Minnesota’s Sustainable Aviation Fuel Opportunity: Forestry

The ambitious goals being set to stimulate sustainable aviation fuel (SAF) production present multiple opportunities for Minnesota’s forestry and agriculture sectors.

Oilseed crops are used in one conversion technology. Alcohols, including corn ethanol and isobutanol, are used in another. Cellulosic feedstocks like crop residues and woody biomass are used in other technologies. Minnesota has an abundance of all.

Part I of our series on Minnesota’s Sustainable Aviation Fuel Opportunity discussed some of the policy incentives and requirements that would nurture a Minnesota SAF industry.

For the agriculture sector, SAF could provide the market needed to expand winter oilseed cover crops. For the forestry sector, SAF demand could support the existing wood products industries and create a market pull that would support forestry management for fire hazard reduction, plus utilize insect-killed wood.

Part II will dig deeper into the opportunity for Minnesota’s forestry sector.

Technology and feedstock match for sustainable aviation fuel production

Eight technologies have been approved to produce SAF, with more in development.

ASTM International, formerly known as American Society for Testing and Materials, approvals require extensive testing and are specific to technology and feedstock. At a high level, producing jet fuel is about building the right mix of hydrocarbon molecules to create a drop-in fuel compatible with current jet engines and infrastructure.

Jet fuel is comprised of multiple hydrocarbons with carbon chain lengths between C8 and C16, providing the characteristics needed for jet turbines operating at high altitudes.

Some SAF technologies are like oil refining in cracking long molecules, adding hydrogen and catalytically rearranging the molecules, followed by distillation. Other technologies start with smaller molecules, building them up catalytically.

Vegetable or waste oil feedstocks are comprised of molecules with carbon chain lengths between C12 and C24. Vegetable oils are converted to jet fuel using the hydrotreated esters and fatty acids (HEFA) process. In this process, purified oils are exposed to chemical reactions with hydrogen and catalysts and then distilled.

With several steps and inputs akin to oil refining, petroleum companies have announced multiple retrofits and new facilities utilizing HEFA. Most of those, however, are currently being tuned for renewable diesel, which has molecules ranging from C8 to C23. SAF and renewable diesel fuels overlap some, so the HEFA process can produce either with the fuels competing for the same lipid feedstocks.

Other technologies take short carbon-chained alcohols like isobutanol (four carbons) or ethanol (two carbons), remove the oxygen, and catalytically build a longer chain SAF. Again, the catalytic steps are well known and have been used by petroleum refineries.

Several technologies begin with biomass that is subjected to high heat through gasification or pyrolysis to produce syngas or biocrude. Woody biomass, crop residues, energy grasses, and various waste streams have been tested in these systems.

Multiple technologies exist for each step—from heating to deconstruct the biomass to cleaning up the produced syngas or biocrude, to the catalytic conversion into hydrocarbons of the targeted lengths.

The concept is not new. The Fischer-Tropsch process was invented a century ago, and ever since, scientists have worked on developing more efficient, cost-effective systems, finding new catalysts, reducing energy inputs, and increasing output.

Minnesota has abundant feedstock supplies that support each SAF technology type, with the potential to expand.

Forest opportunity to produce sustainable aviation fuel

SAF technologies turning woody biomass into liquid fuels could be a boon for Minnesota’s forestry sector if the market pull for SAF feedstocks can overcome the economic challenge of building the supply chain.

“We have millions of tons of material that could be put into play,” says Pete Aube, chair of the Minnesota Forest Resources Council.

Not only are healthy forests a carbon sink, but many forest products sequester carbon, and others, such as SAF, replace petroleum products.

He cites the wood residuals accumulated by the state’s forest product industries—about half of each round log sawn into boards is unusable in the form of sawdust, wood chips, and bark.

“Then, there are underutilized species,” he says, “like ash, tamarack, and larch where we have no markets. These forests are primarily owned by the state. They could be turned into aviation fuel and biofuels in general. This would not only solve problems with forest pest management and forest health creation but also address climate and carbon.”

Aube points out that about half of Minnesota’s forests are state-owned.

“Every legislator and governor should care about Minnesota forestry because we own half of it. If they care about carbon and climate, they ought to get excited about forestry opportunities. They are the ones that determine if it comes to market and is developed,” he says.

Eric Singsaas, bioeconomy director at the Natural Resources Research Institute in Duluth, MN, says that biofuel technologies that can accept a diverse range of forest feedstocks could be important for forest management.

“There’s a saying that there’s no management without markets,” he says.

He explains that the current practice to reduce fire load and fire hazard is to use taxpayer dollars to bring people into the woods and harvest brush and small trees.

