Sustainability: Aircraft Emissions and the Rise of Alternative Fuels

Sustainability Tree

The Background

Global climate change has already had observable effects on the environment.  Glaciers have shrunk, extreme weather events are becoming the norm, plant and animal ranges have shifted, and trees are flowering sooner.  The effects that scientists had predicted in the past are now occurring – a loss of sea ice, accelerated sea level rise, more intense heatwaves.  And scientists have a high confidence that global temperatures will continue to rise in the coming decades due to greenhouse gases produced by human activities.

Before you start understanding the impact of aviation on climate change, you should be familiar with the international targets set at global climate summits, called the ‘Conference of Parties’, or COP.  For nearly three decades, the United Nations has been bringing nations together to discuss climate change.  In 2015, COP21 took place in Paris.  For the first time ever, every country agreed to work together to limit global warming to well below 2 degrees celsius, and aim for 1.5 degrees compared to pre-industrial levels.  In COP21, the ‘Paris Agreement’, was born, where countries committed to bring forward national plans which set out how much they would reduce their emissions – known as Nationally Determined Contributions., or NDCs.

But the problem is, that the commitments laid out in Paris did not come close to limiting global warming to 1.5 degrees, and the window for achieving this is closing.  At the time of writing, COP26 has just taken place in Glasgow.  In the Glasgow Climate Pact, all countries agreed to keep the 1.5 degree target alive, and to finalize all the outstanding elements of the Paris Agreement.

The unfortunate truth is that climate change puts our way of life at threat.  Which is why aviation needs to play its part.

Commercial Aviation is responsible for about 2 to 3% of global carbon emissions.  In the European Union, direct emissions accounted for 3.8% of total CO2 emissions, with the aviation sector creating 13.9% of transport emissions – making it the second biggest source of transport greenhouse emissions after road transport.

So just for scale, if global aviation were a country, it would rank in the top 10 emitters.  Someone flying from Lisbon to New York and back generates the same level of emissions as the average person in the EU does by heating their home for a whole year. Before the coronavirus crisis, ICAO had forecasted that by 2050, international aviation emissions could triple compared with 2015.

Here’s an amazingly simple podcast to follow on climate change…

Aircraft Emissions

Aircraft emit chemicals, and produce physical effects, such as contrails, that affect our climate.

  • Carbon dioxide is known as a greenhouse gas that causes an increase in global temperature.  CO2 may remain in the atmosphere and affect the climate for up to 500 years after it is emitted.
  • Other types of emissions produce impacts with shorter lifetimes – a day, for contrails, to 10 years for the removal of methane by Nitrogen Oxides, or NOx.
  • Water vapor affects the formation of contrails and contributes to global warming by acting as a greenhouse gas – especially in the stratosphere.  The troposphere already has a high concentration of water vapor, so aircraft engine water vapour does not affect here.
  • NOx emissions within the troposphere increases the concentration of ozone.  In the stratosphere, ozone decreases the ability of the atmosphere to filter harmful ultraviolet radiation.  In the troposphere, ozone acts as a greenhouse gas.  So the impact of NOx is short-lived, and occurs where most air traffic is.
  • NOx emissions also reacts with the atmosphere to remove methane, which is a strong and natural greenhouse gas – so in this sense, NOx has a cooling effect.  The overall impact of NOx is uncertain.
  • Condensation trails, or contrails, are formed when warm and humid air from the aircraft’s exhaust plume mixes with cold and dry air in the atmosphere, causing the water vapor to condense on particles from the atmosphere, and particles from the engine itself.  The extent, intensity and persistence of contrails is affected by the engine, fuel combustion and atmospheric conditions.  Overall, contrails tend to have a warming effect.
  • Soot emissions absorb sunlight, and alter surface reflectivity when deposited on the earth’s surface.  Sulphates, like soot, also contribute to climate change as they facilitate contrail formation, and reflect radiation from the sun back into space.

So to summarise, imagine an aircraft flying trans-Atlantic.  Over the first days after the flight, contrails contribute to warming.  Next, ozone also has a warming effect.  And both these effects are regional, so they affect the transatlantic region. Longer timescale impacts are felt globally.  Over years, the reduced methane from the flight has a cooling effect.  Over the span of decades to centuries, a small warming effect from the flight remains because of the carbon dioxide which remains from the flight.

