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Merriam Webster’s Dictionary defines combustion as the “act or instance of burning.” That sounds simple enough—just about everyone knows that some sort of fuel must be burned in a furnace or kiln to create any type of ceramic product. But what, exactly, takes place in the kiln when the product is inside and the doors are closed? How do those reactions affect the quality of the ware and the safety of the employees? And how can they be manipulated to lower emissions or increase efficiency?
The answer to these questions varies depending on the type of operation, and is usually left for the kiln operator to determine. By gaining an understanding of the basics of the combustion process, kiln operators can be better prepared to answer these questions and others that may arise in day-to-day production.
What is Combustion?Combustion is the rapid oxidation of material (the fuel) to release energy (heat). The fuel can be a solid, liquid or gas, and the amount of heat released is normally expressed in BTUs (British thermal units) or Calories. For industrial applications in the U.S., we generally use the term BTU. A BTU is defined as “the amount of heat necessary to raise one pound of water one degree Fahrenheit from 60° to 61°F.” A Calorie is defined as “the energy required to raise one Kg of water one degree Celsius.”
The oxygen required to release the energy from the fuel normally comes from the air and represents 20.9% of the normal atmosphere. The remainder is primarily nitrogen with traces of other elements. Standard air is defined as “air measured at one atmosphere pressure (14.696 pounds per sq. in.) and 60°F with 0% relative humidity.”
In general, one cubic foot of air will release 100 BTUs of heat, regardless of the type of fuel used. (The exact number of BTUs released will vary somewhat with different fuels, but the variation is typically small, so this relationship holds true for all practical purposes.) As fuels are burned with just enough air to release the total BTUs in the fuel, the reaction is said to be “stoichiometric” or burned “on ratio”—when combustion is complete, no free oxygen or unburned fuel remains.
Natural gas is the most common fuel used in ceramic applications. While the actual BTU content of these fuels can vary between 850 to 1350 BTUs per cubic foot, most will average 1000 BTUs per cubic foot. Since one cubic foot of (standard) air will release 1000 BTUs of energy, a simple calculation shows that it will require 10 cubic feet of air to completely burn 1 cubic foot of natural gas (based on 1000 BTU natural gas). This is the basis for the 10:1 terminology used to express the stoichiometric air-to-fuel ratio. This ratio would produce an exhaust gas containing no free oxygen and no unburned combustible matter. However, due to the retention times in most furnaces, such a mixture would show both oxygen and combustibles in trace amounts.
The following chemical balances represent the typical combustion reactions for a hydrocarbon fuel, assuming stoichiometry with no other reactions occurring.
C + O2 CO2 +
2H22 + O2 2H2O +
C = Carbon
H = Hydrogen
O = Oxygen
CO = Carbon Monoxide
CO2 = Carbon Dioxide
H2O = Water
Other equations showing the combustion of carbon and hydrogen will be variations of these balances, depending on the amount of carbon and hydrogen present in those particular fuels.
When excess oxygen is available for the combustion process, we can typically show the following:
C + XO2 CO2 + YO2 +
H + XO2 H2 + YO2 +
X>Y; the difference being used to form CO2 or H2O
The following chemical balance shows the effect of too little oxygen for combustion [sub-stoichiometry].
2C + O2 CO2 + C +
2C + O2 2CO +
The first chemical balance shows the effect of a deficiency of oxygen, and the result can be soot. Once formed, soot is very difficult to destroy. The second chemical balance shows the formation of carbon monoxide (CO), which will burn if the gases are hot enough and there is enough oxygen introduced later in the process. CO is toxic and replaces oxygen in the bloodstream—it must be handled safely. CO levels should be monitored regularly to ensure compliance with current Occupational Safety and Health Administration Standards.
Requirements for CombustionFive components are needed for combustion to occur: fuel, oxygen (usually air), the proper ratio, mixing, and an ignition source.
