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UMaine thermal deoxygenization (TDO)

Mixed-acid fermentation and lipid accumulation are pathways that are well known to researchers around the world. The pathway Wheeler and the undergraduate students in his lab have developed in the last two years is something new to the biofuels canon. The process is promising not only for its relative simplicity, but also for its end result.

The pathway starts again with woody biomass. As with mixed-acid fermentation, any and all dirty (or clean) cellulose is acceptable. A batch of the UMaine TDO fuel was even made from grocery store shelving, boxes and produce.

Rather than break down the biomass in a biological process, Wheeler’s pathway uses a chemical process. The biomass is heated in sulfuric acid and water to around 200 degrees Celsius (392 degrees Fahrenheit), which breaks down the cellulose fibers into molecules. The molecules decompose further into what is known as mixed-carboxylic acids, which are neutralized to form a salt, which is then dried and heated in the same ketonization reactor as that used in mixed-acid fermentation pathway.

But unlike ketonization, thermal deoxygenation reaction removes nearly all the oxygen from the salt — a key step that distinguishes Wheeler’s biofuel from other processes. Oxygen is removed as both carbon dioxide and water, without the need for any outside source of hydrogen. Therefore, most of the energy in the original cellulose source is contained in the new oil.

“Biomass has a lot of oxygen in it. All of that oxygen is dead weight and doesn’t provide any energy when you go to use that as a fuel,” Wheeler says. “If you’re going to make a hydrocarbon fuel, one of the things you have to do is remove oxygen from biomass. You can do it by using hydrogen, but that’s expensive and also decreases the energy efficiency of your process. So if there’s a way to remove the oxygen from the biomass chemically, then you’ve densified it significantly.

“Our oil has less than 1 percent oxygenates. No one else has done anything as simply as this.”

The fuel has a number of other properties that make it superior to many hydrocarbon fuels being widely researched and even those currently on the market. In early analysis, the UMaine oil was found to have boiling points that encompass those of jet fuel, diesel and gasoline. It also has low acidity levels, making it more stable and less corrosive than other biofuels.

Not only is the fuel remarkable, but so are the amounts of the liquid being produced in Wheeler’s lab. Where some researchers are excited about creating a test tube of biofuel from a lab, Wheeler can produce several liters per month.

“The speed with which Clay got his results (on TDO) speaks to the simplicity of that technology and with that, the likelihood of success goes up,” says van Walsum.

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