An airplane flies over a group of trees.
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The recent announcement that Colorado-based Boom is building a new half-billion-dollar plane factory in North Carolina included this incredible claim: Its fuel-guzzling supersonic jet, the Overture, won’t harm the climate. 

“Overture will be net-zero carbon,” the company said in a press release, “capable of flying on 100% sustainable aviation fuels at twice the speed of today’s fastest passenger jets.” 

Is it too good to be true?  

To find out, the Energy News Network reached out to experts to find out exactly what sustainable aviation fuel is, whether it lives up to its moniker, and if propelling ultra-rich passengers from Madrid to Boston in three and a half hours is its highest and best use.

The answers depend largely on the policies the U.S. and other nations enact in the next decade, including how tightly sustainable aviation fuel is regulated and measured and whether airlines will ever be penalized for their continued reliance on fossil fuels.

British Airways Concorde, a supersonic luxury airliner, pictured in 1986. Credit: Eduard Marmet / Creative Commons

‘The pinnacle of aviation’ 

As extraordinarily wealthy world travelers of the last century know, supersonic flight is nothing new. For nearly three decades, British-Franco Concorde jets regularly crossed the Atlantic Ocean in less than four hours, charging about $7,000 for a round-trip ticket in 1996 — $12,530 in today’s dollars.

But the luxury airliner was dogged by concerns over air and noise pollution and never achieved the commercial success executives envisioned. A catastrophic crash outside Paris in 2000, followed by the Sept. 11 terrorist attacks the next year, helped ground Concorde for good in 2003. 

The last Concorde was parked at London’s Heathrow as a kind of a memorial piece, said Andrew Chen, who used to direct emissions reduction strategy for the airport. “When people talk about Concorde, there’s this really deep sense of nostalgia,” he said, “like that was the pinnacle of aviation.” 

Since then, air travel has emerged as one of the most confounding sectors of the economy to decouple from fossil fuels. Today, it’s responsible for about 2% to 3% of global warming emissions worldwide, said Chen, now at the Rocky Mountain Institute pursuing a new pinnacle in aviation: decarbonizing it altogether. 

By midcentury, when nearly everything else has plugged into an electric grid powered by renewable and nuclear energy, airplane travel could rise to 12% of the problem. “Some industries start to stick out like a sore thumb as the rest of the world cleans up at a faster pace,” Chen said, “and aviation is one of those.” 

Short-haul flights, where not replaced entirely by high-speed electric trains, can run on electric batteries with today’s technology. Eventually, electric-powered flights could run three to four hours. But flying hundreds of people 15,000 miles at 30,000 feet is different, Chen said.

“Fundamentally, it’s really difficult to find a source of energy that’s dense enough and efficient enough that isn’t a liquid fuel.” 

The ‘holy grail’ of sustainable jet fuel

That’s where sustainable aviation fuel — “SAF,” to wonks in the field — comes into play. Now made from any carbon-based matter, from used cooking oil to quickly replenishable crops, it’s blended with fossil jet fuel and used in conventional jets. Today’s sustainable aviation fuel is about 70% better for the climate than fossil jet fuel. As technology evolves to use sources like household garbage, the improvement will rise to 80%, experts believe. 

Then, said Chen, there is the “holy grail” of sustainable aviation fuel, a technology called “power-to-liquid.” That’s when renewable electricity is used to split water into hydrogen and oxygen, and the former is combined with carbon captured from the atmosphere. The result is a synthetic hydrocarbon that’s 99% cleaner than fossil jet fuel.

Though power-to-liquid will supplant biofuels in the next 10 to 15 years, Chen predicts, it’s nowhere near commercial viability today. “It’s a future fuel,” he said. 

But sustainable aviation fuel overall is still mostly in the concept phase. It makes up less than 0.1% of aviation fuel worldwide and costs three to four times as much as conventional fuel. Regulations still require passenger jets to use at least 50% fossil fuels.

Plus, skeptics worry sustainable aviation fuel could pan out as poorly as corn-based ethanol, which almost certainly produces more climate pollution than gasoline, since it displaces food crops and causes other changes in land use. Yet since 2005, federal law has required ethanol and other so-called renewable fuels to be blended with conventional gasoline and diesel.

Rocky Mountain Institute and other nonprofits are working with academics and industry on standards and accounting measures for truly sustainable aviation fuel. But until they’re developed and enforceable by governments, there’s an opening for biofuels like ethanol to enter the market that are no better — or even worse — than traditional jet fuels. 

