[Stoves] Correcting a misconception that approaches myth status

[Paul Anderson]

Dear Crispin and Kirk,   (and I hope it reaches Tom Reed).

There are SOOOOOOO many different configurations of DD and UD that 
almost anything can probably be shown.   For example, a forced high 
volume of air into a TLUD can provide sufficient oxygen to consume the 
char that is being created.   Not the normal case, but it can be done.

What is "normal" in down draft (DD) gasifiers?   I am not going to 
define it.   But many of the important DD units have tuyeres (a nozzle 
through which an air blast is delivered to a forge or blast furnace) 
that are rather near the bottom of the device.   This is in contrast to 
having air enter at the very top of a fuel chamber and drawn or push 
downward through the full stack of fuel before reaching the hot zone 
that is ignited at the bottom.   What the tuyeres can do is make enough 
heat by putting air into the bed of created char so that the char 
becomes a significant zone of glowing hot char.   And there is some heat 
that goes upward and causes pyrolysis of the nearby biomass.   The 
chamber is closed, so there is no gas movement upward.   The hot 
pyrolytic gases are forced downward to the hot char where they are 
change / cracked / modified to become the desirable CO and methane and 
H2 and "good" gases that exit the gasifier at the bottom.   Those gases 
can be burned immediately or eventually be used in internal combustion 
engines (usually after cooling and filtering).   And as the char is 
consumed, the created char from above moves into the hot zone.   And new 
raw fuel can be added at the top, but carefully and when there is 
minimal pressure inside the gasifier.)

THAT type of DD bears little resemblance to TLUD processes.

But COULD a container of fuel as described by Crispin have an ignition 
at the bottom and eventually convert it to charcoal with the gases 
exiting the bottom, etc.?   I do not doubt it.  Was that ever done in 
the days of making coal gas for city lighting, etc? I have not studied 
that history, so I do not refute Crispin nor ask for documentation.   
Many things have been done, some are important and some are not.

It is interesting that we are basically discussing what if the basic 
TLUD configuration was turned upside down.   Could we be using that 
method to make more char?   And could that become important as a way to 
make biochar?   That topic is wide open for anyone who wants to look 
into it.

I never asked Tom Reed what type of DD gasifier he was considering  when 
he had his inspiration for the IDD method. Maybe he will send us a comment.

And yes, Crispin and I are friends since about 2002 when I visited him a 
couple of times at his home and business is Swaziland.   In case you are 
wondering, he is almost always argumentative, presenting different 
points of view, and with supporting evidence, often from obscure 
sources.   Not always correct, but none of us are.   Refreshing, 
challenging, and generally very good-natured. Under-estimated by many 
people.

Paul

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

On 12/15/2014 6:14 PM, Crispin Pemberton-Pigott wrote:
> 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
>
>

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