<html xmlns:v="urn:schemas-microsoft-com:vml" xmlns:o="urn:schemas-microsoft-com:office:office" xmlns:w="urn:schemas-microsoft-com:office:word" xmlns:m="http://schemas.microsoft.com/office/2004/12/omml" xmlns="http://www.w3.org/TR/REC-html40">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<meta name="Generator" content="Microsoft Word 15 (filtered medium)">
<!--[if !mso]><style>v\:* {behavior:url(#default#VML);}
o\:* {behavior:url(#default#VML);}
w\:* {behavior:url(#default#VML);}
.shape {behavior:url(#default#VML);}
</style><![endif]--><style><!--
/* Font Definitions */
@font-face
{font-family:"Cambria Math";
panose-1:2 4 5 3 5 4 6 3 2 4;}
@font-face
{font-family:Calibri;
panose-1:2 15 5 2 2 2 4 3 2 4;}
/* Style Definitions */
p.MsoNormal, li.MsoNormal, div.MsoNormal
{margin:0cm;
margin-bottom:.0001pt;
font-size:11.0pt;
font-family:"Calibri",sans-serif;}
a:link, span.MsoHyperlink
{mso-style-priority:99;
color:#0563C1;
text-decoration:underline;}
a:visited, span.MsoHyperlinkFollowed
{mso-style-priority:99;
color:#954F72;
text-decoration:underline;}
p.msonormal0, li.msonormal0, div.msonormal0
{mso-style-name:msonormal;
mso-margin-top-alt:auto;
margin-right:0cm;
mso-margin-bottom-alt:auto;
margin-left:0cm;
font-size:11.0pt;
font-family:"Calibri",sans-serif;}
span.EmailStyle18
{mso-style-type:personal;
font-family:"Calibri",sans-serif;}
span.EmailStyle19
{mso-style-type:personal;
font-family:"Calibri",sans-serif;}
span.EmailStyle21
{mso-style-type:personal-reply;
font-family:"Calibri",sans-serif;}
.MsoChpDefault
{mso-style-type:export-only;
font-size:10.0pt;}
@page WordSection1
{size:612.0pt 792.0pt;
margin:70.85pt 2.0cm 70.85pt 2.0cm;}
div.WordSection1
{page:WordSection1;}
--></style><!--[if gte mso 9]><xml>
<o:shapedefaults v:ext="edit" spidmax="1026" />
</xml><![endif]--><!--[if gte mso 9]><xml>
<o:shapelayout v:ext="edit">
<o:idmap v:ext="edit" data="1" />
</o:shapelayout></xml><![endif]-->
</head>
<body lang="FI" link="#0563C1" vlink="#954F72">
<div class="WordSection1">
<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">Crispin and Norbert, thanks for the comments. Crispin, you are right in that CO2 is the main product in char combustion at low temperatures. See Arthur’s classical equation [3]. According to later experimental research
[4] it depends also on oxygen concentration.<span style="color:red"> </span><span style="color:black">By googling “Arthur’s formula for CO-CO2 ratio” one can find a comparison of results of some authors in a figure. According Arthur’s results estimated from
this figure CO2/CO ratio is around 0.5 at 600 oC, at 800 oC it is ~ 0.15 and at 1000 oC ~ 0.08. These are chars from coal and biomass could different. I just googled a recent paper [5] for char from biomass.<o:p></o:p></span></span></p>
<p class="MsoNormal"><span lang="EN-US" style="color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Pyrolysis and char combustion can occur simultaneously for a fuel particle [6,7] so that the inner part is still in the devolatilizing stage while the outer char layer is burning. Volatiles contain hydrogen producing
water vapor or hydrogen containing radicals, which enhance the oxidation of CO to CO2. This and high temperature could also be a reason for the strong but short-lasting blue blame just after the flaming pyrolysis stage I see frequently through glass window
in my fireplace. There may be some hydrogen producing H2O or radicals formed from H2O enhancing CO combustion [2]. After that, when looking in my fireplace, there is glowing of char particles with yellow reddish color with no visible flame. Some CO might be
partly burned in a very thin flame in the particle´s boundary layer. In TLUD the char combustion on the top takes place simultaneously with pyrolysis below and hydrogen containing radicals may be present in the char layer above the migrating pyrolysis front.<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, it seems that there are two ways (A and B) to reduce emissions of CO in char combustion stage of a stove. (A) Burn at low enough temperature to produce mainly CO2 directly from char oxidation. (B) Burn keeping the
temperature high enough to oxidize CO produced from char combustion. <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">In case A, the char bed be should somehow cooled after the hot flaming pyrolysis stage. Burning rate depends on temperature. However, reduced burning rate and low temperature could also be useful, since in cooking one
may need first a rapid heating and later lower heat input during simmering. <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">In case B, one should keep the temperature high during char combustion stage after the flaming volatiles combustion stage. In my (probably Tom Reed’s design) commercially bought TLUD the secondary air comes through the
