Is anybody concerned about possible emissions from making biochar?

When you bake wood to make biochar it produces some pretty nasty stuff; including methanol, cabon monoxide, methane, and gasified tar. In a perfect setup this all gets cleanly burned, but if you set things up wrong a significant amount may escape and do more damage to the atmosphere than the carbon dioxide you are trying to sequester would have!

Is this a legitimate concern or am I over thinking this?

Methane has a relatively short half-life, despite the bad press it has suffered over the last decade. It has typically degraded within 12 years (source).

I think, in principal, your concern is a legitimate one but I don't think that it as, well, concerning as you might fear.

Simply burning wood will release all of the above, plus the carbon (mostly in the form of carbon dioxide) that would remain in the charcoal. Given that a stable wood-based fuel economy will need to regrow the same biomass of wood that it consumes, over a given period, at least as much carbon is then sequestered from the atmosphere to do so.

If we now examine the biochar situation, we know that some of the carbon remains after the burn and that this is locked up for a period of time in the soil. We also know that this improves the fertility and biodiversity of soil, allowing the soil to sequester further carbon, and providing additional benefits which, themselves, allow people nearby to decrease their emission (such as by growing food locally).

To further this, if the creation of the charcoal has secondary outputs - which it does, in the form of heat - and that these are used, some of the carbon (or other nasties) that is released has been offset. If you cook your dinner using the waste heat from your charcoal kiln, for example, you are saving the wood/gas/electricity that you would have otherwise used for that task. If you usually cook on wood, you can effectively deduct the emissions that quantity of wood would have produced from the negative outputs of your charcoal burn.

My recommendations would be that, if you're intending to produce biochar, you do so using wood obtained from a coppice or other system that promotes biomass (thus resequestering any carbon released) and that you try to utilise the "waste" heat as completely as possible.

What remains to be known is how much the addition of biochar to the soil increases that soils ability to further sequester carbon (without the need to add more biochar). It may be that, after a period of 2 year, adding 25kg of biochar to an area results in it sequestering a further 25kg (beyond the amount that it would have without the amendment). In this fictional scenario, you can feel secure in the knowledge that your actions have a net positive impact on the amount of carbon in the atmosphere. I'm afraid that I don't know of any data to back this up, however.

Further, there is the question of what would happen to a quantity of wood if left on the forest floor. Sure, it provides invaluable habitats for invertebrates (so does biochar, actually), but a large amount of it is digested and ends up as methane and carbon dioxide. Once again, I welcome actual figures for the amounts involved but my belief is that, in producing and digging in biochar, you are building carbon in the soil much faster than by waiting for leaf-litter and wood to compost over the same area (implying that more carbon is "lost" into the atmosphere by the natural processes).

Thank you for your insight. You have given me much to chew on.

Realistically, I think everyone who makes biochar is concerned about this. I have made many batches of biochar, using multiple different methods. I have settled on a process that I have documented here. In my circumstances, with my most common feedstock, a trench method works best. I can get a nice consistent product, making a large amount of char in a labour efficient manner, and with minimal smoke produced. The key is dry fuel, small diameter, and ensuring that there is an strong flame front above the embers. Other methods I tried were much less clean burning.

As Luke Mitchell explained so well, there is a normal cycle of carbon.

Trees grow, they die, their carbon turns into gases, which can increase the greenhouse climate change effect, but can also help the next tree grow. When we normally burn the wood,  or lightning, etc., naturally burns the wood, it's the same cycle: grow, die, (burn) gases, return into trees.

When we make biochar, we are changing that cycle. We do burn the wood and create harmful gases.  However, the carbon doesn't continue through the cycle. In its biochar form, it is remarkably stable.  The original Terra Preta biochar in Brazil has stayed there for over 500 years at least.  When we stabilize this type of carbon in the soil, we are sequestering it for centuries and perhaps more.  Since it doesn't follow its normal growing or burning cycle, we are putting more carbon into the soil, and not in the air, for centuries. This decreases the carbon in the air, and therefore the greenhouse gas effect.  We are no longer warming the planet or causing long term climate change.  Now we are decreasing the amount of carbon in the air and the connected climate change effect.   Biochar also helps plants and fungi grow, and increases the quality of the food that we can grow for human beings.  This is a positive direction, in my opinion.

