After careful seasoning and storing, use a simple drying chimney for firewood. Next generation cookstoves can also be carbon-positive!
Wet firewood makes lots of smoke, even in an improved cookstove. Luckily you can get dry firewood by avoiding the 2 main sources of moisture, from unseasoned or uncovered wood. If you then need even drier wood, just use my design for a simple drying chimney. My drying chimney was made from scrap materials at no cost. Hot air flows up the chimney, down through the damp firewood and out through the vents. It can convert 10kg of damp firewood (20% moisture) to 9kg dry firewood (10% moisture) in around 30 minutes, using around 300g firewood to provide the drying heat. The firewood can be any length that fits the height of the chimney but a maximum diameter of 4cm is advised. A 2" diameter roll of wire mesh is laid around the outside of the base to keep the wood in the path of the heat and away from the vents. Thicker and wetter pieces of wood are placed toward the top. I use a next generation (carbon-positive, biochar producing) cookstove to provide the heat, though any cookstove could work. Spare heat is available at the top of the chimney, for frying, simmering or drying. If available, fire-proof insulation such as mineral wool building insulation can be wrapped around the drying chimney (although I've used it successfully many times without this). After cooling, the dried wood should be stored carefully to avoid reabsorbing moisture from the air or from rain. I use a plastic rubbish bin to store dried wood outdoors and the wood stayed dry all winter. Happy cooking!
Drying chimney in use, showing moisture from the wood coming out as steam. The drying chimney is supported by metal crates and the cookstove heatsource is placed underneath. The drying chimney could also be used above a larger stable cookstove as the weight of the cool unit is easily managed by one person. Use a simmering-level flame and (if needed) a flame concentrator funnel to direct the flame into the chimney pipe.
Category of the action
Who will take these actions?
- NGO or government aid organisations work with me to create an opensource guide
- Community organisations gather suitable materials and build drying chimneys (or holds workshops for DIY)
- Households make and use their own drying chimneys
- In the next stage, we co-create with communities a next generation of carbon-positive cookstoves that produce biochar as byproduct. This is added to soil and supports tomorrow's growth of food and trees in a virtuous cycle.
What are other key benefits?
- Simplified design
- Easy to construct from scrap materials using basic DIY skills
- Can be made at home or in small workshops with no need for powertools or welding
- Uses a minimum of firewood for wood-drying
- Surplus heat is available for other uses
- Warm damp air is available for other uses, such as fixing bends into woodwork
- Firewood can be dried to any desired moisture level (10 to 15% is ideal, complete dryness is unnecessary)
- The design is scalable to suit the volume of wood, size of cookstove and construction materials available
- The combination of drying chimneys and efficient cookstoves proves that biomass is an effective modern fuel and there is no need for fossil-fuels in cooking
- Can be used with carbon-positive biochar-making cookstoves
- Enables net-improvement of atmospheric carbon surplus, soil productivity and forests
Next generation carbon-positive biochar-producing cookstove in use. For use to dry fuelwood, the pot-holder chimney is replaced by the drying chimney and stand.
What are the proposal’s costs?
I used salvaged materials at no cost. An ideal material for the main body is the metal kitchen bins widely available in many countries. These are poorly made and hard to repair but perfect to recycle as drying chimneys.
If not already available, a pair of tin snips is useful ('aviation' tin snips are easiest to use). Typical cost is under $10 including international postage.
A possible unintended side effect would be to enable burning of more wood, adding to pressure on forests. This can be avoided by upgrading from 'improved' cookstoves (based on rocket-stoves) to next generation carbon-positive cookstoves that produce charcoal as the byproduct. This charcoal (called biochar) is added to compost and soil to support sustainable food and forest growth.
- The exhaust from the drying chimney looks like smoke from an open fire but it's actually just steam from the moisture which is removed from the fuelwood as it dries. So long as the cookstove used underneath is smokeless then no new smoke should be made by the drying chimney. The drying chimney should however be used in a safe place outdoors, away from windows, animals and passers-by.
- If you do get strong-smelling smoke then some of the wood has become fully dried and begun to char (turn into charcoal). Remove the flame from underneath and using leather gloves or metal tongs move the charring pieces from the top of the stack to a saucepan or biscuit tin. When you put the lid on this excludes air and stops the charring so the wood can still be used as fuelwood when cool. Avoid this in future by drying for a shorter time. Fuelwood does not need to be fully dry, 15% moisture should work in any cookstove.
- If you get condensation inside the metal cylinder then add mineral-wool insulation around the drying chimney. Or add a second metal cylinder, to create an insulating air gap. Check the vent holes are spaced around the outside and large enough (I used 2 holes each approx. the size of the chimney pipe hole).
Short-term: Make drying chimneys accessible worldwide, to enable successful all-seasons use of renewable feedstocks for cooking. Upgrade all rocket-stove type cookstoves to carbon-positive biochar-producing cookstoves. Minimise reliance on wood as cooking fuel by advancing 'anila' style cookstoves that use mostly agricultural byproducts as fuel.
Medium term: Eliminate all dependence on both fossil fuels and deforestation for cooking. Cooking should produce (not consume) charcoal, which will be added into soils to enable food security, climate rescue and global reforestation.
Presentation at COP21 in Paris, December 2015, about carbon-positive biomass cookstoves for carbon sequestration.