Kilns: Theory and Practice : Combustion

Combustion, the process of combining oxygen with fuel, results in the release of heat. In order to achieve proper combustion, air and fuel are mixed together in a ration that does not leave excess fuel unburnt or deprive the fuel of an opportunity to burn at its maximum rate. The correct proportion is ten parts of air to one part of fuel. An inadequate quantity of fuel in the mixture results in oxidation because the oxygen in the air is not totally consumed while excess fuel results in a "rich mixture" (or an insufficient amount of oxygen), creating a carbonizing atmosphere and incomplete burning of fuel. The ignition which initiates combustion occurs when the oxidation reaction (a flame) is induced by an external heat source and the reaction itself releases heat faster than the heat which is lost to its surroundings. Or, to put it in another way, after introduction of the external heat source, the heat from the oxidation reaction ignites in what is referred to as "spontaneous combustion". All measurements of heat are based upon B.t.u. (British Thermal Unit) - the quantity of heat necessary to raise one pound of water one degree Fahrenheit. The amount of B.t.u given off by any natural or liquid gas burner is determined by the size of the orifice. The orifice allows a given amount of fuel to pass  into the burner chamber, where the fuel is combined with air, and where, when ignited, it creates a flame. A flame may be defined as a zone in which the combustion reaction is occurring at such a rate as to produce visible radiation. The flame front is that place along which combustion starts. When the correct conditions take place, the flame front appears to be stationary, because the flame is moving toward the end of he burner with the same speed that the fuel-air mixture is coming out. If the fuel-air mixture is fed into the burner at too fast a rate, the flame may blow off.  This is identified as a "pop off" of the flame from the lip of the burner which leaves a gap between the rear end of the flame and the front end of the burner. If the fuel-air mixture is fed into the burner too slow, the flame may have a "flashback" into the burner. in some extreme cases the flame may flash back as far as the mixing point just above the orifice hole, causing the burner itself, which gets extremely hot, to become the heat chamber for the flame instead of the kiln. Atmospheric burners using natural or liquid gas have two important and basic components which are necessary for successful operation : primary and secondary air control. Although these components are also necessary factors in burners using dense and hard fuels, they are more identifiable in burners using gas fuels, where they are easier to control. When the primary air combines with the flame at the ignition point of the burner, the cooler air is heated, and as a result the flame increases in velocity, creating a forceful driving flame at the burner tip. This basic principle, known as the Venturi effect, is the same one that powers a jet engine on a 747. The secondary air is hat which combines with the flame at the tip of the burner where proper combustion is taking place and is being driven into the kiln. An excessive amount of secondary air at this location creates a "cool" flame going into the kiln, and insufficient secondary air creates a flame lacking in proper combustion, which results in a reducing or smoky flame. Primary air is controlled by the air shutter located near the orifice head - secondary air by the position of the burner head in the burner port. For any given burner, a change in the fuel-mixture pressure or the amount of primary air will affect the flame shape. Increase in fuel pressure will broaden the flame in most burners while an increase in the primary air will shorten the flame (assuming the input rate remains the same). But the design of the burner has much more effect upon flame length and shape than either of these operating variables. Good mixing, produced by a high degree of turbulence and velocity, creates a short bushy flame, whereas poor (delayed) mixing and low velocity result in a long, slender flame. Interestingly, burners may be ignited at the point of their external heat termination (the end of the burner). If the position of the burner is correct, initial combustion occurs only at this point, often leaving the internal area of the burner totally without flame. Although this creates a soft flowing flame rather than one which has velocity, this flame serves its purpose well by creating a reduction atmosphere within the kiln and still providing necessary heat rise. Another type of burner which does not operate by using atmospheric air as a part of its mixing procedure is the forced-air burner. this type of burner does not require a secondary air intake since the air is being forced into the burner chamber by mechanical means. With this burner, a much greater fuel input into the chamber of the burner is possible, and its flame, which is very forceful, enables a massive amount of B.t.u. to be thrust into the kiln chamber. It should be noted that burners designed for operation at sea level may not work as efficiently at high altitude, where there is less oxygen in the air. Oil burners of various types work most efficiently with a forced-air blower system. However, some oil burners