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

Thank you for your comments, and for checking with an expert on gasification.

First, relating to previous messages.   My “contention” is not that whether up draft (UD) occurs or that top-lit (TL) occurs.   There certainly can be cases of UDTL or UD-TL or TL-UD occurring.   Call those “good scientific designations”.   But there are social and market and layman understandings of what is TLUD, which even has a pronunciation of “Tee-lud”, and years ago was often spelled T-LUD.  And TLUD is certainly associated with biomass fuels, not with charcoal burning stoves.

What is bothersome are attempts to re-direct the TLUD name / acronym toward combustion situations that often are not in the least bit interested in being designate as UD – TL.    For example, the Cambodian charcoal burner that you have described can be sold on its own merits, and even pointing out that it is UP-TL (or MAYBE even TL-UD).   But to push for referring to such stoves as being TLUD or T-LUD has the appearance of either 1) riding on a TLUD tailcoat,  or 2) trying to diminish the importance of “biomass-burning MPF top-lit updraft = TLUD” stoves.  The APPEARANCE is that you want to cause disruption or misunderstanding, not to enhance science.   For the sake of science, just use UP-TL if you need to discuss such features of the Cambodian charcoal or other such stoves.    And let TLUD, T-LUD (tee-lud) stand for what it is understood to be in “contempoary stove language.”

Second, relates to one of your paragraphs:  

He [the experienced gasification engineer] uses terms that differentiate the “MPF” into several discrete zones: drying, distillation and oxidation.

Drying.  Okay, clear.
Distillation.  I prefer the word pyrolysis, being chemical decomposition caused by heat.  Understood to include the vaporization of volatiles that do not require chemical changes, which is part of distillation.   Generally okay thus far.

But Oxidation as a discrete zone?  Let’s look at this.  In a retort, heat causes pyrolysis, witn no oxygen present except from within the hydrocarbons (and we have an on-going separate discussion about the amount and activity of that oxygen).  In the MPF inside a TLUD, cut off the O2 entering as primary air and the heat generation is not sufficient to maintain the MPF. 

And when there is O2 from primary air, that O2 is in close proximity to the combustible pyrolytic gases that are exiting from all sides of the pieces of biomass that are becoming covered with and the becoming entirely charcoal.  Oxidation occurs where those gases are mixing together, and that is not inside the biomass, but close to but not on the charcoal (unless forcing much primary air).  This is within the pyrolysis area (not the drying area) of the MPF.  It is not a discrete zone.

The expert continues with:

The water gas shift reaction takes place in the oxidation zone which he suggests is something that should be enhanced by steam injection or additional fuel moisture.

The water gas shift requires that molecules of H2O are in contact with the high heat of burning (at least glowing) carbon.  The moderately exothermic reaction results in H2 and 2 of CO.  Great if it happens.  Crispin suggests that some experiment be conducted: 

One method would be to measure the gas composition before it is combusted, and quantify the hydrogen content excluding the water vapour.

He said   “measure the gas composition before it is combusted”    Which gas might this be?   Is this the gases precisely as they exit from the multitude of pieces of biomass but before any combustion with the primary O2 as previously mentioned?   Or as the resultant gases just above the hot biomass/charring pieces?     Or higher up as it it passing through the created layer of accumulated char that is no longer glowing and cannot cause the water gas shift reaction to occur there?  

I am all for the experimentation, but I am not yet able to visualize (even in theory) where this water gas shift reaction is occurring and how to measure anything there.  

To me, more fundamental is the sensitivity of TLUD operations to the moisture content (MC) of the fuel.  The char that is being created via pyrolysis/carbonization is bearly glowing, and the object is to have sufficient heat generated to continue the pyrolysis into the center of each piece of fuel.  The water gas shift reaction (to the extent that it might be occurring) is going to take away some (much?) of that heat when making the H2 and CO that can be usefully burned at the level of the secondary combustion.   But heat loss at the pyrolysis zone could be detrimental to the continuation of the necessary pyrolysis.  I am raising this as a question.

