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

[Crispin Pemberton-Pigott]

Dear Andrew

>As Norbert has pointed out there are many top down updraught fires, but I thought we were discussing "migratory pyrolysis front" which is fairly specific to the clean burning class of stoves first mentioned on this list by Tom Reed and Ronal. 

We are and were, however it is subject to interpretation, and then Paul would like reserve the abbreviation TLUD for only those devices which are pyrolysers that produce charcoal. I maintain that he describes a sub-set of TLUD devices. A top-lit updraft fireplace and a top-lit updraft wood stove or coal stove or charcoal stove is properly described as a TLUD device, full stop. There are lots of them and they have been manufactured to operate in exactly that fashion since at least 1959 in this part of the world. By exactly I mean not only as TLUD configuration but in a pyrolysing air-choked, char making mode that, because of local preference, people choose to continue burning until all the char is gone. 

Just because Paul and others want to stop the combustion at a certain point and tip out the char does not mean the term "top-lit updraft" is applicable  ONLY when the conbustion is interrupted. 

>I only used the term TLUD as it is commonly understood on this list. 

So have I, but I didn't limit it to only TLUDs that are extinguished part way through the combustion of the fuel. 

>As I said I would prefer the term descending pyrolysis front and I was quite happy with Inverted Down Draught as Down Draught gasifiers, like imberts were described as that and inverting it meant something to me, it also introduced the
principle of gravity being involved which I liked as buoyancy of
offgas and superficial velocity are key to how it works.

I agree and perhaps it is significant that Tom Reed used the upside down moniker instead of the well known top-lit updraft gasification method name. Do you think he was perhaps clear about the difference? I am unconvinced the stove mob is. 
>... you miss my point, it is the combination of
words that becomes the common term.

Well Tom Reed was aware of the common terms and used the term inverted downdraft because it was a special case of a TLUD that differed from the common implementations at the time. He was clear in naming it IDD to identify it as a separate, distinct method. Is it that perhaps the stove enthusiasts did not appreciate the precision he applied to the distinctive name?

>>We need another term for the moving fire front (if you will) that is descending through the fuel in a downwards migrating direction.

>Well I'm happy with descending pyrolysis front in the case of
describing these type of stoves. I think descending flaming pyrolysis front may be possible under very low rates of superficial velocity but that's another discussion.

I am more comfortable if the discussion used the normal terms to describe the distinct processes with the oxidation layer being what is commonly called the pyrolysis front. The hottest layer is not the point of distillation. That happens immediately next to it. The water gas shift reaction takes place in the hottest zone so that is gas production, not but pyrolysis. 

Your mention of the superficial velocity brings to mind the note from Tom Reed that the batch loaded TLUD gasifier was a particular implementation of a gasifier, in which, with no other change at all, the device could be made to gasify all the fuel from the top down (superficial velocity higher than 1.0) or to make char (superficial velocity lower than 1.0). This is a significant point. 

Paul holds that when that device is operated with a SV less than 1 it is a TLUD and when the SV is more than 1, it is not. It is some other kind of device because that abbreviation is to be reserves for the char-making operating mode only. 

That is unreasonable. If it is burning fuel from the top down, it is a TLUD. If it has a low SV it is a char-making TLUD. It is true that both have a descending pyrolytic front, but with a higher SV it gasifies the char and when it is low, it doesn't. Whether it does or doesn't, it is a TLUD because that refers to the point of ignition and the direction of combustion air. 

Regards 
Crispin