[Stoves] Correcting a misconception that approaches myth status
kgharris
kgharris at sonic.net
Mon Dec 15 22:49:09 CST 2014
Crispin,
In a ND TLDD there will always be a battle between the downward moving air and the hot combustion gasses which will be inclined to rise. I am thinking that gasses cannot go both directions in the same space, so will there be areas in the fuel where the air will win and areas where the hot gasses will win? This could lead to the buoyancy caused convection you mention, with the heavier air displacing the hot gasses, pushing them upward. Could this unevenness be caused by local resistance variations to gas flow in the fuel? Or might it be a temperature thing where the air wins in cooler areas and the hot gasses win in hotter areas? Or am I totally off base here?
Kirk
From: Crispin Pemberton-Pigott
To: stoves at lists.bioenergylists.org
Sent: Monday, December 15, 2014 4:14 PM
Subject: Re: [Stoves] Correcting a misconception that approaches myth status
Dear Kirk
Within the small spaces between lumps of fuel there is normal buoyancy for the hot air or hot gases so it circulates and convects heat upwards, no matter what the direction of the burn. Generally for very dry biomass this is a problem with a DD stove if an attempt is made at the same time to make char. Either it 'cooks' a lot of fuel at once making wet gas or it races through it and is not able to properly pyrolyse the fuel.
Consider this: in a TLUD with a very low flow of primary air, or with damp fuel, or a combination of the two, there is a real possibility it just stops pyrolyzing. It is a limit. The problem is the heat radiated (and a small amount conducted) downwards is not enough to sustain a continuous reaction. Taking those identical conditions and flipping it upside down one finds the pyrolysis continues because more heat, under the same conditions, rises into the fuel bed.
At the other extreme is the high gasification rate. If the fuel is really dry and full of oxygen it can burn quite well with no air at all. This was reported in the news a couple of months ago in Colorado whereby the roots underground kept burning long after the forest fire was 'out'. It makes charcoal doing this. It does it with a combination of water gas shift reaction and combusting the O2 in the carbohydrates.
Well, in a DD burner this situation arises pretty easily if the gap size is right. When the condition arises and the fuel is setting itself alight upward and uncontrollably, turn it upside down. It turns into a TLUD with the heat rising away from the new fuel. That is a second limit.
So depending on whether you want to make char, make gas and not, burn all the fuel in one go, use low or high density fuel, high or low moisture, and importantly, high or low Oxygen content, you choose the burner that is best suited to the range of conditions anticipated.
Consider: suppose you want to run a higher air flow rate in the physical space available and the fuel is relatively dry and you want to make char. A DD stove would give a good yield and work well in those conditions. An UD would in the same conditions burn much more of the char. I hope that is explanation enough.
On another note don't worry about Paul and me yelling ultimatums at each other. We know each other very well and have cooperated for years. He is enthusiastic which I appreciate but sometimes gets things askew. People think we are overly hostile but it is OK - he will tell you the same thing. We communicate frequently and he is a guy I can count on to want things done right.
As things stand, Paul put a decade into TLUD investigation and I put it into BLDD work. I am going to post a few small pictures showing the movement of the MPF vertically in a DD stove fuel hopper. A significant advantage of a DD combustor is that the secondary combustion can be maintained directly next to the pyrolysed fuel because the fuel is always falling against the grate. It means the combustion is very stable once it is running. It doesn't change for hours on end. This is very well suited to process heating and home heating. Cooking is more difficult.
With a TLUD cooking is easier and and space heating for a long time is difficult because of the refuelling.
I have seen drawings for a Russian combustor the fed fuel into a conical TLUD fire that was continuous, burning on a round steel plate that rotated slowly. It could do what a DD burner does in terms of long term burns.
Regards Crispin
Crispin,
I have been following your insights as you have shared them with us. I value what you are saying. I like your earlier statement that the pipe is a solution for a problem that should not exist in the first place. If my stove was providing adiquate air and mixing then my Wonderwerk Strata combustor would have no effect. It would not be necessary.
Reading through this responce to Dr. Anderson I can follow your logic until I get to the last paragraph. I do not disagree with you, I just wonder how it works. The heat of pyrolysis rises through the fuel in a BLDD, heating the fuel and driving out moisture. I wonder what the mechanism is for this raising heat in a down draft stove. Is the gas not moving down, carrying the heat downward? Heat is a property of material, not material itself, and so it is independent of gravity and buoyancy. The only mechanism I can see for heat to move upward is radiant heat, just like a TLUD MPF radiates heat downward. I would like to know more about how this works. I mean no contention here, I just want to know.