In the long term, the use of fossil fuels is not sustainable, and a solution to this problem could be the use of hydrogen as a source of energy to power the aircraft of the future.

The Rise of Alternative Fuels

Hydrogen can be used to power a fuel cell to generate electricity, or directly combusted to power an aircraft’s engines.  And the only waste product is clean water.  For the same kg of fuel, Hydrogen has three times the energy of conventional jet fuel, and more than hundred times that of lithium-ion batteries.  And this is why a number of governments and companies are investing in the technology.

“Hydrogen is one of the most promising technology vectors to allow mobility to continue fulfilling the basic human need for mobility in better harmony with our environment”. 

Grazia Vitaldini, CTO Airbus

In fact, in September 2020, Airbus announced a project named ZeroE, where it presented three concept planes which could be ready for deployment in 2035.  All three aircraft are hydrogen hybrids – so they have gas-turbine engines that burn liquid hydrogen as fuel, and generate electricity via hydrogen fuel cells.

Reducing aircraft emissions - Airbus ZEROe infographic
Source: Airbus 2021

But the use of Hydrogen as an aviation fuel has several challenges which need to be overcome.

First, while Hydrogen has a higher energy per unit mass than jet fuel, it has a lower energy by volume.  So 1 meter cubed of Hydrogen has less energy than 1 meter cubed of conventional jet fuel.  This is because Hydrogen exists as a gas at normal temperatures.  So before we use it in aircraft, we need to compress it and turn it into a liquid by cooling it to extreme low temperatures.  And the storage tanks are complex and heavy – which creates a weight problem for aircraft.  The storage tanks also need to be four times as large.  So that would mean that there would be less space for passengers or cargo, or that we would need to design considerably larger aircraft, which would create a whole range of problems for infrastructure.  Adequate infrastructure would also need to be put in place to manufacture, store and transport liquid Hydrogen.

There is also the question about whether hydrogen can be produced at scale, and without a large carbon footprint itself.  Today, most hydrogen is created by using methane, releasing carbon dioxide as a waste product.  Hydrogen can be produced by a process called electrolysis and driven by renewable energy.  But only 1% of hydrogen production today uses this way, because it is more expensive.

So the general belief of industry experts is that Hydrogen will not be used as the main energy source for our aircraft until 2050 or beyond.  But in the meantime, we might have a solution – Sustainable Aviation Fuels, or SAFs.

Sustainable Aviation Fuels

SAFs can be divided into two categories.  The first are biofuels made through the chemical or thermal treatment of agricultural residues, sustainable feedstocks, fats, oils and solid waste from homes and businesses.  The second category are electro fuels, or E-Fuels.  Sometimes, these are also known as ‘Power to Liquid’ fuels.  E-fuels are made by reacting hydrogen with carbon dioxide to make what is known as ‘syngas’. The ‘syngas’ is then converted into ‘e-crude’, a crude oil substitute which can be refined to jet fuel. 

If the energy required for these processes is derived from carbon neutral sources, then any flight using only SAFs can be carbon neutral.  And the best part about SAFs is that they are ‘drop in’ fuels – they can be used in existing aircraft with little or no redesign, without delay, and without the huge amount of capital that is required to invest and manufacture new aircraft technologies.

SAFs can give a reduction of up to 80% in carbon emissions over the lifecycle of the fuel, compared to conventional jet fuel.  But currently, it is considerably more expensive.  This because of the current availability of sustainable feedstocks, and the continuing development of new production technologies.  As time goes by, the price of SAFs will probably go down.


This article is one of a series of articles and content posted on RAVEN to increase climate change awareness in the aerospace community.

What is Raven doing?

Raven has been actively working on course content in relation to aircraft emissions, alternative fuels, SAFs and other topics related to the future of a more sustianable aviation industry. Have a look at the first course that RAVEN will launch in May 2022, called “Aviation’s Sustinable Future

We plan to work with Subject Matter Experts to develop more courses on Aviation Sustainability. Should you be that person, please get in touch with us on info@raven.aero.

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