<html>
<head>
<meta content="text/html; charset=ISO-8859-1"
http-equiv="Content-Type">
</head>
<body bgcolor="#FFFFFF" text="#000000">
<div class="moz-cite-prefix">Crispin,<br>
<br>
For anyone wanting to burn the char in the device, your
explanation is fine and is much appreciated.<br>
<br>
The biggest difference in our positions is that I and some others
do NOT want to burn the char. Reasons can include wanting it for
biochar, or to burn it in a different device later, or to protect
the fuel container for the high heat of char-gasification, or for
some reason of simplified design such as using less expensive
materials of controls, and probably a few more reasons.<br>
<br>
For centuries, one objective of scientifically designed combustion
devices was to obtain the maximum of energy possible, and that
meant to leave the minimum of carbon behind in with the ashes.
But as you pointed out, things have been changing. <br>
<br>
I got started in 2001 with what has become known as TLUDs by Tom
Reed. Clean combustion was the feature. The TLUD stoves
happened to make charcoal. It was a "by-product" that became a
"co-product" that for some is the "main-product". <br>
<br>
Paul<br>
<br>
<pre class="moz-signature" cols="72">Doc / Dr TLUD / Prof. Paul S. Anderson, PhD
Email: <a class="moz-txt-link-abbreviated" href="mailto:psanders@ilstu.edu">psanders@ilstu.edu</a>
Skype: paultlud Phone: +1-309-452-7072
Website: <a class="moz-txt-link-abbreviated" href="http://www.drtlud.com">www.drtlud.com</a></pre>
On 12/12/2014 1:20 PM, Crispin Pemberton-Pigott wrote:<br>
</div>
<blockquote
cite="mid:COL401-EAS438660B6EE66FA888C6EF0BB1600@phx.gbl"
type="cite">
<meta http-equiv="Content-Type" content="text/html;
charset=ISO-8859-1">
<meta name="Generator" content="Microsoft Word 15 (filtered
medium)">
<style><!--
/* Font Definitions */
@font-face
{font-family:"Cambria Math";
panose-1:2 4 5 3 5 4 6 3 2 4;}
@font-face
{font-family:Calibri;
panose-1:2 15 5 2 2 2 4 3 2 4;}
@font-face
{font-family:Consolas;
panose-1:2 11 6 9 2 2 4 3 2 4;}
/* Style Definitions */
p.MsoNormal, li.MsoNormal, div.MsoNormal
{margin:0mm;
margin-bottom:.0001pt;
font-size:12.0pt;
font-family:"Times New Roman",serif;
color:black;}
a:link, span.MsoHyperlink
{mso-style-priority:99;
color:blue;
text-decoration:underline;}
a:visited, span.MsoHyperlinkFollowed
{mso-style-priority:99;
color:purple;
text-decoration:underline;}
pre
{mso-style-priority:99;
mso-style-link:"HTML Preformatted Char";
margin:0mm;
margin-bottom:.0001pt;
font-size:10.0pt;
font-family:"Courier New";
color:black;}
span.HTMLPreformattedChar
{mso-style-name:"HTML Preformatted Char";
mso-style-priority:99;
mso-style-link:"HTML Preformatted";
font-family:Consolas;
color:black;}
span.EmailStyle19
{mso-style-type:personal-reply;
font-family:"Calibri",sans-serif;
color:#1F497D;}
.MsoChpDefault
{mso-style-type:export-only;
font-size:10.0pt;}
@page WordSection1
{size:612.0pt 792.0pt;
margin:72.0pt 72.0pt 72.0pt 72.0pt;}
div.WordSection1
{page:WordSection1;}
--></style><!--[if gte mso 9]><xml>
<o:shapedefaults v:ext="edit" spidmax="1026" />
</xml><![endif]--><!--[if gte mso 9]><xml>
<o:shapelayout v:ext="edit">
<o:idmap v:ext="edit" data="1" />
</o:shapelayout></xml><![endif]-->
<div class="WordSection1">
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">Dear
Paul and All the Rest<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<div>
<p class="MsoNormal"><span style="color:#1F497D">From Paul: </span>2.
