Forget electric planes: U.S. aviation can be decarbonized using grass

In 1960, 57.7 million passengers flew on US airlines- the majority of which were business travelers as, at the time, only a small percentage of the general public could afford to travel by air.

Compare that to the estimated 926 million travelers who took to the skies in 2019.

Air travel is not so much of a luxury as it is a necessity and is available to more people worldwide- you only have to visit your nearest international airport at any given time to see this. Better yet, it's only projected to increase further.

Still, despite significant progress in the efficiency of aircraft and flight operations over the last 60 years, air travel continues to produce the highest and fastest growth of individual emissions. On top of this, with over 45,000 planes ferrying over 2.9 million people daily, the U.S. is the most significant user of commercial aviation worldwide.

What if we could make almost all US air travel emissions-free? What if we could switch out carbon-intensive jet fossil fuels for greener bio jet fuels made from American-grown rain-fed grass?

This is where a recent study may come into play. Researchers at Arizona State University demonstrate that planting a particular type of grass on unused agricultural land could generate enough biofuels to decarbonize the US aviation sector completely.

Interesting Engineering (IE) spoke with two lead authors, Nazli Uludere Aragon and Matei Georgescu, to delve deeper.

Offsetting aviation emissions is "within reach"

Aragon and Georgesco explained to IE that if emissions from the global aviation industry were to be summed up and compared to individual countries' emissions, it would rank among the top CO₂-emitting nations globally.

There are roughly 100,000 daily commercial air flights globally, which is only expected to increase in the future.

"Identifying solutions that can offset emissions from the airline industry will play a meaningful role in diminishing the overall human footprint on climate," they added. This includes CO₂ that is produced when jet fuel burns and its non-CO₂ emissions.

Utilizing low-carbon liquid fuels is one solution that could help decarbonize aviation, as electrified propulsion of large aircraft is still impractical. However, commercializing such "sustainable aviation fuels," or SAFs, is lagging due to a lack of clarity regarding their potential.

"We demonstrate that it is within reach for the United States to decarbonize the fuel used by commercial aviation- without having to wait for electrification of aircraft propulsion," said Nazli Uludere Aragon.

They sowed 23.2 million hectares of miscanthus

The researchers sowed the grass miscanthus on 23.2 million hectares of currently existing marginal agricultural lands across the United States. Such land frequently lies uncultivated due to its poor soil quality.

They discovered that this soil could sustainably produce enough biomass feedstock to completely meet the aviation industry's liquid fuel needs from biofuels- an amount expected to reach 30 billion gallons/year by 2040.

Still, the researchers recognized that any alternative energy solution must be financially feasible if it is to be adopted. After all, often, it is the economics that drives people's decisions on the ground.

Biojet fuel using miscanthus costs $4.10/gallon on average

With this in mind, they found that producing bio jet fuels derived from miscanthus would cost an average of $4.10/gallon. While this is higher than the average price for conventional jet fuel — typically about $2/gallon — the team argued it is reasonable considering bio jet fuel's potential to cut emissions.

Additionally, jet fuel prices are well known to fluctuate and reach prices above $4/gallon.

The team made a note of the fact that previous studies on the potential of biofuels mainly consisted of isolated evaluations. These have not been adequately integrated, for instance, missing important information about how the changing crop cover affects the local climate.

Real-world solutions to real issues

To avoid this trap, the scientists adopted an integrated systems approach for growing the grass for bio jet fuel that considered both environmental and economic performance.

Firstly, they began with assessing the marginal land base. "I.e., where are the marginal lands? What are they being used for (currently)?" Aragon and Georgescu explained.

They then used a model that could evaluate the interactions between the land cover and the atmosphere.

This enabled them to examine what would happen if there was a change in the ground cover on these marginal lands, for example, from planting miscanthus. "This constitutes the hydro-climate outcome," they described.

"We took the land assessment and the climate data to then evaluate how much biomass feedstock we can produce from growing miscanthus over specific areas that does not cause detrimental impacts on the land," they told IE. "This is called ecosystem modeling."

For example, any region resulting in soil desiccation would not be beneficial.

Essentially, once the team knew how much biomass feedstock was potentially available, they evaluated its economically feasible quantity and spatial configuration: where the biomass is grown, processed, converted into (bio jet) fuel, and ultimately distributed nationwide.

The researchers emphasized the importance of their integrated approach to developing realistic solutions that can scale.

"Marine transport is another sector that is challenging to electrify"

But could miscanthus-derived biojet fuel be applied to industries outside of air travel? Notably, military aviation, whose share of global greenhouse-gas emissions, comparable to that of air travel, is roughly 1-5 percent?

From a recent study, we know that armed forces have a massive carbon footprint absent from global accounting, and the United States military alone emits more greenhouse gases than numerous nations, including Peru, Singapore, and Switzerland.

Even better, the US military would have the highest emissions per person in the entire globe if it were a country (at 42 metric tonnes of CO₂-eq per staff member).

Aragon and Georgescu said, "technically, yes; however, military purchases of jet fuel might be subject to different regulatory or readiness requirements. We cannot speak to that."

Still, "Another potential bio-based fuel need is for marine diesel. We did not directly explore this in our study, but marine transport is another sector that is challenging to electrify," they revealed.

However, the playing field for low-carbon fuel needs to be equal

Now that the researchers have demonstrated a feasible way of using bio jet fuels for aviation, what's stopping the miscanthus-derived bio jet fuel from going to scale?

"The obstacles are primarily economic," they explained to IE. "Because current [fuel] prices do not reflect the cost of GHG emissions, any substitutes for fossil fuels appear much more expensive than they truly are."

Aragon and Georgescu added that putting an explicit price on carbon would level the playing field in favor of low-carbon fuel alternatives and products from a bio-based economy.

That said, more is needed to ensure sustainability in feedstocks and associated land and water use. "Ensuring those requires targeted policies," they said.

"Land conversion of this scale cannot happen overnight"

"A limitation of our study concerns the potential variations in miscanthus productivity under changing climate or extreme weather conditions, which is not widely modeled or easily studied in field trials," they disclosed.

Similarly, the researchers admitted that they need to learn more about the impact on productivity when miscanthus is grown in highly degraded lands. "Different marginal land representations could produce different results," they added.

"In addition, we did not capture market feedback. Land conversion of this scale cannot happen overnight," Aragon and Georgescu highlighted.

They explained that as the (miscanthus) biomass cultivation and bio jet fuel refining industries are established, there will be feedback or "ripple effects" which will impact commodity prices (including livestock), fuel prices, and agricultural land prices.

"In turn, this would affect how (and how fast) the bio jet fuel industry grows," the researchers said.

The next steps for the team are to evaluate the greatest research benefit that also considers the limitations mentioned above. "For example, is it more useful to test our results under different marginal land definitions," they explained. "Or [it is more beneficial to] resolve the uncertainty in biomass productivity?"

The team will also explore different geographies to replicate their analysis.

A template for the future of scaling biojet fuel

According to the researchers, it's critical for the scientific community to cross disciplinary boundaries and move past small emissions reductions to uncover more answers to the Earth's climate catastrophe. Ultimately, they stress the importance of practical, scalable solutions.

"We show that it is possible to make a strong dent, and investments in scaling this [biojet fuel] nascent industry are required as next steps," the researchers concluded.