[Stoves] why does charcoal produce more CO?

[Andrew Heggie]

On 24 January 2018 at 21:00, Darpan Das <darpandasiitb at gmail.com> wrote:

> Dear All
>
> Recently I performed some experiments on some natural draft stoves to
> compare the emissions from charcoal and wood, I found that although the
> particulate matter emissions have decreased the CO emissions have increased.
>

Hi Darpan

I was hoping one of our more erudite contributor's would respond to this
but now I'll have a crack at the question.

The particulate emissions reduce because they are particles of incomplete
combustion of pyrolysis offgas in the secondary flame, once there is only
char left there are no longer pyrolysis products and hence no precursors
for particulates

One normally expects CO to rise at the end of a burn of wood and  when
fresh wood is added to a fire.

 It is necessary to have a flame to burn Carbon Monoxide and the the
requirements for a clean burn are Temperature,  Turbulence and residence
Time.

A charcoal fire is much the same as a wood fire that has been reduced to
hot char and is no longer evolving pyrolysis products

All the time wood is evolving pyrolysis offgas and it has sufficient
secondary air and these three Ts any CO produced low in the fire from
primary combustion has the conditions to combine with the secondary air and
complete its oxidation to carbon dioxide.

Once there is only a bed of coals and the primary air superficial velocity
remains the same as before or the depth of the char bed is low then an
oxygen molecule from the primary air can dissociate on the  hot char and
produce carbon dioxide and a lot of heat is released as this reaction is
exothermic. If there is insufficient  extra oxygen and the fire-bed is hot
and glowing then the hot char can reduce this carbon dioxide to carbon
monoxide, this takes heat out of the char as this reduction is endothermic.

The area above the fire-bed is thus cooler and there is a mixture of
nitrogen from the primary air and the carbon dioxide that hasn't been
reduced plus CO that has been formed. Even if there is sufficient air with
oxygen above the fire-bed the CO is now heavily diluted  by the nitrogen
and carbon monoxide so it has less chance of mixing with an oxygen
molecule. Also even if there is a flame somewhere nearby the carbon
monoxide won't react with the oxygen unless the CO to air ratio is within
the bounds of flammability or the temperature is above  the  autoignition
point of CO.

If you took the glowing coals and put them in a tube so their depth was
much higher and then passed the same primary air through the heap the
superficial velocity of the air is higher and as heat losses radiating from
the char is lessened the stack can become a carbon monoxide generator with
most of the carbon now combining  with oxygen to produce a stream of
nitrogen and carbon monoxide which can be cleanly burned in a lovely blue
secondary flame.

Andrew
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