[Stoves] Comments about T-LUDs

[Paul S. Anderson]

All,

I believe that everything that Jeff reported is correct.  With our 
well-informed
readers who understand DD and UP and IDD (AKA T-LUD), we can discuss the
gasification processes and find the similartities and differences.

Test question:  Among the different types of gasifiers, in which type(s) does
the incoming primary air FIRST create the pyroylis gases and then move to the
zone where char-gasification can take place?

Answer:  (think first, do not just look at the answer)
.
.
.
.
.
.
.
.
.
Answer:  That sequence occurs in both the DD and IDD processes, but with
downward flow of the air in DD and upward flow of the air in IDD (T-LUD).

So, what is the important difference?  The amount of primary air.  In the DD
(downdraft), the amount of air and the air speed or through-put must be
sufficiently large so that there is still O2 in the air when it reaches the
char layer.  Otherwise the device would only be pushing the pyrolysis gases
through the char and into the area of secondary combustion, resulting in a
build-up of char and eventually preventing the addition of more raw fuel.

In a T-LUD (or IDD), the amount of primary air is intentionally controlled so
that the only O2 to reach the char level is the amount sufficient to sustain
the heat generation needed for continuation of the pyrolysis process.  
The heat
generation (exothermic reactions) might be from some in-situ (at the spot)
secondary combustion of some small amount of the pyrolysis gases with a little
of the O2 of the primary air, or from the exothermic reactions when O2 comes
into contact with hot pure carbon.  (I leave the reactions to the chemists to
explain.)

Therefore, if the primary air in the IDD (T-LUD) is significantly increased,
more O2 will reach the char layer and will conduct char-gasification, 
resulting
in considerably less charcoal remaining when the pyrolysis phase ends for that
batch of fuel.

Therefore, there can be control via the primary air of how much char 
will remain
in a T-LUD gasifier, even without utilizing different moisture percentages in
the fuel.

So, why do we have T-LUDs (IDDs) instead of simply having DD downdraft 
devices? Because T-LUD technology lends itself to making very SMALL 
gasifiers.  Others
reading this message might be able to tell us, but I do not know of any truly
small gasifiers that run in the true DD mode.  The reaon is probably 
because it
is tricky to get the small quantities of emerging gases (moving downward) to
proceed nicely to a combustor to give useful heat, and all for less than US$60
(Woodgas Campstove is $55) or even less than $10 or $5 per device that we seek
for the developing societies.

Evaluation question:  I write this stuff and I have very little feedback as to
its usefulness.  For some it is known fact, for others it might still be
unclear, or for some it has helped them understand the workings of T-LUD
gasifiers.  Personal, Off-List replies are welcome, or here on the Stoves
Listserve if appropriate for all.  For me, just to have any discussions about
T-LUDs is refreshing.  There are not very many of us seriously 
developing these
things.

Paul
-- 
Paul S. Anderson, Ph.D., Geography professor - Emeritus
Telephone:  USA-309-452-7072 (residence and office)
Internet site:  www.ilstu.edu/~psanders
For my gasifier stoves info, go to:
http://bioenergylists.org/contributors#Paul_Anderson


Quoting Jeff Davis <jeff0124 at velocity.net>:

> On Thursday 28 September 2006 10:21 am, Roger Samson wrote:
>> I am not sure but think maybe the biggest limitation of T-Luds is the
>> relatively modest energy conversion efficiency which limits the fuels we
>> can use in them. It is great for wastes where you can recover the material
>> as a carbonized soil amendment. However if we are making fuel pellets from
>> dedicated biomass fuels then we need to burn the residual charcoal as fuel.
>
> Dear Roger,
>
> I have always found it interesting when Tom Reed writes that the 
> inverted down
> draft gasifier will produce LESS (or maybe no) charcoal when the fuel is wet.
> The device/fuel will burn hotter also, maybe 900C wet and 750C dry. Because
> of that I have often wondered if wet fuel burning at about 900C would produce
> a more tar free gas than a dryer fuel at 750C. This would be opposite of a
> downdraft gasifier. Maybe dry fuel with steam injection?
>
> A down draft gasifier will gasify charcoal and an inverted down draft 
> gasifier
> is much like a down draft gasifier so I would think that by increasing the
> air blast of the inverted down draft gasifier that you could gasify your
> charcoal. But now you would have to be able to sense this and shift gears (so
> to speak) so this may not be practical.
>
>
> Jeff
>
>
>
>
> --
> Jeff Davis
> Somewhere 20 miles south of Lake Erie, USA
> http://www.velocity.net/~jeff0124
>
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>



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