[Stoves] Request for help on TLUD operating data

[Saastamoinen Jaakko]

Dear Ron,

I have studied propagation of the ignition front in a fuel layer (considering grate combustion) and  published some papers on it. 

There are different stages.

1. Ignition at the top. It takes some time that the ignition front propagation reaches a steady velocity

2. After that the ignition front propagates approximately at constant velocity. There may be some increase in the velocity at the initial stage due to the accumulation of hot char above the pyrolysis front giving more radiation to the non-ignited fuel. The velocity depends on the primary air rate. If the primary air rate is too small, the front does not propagate. Also, if the air rate is too high, the burning is quenched due to cooling by the air so that the flame temperature goes down giving less radiation to preheat the non-ignited fuel and also keeping the non-ignited fuel cool. The velocity of the ignition front has a maxim at certain air rate. There is accumulation of char above the pyrolysis front. The maximum amount char is obtained with low primary air rate (but high enough to keep the front moving). If the air rate is high, also some char is burned above the pyrolysis front due to excess air especially if the fuel is moist. The excess oxygen that is not consumed in the pyrolysis front reacts with char giving less char. Even the ignition velocity is quite constant, the burning rate of the whole batch including the char may increase during the burning with high primary air rate, since the amount and thickness of char layer accumulating above the pyrolysis front is increasing and can react with excess oxygen. Then you would have a positive value for C in your formula (considering the whole weight loss of the batch), if there is much excess air.

3. In the end, the pyrolysis front reaches the bottom and this is may also be accounted by the term C*t^2 in your model.  In this stage the amount of pyrolysing particles at the bottom decrease leaving more excess air to react with the char.  It seems that sign of C depends on the air rate. With high air rate, the sign goes to more positive direction, since the rate of flame propagation is low in the stage 2, but the rate of char combustion becomes high at the stage 3.
 
Jaakko


-----Original Message-----
From: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of Ronal W. Larson
Sent: 14. elokuuta 2014 0:39
To: Discussion of biomass
Subject: [Stoves] Request for help on TLUD operating data

List:

	I am trying to better understand TLUDs - for purposes of improving their performance.  This is to ask all char-making stove developers to report back, either on-list or off-list, on a fundamental characteristic of all TLUDs: the way performance changes over time.

	I have seen plots showing the weight of the stove-fuel combination; basically dropping relatively linearly as the pyrolysis front moves from top to bottom.  My question is on the word "relatively".  Is that weight loss at the end dropping more (concave down) or less (concave up) rapidly than linearly?  

Stated as an equation, for a test run with a single fuel and a single primary air center the average linear equation could be:

	W(t)= Wo-A*t, with A = (W(t=tf)-W(t=o))/tf

but what I need is a quadratic form:

	W(t) = Wo - B*t + C*t^2

where B is certainly positive, but equal to A only if C is zero.

C can be either positive (concave up) or negative (concave downward).   The sign of C is the most important question I am asking - but even an approximate ratio C/B would be a big help.

	I only need a rough plot of the weight - then I can come up with the A,B, C constants.  I for sure need the starting and ending weights, and then at least a few other weights (and their times) near the end.  Knowing more on the stove would be helpful - especially the fuel can diameter.  The fuel  (sticks, chips, pellets, etc) is needed also.

Who can help?  (and leads to published material would be just as good.)

I will summarize and report - and can do this without using any names.

Ron






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