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

Paul Anderson psanders at ilstu.edu
Sat Dec 9 23:00:23 CST 2017


Crispin,        (Friends are not required to agree.)

Some of what you wrote is correct, but you go too far or leave out some 
important distinctions, thereby implying that some things are equal, 
that is, some are not really as worthy and distinctive as others (Ron 
and Paul) have stated.  I give you four cases:

1.  Pyrolysis progresses (migrates) through a piece or pile of biomass 
as heat is able to penetrate.  True.   The Adam retort (and other 
retorts) heat the biomass from many sides, but there is no flame (actual 
combustion) inside.  It is a stretch of language to say that the 
pyrolysis in the retort
> has an MPF [migrating pyrolytic front] proceeding in a roughly 
> horizontal direction.
Sorry, that should say "has external heating proceeding inward from 
several directions from the sides toward the center."   It is no more of 
a "front" than having the heat in a home reach to the center of the 
house from all of the furnace duct registers in each room. Furthermore, 
the processes in a retort are in sequential order during numerous 
minutes to 1) heat virtually all of the biomass to drive off the 
moisture, then 2) add more heat to virtually all of the biomass to drive 
off the low-temperature volatiles to create torrified material, and than 
3) to finally get the temperature hot enough to have actual pryrolysis, 
first in the 400 deg C range and then at higher temperatures if the 
external heating is continued.

In sharp contrast, in a TLUD there is a distinctive zone of a few cm 
depth with actual small amounts of combustion (of some of the initial 
offgases).  That zone migrates through the pile of biomass, heating 
mainly by radiation a small layer of biomass below the MPF, then with  
pyrolysis at a rather uniform temperature (generally from 450 to 700 C, 
depending on USER CONTROLED flow of primary air) to create the char as 
the gases move upward.

You may choose to try to equate the two descriptions above, but that 
would diminish your credibility as a precision-seeking scientist. And to 
equate a smouldering tree stump to MPF is a stretch beyond a stretch.  
Yes, there is fire there, and yes there is pyrolysis occuring.   But 
such comparison is akin to saying that a ladybug and a moose are the 
same because both are living creatures; and in fact both are animals.   
Even a petunia and a whale are both living organisms.  Scientific 
comparisons need precision and detail.

2.  Ron correctly pointed out that in normal operation of TLUD stoves, 
there is zero O2 that gets past the MPF.  That is vastly different from 
having so much primary air enter (as in Rocket stoves and other burners) 
that there is O2 moving upward above the fuel (where the  gases were 
created).  You are saying that is good as preheated secondary O2.  
Maybe, but not really.  It is simply inadequate combustion, with the 
results of undesirable emissions. The excessive primary air (which you 
state should be secondary air) has had a cooling effect on the raw fuel 
that the combustion is trying to get hot enough to give off the 
pyrolytic gases.  In other words, that secondary air that has passed 
over the fuel is part of the problem, not part of a solution because it 
has taken heat away from the fuel.

Your statements therefore are misleading, and a dis-service to any 
novice trying to understand the important differences about where and 
how primary and secondary air enters and performs.

3.  Tertiary air:   Defined as either 1) the needed secondary air that 
did not get into the combustion zone soon enough (meaning that there was 
poor stove design) or 2) excess air that will lower the temperature of 
the hot gases (meaning that this "tertiary air" was NOT needed nor part 
of the combustion process).

Either way, the concept of "tertiary air" is bogus in the context of 
cookstove combustion.  The objective is to get the combustion to be 
complete with the proper amounts of primary and secondary air at the 
correct times and places.

4.  You wrote:
> This [MPF} process is ... how the Terra Preta soils in the Amazon were 
> created over 20,000 years of slash and burn agriculture. 
The origins of Terra Preta are still being debated by the experts in 
that field, which is certainly not your claimed field of expertise.
> The Amerindians cultivated land that was already productive, they did 
> not create it /de novo/. Cecil confirms they farm patches of land that 
> are already productive, not random areas. He observed this when he was 
> doing PhD field research while at Harvard.
I have great respect for Cecil Cook.   But somehow what you have 
attributed to him and his work has not reach my attention previously.   
And I do not accept your statements.   What happened thousands of years 
ago was not witnessed by Cecil.   He could only observe the current day 
activities on lands that somehow became fertile.  You can take that 
topic to the Biochar Listserv, if you want to discuss it further.

Sorry, I reject much of what you wrote.  I would not want you to be 
instructing people about TLUD stoves and char making and Terra Preta.   
IMHO, your comments about pyrolysis in different devices reflect poorly 
on your credibikity regarding your other strongly expressed positions, 
but that is a topic for others to discuss.

