Airlines want to make flight more sustainable. How will they do it?

When airline executives think about how to make air travel more sustainable, the biggest arrow points to the fuel burned to keep planes in the air. "For short-haul flights, there's some encouraging movement for zero-emission planes, such as those running on electricity," Pavlenko says. "But for everything else, it comes down to what fuel you can switch to."

The goal of using sustainable aviation fuels, or SAFs, is to reduce the amount of greenhouse gases emitted during the lifetime of the fuels, from production to combustion, compared with current petroleum-based jet fuels. The scale of that reduction depends on the process used to make the fuel and the carbon source. And even though some SAFs boast significant emission reductions, few are made at large scale.

Pathway to Fischer-Tropsch synthetic paraffinic kerosenes

To make Fischer-Tropsch synthetic paraffinic kerosenes, industrial chemists oxidize the carbon source to synthesis gas, a mixture of carbon monoxide and hydrogen, and then run this gas over an iron, cobalt, or ruthenium catalyst to produce hydrocarbons. They then blend the products with fossil-derived jet fuels before the result can be burned in a jet engine.

Credit: International Council on Clean Transportation, National Energy Technology Laboratory/Yang H. Ku/C&EN/Shutterstock

Sources: International Council on Clean Transportation, National Energy Technology Laboratory
Note: In the Fischer-Tropsch reaction scheme, n indicates the number of carbons in the resulting hydrocarbons. For sustainable aviation fuels, that number is usually between 10 and 20.

Generally, commercial airliners use kerosene fuels called Jet A and Jet A-1. They're mixtures of paraffins, naphthenes, aromatics, and olefins and are mostly derived from petroleum. Some companies, such as Airbus, which declined to be interviewed for this story, are looking to hydrogen as a fuel, since it combusts to produce water vapor. But developing H₂-burning engines is in early stages, so such planes are still far in the future.

Most companies are looking instead at drop-in fuels, or fuels that can work with existing jet engines. They need to have similar properties to jet fuel, including their energy released when burned, performance at low temperatures, and flow. These specifications ensure that the fuels will behave the same way fossil-derived jet fuel does in an airplane's engine, Pavlenko says. One way fuel manufacturers get SAFs to match these specifications is by blending them with conventional jet fuel, he says. How much is blended varies quite a bit, however. Most SAFs need to be blended 50:50 with Jet A-1. And while there's interest in SAFs that don't need to be blended, none have been commercially approved, according to Pavlenko. The standards these 100% SAF fuels would have to meet haven't even been set yet and will take probably 3–5 years to get completed, Rick Barraza, vice president of administration at the alternative-fuel company Fulcrum BioEnergy, says in an email.

There are three main ways to make SAFs: from hydroprocessed esters and fatty acids (HEFA), Fischer-Tropsch synthetic paraffinic kerosene (FT-SPK), and alcohol-to-jet synthetic paraffinic kerosene (ATJ-SPK). All three can be used at similar blend levels, around 50%.

To make HEFA fuels, industrial scientists remove the oxygens from molecules in unused vegetable oils, or waste fats, oils, and greases. Then they treat the mixtures with hydrogen to yield burnable hydrocarbons the right length for jet fuel, usually between 10 and 20 carbons long, according to the ICCT. Compared with Jet A and Jet A-1, these fuels are the most cost competitive SAF technology, Pavlenko says.

To make Fischer-Tropsch-SPK, scientists oxidize a wide variety of plant and human wastes and residues to make synthesis gas, a mixture of H₂and carbon monoxide. Adding a catalyst—usually iron, cobalt, or ruthenium—to this gas triggers Fischer-Tropsch synthesis, which produces hydrocarbons.

The sources for alcohol-to-jet-SPK are crops such as sugarcane and corn, plant and agriculture wastes, and in some cases industrial flue gases. Generally, scientists convert these feedstocks to ethanol or isobutyl alcohol and then upgrade the alcohols to long-chain kerosene by removing water, treating them with hydrogen, and combining short-chain hydrocarbons to form longer ones.

Fickle fuels

Depending on the carbon source, sustainable aviation fuels can produce a large range of greenhouse gases over their lifetimes. Scientists compare these fuels’ carbon footprints by looking at the life-cycle carbon intensity, measured in grams of CO₂ equivalent (g CO₂e) released per megajoule of energy burned.

Sources: International Council on Clean Transportation, International Civil Aviation Organization.
a Not including plastic municipal solid waste.

