Saturday, May 15, 2010

Rocket Retort Rocks!

Yesterday was the christening of my new kiln, the Rocket Retort, a culmination of many months of research, design, contemplation; and a recent spate of hard work. Like so many others bit by the biochar bug, I wanted to create a kiln for my own use. I also recognized the potential that a practical, high-performance, "personal" biochar kiln could have in leveraging distributed production among home gardeners and other small stakeholders, and perhaps ultimately, subsistence farmers worldwide. My prior experience of small biochar kilns, gleaned from YouTube profiles and my own backyard pyrotechnics, had been of barely-contained conflagrations that produced an uncertain sort of biochar. My Rocket Retort design was informed by my work as hardware development manager for a philanthropic-funded biochar project in Costa Rica, involving a much larger kiln designed by Nikolaus Foidl and guided by Stephen Joseph, two of biochar's leading lights. Design criteria for my personal kiln include:

  • Low cost materials
  • Basic shop tools only
  • Low emissions
  • Efficient biomass conversion
  • Controlled firing profile
  • Recycle pyrolysis gases
  • Collect wood vinegar

The 55 gallon drum--durable, affordable, widely available, easily handled--is at the heart of the design. A removable-lid drum stuffed with wood mill scrap serves as the retort. To prime the kiln, I had been considering scaling up one of the newer innovative biomass stove designs, but felt stymied by the challenge of refueling and controlling output. On a suggestion from stover-friend Charlie Sellers, I looked into the Rocket Stove (, a versatile design that addressed my emissions, fuel feed, and control concerns. The rest of the hardware fell into place after a bit of "outside the drum" thinking: Create fire chamber and insulating jackets (two total) by cutting ends off drums, slitting open, and welding inserts cut from a third drum. The tricky bit was opening the slit drums evenly to maintain the roundness of the now-larger cylinders. The nesting Russian doll cylinders rest on staircase ledges in the modified rocket stove base. Each cylinder is topped by a shallow cone-shaped lid with a central exhaust vent made by cutting a sliver wedge out of a sheet metal disc and welding the cut edges together. The lids are secured by bolts welded to the inside rim of the cylinders.

The other design consideration was collecting wood vinegar (natural pesticide and plant growth stimulant) and recycling pyrolysis gases. A two-inch steel pipe was threaded onto the bung hole on the lid of the retort drum, exiting holes cut into the shallow cone lids, and elbowing down toward the stove's fuel feed opening. A "T" fitting and valves enable directing evolved gases toward either a condenser pipe leading away from the kiln to collect wood vinegar, or directly into the fuel chamber to fire the kiln. The fuel feed opening is divided horizontally by a stainless plate, with the lower portion intended for intake air. Being able to block the throat of the upper portion of the feed chamber enables greater range of control and can improve combustion efficiency by limiting excess air.

We were thrilled with our first firing! The rocket stove enables ramping up temperatures gradually, which could be a big advantage when working with high moisture content feedstocks. The cross-over from distillation to pyrolysis was fairly tender. Directing all of the gasses into the stove's fuel chamber resulted at first in an over-temperature condition, which was alleviated by diverting pyrolysis gasses out the vinegar condenser pipe--at one point flames were shooting out two meters (very dramatic!)--stimulating conversation on the various uses to which these surplus combustible gasses could be put.

For future firings the kiln will be fitted with thermocouples and a multi-station digital thermometer so we can approach pyrolysis temperatures a bit more gingerly, with the goal of achieving a longer soak at the lower end of the pyrolysis range to retain more organic compounds in the carbon matrix for a more plant-effective biochar. Separately, I'm  working on a design for a rotisserie-style reactor for making biochar mineral complex (BMC)--a step up from garden variety biochar. Wood biochar, clay, chicken litter, and mineral nutrients (rock phosphate, calcium, etc.) will be blended and loaded into a 55 gallon drum mounted laterally over the rocket stove for tumble-heating at sub-pyrolysis temperatures, to create a substance resembling aged terra preta (based on the pioneering work of Stephen Joseph). 

It is worth noting that labor was not among my design considerations. Although labor cost is crucial in commercial economic analysis, home gardeners are known to lavish lots of time on their gardens, heedless of return on their labors. Likewise, backyard biocharers generally do it for the benefit of their garden and for sport (the thrill of the burn). As for the ultimate target audience, subsistence farmers, the low-value of their labor is one of the snares of the poverty trap. Producing biochar, and improving the productivity of their agriculture, might just help them pick the lock.

For a captioned slideshow, go to: Rockin' Rocket Retort.  We'll get a YouTube together soon. 


  1. baja rob,

    thanks for the wonderful inputs.
    im from the philippines. im an engr and closely researching on biochar technology that would help our subsistence farmer earn extra income thru coconut shell charcoal making.

    i like your system for it would not take so much time which means they could do other things while making charcoal.

    you see, their current charcoal making practice is time consuming and would take 3 to 5 days and night vigils to produce one batch of charcoal.

    my question for you are the ff:
    1. how much time consume from starting your fire to pyrolisys?
    2. does your kiln have chimney? where do smoke go? or is the fuel feed the only opening?
    3. what's your firewood feed volume before gases start to come out?
    4. since you fused two drums into one big contatiner, whats your lid locking mechanism used?
    5. do you weld the two drums?

    thank you so much for your time,

  2. Hi Sir,
    I want to prepare wood vinegar from Eucalyptus tree for plant protection purpose.Kindly suggest me how we can extract wood vinegar from the wood.Give details of the easy method so that I can prepare wood vinegar and use it in research purpose.
    Pravin Patel
    Research Scientist.
    Anand Agricultural University,Anand, Gujarat,India

  3. I know about rocket stoves too, but I do not understand how you could mix the two...
    - A rocket stove burns to ashes,
    - and a gazifier burs to char.

    - A rocket stove is lit near the ground, as you did
    - The gas stove is lit on top... So that the char stays oxygen free and does not burn to ashes.

    That would be great if you could make an article to help understand how yours works.


  4. Thermal decomposition regarding other materials might also be helpful to cut back the load on the earth's landfills.

    Pyrolysis Reactor

  5. May i copy this method and post to ?

    This is my blog about making charcoal at home

    Plz reply

  6. very cool, we can do same in Guatemala in El Salvador thanks

  7. Two comments; one could run a second retort from the out-gassing from the first retort into a gas fed rocket burner and only use enough wood to char the first load. Given the efficiency of the rocket design that wouldn't be much. Two for the price of one (sort of ...) . Second comment; an interesting design change might be to mount the barrels horizontally so a clamp on lid could be attached to the inner barrel allowing access to fill and empty without having to move the darn thing. I envision a horizontal trough along the bottom of the outer barrel from the vertical burn tube of the rocket burner allowing the hot gasses to more evenly heat the longer side of the barrel. Ok, three comments; I wonder if a smaller (less fuel to start) fire in the rocket burner might allow pyrolysis to begin just as the fire burns out so there isn't an overheating problem.

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