[Stoves] Natural draft TLUD turn-down

Fri May 23 17:59:49 CDT 2014

Dear Kirk

This is the sort of detailed analysis that really helps the whole field of
advanced stoves. I really appreciate your taking the time to put it into
writing.

Method three, turned upside down, is a downdraft stove with preheated
secondary air and it is a very good way to burn biomass, especially pellets
which are predictable. There is no need to elevate the fuel because gravity
handles that. I can recommend that anyone trying it use a wire grate, not
bars. This is a development of the BLDD stoves from the SeTAR Centre where
considerable success has been achieved in reducing PM emissions burning
semi-bituminous coals and again in Mongolia burning wet lignite, both with
extremely low emissions. 

Method 2 - using flamelets to maintain the main fire is a useful idea that
can be added to many stoves to overcome some debilitating shortfalls (ie
make them practical devices).  

Can I assume, based on your report, that you do not own a combustion
analyser?  You have long ago reached the stage where you need one to be able
to make better prototypes and to be able to make more generalised
statements. Some of the things said in the document are only correct in
certain cases. In nearly all stove layouts, if there is air entering from
below and exiting the top, the excess air level is too high to get really
good results save in narrow circumstances (low variability). If you want to
have high turndown, you have to control the EA in order to have a flame
remain viable. 

In the case of the flamelets, part of the reason for your success is that
the flamelets are compensating for too much excess air in a turned down
condition. If the EA was lower, the flamelets would perhaps be 'less
necessary'. 

You described having up to three controllable air sources. What I have been
suggesting is that the control of the secondary air can be made automatic if
the bottom-in-top-out architecture is abandoned in favour of downdrafted
preheating. You are correct, you need three controls, but if the structure
is correct, that happens automatically because the heat inside controls the
amount of air entering. With a bottom-to-top air path, there is nearly no
hope of getting that to happen automatically. 

With the downdraft layout, there is a lot to be gained in terms of automatic
control because all air is downdrafted to the fire chamber. A combination
would be downdrafting secondary air, possible the pilot flame air as well,
in the amounts needed to cover the range of power envisaged. The draft
calculator available on the Stoves website can assist with this while only
having a thermocouple to measure gas temps with. You divide the stove into
sections and enter the temps are the top and bottom of each and it will
calculate the total draft. You can either check the draft of something that
works well, or predict what temps or heights you need in order to generate
the draft necessary to run the stove at that power.

I really encourage you to get or borrow a combustion analyser and read the
article in Boiling Point on how to use and calculate the necessary numbers
that provide guidance. 

Regards

Crispin

Crispin,

Attached is the document which introduces the turn-down methods I have been
looking at.  

My comparison is that all three stoves, the Peco Pe, Vesto, and Prime stove
introduce air lower in the reactor from holes in the reactor wall, directing
air at the char.  The Method 2 arrangment introduces the air high in the
reactor, just under (2.5 cm) the secondary air entrance.  This air combusts
wood gas forming a pilot flame just under the secondary flame.  The way it
works is that the pilot flame supports the secondary flame, keeping it hot
and maintaining flame presence which enables the secondary flame to survive
turn-down.  The functions of the two methods, blowing air on the char vs.
forming this pilot flame are very different.  Method 2 at first seems
unlikely to work.  When I first saw it, I was surprised and upset because I
could not see how it could work.  It happened by chance.  I was testing
Method 1 to see what was the best sequence for adjusting the three controls
to lower the flame.  I tried several sequences.  Then I tried opening the
secondary air control and then reducing the early secondary air to form a
small flame (this small flame acted as the pilot flame).  I reduced the
primary air.  To my surprise and shock, the secondary flame reduced to a
strong medium sized flame.  Usually the secondary flame becomes unstable and
produces smoke before an early secondary flame appears at the char.  I
reduced the primary further and the secondary flame reduced even further,
still no smoke.  I kept reducing it until the small secondary flame finally
became unstable.  I was very upset because I had been working on Method 1,
producing an early secondary flame at the char for over a year and here was
a stable and small secondary flame.  I felt like my work was a waste if the
secondary flame can be stable during turn-down.  In time I realized that
this was a seperate method for turn-down, and it became Method 2.  Methods 3
and 4 were found later in a similar manner, by chance while doing tests on
Method 1.  I did not predict or plan these methods, I only found them by
chance.  I found Method 1 by chance also, while doing tests with spinning
the air forming a vortex, and I noted the spinning air sometimes descended
into the stove and kept the flame alive even when I shut the primary air
off.  I saw that the flame could be kept alive with air from above and this
led to Method 1.

The four methods are shown in the attached document.  I will be at stove
camp at Aprovecho in July and will demonstrate all four methods then.

Thank you for your interest,

Respectfully,

Kirk Harris

Santa Rosa, CA. USA

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20140523/04135db5/attachment.html>