Wood and charcoal combustion
Charcoal burns much hotter than wood and in an ultralight blower stove, it can burn at damaging temperatures (~1,000+C) particularly when there is little or no wood in the fuel mix. Understanding of these issues is key to the safe use of charcoal as an abundant free fuel and as an unavoidable byproduct of wood pyrolysis.
Introduction
Charcoal remains after the completed pyrolysis of all the volatile and combustible gases (wood gas, organic gas or smoke) from wood. My ultralight blower stove is optimized for burning a balanced mixture of wood and charcoal. It will reach high and destructive temperature if the blower fan is left running and either the wood component that can be pyrolysed is depleted or pure charcoal is added as the only fuel (see example in the photo below)
Understanding ‘wood’ combustion
The review by Browne published in 1958 provides a wonderfully definitive description of wood and charcoal combustion that can explain most of the unusual burning characteristics that I observe in my little stoves. It is worth reading if you would like to better understand the combustion of wood.
He indicates that charcoal can spontaneously combust at temperatures as low as 150 to 250 or even 100 degree C and this is in total contrast to ignition temperatures of 350 to 750 degree C for pyrolysis gas products from wood. To me, this low ignition temperature explains why a little charcoal when added to the stove before a startup can help it light up very quickly and hasten the establishment of hot coal base in the stove.
Browne’s review also indicates that one half to two-thirds of the combustion energy in wood comes from pyrolysis and flame while the remaining portion comes from the burning of the glowing charcoal.
Also in the review, regarding wood combustion he states that;
“Self-sustaining diffusion flames, from organic fuels, burn at 1100 degree C or somewhat more……. As long as gases pour forth rapidly enough to blanket the wood surface to the exclusion of oxygen, the charcoal formed cannot burn and is left to accumulate..”
By contrast on charcoal burning he states that;
“…….it (charcoal) cannot go forth to meet the air for its combustion, but must burn in place as air reaches it.”
I consider his wonderful words comprehensively explain the observations I have made with my unusual stove designs over many years (ultralight blower stove and ultralight tent stove). Understanding the meaning of these words will greatly help in understanding the peculiar way in which my stoves work.
I speculate that;
- My blower fan makes much more air penetrate ‘Browns blanket of diffusion flames from organic fuels’ and then reaches (and burns) the charcoal and reduces the above-mentioned accumulation of charcoal.
- When the hydrocarbon fuel (wood gas) is depleted, my blower fan delivers much more air to the charcoal than diffusion process can and it does so with a high velocity that causes a lot more turbulence. These changes cause the charcoal to burn rapidly and reach destructively high temperatures. “just like a blacksmiths forge.”
This means that without the fan blowing the charcoal burns slowly at lower temperatures for a long time with only a weakly diffusing air supply. This enables the stove to ‘idle’ or ‘hold fire’ for a long time.” With the fan off, the heat from the slow charcoal burn is excellent for simmering/cooking and maintenance of water near boiling point. From this state, pushing the primed fuel sticks in and starting the blower can quickly run the stove up to full power when required.
While I warn about the potential harm of charcoal burning, it still is a wonderful fuel for an ultralight blower stove because at some campsites it just so abundant in old campfires, it burns so cleanly and it just needs to be mixed with wood stick fuel or burnt with less blower air (See pulsed blower operation) to be a very satisfactory and nondestructive fuel.
Understanding fire chemistry and the resultant heat movement mechanisms
The heat from the charcoal combustion is intense (~1000 C) and it makes very little gas when burnt. The heat is largely on the charcoal and cannot move away as a flame does to sustain burner flue pipe draft in the inverted J-burners of my tent stoves.
My crude description of the chemistry of charcoal burning. Two molecules of carbon (solid) react with two molecules of oxygen (gas) and make two molecules of carbon dioxide (gas). There is no increase in the number of gas molecules in this reaction to drive a strong flue pipe draft. In my stoves that have inverted (or upside down) burners, this means that if the wood ‘feed-in’ stalls, then the combustion regresses to charcoal only (with no flame) and the fuel tube will become the undesirable substitute flue pipe. “This is very hot chemistry, but it is largely confined to the charcoal surface, but wait till I use a USB fire blower on it.”
This charcoal combustion contrasts with the moving and expanding flame that is produced by the burning of smoke from wood pyrolysis. In this reaction, there is a hot tongue of flame and gas that moves out and away from the pyrolysing wood. “It can go forth to find oxygen to complete its combustion and transport its heat to distant places.” The number of gas molecules doubles as the flame moves toward the flue pipe during the complete combustion of hydrocarbons. This ability to move to the flue pipe sustains the gas flow to drive the burner draft and the perpetuation of the combustion. “Without this movement of hot flame and gas, the J-burner stove will stop working.”
