[Stoves] ***SPAM*** RE: Methane emissions during pyrolysis / charcoal production

[Crispin Pemberton-Pigott]

Dear Christa

There are two surprises I came across recently: methane and LPG (CxHy if taken together).

Methane (CH4) from making charcoal by any process (kiln or stove) is not from the fuel nor the product.  It is the result of the stoves design or operation.

For TLUD gasifiers there are two major sources: leakage around the upper combustion zone and departure around the pot (called "sneakage") and PICs resulting from combusting gasses striking the cold pot and being chilled enough to fail to complete the combustion.

There is probably CO mixed in the methane (and CxHy) in all cases.

The second surprise was found measuring the emissions from LPG stoves in Rwanda.  We happened to be using an analyser that reported CxHy.  We found not only dangerously high CO, there was twice as much unburned propane.  This was a shock as everyone has been saying that LPG (usually a mix of propane and butane) is a "clean fuel".  Automatically.  No questions asked about the appliance.

Back to charcoal making stoves:


These have largely been based on the work of Paul Anderson, Tom Reed and to a certain extent by Paal Wendelbo.  The commonality with their designs is the direct cause of CO and methane emitted.  It doesn't matter what the fuel is, the proximate cause is the same.



Consider the top of Tom Reed's camp stove.  It has little holes through which a fan blew combustion air. They were many and they were near the top of the cylinder.  A lot of people copied that.



The Peko Pe was a gasifier even if it wasn't necessarily trying to produce char.  Look at the top: it has a number of small holes quite near the top.   The force which pushed the secondary air into the gas stream was driven by rising air trapped between the combustion chamber and an outer sleeve.  The flow was buoyancy driven by heating secondary air.



Several stoves from Pal Anderson have the same upper end.  There is a series of holes around the upper end of a cylinder, preheated air driving it, and a short vertical distance from the holes to the pot bottom.



All these designs are inherently in danger of creating PICs and CO because they flame does note complete combustion before being pot-cooled.  The secondary air cannot mix with the gases in the centre of the diameter.



Both Paul and Paal added what Paul called a "concentrator ring".   We observed the effect of this on a short coal stove at my factory in ~2003.  The effect was to bring the secondary air closer to the centre and this helped reduce emissions.



Now look at a Vesto burning as a TLUD.  It has secondary air holes that are closer to the centre and can reach the centre in part because it has a smaller diameter than most of Paul's and Paal's stoves.  It also has more holes layered which enhances gas column penetration by the air.  The air is pushed in by heated secondary air trapped in a vertical space and pulled by a chimney effect because there is an empty chimney portion of the combustion zone above the secondary air entrance.  There is more chimney draw than there is push most of the time.



Here is a photo of successful secondary combustion of gases above the char producing zone:



[cid:image001.png at 01DA546E.F8F480C0]



You can see there is no spot of the fuel bed which is not covered by flames.  The CO and methane and PM generated by this fire is close to zero.



If the coverage is less than complete there is some amount of sneakage:



[cid:image002.png at 01DA546E.F8F480C0]



This design is suboptimal because the upper section has secondary air holes which should not be there.



All this is before reaching the pot.  If the flames from any stove touch the pot, it will produce products of incomplete combustion because cold gases cannot burn.



Gas stoves without enough free space below the pot emit CO.  A typical American gas stove top has 4 or 5 burners and there is an arched cast iron put support elevating the pots about 40mm above most of the gas flames.  The target for CO is CO/(CO2+CO2) of <0.8%.



In Europe the same stove has a different cast iron pot support - it is lower because the target emission is <2%.   Being closer increases the heat transfer efficiency a little.  This demonstrates that the design affects the emissions, not the fuel or what was being produced at the time (charcoal).



To a great extent emissions from charcoal making in a field are the same - some produce huge amounts of CO and methane, some burn it off, others burn a portion of it.



For these reasons it is difficult to quantify any of it unless particular appliances are tested and the emissions quantified.  Those numbers are tied to the particular circumstances when tested.



Best regards

Crispin








From: Stoves <stoves-bounces at lists.bioenergylists.org> On Behalf Of Christa Roth (bioenergylist)
Sent: Wednesday, January 31, 2024 7:13 AM

Dear colleagues

Anybody know any figures on methane emissions while making biochar or producing charcoal?

I recently got alerted about unexpected amounts of methane emitted from gasifier cookstoves when making char.

Does that also occur when making biochar or normal charcoal? I guess a retort will be a good option to burn up any potential methane?

During ETHOS quite a bit about the real -life figures about charcoal production were discussed but I am not aware of any methane figures.

Any leads?


Christa Roth
stoves at foodandfuel.info<mailto:stoves at foodandfuel.info>

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20240201/f34a3b1f/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: image001.png
Type: image/png
Size: 63393 bytes
Desc: image001.png
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20240201/f34a3b1f/attachment.png>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: image002.png
Type: image/png
Size: 60336 bytes
Desc: image002.png
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20240201/f34a3b1f/attachment-0001.png>