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

Crispin Pemberton-Pigott crispinpigott at outlook.com
Sun Dec 10 09:04:38 MST 2017


Dear gordon

You almost nailed it. Heating some small portion of the biomass and the temperature of the nearby fuel (not all of it) is sufficient to ignite a reaction that can continue indefinitely provided there is sufficient insulation and the oxygen content of the fuel is sufficient.

There are two approaches that will work, as I understand it: the first is the charring of underground biomass (or coal) in the absence of additional oxygen. Once started, it will continue until the fuel is exhausted.

The other does not require heating, but uses compressed air (oxygen) to provide the necessary reaction oxygen rapidly enough to get the reaction to continue (the Hawaiian invention).

An MPF is the front at which the heat releasing reaction takes place.

You can also consider the way char is formed. In the initial stage the energy needed to drive off the endothermic volatiles is more than the energy released by oxidizing the gases using only the oxygen available in the fuel. As the system heats up, the energy gap between evaporating volatiles and exothermic reactions decreases. At some point, the gap is small enough for the released oxygen to continue in the absence of additional air.

TLUDs are in general too small to see this take place. They cool off too easily so additional air much be provided to sustain the reaction, which burns the rest of the hydrogen and at least some of the carbon.

An underground fire can continue for centuries. All it has to do is get started. Prof Lloyd can explain how these fires can even auto-ignite. You have probably heard of stacks of hay setting themselves on fire by decomposition of the material in a large enough pile to retain the heat. Once it reaches or exceeds about 290 C the reactions are exothermic and from there it is a short trip to charging temperatures. The whole haystack can turn to char save the cool exterior.

Burning coal seams manufacture their own oxygen from water. Coal doesn't have enough oxygen to sustain a pyrolytic front unless it is really hot. But burn they do.

You can determine if there is an MPF but putting in a set of thermocouples and watching the front pass by. A burning log in an open fire has an MPF inside working it's way to the centre. The whole log will be charred long before it splits open.

The interesting thing about a packed bed gasifier is that it can make gas predictably and controllable from ‎small sized fuel generally considered a waste material. It is no small achievement.

Regards
Crispin


I took a look at the Adams Retort design and watched a few videos. A couple of questions come to mind:

1) How does one determine that an MPF exists in an A-R?

2) I am not much of a chemist, but I would be interested to see an explanation of the reaction sequence where O2 is released and then burned in the MPF of an Adams Retort.

It seems to me that if the external heat source is raising the temperature to the gasification point of the volatile components, thereby causing gasification (duh…), what purpose is the MPF serving?

In a TLUD the MPF is burning some of the volatile gases to generate the heat needed to gasify un-pyrolyzed biomass.

Gordon



On Dec 9, 2017, at 10:54 PM, Crispin Pemberton-Pigott <crispinpigott at outlook.com<mailto:crispinpigott at outlook.com>> wrote:

Dear Paul

>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.

Of course there were things left out. I am happy you included below further refinements to the explanation.

>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.

Au contraire. The Adam Retort (which heats the biomass from under a portion of the bottom of the pile, not multiple sides) does indeed have a MPF and the combustion is fed by oxidation using oxygen liberated by the thermal decomposition of the fuel. There is no added oxygen, but oxygen there definitely is. There is enough oxygen in biomass to almost completely combust the hydrogen content. In practise the carbon and hydrogen are both involved in sustaining the MPF. Once the fuel is hot enough, the MPF can move through the pile without any additional heat required.

>Sorry, that should say "has external heating proceeding inward from several directions from the sides toward the center."

That is an incorrect description of the Adam  Retort. There are retorts that function in the manner you describe. His is not one of them.

>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.

That is true for some retorts, not the Adams Retort.

>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 [sic] flow of primary air) to create the char as the gases move upward.

The same also happens in a downdraft pyrolyser, and a bottom-pit updraft pyrolyser such as those produce by Hirendra Chakrabarti in India.

The addition of a control feature makes the process variable, however it does not fundamentally change how the system works.

>You may choose to try to equate the two descriptions above, but that would diminish your credibility as a precision-seeking scientist.

There is no need for that kind of talk. First familiarise yourself with the systems under discussion. Seen from the side, an Adam Retort starts the MPF on the bottom right and it moves over a period of about 2 hours to the top left.  Prof Lloyd can provide examples of sustained MPF’s moving horizontally underground in the Witbank area of South Africa without any addition of primary air. There are conditions where primary is required, and others when it is not. Tiny stoves require it.

>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.

