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A plan for biomass removal that easily beats DAC on cost


Image credit: "Scalable, economical, and stable sequestration of agricultural fixed carbon", pnas.org


Plants and trees are good at capturing CO2 from the air, cheaply. Fast-growing native plants such as switchgrass, common on the North American prairie, need no tilling or fertilizer, and can be grown and harvested for as little as $30-40/ton of CO2 stored. This is far, far cheaper than any man-made CO2 capture system that has been proposed.


The problem with switchgrass is that what biology does, it has also learned to undo. As a general rule, biomass is rapidly consumed by microorganisms who have evolved to extract energy from it as quickly as possible, before competitors have the chance. This decomposition releases CO2 as a byproduct. From the moment the switchgrass is harvested, it releases the CO2 it gained. Plans to harvest biomass for carbon drawdown - a sub-field with the name Biomass Carbon Removal and Storage (BiCRS) - tend to struggle when fighting this natural tendency, especially in the context of scaling proposals to atmospheric relevance.


There are, however, situations where the biomass is preserved, most particularly when the "water activity" level around it is low. Water Activity, the relative humidity in equilibrium with the biomass, is the key to controlling microorganisms in sequestered biomass. When Water Activity is kept below a certain level, the growth of harmful bacteria, yeasts, and mold is inhibited. Decreasing Water Activity further can slow down the metabolic rate of microorganisms and eventually extinguish life. This is because cells need water to transfer nutrients and waste products in and out of their cell walls. As Water Activity decreases, water becomes tightly associated with chemical ions and is no longer available for cells to use. This process is essential to ensure the dryness level needed for preservation in aerobic, anoxic, and anaerobic environments.


Drying out biomass lowers its water activity, but applying heat in this way also requires a substantial amount of energy. This week two researchers from Berkeley proposed a cheaper, less aggressive method to sequester carbon in biomass: Salt it. Salt preserves biomass by tying up it water; a sufficiently salted system will have a water activity level as low as a dried one.. A small amount of salt, costing less than $3 per tonne of biomass, can ensure a Water Activity of less than 0.6, making it an affordable solution.


But what to do with the salted plants? The next step is to design a biolandfill that can store carbon by sequestering salted biomass composites in a dry tomb structure. This structure needs to be built to prevent water from entering, as it could lead to decomposition and the release of greenhouse gases. The researcher's design is based on current best municipal landfill practices, but with enhancements to ensure the biolandfill remains dry and the sequestration process is verifiable. Additionally, the salted plants can be buried deep, with the surface of the biolandfill restored for agricultural production.


To make this approach viable, we need to source biomass from high productivity crops, like miscanthus, switchgrass, and loblolly pine. These crops can be grown in diverse climates around the world, including on marginal lands that don't compete with food production. Carbon can be sequestered at a rate of 1.5 to 2 tonnes of CO2 equivalent per tonne of dry biomass.


The costs involved in growing, harvesting, and sequestering carbon in crops are estimated to be around $60 per tonne of CO2, based on bottom-up analysis and observed prices in today's society. This makes the process an affordable and efficient way to combat climate change.


Could we really do this? The researchers have calculated that using this method for storing approximately half of the world's greenhouse gas emissions (~20 Gt of CO2 equivalent per year) would require about 1/5th of the world's row cropland or 1/15th of the total land area of all croplands, pastures, and forests. This is a lot. It would be in many ways preferable to restore this land to its native state, to secure ecosystem benefits as well as carbon ones. But native ecosystems are not "secure" - they can still decompose biologically, or burn in a forest fire. The long-term capacity of salting is likely greater than the saturation capacity of the biosphere.


I'd still rather plant forests, but given the difficulty of building an offset plan that works, this second-best solution seems entirely viable. Using dryness and salting techniques to eliminate biomass decomposition offers highly economic solution to help reduce greenhouse gas emissions. This is extremely competitive against other methods of direct air capture in particular, and policy-makers should take note.

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