Primary and Secondary air calculation

There ought to be a straightforward formula relating quantity of air required (primary and secondary air) to burn a quantity or BTU rate of wood. Anyone know where I might find a reference?

I'm in the middle of hacking a homemade wood furnace that will use forced air for combustion, via a small variable speed fan (or two - still experimenting). So far I have had some pretty good experiments out in the driveway, using pretty crude fans (hair dryers and heat guns) to simulate a permanent and controlled forced air draft, and am able to achieve completely clear, smokeless chimney exhaust after much fiddling. I'm probably way-overdoing it on air flow, and would like to order some fans with known CFM ratings.

These experiments started with Rocket Stoves built out of 1 gallon empty paint cans, very successful after some adjustments, then scaled up to 5 gallon metal tins, now scaled up to 50 gallon drums. But, no, this isn't a "barrel stove" - Sotz used to sell those and everybody called them "Sootz". It employs several features which should increase efficiency, such as preheated combustion air, secondary combustion, insulated firebox and so on. Great fun, and it isn't costing $10,000 for an outdoor wood furnace. Sure, it will have to be rebuilt in a few years when it rusts out, and that's just more fun that I can anticipate.

So where might I find info about airflow rates for wood combustion (and other wood-burning physics)?

Sounds like something to really geek out on!


Meanwhile, I found 2-3 other resources, indicating generally that 10-12 LBS of air are required to burn 1 LB of wood efficiently. The actual stochiometric ratio (Thanks - having that word allowed me to do a useful Google search instead of stumbing around!) is around 6 but more air is required in real burners.

This slideshow from EPA pg 11 shows that wood combustion requires about:

1100 to 1500 F temperature
Air ratio of 10-12 : 1 air weight to fuel weight
Residence of 204 seconds in high temperature zone
No flame contact with cold surfaces that quench the flame

12 lbs per pound would mean that a wood stove that burned 40 LB of wood in 2 hours would produce about 125,000 BTU per hour, and would burn up 0.33 LBs of wood per minute, needing about 49.4 say 50 CFM of combustion air. YMMV depending on wood species, moisture, and the usual other variables.

A bigger furnace that burned 75 LB of wood in 2 hrs would require 92.5 say 100 CFM of combustion air and produce roughly 235,000 BTU/Hr.

Thus a really small fan should be enough to supply the combustion air. These fans have little static pressure capacity, so generous ductwork would be needed to keep the static pressure under the spec of 0.26 inches W.C. This fan is more expensive but should handle more pressure

Preheated combustion air is a lot more important that I realized, since temperatures under 1100F will put out the flame. My own experiments with a heat gun also showed this - there was clearly a lot more secondary combustion observable when the heat gun was set on "10" and provided hot secondary air, rather than set on "0". Of course in a real stove one would never use electrically preheated combustion air, this was just an experiment. But an outdoor wood furnace could potentially be sucking in flame-robbing cold air below zero - it isn't just the BTUs required to heat the air, heated air helps the flame stay hot. That is really interesting.

TCaldwell said:

the above read you are quoting is a good guideline, there are many different schools of thought on how to divide the p/s air. The easiest place for you to start would be what is considered by some circles to oppose the damper or fan speeds, if primary is 10% then secondary is 90%, 30/70. 50/50,ect. This will give you the basics on cause and effect. A o2 analyser would be a good tool for your experimenting.

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Recommend any O2 analyzers?

TCaldwell said:

keep in mind that with 2 fans you will have a pressureized combustion chamber, so when you open the loading door you will get some smoke roll out. With a induced system and dampers less smoke roll out. When you modulate your fans or dampers you will need to determine a goal, is it flue temp, combustion temp, o2 or a combination of. most sophisticated algorithims use all of the above.

Click to expand...

I will likely start simple and make it more complicated later (that's how these projects always go!) for example start with temperature - easy to measure. What are you using for O2 sensors for your Garn?