Introduction
The issue of ecologic fuels has long been standing in every machine-related industry: land, sea, railroad transport, and, of course, aviation. Sindhu et al. (2019, p. 79) state that “increases in the global demand for energy, high fuel prices, and depletion of fossil fuels have led to the search for alternative strategies for energy production”. While regulators have stepped up pressure on some industries, such as power generation, to demand lower emissions, they have hardly noticed others, such as aviation. Perhaps the reason is in relatively small emissions, the social significance of the sector, and its low profitability, as well as in the absence of simple and cheap methods of decarbonization.
However, nothing lasts forever, and the green agenda in aviation is being discussed at ever higher levels. Moussavi and Bossink (2017, p. 1263) add that “the shift of focus in business from competitiveness alone, to the combination of sustainability and competitiveness impacts firms’ capabilities for innovation.” The US authorities are preparing the transition of aviation to renewable energy sources by 2050. Joseph Biden’s administration have been discussing the environmental transformation of air transport for a while now, as the issue stands rather acute. The federal government is considering incentives to support private sector production of cleaner jet fuel (The White House 2021). There are the possibilities of eliminating the traditional jet fuel by 2050 and the transition of aircraft to fuel from renewable resources.
The European aviation industry has also announced a plan to achieve carbon neutrality in the aviation sector by 2050. Carbon dioxide (CO2) emissions for all flights within the EU and aircraft departing from the EU must be reduced by 45%, with CO2 emissions reduced to zero by 2050 (European Commission 2021). At the same time, experts express doubts that hydrogen-powered aircraft will appear before 2035.
The global demand for jet fuel is currently very high, which is concerning in regards of possible environmental damage. One way to reduce jet emissions is to improve engines that burn less fuel. In practice, this still turns out to be an ineffective way to protect the environment: the less fuel the engines burn, the cheaper it becomes to operate the aircraft. As the cost of air travel decreases, the number of buyers increases, which leads to an increase in the number of flights and, accordingly, emissions into the atmosphere. About 200 billion liters per year is needed for the world aviation industry (ICAO 2018). For comparison: in 2021, from 100 to 120 million liters of sustainable aviation fuel (SAF) will be produced – that is, only 0.05% of the total amount of fuel, according to the International Air Transport Association (ICAO 2018). However, the trend towards an increase in the number of aircraft models capable of operating on alternative fuels is already obvious. This paper explores the possible measures the industry can take towards switching to ecofuels, as well as the potential obstacles to it.
Overview of Sustainable Aviation Fuel
Environmentally friendly jet fuel is a fairly broad concept that expands over various types of fuels. Usually, it is called sustainable aviation fuel (SAF), where sustainable means reduced carbon dioxide emissions into the atmosphere, especially compared to regular aviation kerosene. This kind of fuels takes into account both the actual process of fuel combustion in aircraft engines, and the “carbon footprint” in its production.
There are currently two types of SAF which have various sub-types in them. Synthetic fuels are made from carbon dioxide from the atmosphere and hydrogen through electrolysis. Biofuels can be produced from various raw materials: used vegetable oil and animal fat, agricultural waste, cellulose, algae. Alalwan et al. (2019, p. 127) add that “in contrast to other green energy resources, biofuels can provide liquid fuels essential for transportation; development efforts should be focused specifically on the genetic engineering of algae.” Thus, SAF has two environmental benefits at once: it reduces CO2 emissions and allows waste to be recycled. Also, an important requirement for clean fuel producers is the preservation of the land use structure (no additional areas are allocated for raw materials for SAF, no forests are cut down) and the lack of competition with food crops. Due to the latter, only waste is used for SAF, so farmers do not reduce the area under the varieties used for food and do not sell them to fuel producers to the detriment of the food industry.
Although it is often said that humanity produces a lot of waste and something needs to be done about it, there are not so many raw materials suitable for the production of SAF. One of the largest SAF producers, Neste, currently produces about 100 thousand tons of clean fuel per year with the prospect of increasing this volume to 1.5 million by 2023 (Neste 2021). This is a drop in the sea of aviation fuel consumption. Even in the crisis year for the industry in 2020, airlines needed 220 billion liters (or 176 million tons) of kerosene against 350 billion in 2019 (IATA 2021). There are currently 65 airports on the ICAO list that offer SAF (ICAO 2021). For comparison, there are about 42 thousand airports in the world (Welsh 2021). Thus, current volumes of SAF production are not able to meet the needs of aviation.
