[Stoves] other differance between TLUD and conventional imbert style

[drew]

I agree with Paul's general point but would like to note another very 
important differance in these two gassification systems.

    As the fuel moves downward in a DD system (remember the fuel moves 
in the downdraft system) the fuel decreases in size as it's volitiles 
are being vaporized and it is being reduced to char (carbon).   In a DD 
system at this point it is collected into a much narrower tube (often 
1/10 the original fuel stack size) this means that the *fuel is 
pyrolized in one size bed, and the car is consumed in a second size bed, 
the S/V of each bed are substantialy different*.   This separation of 
beds is important in that the heat generated in the char consuming bed 
largely remains there insulated from the remaining fuel by the layer of 
pyrolizing material.   This is where steam injection  became useful, for 
cooling the char bed, and providing potential momentary extra hydrogen 
for power using a sort of a thermal flywheel, (the char reduction tube).

for some good images of where this reaction separation takes place see 
Greg Mannings  wonderful images under
 Update Nov 6 04
http://www.inetlink.ca/a31ford/cgcmb/old-default.htm

P.S.   the mouse over captions were not working in my browser (firefox 1.5)


Vehical drivers would commonly use this DD steam injection when idiling 
thier engines say at a stop light, they would know from experiance that 
they are building up excess heat in the reduction tube, so as when the 
light changed they would inject (manual control) steam into the char 
bed, cooling the bed, and providing a momentaraly substantialy higher 
btu gas.      As others on the list with far more experiance than me in 
DD systems, have pointed out (Doug for one) the use of steam to create a 
higher btu gas is a trade off, it takes more energy per btu of gas 
created so as you increase the hydrogen ratio in the gas, you will use 
more fuel to get that btu value of gas, it may be hotter faster burning 
gas, but not gas with more heat value.   Making things even trickier, is 
that because you have consumed more total fuel, you will also have 
likely run more of the nasty N nitrogen, nemisis of good gas, through 
your system.   Drivers knew that using too much steam would eventualy 
rob to much heat from the system, cooling the char bed (remember the 
cracking of water into hydrogen / oxygen is strongly endothermic), and 
creating a tarry gas that would clog thier filters.   This skill 
required to run these systems is likely one of the greatest reasons that 
these systems were abandoned as soon as possilble.   

In my opinion for most gassifiers use, having the least N is more 
important than having the most H as the N is the heat robber stealing 
precious heat from your process.

This fuel compression, or collection is vital, and I belive that Doug 
could speak volumes to the importance of having the right size, and 
length of a reduction tube in DD systems.

My solution to creating a similar effect in the tlud was to add a layer 
of refractory spheres on the top of the Tlud fule stack, (composed of 
equal parts fireclay, coarse sawdust, sand and pearlite)  this layer was 
about 3" deep and the balls varied in size from 1" to 2" inches.   These 
balls served to compress the top of the fuel bed (shrinking the 
interstatial distances between the char) and substantialy increasing the 
contact between the gas and the incandesant char) providing a gas that 
was cleaner.  In my mind this created a sort of gravity powered slow 
motion fluidized bed.   The secondary effect of these spheres was to 
substantialy reduce radiant heat losses to the exposed sides of the fuel 
containing vessel, and to the bottom of any pot that would have been 
located on a cookstove (my sytem was not a stove, and so had a closed 
lid). 

I have not heard of anyone duplicating my sphere insulated tlud 
experiments, but would like hear about others experiances in trying them 
if anyone has.

Take Care
Drew

perhaps this should be cross posted to the gassification list?    I'll 
leave that decision to the moderators.




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