are designed to operate without forced air and yet are able to provide an extremely powerful flame, as if forced air were being used. An example of this type is the oil burner which operates by converting oil into a vapor under pressure before it is released at the orifice opening. Naturally, this burner does not require any electrical means to create its forceful flame. After you have acquired a basic understanding of burners, it is important you become aware of the effect that flame has upon that are of the kiln where heat input is being initiated - the area universally referred to as the "firebox". The firebox is the heat energy source for the entire kiln; it is the motor which makes the kiln go! Firebox shape and sizes may differ, according to the type of kiln. Downdraft kilns normally contain well defined fireboxes where the massive buildup of flame goes on before the flame thrusts its heat up into the ware chamber. Updraft kilns usually have the area  below the bottom ware shelf as the firebox, although there are exceptions to this arrangement. in both cases, the fireboxes takes the greatest beating during the firing cycles, since it is subjected to thermal shock at the firing's onset. It must also withstand higher temperatures than the rest of the kiln because of its generating source and continuous flame impingement. Kilns made of bricks - whether they be refractory or insulating - constantly need repair in the firebox area because of these factors. The bricks here show considerable expansion and contraction compared to other parts of the kiln, and it is necessary to "beef up" this are air durability is required. However, as already indicated, with kilns that use ceramic fibers as a hot face covering on the internal walls of the firebox, the material is unaffected by either thermal shock or flame impingement. Also, most of the characteristics of firebox abuse, such as expansion and contraction, are eliminated since the material does not expand. Ceramic fibers do contract slightly (about 2 to 3 percent) if they are taken higher than their given hot-face working temperatures and therefore, if the internal surface of a kiln contains a ceramic-fiber face rated at 2,300°F in the firebox area and heat generated during the firing exceeds this temperature, the material will shrink slightly and become somewhat brittle. It will not, however, expand again once it has contracted. One solution to slight contraction might be to use a higher rated ceramic fiber in the firebox area. For example, some specialized ceramic fibers made of zircon  have working temperatures as high as 4,500°F, but they are extremely expensive and not worth the cost since the lower rated and more available alumina-silica fibers provide the same protection in the firebox area once they have contracted. Natural gas is the cleanest of all natural fuels, followed by propane, butane, and then the oils. Natural gas is lighter than air and therefore problems with burner carbonization rarely occur during the low preheat periods of kiln firings. Liquid gases, however, which are heavier than air, even in a state of vaporization, often present carbonization problems during the preheat period unless the kiln is started at a high, rapid level of heat input. orifices in liquefied petroleum gas burners are often too large to present some carbonization, both on the inside of the burner as well as in the firebox during the low heat prefiring cycles. This factor is all the more evident with oil burners, and one must be aware that carbon residue may be building up inside the burner itself, particularly on the inside nozzle end. The buildup can be very slight; however, after many firings, the carbon builds up significantly to actually constrict the opening of the burner. The result is the operation of a smaller burner than the one that was originally the proper size. Frequently this occurs with homemade burners which use the gas manifold as the burner mount; the orifice is in the manifold and the carbon can constrict the size of the orifice, thus cutting down substantially on the required B.t.u. input of heat to the kiln. When carbonization takes place within the firebox itself - often directly in front of the burner - it is of little consequence to any of the functioning areas of the kiln or the burners because as the heat increases and the temperature becomes extreme (above 1,000°F), all carbonization, regardless of how thick, will burn off by the end of the firing cycle. One exception to this rule would be a kiln which has vertical burners situated directly under the burner port leading into the firebox - that is, a kiln where the firebox is a ceiling suspended directly over the burner. If carbonization builds up substantially on the face of the firebox over the burner, heavy accumulated pieces of carbon can scale off and fall into the throat of the burner, causing a deflection of the flame and resulting in a very unsatisfactory flame shape entering the kiln. Once lodged into the throat of the burner during the early firing stages, the carbon refuse will not burn away since there is little heat generated within the burner itself. Fortunately, carbonization does not affect the new materials themselves since, as already mentioned, they are inert. Conventional materials such as insulating characteristics may be modified by heavy carbonization.