Kevin,
Of course different fuel preparation (such as chopping) is an option.
But I am referring to when such preparation is not very convenient or
maybe is not even an option.
In my mind, there is mainly biomass fuel in the chamber, and the filler
(charcoal or other) is relatively little. Extreme cases can always be
named and found to be lacking or difficult.
We deal with an imperfect world with imperfect stoves and imperfect
fuels (except when significant processing takes place).
More hands-on experimentation needs to be done with charcoal as a space
filler before these debates of "what if..." can be of much use.
Paul
Paul S. Anderson, PhD aka "Dr TLUD"
Email: [email protected] Skype: paultlud Phone: +1-309-452-7072
Website: www.drtlud.com
On 5/6/2013 10:53 AM, Kevin wrote:
Dear Paul
Thanks very much for your detailed explanation.
Fundamentally, a "Perfect" TLUD will work perfectly if it has perfect
fuel, perfect, required air volume, and perfect fan pressure (or stack
vacuum) to deliver the perfect air flow.
Given that you have "off-standard" fuel, with greater average void
space diameter, the pressure loss across the fuel bed will be lower.
The fan or stack will thus deliver more air through the bed. Thus, you
will get excess primary air flow, and if your secondary air porting
was designed for a bed with a greater pressure drop, you will get less
secondary air delivery; this will mess up your intended
Secondary/Primary air ratio.
You can burn virtually any fuel in a TLUd, as long as it is uniform,
and the stove was designed to handle it. Change the fuel
significantly, and you need to change the design, to maintain the
desired SA/PA ratio. If you "change the fuel", but don't change the
design, then the only way to restore the system to "good operation" is
to "modify the fuel" to one having similar a pressure drop across the
bed, similar to one for which the system was designed. This is what
you are effectively trying to do.
Assume, for example, that you have 4" long pieces of straw as fuel.
This will likely give you all the problems you note. Adding char can,
in theory, help increase the bed pressure drop. However, I am guessing
that it will be a real stinker to get the char particles distributed
through the bed. There will likely be "too much" in one area, and "too
little" in another. Channelling is thus very likely. What you will
probably end up with is a fuel bed having non-uniform flow properties,
because of the large differences between straw properties and char
properties. Try mixing various percentages of char with the straw, in
a mixing bucket, then try to take "mixed fuel" from the "mixing
bucket" and place it in the stove. I am guessing that you should see
the non-uniformity of the fuel bed even before you ignite it.
My guess is that your best bet would be to chop the straw, so that it
is free-flowing. This alone will make life very much easier for the
operator, and will enable a greater weight of fuel to be added to the
stove. Longer burn times between re-fuelling. Play with the air flow,
simply by obstructing the fan intake with a piece of paper or
cardboard. Then see if you can get your usual good combustion. If not,
then consider re-drilling or partially plugging, the SA air holes to
get back to the correct PA/SA ratio for good combustion.
Is chopping the straw (or whatever the fuel is) an option you can
consider?
Best wishes,
Kevin
----- Original Message -----
*From:* Paul Anderson <mailto:[email protected]>
*To:* [email protected] <mailto:[email protected]>
*Cc:* Kevin <mailto:[email protected]> ; Discussion of
biomass cooking stoves <mailto:[email protected]> ;
James S. Schoner <mailto:[email protected]> ; Hugh McLaughlin
<mailto:[email protected]>
*Sent:* Monday, May 06, 2013 11:23 AM
*Subject:* Re: [Stoves] [biochar] Charcoal as space filler in TLUD
reactors
Kevin,
The objective of the filler is two-fold:
One reason is precisely to reduce the maximum flow of primary
air. Without resistance, too much primary air can race through
the loose pile of biomass, reach too much of the biomass at the
same time, and have an excessive fire without much control.
Control simply by a "gate" at the entrance of the primary air is
usually insufficient.