One of the world’s first eco-fuels was “biogas”, made from carbon dioxide in the air and hydrogen produced from water by electrolysis. According to Prussi et al. (2019, p. 1), “biofuels to meet aviation carbon neutral growth would account for 16% of the current EU total biofuel demand.” In the simplest case, in the presence of anaerobic bacteria, a reduction reaction occurs and methane is obtained: CO2 + 4H2 → CH4 + 2H2O. However, one cannot simply fill an airplane with methane, as it will pose both significant danger and reduced efficiency. Thus, there are 6 types of SAF that can be used in aviation:
The first is synthesized isoparaffin kerosene that is synthesized via the Fischer-Tropsch method (FT-SPK). From coal, natural gas or biomass, synthesis gas is produced, consisting of hydrogen and carbon dioxide. Then this synthesis gas is catalytically converted into liquid hydrocarbon fuel in a special reactor. There is also a variety of this fuel FT-SPK/A containing aromatic compounds.
Another type is the synthesized isoparaffins (SIP) – a type of fuel similar to the isoparaffin kerosene. However, the method of production is rather different from the Fisher-Tropsch approach described above. Here, hydrocarbon molecules suitable for blending with fossil fuels are obtained during the fermentation of vegetable raw materials with a high content of sugars.
Third type of sustainable fuels refers to hydrotreated fatty acid esters and fatty acids (HEFA). Vegetable oils are used here: both obtained specifically for the production of fuel (for example, from rapeseed crops), and from food production waste – vegetable and animal fats. Oils are deoxygenated and then hydrotreated – that is, chemically converted in the presence of hydrogen under high pressure at high temperature – to become a hydrocarbon fuel suitable for blending with kerosene.
Next is the hydrotreated isoparaffinic kerosene obtained by hydrocarbon synthesis (HH-SPK, aka HC-HEFA). It is obtained from the fats contained in the green microplanktonic algae: Botryococcus Brown – the same raw material is used, in particular, for the production of biodiesel. This type of fuel is, perhaps, one of the most ecologically friendly among all sustainable fuels.
Another type of eco-fuel is the alcohol jet fuel (ATJ), where isobutanol or ordinary ethyl alcohol undergoes dehydrogenation, oligomerization, and hydrotreatment, turning into hydrocarbon fuel. Ethyl alcohol can be obtained by hydrolysis from wood (for example, production waste, sawdust), or bioethanol can be taken – that is, in fact, ordinary food alcohol obtained by fermentation from plant materials. Alcohol-based fuel produces more emissions than HEFA or SPK Fischer-Tropsch fuels because the production of alcohol from starch-based crops requires a lot of energy and releases a significant amount of greenhouse gases in the process. In general, biofuels derived from waste and by-products tend to have lower greenhouse gas emissions than biofuels derived from crops.
Finally, the last type of SAF is the jet fuel produced by catalytic hydrothermolysis (CHJ). Here, heated water is added to fatty acids from waste oils, including energy oils (transformer, turbine, compressor, obtained from oil). Then, cracking, isomerization, and cyclization of paraffins and isoparaffins occur in the reactor at very high pressure and temperature, producing cycloparaffins, respectively, as well as aromatic compounds.
Most of the listed fuels can be mixed with jet fuel in various proportions according to their combustion potential. For this strategy of using SAF, no modifications to existing engines are required – that is, the aircraft flies on the mixture exactly the same as on 100% kerosene. At the same time, industry works to completely abandon kerosene. Boeing, for example, promises to switch all manufactured aircraft to the possibility of using exclusively biofuels by 2030 (McGlaun 2021). The world’s first commercial biofuel flight using a Boeing 777 cargo was successfully completed back in 2018 (Boeing 2021). SAF has a much smaller carbon footprint than conventional jet fuel due to the raw materials it is made from. Such raw materials seem to be an unlimited resource, but they are not. In fact, the availability of sufficient amounts of waste suitable for conversion into sustainable aviation fuel is rather limited.
