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Crispin,
(JSS to post)<br>
<br>
Thank you for your comments, and for checking with an expert on
gasification.<br>
<br>
First, relating to previous messages. My "contention" is not that
whether up draft (UD) occurs or that top-lit (TL) occurs. There
certainly can be cases of UDTL or UD-TL or TL-UD occurring. Call
those "good scientific designations". But there are social and
market and layman understandings of what is TLUD, which even has a
pronunciation of "Tee-lud", and years ago was often spelled T-LUD.
And TLUD is certainly associated with biomass fuels, not with
charcoal burning stoves.<br>
<br>
What is bothersome are attempts to re-direct the TLUD name / acronym
toward combustion situations that often are not in the least bit
interested in being designate as UD - TL. For example, the
Cambodian charcoal burner that you have described can be sold on its
own merits, and even pointing out that it is UP-TL (or MAYBE even
TL-UD). But to push for referring to such stoves as being TLUD or
T-LUD has the appearance of either 1) riding on a TLUD tailcoat, or
2) trying to diminish the importance of "biomass-burning MPF top-lit
updraft = TLUD" stoves. The APPEARANCE is that you want to cause
disruption or misunderstanding, not to enhance science. For the
sake of science, just use UP-TL if you need to discuss such features
of the Cambodian charcoal or other such stoves. And let TLUD,
T-LUD (tee-lud) stand for what it is understood to be in
"contempoary stove language."<br>
<br>
Second, relates to one of your paragraphs:
<blockquote type="cite">He [the experienced gasification engineer]
uses terms that differentiate the “MPF” into several discrete
zones: drying, distillation and oxidation. </blockquote>
Drying. Okay, clear.<br>
Distillation. I prefer the word pyrolysis, being chemical
decomposition caused by heat. Understood to include the
vaporization of volatiles that do not require chemical changes,
which is part of distillation. Generally okay thus far.<br>
<br>
But Oxidation as a discrete zone? Let's look at this. In a retort,
heat causes pyrolysis, witn no oxygen present except from within the
hydrocarbons (and we have an on-going separate discussion about the
amount and activity of that oxygen). In the MPF inside a TLUD, cut
off the O2 entering as primary air and the heat generation is not
sufficient to maintain the MPF. <br>
<br>
And when there is O2 from primary air, that O2 is in close proximity
to the combustible pyrolytic gases that are exiting from all sides
of the pieces of biomass that are becoming covered with and the
becoming entirely charcoal. Oxidation occurs where those gases are
mixing together, and that is not inside the biomass, but close to
but not on the charcoal (unless forcing much primary air). This is
within the pyrolysis area (not the drying area) of the MPF. It is
not a discrete zone. <br>
<br>
The expert continues with:<br>
<blockquote type="cite">The water gas shift reaction takes place in
the oxidation zone which he suggests is something that should be
enhanced by steam injection or additional fuel moisture.</blockquote>
The water gas shift requires that molecules of H2O are in contact
with the high heat of burning (at least glowing) carbon. The
moderately exothermic reaction results in H2 and 2 of CO. Great if
it happens. Crispin suggests that some experiment be conducted:
<blockquote type="cite">One method would be to measure the gas
composition before it is combusted, and quantify the hydrogen
content excluding the water vapour.</blockquote>
He said "measure the gas composition before it is combusted"
Which gas might this be? Is this the gases precisely as they exit
from the multitude of pieces of biomass but before any combustion
with the primary O2 as previously mentioned? Or as the resultant
gases just above the hot biomass/charring pieces? Or higher up
as it it passing through the created layer of accumulated char that
is no longer glowing and cannot cause the water gas shift reaction
to occur there? <br>
<br>
I am all for the experimentation, but I am not yet able to visualize
(even in theory) where this water gas shift reaction is occurring
and how to measure anything there. <br>
<br>
To me, more fundamental is the sensitivity of TLUD operations to the
moisture content (MC) of the fuel. The char that is being created
via pyrolysis/carbonization is bearly glowing, and the object is to
have sufficient heat generated to continue the pyrolysis into the
center of each piece of fuel. The water gas shift reaction (to the
extent that it might be occurring) is going to take away some
(much?) of that heat when making the H2 and CO that can be usefully
burned at the level of the secondary combustion. But heat loss at
the pyrolysis zone could be detrimental to the continuation of the
necessary pyrolysis. I am raising this as a question.<br>
<br>
Also, Crispin can ask the gasification engineer (or combustion
chemists or others) about what was meant by the discrete zone of
oxidation. Was that referring to the zone of char-gasification,
which is the direct burning of the carbon? THAT zone does exist in
full fledged gasifiers. But TLUDs are only utilizing the pyrolysis
part of gasification. [We note that the word gasification is used
