Theory of Design of a Functional Secondary Combustion System

The JUCA design does not depend on air-starvation (air-tight) operation so the problems of excessive creosote production and carbon monoxide production do not represent the great problem existing in most wood burners on the market. For this reason, it was unnecessary for us to try to arrange a functioning secondary air system even when that was the current rage. We quietly said for years that it was unlikely that any of the secondary air systems worked very well, if at all. Independent researchers eventually showed that we were right all along. With this preface, we herein will give the reasoning why all existing secondary air systems don't work well; and the design considerations necessary to make a system that does work as intended (again, for AIRTIGHT products where it could be important).

See other sheets of ours for a description of the overall theory of airtight products and that of products like the JUCA (Sheets 128, 120, 310, 314 and others).

Since the lack of enough oxygen in the fire's vicinity is the cause for the incomplete combustion in an air-starvation burner, it would be advantageous to supply air to it later on to complete the combustion. The hitch is that the primary reaction that needs to occur (carbon monoxide plus oxygen gives carbon dioxide) will only occur above about 1200°F. If it didn't happen while it was still in the flame tips, we may have trouble keeping it hot enough for the reaction to go.

Let's consider an example. The actual flame temperature of a wood fire can range from about 900°F to 2500°F. An "average" fire will commonly be around 1900°F. Almost instantly on leaving the flame tip the smoke mixes with other air or smoke, quickly reducing the temperature. The amount of this temperature reduction is dependent on many variables, some of which are not yet fully understood. For argument's sake, let's say it is at 1400°F. In order to permit substantial secondary combustion to occur, it will be necessary to supply a decent amount of secondary combustion air, generally on the order of the primary air supply.

This is necessary so that the statistical probability of CO molecules being able to "bump into" O2 in the hot zone is high, preferably at least 90%. The molecules will only be in this environment a very short time, but we want the great majority of them to have the opportunity to combine with the oxygen atoms. These conditions are mandatory to ensure substantial and consistent secondary combustion over a wide range of firing conditions.

Some currently available products do seem to be able to support secondary combustion SOME OF THE TIME and TO A LIMITED EXTENT. Under optimal conditions maybe 1/3 of the available fuel is recovered. Under most other conditions, less. The amount of air supplied is too small to allow high probability of the CO and O2 reacting. Just do a molal analysis to see the lop-sided proportion of many CO to few O2 molecules. Actually if pure oxygen was fed, it would work fairly well. Air being 80% Nitrogen just reduces the probabilities of reaction.

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And it represents more material that must be pre-heated so as not to chill the smoke to below 1200°F. Getting back to our example, if we mix our 1400°F smoke with an equal amount of room temperature secondary air, the resultant temperature of the mixture will be less than 800°F, far less than the necessary 1200°F. No reaction. Poor efficiency. A lot of creosote. A lot of pollution. Bad. You can probably see that you are going to need a source of secondary combustion air at about 1000°F or higher under these conditions. A pre-heater will be necessary to boost the room air to 1000°F.

Unfortunately, there are some conditions of low fire (severely held back) where the smoke itself is under 1200°F within inches of the logs. In that case secondary combustion is almost out of the question. It is ironic that in the situation of a severely suffocated fire that most needs the effect of secondary combustion, it is most difficult to obtain. When the fire is burning relatively freely (and therefore cleanly), that is when it is easiest to initiate secondary combustion.

Again let's get back to the example at hand. We need to pre-heat air to 1000°F. It will be necessary to use a heat exchanger to do this. Some current products have a 6" long tube to pre-heat the air as it passes through. We'll see that this isn't even close to enough exchanger surface. Assuming that the stove consumes 35 CFM of primary air for the fire, we will also need 35 CFM of secondary air as described above. To heat 35 CFM from 70°F to 1000°F will take about 35 x (1000-70) x 0.24 / 28 * 60 or approximately 17000 Btu/hr. The 0.24 is the air's specific heat; the 1/28 is the air's specific volume at the mean temperature; 60 is the number of minutes in an hour.

When we are talking about a unit that is only going to develop 20,000 or 30,000 Btu/hr, you can see that we are going to have to use more than half of the capability for pre-heating. The secondary combustion might add 25% to the output (maybe 7000 Btu/hr) but you use 17000 to do it. A losing proposition. Except for the safety considerations, it would be foolish to consider.

Conventional heat exchange analysis (see other sheets in the 300 series) will give the necessary areas of heat exchange for this pre-heater. We will avoid the math here. A two stage boost heater is most logical and effective here, where the first stage heats the air to (600°F in our example). The necessary area in a 700°F part of the stove for this exchanger is 1.6 sq. ft. The air then passes to the second exchanger to be heated further (to 1000°F) in a hotter part of the firebox right over the flame tips. The necessary area of this exchanger is 1.5 sq. ft., assuming the smoke temp is 1300°F in that part of the firebox.

If the supply tube is 2" in diameter, the first exchanger must be nearly 9 feet long (wrapped around inside the firebox) and then the second will also be about 9 feet long. The secondary combustion air supply would have to pass through a total of 18 feet of specially located heat exchanger to ensure good secondary combustion. There would not be much room left in the firebox in most stoves for any exchangers for USEFUL heat. And remember, even then there are conditions when secondary combustion still won't occur. Is there any wonder why currently available products with a stub tube pre-heater don't work?

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