[Stoves] Request for help on TLUD operating data

[Crispin Pemberton-Pigott]

Dear Ronal’n’All

 

Good find!

 

http://gekgasifier.pbworks.com/f/ignition+front+saasta.pdf

 

            b.  Because of my background, I think of the upward flow of
primary air and pyrolysis gases as three resistances and a “current” (gas)
generator.  The lowest chip/pellet region has a resistance RL  that
continually gets smaller as its volume decreases.  The upper (char) region
keeps gaining in height, but is losing weight rapidly as well;  for height
(and other) reasons it (RU) presumably is increasing (at the end of a run,
RL is zero).  The middle pyrolysis zone resistance would not seem to change
much during a run.  And the “current” source also would not seem to change
much during a run  (But maybe it does.)

 

I think that is a good description for the start of the conversation. The
only think I would add is that the overall height changes with time because
of shrinkage. It means the RU (region in the upper section that contains
char) will not only be smaller than the original fuel it will also be
evolving some CO2 and CO (by cracking CO2 from below). 

 

            So my first (electrical analog) observation is that the only way
that we can have a straight line is if the sum of RL and RU is a constant
(call RC)

 

There is a reason why it should not be and that is because (assuming it is a
natural draft device) the temperature of the vertical column varies with
time giving an increase in draft as the colder RL (Region Lower) develops
into a hot RU. The draft increase can be calculated using the Draft
calculator available on this site using the section on the right. All things
being equal, the draft will increase time, resulting in an slow increase in
power which is what is observed using high frequency mass measurements (high
in this case meaning per 10 seconds).

 

            Second - If I had to guess that the change in either would
“exactly” balance the other, I would have said no way.  But for us, it is
decidedly serendipitous/fortuitous.

            The reason for RU increasing must include viscosity changes.  

 

I have not investigated the influence of the temperature of the gases on the
viscosity in an increasing temperature column. If the temperature in RL is
293°K and 493°K in RU then perhaps the increase in viscosity of the
(different) gases will approximately balance. It seems you should put those
two variables into your formula so you can investigate the overall effect of
holding the char at a different temperature. Remember the height of RU as a
% of total will change (as you describe) but also the ‘reference height’
will drop with time. 

 

This doesn’t undermine your initial conceptual description.  

 

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

            [RWL:  Yes, but with controllable primary air, which most TLUDS
allow, there can be large early primary and then a cut back.  And usually
operated exactly that way.]

 

[CPP] Just a comment on the several failures to ignite TLUD’s in Ulaanbaatar
this summer: there are some basic precautions to take. One is that the
secondary air ports have to be closed to get a rapid ignition. We had
several cases of people (a) having secondary air only (!) as the recommended
lighting method, (b) leaving it open or not fully closed during ignition,
(c) not providing a large dominance of primary air. A further refinement for
ignition is that the kindling materials have to heat as much of the surface
as possible as soon as possible. This is not readily accomplished by
lighting a flat surface of fuel. In all cases the results of lighting at the
bottom of a conical depression or a ‘vee’ pushed into the surface results in
more rapid ignition of a larger surface per minute. Radiant heat from the
side of the flames is a booster compared with trying to radiate heat
downwards.


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. 

            [RWL:  This is the first time I have seen this.  I can see a
problem with a fan/blower, but also natural draft?


[CPP] We frequently see this when it is combined with the presence of
secondary air. This season several stoves were ignited and ‘failed to
thrive’ for more than an hour because of the combination. Fuel moisture
(which can be as high as 33%) is a major issue in the early phase.

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. 

            [RWL:  I am sure you can help us with fuel moisture issues.
Should we be “curing/drying” all fuel (maybe above the cookstove)?


[CPP] This is not going to fly In many cases, many places. It is better to
have a good ignition method so once started, it dries the fuel continuously.
There is already strong resistance at village level to trying to prepare
fuel. 

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.

            [RWL:  The design mod I have in mind requires small C, but
keeping below a certain primary air flow rate should not be a major
constraint.   By “excess air” in the last sentence,  I presume you mean
excess primary air?  (we have been using “excess” with secondary air)

 

[CPP] I will add for Jaakko’s entertainment that we have been using the
SeTAR HTP calculation of ‘excess air’ which is a chemically balanced
calculation, not the usual 

 

(O2-CO/2)/(21-(O2-CO/2))    [1]

 

We are using all the available molecules as measured, to consider the
influence on the EA value of O2 present in the fuel. Performing a chemically
balanced calculation provides a more realistic ‘EA equivalent’ telling us
what is actually going on in the combustion and reaction zones. I can
provide more details here if that is needed. It is in the lab manual which
is on line at the WB site.


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.

            [RWL:   I need help with the terms “stage 2” and “stage 3”.   I
think that with controllable primary air, that we can avoid the "high rate
of char combustion” at the end of a run - assuming (as I do) that we want to
maximize char production.   I presume stage 3 is this final stage as the
pyrolysis front reaches the bottom.   And (a question) stage 3 is the 95% of
the time period with a “constant” power level (constant rate of fuel
conversion)?  So “stage 1” is the short start up period as the pyrolysis
from moves.

 

[CPP]  Sort of related to the above: if the standard Excess Air is ‘x’ then
a recalculated version including water vapour might provide some hints as to
where to set the airflow (assuming you are not only finding it by trial and
error).  You can maximise the char yield by dropping the system temperature.
If you need more heat (Watts) just make it larger.

 

Regards

Crispin 

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