Will ethanol fuel a low-carbon future?

Humans have a complicated relationship with ethanol. Of course, many drink the stuff. People have also been working for decades to scale it up as a fuel and a chemical feedstock. The dream is that with the right technology for making and using ethanol, the chemical and energy industries could break their reliance on petroleum and drastically cut their climate impact.

But that dream has a troubling side. Conventional ethanol relies on sugars extracted from corn, sugar beets, and sugarcane. Growing those crops requires fertilizer and fossil-fueled farm equipment, moving them requires diesel-fueled trucks, and fermenting and distilling them requires heating fuel. To top it off, the ethanol fermentation process yields carbon dioxide as a by-product.

Yet ethanol has a good side too—namely, the crops it is derived from pull CO₂ from the air. Critics, advocates, and academics have long debated which of the two sides prevails in the battle to find a fuel and chemical feedstock that has fewer greenhouse gas (GHG) emissions than petroleum.

Starting from cellulose instead of sugar could make ethanol’s climate math much easier. Cellulose, in the form of lignocellulosic biomass, is abundant in agricultural and forestry waste. Examples include corn stover—the stalks, leaves, and husks left over after kernels are harvested—as well as straw, and sugarcane bagasse—the dry, fibrous pulp left over from sugar production. Because the waste material’s current fate is to rot or burn and release methane or carbon dioxide, and because it requires no additional inputs to grow, cellulosic ethanol’s claim to low carbon intensity is strong.

Moreover, transforming such biogenic carbon into durable goods instead of fuel achieves a certain kind of CO₂ sequestration. Some industry watchers think that after decades of false starts, the global push toward net-zero carbon emissions is now poised to combine with an improved suite of cellulosic technologies to usher in a new era for ethanol.

Promises of the past

Despite many attempts, cellulosic ethanol, often called second-generation or 2G ethanol, has proved much more difficult to produce at scale than its first-generation cousin made from starch and sugar. In the 2010s, a handful of ambitious companies spent hundreds of millions of dollars to build cellulosic ethanol plants in the US. Today the plants are all shut down.

Two of those companies were the partners Poet, an ethanol maker, and DSM, a specialty chemical firm. They started producing ethanol from corn stover in Emmetsburg, Iowa, in 2014. The facility was designed to make 95 million L per year. But it struggled right away with handling the feedstock and preparing it for enzymatic depolymerization into fermentable five- and six-carbon sugars.

In 2017, Poet and DSM said those problems were sorted out. But by 2019, the plant was shut down. The firms blamed their failure on the US government, saying that the Environmental Protection Agency was mismanaging its biofuel subsidy program and as a result, the premiums they were able to charge for cellulosic ethanol weren’t enough to make a profit.

Poet says it is still conducting R&D at the Emmetsburg facility. Meanwhile, DSM sold its cellulose-to-sugar enzyme technology to Versalis, a subsidiary of the oil company Eni, in late 2022. Versalis says it will use the technology in its cellulosic ethanol operation in Crescentino, Italy, a plant that the Mossi & Ghisolfi Group sold off during bankruptcy proceedings several years ago.

DuPont’s effort to make cellulosic ethanol also failed. In 2015 it opened a $200 million plant in Nevada, Iowa, promising it was the first of many. That aspiration did not survive the merger of DuPont and Dow, and the plant went on the auction block by 2017. The biofuel company Verbio bought it in 2018 and converted it to make biobased methane.

In a similar story, the renewable energy developer Abengoa started making ethanol from corn stover in 2014 at a facility in Hugoton, Kansas. Citing financial trouble across the company, Abengoa shut the plant in 2015.

It’s a sorry track record, but that isn’t stopping a wave of other companies from building and commissioning a new round of cellulosic ethanol plants. The biggest players are the chemical company Clariant, the chemical engineering firm Praj Industries, and Raízen, a joint venture between the energy companies Shell and Cosan. All three are making cellulosic ethanol today and have big plans for expansion.

Praj opened India’s first commercial-​scale cellulosic ethanol plant in August. Owned by Indian Oil but built and operated by Praj, the plant will yield 30 million L of ethanol per year from rice straw when it reaches full capacity. Praj CEO Shishir Joshipura says the firm is working on sister plants with Hindustan Petroleum and Bharat Petroleum. All three oil companies are owned or part owned by the Indian government.

