As with any tool, the skill of the operator determines how well the work is accomplished. It takes years to learn how to use a hammer or shovel. The Three Stone Fire can be effective and clean or it can be very dirty and wasteful. In some kitchens, large fires use a lot of wood and make a great deal of smoke. Small fires are also made that cook food relatively cleanly. 

Watching indigenous experts cook with fire has led to a better understanding of improved biomass fuel use. Cooks who are trying to conserve wood tend to burn the wood at the tip of the stick making flames. Knowledgeable cooks only need a small, hot fire close to the pot to boil water. 

Improving upon a well-made Three Stone Fire has been more difficult than expected. Learning from expert users helped teach engineers how to make better stoves. Well-constructed Three Stone Fires protected from the wind and tended with care, score between 20% and 30% thermal efficiency. Open fires made with moister wood and operated with less attention can score as low as 5%. 

When Tami Bond achieved 33% thermal efficiency with a Three Stone Fire, ARC started to depend on the pot skirt with a 6mm channel gap to help folks use less fuel to cook food. Expertise with the Three Stone Fire is an important skill that empowers the cook and has to be respected.

One of my favorite reference books is “The Energy Primer” published in 1974. It has comprehensive review articles on solar, wind, water, and biomass energy. The following chart comes from a great article on biomass written by Richard Merrill. When I taught semester courses to college students at ARC, I tried to give students useful comparisons so they would be able to estimate the potential success of alternate technologies (unfortunately, fads that are bound to fail are all too prevalent in the green culture).

How much renewable energy can be grown on an acre of land? Can a family create an energy budget based on yearly production? As seen below, there are big differences in the amounts of energy that can be produced by a one acre grain field or one acre woodlot.

One acre of hay yields something like 29 million BTUs per year. One acre of trees is better, producing an estimated 42 million BTUs per year.

If the hay is turned into alcohol the yield is greatly reduced (6 million BTU/year) and the average yield of 3.5 tons per acre of trees is approximately 8.5 million BTU/year.

If the hay is fed to cows and the manure is turned into methane the energy content is 15 million BTU/yr.

Burning biomass for heating and cooking can be a lot more efficient than making alcohol or methane to be used for the same purpose.

At ARC, after decades of “living on the land”, we think that one or two acres of biomass for energy and five acres for food is a good place to start calculations when planning for a secure and happy family. It’s amazing to own land!

In our Nov. 24 newsletter, we shared a basic description of how wood burns from Samuel Baldwin’s book “Biomass Stoves: Engineering Design, Development, and Dissemination” (1987). Here are more details about the process from the same book:

“The temperature of the hot gas above the wood is typically around 1100ºC and is limited by radiant heat loss and by mixing with cold ambient air. As the volatiles rise they react with other volatile molecules forming soot and smoke and simultaneously burning as they mix with oxygen. Some 213 different compounds have so far been identified among these volatiles. If a cold object, such as a pot is placed close to the fire, it will cool and stop the combustion of some of these volatiles, leaving a thick black smoke.”

“Overall, these burning volatiles account for about two-thirds of the energy released by a wood fire. The burning charcoal left behind accounts for the remaining third. Because the volatiles are released as long as the wood is hot, closing off the air supply stops combustion alone. The heat output of the fire is then reduced but the wood continues to be consumed for as long as it is hot, releasing unburned volatiles as smoke and leaving charcoal behind.”

“As the topmost layers gradually lose all their volatiles only a porous char is left behind. This hot char helps catalyze the breakdown of escaping volatile gases, producing lighter, more completely reacting gases to feed the flames. In some cases, the volatiles cannot easily escape through this char layer. As they expand and force their way out, they cause the burning wood to crack and hiss or spit burning embers.”

“The char layer also has a lower thermal conductivity than wood. This slows conduction of heat to the interior and thus slows the release of volatiles to feed the flames.”

“At the surface of the char, carbon dioxide reacts with the char’s carbon to produce carbon monoxide. Slightly further away (fractions of a millimeter) the greater oxygen concentration completes the combustion process by reacting with the carbon monoxide to produce carbon dioxide. The temperature near the surface of the burning charcoal surface is typically about 800ºC. The endothermic (heat absorbing) dissociation of carbon dioxide to carbon monoxide and oxygen, and radiant heat loss, limit higher temperatures.”

“When all the carbon has burned off only mineral salts remain as ash.  This ash limits the flow of oxygen to the interior and so limits the combustion rate. This is an important mechanism controlling the combustion rate in charcoal stoves.”

“The entire process uses about 5 m³ of air (at 20ºC and sea level pressure) to completely burn 1 kg of wood. To completely burn 1 kg of charcoal requires about 9 m³ of air. Thus, a wood fire burning at a power level of 1 kw burns 0.0556 grams of air per second. Additional excess air is always present in open stoves and is important to insure that the combustion process is relatively complete.”