Electricity as a Means of Sustainable Energy
In modern times, attempts to make a zero-carbon aircraft have first taken the path of an electric aircraft. If electric cars and gadgets could take off after the invention of lithium-ion batteries, then why not apply it to the aviation industry? However, there were several difficulties here, which posed significant limitations for the implementation of electric engines. First, the energy density of lithium-ion batteries is too small to take a large propeller-driven aircraft into the air, as well as to allow a small aircraft to fly for a long time. For example, even with electric drones, flight duration is the biggest obstacle today. Secondly, the charging time for rechargeable batteries is hours, if one wants them to last a long time. Therefore, the aircrafts will have to have removable battery packs, which in any case will complicate operation. Thirdly, lithium-ion batteries are flammable and thus extremely dangerous to use in aircrafts. For example, it was the ignition of a lithium-ion battery that destroyed the flight prototype of the Israeli promising electric passenger aircraft Alice.
There, hydrogen fuel cells (HFC) come to the light, in which hydrogen does not burn, but is electrochemically oxidized, giving electricity as a result. The output is water – only without the potential emissions of nitric oxide that can form when hydrogen is burned in a jet engine – and electricity, which is supplied to the rotating propeller by an electric motor. This technology has the potential of replacing the internal combustion engine in the future.
Hydrogen as a Sustainable Fuel
At the end of the 20th century, oil fuel gave way to gas fuel. The share of oil in the global energy sector decreased, while the share of gas exceeded the expectations of the experts. According to modern ideas of geologists, the potential gas reserves on the planet are dozens of times greater than the reserves of coal and oil combined. Natural gas has long been a common automotive fuel. Today, scientists are thinking about using it in river, sea, and rail transport. Aircraft builders have also come to grips with this problem.
Eco-friendly kerosene is a great marketing ploy; however, it has practically nothing to do with zero greenhouse emissions. Therefore, the European airline concern Airbus is working on fundamentally new airliners with hydrogen engines and zero emissions of pollution (Airbus 2020). It is assumed that these liners will be equipped with a modified gas turbine engine that works by burning hydrogen instead of kerosene. Liquid hydrogen will be stored in special cryogenic tanks that will ensure its safety.
Liquid hydrogen (LHG) was used as aviation fuel – this almost ideal environmentally friendly fuel emits during combustion mainly water and a small amount of nitrogen oxides. In terms of calorific value, hydrogen is three times superior to traditional aviation kerosene. However, at the same time, hydrogen is explosive; it can only be stored and transported in a liquid state at very low temperatures close to absolute zero. This presents a very serious problem for implementing hydrogen in the aviation industry.
The problem of special importance is explosion and fire safety. On an aircraft powered by LHG, it has its own specifics. If the airtightness of the fuel system of an aircraft filled with traditional fuel – kerosene – is broken, it, like a low-evaporating liquid, fills a relatively small volume. Although it is very difficult to detect a leak, the risk of fire or explosion is not so big. However, on an aircraft powered by LHG, things are much more serious. In the event of a gas leak from the fuel system, it quickly fills the airframe compartments, posing a significant danger. Therefore, to avoid possible ignition of methane, all spark-producing electrical equipment should be removed from the compartments, and gas analytical sensors should be installed that will signal an emergency. In addition, forced ventilation has to be provided in the compartments.
In order for LHG flights to become regular, it is necessary to create ground infrastructure at airports. These are, first of all, gas liquefaction plants and gas filling equipment, with strict safety policies. Additionally, since most airports are located near main gas pipelines, where gas is under high pressure, gas pumping and gas distribution stations are also needed. The transition to LHG is a huge challenge for any economy, as different industries need to quickly create new technologies, master the production of completely new products, and deploy new or adapt existing infrastructure. In various countries, there are developments on the use of hydrogen as a fuel for aircraft engines. However, it is not necessary to say that these developments can be commercialized in the shortest possible time. In addition, it must be admitted that the world has not yet formed certification requirements for all aspects related to the use of hydrogen.
Unlike kerosene, methane will have to be supplied to the engine combustion chamber in gaseous form to exclude a two-phase state, which completely eliminates the use of standard fuel units, communications, manifolds and injectors. This greatly complicates the design of the engine, and in some cases makes it impossible to modify it to be powered by two types of fuel. To ensure the safe operation of LHG fuels, it is necessary to have qualitative and quantitative methods for assessing and comparing each type of hazard. Qualitative and quantitative assessment – the determination of the type and degree of hazard – allows for a comparative analysis of condensed fuel according to hazard criteria. In the long term, it also makes possible to formalize the task of choosing technical means and methods for the safe operation of fuel systems using LHG, as well as its storage and transportation.
