[Stoves] Charcoal stove design

[Frank Shields]

Dear Crispin and Saastamoinen,

 

With the GEK I work with I have control of the air and a good long bed of
char to work with. But what is the goal of these small household stoves?
Perhaps with natural draft? I would think a surface reaction where the C >
CO > CO2 all at the surface below the pot. No real secondary and all the
heat produced close to the same location. I would think that would require a
narrow blast of air. 

 

Suggestion: Take a section of plastic pipe that holds about 4 Liters. Cap
the top and bottom. The bottom cap has a ½” pipe fitted with a ball valve.
The top cap has a ½” pipe coming out. That top pipe is connected to flex
tubing and that connected to a stainless tube to draw in gas. Open the ball
valve and set 4L pipe in a water bath such that water enters and fills large
pipe. Close valve and remove from water bath. Place the stainless tube at
exact location you want to sample the gas (say under the pot). Open the ball
valve to let the water drain and gas is drawn into the large pipe. The rate
and volume is the same as the water draining. Leave a couple inches of water
in the large pipe and close the ball valve. Shake to mix the gases. Place
large pipe into the water bath, open the valve, so water filling and the
stove gas exits the top where you can measure it using your tester. You
might add a small amount of acid to the water to keep the CO2 from
dissolving. Or add a known amount of NaOH to capture the CO2 (CO?) for
analysis leaving the other gases for the tester.  With this setup a lot of
the variables are controlled like air flow and temperature. You get an
average or set your tester just below the input to get real time. 

 

Regards

 

Frank

 

 

Frank Shields

Control Laboratories; Inc.

42 Hangar Way

Watsonville, CA  95076

(831) 724-5422 tel

(831) 724-3188 fax

frank at biocharlab.com

www.controllabs.com

 

 

 

 

From: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of
Saastamoinen Jaakko
Sent: Wednesday, September 18, 2013 2:37 AM
To: Discussion of biomass cooking stoves
Subject: Re: [Stoves] Charcoal stove design

 

Dear Frank and Crispin,

 

the main DIRECT product in the reaction of carbon with oxygen is CO but also
some CO2 is directly formed. CO that is formed can be oxidized to CO2
(CO+½O2=CO2) at high enough temperatures in the vicinity of carbon particle
or in later stage in the gas flow. So there are different zones along the
gas flow in carbon particle layer when air (or gas) is flowing upwards
through it: 

 

I)                 Zone where exothermic reaction C+½O2=CO takes place (but
some CO2 is also formed). Here the gas temperature is rather low because it
is close to the inlet and the gas has not heated up enough. So CO is not
burning well.  

II)               Zone where the gas temperature becomes high enough so that
also the exothermic reaction CO+½O2 =CO2 (enhanced by H2O) takes place in
the gas. This leads to even higher local temperature along the gas flow so
that exothermic reactions (C+½O2=CO and CO+½O2=CO2) take place at even
higher rate. Here also the endothermic reaction CO2+C=2CO takes place
because the char temperature is high enough. This rrwaction adjust the
temperature level preventing it to increase very high. Then at the location,
where all oxygen is consumed, the gas temperature and CO2 concentration
reach the maximum values.  

III)              After that the carbon reacts with CO2 producing carbon
monoxide in endothermic gasification reaction CO2+C=2CO. The gas is cooled
due to this endothermic reaction. If the layer is very thick, the gas is
cooled to a temperature at which the reaction rate C+CO2=2CO becomes very
low.

 

So some conclusions:

 

1.      For a thin layer, there is only zone I producing CO. 

2.      For thicker layer (zone II), the exit gas contains much CO2 and some
CO. The exit gas is hot and CO may be burned introducing (preheated)
secondary air. The exit gas is hottest if the thickness of the layer is just
in the  intermediate transfer regime from zone II to III. 

3.      For a thick layer, the gas contains much CO and some CO2. Its
temperature is low and it is difficult to burn CO without highly preheated
secondary air.

 

The reaction CO+½O2=CO2 can take place also in the other direction. This
reverse reaction (dissociation) of carbon dioxide is not significant at
temperatures <1800 K.

 

Regards 

 

Jaakko

 

From: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of
Frank Shields
Sent: 18. syyskuuta 2013 0:55
To: 'Discussion of biomass cooking stoves'
Subject: Re: [Stoves] Charcoal stove design

 

Dear Crispin,

 

So the CO2 > CO is endothermic. But still needs carbon. As I see it the only
difference is the temperature changing as the gases move around the pot.
Wondering if the reading could be affected by temperature? Perhaps amount of
gas entering the instrument or something? Interesting. Could hydrogen react
with one of the oxygen in CO2 making water leaving CO?  

 

Frank

 

From: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of
Crispin Pemberton-Pigott
Sent: Tuesday, September 17, 2013 2:01 PM
To: 'Discussion of biomass cooking stoves'
Subject: Re: [Stoves] Charcoal stove design

 

My conclusion is the CO splits endothermically. It does not happen in stoves
with high EA.

Crispin

 

 

 

Dear Crispin,

 

<snip>

 

I found that directly above the middle of a good stove (under the pot) there
is zero CO – I was amazed. But nearer the edges there is more.

 

[Frank >] How is that possible? I thought going from CO2 > CO could only
occur be in a bed of char. This is very strange. 

 

Regards

 

Frank

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