Principles of Combustion and Incineration Thermodynamics

13.2 Principles of Combustion and Incineration Thermodynamics

The physical and chemical processes of combustion are the same whether the materials are burned in an open fire, an engine, or a refractory-lined chamber like a boiler or incinerator. Combustion requires the presence of organic matter, oxygen (usually air), and an ignition source. The term “fuel” in the context of combustion is used to designate any organic material that releases heat in the combustion chamber, regardless of whether it is a virgin fuel such as natural gas or fuel oil or a waste material. When organic matter containing the combustible elements carbon, sulfur, and hydrogen, is raised to a high enough temperature (order of 300 to 400°C, 600 to 800°F), the chemical bonds are excited and the compounds break down. If there is insufficient oxygen present for the complete oxidation of the compounds, the process is termed pyrolysis. If sufficient oxygen is present, the process is termed combustion.

Pyrolysis is a necessary first step in the combustion of most solids and many liquids. The rate of pyrolysis is controlled by three mechanisms. The first mechanism is the rate of heat transfer into the fuel particle. Clearly, therefore, the smaller the particle or the higher the temperature, the greater the rate of heating and the faster the pyrolytic process. The second mechanism is the rate of the pyrolytic process. The third mechanism is the diffusion of the combustion gases away from the pyrolyzing particles. Clearly, the last mechanism is likely to be a problem only in combustion systems that pack the waste material into a tight bed and provide very little gas flow.

At temperatures below approximately 500°C (900°F), the pyrolysis reactions appear to be rate con- trolling for solid particles less than 1 cm in diameter. Above this temperature, heat and mass transfer appear to limit the rate of the pyrolysis reaction. For larger pieces of solid under most incinerator conditions, heat and mass transfer are probably the rate-limiting step in the pyrolysis process (Niessen, 1978). Because pyrolysis is the first step in the combustion of most solids and many liquids, heat and mass transfer is also the rate-limiting step for many combustion processes.

Pyrolysis produces a large number of complex organic molecules that form by two mechanisms, cracking and recombination. In cracking, the constituent molecules of the fuel break down into smaller portions. In recombination, the original molecules or cracked portions of the molecule recombine to form larger, often new, organic compounds such as benzene. Pyrolysis is also termed destructive distil- lation. The products of pyrolysis are commonly referred to as Products of Incomplete Combustion, PICs. PICs include a large number of different organic molecules.

Pyrolysis is only the first step in combustion. To complete combustion, a properly designed incinerator, boiler, or industrial furnace mixes the pyrolysis off-gases with oxygen, and the mixture is then exposed

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The process of combustion can be viewed as taking place in three primary zones: (1) Volatilization or pyrolysis zone — referred to here as the pre-flame zone, (2) the flame zone, and (3) the postflame or burnout zone. In the first zone, the organic material in the gaseous, liquid, or solid fuel, is vaporized and mixes with air or another source of oxygen. Those organic compounds that do not vaporize typically pyrolyze, forming a combustible mixture of organic gases. Volatilization is endothermic (heat absorbing), while pyrolysis is, at best, only slightly exothermic. As a result, this step in the combustion process requires

a heat source to get the process started. The source of the initial heat, called the “ignition source,” the match or pilot flame for example,

provides the energy to start the combustion reaction. Once started, the reaction will be self-sustaining as long as fuel and oxygen are replenished at a sufficiently high rate to maintain the temperature above that needed to ignite the next quantity of fuel. If this condition is met, the ignition source can be removed. The energy released from the initial reaction will activate new reactions, and the combustion process will continue. A material that can sustain combustion without the use of an external source of ignition is defined to be autogenous.

In order to speed the phase change to the vapor, a liquid is usually atomized by a nozzle that turns it into fine droplets. The high surface-to-volume ratio of the droplets increases the rate at which the liquid absorbs heat, increases its rate at which it vaporizes or decomposes, and produces a flammable gas which then mixes with oxygen in the combustion chamber and burn. Atomization, while usually desirable, is not always necessary. In certain combustion situations, the gas temperatures may be high enough or the gas velocities in the combustion chamber large enough to allow the fuel or waste to become a gas without being atomized.

The phase change is speeded for solids by agitating them to expose fresh surface to the heat source and improve volatilization and pyrolysis of the organic matter. Agitation of solids also increases the rate heat and oxygen transfer into the bed and of combustion gases out of the bed. In practice this is done in many different ways, such as:

• Tumbling the solids in a kiln • Raking the solids over a hearth • Agitating it with a hot solid material that has a high heat capacity as in a fluidized bed • Burning the solids in suspension • Burning the solids in a fluidized bed • If the combustion is rapid enough, it can draw some of the air that feeds the flame through a

grate holding the solids. It is important that the amount of fuel charged to the burning bed not exceed the heating capacity of

the heat source. If this occurs, the burning mass will require more heat to vaporize or pyrolize than the heat source (the flame zone) can supply, and the combustion reactions will not continue properly. In most cases, such overloading will also result in poor distribution of air to the burning bed and improper flow of combustion gases away from the flame. The combined result will be that the flame will be smothered.

