[Stoves] Traditonal Charcola Making Process / retort

[Crispin Pemberton-Pigott]

Dear Tom

> How many stoves per batch and what size kiln do you need?

Well...this will be an interesting piece of math.  I have to heat the oven
at the fastest rate that will not melt the material (PK11).  We melted about
200 stoves working out how to do this!  This problem is visible in the
temperature ramping I described before - as it gets hotter I have to slow
down of the stoves near the elements melt.

The duty cycle of the electric elements is only 50% or less at any time.  I
am hoping that the gas fire will circulate the heat much better than the
electric oven so that I can increase the temperature much faster.  This will
decrease the cycle time and give me more stoves per day.

At the moment the experiments are being run in a 0.7 M^3 kiln with 28 stoves
in it.  It will hold 29 but I can't get good enough circulation of the air
(even temp) unless I move the shelves around.

Thus getting a better kiln design is a top priority.  I am looking at 3
cubic metres with a gas flow path that ensures more even heat distribution.
More heat is wasted using gas but it solves a difficult heat distribution
problem.

So the power required will be higher than an equivalent electric oven
because a) it wastes heat and b) it will be ramping faster and c) there will
be more MCS mass relative to the oven wall mass (which is insulative).  So I
want to model the power demand before ordering the kiln, and the gasifier
will have to match that.

The power demand is non-linear.  The power required at the beginning (on the
present 2 identical ovens) is 35 KW, and the duty cycle is 10% to start
with.  Because of poor air circulation, it is on full power for 10% of the
time on a 30 second cycle.  This allows the heat to distribute well.  This
phase is for drying the clay.  Later the crystal water is driven off at a
higher power rating, then the charcoal burned out at a low power (but at 600
C) then it is ramped as fast as possible without melting the stoves.  The
gas + air movement will really help with that.  At 1170 it is held for 3 hrs
at a low power mostly to ensure the heat distribution is good.  I want to
reduce that to perhaps 30 minutes.  Then the power is shut off.

So the determination of gas volume from the gasifier will be decided by the
ramping rate from 600 to 1170 (or 1250).  I have a reasonable model of what
is happening now which is why I could tell you that there is 3.5 KWH in the
firing of one stove.  In a large stove that will drop - perhaps to 3 KWH or
less.

Ideally we should get the cycle time down to 12 hours so we can do two loads
a day per kiln.  That would be good - and the gasifier would have to produce
for 8 of that I think.  

If it was 3 cubic metres it would hold about 125 stoves.  The total power
demand would be in the neighbourhood of 420 KWH.  With Kevin's analysis
taken into consideration and your 2.75 Kg we are talking about loading
350-400 kg of wood into the gasifier.  Let's go for 450 kg or 1000 pounds.

That sounds manageable.

If the gas fire temp is reliably in the 1300 range I can raise the firing
temp and reduce the production cost which I like as a side benefit.  That
would require going back to the ceramics lab for a new formula but it would
be worth it.

The spreadsheet will not look too complicated and we can bounce if back and
forth.  The things I want to see clearly are

Gas volume produced
Heat content
Variable production rate assured
Temperature max
Sustainable power max
Charcoal yield (which takes a back seat, however but I like the $$)

Thoughts, anyone?

Regards
Crispin