[Terrapreta] More trials

[Greg and April]

Interspaced in Blue.

  ----- Original Message ----- 
  From: MFH 
  To: Terra Preta 
  Cc: jameskater at onetel.net 
  Sent: Monday, May 12, 2008 19:43
  Subject: [Terrapreta] More trials


  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.    We did join the rest of the world ( officaly that is ), it's just that un-officaly most Americans still find it easier to say they want a 6ft 2"x4" than a 2 meter 50mm x 100mm ( or 5cm x 10 cm ). 
    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.    Why heat only from the bottom?    I would think that something like a 5 cm gap between the walls and the retort would alow heating from the sides as well ( without loosing excess heat from below ) - and heat from as many directions would decrease the time needed per batch. 
    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.    How did the tire scraps work out?    Any of that nasty burning rubber smell?    I would have thought that they would have been better used as fuel, once you had a hot fire going. 
    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.    Ok, I'll admit that I have not had my morning caffine yet, what's " MC "? 
    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.    Any reasion that you chose 8mm over another size?    Why only 8?    Was it to prolong the burn and decrease the rate of off gassing? 
    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.    So 5 hrs from start to finish.    How much time did you actualy spend fueling the retort and how material did you use to fuel it? 
    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.    No metal in the tire scraps? 
    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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