For useful combustion, a flame holder or stabilizing device must be provided to anchor the flame and direct the heat to the desired areas. This is normally a burner tile designed for a particular burner. Combustion systems are designed to give the type of flame pattern that is needed for a process—from a short, intense flame to a very long, luminous flame. Changing the tile design can adversely affect the stability and operation of a burner and is not recommended.
The fuel can be any material that will burn. However, for this discussion, it will be assumed that natural gas is the fuel. The oxygen is from the air and represents 20.9% of the total volume of air.
The proper ratio of air to fuel is essential to good, safe combustion. In general, natural gas will burn with stability and self-ignition if the air-to-fuel ratio is between 4.5:1 and 14.5:1 (4.5% rich to 14.5% lean mixture). If the mixture gets too lean (greater than 14.5:1 ratio), the excess air in the system will drop the hot mix temperature below the auto-ignition point—1350°F for natural gas—and the burner will “flame out,” or lose its visible flame. If, however, the mixture gets too rich (less than 4.5:1) there will not be enough oxygen present to release the energy in the fuel, and the mix temperature will remain below the auto-ignition point.
To determine how much air is needed for a given quantity of fuel, the BTU content and the volume of the fuel being used must be known. The chemical composition of the fuel will determine how much air is necessary to release all the BTUs at the highest flame temperature. This highest flame temperature (or theoretical flame temperature) is a calculated value based on the actual chemical composition of the fuel. This temperature is never attained due to dilution by surrounding air and other materials, which absorb the heat from the flame as it is being generated. For natural gas at stoichiometric conditions, this value is 3450°F. *
For the purposes of this article, it will be assumed that the natural gas used contains 1000 BTUs per cubic foot with a theoretical hot mix temperature of 3450°F.
To determine the amount of air required for a given volume of fuel at stoichiometry, use the following formula:
Cubic Feet of Air Required = Cubic Feet of Gas x 10
Cubic Feet of Air Required = BTUs/100.
If excess air is required, the amount of air to provide can be calculated as:
Total Air = ([%Excess Air/100] x [Required Air]) + (Required Air)
Most ceramic processes require an oxidizing atmosphere, and it is important to provide sufficient excess air through the system to allow for the changing composition of the fuel and air. A humidity change from 0% relative humidity to 100% relative humidity at 60°F reduces available oxygen from 20.99% to 20.62%. A temperature rise of only 30 degrees to 90°F, in addition to the added relative humidity, reduces the available oxygen further to 19.99%—a net reduction of 5% of the original amount of oxygen in the air.
If there is insufficient oxygen in the process, a reducing condition will occur. Some special ceramic processes require a reducing condition, especially to develop specific chemical properties for a body or a glaze. There is, however, a substantial cost difference to operate in the reducing mode, and it is important to run with as little excess fuel as possible.
For those processes where an oxidizing atmosphere is required and there is not sufficient oxygen in the air supply, the product may be adversely affected. The fuel seeks oxygen from any source. If it is not available from the air supply, it will remove the oxygen from the oxides of the product. At the least, a slight color change of the product may occur. At the worst, the product can be completely destroyed.
It is possible for a system to have large quantities of excess air in the area of the flame. This can come from secondary air sources or can be caused by running the burners at high excess air rates. The excess air can cause the flame to be quenched before combustion is completed, forming CO and aldehydes (CH3CHO) in the resultant products of combustion.
Measuring CombustionSince it is not always possible to measure the theoretical temperature of a combustion process due to dilution of the gases and absorption of the heat by radiation to the surrounding areas, other methods have been developed to determine whether a combustion process is being properly handled. One method is to determine the energy released by the components in the fuel. However, since this changes with the fuel sources and along a trunk supply line, this is not practical for a continuous process.
Other methods involve determining the amount of products of combustion generated by the reaction and determining how much free oxygen is left in the system after combustion is completed. If all the oxygen has been used and there are combustibles in the process, this can also be measured with proper instrumentation.
