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

Sun Dec 10 12:38:47 CST 2017

Gordon,
You may be asking for too much.  Quality char and good combustion and an
idle standby??....mans reach should exceed his grasp I suppose:)
Alex

On Sun, Dec 10, 2017 at 12:10 PM, Gordon West <gordon.west at rtnewmexico.com>
wrote:

> I would like to add some context to my interest in MPFs. I will also post
> this on the biochar list, but as there is some relation to stoves I’m also
> keeping it here.
>
> Our systems are designed to be community-scaled “industrial”, and because
> of that we have an interest in being able to modulate the pyrolysis
> reaction down to ”idle” when heat is not needed, then to start it back up
> on demand (in a continuous feed device, not a batch system). We have been
> able to do this, but not in a reliably smooth manner. So it is very
> important for us to understand the processes happening at the MPF in detail.
>
> One thing we know is that we can snuff the MPF by sealing off all external
> inputs of air. In our experiments, a small quantity of air leakage, from
> either the bottom through the biomass feedstock, or from the top coming
> down through finished char, keeps the MPF hot enough to restart upon the
> application of sufficient primary air. During idle it produces a small
> amount of very foul smelling gas that is of insufficient density to burn on
> its own. Seal off all air and the MPF snuffs completely within a short time
> and the MPF cools below ignition temperature.
>
> Gordon
>
>
>
>
> On Dec 10, 2017, at 9:41 AM, Gordon West <gordon.west at rtnewmexico.com>
> wrote:
>
> Is there a report on monitoring an Adams retort MPF using thermocouples? I
> am still not successfully visualizing the process.
>
> I did a bit of a search of technical reports on the chemistry of woody
> biomass pyrolysis and have not yet found a reference to the release of O2
> from heating the feedstock and its subsequent recombination with other
> elements resulting in an MPF. Several analyses of syngas that I saw show
> plenty of oxygen but it is bound in various fuel molecules.
>
> Gordon
>
>
>
>
>
> On Dec 10, 2017, at 9:04 AM, Crispin Pemberton-Pigott <
> crispinpigott at outlook.com> wrote:
>
> 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> 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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