in different ways that sometimes exclude pyrolysis and focus on the
char-gasification process. That is why I prefer to say that
gasification consists of pyrolysis and char-gasification in order to
recognize both key processes.]<br>
<br>
This sentence bears repeating here:<br>
<blockquote type="cite">The water gas shift reaction takes place in
the oxidation zone which he suggests is something that should be
enhanced by steam injection or additional fuel moisture.</blockquote>
This is excellent and standard practice in bottom burning downdraft
classic full-fledged gasifiers. In them there is an oxidation zone
that consist of a layer of charcoal (that was earlier produced
inside that gasifier). It is the bottommost layer (zone). Air
(with O2) is injected and it is white-hot, the kind of heat that can
melt metal. So one way to lower that temperature and to obtain the
benefits of the water gas shift reaction is to drip in some
water. -------- The very clear reason that this advice is not
applicable to TLUD gasifiers is that the char bed is starved of
oxygen and the char accumulates for later extraction. <br>
<br>
Sorry that it took me a while to sort this out, but I hope that what
is written is clear. I believe that the expert on gasification will
understand and agree that the comments about the absence in TLUDs of
a discrete oxidation zone that would be the place for the water gas
shift reaction, as in the traditional usage of such phenomena in
char-gasification gasifiers.<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>
<div class="moz-cite-prefix">On 12/13/2017 12:10 PM, Crispin
Pemberton-Pigott wrote:<br>
</div>
<blockquote type="cite"
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<p class="MsoNoSpacing">Dear Friends<o:p></o:p></p>
<p class="MsoNoSpacing"><o:p> </o:p></p>
<p class="MsoNoSpacing">Further to the earlier discussion about
the nature of and terms for the pyrolysis zone. I have
consulted one of the most experienced gasification engineers
in the world to ask how, over his career of 65 years, the
terms used for describing the zones have evolved.<o:p></o:p></p>
<p class="MsoNoSpacing"><o:p> </o:p></p>
<p class="MsoNoSpacing">First, he said that he was unfamiliar
with the term ‘migrating pyrolysis front’. If the term MPF is
meant to describe the whole active zone from the point at
which the fuel dries, then commences pyrolysing up to the
point at which there is hot char no longer releasing gases, we
can refer to that as the active zone.<o:p></o:p></p>
<p class="MsoNoSpacing"><o:p> </o:p></p>
<p class="MsoNoSpacing">He uses terms that differentiate the
“MPF” into several discrete zones: drying, distillation and
oxidation. The water gas shift reaction takes place in the
oxidation zone which he suggests is something that should be
enhanced by steam injection or additional fuel moisture.<o:p></o:p></p>
<p class="MsoNoSpacing"><o:p> </o:p></p>
<p class="MsoNoSpacing">As was reported by Tom Reed and
confirmed many times since, adding moisture to the fuel
reduces the char yield and gives more gas, and it should be
higher energy gas. The water gas shift reaction produces CO
and H2 from the fuel moisture extracting heat from the
oxidation zone. This phenomenon has been discussed on this
list but if I recall correctly, it was always on the basis
that the fuel moisture ‘required more energy to remove it’,
not that there was a water gas shift reaction taking place
generating more combustible gas, using the carbon in the
process.<o:p></o:p></p>
<p class="MsoNoSpacing"><o:p> </o:p></p>
<p class="MsoNoSpacing">Can anyone think of an experiment to
prove it? One method would be to measure the gas composition
before it is combusted, and quantify the hydrogen content
excluding the water vapour. If it exceeds the mass that is
available from the fuel, then the water gas shift reaction
could explain it. <o:p></o:p></p>
<p class="MsoNoSpacing"><o:p> </o:p></p>
<p class="MsoNoSpacing">An advantage of the three-zone
description of what takes place in the MPF is that it an be
applied to all gasification, not just pyrolysation. Described
previously is the charcoal TLUD that can be described as
having a descending oxidation zone in which gases are created
resulting in the reduction of char to ash instead of biomass
to char. It is incorrect to call it pyrolysation, and MPF is
therefor inappropriate. An “oxidation zone” applies to both
the pyrolysis and gasification processes. Thus they both have
a migrating oxidation front descending into the fuel. One has
a migrating pyrolysis front that includes in it (if so
defined) the oxidation, drying and distillation zones.<o:p></o:p></p>
<p class="MsoNoSpacing"><o:p> </o:p></p>
<p class="MsoNoSpacing">This three-zone description applies to
both the TLUD and BLUD gasifiers and pyrolysers though they
produce different gas compositions.<o:p></o:p></p>
<p class="MsoNoSpacing"><o:p> </o:p></p>
<p class="MsoNoSpacing">Regards<o:p></o:p></p>
<p class="MsoNoSpacing">Crispin<o:p></o:p></p>
<p class="MsoNoSpacing"><o:p> </o:p></p>
<p class="MsoNoSpacing"><span style="mso-fareast-language:EN-CA"><o:p> </o:p></span></p>
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<br>
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