When the MPF (migratory pyrolytic front) approaches the
bottom, the radiant heat that goes downward has no "next
layer" of biomass to dry, torrify and pyrolyze. Instead,
it radiates to the grate and back to the already hot fuel,
making it hotter and pyrolyzing faster. <o:p></o:p></p>
</div>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">That
is true but it is not really helping us understand what to
do about the ‘final days’ of the wood gas phase and the
beginning of the charcoal gasification phase.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">First
things first. Does the designer intend the user intend to
burn the charcoal, or make charcoal? These are quite
different goals. Originally the use of wood gas was to
overcome the problem of not knowing how to burn wood
properly. Burning the gases only made a heck of a less
smoke. In consequence there as charcoal produced. In the
meantime some people want to make charcoal. Times have
changed on several fronts so we should be clear what the
intention is. <o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">If
you are going to burn the char, then you should not try to
make a lot of it. To optimise char production, you are not
going to easily turn around and make it burn well. The
conditions are different. My point is there is no need to
overcome problems that shouldn’t be there in the first
place, if in the first place you have avoid those problems.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Second,
having decided that you are going to burn the char, you can
ask if the original reason for making it as because you are
avoiding PM creation by making and burning wood gas first?
If so, then perhaps that line of thinking needs review. If
you have very low emissions does it really matter how you
got to that point?<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">So,
first take a lesson from John Davies’ packed bed coal chip
gasifier, the complete plans for which are on this website.
He formed a secondary injecting cone that directs secondary
air at the top of the fuel bed. The angle was quite high.
One of the effects is to burn the char at the top of the
fuel bed – slowly – which maintains a very high temperature
at the top of the fuel stack. It is not necessary to burn a
lot, it is necessary to keep a high temperature.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">The
effect is that when the pyrolysis is complete and the switch
is made to char burning, the flame is not maintained only by
the gas combustion. This is the key point to solving the
transition problem. <o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">There
are ‘among us’ two ideas about gasifier building: make gas
and burn it separately with no thermal connection between
the fire and the gas generator, and gas burning within close
proximity to the fuel bed so there is a strong heat exchange
between them. The latter case is called ‘close-coupled’. It
is much easier to get the desired result (transition) using
a close-coupled design.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">The
problem of the flame going out (making smoke – smoke being
condensed combustible volatiles, water droplets and very
small bits of carbonaceous fuel) is that the gas composition
changes at the same time as the heat available drops, while
the primary/secondary air split needs to change by a factor
of about 6. Trying to burn the gas continuously without
thermal support is just asking for problems. <o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">There
are several solutions that support the flame. High
temperature secondary air will limit the problem by keeping
the flame hot. Sending it downwards into the top of the fuel
will keep some of the char burning throughout the pyrolysis
phase. It also heats the flame from below. Having a high
mass combustion chamber with enough heat stored to support
the flame during transition adds free heat at the critical
moment, but <i>only</i> if it is conductive enough to feed
that heat back to the wall surface at a rate that stops the
wall temperature from dropping. This was discussed in a
separate post related to whether or not it was ‘better’ to
have an insulative or heat conductive combustion chamber
lining. Dean has always felt that it should be insulative so
there is an early, fast rise in the combustion temperature.
I have found that the advantage of that is brief and
limited. If the combustion chamber has a mass of, say, 1 kg,
and it is conductive enough to feed heat back into the fire
by convection and radiation, it will support the combustion
of the blast of CO that accompanies the transition. If the
CO burns, the heat is maintained, and the flame remains lit.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">While
this is still a good idea (a ceramic lining that can support
CO combustion) it is not as good as close-coupling the
combustion.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Because
the intention is to burn the char, there is no need to try
to preserve it in the first place. Thus some ‘standard’
approaches to building the burner can change. The secondary
are does not need to be fed in above the initial fuel level.
The secondary air has to ‘be there’ by it does not have to
be wafted over the surface in the initial stage. The fuel
can remain at the same level and the secondary air holes can
be below the fuel. This is only going to work if the fuel
doesn’t fall into the holes. See the posts about hole sizing
for secondary air. If the fuel will fall through the holes ,
make sure there is an easy way to get it out later, and
place the air exactly at the top of the fuel. If the fuel
will not fall through, place the secondary air holes below
the top of the fuel. As it burns the fuel will shrink and
sink, uncovering the holes. They will blow secondary air
across the char burning some of it and maintaining a very
hot top layer. That was John’s intention. In this way it
does not have to be blown downwards (very simple to make).