The extra steam from the water in the damp fuel (bush sticks) and the heat expansion of all the gases, including the ~78% nitrogen in the air, all add to this burner draft driving effect.
According to Dr Karl, Australian hardwood is made up of about 42% cellulose, 26 %, hemicellulose and 26% lignin. So this accounts for about 94% of the ‘stuff of wood and so I probably should use cellulose, rather than hydrocarbon as above) to better approximate the chemistry of the combustion of wood.
Metal protected by smoke?
I also consider that the presence of smoke in the burner may also chemically protect the stove metal from oxidation by creating reducing conditions in the burner. I can not find any reference for this. Smoke must be in the burner for the stove to have a steady moving gas/flame front that finishes its combustion outside the main burner over the fuel sticks and under the second pot (and beyond )in a blower stove. Or alternatively, it can often finish combustion in the lower reaches of the flue pipe in my dome stoves.
Now I can hear some of you saying; “How can carbon that is a fuel protect a metal surface?” I have a fused quartz glass burner tube on one of my tent stoves to provide a little light inside my tent. It is only a partial success as carbon deposits slowly build up on the inside of the glass. Now despite the high temperature on the surface of the glass (est to be up to 1,000C, by colour), stubborn carbon deposits persist for a long time and are difficult to burn off completely.
“This does not surprise me. Carbon is a wonderful element, from diamond jewels, abrasives, carbon fibres, purifiers, gun powder, activated charcoal, capacitors, insulators, conductors, electrodes, motor brushes, graphite lubricants, the building blocks of terrestrial life and coal, a vexatious element involved in climate change (if you have not got your head firmly stuck in the political sand or some other orifice). What an element, so full of wonder.”
Me
Note: Since preparing this post I came across a video (below) where two rocket stove experts discuss the causes of metal decay (including lack of carbon) in parts of rocket mass heaters. “I now no longer feel that I am a lone voice on this issue.”
My Experimental ceramic burner with a fused quartz charcoal-burner glass and USB fire blower demonstrates these points. By design, the burner separates the combustion of charcoal from wood. Air is injected into the charcoal to preferentially bun it. Consequently, the temperature generated is so high that no carbon deposits on the glass and there is a slow erosion or devitrification of the fused quartz that the glass tube is made of.
Wood selection
I select a few dry decaying sticks for fire starting and while continued use of this wood will sustain the burn with a big flame the wood will burn quickly and leave negligible coals.
A more satisfactory burn can be had with dense sticks with solid wood and even intact bark. The dense wood still contains most of the original energy of resins, gums and oils, even if a little damp. This wood is better for sustained heat for convenient cooking with less need for refuelling and will maintain a substantial bed of hot coals. It also means that the flames from the burner will be less prone to setting premature fire to the incoming fuel sticks.
If more heat is required in the second cooking position then some dryer and or thinner fuel sticks should be used on top of the damper sticks and conversely damp sticks on top will suppress premature burning. Lastly, when there is an established hot coal bed larger dense sticks make better fuel because their smaller surface area/volume means they burn slower and they make better, bigger more solid coals that also burn slower.
Damp and wet fuel sticks
On my winter wonderland adventures, damp, wet or even frozen sticks are sometimes my only wood fuel. However, this is not a problem as they still make an excellent fuel in my well-designed stoves.
All that is needed is a little dry wood or DIY ultralight fire starter accelerant sticks to get started and the abundant heat will do the thawing and drying automatically.
For my tent stoves with inverted burners, they perform better with thin damp fuel sticks rather than dry sticks. This is because the damp fuel sticks discourage ‘reverse burning’ or pre-ignition in the input tube. A mixture of damp and dry sticks is also suitable. These issues are discussed in detail in my Experimental ceramic stick burner post where I made a ceramic burner platform to test various inverted burner configurations without fear of thermal destruction.
Splitting sticks
Another way of dealing with wet sticks is to split them finely so that they dry quickly and burn rapidly and easily. This method can be used to start the burner if you have no dry wood.
Conclusion
Understanding combustion from first principles is key to harnessing the wonderful heating power of almost any wood.
The culmination of this experimentation, tinkering, thinking and playing with stick stoves has resulted in my simple compact KISS tent stove and outside blower stoves that can be the one stove. They/it tick all the boxes according to my rules for near perfect combustion, even under the worst of conditions.
Tim
