flight. Its energy content should exceed a minimum value (42.8 MJ/kg) in order for the aircraft to achieve its operational range with a constrained mass and volume of fuel. There are also additional constraints associated with safety concerns, such as flammability, or with design features of aircraft engines (for example, jet fuel is used as a cooling fluid and a lubricant in engines).
The properties that any batch of fuel has to satisfy in order to be accepted on board an aircraft are defined by specifications, the main onesfor conventional jet fuel being the DEF-STAN 91-91[1] and the ASTM[2] D1655. These fuel specifications do not fix any precise composition for the fuel. They instead define the nature of the fuel and of the process for its manufacturing, as well as the limit values for a number of its properties. The experience with the fuel ensures that checking this limited set of properties is sufficient to guarantee suitability and safety.
To allow their use in aviation, specifications had to be created for alternative fuels. Prior to this, the fuels had to undergo an approval process that achieved a detailed assessment of a large number of theirphysical and chemical properties, as well as an in depth testing of the fuels behaviour in aircraft and engines systems, in order to demonstrate that there was no harm to use them. This approval process was developed for the approval by ASTM of Fischer-Tropsch fuels, the first alternative fuels for aviation. ASTM has now defined a standard, ASTM D4054, that frames this process.
D4054 is an iterative process that involves many experts in the ASTM’s aviation fuels subcommittee, in particular, the Original Equipment Manufacturers (OEM). The OEM review the research report produced by the candidate fuel producer with testing results covering basic specification properties, additional properties referred to as “fit-for-purpose properties”, components testing and, if deemed necessary, full-scale engine testing. Approval of the fuel requires formal ballots and results in a fuel specification. For the specification of synthetic fuels, ASTM has created a specific standard, D7566, which includes a dedicated annex for each newly approved fuel. This annex defines the final list of properties that need to be checked for the acceptance of the fuel batches. As the approval process ensures that the fuel is drop-in, any fuel qualified under D7566 is automatically qualified under D1655 and can be considered as Jet A-1. Accordingly, it can be used without any recertification of aircraft.
- What are the potential benefitS of alternative fuels ?
The major potential benefit of introducing sustainable alternative fuels in aviation is to reduce the contribution of aviation to climate change through the reduction of aviation greenhouse gas emissions.
Alternative fuels may also have additional environmental benefits for local air quality.
How sustainable alternative fuels reduce aviation GHG emissions?
What are the other potential environmental benefits?
How sustainable alternative fuels reduce 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 (include link to previous question), this does not mean that there is no GHG emissions associated to the use of sustainable alternative fuels. The full life cycle (link to the box related to LCA)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
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, emissions are associated to crude oil extraction and refining, as well as final fuel transport and distribution. In the case of biofuels, a significant part of the emissions can be associated to the cultivation and the transportation of the feedstock.
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.
(this would be included in a box that is open when clicking on life cycle)
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 underground[3]. 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.
References:
Stratton & al. - Life Cycle Greenhouse Gas Emissions from Alternative Jet Fuels – PARTNER, Project 28 report, 2010.
Prieur & al. – Life Cycle Analysis Report – SWAFEA European Study, 2011.
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.