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| Gasification Archive for September 2002 |
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| 114 messages, last added Tue Nov 26 17:18:28 2002 |
[Date Index][Thread Index]
Re: GAS-L: Re: Fw: RETORT FOR CARBONISING COCONUT SHELLS - REG.
On Mon, 16 Sep 2002 11:37:52 -0300, Kevin Chisholm
<kchisholm@ca.inter.net> wrote:
>A temperature of 300 C = 570F would seem to be OK.
>Ordinary mild steel, such as would be had in 45 gallon
>drums, doesn't scale significantly until well above
>700F.
Woops! here I am back again for another three cents this time. The mild
steel problem is not just temperature it is temperature plus water vapour!!
And, hot steam is very aggressive on mild steel, well below 700oF. Its
not too much of a problem with very dry wood, but even so there is a lot of
heavy black scaling which is some fancy form of iron oxide, but indicative
of the presence of water. As for those wanting to burn off the smoke,
been there too! A couple of steps up from Tongan kiln is when you weld
three or four drums end on end with a clamp on sealable door on the front
end and a 3" pipe out of the back, which curls down and along the
underneath side of the kiln with 1/4" holes every -3-4" along it. This, is
ideal for making charcoal out of long sawmill slabs, saplings, bamboo,
Leucaena coppice wood and anything which is reasonably straight for 2-3
metres and saves a lot of extra cutting. I believe it was invented by or
for Ken Newcombe, who was Director of Renewable Energy Sources for Papua New
Guinea during the 2nd OPEC crises in the early 80's, until he got his 'come
uppance' and ended up working for the World Bank!! We called it the
Newcombe Rocket! and that was how we reckoned he got to places on High!!.
You lit a fire underneath it and tended it patientially while all the steam
tried to blow the fire out and then when the gas started to come through
you hid behind the soil bank which you had dug it into, as it took off with
an uncontrolable roar!! From that point on until it finally sputtered when
the gas gave out the drums were red hot and unless it was bedded into a
firm earth support with lots of heavy steel cross bars, it would collapse
the steel into some really grotesque but expensive shapes from which it was
impossible to retrieve your charcoal let alone get the next batch of wood
into it. We rarely got more than 5 burns out of one set of drums. To work
it you really had to have complete control over the moisture content of your
wood, with a blend of wet and dry. The dry in the bottom, to ignite fairly
quickly, and the wet on top, to control the temperature, but unfortunately
not that corrosion, by blending in the steam with the gas. And even then
a good high pressure hose close by is a wonderful insurance policy.
O.K. a long involved tale of hair singeing adventure, but may it serve to
make the point that nothing is ever as easy as it seems from the armchair,
and if you do want to make a more controlled and efficient combustion
system, then it either has to be very very good, with lots of controlled
feedback, or else you have to stand there and watch it anyway. Yes you
should be able to fill a metal kiln to the top with wood all in one go and
then light a little fire and close it up and wait for several days for it to
burn all the way through by controling the air supply, but if you can't see
into the kiln, or keep adding fresh wood where it is burning out too quickly
and mollycoddling the side that doesn't burn evenly you will have an awful
lot of unburnt wood to feed back into the next load of the kiln. There is
nothing more frustrating than watching one side of your develop a bright
red patch and start to buckle when there isn't a hole/cap nearby to get a
hose into and even things up. The big problem with fancy metal kilns is to
control the airflow at the end to let them actually stop burning and cool
down. It only needs the slightest buckle in one ring of a TDRI kiln and
when you turn up next day to unload it you find that its still red hot and
half of your coal is gone! And don't dare say she'll be right and start to
unload it before it is completely stone cold, or you will be surrounded by
heaps of charcoal or worse more sacks of charcoal bursting into flames all
around you. We nearly lost a truck once when the load of bagged charcoal
going accross to the screening plant lit up half way there!
As for all those temperature gauges, forget em! If you can tell the
difference between dull red, red, bright red and cherry red, you can make
any quality of charcoal you want by eye! As Tom commented previously,
there is very little smoke from these simple kilns and you can most of the
time get to look at the colourof the glowing charcoal building up in the
bottom of the kiln, even though you keep piling extra fresh wood on top. To
achieve more burn off of volatiles, ie, burn at a higher temperature, you
add an extra controlable air bleed into the base of the kiln to maintain
combustion. Or, if it is a big one, like Tom's ring kilns, which we called
the African or TDRI kilns you needed a ring of 1" pipe nipples welded
into the circumference on the middle of each ring. Then by adding rings and
opening and closing nipples you had only a shallow zone of combustion,
less than half of the depth of each ring.
These let in enough air to give incomplete combustion under the blanket of
hot cover gas, and once they had got reasonably covered with a brighter
red charcoal you screwed caps on them. A very rough and ready form of
control, but its amazing what you can do with very little. Well that's
about 4 bobs worth rather than 4 cents, but Tom, I hope you won't have too
many questions after this diatribe!!
Ken C.
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