I have always been a big fan of 'tip mulch'. Our local tip (sorry, waste transfer station) get a lot of garden waste. Imagine a pile of lawn clippings, prunings, logs and old fences 20 metres wide, 100m long and 6m high. That's a month of garden waste at our tip. Melbournians are constantly chopping, hedging and rebuilding their gardens, and nobody seems to want to keep the waste.
So the council have a problem, what to do with it all. Once upon a time they buried it, hoping to convert it to coal for future use. This has become too expensive so now they pile it up in huge mountains, then run it through an industrial scale munching machine which knockes everything down to a mixture of pieces ranging from about 20cm long to dust. This sits in another pile and steams happily for another month. Finally, weird people like me drive to the tip with an empty trailer and they fill the trailer as high as they can (often higher then the car), pat the whole thing down and send us on our merry way. They charge 5 for the petrol. If you are feeling energetic you can shovel your own for free.
I brought a trailer load of this home and dumped it into a pile. I estimate the pile's volume at between 2m^3 and 3m^3. One nice thing about this pile is that I got it whilst the pile was still actively decomposing. It steamed when I took it out of the trailer, and it smelt, as Sarah put it, like off eucalyptus oil (which is quite accurate really :).
Normally I would shovel the mulch around the garden immediately, or leave it uncovered in a pile if it wasn't fully rotted. This time I piled the mulch as compactly as possible and covered it in a plastic sheet. A most peculiar thing happened. In the past the pile has been quite slow biologically, rotting over a period of months. This time the mulch is decomposing so fast that the top of the pile is actually hot to touch. This will kill weeds and pathogens quite effectively, but furthermore, it can be used to keep household compost warm too.
The mulchinator is a small scale high speed composting system for rapidly breaking down arbitrary waste, in particular, ex- rabbit cage straw. It consists of a large pile of tip mulch with household waste piled on top, the whole thing is covered in clear plastic sheet.
Considering the remarkable amount of heat being produced I am wondering if this heat could be used for household heating (heat water in pipes, release in house). The significant advantage over directly burning the green waste is the fact that rather than ash and gases the mulchinator produces useful compost for soil improvement, and it leaves a good deal of the carbon sequestered. The resulting mix is very similar to peat in consistency and pH and can be sterile if good yard practices are used.