[Stoves] Correcting a misconception that approaches myth status

[Paul Anderson]

Stovers,

Truth:  TLUD stoves are very different in combustion from simply turning 
a down-draft gasifier upside down.
Myth and misconception:  That TLUD and DD combustion is the same except 
upside down.

My comments are stimulated by what Crispin Pemberton-Pigott who wrote:
>
> A TLUD is an upward burning downdraft stove and there is a great deal 
> in common with them. There are investigations and patents going back 
> to the 17^th century. FD-downdraft stoves are rare though known. There 
> is a good reason why Dr Tom's stove was called an upside down 
> downdraft combustor. It was a downdraft stove turned over, burning the 
> same fuel in the same manner internally, with the fire on top.
>
Tom Reed called his innovation "Inverted Down-Draft" or IDD for short in 
the 1980s and 90s.   He was inspired by what he knew about true Down 
Draft (DD) gasifiers.   But names and reality are not always clearly 
matching.   In 2004 - 05 I re-named it Top Lit UpDraft (TLUD) which is 
also not totally accurate.   None of the names acknowledge the Migratory 
Pyrolytic Front (MPF) that is the dominant feature of what is commonly 
called TLUD micro-gasification.

Note:
A DD gasifier is ignited at the BOTTOM, and the fire (hot zone of 
gasification) REMAINS at the bottom, and the unit can be operated with 
continuous fuel entry into the top.

A IDD or TLUD or MPF gasifier is ignited at the TOP, and the fire (hot 
zone of gasification) slowly MOVES to the bottom, after which time the 
fire zone stays at the bottom and the unit operates as if it were a 
regular UpDraft (UD) gasifier if anyone puts more fuel into the top.

Fundamentally different.

Because of Tom Reed and also Paal Wendelbo who worked totally 
independently, we have something new for cookstoves.   They did not 
invent pyrolysis.   They were not the first people to have a fire in a 
barrel making gases and have the flames only up at the top where 
secondary air was available.   (I saw that in the 1950s in a burn barrel 
at my home.) But Reed and Wendelbo were the ones who CONTROLLED it and 
understood it and made it all small enough to be useful in a 
cookstove.   And the rest is history.   And TLUD history is still being 
made.

Paul

Doc  /  Dr TLUD  /  Prof. Paul S. Anderson, PhD
Email:  psanders at ilstu.edu
Skype: paultlud      Phone: +1-309-452-7072
Website:  www.drtlud.com


> >For now what we need are some very basic empirical studies on ND-TLUD 
> performance, so that builders of stoves can make more deliberate 
> decisions, so that rural extension workers can have confidence that 
> their advise is well-grounded, and so that both can respond 
> effectively to unexpected situations.
>
> That would be great. I wouldn't second-guess what people are doing or 
> have done, and not published it in a way you could find it. There is a 
> lot in Chinese which is not very accessible.
>
> >We need to start building ND-TLUD science at a very basic 
> empirical level.  Nobody has actually measured primary and secondary 
> air flow, so nobody really knows the their optimum proportions for low 
> emissions and how that changes with gasification temperature, types of 
> burner and fuel.
>
> I believe that has been done for BLDD gasifiers long ago. Such 
> gasifiers were used for making town gas. It is (as I understand it) 
> not wise to estimate that the gases produced somehow set the emissions 
> of the fire. That is a bit like saying ethanol is a 'clean fuel'.  I 
> can suggest that you try two sets of experiments: use a fixed burned 
> design and vary the gas production parameters, then see how the burner 
> handles the gaseous fuel.
>
> Then leaving the gas production in one of several starts, vary the 
> burner to get the best burn for the gas it happens to produce. What 
> you will find is that there are dozens of combinations that produce 
> very low emissions. The combination of a gas generation method 
> (creating a significantly different gas composition) will burn best in 
> a particular architecture of burner, mixing and secondary air 
> pre-treatment regime.
>
> >Nobody has actually proven that preheating secondary air is effective; 
> it is just assumed to work, even though we are taking heat from the 
> reactor to create more viscous and less dense air.
>
> I have personally conducted many tests involving this and so has Roger 
> Samson who has been manufacturing rice hull gasifiers for years. I 
> have been making downdraft stoves more than 10 years and they all have 
> (now) preheated secondary air because the benefit is so great.  The 
> effect of preheating is strong, reproducible, and is best demonstrated 
> when using a gas analyser which tracks the CO/CO2 ratio. Completeness 
> of combustion is a good metric. Taking heat out the combustion process 
> generates CO sooner than PM.
>
> ND-TLUD design has been investigated by me and Prof Lodoysamba many 
> times in Mongolia, first over a period of 18 months, and later for 
> several shorter periods.  We have many hours of tests showing the 
> clear advantage of preheating the secondary air and how to introduce 
> it, and more hours of showing the poorer results of not doing so. Two 
> stoves that failed to make the cut this year were failed precisely 
> because they did not incorporate secondary air preheating, combined 
> with poorly conceived operating instructions.
>
> >If preheating makes no difference, then it becomes easier to design 
> the gas burner and the gasification reactor as separate modules, that 
> are mixed and matched according to need.
>
> Yeah, but it is a big damper on combustion efficiency (pun intended).  
> Cold air entering the gas stream simply cannot compete with hot air 
> entering the same gas stream. Cold air injection produces a combustion 
> efficiency that will meet US and EU CO targets, but the approved 
> stoves in Ulaanbaatar demonstrate a far lower sustained CO level -- 
> often 0.03% or lower. This is only achieved in those TLUD's that have 
> preheated secondary air and have it injected in the right amount in 
> the right manner.
>
> >Nobody has shown that TLUD biochar has safe levels of polycyclic 
> aromatic hydrocarbon and dioxins.
>
> No idea. I have not researched what is in biochar though I think the 
> Japanese have been looking at that for some time.
>
> >All this, and more, needs clarification in replicated, controlled 
> experiments ... and open access publication.
>
> There is a significant problem with that in that as the money pool 
> spent on stoves drives the creation of larger private participants, 
> they have proven to be less likely to share as openly as in the past 
> (unless it is in the patents they file). This group is far more likely 
> to participate in a sharing exercise, but even here, times they are 
> a-changing.
>
> I appreciate your willingness to share your work and thoughts about 
> your investigations.
>
> Regards
>
> Crispin
>
>
>
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