Enviva’s Ahoskie, North Carolina, facility produces wood pellets, many of which are shipped to international utility customers in Europe, where they are a popular coal substitute. Credit: Dogwood Alliance / Submitted

‘I do not see any carbon logic’ 

Enviva, the world’s-largest wood pellet maker with four mills in North Carolina, may be seizing that opportunity. Long focused on supplying electric power plants in Europe and Asia with pellets to burn in place of coal, the company has signed a 10-year contract with a European company that produces biofuels, including aviation fuels, WFAE reported in November. 

Last month, Enviva announced its first deal with a U.S. “cleantech” company, “with a focus on the customer’s operations in the U.S. Southeast.” Under the contract, the company said in a press release, “the customer’s refining process will convert Enviva’s woody biomass into a drop-in replacement for crude oil used for producing aviation fuel.” 

The new customer remains unknown, and Enviva didn’t respond to questions about the announcement for this article. But Sami Yassa, a senior scientist at the Natural Resources Defense Council whose specialty is carbon accounting, said there’s no way woody biomass could produce aviation fuel that benefits the climate.

Wood-sourced biofuel releases carbon into the atmosphere when it’s burned, Yassa points out. Even if the 20- to 40-year-old forests that supply Enviva are replanted, as the industry claims, they won’t grow back to reabsorb that carbon “in time frames to address climate change,” Yassa said. “We’ve got about ten years.”

Though the company argues it uses only parts of trees or whole trees that are unsuitable for lumber, several media outlets and Enviva’s own data have shown it relies at least sometimes on entire clear-cuts of 50 acres or more, including of ecologically valuable hardwoods.  

“There is extensive evidence that the wood pellet industry is degrading forests in the Southeast,” Yassa said. “When you cut more than you can grow, you are eroding and undermining the ability of the forests to sequester carbon.”

If Enviva supplies its mystery customer with actual wood pellets — not just the wood biomass used to make them — the resulting biofuel’s carbon footprint could be even larger. Yassa’s research shows that all the front-end processes that go into pellet production, especially drying them extensively to give them coal’s combustion properties, produce climate-warming pollution in and of themselves. 

“Whether you transform them into aviation fuel and burn them, or whether you burn them in a power plant, when you manufacture wood pellets, you use an enormous amount of energy — which produces CO2,” Yassa said. “I do not see any carbon logic in using pellets for aviation fuels.” 

‘It doesn’t keep me awake at night’

What type of sustainable aviation fuel ultimately powers Boom’s supersonic jets will depend on its airline and airport partners, not Boom itself, according to the company.

For example, United Airlines, which is buying at least 15 Overtures, plans to purchase 1.5 billion gallons of fuel made from “cellulosic feedstock,” and has made a “$30 million investment” in fuel from municipal solid waste, a Boom spokesperson said.

She added, “Boom believes that power-to-liquid fuels is one of the most sustainable and scalable types of SAF.”  

Yet even if the Overture employs the cleanest sustainable aviation fuel imaginable, there’s still the question of efficiency. A recent study by the International Council for Clean Transportation found that supersonic jets would burn seven to nine times as much fuel per passenger per kilometer as subsonic versions. 

Using sustainable aviation fuel for supersonic flights is “an insane use of a scarce resource,” Cait Hewitt, policy director of the U.K. nonprofit the Aviation Environment Federation, told the Guardian last year.  

Chen doesn’t dispute the fact that supersonic jets use enormous amounts of fuel, or that little to no air travel would reduce more climate-warming pollution than even the cleanest jet fuel replacements. But he insists that aviation is a net cause for good, and the goal shouldn’t be its eradication. 

“The enemy isn’t aviation,” he said. “The enemy is carbon from aviation.” 

To the extent the buzz over the return of supersonic flight gets sustainable aviation fuel into the spotlight, Chen said, that’s a good thing at a time when the market for the fuel needs to be scaled up dramatically. “I need us to be having that conversation.” 

But as for the climate implications of a luxury product that will reach an infinitesimal fraction of the global population, Chen said, “it doesn’t keep me awake at night.”

Elizabeth Ouzts

Based in Raleigh, North Carolina, Elizabeth has covered the state’s clean energy transition for the Energy News Network since 2016. She has also produced features for Environmental Health News and SEJournal, the news magazine of the Society of Environmental Journalists. A former communications director for the nonprofit Environment America, Elizabeth brings over two decades of environmental and energy policy experience to her reporting.