Sequestering Biomass and Carbon More Effectively by Adding Salt

Reducing global greenhouse gas emissions is critical to avoiding a climate disaster, but current carbon removal methods are proving to be inadequate and costly.

Now researchers from the University of California, Berkeley, have proposed a scalable solution that uses simple, inexpensive technologies to remove carbon from our atmosphere and safely store it for thousands of years.

In an article “Scalable, economical, and stable sequestration of agricultural fixed carbon” in PNAS, scientists propose growing biomass crops to capture carbon from the air, then burying the harvested vegetation in engineered dry biolandfills. This approach, which researchers call agro-sequestration, keeps the buried biomass dry with the aid of salt to suppress microbials and stave off decomposition, enabling stable sequestration of all the biomass carbon.

The result is carbon-negative, making this approach a potential game changer, according to Eli Yablonovitch, PhD, lead author and professor in the graduate school in UC Berkeley’s department of electrical engineering and computer sciences.

“We’re claiming that proper engineering can solve 100% of the climate crisis, at manageable cost,” said Yablonovitch. “If implemented on a global scale, this carbon-negative sequestration method has the potential to remove current annual carbon dioxide emissions as well as prior years’ emissions from the atmosphere.”

Different from previous efforts

Unlike prior efforts toward carbon neutrality, agro-sequestration seeks not net carbon neutrality, but net carbon negativity. According to the paper, for every metric ton (tonne) of dry biomass, it would be possible to sequester approximately two tonnes of carbon dioxide.

“We describe a scalable, economical solution to the carbon dioxide problem. CO₂ is captured from the atmosphere by plants, and the harvested vegetation is then buried in an engineered dry biolandfill. Plant biomass can be preserved for hundreds to thousands of years by burial in a dry environment with sufficiently low thermodynamic “Water Activity,” which is the relative humidity in equilibrium with the biomass,” write the investigators.

“Maintaining a dry environment within the engineered dry biolandfill is assisted by salt that preserves biomass, which has been known since Biblical times. A “Water Activity” <60%, assisted by salt, will not support life, suppressing anaerobic organisms, thus preserving the biomass for thousands of years. Current agricultural costs, and biolandfill costs, indicate US$60/tonne of sequestered CO₂ which corresponds to ~US$0.53 per gallon of gasoline.

The technology is scalable owing to the large area of land available for nonfood biomass sources. If biomass production is scaled to the level of a major crop, existing CO₂ can be extracted from the atmosphere, and will simultaneously sequester a significant fraction of world CO₂ emissions.”

Agro-sequestration: A way to stably sequester carbon in buried biomass

The idea of burying biomass in order to sequester carbon has been gaining popularity, with startup organizations burying everything from plants to wood. But ensuring the stability of the buried biomass is a challenge. While these storage environments are devoid of oxygen, anaerobic microorganisms can still survive and cause the biomass to decompose into carbon dioxide and methane, rendering these sequestration approaches carbon-neutral, at best.

But there is one thing that all life forms require—moisture, rather than oxygen. This is measured by “water activity,” a quantity similar to relative humidity. If internal water activity falls below 60%, all life comes to a halt—a concept underpinning the UC Berkeley researchers’ new agro-sequestration solution.

“There are significant questions concerning long-term sequestration for many of these recently popularized nature- and agriculturally-based technologies,” said Harry Deckman, co-author of the study and a researcher in the department of electrical engineering and computer sciences. “The agro-sequestration approach we’re proposing can stably sequester the carbon in dried salted biomass for thousands of years, with less cost and higher carbon efficiency than these other air capture technologies.”

Hugh Helferty, PhD, co-founder and president of Producer Accountability for Carbon Emissions (PACE), a nonprofit committed to attaining global net zero emissions by 2050, sees great promise in this solution.

“Agro-sequestration has the potential to transform temporary nature-based solutions into permanent CO2 storage,” said Helferty, who is not involved with the study. “By developing their approach, Deckman and Yablonovitch have created an invaluable new option for tackling climate change.”