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

[Andrew Heggie]

On 14 December 2017 at 15:20, Crispin Pemberton-Pigott
<crispinpigott at outlook.com> wrote:

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

As Paul has pointed out once you use the acronym TLUD, whether we
think it particularly appropriate or not, it is commonly understood,
on this list and probably globally, to mean the type of stove, top lit
with a descending pyrolysis front, that can be extinguished when much
of the volatiles have been flared off to leave a residue of char, for
whatever purpose.
<snip>
>
> 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?

Yet that name didn't stick outside of this list and Paul promoted TLUD
and has been instrumental in getting this type of stove accepted in
some special circumstances, I'm prepared to honour that contribution.

> 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.

Well I suspect you have got that wrong because the temperature and
massflow considerations do not favour that type of reaction and I
suspect the pyrolysis zone we are discussing is too narrow for it to
happen. As Phillip points out blowing with oxygen does it in a deeper
bed but that's a different ball game. Yes what happens in that narrow
descending pyrolysis zone is probably complex but we can look on it as
a moving black box inside which we do not need to know the reactions
but that the offgas is devoid of oxygen at certain superficial
velocities of primary air, char above the  black box is thus preserved
and heat from the black box is transferred to the layer below .
>
> 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.

Yes it is, but not relevant to this discussion, He also suggested the
char layer would need to be 20 particle diameters deep in order for
the char to be oxidised to CO, and that the reaction took place from
1100C down to about 800C, the zone we are discussing is narrower and a
lower temperature.  I have destroyed a number of flue tubes by forcing
air or oxygen through the biomass  to see what gasification I can
achieve but that's again a different ball game.
>
> 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.

I hadn't seen that , I would have to find some old notes to see what
superficial velocitiies I used but I suspect the Excess Air Value was
well under one or else you would have no char left.
>
> That is unreasonable. If it is burning fuel from the top down, it is a TLUD.

No it is a top lit updraught fire ;-), anyway that's Paul's fight.

Andrew