Thank you,
Kirk
----- Original Message -----
From: Crispin Pemberton-Pigott
To: 'Discussion of biomass cooking stoves'
Sent: Monday, December 15, 2014 12:39 PM
Subject: Re: [Stoves] Correcting a misconception that approaches myth status
Dear Paul
You can’t just jump in with such classifications without asking first.
>…In 2004 - 05 I re-named it Top Lit UpDraft (TLUD) which is also not totally accurate. None of the names acknowledge the Migratory Pyrolytic Front (MPF) that is the dominant feature of what is commonly called TLUD micro-gasification.
I think you should have a look at a BLDD working and watch the migratory pyrolytic front rising through the fuel instead of descending into it. You might change your opinion.
Note:
A DD gasifier is ignited at the BOTTOM, and the fire (hot zone of gasification) REMAINS at the bottom, and the unit can be operated with continuous fuel entry into the top.
No that is not necessarily true at all! One could make the same claim for all top lit stoves with a high superficial air velocity. In a BLDD there is a gas production process started by a fire at the bottom. Same as TLUD. There is a migratory pyrolytic front that works its way upwards. Same as a TLUD. When it reaches the top, there is a fuel chamber full of char, same as a TLUD. The gas produced can be burned immediately under the grate or taken elsewhere to be burned in a gas burner. Same as a TLUD.
A IDD or TLUD or MPF gasifier is ignited at the TOP, and the fire (hot zone of gasification) slowly MOVES to the bottom, after which time the fire zone stays at the bottom and the unit operates as if it were a regular UpDraft (UD) gasifier if anyone puts more fuel into the top.
Fundamentally different.
Again, no. It is the same! When the migratory pyrolytic front reaches the top of a BLDD fuel chamber the combustion continues at the top drafting air in (if allowed) and does exactly what you describe but upside down. The big difference is that the BLDD can be refuelled by placing more fuel on top and the MPF continues to work its way into the new fuel, upwards, until it again reaches the fully charred condition.
Because of Tom Reed and also Paal Wendelbo who worked totally independently, we have something new for cookstoves. They did not invent pyrolysis. They were not the first people to have a fire in a barrel making gases and have the flames only up at the top where secondary air was available. (I saw that in the 1950s in a burn barrel at my home.) But Reed and Wendelbo were the ones who CONTROLLED it and understood it and made it all small enough to be useful in a cookstove. And the rest is history. And TLUD history is still being made.
That may all be true, but the BLDD stoves with MPF have been in use literally for centuries. BLDD combustors with large fuel chambers were used for making coal gas and the only way a useful gas can be created is by having a MPF in the fuel bed. Obviously it was controlled or they would not have been able to get a reliable product.
A TLUD is a bottom-lit downdraft stove turned upside down so that can’t be refuelled. Deal with it.
The only practical difference is that the heat from pyrolysis in a DD stove rises within the fuel supply, heating it and driving out moisture sooner than it would in a TLUD where the heat all rises, save by radiation. There is an advantage to this with coal or wet charcoal or wet wood. Otherwise, not really any practical difference. I find that with a BLDD and wood pellets, for example, the airflow velocity through the interstitial spaces becomes more important in a DD stove than an UD one in order that the rising heat does not create run-away gas generation inside the fuel bed. If too much gas is produced it is hard to burn it all. Apart from this small consideration, and having built lots of each, I observe no difference at all between these architectures in terms of how the fuel is pyrolysed and the options for burning the gas.
Regards
Crispin
----------------------------------------------------------------------------
_______________________________________________
Stoves mailing list
to Send a Message to the list, use the email address
stoves at lists.bioenergylists.org
to UNSUBSCRIBE or Change your List Settings use the web page
http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org
for more Biomass Cooking Stoves, News and Information see our web site:
http://stoves.bioenergylists.org/
------------------------------------------------------------------------------
_______________________________________________
Stoves mailing list
to Send a Message to the list, use the email address
stoves at lists.bioenergylists.org
to UNSUBSCRIBE or Change your List Settings use the web page
http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org
for more Biomass Cooking Stoves, News and Information see our web site:
http://stoves.bioenergylists.org/
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20141215/85a1c504/attachment.html>
More information about the Stoves
mailing list