Efficient mitigation of aviation’s climate impact

Pre-covid, the greenhouse gas emissions stemming from planes taking off from Denmark totalled 3 million tons of CO₂ in 2019 in, accounting for half of the total emissions from Danish passenger vehicles. In addition, non-CO₂ effects from aviation must be considered, such as condensation trails – so-called contrails – arising from emissions of soot particles, nitrogen oxides, water vapour, etc. which change the chemical composition of the atmosphere and cloudiness, perturbing the radiative forcing. The magnitude of the non-CO₂ effects is uncertain but is estimated to be at least equivalent to the CO₂-effect[1]. A study published by the EU Commission estimates the non-CO₂ effect to be twice the CO₂-effect[2].

An efficient mitigation strategy addresses both CO₂ and non-CO₂ effects

In Denmark, the political emphasis has primarily been on mitigating the CO₂ emissions from aviation through the use of sustainable aviation fuels (SAF)[3]. Using SAF is however one of the most expensive ways to reduce emissions, as identified in the Danish Government’s Power-to-X strategy. The production costs of e-kerosene far outweigh those of green hydrogen or ammonia, which can be used for steel-production or fertilizers with a similar climate impact[4].

Producing e-kerosene necessitates the use of a CO₂ in addition to large amounts of renewable electricity; alternatively, the CO₂ could have been pumped underground and stored permanently (a process known as CCS). Thus, deploying e-kerosene to decarbonize aviation is capital-intensive and requires large amounts of renewable electricity and carbon, all of which can be used more efficiently elsewhere, with greater climate benefit. In contrast, the opportunities for addressing the non-CO₂ effects are affordable and require only limited resources. Additionally, such initiatives can be implemented quickly using currently available technology.

Through better route management and adjusted flying altitude it is possible to avoid areas where atmospheric conditions entail a higher contrail cirrus occurrence[5]. CONCITO calls on the Danish Government to advocate for such regulation at EU level.

Furthermore, hydrogen can be blended into the currently produced fuels at the refineries to reduce the aromatics content in kerosene (Jet A-1 fuel), which cause contrail formation as the aromatics encourage particulate matter formation such as soot upon combustion on which water vapour in the atmosphere nucleates.

Kerosene has an average aromatic concentration of around 17 volume percent. The International Civil Aviation Organisation ICAO prescribes a concentration of a least 8 volume percentage and a maximum 25 volume percent for the sake of safety as older airplanes need the aromatics to ensure the performance of elastomer seals, hence, a minimum limit of aromatics is needed.

Blending hydrogen with kerosene can reduce the content of aromatics using hydrotreating. Hydrotreating remove hetero atoms in refinery streams by reacting the streams with hydrogen in the presence of a catalyst, this is widely used in the refining industry[6]. It should be noted, that existing hydrotreating units cannot move from processing one product to another e.g., jet fuel due to unique configurations. Further, the potential cost of hydrotreating does, among others, depend on scale and whether the refinery has a hydrocracking facility or not[7].

To reach the limit of 8 volume percent, approximately 1 percent of hydrogen, in terms of mass, needs to be added. As a result, the hydrogen required to reduce the aromatic content to the minimum limit is around 1/40 of the hydrogen needed to produce an equal amount of e-kerosene.

In Denmark, a 1 percent annual consumption of 1 million tonnes of kerosene (2019-levels) corresponds to 10,000 tonnes of hydrogen. 10,000 tonnes of hydrogen require approximately 0.5 TWh of renewable electricity. 0,5 TWh of electricity is only half of the electricity needed to produce e-kerosene to cover the demand of domestic aviation (2019-levels). To cover both the domestic and international annual fuel demand with e-kerosene would require around 20 TWh of electricity corresponding to around 60 percent of Denmark’s total electricity consumption.

Adding hydrogen reduces the contrail cirrus occurrence while simultaneously reducing CO2 emissions by increasing the fuel efficiency of the aircraft, as the fuel density (mass per volume) is increased without adding energy density (energy content per volume). Furthermore, instead of being used in e-kerosene, the carbon can be stored underground.

