What comes to mind when you hear the term engineered climate technology? For me, it’s usually an expensive metal machine sucking carbon out of the air with a lot of moving parts, a lot of engineering schematics and a lot of complicated chemical explanations.
A less popular image of carbon removal is a hole in the ground filled with tree trimmings. But biomass burial, the technical term for this hole, is one of the most promising and uncharacteristically simple engineered approaches to removing carbon from the atmosphere.
"We call this a hybrid nature engineering method," said Ning Zeng, professor at the Department of Atmospheric and Oceanic Science and the Earth System Science Interdisciplinary Center at the University of Maryland. "It doesn't really fall neatly into any particular big boxes they have defined."
But it is, in fact, an engineered solution to climate change. Burying wood trimmings about 6.5 feet underground prevents the decomposition process, as it preserves the carbon in the wood instead of releasing it into the atmosphere.
"[Without biomass burial], you waste all that photosynthesis capacity from a carbon point of view," Zeng said. "The plants pump CO2 into their body, but then they die, it goes back [into the atmosphere]."
Nature’s already tackled the hard part of capturing CO2 from the atmosphere, leaving us to configure a storage solution. But how durable that storage is depends on engineering the wood vault where the trimmings are stored in a specific way, and using materials that create an anaerobic (oxygen-free) environment.
The vault itself can be created from the soils at the site of burial if the soil can insulate from termites, fungi and water. According to Daniel Sanchez, chief scientist for biomass carbon removal and storage at Carbon Direct, dense, nonporous soils such as clay, silt and sand are the best for keeping an underground environment stable in this way.
"You absolutely need to think about how you design different vaults," he said. "They're going to have different considerations and different abilities. The science of the wood vault is still really evolving."
But with good engineering, the carbon stored in wood vaults can be preserved for thousands of years, according to scientific models. That number has pricked the ears of companies, investors and sustainability experts alike. And the carbon crediting machine has already started lurching into motion on biomass burial. Puro.Earth has a few biomass burial projects listed on its marketplace, including the Potomac Project of Carbon Lockdown a startup founded by Zeng’s demo projects in Montreal where the carbon removal is selling for $170 a ton. And last March, Puro released a methodology for biomass burial, specifying where the wood can come from, how to ensure additionality and making sure to remove other greenhouse gasses such as methane.
Biomass burial is one of the most promising and uncharacteristically simple engineered approaches to removing carbon from the atmosphere.
Of course, the climate tech startup ecosystem has already started roaring to life around this technology, including newcomer Kodama Systems. They are hoping to go beyond carbon accounting and solve three problems at once — decreasing the amount of carbon in the air, reducing wildfire risks and increasing the workforce for forest management, a woefully understaffed career track. The company is working with U.S. Forest Service partners for tree thinning and burying the waste materials from forest thinning operations in a wood vault in the Nevada desert.
"The reason we need to restore these forests is we've actively suppressed wildfires for the past 100 years," Kodama CEO Merritt Jenkins said. "Many forests are overstocked beyond their natural carrying capacity and there literally aren't enough people to do the work that needs to be done in these forests."
According to Jenkins, the company is planning to bury small diameter trimmings that would have been burned without Kodama’s project. The company is working on using automation to support workers to cover the vast amount of acres that need managing.
"Carbon management is a waste management problem," Jimmy Voorhis, head of biomass use and policy at Kodama, said. "We think forestry residue management is functionally right now a waste management problem and we really need to be driving towards solutions that are scalable and don't have a lot of negative externalities."
The pilot project will bury 4,500 tons of wood from the Eastern Sierras near Mammoth Lakes, and remove around 3,200 tons of CO2 when accounting for the possible emissions losses. The project has promised the first tranche of carbon credits to Frontier for $250,000 if they hit certain baselines.
Of course, this begs the question: Why not repurpose these waste materials into a tangible product or for energy generation in lieu of their subterranean future?
"There are economic challenges to building facilities that can turn this biomass into a product," Jenkins said. "And the main economic challenge is that centralized facilities are high [capital expenditure]."
It is very expensive for a biofuel or manufacturing facility to create building material out of these small trimmings. To secure bank loans, would-be entrepreneurs need to prove their access to a long term supply of trimmings. And then there is the matter of trying to build a facility on fire prone land, all to make the best emissions-saved to emissions-released ratio. According to Voorhis, there is always going to be more emissions released from making energy out of biomass than a biomass burial project, where the target is around a 90 percent conversion. The burial method makes much more economic and environmental sense and is more easily scalable than other engineered carbon removal options.
Many more engineered carbon removal technologies are still in their infancy, in need of decades for maturity. And, according to Zeng, we don’t even know if those ventures will succeed. He prioritizes the relatively secure investment into this technology that can take advantage of the wood residuals that are otherwise creating a forest fire threat.
"[Biomassburial] does not require unknown technology and it can be scaled up immediately," Zeng said.