[Stoves] Understanding TLUDs, MPF and more. (was Re: Bangladesh TLUD )

Nikhil Desai pienergy2008 at gmail.com
Thu Dec 14 12:59:31 CST 2017


Crispin:

This discussion has been very informative for me. Thank you.

However, while I understand agree that "adding moisture to the fuel reduces
the char yield and gives more gas," I am puzzled by the dogma that "fuel
moisture ‘required more energy to remove it’."

I interpret this to mean that higher-moisture fuel, for a given stove,
scores low on the efficiency metric. Am I correct?

Should it not be that one takes moisture level as a given - there goes the
theology of oven-dried wood and computing efficiency of boiling water - and
design stove functions around it, the actual fuel? `

That is what I interpreted from Frank's insistence on fixing the fuel.
Where possible, the moisture content can be adjusted - either drying in the
sun and storing for rainy days or deliveries of pellet or charcoal.

As far as I can tell, "wood gas" and "char making" are enough rationales
for "more complete union" of carbon and oxygen. The obsession with
efficiencies

Energy should be wasted. Economically. Then it is not waste.



Nikhil


On Dec 13, 2017, at 1:10 PM, Crispin Pemberton-Pigott <
crispinpigott at outlook.com> 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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