These three alternative fuels don't have the same impact on the environment. The ICCT recently released a report showing the amount of greenhouse gases emitted during various alternative fuels' life cycles, including growing or collecting the carbon sources, synthesizing the fuels, and combusting them in an engine. The data came from the International Civil Aviation Organization's Carbon Offsetting and Reduction Scheme for International Aviation program, a United Nations effort.

The ICCT found that alcohol-to-jet-SPK fuels tend to have higher emissions than HEFA or Fischer-Tropsch-SPK fuels because making alcohols from starch-based crops takes a lot of energy and emits substantial amounts of greenhouse gases. In general, biofuels made from wastes and by-products tend to have lower greenhouse gas emissions than crop-based ones, the ICCT's Pavlenko says.

The SAF industry is shifting more to such waste-based fuels, according to Aaron Robinson, senior manager of environmental strategy and sustainability for United Airlines. Ten years ago, alternative-fuel companies focused on growing crops for biofuels. "Two out of our first three SAF flights were powered by agriculturally grown material," he says. "That's the way we thought the industry was going to be going." But life-cycle analyses have shown how environmentally costly that route can be. When the fuel source is a food crop, the process contributes to deforestation because more land is needed to grow the additional crops, Pavlenko says.

No fuel companies currently produce Fischer-Tropsch-SPKs, so the fuel currently in production with the lowest lifetime GHG emissions is HEFA, depending on the feedstocks. "Some SAFs actually don't offer very many, if any, greenhouse gas savings at all, such as palm oil–derived biofuel, whereas others can have greenhouse gas reductions approaching 100%," Pavlenko says. Used cooking oil and waste animal fats have lower lifetime emissions and are more popular than palm oil, he says.

The company closest to commercial production of Fischer-Tropsch-SPKs is Fulcrum BioEnergy in Pleasanton, California. This company's technology uses municipal solid waste, also known as trash, to make jet fuel. Fulcrum plans to start producing biofuel in the last quarter of this year at its plant just east of Reno, Nevada, Vice President Barraza says. The company plans to build eight other plants with a total production of about 1.5 billion L per year, enough to meet the needs of United and other partners, which include Cathay Pacific Airways and Japan Airlines, Barraza says.

Using municipal solid waste as a feedstock for jet fuel could provide significant greenhouse gas emission savings. Generally, the breakdown of municipal solid waste in landfills produces methane, which has over 28 times the climate change impact of CO₂ over 100 years. "By diverting [municipal solid waste] away from the landfill, we are thereby avoiding all that methane from being generated," Barraza says. The overall greenhouse gas life-cycle analysis for municipal solid waste fuel shows that this pathway can reduce greenhouse gas emissions by over 85% compared with fossil fuels. But the key is the company must first remove plastic waste from the trash.

Plastic sitting in a landfill is actually a form of carbon storage, the ICCT's Pavlenko says. "But if you're converting it into fuel and combusting it, that carbon that would have been safely in the ground for a long time, now it's in the atmosphere," he says. Barraza says the waste feedstock the company uses is mostly organic or biogenic material. "We have the ability to remove a fair amount of the high-value plastics and metals from the raw MSW as part of our feedstock preparation process," he says.

Powering airplanes with waste is certainly the dream for sustainable flight, and a viable one, United's Robinson says. "You could power all of United using just 20% of US landfill waste."

But the reality is that relative to total amounts of jet fuel, airlines aren't using a lot of SAFs right now. "It's less than 0.1% globally," Pavlenko says. For example, United has used about 3.8 million L of SAFs per year in recent years, compared with more than 15 billion L per year of conventional jet fuel, Robinson says. To bump up the amount of SAFs it has access to, United has struck deals with Fulcrum and World Energy, he says. Delta and JetBlue have agreements to purchase SAFs from the Finnish company Neste. These agreements give airlines both price and source certainty for future fuels and provide the fuel companies with a future market for their products.

Fulcrum's Barraza declines to provide details about the timeline for delivering the fuel the firm has promised to United. The big question, Robinson says, is how much the fuel is going to cost and if United can get it soon enough to make a difference in lowering emissions.

The materials used in every part of a plane can determine its weight, aerodynamics, and resistance to the wear and tear of whooshing through the atmosphere. What these materials, and the coatings applied to them, are made of and how they perform can therefore significantly affect a plane's environmental impact. Companies and researchers are now investigating lighter and less toxic materials and coatings for the more sustainable planes of the future.

