[Stoves] Seeking blue flames

[Jaakko Saastamoinen]

Dear all,

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.

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.

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.

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]).
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?

[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.
[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.

Jaakko


From: Stoves <stoves-bounces at lists.bioenergylists.org> On Behalf Of Aaron Wingle
Sent: torstai 10. tammikuuta 2019 1.34
To: stoves at lists.bioenergylists.org
Subject: [Stoves] Moderated Message

I couldn’t figure out how to reply directly to the threads about Paul Oliver Rice Husk Burner.

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.

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.

Does anyone know why rice husks are magical?

As an experimenter of these TLUD stoves I find misrepresentation of the cleanliness and efficiency to cause disheartened stove designers.

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.

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