[Terrapreta] More trials

[MFH]

If you can bear with me here is some info from last weekend's trials.
Various conclusions are probably of little scientific merit and may well be
blindingly obvious but I'll include for those who maybe don't have one of
these exciting toys.

 

1.	The original second-hand house brick kiln had 15cm/6" (when will the
US join the rest of the world?) gaps between the drum and the bricks on both
sides, and a relatively shallow space under the drum for the initial fire.
The idea was that it would be easier to add fuelwood on the sides, but in
fact this reduced the effectiveness of the insulation.
2.	I re-laid the bricks to give a greater fire space under the drum for
the initial fire, and moved the side walls inwards so that the only gap was
between the ridges of the drum and the bricks. The basic concept was to
apply the heat from underneath, and to insulate as best possible (under the
primitive circumstances) against any unnecessary heat losses
3.	The drum was loaded with around 100kg of old dry dense hardwood,
plus 2 x 75mm thick telephone books and some tyre scraps I had collected
from beside the highway. 
4.	Scrap dry wood was loaded under the drum and fired at 17:00. Once
that achieved a significant burn I added bricks to the open front to further
improve insulation
5.	I've learnt that a slow initial burn is best as opposed to a blast.
The assumption here is that the mass of material in the drum (despite MC of
maybe less than 12%), needs gradual heat (given the substantial insulating
properties of dry dense wood) well before the stage when pyrolisis can begin
and be sustained. I've done the opposite -high initial heat, quick
gasification, and then no continuation. There is a lot to discuss here,
including the use of 'waste' heat to raise the temp and reduce MC, in the
following batch.
6.	By 18:00 the first gas burn had started and by 18:15 the 8 x 8mm
holes in the base of the drum were all roaring
7.	This was about the 10th trial, and with each the seal on the drum
lid has become less effective. This photo shows the burn of the escaping
gases through these leaks. In a totally un-scientific guess I'd suggest that
at least a litre of gas/second was burning happily through the gaps. None of
this energy was in any way contributing to the char process. These waste
gases burnt for 2 hours.

 



 

8.	With all the jets alight I then added bricks to the top of the drum,
giving better insulation.

 



 

9.	By 19:00 the drum was glowing red hot when seen through the gaps in
the top bricks, except for a small strip down the centre of the top. I
dropped some glass from a broken bottle in a couple of the gaps, and within
minutes the glass became malleable.
10.	Around 21:00 the gas burn started to slow down, and by 22:00 the
last flame was gone.
11.	The front bricks were removed at dawn, and by midday the drum was
cool enough to be opened without a risk of the char catching alight.

 



 

12.	The charring was complete, including the tyre rubber, the 2 phone
books, and dense hardwood as large as 20cm/8" in diameter.
13.	Volume loss was in the region of 20% at a guess.

 

It is the energy output that continues to stun me. The volume of gas that
escaped through the poor lid seal was very substantial and burnt for over 2
hours. In addition, the gas burning under the drum was obviously far in
excess of the volume required to maintain the char process, just using the
red heat of the drum as an indicator. And on top of that was the vast heat
energy given off to the atmosphere despite the attempts to provide
insulation.

 

I'll continue making batches using this crude system every weekend, but
there's not a lot more to prove and I now really need to take the lessons
learnt and build a decent drum and kiln. In particular the effectiveness of
the insulation will be a considerable determinant in the efficiency of the
process. I will aim for a castable refractory kiln in a similar shape to the
current brick one, with relatively narrow gaps between the drum and the
refractory except for the "firebox" underneath. It will have two hinged
doors at the front - the upper one allowing the drum to be slid out above
the lower firebox door. A similar upper door also for the rear, and this
will also have an adjustable vent to allow heat to escape rearwards. This
would lead into a second chamber where another drum loaded with wood is
waiting its turn in the queue, being pre-heated at the same time. When one
drum has completed the char process, it will be slid out to cool, the drum
in the heat chamber at the rear is slid in to take its place, the refractory
is at high temp already, the gas jets are lit, doors closed, the third drum
is loaded and slid into the warming chamber..

 

The drums to be fabricated from boiler plate, and maybe with domed lids and
toggle screws to clamp down. Then I need to work out how to plug in a pipe
or hose to vent off excess gas, plus a compressor and a pressure vessel to
store. And that pre-supposes a capacity to record temperatures inside the
drum so that this info can be fed to a controller that will make decisions
when and if to pipe off some gas for storage. Plus a serious gas burner
system under the drum, because I believe we can eliminate the need for wood
fuel and just use some of the stored excess gas. And then some boiler tube
at an upper level through which water can be piped and fed into a large
storage tank as a heat bank, and then into the house and/or a greenhouse in
winter through sub-floor piping, radiators, or a concrete storage tank under
the slab. I don't have a house at the farm yet or even a greenhouse much
less an electricity supply but that just adds some more interesting
challenges. Its down to time and dollar availability.

 

In the meantime I'm continuing with the garden trials, and certainly there
is visible evidence of improved growth and vigour in the plots which had the
char added. The best is the one that also had some cocopeat organic matter
added, as well as some worm castings. Digging down a few inches and grabbing
a handful gives this sweet-smelling crumbly mix, laden with organic matter
and just seeming to be bursting with goodness. Hardly a scientific analysis
but I've been handling and smelling soil for a long time and this lot is
just about good enough to eat.

 

Max H

 

 

 

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