space between the outer and inner walls and is preheated to some extent. If one adds insulation around the device and fins in the air passage, the temperature would be higher. However, the material may not suffer so high temperature. Storing some heat for
later use for example in the walls of the stove or in a separate fixed bed could be one possible way to keep air temperature high. I have studied heat transfer in regenerators and fixed beds [8-10]. Regenerator is a heat exchanger, where heated gas (here
combustion air) and cooled gas (flue gas) flow alternatively through a solid matrix. This can be achieved by a rotating device or with two fixed beds of solid material. The latter option could be cheap, one needs two buckets of gravel. Hot gas flows (by a
fan) through solid material structure heating it and then the flow is switched and cool gas is heated.<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 use of regenerative heat exchanger is illustrated in [9] for wood combustion. It is possible to burn low heat value fuel for example moist wood, if the combustion air is preheated. I once calculated the operation
of a regenerative heat exchanger, which burned low heat value gas (mixture of air and “smell”). It was exhaust air from a paint factory which had about 1-2 grams vaporized burnable material in 1 cubic meter of cool exhaust air. The aim was to stop the emissions.
This was a very low caloric gas mixture. The plant burning it with a regenerative heat exchanger was built. This could also be used in a small scale in cooking. It would be costly to make it operate automatically. But if the cook is around, he could change
the valves directing the gas flows by hands now and then.<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">I like to watch burning in a fireplace during winter. I have found optimum size, shape and placing of wood logs to burn the batch nicely and long time. But the fuel wood available is not usually not the optimum size and
shape, so I have to think how to burn them. After some practice, it is not quite complicated in a masonry fireplace to burn the batches without smoke with dry wood of all size and shape. The firing in stoves for cooking is more complicated. In addition to
fuel, one has to think the heat release as function of time needed in cooking. Optimal stove concerning CO is then not necessarily the most energy efficient and practical solution for cooking.
<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">Jaakko<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">[3] Arthur, J.R. Reactions between carbon and oxygen. Transactions of the Faraday Society 47, 1951, 164-178.
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">[4] Tognotti, L. Longwell, J.P., and Sarofim, A.F. The products of the high temperature oxidation of a single char particle in an electrodynamic balance. 23rd Symposium on Combustion. The Combustion Institute, 1990, 1207-1213,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">[5] Anca-Couce, A., Schaler, R. CO/CO2 ratio in biomass char oxidation, Energy Procedia 120, 2017, 238-245.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">[6] Fuel 72, 1993, 599-609;<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">[7] Combustion and Flame 116, 1999, 567-579.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">[8] ASME Journal of Heat Transfer 117, 1995, 510- 512<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">[9]<a name="_Hlk535249728"> International Journal of Heat and Mass Transfer</a> 42, 1999, 3201-3212, 1999
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">[10] International Journal of Heat and Mass Transfer 46, 2003, 2727-2735.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<div>
<div style="border:none;border-top:solid #E1E1E1 1.0pt;padding:3.0pt 0cm 0cm 0cm">
<p class="MsoNormal"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> Stoves <stoves-bounces@lists.bioenergylists.org>
<b>On Behalf Of </b>Crispin Pemberton-Pigott<br>
<b>Sent:</b> perjantai 11. tammikuuta 2019 1.58<br>
<b>To:</b> Discussion of biomass cooking stoves <stoves@lists.bioenergylists.org><br>
<b>Subject:</b> Re: [Stoves] Seeking blue flames<o:p></o:p></span></p>
</div>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span lang="EN-CA" style="mso-fareast-language:EN-US">Dear Jaakko<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA" style="mso-fareast-language:EN-US">What a great contribution. Thanks.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA" style="mso-fareast-language:EN-US">I suggest there is a third sustainable condition of combustion at a lower temperature than these two below. It is when carbon goes directly to CO2 without an intermediary C=> CO=> CO2
stage. The temperature at which the C=>CO2 reaction takes place on the surface is about 400 C. The result is enough heat to permit the reaction to continue.
