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Dear Crispin, Tom and other Stovers,<br>
<br>
Crispin has given his interpretation about understanding
gasification, and much is correct. <br>
<br>
He did a disservice (in my opinion) by presenting the concept of
distillation ALMOST as if it is the same as pyrolysis, which I am
sure has some (perhaps many) advocates for that word. I am not one
of them. But as long as whomever is writing defines how he or she
is using the words, we can live with people saying things in
different ways. So, if you want to discuss distillation of wood
(such as making wood vinegar), or distillation as making charcoal
plus gases, or distillation in the common usage as what is stated in
the dictionary, please state that clearly. I will use pyrolysis
without splitting some hairs.<br>
<br>
Question: If a piece of biomass (being carbohydrate molecules) is
heated in the absence of O2 from air, and the oxygen atoms inside
the biomass are rearranged (which is recognized as happening in
pyrolysis and carbonization), with some of them becoming CO or CO2,
is that to be called oxidation? <br>
<br>
And if oxidation is "burning", is the "oxidation zone" occurring
outside of the piece of biomass or inside at the moment of
pyrolysis? The atoms reorganized. Pyrolysis and carbonization
(same process viewed from two opposite sides of the same coin) have
occurred. But did oxidation occur? I find no reason to use the
term oxidation in the context of what happens to oxygen atoms during
pyrolysis. <br>
<br>
Crispin says that TLUD gases are not tarry gases like those of
standard Bottom Burning UD gasifiers (similar to incinerators).
TLUD gases (not yet combusted with secondary air) certainly are
tarry, smoky, long-chain hydro-carbons. TLUD pyrolytic gases are
NOT cleaned up as they pass through the accumulated char that is
above the pyrolytic zone. SOME tars are deposited on the char, but
nothing remotely close to being a cleaning process. This is Tom
Reed's IDD Inverted Down Draff, and that would certainly be
interpreted as saying it is an upside-down BBDD (bottom burning down
draft). And yes, it is lit (ignited) at the bottom, but the main
burning always stays at the bottom. So BLDD is not as accurate as
BBDD. It is only because of the recognition and "popularity" of
TLUD (Top LIT) that people started saying BL when they really refer
to BB.) <br>
<br>
In contrast, a true BBDD gasifier is different from an inverted BBDD
(or IDD). <br>
1. The pyrolytic front does not have the distinctive downward
progression (nor upward in the sense of a clearly defined zone, but
pyrolysis could spread upward to become a meter thick area being
pyrolyzed at lower temperatures as one goes further up.)<br>
<br>
2. In a BBDD, the charcoal that is created is at the bottom.
Right, that is opposite of the char on the top in a TLUD (IDD).
However, that char in the BBDD is being fed air (O2) via tuyeres
(nozzles). That char is HOT, super hot, like a forge, which is not
the case in the TLUD (IDD). <br>
<br>
3. The TLUD pyrolytic gases move upward, away from the hotest zone
at the pyrolytic front. The BBDD also has pyrolytic gases that are
moving downward (correct, that is opposite of the TLUD direction)
(downward movement because of either being pulled (sucked) to a
bottom exit under the grate or pushed because the BBDD is sealed and
the created gases are increasing the pressure and the only exit is
through the grate.) There is no pressure increase in the TLUD (IDD)
(just another difference being pointed out.)<br>
<br>
4. Well, when those pyrolytic gases in the BBDD reach the super hot
burning charcoal, they undergo chemical reactions the (ideally) will
break down those tarry pyrolytic gases to become CO, some but
hopefully not much CO2, H2, methane, and short chain hydrocarbons.