John S
PDX OR

I think this was a great question and the responses have been excellent.
I would love to have an analysis of both the particle and gaseous emissions that a tlud, pit, trough  or retort produces.

Someone with a Testo meter could tell  us a lot.

Biochar production should produce very little to no smoke.

In flame cap kilns and TLUDs the smoke is burned directly and in retort systems the smoke can either be condensed so that only the non condensable gasses are flared off ot all the smoke is fed back into the fire box that heats the retort.

No smoke doesn't mean there's no pollution though. An entirely smokeless TLUD for example can still produce soot.

William - ask and ye shall receive:

https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0154617

I'm seriously considering either getting an engineering professor who does lots of biochar research at Massey to sponsor a round of testing, or fundraising to get a combustion gas analyzer, either a multisensor Testo 350 or the Microtector II G460. The first avenue might be tricky, as the professor believes that the only proper way to produce biochar is in a retort with a stack reburner, and he talks down flame cap technology every time he gives a presentation.

Long story short: The carbon embodied in biomass in the environment will all end up in the atmosphere, whether it take days or decades. When we make biochar, we offgas half the carbon and turn the rest into a stable form that will be safe for hundreds to thousands of years (as long as no one burns it, so we put it into the ground to be certain).

The responsibility we have in making biochar is to limit the amount of carbon that turns into problematic stuff, especially CH4 and particulates. Flame cap burns are really good at this as long as the fuels are dry and the fires are well tended...that 750 degree combustion zone and the rolling vortex of turbulence are the key.

Wow Phil, thank you!

I'm not science savvy enough to know how the amount of gasses emitted balance with their effect on global warming,  but somebody here probably is.
I also don't know how to compare this to rocket stoves, but that would be useful.

I wonder, does a Tlud count as a flame cap kiln?
My current project is a Tlud that has an insulated chimney, which I'm hoping will boost the heat, efficiency and cleanliness of the burn.

William Bronson wrote:
I wonder, does a Tlud count as a flame cap kiln?
My current project is a Tlud that has an insulated chimney, which I'm hoping will boost the heat, efficiency and cleanliness of the burn.

Not in my opinion, a TLUD burns from the top down and all smoke passes through the glowing embers and biochar above it. A flame cap kiln is also lit at the top but after the initial bed of embers is established it is fed at the top so smoke doesn't pass through already made biochar or glowing embers.

Also in a TLUD there's primary air going through the column of biomass, in a flame cap kiln all air has to enter from the top and the fire is closer to the point of smoke production. In a TLUD the fire is burning at the point where secondary air and smoke meet, which can be tens of centimeters apart from the place where smoke is generated, especially towards the end of the burn.

Sounds like a TLUD would be better for decreasing smoke.  Also, with the chimney, how I use it, it might be even better.  Hotter flame, more chance to reburn as it rises.

John S
PDX OR

Great explanation of the differences between a tlud and a flame cap.

John, do you have any opening for tertiary air in your chimney?
It might not even be needed, just an idea.

Rocket stoves have evolved to have sophisticated methods of introducing secondary air and increasing the time factor through turbulence.

I hope to help this happen with charcoal producing stoves.
The particulates are a personal threat to me, being an asthmatic, oddly though, woodsmoke never seems to induce an attack.
There seem to be some commercially available  electrostatic filters for wood stoves but the cost makes them daunting.
DIY  electrostatic filters are a thing, but a little to techy for me.

I did find a lead about diy wet scrubbers here:
https://www.watertank.se/projects/wet-scrubbers-a-key-tool-against-air-pollution-caused-by-wood-burning-household-stoves/

Reading it lead me to learn more about wet scrubbers, and their ability to capture gasses , which surprised me.
The information on the site is a little light on details, but they did build a scrubber which reduced the CO2 in the emissions by a lot and the  hydrogen sulfide completely.

William,
I would love to answer your question, but I'm not sure how tertiary air works. I am only guessing at secondary air (derriere- ha! shows how sophisticated I am :)

You all teach me a lot, but I'm not really a leader in metallurgy/engineering/technology.

John S
PDX OR