Since I first built my first RMH, I've always thought that small diameter fuel would burn hotter and more efficiently than larger diameter fuel of the same material. I have been given a large supply of long hardwood skids (pallets) as some would call them. They are in pieces of 1"x3", 1"x4", 1" x6" and 3"x3" and 4"x4" hardwood which I cut into 24" lengths. I spent a lot of time with a woodsplitter splitting these pieces to roughly 1"x 1 1/2" to "2" thinking that they would burn best that way.
But just yesterday I decided just to try to see how the larger pieces would burn. So I loaded my 24" high burn tube with 1x3's 1x4's even a 1x6 and a3x3.
With the smaller sized fuel I have rarely registered a barrel top temp at 600F (my barrel top is a full 8 feet from point of initial combustion.) But with this large fuel it easily soared over 750F! That was with a strong wind outside which drew a stronger than usual draft, but on two other burns with no wind or very little wind my barrel top temps went over 650F easily. My RMH is detailed here at https://permies.com/t/69632/Building-tube-steel-air-cooled
BTW, my ash volume is considerably less burning large diameter fuel than it was with smaller diameter fuel of the very same material (hardwood).
I'm curious! What is the physical explanation for this? Even tho I don't understand it, I'm certainly not going to waste precious time splitting my skid wood smaller in the future!!!
Well Bruce, there are several things at work here and this is my take on what is going on with the fire. It seems counter intuitive at first but if we start with extreme examples it'll hopefully make sense. In our example we have two batches of wood, each is one cubic foot, each is made of yellow pine, each has the same cellular density. The first is a solid block 1'x1'x1', the second is a box 1'x1'x1' filled with tooth pics. Which do you suppose will have the greater mass, and therefore the the greater stored energy? There is more fuel in the block and therefore greater potential energy. If you can get both batches up to the same temperature so they both combust at the same rate (for our example: 1 cu. ft. per hour ), the block will produce more heat.
But with any dynamic system there is going to be a sweet spot, and this is usually a balancing act between constituent parts. With fire there are three constituent parts, fuel, air, heat. If the fuel is so large that you don't get enough air the heat will be low. If there is too much air it dilutes the heat with cold air. So for best combustion you want to get the largest fuel that will burn efficiently in your stove and still stay hot enough to combust at a high rate. This may require some smaller pieces in the beginning of the burn, or even throughout the burn depending on the stove's volumetric efficiency, to support rapid combustion and once the burn chamber is up to temp you can use larger pieces and they will continue their continuous conflagration.
For my current J-tube I start out with a handful of kindling and two 1"x1" splits. But after those are nearly consumed I put in two or three ~ 2"x3" splits. They will burn for about 40 minutes unattended and end up as fine ash. With my older unit, it couldn't handle splits larger than 1.5"x1.5" or it would lose its rapid burn and begin to cool down and smoke.
Good observations on your fuel Bruce. I think Duane gave good info. As he said, there is likely a sweet spot that you will find which generates the optimum burn temperatures.
It should be noted that fuel is often split to facilitate drying, as dry wood burns hotter for a number of reasons.
Assuming that your wood of all different sizes in an experiment were equally dry, then the fire triangle comes into play as another key factor.
The triangle has fuel, oxygen, and temperature as it's components. When all of these meet the minimum requirements, fire occurs.
On top of this, the fuel type, the oxygen levels, and the adequate temperature for combustion also all have variables; red cedar for instance burns more rapidly and hotter in the short term than many other woods due to it's airy consistency and highly flammable oils, but it will not produce a long lasting burn that say an equal volume of oak does. On top of this, as Duane mentioned, the volume of a stick when starting a fire is much different than when a fire is already rocking, and fuel types (species) would make a difference here as well. Larger cedar chunks will last longer, and produce high heat, whereas smaller chunks will produce a short burst of heat, and that heat might not be quite as substantial as that produced by somewhat larger fuel; it burns too rapidly and does not have as much opportunity to radiate. Different fuel species will likely have very different sweet spots in this regard, but generally, in the bush I go from match stick size to finger sized to thumb sized to wrist sized, and once you get to forearm sized you can burn anything, but if you try to go from matchstick sized to forearm size you are going to have issues. The fire just isn't hot enough, or have enough lasting power to ignite the larger wood.
Where the oxygen is delivered to the fire and in what volume makes a large difference on the effectiveness of this component as well. People split the large wood block to expose more of the cellular resins to ignition temperature so that they will vaporize and ignite, but this does the extra purposes of giving it oxygen during the firing and drying it beforehand. As in most Rocket stoves, and RMH's in particular, keeping the main combustion area at a certain optimum temperature is a design feature, usually done by insulating the combustion zones. I know in your case, with steel and air cooling you are doing something quite different. I'm just stating that all of these factors in the triangle are not only necessary, but they are in a flux depending on the components. So in the case of your pallet wood, there is a sweet spot where splitting down to a point is essential, and a point where you are going to have diminishing returns, based on the density of the fuel, it's relative combustability at your ignition point, the heat that is potentially generated at that location and throughout the combustion zone, and how the oxygen is supplied to it in the initiating burn location as well as throughout the gas burning area, and all of that combined with the draw that you are receiving that is pulling the air/gasses through the rest of the system.
Lots of things to experiment with. Again, good observations, ..........and keep tinkering!
"Never doubt that a small group of thoughtful, committed citizens can change the world; indeed, it's the only thing that ever has."-Margaret Mead "The only thing worse than being blind, is having sight but no vision."-Helen Keller
Location: S. Ontario, Canada
posted 2 years ago
Thank you both Duane and Robert! Your answers make a lot of sense. This whole experiment has shown me I sure don't need to split this pallet wood smaller than it is already cut! And since it is already very dry, drying is NOT an issue here. I'm just greatly relieved not to have to do all the splitting I was anticipating. As for starting, I don't even notice a huge difference there! I usually load my feed tube full, put a handful of pine cones at the base, crumple one page of newspaper, light it with a match and away it all goes! The large pieces do take slightly longer to get going but not long at all. Thanks for your explanations as I continue to tinker with this fire eating monster!
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