NONE of that occurs in the TLUD (IDD) gasifiers.<br>
<br>
Crispin wrote (next to last paragraph before the conclusion) (my
emphasis added):
<blockquote type="cite">
<pre wrap="">The TLUD was described by Tom Reed as an inverted downdraft gasifier. In fact, this is a good description. The same thing happens ‘upside down’ in a downdraft gasifier: the oxidation zone heats and dries and distills the fuel above it, a<b><u>nd all gases pass through the hot char. Identical result: low tar low, energy gas.</u></b>
</pre>
</blockquote>
Patently NOT correct. I respect Cripsin's world expert on
gasification, but I think he is not saying what Crispin wrote. I
hope not. Tom Reed will celebrate his 92nd birthday is week, but
he is less active now. So I cannot pass this debate to him. But
what I wrote can be confirmed by others.<br>
<br>
So, if anything else that Crispin wrote below does not match with
what I have written above, please put it in a file for "we
respectfully agree to disagree."<br>
<br>
I liked the reference to John Davies (who taught me much about
gasification back in 2001 - 2003) and the coal burning first
downward and then upward. Pyrolysis on the downward path and
char-gasification on the upward path. Great for steam
locomotives. Interestingly, this is analogous to the TLUD stove IF
(but not recommended) the charcoal that was created on the downward
path of pyrolysis was to then be burned (char-gasified) after the
pyrolysis batch was finished. This is not recommended (as has been
discussed in other messages) because a TLUD stove and the fire-box
of a steam locomotive are rather different in construction and
purpose.<br>
<br>
In a slightly later comment posted by Crispin on this topic to the
Stoves Listserv, he wrote something that I really like:
<blockquote type="cite">
<pre wrap="">I see the next big step for micro-gasifiers to be the development of the crossdraft burners. They have great potential to burn a wide variety of fuels extremely cleanly by using coke/char bed breakdown of the long chain gases. The multiple variables you describe are greatly 'normalized' by this post-evaporation treatment. If it can be done under 4 kW it is going to be a winner in the convenient cooking sweepstakes.</pre>
</blockquote>
Unfortunately, that final sentence starting with "If it can be done
under 4 kW..." is still an IF statement. I sincerely wish everyone
who works on crossdraft burners great success. Meanwhile, we have
the TLUD stoves with over 30 years of development and several years
with highly successful stove projects where they are already being
put into well-accepted use in ten's of thousands of impoverished
households in West Bengal. I will spend my time expanding that and
other TLUD efforts, and I hope many of you will join the efforts
there and around the world. <br>
<br>
Happy New Year to all !!! (and a major thank you to Tom M. for
maintaining the Stoves and other Listservs.)<br>
<br>
Paul<br>
<br>
<pre class="moz-signature" cols="72">Doc / Dr TLUD / Prof. Paul S. Anderson, PhD
Email: <a class="moz-txt-link-abbreviated" href="mailto:psanders@ilstu.edu">psanders@ilstu.edu</a>
Skype: paultlud Phone: +1-309-452-7072
Website: <a class="moz-txt-link-abbreviated" href="http://www.drtlud.com">www.drtlud.com</a></pre>
<div class="moz-cite-prefix">On 12/31/2017 12:37 PM, Tom Miles
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:004801d38266$5a1e3300$0e5a9900$@trmiles.com">
<pre wrap="">Crispin,
Nice post, Thanks.
Some additional thoughts: As very stover knows, solid fuels, especially biomass, present some complicating factors so that the reactions are not uniform through the fuel bed. One is variability in the morphology of the biomass. There are differences between grasses and wood, and between hardwood and softwood. The wall thickness of grasses is very thin, so it reacts quickly. Straws contain enough air to support combustion, so you often get smoldering combustion which leads to incomplete conversion of solids to gas. Hardwoods are "surface reactors" compared with softwoods. As you heat hardwoods in the presence of CO2, as in a gasifier, you find that particles tend to reduce from the surface of the particle towards the center, chunks or wood "melt" like ice cubes as the solid charcoal converts to CO and H2. Softwoods have structural features that make them porous and react throughout the particle. Particle size is also a key factor as the interior only reacts when the heat gets to it and wood has great insulating properties. Thinner chips make more uniform reactions. In the 1970s we supplied researchers in pyrolysis and gasification with uniformly sized "designer" chips down to 3 mm cubes to reduce the effect of particle size.
These variations lead to variations in reaction rates and intensities in reactors. "Torrefaction" occurs as you heat biomass in the absence of air to about 285 C to remove moisture and volatiles, thereby increasing the energy density of the fuel. Producers of torrefied wood say that above 285C it is easy to go exothermic. Hot spots are created by variations in the fuel.
In short, the reactions in the TLUD or BLUD depend on the characteristics of the fuel, the design of the reactor, and how it is operated.
