More Trials
Max Henderson,May 12, 2008

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 10^th 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