[Stoves] Clean burning with a bit of moisture

[Crispin Pemberton-Pigott]

Dear Neil

Hey! Long time. My theory which is only informed by way too much time spent trying to burn everything under the sun is this:

Fuel moisture moderates the rate at which gases evolve from biomass, with other factors being the oil content (gummy wood), density, porosity, ash content and the volatile fraction of the total mass (which is to say, the ash-free carbon content in reverse.

If you want to make smoke, you heat a mass of fuel all at once and starve it of sufficient air. This is easy to demonstrate - toss a big pile of leaves onto a fire making sure to cover all visible flame. It will smoke like crazy.

Set the same pile of leaves in a long "chain" on the ground and light it one end. It will burn rather cleanly to the other end.  What is the difference?  In case one you have a lot of fuel being heated underneath by char and hot fuel with hardly any air to complete the combustion of gases generated.

Take a cylindrical stove and put in wood with a fire, and slow keep adding fuel until at some point, the flames are heating enough wood to produce more gases than there is air to burn it properly. Now it is smoking. Whatever amount that is, instead of putting in less fuel, use wetter wood, say 3% more moisture. This will moderate the burn and it will be clean(er).

If you have a really decent fire  burning away in a fireplace or stove, and add totally dry wood, it will nearly explode into gases as fast as it can, with big self-accelerating flames pushing out combustible gases.

One can build a stove to handle very dry material, but most are not tuned to that. So they work best with 12% Denver dry wood.

I have observed that to burn wood grown in a desert, like Chad or Niger, the designed has to treat it as dry: limited fuel heating by having no primary air preheating, preheated secondary air. That fuel is gummy and dry. Terrible stuff to burn clean.

If you know it is green and damp like an EPA wood stove test, you preheat the fuel and air well, then try to limit the power by total air restriction/control. Retain the heat, roast the fuel slowly and have adequate secondary air. But not too much. Most stoves have way too much air going through them, a rocket stove on medium power, for example. They have 6 to 10 times too much air going in.

The hot water vapour has an effect on the particle agglomeration somehow. Smaller particles are easier to burn out - acting more like gases.

Somehow it all adds up.
Crispin


From: neiltm at uwclub.net
Sent: October 12, 2022 2:06 PM
To: stoves at lists.bioenergylists.org
Reply to: stoves at lists.bioenergylists.org
Subject: [Stoves] ***SPAM*** Re: ***SPAM*** Re: ***SPAM*** Re: ***SPAM*** Re: Stoves Digest, Vol 146, Issue 3

On 12 Oct 2022 at 0:46, Crispin Pemberton-Pigott wrote:

> There is a general rule which that biomass burns best (most cleanly and
> completely) when it is NOT completely dry, unless you have a special
> combustor.

Can you explain why that should be so Crispin?

I feel this is right from my own experience, but also remember those
running internal combustion engines from woodgas said the same thing.
 Their theory was that they were 'cracking' the water into hydrogen
and oxygen, and got better performance than with totally dry wood.

Best wishes,   Neil Taylor, (still regularly cooking with wood, but
mostly these days with my original StoveTec rocket stove when not
camping, and utilising a large BBQ hand crank fan for forced air,
turned slowly in the early stages to avoid smoke.)

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