[Stoves] Understanding TLUDs, MPF and more. (was Re: Bangladesh TLUD )
Paul Anderson
psanders at ilstu.edu
Wed Dec 13 23:13:10 CST 2017
Crispin, (JSS to post)
Thank you for your comments, and for checking with an expert on
gasification.
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.
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."
Second, relates to one of your paragraphs:
> He [the experienced gasification engineer] uses terms that
> differentiate the “MPF” into several discrete zones: drying,
> distillation and oxidation.
Drying. Okay, clear.
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.
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.
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.
The expert continues with:
> 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.
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:
> One method would be to measure the gas composition before it is
> combusted, and quantify the hydrogen content excluding the water vapour.
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?
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.
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.
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.]
This sentence bears repeating here:
> 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.
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.
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.
Paul
Doc / Dr TLUD / Prof. Paul S. Anderson, PhD
Email: psanders at ilstu.edu
Skype: paultlud Phone: +1-309-452-7072
Website: www.drtlud.com
On 12/13/2017 12:10 PM, Crispin Pemberton-Pigott wrote:
>
> Dear Friends
>
> 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.
>
> 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.
>
> 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.
>
> 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.
>
> 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.
>
> 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.
>
> This three-zone description applies to both the TLUD and BLUD
> gasifiers and pyrolysers though they produce different gas compositions.
>
> Regards
>
> Crispin
>
>
>
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