A friend of a friend on Facebook posted photos of a compact wood cooking stove he made which pyrolizes wood or other dry biomass, and it sent me reading. I though I’d try to document some of what I learned and note some of the more interesting links.
Pyrolization is the process of thermally decomposing hydrocarbons in an oxygen poor or oxygen free environment. The products are char, ash, various gases, liquids and tars. Charcoal is made by pyrolization by heating wood in a low oxygen environment. Often, the heat is provided by partial combustion of the wood. The partial combustion drives much of the fuel content is driven off as smoke, and leaves behind charcoal. Pyrolizing wood stoves harness the energy content in the smoke by adding hot air to it and burning it thoroughly. The heat is used for cooking or heating. The resulting charcoal can be burned for other uses, or composted to improve soil quality and provide a way to sequester carbon long-term to help fight global warming.
My own interest in pyrolyzing wood stoves is motivated by a few things. I like tinkering and highly constrained engineering problems, I’m interested in renewable energy and sustainability, and I’m concerned about how I’m going to eat and stay warm if the US and global economies collapse because of declining oil production. Also, I like fire.
I don’t seem entirely alone in my motivations for being interested in stoves. Stoves in genreal have received a lot of attention over the last decade or more. People have looked for ways to improve efficiency and improve flexibility of fueling while working within tight cost-constraints and limited available materials and tools. Improved efficiency reduces fuel consumption. It also reduces pollution, both due to reduced fuel consumption to cook a given amount of food, and more thorough combustion, which produces less smoke from a given amount of fuel. Reduced fuel consumption frees money and/or time that would otherwise be spent on procuring fuel. It also reduces the environmental toll of collecting fuel. Stoves which can adapt to different sources of fuel, or adapting efficient stove designs to local fuel sources also helps reduce the environmental toll. Some stoves let people cook with dried dung, or grass, rather than having to use already scarce wood, or let people cook with waste, like rice chaff or waste paper, rather than having to make special efforts to get cooking fuel.
Balancing these objectives against local conditions has lead to a wide variety of stove designs. Many seem driven solely by efficiency and low cost. Whether or not pyrolyzing wood stoves really hit the sweet spot there depends on your point of view. They can certainly be quite inexpensive; You can build one suitable for a single-family cookpot out of a couple of coffee cans, some tin-snips, and something you can punch holes in the sheet metal with. Whether or not they are efficient depends on what you are comparing them too. If you are comparing them to a good stove that completely consumes all the wood fed into it, then the charcoal from a pyrolyzing stove represents energy that could have gone into cooking. If you step back and consider that the charcoal produced by the stove will offset charcoal that might have been produced in a manner that just dumped the tars and volatile gases from the wood into the air as smoke, then the pyrolizing stove looks a good bit better. As I mentioned earlier though, the ability of “biochar” to serve as a soil amendment and carbon sink has boosted the interest in pyrolysing stoves.
There is a huge amount of information out there about all manner of biomass burning stoves, and I’m still trying to get my head around it, but in the meantime, I thought I’d share some of the more interesting sources I’ve found:
- BioenergyLists.org is a clearinghouse of information from a variety of sources and authors. They cover fuels, a variety of different types of stoves and cookers, important design approaches and other considerations, like construction techniques and materials, measurements of performance and emissons. They also host mailing lists with active discussions of these and other stove-related topics.
- StoveTec sells very efficient “rocket”-style stoves for $35-40 in the US made from a combination of metal and light-weight ceramic. They also sell their stoves outside the US for $3-12 per unit when purchased a shipping container at a time.
- The Aprovecho Research Center is connected to StoveTec. They’ve been working on designing and implementing improved biomass cooking and heating technologies for almost 30 years.
- WorldStove also produces stoves for developing countries, as well as models intended for customers in developed countries. They differ a bit from many others in that some of the parts use advanced manfacturing techniques to produce an air-inlet that creates a vortex, leading to more efficient combustion.
- GoodStove.com is a gateway to a number of different stove designs created by Dr. N. Sai Bhaskar Reddy.
- Lanny Hanson has some nice, concise YouTube videos demonstrating the efficacy of some of his stove designs.
Most of these designs are “open source,” in the hope that people will make improvements and share what they learn.
I really need to do more research. The more I read the less I think I understand. I’ll try to update this post or write new posts as I learn more.