[Stoves] Comments about T-LUDs

[Jeff Davis]

Dear All,

To be honest, I struggle with understanding this stuff but here goes it!

Paul wrote:
> 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.

I would think that the amount of air per amount of gas would be about the 
same. The difference is the velocity of the air and much easier to push the 
air through the IDD than the DD. Also, in the IDD the pyrolysis zone moves 
not the fuel as in the DD. Of course in a tuyerless DD (stratified) the 
pyrolysis zone can move. The IDD also runs cooler than a tuyered DD.


 >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.


The air creates the flaming pyrolysis zone and the gas exiting here is CO2, 
H2O, CO, and H2. Next is the glowing char zone where the CO2 is broken down 
to CO and some H2 out of the H2O (both being endothermic). The char acts like 
a catalyst. The gas entering the char zone may be of a higher temp than the 
char and this would help with the temp balance but I am not sure about that.

Next the gas should be cooled so that the CO doesn't revert back to CO2.


> 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.

Or is it that more air (higher temp, er ratio) will keep more char glowing red 
and able to react more. 
Once the char temp drops too low it is not going to react with the gases. The 
sens

> 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. 

Or is it that it is much easier to move the air through the IDD than the DD?



> 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.  

What if one blows hard through a cigarette?



Paul wrote,
> 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

-- 
Jeff Davis
Somewhere 20 miles south of Lake Erie, USA
http://www.velocity.net/~jeff0124