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

On 12/9/2017 9:03 PM, Crispin Pemberton-Pigott wrote:
> Dear Ron
>
> I see no systematic differences between the uses of the terms. You are 
> trying to create some special meaning for quite ordinary processes of 
> combustion.
>
> It is unfortunate you mix baseless assertions, projections of motive 
> onto others and micro-aggressions ‎into your erroneous analyses and 
> combustion theory. It is wearying trying to separate your mistakes 
> from your agendas.
>
> For the rest of our readers, these definitions may be useful:
>
> Primary air refers to that atmospheric oxygen used to convert a 
> combustible fuel into a gaseous state through thermal decomposition. 
> It is needed to combust gaseous fuels if the gas is not composed of 
> molecules that can be oxidised directly (evaporated kerosene, for 
> example).
>
> Secondary air is that atmospheric oxygen which is used to combust 
> those ‎gases at some location generally distinct from the solid fuel.
>
> Tertiary air is that atmospheric oxygen separately added to the gas 
> combustion phase in order to complete the combustion and or to 
> moderate the gas temperature‎ to fall within some desired range (for 
> example a low-NOx burner or a fish dryer).
>
> While a TLUD's migrating pyrolytic front (MPF) can produce char from 
> biomass, there are other methods of doing so. Two that have a higher 
> theoretical char yield ‎are the charcoal kilns designed by Chris Adams 
> (the Adam Retort) and the World Stove. Their working principles are 
> not the same as the TLUD. Both produce a combustible gas that is 
> applied to a process outside the charring of biomass.
>
> In the char producing ‎section of the Adam Retort there is no primary 
> air supplied at all but it still has an MPF proceeding in a roughly 
> horizontal direction. The World Stove supplies some primary air in the 
> beginning but only secondary air later. Still later, primary air is 
> again supplied if the operator wishes to burn the char produced.
>
> All three of these approaches have proven effective. Of the three, the 
> Adam Retort has the highest char production fraction per kg of input 
> material, the widest range of usable materials and the least possible 
> preparation required. The seven drum kiln designed by Dr AD Karve 
> operates on the same principles and is specialised to char leaves. A 
> small percentage of the fuel ends up in the ground as biochar.
>
> A higher fraction of the char ends up in the ground‎ when it is 
> produced using the earth pit charcoal production process. I have not 
> seen this discussed on the Stoves list. An investigation by Cecil Cook 
> in Mozambique into the ultimate destination of char conducted while 
> working for ProBEC/GTZ showed that a considerable fraction of char 
> produced ended up under the pot and the rest ended up on or in the 
> ground, which you have termed variously as 'sequestration' and some 
> form of fertilization.
>
> ‎There is nothing magical or new about an MPF and it does not need a 
> special vocabulary to describe it. In Southern California the 
> devastating forest fires are leaving behind millions of smouldering 
> tree stumps each of which has an MPF moving along its stump and major 
> underground roots. This process is as old as fire. It is how the Terra 
> Preta soils in the Amazon were created over 20,000 years of slash and 
> burn agriculture. The Amerindians cultivated land that was already 
> productive, they did not create it /de novo/. Cecil confirms they farm 
> patches of land that are already productive, not random areas. He 
> observed this when he was doing PhD field research while at Harvard.
>
> I hope others can get contribute to especially the technical aspects 
> of the discussion. I will close with a reminder that the highest char 
> production fraction I have encountered (45%) is the Hawaiian 
> pressurised kiln which uses a controlled primary air supply and an MPF 
> to char a five foot long log one foot in diameter in 45 minutes. "45 
> in 45".
>
> ‎Amazing.
>
> Regards
> Crispin
>Ron Larson wrote:
> Todd:
>
> Thanks for the comment.  I urge readers to visit 
> https://www.silverfire.us/ 
> <https://eur03.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.silverfire.us%2F&data=02%7C01%7C%7C2f4e87e571974e97b4cf08d53f3a03ae%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C636484440759723211&sdata=2uvPfW50fAZhf81ZYqCzad40wiN2th9I1xzu7%2BCy7wM%3D&reserved=0>. 
>  Some nice looking designs.
>
> I can’t quite accept your message below (and one from Crispin that we 
> received 1:23 hours later).  You are both using the terms primary and 
> secondary in a different (not wrong - just different) sense from those 
> of us interested in char-making.  We char-makers (or at least the TLUD 
> design - Nat Mulcahy and World Stove have a still different meaning to 
> this nomenclature) use primary air (meaning oxygen) as something that 
> is totally depleted at/near the pyrolysis front.  No oxygen in the 
> exiting pyrolysis gases, where they are met by secondary air (oxygen).
>
> I am pretty sure you and Crispin (who is also discussing char-using 
> stoves) do not mean the above.  Rather “primary” means plenty of air 
> (controllable to a minimum degree) to both create and consume char. 
>  Your and most Rockets (regardless of pre-heating) find char to be a 
> defect;  you would have preferred to consume it, I believe. (I’d love 
> to hear that you welcome the char.)  So your “secondary” is to be sure 
> that the “small” amount of uncombusted gases leaving the fuel bed are 
> combusted.   I am NOT disputing your phrase below (“Secondary 
> combustion /is possible /with a rocket stove”).  I am only saying that 
> secondary air is mandatory with TLUDs, and has a totally different 
> function than “primary” air.   Most of your primary air consumes char.
>
> The dialog between Paul and myself below on secondary air is different 
> than your own explanation below on its preheating.
>
> Do we agree that the terms  “primary” and “secondary” have very 
> different meanings in these two different parts of the stove world?
>
> Ron
>
>
>> On Dec 9, 2017, at 12:32 AM, Todd Albi <todd.r.albi at gmail.com 
>> <mailto:todd.r.albi at gmail.com>> wrote:
>>
>> Ron:
>>
>> Not quite correct.  Secondary combustion/is possible/ with a rocket 
>> stove.  Our Survivor Rocket stove launched 5 years ago introduced 
>> preheated ventilation ductwork at launch.  In fact we only offer a 
>> primary & secondary combustion design rocket stove. Thats why our 
>> stove incorporates 360 degree ventilation through base of stove, tied 
>> to ventilation ductwork behind combustion chamber walls. The double 
>> insulated walls allow preheated air to travel behind firebox and mix 
>> at chimney base, before exiting cooktop.  Others have also now added 
>> preheated channels and secondary gasification to rocket stoves. It is 
>> not an inclusive to TLUD designs and can be adapted to any stove 
>> firebox.
>>
>>

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