Tom
-----Original Message-----
From: Stoves [<a class="moz-txt-link-freetext" href="mailto:stoves-bounces@lists.bioenergylists.org">mailto:stoves-bounces@lists.bioenergylists.org</a>] On Behalf Of Crispin Pemberton-Pigott
Sent: Sunday, December 31, 2017 6:08 AM
To: 'Stoves (<a class="moz-txt-link-abbreviated" href="mailto:stoves@lists.bioenergylists.org">stoves@lists.bioenergylists.org</a>)' <a class="moz-txt-link-rfc2396E" href="mailto:stoves@lists.bioenergylists.org"><stoves@lists.bioenergylists.org></a>
Subject: Re: [Stoves] Understanding TLUDs, MPF and more. (was Re: Bangladesh TLUD )
Dear Friends
This post is long and refers to the earlier consultation about the products that can be produce from biomass heated in the absence of air, and the energy released when doing so.
It is a follow up on the messages of 12-12-2017. It is attached as a PDF for convenience.
Regards
Crispin
++++++++++++++++++
Dear Friends of Biomass Gasification
I have consulted at length one of the world’s leading authorities on the subject of the production of combustible gases and the making of charcoal from biomass. His name Hirendra Chakrabarti is known to some of you as he served on Task Group 1 “FUELS” in ISO TC285. He is the author of one of the “top 50 papers in the past 50 years” in the solid fuels sector, which was not written decades ago, but in 2013 at the age of 87. For the test method historians, he published a method for calculating the efficiency of heating while cooking in 1960.
The questions I put to him related to the idea of chemical transformation of biomass into other arrangements in the absence of air, something generally concluded by the group here as “not happening to any meaningful extent”, with a couple of hold-outs.
If you recall, I started by saying that if the elements in biomass were re-ordered, they would produce CO2 and water vapour releasing in the process, about 18.5 MJ/kg (LHV). If the biomass was heated in the absence of air, I said 93.6% of the hydrogen could in theory be oxidised by the oxygen contained in the fuel. Other opinions held that such oxidation does not happen, not for carbon and not for hydrogen.
Prof Lloyd provided links to papers that showed the exothermic decomposition of biomass in the absence of air does release a significant spike of energy at 360˚C. This is sufficient to cause thermal runaway in a heated pod of fuel provided it is not cooled excessively. This reaction requires some change in the biomass including a rearrangement of the elements supporting the case that such reactions do occur in the absence of air.
Hirendra was kind enough to provide a comprehensive reply over the past weeks on matters relating to the construction and operation of gasifiers and from this I have extracted the things that are most relevant to the TLUD and BLDD micro-gasifiers. Initially there was a struggle with nomenclature because the TLUD community, if I may call them that, is not using the standard terminology common in the gasification industry. Professionals use more precise terms and subdivide the fuel processing into distinct stages, which you will easily recognise.
Distillation: This normally refers to heating biomass like wood in a closed retort without any addition of air. One such wood distillation plant was at Bhadravati in South India. The wood was distilled in an indirectly heated horizontal retort. It used to make several products at the same time: charcoal as the main product, methanol and acetic acid, wood tar and wood gas. Charcoal making without adding air can be done on an industrial scale.
Note that the production of wood gas requires that the hydrogen, oxygen and some carbon be liberated during the process, demonstrating that indeed, biomass does decompose and re-arrange the elements in the absence of air. The chart I posted shows that the reactions are not endothermic save at 360 degrees. Just because they are not exothermic does not mean that the atoms are not re-arranging to produce combustible gases in the complete absence of air.
The wood gas produced by a retort contained 35-40% carbon dioxide and has a calorific value of about 12.6 MJ/m3. This should be sufficient evidence that gasification can take place in the complete absence of air, and that the elements do in fact re-arrange themselves to create CO2, H2, CH4 and other combustible gases.
That is one of the fundamentals. Hirendra writes: “True it is that unless the fundamentals are cleared in person nothing can progress.”
The distillation of coal is no different. Being very old biomass, it is subject to the same decompositions and formation of gases, combustible or not. Coke making is described in the book “Coal and Coke Chemicals” by Wilson and Wells (MacGraw Hill). The reactions that take place during coal distillation or coal carbonization are widely known by the coke making people, as the subject is very old. The treatment of biomass is essentially the same.
This processing in a retort is “pure distillation without any infiltration of air”. The only inputs are the elements in the biomass to mane charcoal (or coal to make coke).
Coal is originated from wood having dry ash-free analysis like this:
Wood:
carbon content 50%
hydrogen 6.5%
oxygen 40%.
Peat / lignite
carbon content 65-70%
hydrogen ≈6%
oxygen 23-25%.
Bituminous coal of various types
carbon content 78-89%
hydrogen 5-3.5% and
oxygen 11-6%.
