Monday, March 8, 2010

The Scalability of Biochar

Visualization of approximate amount of wood that would have to be charred and buried annually to offset carbon emissions of one United States resident.  (Picture credit)

A popular idea at the moment to address climate change is biochar - essentially taking organic materials, charring them, and burying them in the soil.  As the Wikipedia explains:
Biochar is charcoal created by pyrolysis of biomass, and differs from charcoal only in the sense that its primary use is not for fuel, but for biosequestration or atmospheric carbon capture and storage.[1] Charcoal is a stable solid rich in carbon content, and thus, can be used to lock carbon in the soil. Biochar is of increasing interest because of concerns about climate change caused by emissions of carbon dioxide (CO2) and other greenhouse gases (GHG). Carbon dioxide capture also ties up large amounts of oxygen and requires energy for injection (as via carbon capture and storage), whereas the biochar process breaks into the carbon dioxide cycle, thus releasing oxygen as did coal formation hundreds of millions of years ago. Biochar is a way for carbon to be drawn from the atmosphere and is a solution to reducing the global impact of farming (and in reducing the impact from all agricultural waste). Since biochar can sequester carbon in the soil for hundreds to thousands of years[2], it has received considerable interest as a potential tool to slow global warming. The burning and natural decomposition of trees and agricultural matter contributes a large amount of CO2 released to the atmosphere. Biochar can store this carbon in the ground, potentially making a significant reduction in atmospheric GHG levels; at the same time its presence in the earth can improve water quality, increase soil fertility, raise agricultural productivity and reduce pressure on old growth forests.
Giving credit were due, this practice was invented by pre-Columbian Amazon tribes, and Wade Davis, in his book The Wayfinders reports that an area in the Amazon the size of France may once have been managed in this way.  James Lovelock is on record with the idea that this is the last best hope of humanity.

Now, the biofuel story has given me a bit of a horror of ideas that sound cool to environmentalists, are fine on a small scale, but are a disaster when scaled up by industrial society.  So I wanted to do a few quick back-of-the-envelope calculations of the limits of this approach.

US carbon emissions currently run about 1.8 billion tonnes of carbon each year, and the US population is about 300m, so emissions per person are 6 tonnes/year of carbon.  To get a feeling for this, let's express it in wood terms, approximately.  Let's figure wood is basically carbohydrate with a formula of CH2O, so that 6 tonnes of carbon corresponds to 6 * (12+2+16)/12 = 15 dry tonnes of wood.  The density of the softwoods that are mainly used commercially in the US are around 1/2 tonne/m3, so we would need to char and bury about 30 m3 of wood to offset the emissions of each person in the United States - roughly a medium sized tree.  Each year.

30 m3 is about 480 board feet of lumber (gross - much of a tree would only be good for firewood).  There's about 3000 board feet of lumber in a 1000 sq foot US house, for comparison.  So a typical American family's share of carbon emissions is equivalent to all the lumber in their house every three years or so (figuring on a family of four in a 2000 sq foot house).

Looking more globally, here is an estimate of the amount of the carbon flowing into the global economy through the major channels of wood and agricultural products (from this TOD story):

Note the y-axis scale.  The data are a few years out of date, but you can pretty much see where the trend was going.  Next, here's fossil fuel carbon emissions:

As you can see, the entire carbon inflow from the biosphere into the economy currently is only about one third of the fossil fuel throughput.  So in order to also offset carbon emissions entirely with biochar, we'd need to char and bury an amount of carbon three times larger than current off-farm usage of biological products in the economy.

Given that the existing level of take that humanity makes on the biosphere is pretty impactful, what you don't want to do is create some kind of general payment incentive for commercial operations to char and bury carbon.  That would be disastrous and lead to bulldozers wiping out tropical forests on a huge scale in order to pile them up, char them and bury them.

What would be potentially more reasonable is an incentive, on existing farmland only, to do biochar of agricultural residues.  That might be environmentally beneficial on the whole (improve the soil in-situ, without incentivizing spillovers onto marginal soils or tropical forest ecosystems) though the interaction with current no-till agricultural practices should be thought about carefully.  You might expect to get, very roughly, about the same amount of carbon in the residues as there is in the food supply - a couple of gigatonnes globally.  Given about 1.5 billion hectares of arable land globally, or 4 billion acres, 2 gigatonnes of carbon is about 1/2 ton of carbon/acre, which is 1.25 tonnes/acre in carbohydrate terms, which is consistent, for example, with current estimates of harvestable corn stover of 1-1.5 dry tons/acre.  Globally, it could be somewhat more or somewhat less, but that's the ballpark.