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA" style="mso-fareast-language:EN-US">Regarding creating nearly zero CO combustion, I have not found that a blue flame is needed. I offer this photo:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA"><img width="212" height="378" style="width:2.2083in;height:3.9375in" id="Picture_x0020_1" src="cid:image001.png@01D4AC30.F4928E00"></span><span lang="EN-CA" style="mso-fareast-language:EN-US"><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA" style="mso-fareast-language:EN-US">The colour of the flame (which is difficult to see as it is almost invisible) is pale yellow to slightly pink. The vertical depth of the flame is about 300mm and can properly be described
as “translucent”. The CO is extremely low, with a CO/CO2 ratio of 0.05% to 0.03%.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA" style="mso-fareast-language:EN-US">There is no blue colour visible at all, yet it is a gasifier flame, meaning it is burning gases produced in a separate chamber a few inches away. The water vapour content of the fuel
is quite low, though there is of course hydrogen burning so there is H<sub>2</sub>O in the flame.
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA" style="mso-fareast-language:EN-US">I have not sampled the gases immediately prior to combustion, for example at the bottom of the visible flame just above the char bed. That is possible in the immediately future as there
is now some new equipment at the CAU lab in Beijing. I am interested to know it they are short hydrocarbons and not CO + H<sub>2</sub> + CH<sub>4</sub>.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA" style="mso-fareast-language:EN-US">For your information, the Chinese Academy of Science in Beijing has modeled the combustion of coal in a crossdraft gasifier including the combustion of particles. It runs on a 100 petaflop
computer (about 2% of its capacity) and runs for about 3 days. It is pretty impressive. They have produced (completely independently of our work) a working device for heating water that, in terms of the combustor architecture, is in many respects is within
a centimeter the same as the KG4.4 and MN.2 stoves in Central Asia. To me, that means we are both on the right track.
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA" style="mso-fareast-language:EN-US">So far we have not been able to get our hands on one of the prototypes or production models (if they are in production) to test it using a contextual test sequence to find out how it
differs from its friends.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA" style="mso-fareast-language:EN-US">Regards<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA" style="mso-fareast-language:EN-US">Crispin<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<div>
<div style="border:none;border-top:solid #E1E1E1 1.0pt;padding:3.0pt 0cm 0cm 0cm">
<p class="MsoNormal"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> Stoves <<a href="mailto:stoves-bounces@lists.bioenergylists.org">stoves-bounces@lists.bioenergylists.org</a>>
<b>On Behalf Of </b>Jaakko Saastamoinen<br>
<b>Sent:</b> Friday, January 11, 2019 3:37 AM<br>
<b>To:</b> Discussion of biomass cooking stoves <<a href="mailto:stoves@lists.bioenergylists.org">stoves@lists.bioenergylists.org</a>><br>
<b>Subject:</b> [Stoves] Seeking blue flames<o:p></o:p></span></p>
</div>
</div>
<p class="MsoNormal"><span lang="EN-CA"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">Dear all,<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"><span lang="EN-US" style="mso-fareast-language:EN-US">I once made a model and a computer program of combustion of a layer of wood. The model for the char bed combustion stage was simplified as a quasi-steady model [1]. This simplification
means that “the past” is not included in the model equations, because the “history term” (meaning the sensible energy storage in the particles) in the energy equation is quite small compared to the heat of combustion and heat transferred to the surroundings.
