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.”

Sometimes it’s good to step back and review the very basis of stove work – fire. Samuel Baldwin gives a good description of how wood burns in his book “Biomass Stoves: Engineering Design, Development, and Dissemination” (1987).

“The combustion of wood and other raw biomass is very complicated but can be broken down crudely into the following steps:”

“The solid is heated to about 100ºC and the absorbed water is boiled out of the wood or migrates along the wood grain to cooler areas and re-condenses. At slightly higher temperatures, water that is weakly bound to molecular groups is also given off.  Heat transfer through the wood is primarily by convection.”

“As the temperature increases to about 200ºC, hemicellulose begins to decompose followed by cellulose. Decomposition becomes extensive at temperatures around 300ºC. Typically only 15% of cellulose and hemicellulose remain as fixed carbon and the remainder is released as volatiles gases. Roughly 50% of the lignin remains behind as fixed carbon”

“The volatiles produced by this decomposition may escape as smoke or may re-condense inside the wood away from the heated zone. This can often be seen as pitch oozing out of the non-burning end of the wood. Heat transfer into the wood is still primarily by conduction, but the volatiles flowing out of the heated zone carry some heat away by convection.”

“As the volatiles escape the wood, they mix with oxygen and, at about 550ºC, ignite producing a yellow flame above the wood. Although radiant heat from the flame itself (not counting radiant emission from the charcoal) accounts for less than 14% of the total energy of combustion, it is crucial in maintaining combustion. Some of the radiant heat from this flame strikes the wood, heating it and causing further decomposition. The wood then releases more volatiles, which burn, closing the cycle. The rate of combustion is then controlled by the rate at which these volatiles are released. For very small pieces of wood, there is a large surface area to absorb radiant heat compared to a little distance for the heat to penetrate or for the volatiles to escape. Thus, fires with small pieces of wood tend to burn quickly. This is also why it is easier to start a small piece of wood burning than a large thick one. A thick piece of wood has less area to absorb the radiant heat from the flame compared to the greater distances through which the heat and volatiles must pass within the wood and the larger mass that must be heated.”