Thus, replacing oil and gas supplies with hydrogen appears too problematic as of yet. When it comes to the future transition to hydrogen during decarbonisation, the industry abandons not only directly oil or natural gas, but also the possibility to obtain hydrogen from them. That is, the technology to produce “brown”, “gray”, and “blue” hydrogen. Green hydrogen is produced mainly from water through electrolysis technology using more environmentally friendly energy – in particular, wind or nuclear. Other “colors” can be used as a kind of transitional stage, while the volumes are insufficient and the cost of real green hydrogen is high.
Despite all its prospects, hydrogen is a very “capricious” type of fuel, as its storage and transportation are fraught with serious difficulties. For example, storage of hydrogen is even more expensive than production. First, storage systems must withstand either high pressure or cryogenic temperatures. Secondly, they must contain active materials that interact with water and air. The hydrogen can move through pipes, ships and tankers – in the future, the cheapest way would be hydrogen pipelines, but so far they mainly exist only within the areas of chemical plants and refineries. The use of existing gas pipes for the most part is not possible due to technological features. To transport large volumes of hydrogen, special pipelines are needed that pressurize the gas or cool it down to liquid.
So far, for storage and transportation, the technology of mixing hydrogen with ammonia, the density of which is higher, seems to be the most preferable. However, in that case, about a third of the supply volume will be lost during the conversion and reconversion of the mixture. Overall, the issue of LHG storage and movement remains the most acute and so far unresolved problem of hydrogen energy. In addition, it takes a different amount of energy to transmit the same amount of energy in the form of natural gas and hydrogen through the same pipelines.
Biodiesel as a Sustainable Fuel
A promising direction for obtaining environmentally friendly fuel based on renewable biological resources is the production of the so-called biodiesel. The main difference between biodiesel and conventional diesel fuel is its environmental friendliness. Exhaust gases from industries powered with such fuel are, according to some indicators, significantly cleaner than machines running on petroleum diesel fuel.
Biodiesel fuels are monoesters of vegetable oils or animal fats – they are obtained as a result of a chemical reaction of oil and fat transesterification with alcohol in the presence of a catalyst. The reaction products are monoesters known as fatty acid methyl esters which are directly used as fuel for diesel engines, and glycerin that is used in soap making and pharmacology. The alcohol required for a reaction is methanol and ethanol, but due to the lower price, methanol is used more often.
In Europe and the US, fuels such as rapeseed oil methyl ester (RME) and soybean oil methyl ester (SOME), are already being used as alternative diesel fuels and additives to conventional fuels. The use of vegetable oils as a motor fuel is far from a new idea; even Rudolph Diesel in his time invented an engine that actually runs on vegetable oils, in particular, peanut oil. However, due to the availability of cheap petroleum oil, phytofuels have been largely ignored for a long time. Today, finally, the industry is faced with the costs associated with its dependence on oil, as well as with local and global pollution of the planet. The advent of the post-fossil fuel economy is approaching, and the industry must do everything possible to accelerate this transition.
The use of biodiesel fuels reduces the emissions of almost all harmful substances in comparison with petroleum diesel fuels. Also, after processing used oil into sustainable aviation fuel, the energy performance is even slightly better than kerosene which means that less SAF is needed. The fact that biodiesel does not contain impurities contributes to its better performance. In addition, biodiesel (for example, spilled) completely decomposes into components that are non-corrosive to the environment. This is due to the fact that it is produced not from petroleum products, but from vegetable oil. Absolutely any oil can be used: olive, vegetable, animal fats. Moreover, any oil used for cooking will work for bioaviation fuel (Slav 2021). Developments are currently underway related to the possibility of using kitchen oil processing waste so that aviation needs do not affect the food industry. Major restaurants are already selling used cooking oil for bio-fuel processing. The more people eat fried food, the more it will help the industry.
