[Stoves] ​ Seeking blue flames

[Norbert Senf]

Very interesting, Jaakko. Thank you.
It relates to testing we have done on masonry heaters.

On the Finnish type, we now use 2 small grates running transversely at the
front and rear of the firebox, and push the charcoal there to speed up the
combustion at the end.
These heaters typically have a flue damper that gets closed, and the coals
have to be burned before you can do that.

On the Austrian type, in Austria flue dampers are not allowed due to
potential carbon monoxide danger. In this case, the heater has to be
airtight, and you
simply close off the combustion air supply at any point once you get to the
coal stage. You avoid a lot of the CO emissions at the end, and end up with
pile of charcoal. The charcoal can be removed (biochar), or can be burned
up in the next fire.

The Building Research Division of Canada Mortgage and Housing Corporation
(CMHC) did some very interesting work in the 70's and 80's regarding
venting and pressure issues in housing. One project was to develop a whole
house simulator, including a program called Woodsim, that modeled an open
wood burning fireplace and how it related to house pressures created by the
venting system and house envelope leaks. The project included building a
house that was instrumented for research purposes to calibrate the software
simulators. We have some info on it here:
https://mha-net.org/html/software.htm

Of most interest probably would be this report describing the software,
particularly the modeling of the fuel pile:
heatkit.com/research/Woodsim5.pdf

Best .............. Norbert


----------------------------------------------------------------------
>
> Message: 1
> Date: Thu, 10 Jan 2019 19:36:42 +0000
> From: Jaakko Saastamoinen <Jaakko.Saastamoinen at lut.fi>
> To: Discussion of biomass cooking stoves
>         <stoves at lists.bioenergylists.org>
> Subject: [Stoves] Seeking blue flames
> Message-ID:
>         <
> AM0P195MB03696347FDA4A647B90564E986840 at AM0P195MB0369.EURP195.PROD.OUTLOOK.COM
> >
>
> Content-Type: text/plain; charset="utf-8"
>
> 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., Kj?ldman, L., Oravainen;
> H, and Bostr?m, 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 Norstr?m, 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
> (snip)  -----------------------------
>
> End of Stoves Digest, Vol 101, Issue 8
> **************************************
>
-- 
Norbert Senf
Masonry Stove Builders
25 Brouse Road, RR 5
Shawville Québec J0X 2Y0
819.647.5092
www.heatkit.com
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20190112/9ab36b87/attachment.html>