If you have fuel-in-hole problems then you have to use
John’s cone solution.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">You
may need two or more rows of secondary air holes – I use
three. Some can be below the initial fuel level – say 1/3 of
them. That ensures the combustion of some of the char which
will later maintain the flame during transition.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">As
Dean has pointed out, there is a moment when the cook might
realise that the combustor has changed (end of fuel
approaching) and they can open the primary air. I find this
works and in fact if it Is anticipated by perhaps 2 minutes
there is a smooth transition, but there are two problems
with this approach (even though people get used to it). The
first is the cook may be busy and not notice. The second is
that there is no need to do it. Using the ‘burning paper’
solution is very simple and easy and automatic. The paper
covers the extra holes needed and as the fire approaches, it
burns away allowing air to flow. This can be done before the
MPF reaches the bottom by having the additional primary air
holes on the side, above the grate, pushing air into the
fuel. An advantage of this is that the air does not drive
the combustion ‘at the grate’ it drives it into the fuel
which is them combusted. Thus avoiding melting the grate and
accomplishing the goal of burning with additional primary
air.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Time
for a digression – is the plan to burn cleanly, or to make
charcoal and then burn it? You have to decide. Always try to
avoid having to solve problems that should not be there in
the first place. If the superficial velocity was higher to
begin with, there would be more combustion of the carbon in
the fuel during gasification. Close-coupled and having
higher CO and a generally higher pyrolysed fuel bed
temperature is an advantage, not a disadvantage. Don’t be
fanatical. MPF combustion is not magical; you do not get
clean burning just because of that. You can in fact get very
dirty combustion with an MPF gasifier, as with anything and
any fuel. If clean burning is the goal, make the initial
conditions much close to the final conditions and you will
not have such a big transition to manage.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Now,
back to design if air supply: The primary/secondary air
split for wood burning is 1:3. For charcoal it is about
3:1. That is a change of 9:1 but in practice you can’t do
that. I mean, you can’t increase the primary 9x and leave
the secondary the same. Won’t work well in the same physical
space because the power and temps will be really wrong. You
have to cut the secondary and open the primary and it should
happen automatically. How do you do that? With
self-balancing air supplies. I can’t explain that here – it
needs drawings and math. But basically you limit the total
air and supply it through a channel that <i>would </i> feed
it to the primary in the right proportion for char burning
(final configuration) but which can’t flow because some
primary air holes are blocked. Therefore it becomes
secondary air until the change is made. When the primary
holes are unblocked, it taps the secondary air flow and
changes the proportions automatically. Follow?<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">There
are several ways to achieve that so use your imagination.
The goal is for it to be as automatic as possible. I hope
to see at least one product using this approach coming to
market in the next few weeks. <o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">A
warning: if the fuel is itself a significant element of the
control of the air flow, there are going to be problems –
unless the fuel is homogenised and completely predictable
like pellets or briquettes. “Wood burners” will have to have
control of the air by design, not by fuel packing, because
wood is a lot less predictable. <o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Final
recap: If the desire is to burn the char, set up the initial
design to make it less sharp a transition by running a
higher carbon burn rate throughout the cooking session. Make
the change in primary/secondary split automatic, even if it
has to be manually initiated.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Regards
to all<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Crispin<o:p></o:p></span></p>
</div>
<br>
<fieldset class="mimeAttachmentHeader"></fieldset>
<br>
<pre wrap="">_______________________________________________
Stoves mailing list
to Send a Message to the list, use the email address
<a class="moz-txt-link-abbreviated" href="mailto:stoves@lists.bioenergylists.org">stoves@lists.bioenergylists.org</a>
to UNSUBSCRIBE or Change your List Settings use the web page
<a class="moz-txt-link-freetext" href="http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org">http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org</a>
for more Biomass Cooking Stoves, News and Information see our web site:
<a class="moz-txt-link-freetext" href="http://stoves.bioenergylists.org/">http://stoves.bioenergylists.org/</a>
</pre>
</blockquote>
<br>
</body>
</html>