Let’s play optimist for a moment and assume that electric vehicles do account for 60 percent of new car sales in a decade or so — as they must to stay on track for zero net emissions by 2050. That rosy-hued scenario would still leave hundreds of millions of internal combustion engines (ICE) on the road for many, many years, not to mention millions of engines powering airplanes and ships. So it is hard to escape the reality that the world really needs clean-burning sustainable fuels for its legacy fleets, no matter how fast electrification progresses.

The good news is that, with some minor modifications, ICEs can run on a remarkable variety of fuels — everything from alcohol to propane or even hydrogen — that are cleaner than gasoline and diesel. Indeed, some cars on the market today can shift from one fuel to another with the flick of a switch. States with low-carbon fuel standards in place have already reduced emissions through the increased use of corn-based ethanol alcohol and soy-based biodiesel, which are blended with petroleum-based products.

Curb your enthusiasm for at least a moment. Ethanol and biodiesel can both be processed in existing refineries, are broadly available and make little perceivable difference to engine performance. But peer-reviewed studies have challenged the green credentials of those fuels, claiming they merit thumbs-down when all of their negative externalities are accounted for. And, for now, anyway, there isn’t enough raw material or processing capacity to scale to the massive volumes needed to displace a good fraction of the oil used in transportation.

The goal is to make these new fuels from household and commercial waste otherwise bound for landfills or incineration and from the biomass left after trees are harvested that is now left to rot or be burned. Synthesis has advanced beyond proof of concept — British Airways has already flown a plane powered by sustainable aviation fuel (SAF) — but makers still need to demonstrate that they can come up in scale and down in cost.

“Even with not selling any more ICE cars by 2035, what does that mean?” asks Jeremy Martin, director of fuels policy at the Union of Concerned Scientists. “In this sort of aggressive electrification case, we can reduce liquid fuel use in the U.S. by 85 percent. But the 15 percent that’s left is still quite a lot. It’s clear we can’t just use more corn and vegetable oil.”

Diesel or Doughnuts, the Biofuel Conundrum

For readers — and writers — of a certain age, the very idea of biodiesel brings to mind a long-haired hippy scavenging used grease from the fry baskets at local diners to fuel a Mercedes 190D. People still do that, but the serious players in the biodiesel game have names like ExxonMobil, Phillips 66 and Marathon Petroleum, all of which are adding fuels derived from animal or vegetable fats to their products in response to government incentives.

This, alas, is a zero-sum game that pits the oil giants against big food companies like Krispy Kreme, Bimbo Bakeries and Pepperidge Farm, all members of the American Bakers Association, which has asked the U.S. Environmental Protection Agency to lower federal mandates for biofuels.

“We support renewable fuels and the green agenda, but soybean oil [prices] have tripled,” Robb MacKie, the trade group’s chief executive, told the Financial Times. “Our members are worried that they may not be able to buy any oil.”

Soybean oil used to make biofuels in the U.S. is expected to total 11.5 billion pounds this year, up by a third from 2019 and accounting for more than 45 percent of domestic consumption. That hefty non-food demand has prompted increased production and a raft of joint ventures between petroleum and farming companies. But if you Google “soybeans and tropical deforestation,” the problematic nature of this feedstock is clear. A voluntary soybean moratorium appeared to slow deforestation in Brazil, only for it to accelerate in Paraguay and Bolivia.

The soybean dilemma recalls some of the issues plaguing corn-based ethanol. Ethanol burns more cleanly than gasoline. And Iowa’s premier place in presidential primary elections ensures that politicians bow before the corn lobby, which never turns its back on a new source of demand. But food producers have similar objections to its use as a fuel. What’s more, as an iconic article in the peer-reviewed journal Science made clear, shifting forest and grasslands to corn or other fuel feedstock crops does more harm to the climate than good.

“Corn-based ethanol, instead of producing a 20 percent savings, nearly doubles greenhouse emissions over 30 years,” the authors concluded. “Biofuels from switchgrass, if grown on U.S. corn lands, increase emissions by 50 percent.”

It’s also a question of scale. “Ethanol’s problem is that to replace [petroleum-based] fuel in the U.S. you’d need five times the surface area of the inhabitable land on earth,” explained Philip Leduc, a professor working at the intersection of mechanical engineering and biology at Carnegie Mellon. “I think the biofuels are going to have a challenging time.”

Ethanol and biodiesel are chemically different from gasoline and diesel, which limits their portion in blended fuels suitable for unmodified vehicles to about 10 percent. Now, so-called renewable biodiesel, which is produced by adding hydrogen to plant or animal fats, can be used at 100 percent and can be produced in existing refineries. Bloomberg NEF estimates that global renewable diesel manufacturing capacity will triple by 2024, to more than 20 million tons (7 billion gallons). Good news for the transition, sure. But look closely at the numbers: 7 billion gallons is a drop in the fuel tank, and feedstock limitations would pinch in any serious effort to scale up.

Garbage In, Fuel Out

There are feedstocks — domestic and commercial solid wastes — with no supply constraints on the horizon at all. The pressure to achieve carbon neutrality in this lifetime has spurred a raft of companies, from starry-eyed startups to seasoned petroleum players, to take a fresh look at technologies that can transform garbage to fuel. Indeed, these fuels represent one of the few low-carbon technologies with the potential to replace oil in aviation and are the only feasible options in the near term.