In Brazil, Raízen has been running a 34 million L plant on sugarcane bagasse since 2015. A facility the company is finishing now, plus two more slated for construction in 2024, will bring the firm’s capacity to 280 million L. By 2030, Raízen plans to have 20 cellulosic ethanol plants with a total capacity of 1.6 billion L per year.

Clariant’s 63 million L plant in Romania started production in June with wheat straw as a feedstock, and the firm has already sold five licenses for its process. Start-up troubles led the firm to write off most of the plant’s value in December, but owning and operating such plants was never the main business plan, says Ralf Hortsch, Clariant’s head of marketing and strategy for biofuels and derivatives. The plant will nonetheless continue producing cellulosic ethanol; Shell already bought the first several years of output, and Clariant will also use the site to showcase and further develop the technology.

“We are not a fuel producer. It was always the strategy to go into technology licensing,” Hortsch says. “We also sell the microorganisms that produce the enzymes integrated in the plant and the yeast that ferment the sugars to ethanol.”

Making ethanol well

When you sit down with the executives, engineers, and investors who have been trying to make cellulosic ethanol work, talk of market woes and parent-company politics soon falls away. The hardest problem to solve, the lead weight that dragged so many previous efforts down, is the prosaic task of procuring and handling the feedstock.

One hurdle is just getting the cellulosic biomass to the factory, Hortsch says. The firm picked Podari, Romania, for its first commercial plant in part because the area grows a massive amount of wheat and wasn’t doing much with the leftover straw.

But because the material had no buyers, there was also no mechanism in place to collect and distribute it. “If nobody’s buying the straw, why should somebody come and pick it up from the field? Our local team had to build a supply chain basically from scratch,” Hortsch says.

The same was true with rice waste in India. Joshipura says his firm has won some government support because of the air quality improvements made when farmers sell Praj their rice straw instead of burning it in huge bonfires.

Once cellulosic biomass is hauled into a plant, it’s dirty, abrasive stuff with a habit of destroying the pipes, screws, and tanks that move it around and prepare it for enzymatic digestion. “The biggest challenge for the new technologies 10 years ago was that they were not reliable enough,” Joshipura says.

Cellulosic biomass is also a diverse category of materials. Earlier attempts often assumed that the same basic equipment would handle any biomass. A painful lesson for the industry is that every cellulosic feedstock is unique. “It was important for us to understand the feedstock constituents, their different behaviors, and how they are interrelated to each other,” Joshipura says.

After raw material procurement and handling comes biomass pretreatment: separating cellulose and hemicellulose—natural polymers built from C₅ and C₆ sugars—from lignin, an amorphous phenolic polymer. Praj’s pretreatment method employs catalysts to break the links between lignin, hemicellulose, and cellulose. Clariant and Raízen use steam explosion, which is basically the same process that breakfast cereal makers use to puff rice.

From there, the processes converge. All the companies use fungal enzymes called cellulases to depolymerize the cellulose and hemicellulose into sugars, and they use specialized yeast to ferment those sugars into ethanol.

“The key to success is going to be who brings the ability to understand the feedstock and what to do with it, and that’s what will differentiate us. Once sugars are made, everybody knows what to do with that,” Joshipura says.

With all that chemistry and engineering to think about, it’s easy to lose sight of the point: low greenhouse gas emissions. “Cellulosic ethanol is more and more coming into focus, mainly because you can produce a much higher carbon dioxide savings compared to conventional starch-based ethanol,” Hortsch says.

Conventional industrial ethanol is made from sugar and starch in basically the same way people have been making hard liquor for at least 1,400 years. Still, the industry isn’t sitting around waiting to be overtaken. As cellulosic ethanol developers were honing their processes, traditional ethanol makers starting from starch and sugar were trimming emissions to keep their grip on federal, state, and local environmental subsidies.

