There is currently no good way of disposing of garden waste. Composting doesn't really work, shredding is time consuming and the resulting mulch still needs to be put somewhere, incinerators tend to be smoky, dumping it requires equipment (e.g. trailer) time and energy, and it is expensive to have it taken away.

What is needed is a backyard pyrolyser, i.e. something to turn garden waste into charcoal.

I envisage this would be a well insulated large metal pot with a securely fastening lid with a small exhaust outlet (essentially a large pressure cooker). A propane burner heats the pot to a few hundred degrees C.

The exhaust would be fed through a basic gas scrubber (a drum of water which the exhaust bubbles through). The scrubbed exhaust should have a high content of hydrogen so could be fed back to supplement the propane at the burner.

The charcoal would then be dug into the garden to make terra preta.

I know that making charcoal with garden waste is not a new idea. However, this is generally done on a large scale after it has been dumped. When it is done on a small scale it is generally inefficient and dirty. The new idea here is a simple product that could be placed in a residential backyard.
-- xaviergisz, Apr 24 2011

illustration http://imgur.com/a/XZm4z
should be self explanatory. It will need one-way valves at obvious locations. [xaviergisz, Apr 25 2011, last modified Dec 13 2011]

illustration http://i.imgur.com/AtZpF.jpg
lid closed [xaviergisz, Apr 25 2011]

A somewhat larger-scale version Solar_20Tar_20Stills
This Idea seems to have recurred. [Vernon, Apr 25 2011]

Another home-sized version Home_20Solar_20Charcoal_20Distiller
This Idea seems to have recurred. [Vernon, Apr 25 2011]

The scrubber might not be needed. I think pyrolysis gas burns fairly cleanly if the flame is hot and there's enough oxygen.
-- spidermother, Apr 24 2011


//Composting doesn't really work//

Really? Composting doesn't happen overnight, but it definitely works, Vermiposting even more so.
-- MechE, Apr 25 2011


I've found composting works well with food scraps, but not so well for grass clippings, sticks etc. What's your secret?
-- xaviergisz, Apr 25 2011


There are limits to how quickly wood will break down, but small sticks are okay.

The real problem with grass clippings are density and moisture. It also has a high nitrogen content which needs high carbon to balance it. You need to mix them fairly well with bulky dry materials to break them up and allow aeration (those sticks, or leaves, or dry plant material are perfect for this, you can use paper if needed). Also turn the pile more often to break up and mix in the grass.
-- MechE, Apr 25 2011


Once you have the charcoal, why not use the water gas/producer gas cycle to produce" green" domestic fuel ?
-- 8th of 7, Apr 25 2011


[8th...] probably better to bury the carbon dust this will produce, or were you thinking a F-T process?
-- 4whom, Apr 25 2011


Bubbling through water would be a good enough 1- way valve.
-- marklar, Apr 25 2011


F-T is energetically inefficient and quite polluting, but it does peoduce octane and indeed aviation spirit, so yes.
-- 8th of 7, Apr 26 2011


//Is it worth combining this with a dehydrating greenhouse? Shred the waste, place on shelves in the greenhouse till dry, then pyrolyse?//

You could if you wanted to. You'd certainly fit more in the pyrolyser and expend less energy pyrolysing it.

But the main benefit of this system is the simplicity. Put in any garden waste regardless of moisture content, leave burners on long enough, and eventually charcoal comes out.
-- xaviergisz, Apr 26 2011


Composting works well - the trick is (as [MechE] said) to get the Carbon/Nitrogen mix right. Paper (from your paper shredder's bin) can be used to add Carbon. Leaves don't compost well - the best way to deal with them is to wet them and put them into a large plastic bin-liner. Tie the top, make a few air-holes and leave for a year, after which the leaves will have turned into a rich mulch.
-- hippo, Apr 26 2011



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