“Woody biomass, municipal solid waste and crop residues are feedstocks that can be sourced on a sustainable basis,” confirmed Jeff McDaniel, vice president for new projects at Velocys, a U.K. startup that has a joint venture with British Airways for sustainable aviation fuel. “We see the combination of three elements as being critical: sustainable feedstocks, power from a renewable source, and sequestering the CO₂ produced.”

Waste materials are fed into a gasifier, a high-temperature pressurized vessel that produces so-called syngas, a mix of hydrogen and carbon monoxide. This gas is purified and pressurized in the presence of iron-based catalysts, producing hydrocarbons that store a lot of chemical energy. The hydrocarbons are then cooled into liquid form to fuel airplanes, buses and trucks. 

Note, however, there’s no free lunch here. This metamorphosis uses considerable energy and generates considerable CO₂, so additional technology must be applied to clean it up.

Velocys is developing a plant in Mississippi that will make jet fuel from the “logging slash” left on the ground after pine trees are harvested for lumber or paper. The facility, Bayou Fuels, will produce 25 million gallons of SAF per year. Velocys has partnered with Oxy Low Carbon Ventures for carbon capture and sequestration.

“The pathway to converting biomass to SAF has not been fully proven at scale,” McDaniel said. “With our project, each of the individual technologies has been proven, but not all in concert. We see a really large market opportunity, lots of feedstock and a big market.”

Velocys has plenty of competition. Fulcrum Bioenergy’s Sierra BioFuels plant near Reno, Nevada, plans to convert 175,000 tons of prepared municipal solid waste into approximately 11 million gallons of zero-carbon “syncrude” annually, which will then be upgraded to transportation fuels such as sustainable aviation fuel, renewable diesel and renewable gasoline — all drop-in replacements for their petroleum equivalents. Oregon-based Red Rock Biofuels will convert approximately 166,000 dry tons of waste wood biomass into 16.1 million gallons/year of low-carbon, renewable jet and diesel fuels. Even Shell Oil is sticking a toe in the… whatever you want to call it.

Sustainable fuels will come from “a combination of old oil companies and new technology,” predicts Daisy Robinson, an analyst with Bloomberg NEF. “The cost is around two to five times the price [of petroleum-based fuels], and that multiple can go up to eight times if you’re looking at the sustainable aviation fuel. Going forward, policy is a big driver, particularly as governments set new regulations. Also, we can’t ignore the corporations, like Amazon and DHL, setting net-zero goals.”

Fuel From the Sky

In the race to net-zero, two promising but controversial technologies are hydrogen synthesis and carbon capture. Hydrogen is the most common element in the universe. But found in nature, it is always bound to either carbon or oxygen, and most hydrogen in use today is refined from fossil fuels. So-called green hydrogen, produced by splitting oxygen from hydrogen in water through electrolysis powered by renewable electricity, remains scarce and expensive. Carbon capture at industrial plant sites is growing rapidly, but direct-air capture, pulling CO₂ out of the atmosphere, is still in its pilot phase.

So what could be better than a fuel that combines the two? The idea is simple enough: capture CO₂ from the air and then combine it with green hydrogen to produce a hydrocarbon fuel. Burning the fuel in jets or internal combustion engines would, of course, release the carbon back into the atmosphere, but it’s not “new” carbon.

The catch: e-fuels are bound to be expensive. A few entrepreneurs, though, think it’s worth a try. One group is Carbon Engineering, which was founded in Calgary by Harvard professor David Keith in 2009. Carbon Engineering plans to open its first commercial-scale facilities in Canada, the U.S. and the UK in 2022, with an extensive rollout on multiple continents in 2030. A stretch, yes. But one to be taken seriously: the company’s backers include three of the world’s largest energy companies — Occidental, BHP (the mining giant) and Chevron.

Carbon Engineering is counting on government intervention to make its business viable. “While policy support for direct air capture, and emissions reductions overall, has been limited in the past, this is now starting to change,” the company says on its website. “Leading jurisdictions like California, Oregon, Washington, British Columbia, and the EU have implemented effective market-based policies to reduce greenhouse gases and promote clean energy. As these policies continue to spread and tighten around the globe, we see our carbon removal technology having significant potential for widespread global deployment.”

Would You Buy It?

Three years ago, researchers at Yale and George Mason University reported that most Americans think that “clean energy” and “renewable energy” are good things. Yet half of those surveyed said they wouldn’t pay a premium for it, and even those who would pay more weren’t willing to pay much more — on average $16.25 a month. At current prices, $16.25 doesn’t buy one gallon of fuel made from garbage or pulled from the air.

Which raises an obvious question: What if someone builds it and they don’t come? What if companies succeed in creating a supply of sustainable fuels but aren’t competitive in the market?

This is where boosters of sustainable fuel say public policy will play a crucial role. “My view of how we decarbonize transportation is not getting consumers to demand it at the gas station,” explains Jeremy Martin of the Union for Concerned Scientists. “You incentivize producers.” Indeed, given the urgency of the problem of climate change containment, it’s hard to quibble with the idea that clean transportation initiatives should boost electric vehicles and clean fuels alike. “Liquid fuels are not an alternative to electrification,” Martin emphasizes, “they’re really a complement.”