A Model for Combustion and Even Heat in Kilns

by Louis Katz

Note: When written I believe this a good model for reduction. In fact when the purpose of the model is to even the heat distribution in kilns, it still works. Reduction with hydrocarbons also involves some hydrogen as a reducing agent.

As a ceramics student, I was eager to learn about firing. I fired a few kilns, and read what I could on oxidation, reduction, primary and secondary air, etc. Mostly I fired to Cone 10 in downdraft kilns. Fortunately, slow Cone 10 firing in downdraft kilns tend to be even in temperature. Had the firings been uneven, I would not have known how to correct them.

I had been told a few rules that others had found useful in their times of need. These included, "closing the damper will force the heat down" and "oxidation will heat the top." While I'm sure that these "rules" have worked at times, they don't work in all kilns under all circumstances. What I needed was a scientific model or picture of what happened to gas, air, and heat inside a kiln.

My engineering background helped me develop and polish such a model. This article is written not for those who have never fired, but to clarify some things for those who have. While I believe in this model, I understand that it is imperfect; it approximates actual circumstances.

To begin with, in order to have fire there must be fuel, oxygen, and enough heat for combustion. This relationship is often represented as the fire triangle. If you leave out the fuel, oxygen, or heat, no burning takes place.

Another important fact is that fuel and oxygen must be in close contact in order to burn. A log in a fire burns from the outside, where it is in direct contact with air. Gas in a kiln burns only after being mixed with air

The principles are the same for most fuels, with a few exceptions (explained at the end of this article) for solid fuels as well as some oil-burning systems. Because many reduction kilns are fired with natural gas using venturi burners, for simplicity's sake, I will focus on kilns equipped with this type of burner. The fuel discussed is methane, the main constituent of natural gas.

Methane's chemical formula is CH4. This means that for each atom of carbon in methane there are four atoms of hydrogen. The chart shown represents several theoretical types of flames. (No real flame exhibits any one of these reactions exclusively. Some incompletely burned fuel and oxygen are always left over). The first equation, oxidizing, is read, "One molecule of methane plus three molecules of oxygen yield one molecule of carbon dioxide plus two molecules of water with one molecule of oxygen left over." It gives off 896 BTUÕs (British thermal units) of heat for each cubic foot of methane. A BTU is the amount of heat required to raise the temperature of 1 pound of water 1 degree Fahrenheit. Notice, in the column labeled "equations", that from top to bottom the amount of oxygen decreases. The upper equation provides oxygen in excess of what is needed for complete combustion. The bottom two formulas represent reduction, and lack the oxygen necessary to completely burn the fuel. Gases other than oxygen in the air have been ignored. They generally pass through kilns unchanged except for the heat they absorb.

A good understanding of the theoretical neutral flame is essential for understanding firing. If you have a neutral atmosphere in a kiln and the ability to sample the flue gases, on examination you would find neither oxygen, carbon, nor carbon monoxide in these gases. This means that if more methane were introduced, it would not burn. If, instead, more oxygen were added, you would only have more air to heat up. Theoretically, neutral flames give the highest flame temperature, and transfer the most heat to the ware. The reason for this is imperfect mixing of air and fuel.

Air comes in to most kilns in three places; Air coming through the burner itself is primary air. Air that enters through the burner ports, but around and not through the burner is secondary air, as is all air that enters the kiln through various refractory leaks.

In venturi burners, the force of the fuel coming through the orifice pushes air through the burners and into the kiln. Forced-air burners use a blower. The amount of air coming through the venturi burners is usually controlled by an adjustable disk covering the back of the burner. The quantity of air is also, to a smaller extent, affected by the amount and pressure of the fuel flowing through the orifice and by the kiln's draft.

Secondary air is controlled by the damper and is affected by the temperature of the chimney, the amount of wind blowing across the top of the chimney, and the size of the burner ports.

When fuel burns, energy is given off as heat and light. The light is what we call flame. The end or tip of this flame is a good approximation of where the last bit of heat is gained by the combustion of gas. The part of a kiln where the flame ends tends to be the hottest. By controlling the flame's link, we can control the distribution of heat.

The sooner the methane is able to mix with enough air for combustion, the sooner it burns and the shorter the flame. In reduction, there is not enough air in the kiln to burn all the methane so the flame extends into the flue and out the peepholes. The fire burns slowly with a long flame as it consumes the last traces of oxygen in the kiln. In oxidation, the excess air allows quicker air/fuel mixing, resulting in a shorter flame.

