Burying wood chips to solve the climate crisis

Carbon dioxide removal is apparently the holy grail of climate mitigation. How hard can it be?

Savior of the climate, maybe. Previous spruce forest in Finland. From Wikimedia Commons.

Sometimes, admittedly very seldomly, I venture out in the big world of Substack. Out there you never really know where you will end up. Last week, when I was on one of these unusual excursions I ended up on a carbon removal blog, that is a blog that is wholly focused on exploring and explaining the subject of removing carbon dioxide from the atmosphere and dumping it somewhere that is not the atmosphere. 

I have heard about carbon removal before, but it is not really one of my main interests. I can not really say that reading an entire blog about it made me more interested in the subject. But it did make me a bit curious. Could this really be complicated enough to merit an entire blog?

What is carbon removal?

Carbon dioxide removal is, not that surprising, the art of removing carbon dioxide from the atmosphere. This line of business has not been that successful yet. Not because it is very difficult to remove carbon dioxide from the atmosphere but rather because it is expensive to do it. At least more expensive than it is to not place the carbon dioxide in the atmosphere in the first place. 

This equation is starting to change as the carbon dioxide content of the atmosphere builds up and as efforts to limit carbon dioxide emissions seem more and more futile. Many climate opiners seem determined that atmospheric carbon dioxide levels need to go down, rather than up or remaining the same. This, naturally, necessitates carbon dioxide removal on a large scale.

From the negligible texts on the subject I have read, carbon dioxide removal currently consists mostly of nature doing its thing. Humans only do what they usually do, albeit slightly differently. The main method of certified carbon removal today, that is carbon removal where someone gets paid to remove carbon from the atmosphere, is reforestation. The logic being that standing trees contain a lot of carbon and if you place trees where there previously were none, you will have locked up a lot of carbon that will no longer float around in the atmosphere.

This works, of course, but it is not very impressive. The carbon storage is only active as long as the trees stand. And someday, someone will remove those trees which will most probably release the carbon back to the atmosphere. Also, reforestation does not scale very well. A hectare of fully-grown (Swedish) forest contains something like 200 tons of biomass, equating to maybe 350 tons of carbon dioxide.

That is trifling compared to current emissions. To offset the current global carbon dioxide emissions of 36.8 billion tons would require about a million km² of reforested land. That is an area the size of Texas and California combined. Each year. And only for offsetting current emissions.

For reasons like this, more high-tech solutions are usually expected to do the heavy lifting of carbon dioxide removal. Carbon capture and storage (CCS) is already a working technology. Carbon dioxide is extracted from the atmosphere and injected deep underground. This works, but it is prohibitively expensive. It is costly both to extract the carbon dioxide from the atmosphere and to drill the hole and place it in the ground.

The cost of storing away carbon dioxide this way can be as low as 20 dollars per ton. But that is only under very specific circumstances where underground storage is easily accessible and when the carbon dioxide is already captured, which usually means adjacent to a fossil fuel burning power plant, which sort of makes a mockery of the whole exercise. The cost of capturing carbon dioxide directly from the atmosphere and storing it away is "in the high triple digits" per ton.

A bright idea, maybe

I do not know very much about carbon dioxide removal. But I do know quite a lot about forestry. That is handy knowledge, because forests are, to a large extent, made of carbon. My first thought when reading about carbon dioxide removal was that it should be possible to do using biomass from forests. After all, the oil and coal in the ground was once upon a time forests, or at least plants, above the ground.

Google quickly informed me that this is already being done or, probably more correctly, already being talked about. MIT Technology Review has a good piece summarizing the store-away-biomass-market. The company featured in their article had a bright idea moving biomass, mostly brushwood, from wet, temperate areas in the northeastern USA to a very dry area in Nevada. The theory being that an extremely dry environment will preserve the carbon in the biomass instead of releasing it back to the atmosphere. 

The MIT article also mentions a few other methods of wood preservation currently being explored, for example burying biomass in clay-lined pits or impregnating wood with salt water. 

One thing they do not mention is simply burying the biomass underground, in temperate areas with high ground water tables. Down in the ground where there is always water the environment is more or less oxygen-free, meaning that organic material hardly decomposes at all.

This was the first thing I thought about when reading about carbon removal. Using cheap wood from Swedish forests and bury it below the water table to ensure it does not decompose. Strangely, I can not find any mention of this particular bright idea out there, although, considering the vastness of cyberspace, it probably is there somewhere. What definitely seems to be lacking is Swedish carbon removal projects. Which is odd considering our abundance of forests with their carbon removal potential. 

What instantly drew my attention is the fact that the Swedish style of forestry seems to integrate very well in a carbon removal system. In Sweden we almost always clear-cut our forests, taking away the logs and leaving a lot of roots, branches and tree tops on the clearcutting. These logging leftovers are then raked together, processed into wood chips and burnt in the nearest heating plant. It would be very simple to not truck away the wood chips but instead dig a deep hole and just dump the chips into it. As long as the hole is deep enough to reach below the water table, the wood chips and the carbon they contain should be gone for the foreseeable future.

Doing the maths

That sounded a bit too simple. Would it work? I had to do some calculations.

Wood chips being a leftover product from logging in Sweden means that they are very cheap. The price is not much more than 10 dollars per cubic meter. A cubic meter of wood chips weighs about 300 kg which translates into a little more than 500 kg of carbon dioxide. Put differently, that is 20 dollars for 1 ton of carbon dioxide. Which is a great price. But what about storing it?