Airplane coatings

Every part of a plane needs a coating to improve its function. Here are some examples from outside and inside a plane.

Credit: C&EN/Shutterstock

1. Cockpit windows: In addition to heat- and ultraviolet-resistant coatings, cockpit windows are coated with a conductive oxide material. Pilots can apply a voltage to the material to melt ice off the windows, saving deicing time and reducing delays.

2. Engines: The combustion chamber of a jet engine can reach almost 1,400 °C, and newer, more efficient engines need even higher fuel compression and combustion temperatures. All parts of airplane engines are coated with materials, often ceramics, to help them withstand these extremes.

3. Fuselage: The outsides of planes need coatings such as chrome to help resist rust, and polyurethanes and acrylics to protect against damage from ultraviolet light and improve aerodynamics. Coatings are also used to add airline logos to planes.

4. Passenger windows: Windows get coatings made from plastics and stretched acrylics to make them heat and ultraviolet resistant. New technology in development could add an electrochromic layer. That would allow passengers to dim the windows, eliminating the need for window shades.

5. Landing gear: Protection against rust and impact resistance are especially important in the landing gear, critical parts of the aircraft that have to withstand harsh conditions and intense forces. Coatings for landing gear include chrome; hard, diamond-like carbon; and anodized metals.

6. Passenger seats: Parts inside the plane that passengers don’t see, such as the mechanism that lets passengers recline their seats, also need coatings to help them resist friction and wear.

7. Tray tables: Plastic tray tables get coatings to prevent food stains, kill viruses and bacteria, and resist damage from cleaners used 10–15 times a day. These polymer materials have embedded nanoparticles or are treated with nonstick coatings and quaternary ammonium compounds.

An airplane's weight significantly contributes to its carbon footprint because lighter aircraft need less fuel to operate, and less fuel burned means lower emissions. One way to make airplanes lighter is to change what they're made of.

Airplanes were historically made of metal, usually an aluminum alloy. Now some new airplanes, such as the Boeing 787 and Airbus A350, are about 55% composite materials instead, says Samit Roy, an aerospace engineer at the University of Alabama. A composite is anything made up of two or more materials. For example, some companies build aircraft wings, tails, and parts of the fuselage with polymers that have embedded carbon or glass fibers. Composites can reduce the weight of airplanes by up to 20%, Roy says.

Roy's group is working on new composites consisting of carbon fiber with embedded nanoparticles. In addition to being lightweight, these materials can be 3D printed. Printing would reduce material loss because parts can be produced in the exact size and shape needed instead of being cut out from a larger piece of material, as is done with aluminum, Roy says.

Roy and colleagues also want to make these composites conductive. "If an aluminum aircraft like a 747 gets struck by lightning, the lightning passes right through on the skin of the aircraft, doing minimal or no damage," Roy says. But most composite materials used in aircraft are not conductive on their own, so a direct zap could do serious damage. To protect against this, aircraft manufacturers like Boeing and Airbus put copper mesh over the fuselage of their composite planes, but that layer adds cost and weight, Roy says. To improve electrical conductivity and let planes ditch the heavy copper layer, scientists are developing reinforced composites with nanographene or carbon nanotubes, and conductive polymers.

Coatings' main purpose is to add new functions and properties to surfaces, says Lars Haubold, manager for coating technology at the diamonds and coatings division of Fraunhofer USA Center Midwest, a research nonprofit that is partnered with Michigan State University. Coatings provide corrosion protection, add insulation, reduce air friction, or simply decorate the plane with a company's logo. Pretty much every part of a commercial airplane gets some kind of coating, whether it's the windows to add ultraviolet light protection, the landing gear to keep the mechanics from rusting, or the tray tables to make them stain resistant.

One of the primary jobs of some coatings is to keep a part from rusting and breaking down. For many years, the go-to corrosion inhibitor in airline industry coatings was hexavalent chromium, also called chrome. Unfortunately, the compound is a known carcinogen and is harmful to the eyes, skin, and respiratory system. Cr(VI) has contaminated water supplies in a few well-known cases, such as the one featured in the movie Erin Brockovich. Because of Cr(VI)'s health and environmental issues, the US Environmental Protection Agency limits chromium emissions in electroplating and levels in drinking water. The US Occupational Safety and Health Administration also lists the compound as a carcinogen and regulates workers' exposure to it.

