CO2 Utilization: Biofuel from the Stack

by Katie Schroeder (Ethanol Producer Magazine) Utilization technologies are stimulating interest in ethanol producers’ constant coproduct, carbon dioxide. An initiative from the U.S. Department of Energy has formed the CO2 Reduction and Upgrading for E-fuels Consortium—or CO2RUe—made up of labs across the country working on developing technologies to upgrade CO2 into valuable drop-in electrofuels, also known as “e-fuels.” Ethanol plants are the best source of 99 percent pure, biogenic CO2, and are made a more attractive target for utilization technologies because of how many of them are in close proximity to renewable energy such as wind power.

Valerie Reed, director of the DOE’s Bioenergy Technologies Office, or BETO, explains that the consortium will research what’s required to make CO2 derived fuels a reality through applied R&D, with the goal of overcoming technical barriers as well as researching the economic side of deploying these fuels nationwide. “Those things are all incorporate[ed] into the thinking of the consortia, and its ultimate vision is to give us e-fuels as another tool in our toolbox to reduce CO2 emissions, particularly from the hard-to-decarbonize sectors like sustainable aviation fuel,” Reed says.

Laboratories involved in the consortium include NREL, Berkeley, Argonne, Lawrence Livermore and Oak Ridge. There are also several partners and representatives from various industries involved in the research.

Electrofuel production requires electricity and CO2, as well as hydrogen, depending on which process is used, Reed explains. If the CO2 is first changed into an intermediate like carbon monoxide, ethanol or methanol, and upgraded to a drop-in fuel, then hydrogen is not necessary. Because CO2 is an oxidized molecule without energy left to expend, large amounts of electricity are needed to upgrade the molecule. It is vital that the electricity used comes from renewable sources in order to achieve the necessary carbon reduction.

Grim (Gary Grim, staff scientist with NREL) outlines why the purity of CO2 generated at ethanol plants makes it a great feedstock option. “It’s very helpful on our side because it cuts down on purification costs and potential issues of ‘poisoning’ downstream with various catalysts and other processes. So, that’s [principally] why we like bioethanol CO2,” he says.

Implementing conversion technology, an ethanol plant’s CO2 would be converted into ethanol or another intermediate, then upgraded to a drop-in fuel, such as SAF or renewable gasoline. CO2RUe’s research is primarily focused on the first step of turning CO2 into a molecule with a higher carbon number, because most of the processes that turn intermediates into the final product are more mature, Grim explains. Because CO2 is a fully oxidized molecule with no energy, a lot of electricity is needed to convert the C1 molecule into a molecule with a higher carbon number, such as benzene. “It’s really difficult to make a fuel-range hydrocarbon from CO2 in a single step,” he says. “You’re unlikely to get from a C1—carbon dioxide—to … octane or a dodecane in a single step.”

The intermediate used will depend greatly on the final product you are trying to make, Grim explains. For example, ethylene is a good intermediate for upgrading to jet fuel and use within the plastic industry. Carbon monoxide can be combined with hydrogen to make syngas. Methanol, ethanol and methane are also “intermediates of interest” with a variety of uses. Intermediates produced using biological methods include acetic acid, which can be used for vinegar, formic acid and fatty acids.

CO2RUe did a case study of what implementation of these technologies could look like at a 40 MMgy ethanol biorefinery. A first-generation ethanol plant that size would produce around 14 tons of CO2 per hour, Resch explains. With the introduction of CO2 conversion technology, the carbon dioxide could be converted to ethanol using electrochemistry and gas fermentation, and the plant’s output would increase 30 percent—up to 58 MMgy of ethanol. However, this increase in output would come with an added 47 megawatts of power needed for the conversion process, which would need to come from renewable sources. To produce that much renewable energy, a total of 94-100 wind turbines would be needed.

If an ethanol producer is interested in integrating electrofuels technology into their plant, Grim recommends pursuing opportunities for systems integration, to identify the synergies of combining an ethanol biorefinery and a CO2 conversion process.

One of the primary limiting factors to the development of electrofuels is the amount of cheap renewable electricity available on the market. Resch believes that more access to renewable energy will be needed before e-fuels can become widespread. READ MORE