That is why the discussion is on principles not ladybugs. A dry tree stump can be ignited to char the stump and the roots underground, and in the 1800’s one could buy a kit for doing so. I was told of a different method when I was young involving a steel pipe and kerosene. The point was to get the stump hot enough so that the pyrolysis would continue into the ground. The TLUD stove with an MPF is no more than a containerised version with controllable primary air.

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).

Ron is correct about the O2. It is indeed different (I don’t know about ‘vastly’ from stoves where the fuel and flames are ‘attached’ to the solid fuel and the gases oxidise as and when they can. it is also true (but I didn’t expound on it) that within a fuel bed there are zones without no available O2, just as in the zone above the char.

>You are saying that is good as preheated secondary O2.

Functionally that is secondary air provided simultaneously, at least in some cases. My definitions are general cases and can be applied to the situation you describe.

>Maybe, but not really.  It is simply inadequate combustion, with the results of undesirable emissions.

That is speculation and cannot be true in all cases.

>The excessive primary air (which you state should be secondary air)...

Which I state has the function of providing 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.

That is not a general case. It could happen.

>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.

That is not the case. What ‘problem’? Heat passed through a metal wall to secondary air is not ‘different heat’ from that gained by air passing through a fuel bed, or along a hot combustion chamber wall. There are many roads to Rome. Air that is consumed after preheating is indifferent to hwo the heat was gained.

>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)

I see this as inaccurate on two counts: 1) the secondary air may be added in a way that decomposes complex gases further before final combustion, and 2) the bit about poor design is unfair comment. There are many good designs you may not know about.

>…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).

Tertiary air may be provided for a number of reasons, but normally to provide the oxygen needed to complete combustion with a desirable level of free O2 in the gases. Under certain circumstances, for example natural gas boilers, the level of free O2 is very low (1% by volume). To maintain such a low level of excess air it may be necessary to have tertiary or even more air injection points.

>Either way, the concept of "tertiary air" is bogus in the context of cookstove combustion.

That is opinion only, contradicted by existing technologies that use it. One of the highest performing stick-burning stoves available has tertiary air injection – the Rocketworks stoves from Adrian Padt in Durban, tens of thousands of which are sold each year.

>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.

That is true in some cases and not in others. Most Rocket stoves (per Aprovecho design guides) have only a single air entrance. The Rocketworks stoves have three distinct 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.

I am sorry to hear you talk about ‘fields of expertise’. Facts in evidence speak for themselves and are not invalidated by any messenger claims.

>>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.

So what?

>And I do not accept your statements.

I don’t care. I cared to know, by interviewing him, how the Amerindians farmed and what role char had in their agriculture. I reported this previously to this list. Maybe you forgot. It is in the archives. At the time Ron replied that did didn’t accept the observational reports from Cecil who lived in the jungle for 4 years learning how their agricultural society functions.

>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.

He reported observations, the most important of which is that they do not practise slash and burn farming over whole areas, but only the fertile bits. The practice of slash and burning creates a lot of char in the ground, something contested by Ron at that time and diminished by your message regarding the tree stumps burning in California. If you don’t believe me, you can visit the area right now and witness the MPFs moving into the ground to charcoal the root systems. A major part of the work of firefighters is extinguishing these MPFs. This is sufficient to convince me they exist. If they exist, they are producing biochar in the ground.

>You can take that topic to the Biochar Listserv, if you want to discuss it further.

I have no interest in discussions on another list. I receive information from the Biochar-Ontario group.

>Sorry, I reject much of what you wrote.

Don’t be sorry. Learn and be happy! We are sharing perspectives here.

>I would not want you to be instructing people about TLUD stoves and char making and Terra Preta.

That is none of your business over which you have no control.

>IMHO, your comments about pyrolysis in different devices reflect poorly on your credibikity [sic] regarding your other strongly expressed positions, but that is a topic for others to discuss.

And I feel sometimes you could learn a thing or two before posting comments about other technologies. Like everyone else, I find your contributions worthy and partially correct.

Perhaps there is something you could answer: Is there something about char making that requires or drives their proponents to make completely unnecessary personal attacks and derisive, inaccurate speculations about motives?  Outside the agricultural sector and the production of char-enhanced fertilisers, I find the biochar enthusiasts as a group to be cloistered and fanatical, frequently engaging in divisive speech about something they have recently discovered.

I should take a minute to compliment you, Xavier, Julien, Jonas Haller and Dr Nurhuda for being very open to exchange with me advice, explanations and observations without reservations about how or where the information originates. All of you have produced very interesting and effective products suited to certain communities.

Best regards for a better burn
Crispin

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