Sweden is a rather wet country and the water table is seldom more than a few meters below ground. It varies over the year but finding spots where the water table is never lower than 3-4 meters below ground should be possible in most locations. A 24 ton excavator is able to dig to a depth of at least 5-6 meters which gives decent storage volume below the water table.

The cost of a 24 ton excavator is something along the lines of 150 dollars per hour. With a 1 cubic meter bucket and 30 seconds cycle time the excavator moves 120 cubic meters of soil per hour. For each cubic meter of storage volume below the water table it is probably necessary to remove 5-10 cubic meters above the water table. It is also necessary to refill the hole once the wood chips have been placed at the bottom. In total, storing away 1 cubic meter of wood chips should require moving 10-20 cubic meters of soil, which would cost 12-25 dollars.

Now we are getting close to a firm number. Since a cubic meter of wood chips equaled 0.5 tons of carbon dioxide, storing away 1 ton of carbon dioxide would cost 25-50 dollars. Combine that with the cost of the wood chips and we get a total cost for getting rid of 1 ton of carbon dioxide of 50-70 dollars. This is far cheaper than any other form of atmospheric carbon capture and storage.

Countering the counter-arguments

If something is too good to be true, it usually is. Why should this particular type of carbon removal not work? Well, I can think of several different ways.

Most obviously, there could be something seriously wrong in my cost calculations. The price of wood chips is very low in Sweden due to our particular type of forestry. When I google around a bit I find prices in international markets to be double or triple the Swedish price. That is perfectly logical since it matches the Swedish price for non-timber logs, that is logs that are unsuitable for timber and will become paper pulp or firewood instead. But even when doubling the price of the raw material the cost of this way of carbon removal will still be very competitive, at 70-90 dollars per ton of carbon dioxide.

The cost of digging is another potential error. Swedish woodlands are generally very stony and it might not be able to come even close to the excavating capacity I am imagining. On the other hand it should be possible to choose the spot for excavation quite freely in the clear-cut area which should simplify the process. The water table depth of 3-4 meters is also conservatively estimated. On most sites it should be possible to find locations with less water table depth than that.

There is another potential silver lining to the digging part. In Sweden, when planting saplings on clear-cut areas it is customary to prepare the soil first, to give the saplings a better chance of survival. This soil preparation is often done by a heavy excavator. In other words, the necessary equipment for burying biomass underground will already be present, meaning small but not insignificant savings.

But does it work?

Minor accounting tricks aside, the main objection will probably be that this cannot possibly work in practice. If it did, someone else would obviously already be doing it. It would have been great if I had irrefutable proof that this method of carbon removal does work. Unfortunately, I have not. But I do have some circumstantial evidence.

To start with, burying biomass underground is the basis for fossil fuels. Any geologist can tell you that. The exact process is regrettably not well-known, but the general principle is not in doubt. Biomass is prevented from reacting with oxygen and with time; heat and pressure will transform the fluffy biomass into energy-dense coal, oil or gas.

This is all very similar to how peat works. Peat is an accumulation of organic matter in wetlands where the biomass can not decompose due to lack of oxygen. Peat is generally regarded as the first stage on the road to fossil fuels. Burying wood chips underground is not that dissimilar from peat formation.

Only much faster. Peat deposits grow when organic matter is added to them. This organic matter comes from the mosses and lichens that grow in the wetlands. These plants are not nearly as efficient in capturing carbon as trees. And some proportion of them decay and release their carbon again before being buried in the wetland. Wood chip burying would copy this natural process, only speeding it up by an order of magnitude or more.

How sure can we be that groundwater will really preserve the wood placed in it, or at least its carbon? Testing this hypothesis would take thousands of years. Luckily there is a scientific discipline looking at things on a thousand-year horizon, namely archaeology. And archaeologists have long known that wooden artifacts survive very well in anaerobic environments, for example in a bog or a swamp.

Or in ground water. Archaeologists have found some truly remarkable wood artifacts in wet environments. Oldest of them all is the Clacton Spear, which was found in wet soil close to the Thames estuary in southeastern England. When it was unearthed it had been lying in the ground for over 400.000 years without decomposing and releasing its carbon.

400.000 years old but as good as new (carbon dioxide wise). From Wikimedia Commons.

Certify this

The thing that I like most with wood chip burying is that it is based on an existing business. The forestry business is huge in Sweden and in many other countries as well. Burying the wood chips in situ instead of trucking them to a heating plant is but a minor tweak to an existing business model.

This hijacking of an existing business model means that it will be easy to scale up. According to these statistics Sweden produced 4 million tons of wood chips from clearcuttings in 2021. That corresponds to 7 million tons of carbon dioxide. This is trifling compared to the 36.8 billion tons of annual global carbon dioxide emissions. But it is still more than all other methods of carbon dioxide capture and removal combined (which stand at 2 million tons).

Unfortunately, removing carbon dioxide from the atmosphere might be the easy part. One thing I did learn from reading a carbon removal blog was that the real challenge with carbon removal is not so much removing the carbon as getting certified for removing the carbon. And since the latter is a prerequisite for getting paid it is by far the most important.

This means I will never be involved in this. I hate certifications intensely and I will not have anything to do with something that is at its heart a certification business. Burying wood chips in the ground to mitigate the climate crisis might still be a good idea. But if it ever happens it will not be thanks to my assiduous toil.