Second reason is that burning embers at the top of loosely packed
fuel can sometimes fall to lower areas of the fuel bed and ignite
the raw fuel there. This defeats the process of the pyrolysis
front that starts at the top and should progress slowly and
uniformly downward through the bed of fuel. That migrating
pyrolytic front is THE most important and distinguishing feature
of the TLUD stoves. Ignition at the top and having updraft are
not the single-most defining characteristics of TLUD stoves (even
though that is what the name says). Maybe I should have called
it Migrating Pyrolytic Front Gasification (or MPFG), but TLUD is
the accepted name now. [And Tom Reed always thanks me for
getting away from the Inverted DownDraft (IDD) name that was not
well understood.]
Important note: When the pyrolytic front correctly reaches the
bottom of the batch of fuel, the combustion style changes to be
Bottom-Burning UpDraft (call it BBUD if you must have an acronym,
but note that at the start it was NOT IGNITED or lit at the
bottom). And there is no more migration/movement of a
"gas-making" zone.
Also note: When the batch has been pyrolyzed, the burning at the
bottom is "char-gasification" and can be at forge temperatures
that can damage the metal pieces. There is still restricted flow
of primary air. The hot gases go upward. IF additional raw
biomass fuel is placed onto the top of that charcoal, it will be
heated, dried, torrified, and eventually pyrolyzed, giving
additional pyrolytic gases that can be combusted where the
incoming secondary air enters. But this is NOT operating as a
TLUD stove (with MPFG). This type of bottom-burning gasifier is
well illustrated by the Oorja stove (former BP, now First Energy)
in India. It has a cast-iron cup in the bottom to protect the
other metal parts, and that cup glows red-hot after continual
use. [Technical note: Stove testing should measure separately
the emissions during each of the different combustion modes
instead of just reporting averages that include emissions from two
or more combustion modes. I think we can do some of that at this
summer's Stove Camps at CREEC - Uganda and at Aprovecho -
Oregon-USA where emissions equipment is available.]
About terminology: A bucket stove or mud stove or Rocket stove
and many others can be ignited at the bottom of a container and
they do have updraft, BUT they are NOT GASIFIER devices. So the
designation BLUD is not relevant. UD and DD and TLUD are
designations historically for gasifiers, which means that the
gases are created in one place that is NOT the same place as the
combustion or other use of the gases.
Paul (James, please get this onto
the drtlud.com website in edited format.)
Paul S. Anderson, PhD aka "Dr TLUD"
Email:[email protected] Skype: paultlud Phone: +1-309-452-7072
Website:www.drtlud.com
On 5/6/2013 1:17 AM, Kevin wrote:
Dear Paul
----- Original Message -----
*From:* Paul Anderson <mailto:[email protected]>
*To:* Discussion of biomass cooking stoves
<mailto:[email protected]>
*Cc:* Kevin <mailto:[email protected]> ;
[email protected] <mailto:[email protected]> ;
James S. Schoner <mailto:[email protected]> ; Hugh McLaughlin
<mailto:[email protected]>
*Sent:* Sunday, May 05, 2013 6:33 PM
*Subject:* Re: [Stoves] [biochar] Charcoal as space filler in
TLUD reactors
Kevin,
The "charcoal as filler" is not about consuming the charcoal.
# Sorry, I missed that.
The charcoal is "almost" non-active in the pyrloysis of the
new biomass. This is a discussion about limiting air flows
with a filler that mostly is inactive in environments that
are at 650 C without oxygen.
# Why do you feel it would be advantageous to limit air flow
with an inert filler? If the char was significantly larger or
smaller than the biomass fuel, it could significantly
increase pressure drop through the bed, and would likely
reduce maximum flow.
# Thanks.
[ Note that I avoid using the word "inert" in this
discussion.]
Best wishes,
Kevin
Paul
Paul S. Anderson, PhD aka "Dr TLUD"
Email:[email protected] Skype: paultlud Phone: +1-309-452-7072
Website:www.drtlud.com
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