Also, Crispin can ask the gasification engineer (or combustion chemists or others) about what was meant by the discrete zone of oxidation.  Was that referring to the zone of char-gasification, which is the direct burning of the carbon?   THAT zone does exist in full fledged gasifiers.   But TLUDs are only utilizing the pyrolysis part of gasification.  [We note that the word gasification is used in different ways that sometimes exclude pyrolysis and focus on the char-gasification process.  That is why I prefer to say that gasification consists of pyrolysis and char-gasification in order to recognize both key processes.]

This sentence bears repeating here:

The water gas shift reaction takes place in the oxidation zone which he suggests is something that should be enhanced by steam injection or additional fuel moisture.

This is excellent and standard practice in bottom burning downdraft classic full-fledged gasifiers.   In them there is an oxidation zone that consist of a layer of charcoal (that was earlier produced inside that gasifier).  It is the bottommost layer (zone).  Air (with O2) is injected and it is white-hot, the kind of heat that can melt metal.  So one way to lower that temperature and to obtain the benefits of the water gas shift reaction is to drip in some water.    ——–   The very clear reason that this advice is not applicable to TLUD gasifiers is that the char bed is starved of oxygen and the char accumulates for later extraction.   

Sorry that it took me a while to sort this out, but I hope that what is written is clear.  I believe that the expert on gasification will understand and agree that the comments about the absence in TLUDs of a discrete oxidation zone that would be the place for the water gas shift reaction, as in the traditional usage of such phenomena in char-gasification gasifiers.

Paul

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

MWHPR22MB07845074B2911AF0C2547F96B1350@MWHPR22MB0784.namprd22.prod.outlook.com“>

Dear Friends

 

Further to the earlier discussion about the nature of and terms for the pyrolysis zone. I have consulted one of the most experienced gasification engineers in the world to ask how, over his career of 65 years, the terms used for describing the zones have evolved.

 

First, he said that he was unfamiliar with the term ‘migrating pyrolysis front’. If the term MPF is meant to describe the whole active zone from the point at which the fuel dries, then commences pyrolysing up to the point at which there is hot char no longer releasing gases, we can refer to that as the active zone.

 

He uses terms that differentiate the “MPF” into several discrete zones: drying, distillation and oxidation. The water gas shift reaction takes place in the oxidation zone which he suggests is something that should be enhanced by steam injection or additional fuel moisture.

 

As was reported by Tom Reed and confirmed many times since, adding moisture to the fuel reduces the char yield and gives more gas, and it should be higher energy gas. The water gas shift reaction produces CO and H2 from the fuel moisture extracting heat from the oxidation zone. This phenomenon has been discussed on this list but if I recall correctly, it was always on the basis that the fuel moisture ‘required more energy to remove it’, not that there was a water gas shift reaction taking place generating more combustible gas, using the carbon in the process.

 

Can anyone think of an experiment to prove it? One method would be to measure the gas composition before it is combusted, and quantify the hydrogen content excluding the water vapour. If it exceeds the mass that is available from the fuel, then the water gas shift reaction could explain it.

 

An advantage of the three-zone description of what takes place in the MPF is that it an be applied to all gasification, not just pyrolysation. Described previously is the charcoal TLUD that can be described as having a descending oxidation zone in which gases are created resulting in the reduction of char to ash instead of biomass to char. It is incorrect to call it pyrolysation, and MPF is therefor inappropriate. An “oxidation zone” applies to both the pyrolysis and gasification processes. Thus they both have a migrating oxidation front descending into the fuel. One has a migrating pyrolysis front that includes in it (if so defined) the oxidation, drying and distillation zones.

 

This three-zone description applies to both the TLUD and BLUD gasifiers and pyrolysers though they produce different gas compositions.

 

Regards

Crispin

 

 

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