Biochar Rocket Stoves

There are environmental and economic advantages to producing your own biochar as a byproduct of generating domestic heat and hot water.

There are various programmes implementing clean burning cookstoves which produce biochar in various majority world countries. Variations of the tlud and the anila stove are the two stove designs used to do this.

I have hybridised a rocket stove with an anila stove to produce a stove similar to a rocket stove but with 2 advantages;

1, It produces biochar.

2, It produces more heat than a conventional rocket stove.

In particular situations it has advantages over anila and tlud stoves;

1, It is lit from the front and so can be used indoors under a flue pipe and heaters.

2, It is easier to control the stove temperature by controlling the amount of wood being fed into the combustion chamber.

biochar rocket stove

Included is a leaflet with a diagram of this modified rocket stove. This diagram shows dimensions which are known to work. These dimensions can be scaled up but this size is suitable, for example, for heating a small scale domestic water heater or a large oven. (Thank you Diane for this drawing.)

When it comes to using a pyrolisis stove indoors this stove has several advantages over other biochar stoves. The fire is lit and fed from the front, as with rocket stoves, rather than from the top. This means that the stove can be placed under a flue, oven, water heater or thermal mass and the flue gases effeciently taken out of the room during lighting and throughout the burn. (A door can be fitted to the front of the stove to ensure no emissions escape into a room when the stove has finished burning).

A biochar rocket stove in a thermal mass heat bench

It is my belief that the principles of biochar cook stoves can be applied to developed as well as developing countries. This would help us to reduce our ecological footprint, sequester CO2 and produce a greater proportion of our food locally. The modified rocket stove is clean, easy to light, does not require charcoal to be quenched, can be fitted with a self filling hopper and auger and is more suited to domestic use in developed countries than are the anila or tlud.

How they work.

A rocket stove can be hybridised with an anila stove by replacing the insulation of a rocket stove with biomass such as woodchip.

The inner elbow becomes the combustion chamber and the outer chamber (insulation chamber in a rocket stove) is now a retort.

Holes need to be cut in the bottom of the combustion elbow in order that the wood gases can pass from the retort into the combustion elbow (and to prevent pressure build up and explosion in the retort).

Like anilla stoves, the only place for the wood gases produced in the retort to go is through the holes in the bottom of the combustion chamber where they then combust, adding to the heat being produced.

(Again, refer to the instruction leaflet for a diagram).

The stove shown in the diagram is not fitted with a hopper to allow for ease of filling the woodchip or emptying the biochar. A hopper and removable bucket can be fitted (ensuring the retort remains air tight) in order to make filling and emptying easier.

A fan has been fitted which forces air into the combustion zone, immediately above the point at which the pyrolisis gases enter. Although the stove will burn and produce biochar without the aid of a fan, the flames produced are orange in colour, indicating unburnt soot or carbon. Unburnt carbon released into the atmosphere is a contributor to climate change. The heat output of the stove is significantly increased with the forced air, as the particles are then fully combusted (the flame should turn whiter, less orange).

The fan also makes it easier to ensure complete pyrolisis with less wood.

Using the Modified Rocket Stove

Without a feed hopper, the stove has to be removed from under the heat exchanger or flue and turned upside down.

The 4 nuts (or catches) attaching the base plate need to be removed and the base plate taken off. Biochar from a previous burn can then be emptied into a bucket for use. Wood chip or any other suitable biomass is then fed into the retort chamber. The base plate and the 4 attaching nuts are then replaced. The stove can then be placed back under the flue or heat exchanger ready to be lit.

As with a rocket stove, a fire is simply lit in the elbow of the combustion chamber. Turning the fan on makes lighting the stove easier. Putting a hand full of dry wood chip into the combustion chamber makes it easier to light. As with rocket stoves, the aim is to establish a small pile of charcoal in the combustion elbow. Around 3 sticks of wood at a time can be fed into this charcoal, end grain first.

After around 5 or 10 minutes, the flame should noticably increase in size. This indicates that pyrolisis of the wood chip has begun. The fire should be mantained and the fan turned on until this flame dies down (30 to 60 minutes depending on the biomass used). It is good to continue running the stove as a normal rocket stove for at least a further 20 or 30 minutes in order to ensure complete pyrolisis.

The fan only needs to be on when there is a flame present, the aim is to maximise the efficiency of combustion.

The stove can be left for a couple of hours before being removed and the biochar emptied. This will ensure a more complete pyrolisis. If left for longer still then this will result in less smoke being emitted when emptying the stove and will prevent the need for quenching the biochar with water to prevent it from combusting when in thepresence of air (although this is a good safety precaution).

Other Considerations.

It is important that the biomass in the retort as well as the sticks used in the combustion chamber are properly dried. Moisture in green or wet wood will cool down the combuston resulting in;

1, heat wasted.

2, Increased emissions.

3, Possible incomplete pyrolisis.

Woodchip that has not been dried will not store and will rot.

It is important that the biomass used comes from a sustainable source, either horticultural or other waste or from sustainably managed coppice woodland. There should not be any need to cut down trees for fuel. Rather, the transition from fossil fuel dependence should support and go hand in hand with an increase in sustainably managed woodland.

Useful Links

1. This early video was from a pizza party May 1st, Beltane 2011. They’ve come a long way since then, cleaner and smokeless.

2. More recently, at the Engineers without borders festival August 2011/

3. And Beltane 2012, added computer fan for cleaner burn and more heat.