Replacing aviation kerosene with biodiesel has the highest potential to achieve reductions in aviation carbon dioxide emissions. This is important because this type of fuel can be used in existing aircraft. It can be added to tanks and mixed with jet fuel or kerosene. Kim et al. (2019, p. 317) report that “due to high R&D costs and long lifecycle of aircrafts, drop-in biofuels stand as a promising solution for addressing aviation’s environmental sustainability.” There is already a proven safe technology that gives effective results. The difficulty today lies in the fact that only a few plants produce bioaviation fuel, while every year, more than hundred new processing plants are needed.
So, the industry is yet quite far from the figures for the required number of deliveries. Moreover, the industry’s first priority is that the biodiesel must be safe and freeze-resistant for aviation use. There is also another issue: according to O’Connell et al. (2019, p. 54), “aviation biofuels production method can have a considerable effect on GHG, as they may produce higher GHG than standard aviation fuel.” However, the price still remains the main problem of switching to bio-aviation fuel, since its cost is several times higher than the cost of jet fuel. Additionally, delivery is currently generally carried out by trucks, which significantly increases the cost.
Private aviation is an important productivity tool for companies as it connects markets in an increasingly globalized world. However, it also poses environmental risks, forcing aviation sector players to innovate through design development, new consumption patterns, more efficient and less polluting jets. The aviation industry, not just commercial airlines but private aviation as well, is committed to reducing carbon emissions. Still, governments need to take appropriate policy action to accelerate the growth in the production and use of cleaner fuels. Indeed, its production would require long-term political security to reduce investment risks and focus on research, development, and commercialization of improved and more efficient production technologies.
Additionally, opportunities need to be found to cover the difference in fuel costs. Airlines are not able to cover the difference in price on their own, as they operate on low profits, and incomes are not large enough to afford to use fuel several times more expensive. The cost of bio-fuel is the biggest expense for airlines. The government and other organizations should help to compensate for the difference in price. For example, all emissions that will not be reduced by biofuels, fuel efficiency improvements and other technologies are planned to be covered through the use of green loans that airlines will buy from certified companies. If a company has emitted 100 tons of CO2, it will buy a certificate from a company that, for example, is engaged in planting forests. It is assumed that these same forests will absorb 100 tons of gas thrown out.
Obstacles for Using SAF
However, the majority of airlines is not yet fully ready to switch to SAF in any proportion. The main problem behind it is that SAF is on average significantly more expensive than regular kerosene. However, according to several experts, demand for biofuels is likely to grow from European airlines, whose customers are increasingly more willing to pay a certain amount for the sake of a more environmentally friendly flight. According to Sindhu et al. (2019, p. 79), “the challenges associated with the commercialization of biomass-derived biofuels can be overcome by process integration as well as fine-tuning of various process variables affecting production”. Subsequently, the growth of demand invariably leads in a market economy to increased competition, scaling up production and lowering prices.
Government incentives will also help the industry switch to SAF with minimum losses. For example, tax incentives for US refineries could allow them to produce more green fuel branded fuel. If the World Energy plant received $2 from the government for the production of one gallon of SAF, then the cost of biofuel and standard jet fuel would be equal in this case. Therefore, the federal government, as well as the industry itself, should lobby for its use and try to reduce the costs or provide subsidies for companies that use SAF. In particular, new ICAO requirements come into force from 2027, according to which countries from which international flights are operated must either use SAF or pay a fee for not using it. More precisely, the obligation is not to use the fuel itself, but to control the carbon dioxide emissions on flights – they must be reduced by at least 10% compared to pure kerosene flights. Those who have not reduced the emissions will have to buy a quota for carbon dioxide emissions – these funds will be used to develop the production of alternative fuels.
Another problem is that kerosene is different in different countries. In most countries it is the international standard Jet A-1, and the permissible proportions of mixing with SAF are calculated based on it. However, Australia, Brazil, the UK, Spain, Canada, China, Russia, France, Sweden, and Japan have their own national standards for aviation kerosene. Thus, the harmonization of standards and certification of the respective mixtures are required in order to at least correctly calculate emission reductions, and the size and cost of quotas.
Consequences for the Planet
If aviation remains at the same level without changes, there will be detrimental consequences for the planet. The industry’s emissions are growing much faster than they should. Unless efforts are made to change, aviation will continue to emit more and more carbon dioxide. However, the world cannot stop flying so the industry needs to decarbonize the flights. These steps are costly at the moment, but the longer the industry stay idle, the more carbon will be released into the atmosphere and the more damage will be done to the planet. Companies may even have to fly more expensive to reduce pollution.