Since there is 21% oxygen in normal air, this can be a measure of combustion efficiency. Ideally, a flue gas analysis of 0% combustibles would be achieved. However, since the reaction time of fuel with oxygen exceeds the time the gases are often in the combustion chamber, it is more practical to expect approximately 1/2% oxygen.
Several types of instruments are available to measure either of these gases as a means to control combustion. However, since the CO2 analysis of a gas can vary considerably in any fuel, depending on its carbon-to-hydrogen ratio, this is not a reliable method of analysis.
The oxygen level of the atmosphere, however, remains constant, and any excess amount of oxygen over 1/2% indicates a process that is not fully using all the available BTUs of a fuel. Note that some processes must operate at oxygen levels well above this due to temperature sensitivity. Refractories, for example, can be fired at stoichiometric ratios, while china may require as much as 50-100% excess air.
Types of Heat TransferOnce the fuel has been burned and the BTUs (or heat) released, the heat must be transferred to a product. There are three basic types of heat transfer: convection, radiation and conduction.
Convection is the form of heat transfer whereby the temperature of a fluid (gas or liquid) passing across another object, normally a solid, results in a transfer of energy (temperature) from one object to the other. We normally think of this form of heating as going from a hot gas to a cold, solid object. The same, however, can be held true of a cool fluid passing over a warm surface, changing the temperature of both the fluid and the object being cooled. The more turbulent the flow, the greater the heat transfer by convection.
Convection heating is the most commonly used and important method of heat transfer for the ceramic industry since it allows the hot gases to come into contact with all the surfaces of the product. To transfer energy to any object via convection, the energy must penetrate multiple layers of air that are electrostatically bonded to the surface (see Figure 1). Air, an excellent insulator, makes this transfer difficult. Therefore, the faster that a fluid (gas stream) passes across the surface, the more rapidly these air layers will be swept away, to be replaced by hotter gases. This continuing process is convective heat transfer, and it has been made more efficient for the heating industry through the development of the high-velocity burner.
Radiation is the transfer of energy (heat) between surfaces at different temperatures without the two being in physical contact with each other. The most common example is the sun and the earth. The amount of heat transfer via radiation is proportional to the fourth power of the temperature difference between the heat source (emitter) and that which is being heated (receiver). For example, in Figure 2, Object “A” at 200°F (800°F differential) gets 16 times as much radiant heat from the source as does Object “B” at 600°F (400°F differential).
The problem with this type of heating is that only those surfaces that “see” the heat source will receive the heat waves and have their temperature raised. This becomes a major problem in heating industrial products where all the products cannot see the radiant energy source—generally the refractory walls. This is the reason that many of the older kilns were designed with a very wide hearth in the high heat sections. The added refractory area provided additional sources of radiation in an attempt to achieve more uniformity in the center of the load. This, however, also provided more heat loss through the crown area since the heat was greatly increased in those sections. The advent of the high-velocity burner has increased firing efficiency, allowing smaller kiln sections to be used.
At the higher temperatures, radiation is the most intense form of heat transfer—but only in straight line radiation.
Conduction is the mode of heat caused by the increased activity of molecules within a body. An object heated at one end (by convection and/or radiation) will cause the opposite end to get hot by the molecules passing along the heat (energy). The speed at which this transfer occurs is a function of:
While conduction is generally the slowest of the heat transfer mechanisms and depends on the molecular structure of the material, it is the only way to completely heat an object once the energy has been transferred by convection and radiation to the surface of the object being heated.
Controlling the ProcessWhen a kiln car full of ceramic product enters the kiln, a chain of events takes place. Within each of the burners, air mixes with fuel—ideally at a 10:1 ratio—and is ignited to create a flame. Once the fuel has been burned and the heat released, the heat is transferred to the product through convection, radiation and/or conduction. These events continue for a set period of time until the product has achieved its final state.
Understanding what happens inside the kiln can allow kiln operators to control the variables—air-to-fuel ratio, burner and tile type, length of firing time, amount of emissions—and ensure a safe, efficient firing process.