A key score for both types of ethanol is carbon intensity (CI), which is usually defined as the grams of CO₂ emitted per megajoule of fuel produced. Gasoline has a CI score of about 96 g CO₂/MJ, and the ethanol being produced in the early 1990s was worse: CI scores were north of 99 g CO₂/MJ, according to a recent meta-​analysis of published data (Environ. Res. Lett. 2021, DOI: 10.1088/1748-9326/abde08).

Though the score validates corn ethanol’s early environmental critics, a typical number today is 55–60 g CO₂/MJ, according to Geoff Cooper, CEO of the Renewable Fuels Association, a trade group representing ethanol makers. Cooper’s estimate aligns with recent CI scores in the Environmental Research Letters paper. The biggest improvements have come from better farming, fermenting, and distilling practices. Corn growers are using less fertilizer and fossil fuels, and ethanol makers have halved their energy consumption per liter of product.

“We don’t have a single member company that isn’t looking at or already investing in steps to lower the carbon intensity of the ethanol they’re producing,” Cooper says. The CO2 released during fermentation is an obvious place to start. Around 25% is captured in the US, Cooper says, and industrial gas firms sell most of it for use in beverages, food processing, and refrigeration.

But those markets are only so big. The agriculture giant ADM recently started collecting the CO₂ produced at two ethanol plants in Iowa and injecting it into underground storage in Decatur, Illinois. And on Jan. 30, Cardinal Ethanol and the carbon management developer Vault 44.01 announced plans to install capture equipment and a storage well at Cardinal’s plant in Union City, Indiana.

Ethanol plants can also decrease their CI scores by using biogenic methane instead of fossil fuels for heat and transportation and by switching to renewable or nuclear power. Praj, which has substantial first-generation ethanol operations, is also in the business of upgrading such sugar ethanol facilities. Process integration such as heat and by-product reuse at a first-​generation ethanol plant can reduce the CI score by almost 30 g CO₂/MJ, Joshipura says.

New projects offer the opportunity to start with the most carbon-efficient processes. A group called California Ethanol & Power is taking farmland currently used to grow forage crops for overseas racehorses and converting it to grow sugarcane. After making ethanol from the sugar, its plant will anaerobically ferment the leftovers into methane and burn for electricity what neither fermentation process can use. The firm expects its ethanol to have a CI score around 22 g CO₂/MJ.

Cellulosic ethanol retains a head start, however. Cooper says the CI scores from the plants coming on line now are around 25 g CO₂/MJ, and improvements are on the way. Clariant left room for carbon capture at its plant in Podari, Hortsch says. And Joshipura says a cellulosic ethanol plant that uses the waste material from a sugar ethanol plant and is fitted with carbon capture can reach negative CI scores, meaning it would bring a net reduction in GHG emissions.

And more help converting cellulose is on the way at the National Corn-to-​Ethanol Research Center, part of Southern Illinois University Edwardsville, Yan Zhang and her team are developing better methods to convert cellulose.

One promising line of research is a chemical-and-enzyme cocktail that Zhang sprays on corn stover just before it’s bound up into bales. In 2 weeks, while the bales are shipped and stockpiled, the mixture handles almost all the pretreatment with no additional effort, energy, or costly equipment.

There are also other ways to get ethanol. The green chemistry company LanzaTech uses engineered microbes to ferment ethanol from syngas, a mixture of carbon monoxide and hydrogen. LanzaTech and other firms are also developing projects to make ethanol from trash and from gases emitted by treatment plants for water and solid waste.

Best use of biomass

People know ethanol well, and it has the advantage of being made at scale today. In the long run, though, it may not be the best use of biomass, for both environmental and economic reasons.

In a 2022 paper in ACS Sustainable Chemistry and Engineering, Raoul Meys, now with the life-cycle analysis firm Carbon Minds, and coworkers looked at 46 processes that use fermentation to convert sugars into chemicals. The team, then based at RWTH Aachen University, calculated the GHG emission savings of making a biobased fuel or chemical instead of its petroleum counterpart (DOI: 10.1021/acssuschemeng.2c03275).

In that analysis, ethanol landed squarely in the middle of the pack—better on GHG emissions than gasoline but not nearly as useful for fighting climate change as succinic acid, 1,4-butanediol, or nine other chemical intermediates that can be made from sugar.