“They literally pile it and set it on fire. And there’s not enough taxpayer funding to do all the management,” he says.

The biggest challenge in managing overgrown stands, stands that are above healthy density in the understory or overstory, is economics.

“It’s fairly efficient to take large trees,” Singsaas says. “If you’ve seen modern forestry equipment, mechanization keeps costs down. But when taking shrubs and small trees, it’s generally done by hand by people with chainsaws. Then you still have to bring it to a landing, put it on trucks, and ship it.”

He adds that logs stack nicely for shipping, but the brush is fluffy. People are experimenting with densification systems, but the high cost of harvesting, getting material to a landing, bundling, and shipping has been a major barrier to developing new markets.

Another barrier is public perception.

“There’s this idea that if you make anything from a tree—houses or paper, or jet fuel—the forests are going to go away forever. And that’s just not true,” Singsaas says.

While clear-cutting was a problem more than 100 years ago and still is in some countries, it is no longer done in the US. Although voluntary, best practices for sustainable forestry management are widely followed.

“The major reason for loss of forest lands in the United States has been conversion of forest land to agriculture or houses,” he says. “And that’s because if there are no markets for forest products, you wind up selling it for vacation homes or agriculture.”

Agroforestry is another opportunity for Minnesotans to consider, continues Singsaas. “Things like hybrid poplar are trees grown for a biomass crop. If you’re interested in cellulose, they are much more productive per acre than corn or other crops.”

He explains that trees are typically ready to harvest in 5 to 12 years and can be harvested with machinery, which reduces costs. Combining hybrid poplar with mill residuals and forest thinning offers the ability to mix and match sources to optimize production.

“That’s where the technology research and development opportunities are now,” he says. “We have a number of technologies that work that get us from forest biomass to jet fuel. How do we match that up with the biomass we have, that you can obtain in volume at the right price in order to make this an industry?”

The link between sustainable aviation fuel and the insect-damage crisis

The potential for a rapidly expanding market for feedstocks to make SAF would address another growing problem for Minnesota. First spotted in the state in 2009, emerald ash borer (EAB), a destructive wood-boring pest, has now spread to at least 35 counties.

Five-year projections suggest the peak in ash trees killed by EAB will see more than a half million tons of wood waste each year in the Twin Cities metro area alone, according to an October 2022 report from the Partnership on Waste and Energy, Twin Cities Metro Area Emerald Ash Borer Wood Waste Study.

Currently, District Energy in St. Paul, Minnesota, consumes roughly two-thirds of the metro’s waste wood to generate electricity and thermal heat. The combined heat and power plant uses around 260,000 tons of woody biomass annually.

Before EAB, the majority of District Energy’s woody biomass supply came from the metro area but was supplemented by wood collected from a 60-mile radius and, occasionally, trucked in from further north.

In the past few years, as EAB material has accelerated, they take much less from further out and have even closed their wood yard at times because it was too full to safely handle more material.

“Minnesota has more ash trees than any other state,” says District Energy CEO Ken Smith. “In the Twin City metro, ash makes up around 15 percent of the canopy. Some communities have 20 percent. There are communities in Minnesota that have 60 percent, plus we have black ash in northern Minnesota.”

He explains that EAB has about a 20-year life cycle from when it shows up until all the trees in that area are gone. It was first spotted in 2009.

“EAB is creeping its way from the southeast corner of the state. It’s just now gotten into the central part, and it’s already in Duluth,” he says. EAB was first spotted in 2009.

Unfortunately, EAB is not the only insect causing tree damage.

“The pine bark beetle, the emerald ash borer, and spruce budworm are killing tamarack and balsam fir,” Singsaas says. “And when you have dried out timber in unmanaged forests, you can get a lightning strike Minnesota’s big forest fires two years ago spread in unmanaged forests.

“Minnesota has about 2 million acres of forests at risk of disease and insects that could be made healthier with utilization,” Aube says. “Much like cancer, when you start to see infestations, you cut those trees out, and there are no longer breeding sites for the insects or disease.”

“When you cut live trees, they either seed or sprout, and you get the next forest. The forests regenerate, and they do what you want forests to do—sequester carbon, grow wood, and create wildlife habitat,” he says.

The decades-long struggle to find markets to support forest management makes the SAF opportunity tantalizing.

“I see the climate challenge not as a threat but an opportunity for forests to have a greater play and relevance and significance,” Aube says. “Most people don’t realize what that opportunity that is.”

About the author: Susanne Retka Schill, Langdon, North Dakota, is a semi-retired freelance writer who has covered agriculture and renewable fuels throughout her career.