[Stoves] why does charcoal produce more CO?

Darpan Das darpandasiitb at gmail.com
Thu Jan 25 19:31:35 CST 2018


Thanks Andrew.

I was also wondering if the decrease in bound oxygen content would have any
role to play in the increase in co emissions. My ultimate analysis results
for wood and charcoal show that charcoal has negligible O/C content. This
probably is because of the pyrolysis process for wood has already removed
the volatiles. Now the charcoal is more energy densified containing most of
it part as carbon.

Can this less O present in the solid fuel matrix be attributed as a reason
for formation of high CO?

Regards
Darpan



On Fri, Jan 26, 2018 at 3:18 AM, Andrew Heggie <aj.heggie at gmail.com> wrote:

>
>
> 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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-- 
Darpan Das
PhD Student
Centre for Environmental Science and Engineering
IIT Bombay
Powai, Mumbai 400076

91 916 73 491 63
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