Coatings companies have been trying to eliminate the compound for a very long time, says Robin Peffer, the global marketing manager for aerospace coatings at the paint and coatings company PPG Industries. The company is looking to alternatives with alkali earth, rare earth, and transition-metal compounds. In addition to being less toxic, chrome-free materials can potentially be lighter. "So if we have a primer that's 20 or 30% lighter than standard chromated primers today, that's going to have a direct impact on fuel efficiency for the aircraft," Peffer says.

Changing the way coatings get applied is another way to improve sustainability. Chrome-based coatings typically need to be sprayed on, Peffer says. Many of the alternative anticorrosion coatings can be applied by immersing the parts in a coating solution instead. This process allows coatings companies to apply the material more efficiently, especially to complex shapes. "We can get upwards of 75% or more weight savings because we're putting the coating on more uniformly across the part than we can with spray," Peffer says.

Plasma electrolytic oxidation is another potential way to apply coatings. This method is similar to anodizing, which electrochemically converts a metal to its oxide, making it more durable and corrosion resistant. In plasma electrolytic oxidation, scientists apply between 200 and 1,000 V to a metal submersed in a coating solution, heating the material up to its plasma state, around 10,000 °C, says Ankit Khurana, vice president for engineering at Keronite, a start-up surface technology company. The material's lattice structure then opens up, allowing the material to be infused with particles from the surrounding solution. This process embeds the coating material into the metal while it converts into an oxide layer, he says.

The resulting layer is both thinner and harder than one created by traditional anodizing, Keronite CEO Matt Hamblin says. "The coatings are anywhere from 5 to 35 μm thick, so they really carry no weight at all," he says.

Plasma electrolytic oxidation coatings also are three to four times as wear resistant, so they can be replaced less frequently, Hamblin says. Commercial airliners fly an average 3,500 h per year, so their coatings need to be replaced at regular intervals, some as often as every 3 months. A longer-lasting coating means less time in the repair hangar and less overall material used, Hamblin says, providing both cost savings for the airline and improved sustainability.

One thing to keep in mind, Hamblin says, is that a lot of these technologies are going into airplanes currently being designed that will be flying years from now, not what's currently in the air. Airline companies have "a tendency to redesign or reuse what they've already got, maybe with a slight tweak," he says. "You don't necessarily see these huge shifts." And because airlines use their planes for a long time, current coating technologies will probably still be used on planes in the air for the next 25 years.

The University of Alabama's Roy agrees that these technologies will find commercial use down the road. "That's what research is all about, the look ahead," he says. But as time goes on, more and more airline companies and science agencies are becoming aware of how important sustainable materials technology is, Roy says. "I think now they're waking up more to it. The possibility."

Supersonic air travel has had a mixed record of success. Operating from 1976 to 2003, the Concorde could fly between London and New York City in around 3 h and was considered a marvel of aerospace engineering. But the plane was retired because both airlines that operated the flights, Air France and British Airways, were losing money on the supersonic routes.

Gas guzzler

According to a 2018 study by the International Council on Clean Transportation, the average supersonic airplane would burn between five and seven times the fuel per passenger that a conventional, subsonic flight on the same routes does.

Source: Anastasia Kharina, Tim MacDonald, Dan Rutherford, Environmental Performance of Emerging Supersonic Transport , The International Council on Clean Transportation, July 17, 2018.
Note: Fuel estimates are averages and vary depending on aircraft specifications and environmental policies.

Since then, supersonic airline companies have popped up and petered out. One new company, though, is getting attention for its sustainability goals. Boom Supersonic, an aircraft start-up based in Denver, made headlines in June when United Airlines agreed to buy 15 of its supersonic Overture airplanes, with an option to buy 35 more. Both Boom and United have highlighted the deal as helping United meet its sustainability initiative to reach net-zero carbon emissions by 2050. In numerous press releases, Boom has announced that Overture is "expected to be the first large commercial aircraft to be net-zero carbon from day one, optimized to run on 100% sustainable aviation fuel." Boom has rarely spoken to the press about the details of its sustainability plans and declined to be interviewed for this story.

Critics, however, aren't so sure about the company's sustainability claims. Supersonic flight would have to overcome some significant obstacles to become both sustainable and financially viable. Here is a look at some of those hurdles.

Disclosure: The author has a relative that works at an airline not mentioned in the story.