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Mixed-acid fermentation

When wood arrives at a pulp mill, it goes through extraction to remove liquids from the solids. The solid material is used in the pulping process, transforming it into some form of paper. The liquid extract is the jumping-off point for mixed-acid fermentation.

Mixed-acid fermentation has not been proven to produce jet fuel. But with hydrogen upgrading, it can produce crude gasoline. It’s attractive as a pathway because it doesn’t require a clean feedstock — meaning, anything that contains cellulose can feed the process.

Van Walsum, who has worked on biofuels production for decades, including several years in private industry, has even made fuel precursors from seaweed and seaweed processing waste from a plant in Rockland, Maine.

In many fermentation processes, van Walsum says, a clean feedstock is necessary because the organisms engineered to consume sugars and degrade the liquid are particular about what they ingest. In mixed-acid fermentation, the organisms consume anything from proteins and fats to cellulose and starches.

Another benefit to the “dirty” process, as van Walsum calls it, is lower production cost because the fermentation does not require a stainless-steel reactor or clean, genetically engineered nutrients. In van Walsum’s lab, the process is done in a plastic tub.

“A very highly engineered organism would probably want to have glucose and that’s a refined sugar, mostly from corn or sugar cane, so right away you’re looking at food sources,” van Walsum says. “What we can do is take raw wood or garbage wood or wastes and not use a sterile culture, but rather completely open mixed cultures, like a septic tank — anything that will grow on what will feed it. But what we do is constrain the growth conditions so they only produce the type of product we can use, which is an organic acid.”

At the fermentation stage, the organisms anaerobically digest the liquid extract, decomposing it to produce a mix of organic acids, including acetic, lactic, propionic and butyric acids. Calcium carbonate, or limestone, is added to the fermentation stage to neutralize the acids.

The acids are then dried, which is one sticking point for this pathway. Drying is energy-intensive, van Walsum says, and therefore expensive.

At this stage, the acid has been transformed into a dry salt, which then goes into a process known as ketonization. In a reactor, the salt is constantly stirred and heated to 450 degrees Celsius (842 degrees Fahrenheit). The reaction turns the salt into a liquid fuel.

The liquid resulting from ketonization will burn, but not in the way it needs to in order to serve as jet fuel. Following hydrogen upgrading, however, the mixture could be used as gasoline.

“We’ve managed to get relatively good conversion on our mixed culture to organic acids,” says van Walsum, who can produce the fuel on a bench scale. “It’s very robust and can handle a dirty stream because everything ultimately biodegrades one way or the other. The trick is to keep it making these acids, and that’s not too hard to do.”

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