Consider one of the simplest forms of combustion, that of a droplet of liquid or a particle of solid (the fuel) suspended in a hot oxidizing gas as shown in Fig. 13.1 . The fuel contains a core of solid or liquid with a temperature below its boiling or pyrolysis point. That temperature is shown as T b . The

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Flame zone

Post-flame zone

Liquid

Products of comb

ustion

ature

Temper T i

ation

r Radial distance T f Flame temperature T g Bulk gas temperature T I Ignition temperature T B Boiling point O b Bulk gas oxygen concentration

Oxygen concentr

FIGURE 13.1 Combustion around a droplet of fuel. (Reproduced courtesy of LVW Associates, Inc.) liquid or solid core is surrounded by a vapor shell consisting of the vaporized liquid and the products

of pyrolysis of the fuel. The fuel and its surrounding vapor are the first zone of combustion. The vapor cloud surrounding the liquid core is continually expanding or moving away from the core. As a shell of gases around the droplets expands, it heats and mixes with oxygen diffusing inward from the bulk gases. At some distance from the core, the mixture reaches the proper temperature (T i ), and oxygen-fuel mixture ignites. The actual distance from the core where the expanding vapor cloud ignites is a complex function of the following factors:

• The bulk gas temperature • The vapor pressure of the liquid and latent heat of vaporization • The temperature at which the material begins to pyrolyze • The turbulence of the gases around the droplet (which affect the rate of mixing of the out-flowing

vapor and the incoming oxygen) • The amount of oxygen needed to produce a stable flame for the liquid • The heat released by the combustion reaction

Ignition creates the second zone of the combustion process, the flame zone. The flame zone has a small volume compared to that of the pre- and postflame zones in most combustors, and a molecule of material will only be in it for a very short time, on the order of milliseconds. Here, the organic vapors rapidly react with the air (the chemical reaction is discussed below) to form the products of combustion.

The temperature in the flame-zone, T f , is very high, usually well over 1700°C (3000°F). At these elevated

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The very high temperature in the flame zone is the main reason one can consider the major chemical reactions that occur in combustion to be functions of the elements involved and not of the specific compounds. The vast majority (on the order of 99% or more) of the organic constituents released from the waste and fuel are destroyed in the flame zone.

The flame around a droplet can be viewed as a balance between the rate of outward flow of the combustible vapors against the inward flow of heat and oxygen. In a stable flame, these two flows are balanced, and the flame appears to be stationary.

The rapid chemical reactions in the flame zone generate gaseous combustion products that flow outward and mix with additional, cooler, air and combustion gases in the postflame region of the combustion chamber. The gas temperature in the postflame region is in the 600 to 1200°C (1200 to 2200°F) range. The actual temperature is a function of the flame temperature and the amount of additional air (secondary air) introduced to the combustion chamber.

The chemical reactions that lead to the destruction of the organic compounds continue to take place in the postflame zone, but because of the lower temperatures, they are much slower than in the flame zone. Typical reaction rates are on the order of tenths of a second. Because of the longer reaction times, it is necessary to keep the gases in the postflame zone for a relatively long time (on the order of 1 to

2 sec) in order to assure adequate destruction. Successful design of a combustion chamber requires that it maintain the combustion gases at a high enough temperature for a long enough time to complete the destruction of the hazardous organic constituents.

Note that the reaction times and temperature ranges that are given above are intended only to provide

a sense of the orders-of-magnitude involved. This discussion should not be interpreted to mean that one or two seconds are adequate or that a lower residence time or temperature is not acceptable. The actual temperature and residence time needed to achieve a given level of destruction is a complex function, which is determined by testing the combustor and verifiying its performance by the trial burn.

The above description of the combustion process illustrates how the following three factors, commonly referred to as the “three Ts”: (1) temperature, (2) time, and (3) turbulence, affect the destruction of organics in a combustion chamber. Temperature is critical, because a minimum temperature is required to pyrolyze, vaporize, and ignite the organics and to provide the sensible heat needed to initiate and maintain the combustion process. Time refers to the length of time that the gases spend in the combustion chamber, frequently called the “residence time.” Turbulence is the most difficult to measure of the three terms. It describes the ability of the combustion system to mix the gases within the flame and in the postflame zone with oxygen well enough to oxidize the organics released from the fuel.

The following three points illustrate the importance of turbulence:

1. The process of combustion consumes oxygen in the immediate vicinity of pockets of fuel-rich vapor.

2. The destruction of organic compounds occurs far more rapidly and cleanly under oxidizing conditions.

3. In order to achieve good destruction of the organics, it is necessary to mix the combustion gases moving away from the oxygen-poor pockets of gas with the oxygen-rich gases in the bulk of the combustion chamber.

