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<p class="MsoNormal"><span lang="EN-US">Dear stovers,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">the question of CO release from char bed is interesting. We developed models for the combustion of a char bed [1] and CO [2].
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">[1] Saastamoinen, J., Huttunen; M., Kilpinen, P., Kjäldman, L., Oravainen; H, and Boström, S., Emission formation during wood log combustion in fireplaces - Part II: Char combustion stage. Progress in Computational Fluid
Dynamics 6 (4/5), 209-216, 2006.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">[2] Saastamoinen, J.J, Kilpinen, P.T, and Norström, T.N., New simplified rate equation for gas-phase oxidation of CO at combustion. Energy & Fuels, Vol. 14, No 6, pp. 1156-1160, 2000.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Input values in the model are the particle size, bed thickness, inlet air rate and air inlet temperature (and inlet air composition, H2O etc.). Equations describing the particle and gas temperatures and the gas composition
(species O2, CO, CO2, H2O, H2, NO) as function of location (distance above the air inlet) are solved. Oxygen reacts with the char particles producing CO and CO2 in exothermic reaction. There is some literature on the product ratio CO2/CO on the char surface.
Endothermic gasification reactions, where H2O and CO2 react with char take also place. CO may be burning in the gas depending on the temperature. All the reaction rates (kinetics) depend on the temperature.
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Initially one has to assume some temperature for the char bed. Then iterative calculations are needed using the equations of the model, which lead to actual temperature and species distributions as function of the distance
from the air inlet. Interestingly two solutions for the same problem were found depending on the initial guess. If one assumes relative low temperature (for example 700 oC), the iterative calculations lead to one stable solution for the temperature and species
distributions. If one assumed higher initial temperature, iterative calculations lead to another solution at higher temperature level. After some consideration, it was reasoned that both solutions could be existing. Assuming low temperature, CO produced at
char surface did not burn in the gas and the temperature level remained low but assuming higher temperature it did burn producing heat keeping the bed at higher temperature level and sustaining burning of CO. Actually, there are three stable solutions. Assuming
initial temperature 20 oC, there is no burning.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">So there are some reasons for high CO emissions:<o:p></o:p></span></p>
<ul style="margin-top:0cm" type="disc">
<li class="MsoListParagraphCxSpFirst" style="margin-left:0cm;mso-add-space:auto;mso-list:l0 level1 lfo1">
<span lang="EN-US">The fuel bed is disturbed so that temperature deceases (for example too high air rate cools it). Then it may be difficult to get CO again burning. Preheating of combustion air could promote burning of CO. Burning of CO is shown by the blue
flame.<o:p></o:p></span></li><li class="MsoListParagraphCxSpMiddle" style="margin-left:0cm;mso-add-space:auto;mso-list:l0 level1 lfo1">
<span lang="EN-US">In the end stage usually, the temperature becomes too low to sustain burning of CO. I usually burn thin sticks in the end stage in my fireplace on the char bed, so that much of CO from the larger char particles is burnt with the volatile
flame from the sticks. <o:p></o:p></span></li><li class="MsoListParagraphCxSpMiddle" style="margin-left:0cm;mso-add-space:auto;mso-list:l0 level1 lfo1">
<span lang="EN-US">In a thick char bed, CO may burn inside the bed producing CO2 and rising the temperature. However, CO2 may again react with char upper in the bed producing CO.<o:p></o:p></span></li><li class="MsoListParagraphCxSpLast" style="margin-left:0cm;mso-add-space:auto;mso-list:l0 level1 lfo1">
<span lang="EN-US">Burning rate of CO depends on the H2O level even in the simple reaction CO+O2/2=CO2 there is no H2O. In reality the reaction scheme is more complicated, where radicals formed from H2O promote the reaction rate. If there is no H2O in the
air, the burning rate of CO becomes very low. Usually there is some H2O in the air, but in dry weather conditions, CO release may become higher.<o:p></o:p></span></li></ul>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Jaakko<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><a name="_MailEndCompose"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></a></p>
<span style="mso-bookmark:_MailEndCompose"></span>
<p class="MsoNormal"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> Stoves [mailto:stoves-bounces@lists.bioenergylists.org]
<b>On Behalf Of </b>Andrew Heggie<br>
<b>Sent:</b> perjantaina 26. tammikuuta 2018 20.45<br>
<b>To:</b> Discussion of biomass cooking stoves <stoves@lists.bioenergylists.org><br>
<b>Subject:</b> Re: [Stoves] why does charcoal produce more CO?<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On 26 January 2018 at 01:31, Darpan Das <<a href="mailto:darpandasiitb@gmail.com" target="_blank">darpandasiitb@gmail.com</a>> wrote:<o:p></o:p></p>
<blockquote style="border:none;border-left:solid #CCCCCC 1.0pt;padding:0cm 0cm 0cm 6.0pt;margin-left:4.8pt;margin-right:0cm">
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<p class="MsoNormal"><img border="0" id="_x0000_i1025" src="https://mailtrack.io/trace/mail/e69f256f0f88a1aa36e8809aa9d03574740386e1.png?u=1788967"><o:p></o:p></p>
<p class="MsoNormal"><br>
Thanks Andrew.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<div>
<p class="MsoNormal">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.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">Can this less O present in the solid fuel matrix be attributed as a reason for formation of high CO?<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
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</blockquote>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
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<div>
<p class="MsoNormal">Darpan<o:p></o:p></p>
</div>
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<p class="MsoNormal"><o:p> </o:p></p>
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<div>
<p class="MsoNormal">Crispin has replied to this.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">By the time the fire is reduced to a glowing bed of coals there will be little oxygen bearing compounds left.
<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
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<div>
<p class="MsoNormal">In the early stages of pyrolysis CO is given off and this is a result of the decomposition of wood, so yes the CO is a product from the decomposition of oxygen beariing compounds in the wood.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">The reduction of CO2 by carbon to CO only takes place at higher temperatures by which time the char is nearly all carbon with no oxygen bearing compounds remaining.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
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<div>
<p class="MsoNormal">If the firebed is thin and able to radiate its heat freely so it looks like "black" heat the conditions for making CO from char don't exist and the char should oxidise directly to CO2, as the bed gets deeper and the heat cannot escape then
the conditions are there.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">I cannot imagine what -56C is like +40C was bad enough<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
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<div>
<p class="MsoNormal">Andrew in balmy England<o:p></o:p></p>
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