Advanced BioFuels USA – Will Ethanol Fuel a Low-Carbon Future?

by Craig Bettenhausen (Chemical & Engineering News) After decades of false starts, cellulosic ethanol may arrive just as the chemical and fuel industries clamor for a low-carbon feedstock -- Cellulosic ethanol has been 5 years away for decades. It’s a trope, but it’s been true as plant after plant trying to make ethanol from corn stover and other agricultural waste has opened to great fanfare and then closed in ignominy. But this time may be different. The biobased business community seems confident that a round of companies starting up plants has cracked the feedstock problems that stymied earlier attempts and has improved cellulose depolymerization. If the technology works, a host of customers in the chemical and fuel industries is eager for ethanol as a low-carbon feedstock.

Yet ethanol has a good side too—namely, the crops it is derived from pull CO₂ from the air.

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.

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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.

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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.

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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.

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But because the material had no buyers, there was also no mechanism in place to collect and distribute it.

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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.

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A painful lesson for the industry is that every cellulosic feedstock is unique.

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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.

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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.

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Clariant left room for carbon capture at its plant in Podari, Hortsch (Ralf Hortsch, Clariant’s head of marketing and strategy for biofuels and derivatives) says. And Joshipura (Praj CEO Shishir 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.

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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.

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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.

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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.

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Lignocellulose has functional groups such as ether bridges and double bonds that can give chemists useful handles for creating valuable chemicals and materials.

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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.

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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. READ MORE