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Does it make sense to just bury biomass?

Fossil fuels were created from organic matter that was buried, and transformed over millions of years exposure to heat and pressure. One solution to mitigating the carbon dioxide we have emitted by burning fossil fuels is to try to transform plant matter ourselves, and bury it to more or less replace what we took.

One of the most popular ways to affect this transformation is to make biochar, fairly inert carbon source produced by heating plant matter to between 500-800°C. This biochar can be use in any environment, including application to soil, and it will oxidize to CO2 extremely slowly .Thus plant matter, which would otherwise decompose in years or decades, can be made stable.

Another solution is 'torrefaction', a process similar to biochar manufacture, but operated at a lower temperature. Torrefaction removes the water from biomass, and converts fibers to a grind-able char, but does not render the biomass completely inert. It requires less energy to produce than biochar, but must be buried. Biomass such as switchgrass only costs about $40/dry ton. So is torrefaction and burial an economically competitive alternative to DAC?

A recent academic paper looked at the opportunity, considering small, mobile torrefaction reactors with a throughput of 60 T/day. The proposed capex required to perform these moderate-temperature processes are fairly inexpensive, adding just $5/ton in capex costs. The reactors are not commercially available, so it's hard to evaluate the realism here. But the authors estimate the overall cost of storage using this process at just $135/ton.

This is pretty good. In fact, other papers estimate a cost only half as high. Burial doesn't get as much attention as direct air capture, but maybe it should.

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