The authors offer a sobering takeaway for the ethanol industry: “The overall global warming impact reduction of the biobased industry could be significantly increased by allocating biomass currently used in ethanol production . . . to other biobased chemicals . . . even if the reduction in fuel ethanol production results in a higher consumption of gasoline.”

Chemistry also offers attractive alternatives for converting biomass. The most familiar chemical transformation, of course, is combustion. Burning biomass is carbon neutral in principle because the CO₂ emitted came from crops and other plants that absorbed it from the air. If a project captures enough of the CO₂ it releases, it can become a net carbon sink, a concept called bioenergy with carbon capture and storage, or BECCS for short.

The renewable energy company Drax is developing a BECCS project in northern England that will burn biomass to generate electricity. The facility will capture the CO₂ and permanently sequester it under the North Sea.

A more chemically sophisticated pair of processes is pyrolysis and gasification, which thermally decompose biomass in the absence or near absence of oxygen, respectively. Pyrolysis yields a biobased crude oil that can be fed into petrochemical plants. Gasification yields syngas which can become hydrocarbons through Fischer-​Tropsch conversion.

Alder Fuels, for example, makes pyrolysis oil from forestry waste and other woody biomass and then upgrades it into jet fuel. It has pilot and demonstration plants in the Netherlands and is eyeing commercial facilities in Europe and North America. Alder is working with Boeing to test its fuels in commercial airliners, and it has a purchase agreement with Flexjet, a private-jet-sharing company.

The start-up USA BioEnergy, meanwhile, is planning a wood waste gasification and Fischer-Tropsch plant in Texas that will make jet fuel, diesel, and naphtha. By capturing and sequestering CO₂ from fermentation and heat generation, the plant will yield “deeply negative CI fuels,” the firm says in promotional materials.

John Bissell argues for using chemistry more creatively than degrading biomass into base carbon and using that to make fuels. Bissell is co-CEO of Origin Materials, which uses acid and catalysts to convert lignocellulose to chloromethyl furfural, a possible industrial intermediate for polyethylene terephthalate, which is currently made from petrochemically derived terephthalic acid and ethylene glycol.

“The way you get value out of biomass is by retaining interesting structure, which is why we think furans are the right molecule. You’re keeping a bunch of that semiaromatic structure,” he says. Lignocellulose has functional groups such as ether bridges and double bonds that can give chemists useful handles for creating valuable chemicals and materials.

Origin is finishing a furan plant in Ontario and plans a bigger version in Louisiana for completion in 2025.

What to do with ethanol?

The world made more than 102 billion L of ethanol in 2021, most of it first generation, according to the Renewable Fuels Association. The cellulosic ethanol plants being developed this decade will, if all goes as planned, add billions of liters to that supply.

Almost all ethanol is blended with gasoline and burned in passenger cars. In the US, about 200 ethanol plants consume 35% of the annual corn crop, yielding about 53 billion L, which goes into gas tanks at around 10.5% by volume. Other regions blend at higher or lower rates. Brazil is the highest, with 85% ethanol fuel available at most gas stations.

In the spring of 2020, when the pandemic caused US gasoline consumption to drop by about 30%, roughly half of the nation’s ethanol plants shut down. If electric cars continue to grow in popularity, the demand for gasoline will sink permanently. So where is all the ethanol going to go?

One answer: into a bigger slice of the shrinking pie. “Let’s say we go from 135 billion gal of annual gasoline consumption, which is essentially where we’re at today, down to 100 billion gallons by 2035,” the Renewable Fuels Association’s Cooper says, citing US Energy Information Administration predictions. Moving from 10.5% ethanol to 15% ethanol nationwide would keep the total volume stable, he says.

There’s no technical reason to stop there. Flex-fuel vehicles that can run on up to 85% ethanol have been on the market since the mid-1990s; it’s mostly a question of switching to gaskets and O-rings made from compatible plastics. Even normal engines are probably fine at much higher levels than they use now.