Therefore, turbulence can be considered the ability of the combustor to keep the products of com- bustion mixed with oxygen at an elevated temperature. The better the furnace’s ability to maintain a high level of turbulence (up to a point), the higher the destruction of organic compounds it is likely to achieve.

Complex flames behave in an analogous manner to the simple flame described above. The major difference is that the flame is often shaped by the combustion device to optimize the “three Ts.” To illustrate, consider a Bunsen burner flame. The fuel is introduced through the bottom of the burner’s tube and accelerated by a nozzle in the tube to increase turbulence. Openings on the side of the tube

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The Bunsen burner is designed for gaseous fuels. The fuel is premixed with air to minimize the amount of oxygen that must diffuse into the flame to maintain combustion. Premixing the fuel with air also increases the velocity of the gases exiting the mouth of the burner, increasing turbulence in the flame and producing a flame with a higher temperature than that of a simple gas flame in air. Liquid combustion adds a level of complexity. Liquid burners consist of a nozzle, whose function is to atomize the fuel, mounted into a burner, burner tile, or burner block that shapes the flame so that it radiates heat properly backwards and provides good mixing of the fuel and air. The whole assembly is typically called the burner. The assembly may be combined into a single unit, or the burner and nozzle may be independent devices. The fundamental principles of operation of liquid burners are the same as those of a Bunsen burner, with the added complexity of atomizing the fuel so that it will vaporize readily. In all liquid fuel burners, the fuel is first atomized by a nozzle to form a finely dispersed mist in air. Heat radiating back from the flame vaporizes the mist. The nozzle mixes the vapor with some air, but not enough to allow ignition. The mixture is now equivalent to the gas mixture in the tube of the Bunsen burner; it is a mixture of combustible gases and air at a concentration too rich (too much fuel or not enough oxygen) to ignite.

As the fuel-air mixture moves outward, it mixes with additional air, either by its impact with the oxygen-rich gases in the combustion zone or by the introduction of air through ports in the burner. As the gases mix with air, they form a flame front. The flame radiates heat backwards to the nozzle where it vaporizes the fuel.

Since most nozzles cannot tolerate flame temperatures, the nozzle and burner must be matched so that the cooling effect of the vaporizing fuel prevents radiation from overheating the nozzle. Similarly, if the liquid does not evaporate in the appropriate zone (if, for example, it is too viscous to be atomized properly) then it will not vaporize and mix adequately with air. Proper balance of the various factors results in a stable flame. Clearly, it is important that all liquid burned in a nozzle must have properties within the nozzles design limits. A flame that flutters a lot and has numerous streamers is typically termed “soft.” One that has a sharp, clear spearlike (like a bunsen burner flame) or spherical appearance is termed “hard.” Hard flames tend to be hotter than soft flames.

Nozzles operate in many ways. Some nozzles operate like garden hoses, the pressure of the liquid fed to them is used to atomize the liquid fuel. Others use compressed air, steam, or nitrogen to atomize the liquid. Nitrogen is used in those cases when the liquid fuel is reactive with steam or air. A third form of nozzle atomizes the liquid by firing the liquid against a rotating plate or cup. The type of nozzle used for any given application is a function of the properties of the liquid.

A great deal of information about the fuel and about the combustion process can be gained by looking at the flame in a furnace. CAUTION — Protective lenses must always be worn when examining the flame. The flame’s color is a good indicator of its temperature. However, this indicator must be used with caution, as the presence of metals can change the flame color. In the absence of metals, red flames are the coolest. As the color moves up the spectrum (red, orange, yellow, blue, indigo, and violet) the flame temperature increases. One will often see different colors in different areas of the flame. A sharp flame formed by fuel with high heating values will typically have a blue to violet core surrounded by a yellow to orange zone. Such a flame would be common in a boiler or industrial furnace where coal, fuel oil, or similarly “hot” fuel was being burned.

Another useful piece of operating information is the shape of the flame. A very soft (usually yellow or light orange) flame with many streamers may indicate that the fuel is inhomogeneous and probably

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a large amount of soot is observed. Soot (black smoke) released from a flame is indicative of localized lack of oxygen. While a small amount (a few fine streamers) of soot is common in a soft flame, large amounts of soot or a steady stream of soot from one point indicates some form of burner maladjustment.

There are two common causes of large amounts of soot emanating from the flame. First, the burner may not be supplying enough air to the flame. The system should be shut down and the burner inspected for blockage in the air supply. Second, the fuel could contain too much water or other material with a low heating value such as a heavily halogenated organic. In this case, improved fuel (waste) blending may resolve the issue. The production of large amounts of soot is usually associated with a rapid rise in the concentration of CO and hydrocarbon in the flue gas. A CO monitor is often a useful tool for assuring that burners are properly adjusted.