Non-caking bituminous coal
carbon content 79-85%
hydrogen ≈6% and
oxygen 11-12%.
Caking type coals to coking types, which on heating melts at 400-450˚C, and on further heating hardens to produce coke. The process ends at about 1050-1150˚C.
The gas quality also changes with the maturity of the fuel:
Wood emits 45-50% CO2
Lignite emits 30-35% CO2
Non-caking coals emit 10-15% CO2
Caking and coking coals emit 3-6% CO2.
The older the coal, the less oxygen is available to react to produce CO. This means as the product ages, the heating value of the gas increases. The typical composition of coke oven gas produced from coking coals is:
CO2 3-5%,
Unsaturated heavy hydrocarbons (HHC) 2.5-2.8% oxygen < 1% CO 8-9% methane 25% hydrogen 50-55% and infiltrated oxygen with air 4-5%
Calorific value of coke oven gas is 23 MJ/m3.
Note that all the oxygen in the coal turns into gases, without added air. This shows the oxygen is ‘pulled out of the fuel’ by heating. (The 4-5% infiltration is apparently what industry expects from imperfect equipment.)
Driving the report to 1250 degrees produces distillation gases with carbon dioxide ≈20% unsaturated HHC 2.0% oxygen ≈1% CO 12-13% methane 20% hydrogen 40-45%.
With respect to biomass heated without air, the fuel oxygen reacts with fuel carbon to produce CO2. He never mentioned once the formation of water vapour from the oxidation of hydrogen. Water is in fact an intermediary species:
CH4+H2O => CO+ 3 H2
“I think people who have not worked in this area cannot understand the subject other than getting into utter confusion.”
Well, let’s try to end the confusion.
Pyrolysis
The term pyrolysis means breaking down of a material with heat (pyro). This applies to what is happening in the distillation zone. It does not refer to a particular manner of heating. Driving carbon off the surface of char is also pyrolytic phenomenon.
In a gasifier with added air, there is, apart from the exothermic reactions of the fuel oxygen, exothermic reactions driven by the added oxygen. This is the type of micro-gasifier we hear much about on this discussion list.
There are two popular methods of creating a combustible gas using oxidation of some of the fuel to provide the distillation energy. Because the distillation takes place in the presence of air in the TLUD with a downward moving hot zone, and in the absence of air in the upward moving hot zone, the discussion must be separated at this point to cover both types.
BLUD: Upward moving hot zone (centre-burning, up-drafting, ash removed from the bottom, fuel added on top) This type of gasifier is fueled from the top, and can be refueled indefinitely. The hot zone is a layer with fuel being oxidised by the oxygen (air) provided from below. In this case new fuel is placed on top, and heat rises from below. When it is hot enough, water evaporates drying the fuel. When it gets closer to the heat, distillation products are driven upwards as gases and vapours. The temperature in this zone can be well over the ignition point of hydrogen.
Let’s review: The drying zone at the top of the fuel is where some residual heat is utilized to dry the mass. Below, the rising heat pyrolyses the fuel mass causing distillation of its material. The distillation zone is totally endothermic provided it does not exceed 360 (where the spike shown earlier occurs). As the temperature reaches 360 there is a self-heating that takes place. This is followed by heating to 850 or more depending on the device. Above 550 there are no more tarry gases produced.
At the bottom is the zone where oxidation of fuel takes place using air provided from below. Note that in this arrangement, the oxygen in the biomass above has already been combined with carbon and hydrogen and traveled upwards, so the burning is dependent on air from below to keep the oxidation zone going.
“How much heat is liberated or absorbed in different zones have been defined in various literature and by and large the data given do not differ.”
Using this system, the carbon in any biomass leads to production of CO2, methane, CO and other organic compounds including tar. The production of tar is about 2.5%. This tar is made of 80-85% carbon and 10-12% hydrogen.
TLUD: Downward moving hot zone (centre-burning, up-drafting, no ash removal, no fuel added) This type of gasifier is fueled from the top, lit on top and for all intents is not to be refueled save from below (some rice hull gasifiers). The hot zone is a layer with fuel being oxidised by the oxygen within the fuel and from air supplied from below. We already had quite a discussion about whether or not the fuel oxygen is reacting with other fuel elements, and it should be clear now that it does, under all conditions, provided it is first heated.
The point of burning of the fuel is called the oxidation zone. In most micro-gasifiers, this process is not hot enough and not well-insulated enough to continue without at least some air being added to overcome heat losses.
