Sustainable Aviation Fuel, Changing What Is Possible

By Dr. Patrick Gruber, PhD, CEO, Gevo, Inc.

Air travel has taken some serious lumps over the last couple of years. The pandemic hit, and fear of infection and crowded airports and planes combined with travel restrictions to reduce passenger flow to a trickle. Social media became a forum to highlight scenes of reduced air pollution, and the clean-air benefits of a worldwide lockdown. Airlines, and the industry at large needed financial help to survive. Even in the midst of a financial crisis, we see the airline industry renewing its focus on reducing GHG emissions from the burning of fossil fuels. As always, there is lots of “hype” around new technologies. As one of my colleagues keeps reminding me, “a new, shiny rock provides distraction.” At Gevo, we want to solve the big problems and we don’t shy away.

When we look at GHG emissions we look across the whole of the business system from the source of carbon, through production, to the use as a fuel, to the impacts or benefits to other related business systems. I think most people understand that all electricity is not created equal. Some is made from coal, some from natural gas, some is made from wind, some from solar, some from nuclear. How do we tell which is better from a GHG standpoint? We count fossil carbons used in the production of the electricity. Clearly solar and wind are considered low carbon, reducing or eliminating the GHGs associated with the production of electricity. Hydroelectric and nuclear production likewise eliminate GHG’s from the production of electricity, but these also bring into play other considerations. Is it ok to dam up more rivers to make more electricity? Is the building of more nuclear plants ok? Norway has done a very good job with hydroelectricity. France has done well with nuclear power. 

What will it take to eliminate the fossil footprint of jet fuel? The concept is simple: Eliminate the fossil carbon from across the system—the same approach as in making electrity. The burning of fossil carbon always results in increased GHGs. Always. Whether it is coal, natural gas, from incineration of fossil-based refuse or waste, the answer is always the same: Burning of fossil carbon leads to an increase in atmospheric CO2. Full stop. If we want to make net-zero carbon footprint fuels, we need to eliminate the fossil carbon from the whole of the business system as much as possible. But how?

Electric airplanes have some potential for short-haul flights. Lots more innovation needs to be done to accomplish long haul flights. New types of airframes, batteries, etc, will all need to be developed. Plus more than 60 percent of the electricity generated in the world is fossil-based. And 30 percent of people in the world don’t even have access to electricity. So there is lots of work to be done to make electric planes viable.

Hydrogen is an interesting fuel. It certainly burns clean. In order to use hydrogen as a fuel in an application like an airplane, it would need to be compressed to a liquid—new engines and airframes would need to be designed. The energy density of liquid hydrogen is roughly one quarter the density of hydrocarbon jet fuel. Simplistically, that means that it would take four times the space to carry the same volume of energy. Of course, the hydrogen needs to be “green” meaning it is made from renewable energy sources, otherwise it too would have a fossil-based GHG footprint.

What about renewable fuels? For these, we have to accomplish the reverse of burning. In essence we have to capture CO2 and pack it back into the right structure and shape of hydrocarbon fuel molecules so that they can be a “drop-in,” meaning no changes to planes, engines, or infrastructure would be required. To make a hydrocarbon fuel, carbon is required. The non-fossil carbon source is CO2 in the atmosphere. The CO2 needs to be collected and all the energy of the fuel needs to be put back into the hydrocarbon molecules. It takes energy to collect the CO2 and get it into a form whereby the chemical processes needed to transform it into hydrocarbons can be done. As it turns out, photosynthesis was designed by Mother Nature to capture CO2, the primary GHG. Photosynthesis captures CO2 and transforms it into carbohydrates, accomplishing a lot of the chemistry needed to be done on the way to making a hydrocarbon. By using fermentation techniques the energy of the carbohydrate itself can be used to move further down the path. Finally with simple catalytic chemical steps, finished drop-in hydrocarbons jet fuel is made. In measuring GHGs across this system, we need to take into account the energy sources, and since growing something is involved, we have to look at the sustainability of growing practices. Since growing practices involve land, then we also have to look at land use and changes. It’s complicated. But we can do it. We are doing it. We have to do it. 

We have found residual carbohydrates from the production of protein are a very low carbon feedstock for the making of hydrocarbon fuels. Farming techniques and equipment has changed over the last decades. Now techniques such as precision application of fertilizer and chemicals, where only the amounts needed are applied, are common. Cultivation techniques have likewise changed. In the Midwest U.S., tillage has changed too. Strip-tilling and no-till cultivation and planting is common. With these techniques, carbon is being captured in the root systems, and it remains there, building up soil carbon—it’s a form of carbon sequestration. The farmers do it because they get better yields of grain, the need for fertilizers can decrease, and the water retention in the soil better. It’s not the farming of old. No way. As an example corn in South Dakota has a carbon footprint of -5 gCO2e/MJ (based on Argonne National Laboratory’s GREET model, the premier science-based life cycle inventory model for determining GHGs and other sustainability attributes across the life cycle of a fuel. Corn makes the difference, when grown the right way—producing crops causes soil to capture carbon—a paradigm shift. We think that by rewarding farmers to improve their agricultural practices, by capturing carbon, by reducing run-off, by producing large amounts of protein, we can address several problems at once, and make this world a better place, with better nutrition, while eliminating fossil based GHGs.  

When we separate the protein from the carbohydrates in a kernel of corn, we are making a better feed product: It has both the lower carbon footprint, and the non-nutritive carbohydrates removed. When cows eat simple carbohydrates, they burp methane. Not good. We can reduce the burping by changing the feed—low-GHG-footprint cows. It is possible. We instead can take the carbohydrates for our hydrocarbon fuel process, and when we use fully renewable energy in production (wind electricity, electricity generated from biogas, and biogas) we can eliminate the GHG footprint of jet fuel when counting carbons across the whole of the business system. We’ve measured it. It can be done.

At Gevo, we like to challenge paradigms. We like to measure data and get the facts. We like to solve problems. We believe our Net-Zero Fuels as part of the bigger business system can drive better protein, better use of land, capturing carbon, AND generating raw materials. We are committed to measuring and improving sustainability across the whole of our business system. It is what makes our products special.