CORSIA, an international agreement to reduce carbon dioxide emissions from international air travel by 2020, is a good, responsible start. However, it is also necessary to find volunteer ways to significantly reduce emissions in the aviation industry itself. To do this, the industry needs to either decarbonize fuel and aircraft, or burn significantly less jet fuel. These should be priorities and top actions in addition to CORSIA. Only a few countries in North America (Canada, USA) and Europe (Scandinavian countries, Germany, Great Britain, the Netherlands) have investors ready to cooperate. Brazil has shown it can lead the way in biofuel production from sugar by producing ethanol that can be processed into biofuel. There are also possible initiators in the Middle East and South Africa which use tobacco oil, and Australia and New Zealand have capital ready for investment. The industry needs to start small and move on to countries that do not have enough development and investment opportunities for SAF production.
Conclusion
The industry has failed to keep up with efficiencies, and the industry does not present sufficient results in achieving decarbonisation. A number of factors are to blame for this: airlines have optimized the “passenger load” but are holding empty seats, and additional aircraft flights are required to meet increased demand, which increase fuel consumption. Moreover, the kerosene prices remain low compared to biofuels, as well as aging aircraft leads to increased fuel costs.
How does the climate change under the influence of aviation? Stronger storms, stronger wind/turbulence, and, most importantly, carbon dioxide emissions. Frequent delays require more fuel, big storms require longer flights to avoid extreme weather conditions. Higher ambient temperatures require more takeoff power to compensate for reduced lift. Carrying additional fuel for contingencies requires more energy to propel a heavier aircraft. Higher wind speeds require more engine thrust when flying into a headwind. All of these factors contribute towards environmental damage.
There is still no clear path to revolutionary aviation despite all efforts. Blended wing conversion of commercial aircraft and other breakthrough technologies can generate at least a half of fuel savings compared to existing designs. These aircraft can lead to significant reductions in carbon dioxide emissions. However, there is no defined strategy for using these designs in commercial aircraft. Massive investment is needed to shift traditional manufacturing processes. Current practices offer economical aircraft size options for airlines with limited cabin lengths, while mixed wing models violate this advantage, with airlines preferring uniformity in their aircraft to maximize flexibility. Airports will require significant infrastructure changes, including wider taxiways. An all-electric aircraft may only become a reality after 2030. Therefore, all of the above factors will not contribute to a significant reduction in carbon emissions over the next two decades.
There are many barriers to low carbon fuels, despite all their advantages. Sustainable aviation fuel (SAF) offers the greatest potential for reducing carbon dioxide emissions and is a good alternative to jet fuel. Bio-aviation fuel such as biodiesel has been proven to be as safe and effective as jet fuel. However, there are many obstacles to the adoption of sustainable SAF, which is currently at least several times more costly fossil fuels, and there is only one dedicated production – one refinery operating in the world. The International Civil Aviation Organization (ICAO) states that many more new refineries are needed each year to meet the aviation industry’s goals. The rapid growth of SAF should take place in about a decade ahead of today. There is little time left to reduce the cost of biofuels and invest in the thousands of necessary production capacities, thus, the SAF industry must move forward. Working together, it is planned to soften competition in the industry and generate practical market-based answers to critical questions facing the industry.
To survive, aircraft manufacturers must take the initiative to compensate for the damage caused by aircraft. With the exception of bans or taxes, there are three options. First, it is possible to neutralize the damage from emissions by planting trees that feed on carbon dioxide, installing wind turbines, or supplying environmentally friendly materials to third world countries. Second, the search for new ways to reduce aviation fuel costs can continue. For example, ceramic-based paintwork reduces drag and fuel consumption. Lower fuel consumption could lead to lower airline fares and more flights, but this should be a net benefit for business aviation, where lower costs are not as significant.
However, the best solution to the problem is to switch to low-carbon, and ultimately clean, ecological fuels as soon as possible. Sustainable aviation jet fuel, which is made from a mixture of biomass, oils, waste, and other means, is carbon-free and can lead to significant reductions in harmful emissions. Today, a gradually increasing tax on aviation carbon emissions can provide funds and market incentives for development and push us to use low-carbon and carbon-free fuels.
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