Because the fresh fuel is below the oxidation zone, no heat rises from below into the drying zone. The fuel (pellets for example) are dried from above and the water vapour travels into the oxidation zone, cooling it. Between the drying zone and the oxidation zone is the distillation zone. It has plenty of air, even in an air-constricted situation because the air reaches the distillates before it reaches the oxidising fuel. Because both air and distilled gases, including tarry gases, enter the burning char, the tarry gases are broken down either within the hot red char or in the hot char immediately above that layer.
The temperature in the char burning layer is well above 550 so tarry gases tend not to survive to the upper side. As a result the gases produced for the cooking zone have a relatively low energy content (12-14 MJ/m3). If the fuel is damp, it may be only 9.5MJ.
Those are the two different methods which for convenience of have called TLUD and BLUD.
The TLUD was described by Tom Reed as an inverted downdraft gasifier. In fact, this is a good description. The same thing happens ‘upside down’ in a downdraft gasifier: the oxidation zone heats and dries and distills the fuel above it, and all gases pass through the hot char. Identical result: low tar low, energy gas.
In the BLUD, the process is quite different, producing a tarry, high energy gas (about double). Both will work in the presence of air and the absence of external heating because they are burning some of the fuel.
Conclusions:
1. Biomass can be gasified in the presence or absence of additional air. The process is called distillation.
2. Biomass heated in the absence of air reforms the elements to produce CO2, CO, CH4, H2 and other gases.
3. In the absence of air. it is strongly endothermic at 320˚C and strongly exothermic at 360˚C.
4. Biomass heated by admitting and oxidising some of the fuel can still produce combustible gases if the oxidation is sub-stoichiometric (air is inadequate to burn fuel completely).
5. There are two quite different ways to distill biomass into gases and liquids: in the presence of air adjacent to the oxidation zone, or in the hot exhaust side of the oxidation zone where there remains no oxygen save that contained in the biomass.
6. The use of the term ‘pyrolysis zone’ is probably misleading as it is being used colloquially to describe the distillation zone and the oxidation zone as if they were one and the same. They are not. In the common TLUD the distillation zone is under the oxidation zone. In the BLUD it is above. In both cases, air is supplied from below and exhaust is upwards. Depending on which side of the oxidation zone the distillation zone is located, the chemistry of the gases produced differs (a lot).
7. The term ‘migrating pyrolysis front’ is interesting to parse. All biomass fires have a pyrolysis front somewhere in the fuel because the fuel is being decomposed by heating. What I think has been referred to is the “migrating oxidation zone” that passes through the fuel downwards or upwards. As that zone migrates, it distills the biomass, above or below, either in the presence of air or not, depending on the architecture and the air flow direction.
8. Because of the release of heat at 360 degrees, it is theoretically possible to construct a gasifier that once started, could continue indefinitely producing combustible gases containing 35% CO2 using only the oxygen contained in the fuel, and powered only by heat released in their formation.
9. Such a gasifier is only a ‘pyrolyser’ in the sense that heat is involved. An open fire is also a pyrolyser. An open fire is not a gasifier, which in common parlance refers to a device producing a combustible gas you can pipe to another location.
10. The distinction between devices being either a gasifier or a pyrolyser is false. A gasifier is a device type (produces gas in a pipe, if you wish). Pyrolysis is a process internal to the gas production process. What then is a pyrolyser? Anything that breaks down fuel using heat is a pyrolyser which includes all biomass fuel burning stoves.
11. The term “moving pyrolysis front” is technically correct but inadequate to characterise the process. It is only moving because the oxidation front is moving. It is a consequence, not a cause. Oxidation of the fuel is not the gas-forming process, it is the exothermic fuel consuming process. It is the distillation of the biomass that produces the combustible gases (for simplicity of the demonstration, I am ignoring for a moment the complexities of gas reformation that takes place in the vicinity of the oxidation).
12. In a BLUD the pyrolysis zone is above, but not dependent on, proximity to the oxidation zone. This more clearly separates the two processes. A large top-loaded BLUD gasifier might have a pyrolysis zone a metre deep, not at a ‘front’. In a TLUD or IDD the zone is very shallow and immediately adjacent to the oxidising fuel. The principles of what is happening are the same: the distillation and oxidation are distinct phenomena, and it is the oxidation zone that migrates.
END
Crispin Pemberton-Pigott
29 December 2017
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</pre>
</blockquote>
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