The potential mitigation effect of lowering the aromatics content is subject to great uncertainty, and ongoing research is being done to improve knowledge on this topic[8], however, there is one significant advantage. According to research, cutting the aromatics content in half would reduce the non-CO2 effect by about 20 percent[9]. Assuming the non-CO2 effect constitutes two-thirds of international aviation’s total climate impact, adding hydrogen, as stated above, would reduce the total climate impact of aviation by 13 percent, corresponding to 1.2 million tons CO2-e. To summarize, the climate impact of adding hydrogen is eight times that of converting domestic flights to run solely only on e-kerosene. In addition, the CO2 effect of lowering the aromatic content of kerosene by blending hydrogen is similar when using electricity to produce e-kerosene and lowering the aromatic content.

It is uncertain whether e-kerosene will be cost-competitive

Adding more hydrogen can remove the aromatics completely from the fuel. It is thus possible to achieve equally clean combustion with fossil aviation fuel as with e-kerosene. Whether it is possible to use aromatic-free kerosene depends solely on aircraft design[10]. As a result, the advantage of e-kerosene is therefore reduced to avoidance of emitting fossil CO2 into the atmosphere.

E-kerosene is, thus, a costly method to mitigate emissions, as identified in the Power-to-X strategy published by the Danish Government[11]. However, investments in costly technologies can be justified with the goal of developing and scaling emerging technologies to bring down costs in the long term. Nonetheless, there are no evident analyses or research indicating that e-kerosene will become cost-competitive with (fossil) kerosene, even when considering a high carbon price.

According to international studies, capturing carbon from the air and storing it underground will be far more cost-effective and resource-efficient than producing e-kerosene from the captured CO2[12].

While this will not make aviation free of fossil fuels, the climate impact of capturing CO2 and abating fossil fuels with e-kerosene is similar. If future regulations allow aircraft operators to comply with mitigation requirements by either using e-kerosene or storing of CO2, it seems likely that they will choose the latter because it is less costly. Therefore, it is not given that investing capital in developing an e-kerosene supply chain is required. But in any case, developing direct air capture technologies is a prerequisite for scaling both e-kerosene and storing CO2.

A passenger tax ought to be introduced to ensure the ‘polluters pay principle’

Regardless of the technological solutions, there is a need to regulate aviation’s climate impact ensuring that the impacts are reflected in ticket prices. The polluters must pay for the damages while also working to reduce emissions.

Several factors make the direct taxation of aviation’s climate impact difficult. In practice, the most efficient solution is a passenger tax, as already implemented in Denmark’s neighboring countries.

As a self-proclaimed climate frontrunner, the Danish Government ought to introduce passenger taxes that are at least comparable to those in neighboring countries. CONCITO therefore recommends a passenger tax supplementing the EU ETS taxation, to ensure that also the climate damage from long-haul flights is taxed at least at the same level as shorter flights inside EU.

According to EU Commission figures, a passenger tax similar to the one in Germany will result in a 6 percent reduction in air traffic resulting in both CO2 and non-CO2 emission reductions from the moment the tax goes into effect. This will result in a reduction amounting to 0.5 million tons of CO2e, which is three to four times greater than converting all domestic flights to e-kerosene by 2030. Furthermore, the immediate effect of the passenger tax is important to consider because it provides an additional climate effect when compared to e-kerosene, which is phased in gradually towards 2030.

A such passenger tax can generate tax revenue of €235 million annually when adjusting for changes in behavior and revenue losses. Foreign nationals returning from trips in Denmark will pay nearly half of the tax revenue, resulting in a net socioeconomic revenue from the passenger tax[13]. Because the passenger tax will primarily target high-income groups, who travel the most and for the longest distances, it is a socially balanced tax. [14]. The passenger tax can furthermore be differentiated based on seat size or class and potentially also encourage the use of sustainable aviation fuels[15].

National efforts to reduce aviation emissions are needed regardless of the Fit for 55 package’s ambition level

The EU’s Fit for 55 climate package includes several files directly regulating the aviation sector contributing to cut emissions from aviation, here the phase-out free allowances granted to aviation and the blending of sustainable aviation fuels are highlighted. However, efforts at the European level is inadequate. The legislative proposals presented do not address non-CO2 effects nor the emissions from international flights to destinations outside EU. International flights, which have the greatest climate impact, are not covered by the European emission trading scheme or national duties. The Danish government should reconsider its position and work to address this shortcoming at the EU level[16], while in parallel introducing passenger tax at a national level, as neighboring countries have done.