To demonstrate that notion, the Nebraska Ethanol Board, a state agency, is testing a 30% blend in 825 unmodified light-duty vehicles in the state fleet in collaboration with the University of Nebraska–Lincoln. A little more than 3 months into the 3-year trial, the results are promising, says Reid Wagner, the board’s executive director. “We have had zero maintenance issues, zero check-engine lights coming on. Even on one of our coldest days, when it hit –39 °F [-39 °C] with windchill, everybody was able to get from point A to point B just fine.”

Chemicals are another potential outlet for ethanol. The catalytic dehydration of ethanol to ethylene is straightforward, and ethylene is the starting point for an enormous range of chemical products currently made from fossil resources.

Polyethylene, the world’s most-produced plastic, is a short chemical journey from ethylene. The petrochemical giant Braskem’s I’m Green line of polymers already includes more than 40 polyethylene and ethylene–vinyl acetate copolymer products made from the dehydration of sugarcane-derived ethanol. And last month, Braskem said it is looking at the US for what it calls the world’s first industrial-​scale biobased polypropylene plant.

Ethylene oxide, made by the oxidation of ethylene, is used to make the ethoxylated surfactants used extensively in the home and personal care industry as well as in the polyester consumed by the clothing industry, two consumer-facing sectors that are aggressively pushing toward net-zero GHG emissions. Clariant, Ineos, and Croda all already offer biobased ethylene oxide derivatives aimed primarily at makers of cosmetics and cleaning products.

Companies such as Gevo and LanzaJet are commercializing processes that convert alcohol into ethylene and then jet fuel. Minor variations in catalysts and process conditions yield diesel fuel.

And ethylene isn’t the only option. At the end of January, the joint venture Blue Blade Energy launched with $50 million in funding to commercialize a suite of catalysts that convert ethanol directly to 2-pentanone and 2-heptanone. Sustainable aviation fuel is the first product target—United Airlines is one of the firm’s three backers—but the ketones could flow into any number of other synthetic pathways.

Though less than 2% of ethanol goes anywhere other than gas tanks today, Cooper says, cellulosic ethanol’s low CI scores may push it into products that can command a strong green premium. “We do expect to see some of the first movers in the space really focusing on chemical markets,” he says.

Between the market pull for low-​carbon chemical products and the galloping electrification of cars and trucks, “I would not bet so much on fuel. . . . I would look to materials,” Carbon Minds’ Meys says. “That’s also what the environmental analysis says.”

An evolving market

The timing is good for cellulosic ethanol, according to Jim Lane, editor of Biofuels Digest. Recent policy and market changes are creating demand for biobased fuels and chemicals with low CI scores right as the technology finally seems to be ready.

Lane says government programs have also become better at incentivizing actual GHG reductions instead of encouraging biofuel use regardless of carbon footprint. “Years ago, it was a volumetric standard. You didn’t have to necessarily make low-carbon-intensity ethanol,” he said in a recent webinar. “Now you can do it and get a reward.”

Overseas, the European Commission is attempting to phase out “food-based” biofuels, which would include almost all first-generation ethanol. Alongside growing biofuel mandates in the region, the move could create strong demand for cellulosic ethanol.

And worldwide, many companies are making net-zero GHG pledges in response to shareholder and consumer pressure. That sustainability trend moves the needle for biobased fuels and chemicals independent of government encouragement.

Many of those factors apply to any renewable molecule with a low CI score. But ethanol is already being produced and used at scales that other biobased intermediates are nowhere near.

“The ethanol base is almost 200 plants operating in the US and almost an equal number in Brazil, and of course a large component in Asia and Europe,” Lane says. Potential business partners throughout the value chain already understand the ethanol industry, he says. “And that means that when you’re bringing in advanced ethanol, you’re bringing technology, but you’re not asking someone to establish your market.”

“It’s the carbon story,” Cooper says. “Whether it’s ESG [environmental, social, and governance] commitments that companies are making, whether it’s new tax policy like the Inflation Reduction Act, whether it is climate commitments that various countries have made—what’s driving all of this is the push to continue reducing carbon intensity. And it’s not just in the transportation fuel sector; it is in plastics and chemicals and really across the board.

“After many fits and starts, it does feel like we are on the precipice of some significant breakthroughs in the commercial production of cellulosic ethanol,” Cooper says.