Another factor determining the rate of mixing is whether the air enters the kiln through the burner as primary air or from around the burner as secondary air. Primary air mixes with fuel in the burner prior to entering the kiln. This results in a shorter flame than with secondary air which mixes in the kiln.

In updraft kilns, which are generally difficult to fire evenly, the flame enters at the bottom of the kiln and exits through the flue in the top of the kiln. Now suppose that we are firing an updraft, that we want to keep it in reduction, and that it's hot on top. If it didn't matter about the atmosphere, we could oxidize. This would shorten the flame, (by exposing the methane to oxygen sooner), thereby providing more heat to the bottom of the kiln. However, because we want to keep a reducing atmosphere, we have to find another way to shorten the flame. Because primary air mixes sooner than secondary air, it produces a shorter flame. If we increase the primary air by opening the disks at the back of the burners and we close the damper to keep the kiln in reduction, we would have a greater ratio of primary to secondary air, while maintaining a reducing atmosphere.

If the bottom was hot, we would open the damper and close the burner's air regulating disks. This would increase the secondary air while decreasing the primary air, slowing the mix and lengthening the flame.

The technique is the same regardless of atmosphere: To lengthen the flame, increase the proportion of secondary air; to shorten the flame, increase the proportion of primary air. However, there are limits to the amounts of primary and secondary air available without side effects. If you cut the primary air down very low, the fuel first burns in heavy reduction, producing carbon particles (soot). One effect of this is likely to be carbon trapping in glazes; another is that such "free carbon" is very slow to burn and a good proportion of it may travel out through your chimney, thus wasting fuel.

Low primary air settings may also cause backburning, that noisy state where unmixed fuel ignites at the burner's fuel orifice. Backburning occurs when the speed at which the mixture is burning back into the burner exceeds the velocity at which the mixture travels forward through the burner.

On the other hand, too much primary air can cause flame blowoff. This occurs when the speed of gas/air coming out of the burner exceeds the speed at which the mixture is burning back towards the burner. It results in the flame blowing out or burning erratically several inches in front of the burner tip.

Downdraft kilns are a little more complex, but follow the same principles. In a downdraft, the flame is drawn in at the floor of the kiln and up over a bag wall. It leaves through a flue below or level with the floor. If the floor is hot and and the kiln is reducing, shorten the flame by increasing the primary air and decreasing the secondary. However, in some downdrafts, extremely short flames may aggravate the problem by heating up the bottom of the bag wall. This heat then radiates from the bag wall, further heating the bottom of the ware chamber. If the top is hot, lengthen the flame.

In my experience, a hot middle is most often the problem in downdrafts. To even the temperature at the top, one solution seems to be to raise the bag wall-- not an easy job with the kiln firing. Pulling a brick fro just under the kiln arch seems more practical. This allows the kiln to act in part as an updraft, pulling the flame up to the top of the kiln. To even the heat at the bottom, lengthen the flame by increasing the secondary air and decreasing the primary air; or, if it's a reduction firing, you might try reducing a little harder.

Backpressure is often used with reduction. Backpressure is a condition where the pressure in the kiln forces gases out of cracks and peepholes. You can have backpressure in oxidation, but the gases coming out of the kiln won't flame. Likewise you can have reduction without the backpressure.

A related term for which I often find need is forepressure. When you don't have backpressure, forepressure sucks air into the kiln through cracks, and in reduction firings can cause localized areas of oxidation. In oxidation firings, these leaks occasionally cause cool areas. These notoriously occur where the backpressure is the least and the kiln the leakiest; that is, at the bottom of a kiln by the door. To avoid these areas of oxidation, increase the backpressure by closing the damper and opening the burner's primary air disks. Unfortunately, this also changes the flames' length. This alone is good reason for a tight kiln.

One important difference between gas and other fuels is that with other fuels the ratio of primary air can affect the amount of fuel that will be burned in a given amount of time. In order to burn effectively, these other fuels must first be turned into gases. In the case of oil, this is sometimes accomplished by atomizing-- blasting the fuel into minuscule droplets. In atomizing burners, limiting the primary air may also limit the amount of fuel that can be atomized. Other solid and liquid burning systems rely on a hot firebox. Decreasing the ratio of primary to secondary air (for example, decreasing the primary air) lengthens the flame and cools the firebox. This limits the amount of fuel that can be consumed during a given amount of time.

It is important to note that the preceding is only a model; none of the reactions above occurs purely. Analyses of flue effluents, regardless of firing conditions, always show some free carbon, carbon monoxide, and oxygen. Also, changes in temperature take time to occur. The best advice is to correct problems as early as possible.

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copyright 1997 Louis Katz