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

[Crispin Pemberton-Pigott]

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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