[Stoves] Crispin´s kiln-was Re: Traditional Charcoal Making

[Tom Miles]

> A small prototype would be the next step.

When you have a system that can be dropped into Mozambique to produce 200
stoves per month let us know. 

After studying comments on this list and other sources it is clear that a
well designed and well built 24 ft3 (0.7 m3) wood fired kiln of the kind
that Manny Hernandez has been building can do all that Crispin needs.

It is also clear that the problems with the electric kiln may be due to many
other reasons besides the fuel source. These same reasons are probably why
we hear that most kilns in developing areas do not reach temperatures higher
than about 750C or 950 C.  

My suggestion would be to design and build a good wood fired kiln, that
could also be fired with producer gas, and go into production. It appears
that a 28 stove load could be fired to cone 10 and cooled in less than 24
hours with about 0.68 m3 (.25 cord) of solid wood which at 480 kg/m3 (30
lb/ft3) would be about 326 kg (720 lb). Total fuel input would be about 1522
kW (4.6 MMBtu) or about 54 kW/stove.

It is interesting to look at the power requirements for electric, gas and
wood fired stoves. At 35 kW/24 ft3 Crispin's kiln is pretty typical of what
is available commercially but at the low end of what is recommended by kiln
designers. The high end for electric kilns would be 50 kW/24 ft3. Electric
kilns are apparently well known for heat distribution problems. Other
factors in the design and construction, controls and power supply could
easily make it difficult to achieve the desired cone 10 conditions.

The same capacity in a gas kiln would require a heat input capacity of
147kW+ (400-700,000 Btuh) depending on kiln design and construction. This
equates to 20-30 MJ/m3 kiln capacity (20,000-29,000 Btuh/ft3). It is said
that gas fired kilns typically have far more capacity than required and a
capacity of 16,000 to 19,000 Btu/ft3 is recommended. 

If you derate the gas kiln design about 40% for producer gas you would need
a heat input of about 240 kW+ (830-1,200,000 Btuh) depending on kiln
construction. This equates to 35-50 MJ/m3 (34,000-48,000 Btuh/ft3).Using
this factor Paul's 59 kW (200,000 Btuh) gasifier would fire a 5.8 ft3 kiln
capable of curing about 5 stoves (24/28=.86 stoves/ft3 x 5.8 ft3) at a time.


The best 24 ft3 (.7 m3) wood fired downdraft kilns seem to be designed for a
heat input of about 293 kW (1 MMBtuh). The heat input is only slightly
greater than for producer gas which would generate about the same gas
volume. The firebox design is integral to the kiln as you see in Manny's
kilns. There are designs and kits for building these kilns. They are readily
expandable lengthwise (to 100+ ft3 for example) as needed. The side loading
gasifier Peter found is called a "Hob firebox" and there are layouts for
this design.

Does it make sense to make charcoal at the same time as you are firing the
kiln? A 343 kW (1.2 MMBtuh) design that does not require fuel preparation
could be used to fire a 24 ft3 kiln. Char could be removed manually
periodically. That's a good design challenge.

Tom