We’ve all heard of electric cars and semi trucks that are powered by hydrogen fuel cells, but what about airplanes?
“The problem with aviation is that if you want to move to a new technology like hydrogen, it’s going to take a long time,” says Paolo Patelli, a scientist in the Information Systems and Modeling group at Los Alamos National Laboratory. “Designing an airplane is complicated, and there are a lot of risk factors to consider compared to designing a truck.”
While more sustainable aircraft are designed and tested—which could take decades—Patelli says sustainable aviation fuel (SAF) is a good nearer-term solution. SAF is made from renewable biomass and waste resources, and according to the Department of Energy, has “the potential to deliver the performance of petroleum-based jet fuel but with a fraction of its carbon footprint, giving airlines solid footing for decoupling greenhouse gas emissions from flight.”
However, aviation fuel—of any type—is more complex than automobile fuels. Aviation fuel must be able to withstand extreme environmental conditions, including fluctuations in temperature and air pressure.
When it comes to SAF, researchers at Los Alamos are optimistic and involved in several projects that are exploring the future of sustainable aviation fuel.
One project, which includes Patelli and fellow scientist Chloe Zhou, involved collecting data on where potential biofeedstock (things like plant oils and algae that are used in SAF production) is located and where demand for SAF will be highest. Then, the team derived an algorithm to connect supply with demand and model two production pathways.
The first pathway is a centralized model of vertically integrated production. That means one plant would be responsible for all production steps. The second pathway is a two-step process where biofeedstock would be converted into methane, transported using gas pipelines, and then chemically transformed into SAF by many smaller, decentralized SAF production facilities.
“We will need to produce SAF in high quantities if we want to meet decarbonization goals,” Patelli says. “That’s why decentralized production might be beneficial. Those plants are smaller projects, smaller investments that are much easier to bootstrap.”
Currently, commercial-scale efforts to produce SAF are few and far between because aviation companies don’t have any incentive to switch fuels. This is where a second Lab project comes in.
A team led by Babetta Marrone of the Bioscience division received funding for a three-year Laboratory Directed Research and Development project called Versatile Synthesis Platforms for Advanced Biomanufacturing (VESPA). VESPA focuses on developing a SAF production process that is both economically and environmentally sensible. The team will explore three different areas. First, a techno-economic analysis will evaluate the economic viability of production processes.
“It’s really important to use the economic analysis to guide the biological and chemical research,” says Bill Kubic, a research and development engineer who helps lead the analysis. “Understanding things like cost can direct research toward the processes that are economically appropriate and more competitive in the market.”
Although Kubic admits production of SAF might initially be more expensive than traditional jet fuel, he believes SAF could eventually become competitive with petroleum prices. This would mean a shift away from a decentralized production model to a centralized model where economies of scale could be leveraged to make the process more economical.
Once Kubic has completed his analysis, scientists Raul Gonzalez and Xiaokun (Claire) Yang will step in. Gonzalez and Yang will work on two parallel tracks for the biological and chemical processes with the aim of merging the two processes in the most efficient manner possible.
The biology hinges on photosynthetic organisms called cyanobacteria, which are fast growing microbes that capture, use, and transform carbon dioxide molecules from the atmosphere. Cyanobacteria can be engineered to produce isoprene, ethylene, and polyhydroxalkanoates—three carbon-rich molecules that can be chemically modified to become jet fuel compounds. The biologically produced molecules can also be converted to valuable co-products, such as plastics, thus helping to offset the jet fuel production costs.
After the initial genetic transformation is complete, the team will determine the percentage of fixed carbon that turns into the desired molecule. “Once we have done that, we just begin an iterative process called design, build, test, learn,” Gonzalez says. “Whatever we learn at the end, we will use to try and improve our product yields.”
The team will continue this process until it produces a percentage of intermediate molecules that is considered economically competitive with petroleum. The available carbon within a cyanobacterium’s cell is used for a variety of metabolic processes, and new engineered reactions may not perform efficiently. The target amount for each project will be dictated by Kubic’s analysis, but Gonzalez estimates they will need to increase photosynthetic efficiency by at least 20 percent.
Meanwhile, Yang’s team will be focused on chemically converting the key bio-derived intermediates, such as isoprene, to SAF molecules. SAF is a complex mixture of non-oxygenated hydrocarbons with 8 to 14 carbon atoms and properties defined by strict fuel standards. Selective chain-extension reactions are key chemistry operations that have been established and expanded upon by Yang’s team to synthesize SAF-range molecules with application potential from the bio-derived intermediates.
“Controlling carbon chain lengths is a major challenge,” Yang says. “You are coupling multiple active species together, but you also need to cut off the reaction before the chain overextends.”
Each of the different carbon compounds performs a different function in jet fuel and synergistically determines the fuel’s properties like heating value, viscosity, freezing point, and more. Therefore, it is crucial that Yang’s team creates a blend of compounds that meets the properties and specifications of jet fuel.
“Once we produce the jet fuel range compounds, then there are a series of in-house property tests we conduct to ensure the mixture meets the property specifications of jet fuel,” Yang says. “The percentage of our projects to blend in current airplane engines can then be determined after blending tests. Our ultimate goal is to achieve 100 percent renewable fuels blending.”
Throughout the project, Yang and Gonzalez will work together to integrate the two processes so that the biology team’s products are efficiently fed into the chemical reactions. From there, it is a matter of evaluating how scaling up to commercial-scale production would work.
“One of the problems here at the Lab is we can develop all of these materials and processes, but we are doing so at bench-scale,” Kubic says.
Members of the team believe reaching full commercial-scale production could take between 8 and 10 years.
“It’s difficult, but I think we’re on the right track,” Gonzalez says. “The multidisciplinary nature of the Lab helps. Bringing together people with different areas of expertise is how we’re going to solve this.” ★