So a few gigatonnes of biochar carbon sequestration would be very useful, but it's not a panacea for even the current level of 8.5 gigatonnes of fossil fuel emissions, let alone the growth trajectory.


Gary said...

I completely agree with your argument and would also add that even as an agricultural soil amendment, the jury is still out as to the benefits. In some soils biochar has significantly improved yields - but this is usually on poor soils to begin with. Nevertheless, I am a big fan of biochar used wisely. For example - in my neighborhood there is a lot of logging going on. The mills produce chips, bark, and general slash - some of which makes its way into products like chip board - but much presently gets composted. Now a big mill here is about to go on line with a 20MW electric cogen plant - burning all of their waste for electricity. They can do this economically mostly because of how we define "renewable" energy in state legislatures. Market price for base power from big hyro is <$50/MWh - whereas "renewable" energy for mandated "portfolio" standards is going for >$100/MWh. The win-win would be to pyrolyze the waste, burn the light fraction for electricity, and sell the char to the guys who make compost as a "carbon negative" soil amendment. The marketing even works! But burning that "renewable" carbon is more profitable.
I have a more detailed discussion of the issue with this particular installation here:

kjmclark said...

What did you do with the energy from the pyrolysis gas? Seems like that would reduce your family's emissions, since it's from a renewable source.

Also, what about the fertilizer value of the charcoal? Gary's right about the range of fertilizer benefit, but if it improves the biomass yield of poor soils, that further improves the benefit.

You're right, of course, that we shouldn't try to handle all our emissions that way, but it would seem to make more sense in places without other renewable resources and where biomass grows well.

Stuart Staniford said...


You're right that this is a very rough calculation. I'm also treating it as 100% capture of the carbon, where the current reality is about 50% is captured.

My point is certainly not "this never makes sense", but rather "this is no silver bullet, and could very easily backfire unless the incentive structure is very carefully designed".

Mike Aucott said...

The nice thing about biochar is that it has the capability of actually pulling carbon out of the atmosphere and at the same time producing energy instead of consuming energy as other carbon sequestration schemes would.

Even if we humans can significantly cut our carbon emissions, we will almost certainly still be faced with the long-term legacy of excessive carbon in the atmosphere causing massive ice melt and sea level rise. Probably the only way to permanently remediate this would be to systematically lower the atmosphic CO2 level.

Given that the annual capture of carbon by terrestrial plants is on the order of 60 gigatons, it seems like there might be a way to siphon off a meaningful portion of this into stable reservoirs of char. If done in conjunction with dramatic reduction of carbon emissions, carbon sequestration via biochar could permit a gradual reduction of the atmosphere's burden of carbon without exacerbating energy shortages.

Jason Aramburu said...


Interesting article. I'm the founder of re:char. We do small scale fast pyrolysis to produce biochar and liquid fuel. First of all, point of clarity-- I believe US carbon emissions are more like 15-16 gigatonnes annually, with a per capita emission of around 19 tonnes.

That said, I will agree with you that the task of offsetting all of our current emissions with any scheme or technology is not feasible. However, I think biochar represents an important piece of the puzzle. There are folks who worry that a biochar offset mechanism will lead to clear-cutting and large scale production. I don't think these fears are justified, as virtually everyone in the biochar industry understands a concept called 'maximum collection radius.' If the distance travelled to collect biomass is high, as would be the case with a large biochar plant, any carbon benefit is lost in the logistics.


Jason Aramburu

Stuart Staniford said...


According to the BP Statistical Report on World Energy, 2008 US carbon dioxide emissions were 6371 million tonnes, which is 1.737 GT of carbon.

jewishfarmer said...

Stuart, thanks for doing this. I'd love to see you do a comparable analysis, if you ever have a chance, of the impact of biomass co-generation. This is being discussed a bit too freely by environmentalists - Joe Romm has gone as far as to claim it will be essential, and I find it equally, perhaps more worrisome.


Anonymous said...

Hi Stuart:

Great, thoughtful stuff here.

As somebody who has heated with firewood, your tree looks a bit small. 1 board foot of lumber is 1 twelfth of a cubic foot (it's a 1 foot long chunk of 1x12, or the volume equivalent), so 480 board feet is only 40 cubic feet, or a little over 1 cubic meter.

It would be an efficient house indeed that could be heated for a year with one modest pine tree (let alone all the other per-capita carbon use). 30 cubic meters is more like 7 or 8 cords of firewood, and it is a very big tree that contains even 1 cord of wood.

The sustainable productivity rate of forest land (at least here in the northeast) is around half a cord per acre per year. So the average person is emitting carbon equivalent to the growth of about 16 acres of land - the land area of the US is only ~8 acres per person, and a lot of that isn't capable of growing much of anything.