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

[Philip Lloyd]

Dear Crispin

 

This is exactly how the Lurgi gasifiers at Sasol work, except they use
oxygen not air.

 

Philip

 

From: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of
Crispin Pemberton-Pigott
Sent: Wednesday, December 13, 2017 8:10 PM
To: Discussion of biomass cooking stoves
Cc: riaz.ch_a at yahoo.com
Subject: Re: [Stoves] Understanding TLUDs, MPF and more. (was Re: Bangladesh
TLUD )

 

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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