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US">One had to guess some temperature (“past”) and then by iteration the program calculated the temperature distribution and combustion in the bed. I gave the program to a fellow researcher
as a sub-model to be connected in a comprehensive model of a fireplace. He then noticed that my program gave two different “exact” stable solutions in some cases. Depending on the initial temperature guess the iteration led to either of the two steady state
solutions, the high temperature or low temperature char bed temperature . After some thinking I figured that if one assumes initially a low temperature, the temperature is too low to sustain combustion of CO, but if the guess is high enough, CO is burning
and the temperature remains high due to heat generated and burning of CO continues.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US">The yellow flame usually means burning of volatiles. After that in char combustion stage, the blue flame is an indication that CO is burning to CO2 in the gas. Of course the fuel itself
could have some effect to the color as it is maybe the case of rise husks discussed, but the blue flame could also be an indication of char burning to CO2 on the top of the bed. The sensible heat storage term is small in the energy balance equation for combustion,
but it is, however, significant “remembering the past” temperature. The conclusion is that the blue flame stage can be sensitive. I am using a masonry stove during winters. When I see blue flame in the end of the burning, I try not to disturb it anyway to
keep temperature high and keep combustion of CO continuing producing CO2 instead of emission of CO. After blue flame goes off I may add small thin wood sticks to increase the temperature by the volatiles burning. When the blue flames vanish, it means that
the temperature is too low to burn CO and the product is CO in the chimney. So the burning is quite sensible, if one disturbs it so that temperature decreases, then CO is no longer burning.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US">According to model calculations the temperature in the char bed depends on many things. It the air rate is too high, it cools the bed and CO does not burn. If it is too low, then also
the temperature becomes too low. The char particle size and bed thickness which are decreasing during the combustion of a fuel batch are also important factors affecting combustion of CO. It is also well-known that water vapor in the combustion air enhances
burning of CO (see e.g. [2]).<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US">So we should seek for and plan devices producing long stage of blue flame meaning less CO release and more usable energy. The blue flame could be due to fuel, its particle size, proper
air rate regulation and burner insulation to keep the temperature high enough just above the char bed. One could design a combustor with some heat storage in the stove material and also heat up secondary air going above the bed to keep the temperature high
for burning of CO longer time. It is an interesting and meaningful question, how to keep blue flame burning long time?<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language: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" style="mso-fareast-language: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" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US">Jaakko<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> Stoves <<a href="mailto:stoves-bounces@lists.bioenergylists.org">stoves-bounces@lists.bioenergylists.org</a>>
<b>On Behalf Of </b>Aaron Wingle<br>
<b>Sent:</b> torstai 10. tammikuuta 2019 1.34<br>
<b>To:</b> <a href="mailto:stoves@lists.bioenergylists.org">stoves@lists.bioenergylists.org</a><br>
<b>Subject:</b> [Stoves] Moderated Message<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
<div>
<p class="MsoNormal">I couldn’t figure out how to reply directly to the threads about Paul Oliver Rice Husk Burner. <o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">Through my observations I have noticed there is something magical about rice husk pyrolysis and forced air draft stoves. They always burn clean and blue or violet in crazy ways that I haven’t witnessed on many wood stoves. Alex English
has a blue flame wood stove and one other possible person and myself. <o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">I would like to see a video or photographs showing that stove burning on wood pellets or chips. My guess is a mostly yellow flame is produced. <o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"> <o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">Does anyone know why rice husks are magical?<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">As an experimenter of these TLUD stoves I find misrepresentation of the cleanliness and efficiency to cause disheartened stove designers. <o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">If I am wrong and that stove burns blue while burning wood chips or pellets, I’ll add Paul Oliver to my TLUD designer shrine of Worship. <o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
</div>
</body>
</html>