2.8: Biomass burial

One option that various companies are exploring is to do some form of harvesting and burying biomass. This comes up in a few different forms like burying trees on land, sinking trees in oceans, growing and sinking kelp and I’ll even include here making bio-oil to inject it back into oil wells.

There are some questions you might have at the moment like how expensive these methods could be, if they even work out from a life cycle analysis or how much they could scale. The answer is largely that depending on the exact form you’re talking about you can get lots of different answers to those questions.

The basic idea is you grow biomass which has the advantage of using the energy input from the sun to basically suck CO₂ from the atmosphere. This is then transformed into carbon matter through photosynthesis and you then take this biomass and put it somewhere it won’t decompose and release the carbon back out again. Where you store it is kind of up to you but ideally you want a low oxygen environment and somewhere where there won’t be much bacteria which could decompose your organic matter. The two best places for this are basically deep in the ocean or in a big hole.

The deep ocean is pretty great for this as it doesn’t mix with higher layers of water on any worrying timescale, maybe less than every 1000 years, and the pressure down there, as well as the lack of light and nutrients will mean the organic matter is likely to stay put and not attract any microbes which might come and decompose it.

Holes are a bit trickier. You could go deep and narrow in areas that are very clay heavy as this will do a good job preventing oxygen from getting in. You could also use purpose made capsules which are sealed to prevent oxygen and other things like bacteria in.

One thing that might be on your mind is whether it would not just make more sense to burn the biomass

Burying Trees on land

We could start by considering how much sense it would be to bury trees in holes on land. Let’s take as an example wanting to bury enough wood to keep 0.1Mt of CO₂ out of the atmosphere. 0.1Mt of CO₂ is roughly 30,000t of Carbon which is roughly 60,000t of wood.

If we go off of this video it talks about using a land area of around 2,500km² to get this amount of wood.

If we assume that we can use a logging truck with a maximum capacity of 40t then we would need 1,500 truckloads of wood. Assume the average distance from our site is 20km then we are looking at 1500 * 40 * 20 = 1,200,000 t-km.

⊕The good thing here is that even if we are off by a lot we still have a hefty amount of wiggle room to get a good life cycle analysis.We can ballpark emissions from travel at around 1gCO2/km so the travel part of this would emit around 1.2t CO₂. We then obviously have emissions from digging the hole,making capsules and burial but even if we assume these add an order of magnitude we are looking at somewhere around 10t CO₂ to remove the equivalent of 100,000t in this example.

You might then wonder about how large the hole we have to dig is and it turns out the answer to that would be large but not crazy. Wood, let's say, has an average density of 800kg/m³ and so 60,000t of wood would take up around 75,000 m³ of storage space. Put in context this is roughly the size of a football field 13.5m deep or 30 olympic swimming pools.

⊕You also don’t lose the land as you can just re turf the hole you dug and get back to a natural setting.In terms of cost if we go with a freight figure of around $0.4 t-km then we’re looking at around $240,000 which works out at just over $2 per ton of CO₂ on the travel side. Again let’s say the other costs add an order of magnitude to everything, we are still looking at something like $20-30 per ton of CO₂ which would be remarkably competitive.

One company starting to do this is Exaquest. They store wood biomass in anoxic (oxygen free) environments on land, mainly through the use of specialized containers. They plan to obtain the biomass from forest thinning operations and state that in California alone they currently thin at least 250,000 acres of forest per year, yielding more than 20 dry tons of wood waste per acre. → 5m t/yr of waste wood → (from above conversion of wood → CO₂e) 9m t/CO₂e/yr. Although again not all of this wood would be “waste” as some is used for construction and other activities.

Obtaining the wood responsibly and in a way that makes the burial as additional as possible is absolutely key to this process actually delivering results. Otherwise we might find ourselves in the strange position where you could at the same time plant a forest, not cut down a forest or cut down a forest and be paid in all cases.

Burying land grown biomass in the oceans

The canonical paper in this area has to do with taking crop residue from land and sinking them into the deep ocean.

The paper is rather old but states:⊕

Note that 1Pg = 1Gt. This would suggest that at maximum this approach couldn’t even get to a Gt scale.

Global large-scale agriculture produces about 2 Pg crop residues C annually. If the average fraction of above-ground crop residue that can be removed sustainably is assumed to the same as that for corn, 0.3, the global crop residue supply is about 0.6 Pg C.

The good news is though that this really doesn’t take up much space:

If the residue were deposited on the sediment in an annual layer 4 m deep, the total sediment area required would be approximately 260 km2, about 0.02% of the area of the Gulf of Mexico.

The paper suggests using barges to transport corn stover to deep ocean areas and then weigh them down with ballasts and comes up with a cost of around $90 per ton of CO₂ sequestered which comes from this breakdown:

The final density estimates range from 1 microfarm per 16 km2 in a high-dispersion estimate (example, 200,000 microfarms released at once in the open North Atlantic) to 1 microfarms/ km2 in a low-dispersion estimate (example, 200,00 microfarms released at once into gyre conditions). In practice we will seek a mixture of these conditions.

We can take the geometric mean of 1 and 16 to give a rough average for density of 4 (which is also what they say would be the most likely uniform density), let’s also take the entire surface area of the world's oceans. Not all will be suitable and there would be other logistical challenges but still for our upper bound we’ll take it. This would give us a surface area of around 360 million km² of ocean at 4 microfarms per km² then that gives us ≈1.5bn microfarms. The expected CO₂ removed by a micro-farm is 70kg*0.75 ≈ 53kg. Combining these figures we get ≈80bn kg. However, we have to remember that the growing cycle is only around 6 months so we could do this all twice a year doubling our removal rate to 160bn kg or 160m tCO2.

I don’t know the exact reasons for their density estimates and it also seems like if you went for an approach that was more static you could maybe get much much higher densities which could massively increase this value.

One current issue they, and other similar projects are having is that this approach is much more difficult to monitor than others, say where you bury trees in a capsule and then just measure carbon leakage in that capsule. This means that verification/success will likely have to use a more of a computational approach, modeling how many microfarms are likely to sink in the right places and using models of carbon leakage from sunk farms, rather than measuring each farm individually.

One thing I haven't quite got clear is under which conditions some particular approach is “the best”. If you take wood, crop residue, and kelp it really does seem that under certain assumptions maybe of technology (like being able to pyrolise efficiently) there should be some dominant method from a carbon removal standpoint.

It seems like tree burial and bio-oil are basically direct competitors to things like BECCS or Biochar but surely there is only one best thing to do with the organic matter? It might be that one of these is better today but let’s say charm does get to their $50t target, would it then really be better to use the crop residue for any other approach?

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