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Module 3: Characteristics of Particles - Particle Formation
Introduction
Physical Attrition
Combustion Particle Burnout
Homogeneous and Heterogeneous
Nucleation
Droplet Evaporation
Summary
Practice
Problems
Objectives
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Describe the five main particle formation processes important at
air pollution sources.
- Identify the particle size range associated with each of the five main particle formation processes.
The range of particle sizes formed in a process is largely dependent on the types of particle formation mechanisms present. The general size range of particles can be estimated by simply recognizing which particle formation mechanisms are most important in the process being evaluated. The most important particle formation mechanisms in air pollution sources include the following:
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Physical attrition/mechanical dispersion
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Combustion particle burnout
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Homogeneous and heterogeneous nucleation
- Droplet evaporation
Several particle formation mechanisms can be present in an air pollution source. As a result, the particles created can have a wide range of sizes and chemical compositions.
Physical attrition occurs when two surfaces rub together. For
example, the grinding of a metal rod on a grinding wheel yields small
particles that break off from both surfaces (Figure
1). The compositions and densities of these particles are
identical to the parent materials.
The tertiary stone crusher shown in Figure 2 is
an example of an industrial source of particulate matter that involves
only physical attrition. The dust particles formed range from less
than 10 micrometers to almost 1000 micrometers. However, due to the
limited energy used in the crushing operation, very little of the
particulate matter is less than 10 micrometers. Physical attrition
generates primarily moderate-to-large sized particles.
In order for fuel to burn, it must be pulverized or atomized so that there is sufficient surface area exposed to oxygen and high temperature. As indicated in Table 1, the surface area of particles increases substantially as more and more of the material is reduced in size.
Accordingly, most industrial scale combustion processes use one or more types of physical attrition in order to prepare or introduce their fuel into the furnace. For example, coal-fired boilers use pulverizers to reduce the chunks of coal to sizes that can be burned quickly. Oil-fired boilers use atomizers to disperse the oil as fine droplets. In both cases, the fuel particle size range is reduced to primarily the 10- to 1000-micrometer range by physical attrition.
When fuel particles are injected into the hot furnace area of the combustion process, such as in fossil-fuel-fired boilers (Figure 3), most of the organic compounds in the fuel are vaporized and oxidized in the gas stream. Fuel particles become smaller as the volatile matter leaves and they are quickly reduced to only the incombustible matter (ash) and the slow burning char composed of organic compounds. Eventually, most of the char will also burn leaving primarily the incombustible material.
As combustion progresses, the fuel particles, which started as 10- to 1000-micrometer particles, are reduced to ash and char particles that are primarily in the 1 to 100 micrometer range. This mechanism for particle formation can be termed combustion particle burnout. Figure 3 shows where both combustion particle burnout and nucleation occur in a fossil-fuel-fired boiler. Nucleation is discussed in the next section.
Homogeneous and Heterogeneous Nucleation
Homogeneous nucleation and heterogeneous nucleation involve the conversion of vapor phase materials to a particulate form. In both cases, the vapor-containing gas streams must cool to the temperature at which nucleation can occur, which is the dew point. Each vapor phase element and compound has a different dew point. Therefore, some materials nucleate in relatively hot gas zones while others remain as vapor until the gas stream is cold.
Homogeneous nucleation is the formation of new particles composed almost entirely of the vapor phase material. The formation of particles by homogeneous nucleation involves only one compound.
Heterogeneous nucleation is the accumulation of material on
the surfaces of existing particles (see Figure 4).
In the case of heterogeneous nucleation, the resulting particle
consists of more than one compound.
There are two main categories of vapor phase material that can nucleate in air pollution source gas streams: (1) organic compounds, and (2) inorganic metals and metal compounds. In a waste incinerator, waste that volatilizes to organic vapor is generally oxidized completely to carbon dioxide and water.
However, if there is an upset in the combustion process, a portion of the organic compounds or their partial oxidation products remain in the gas stream as it leaves the incinerator. Volatile metals and metal compounds such as mercury, lead, lead oxide, cadmium, cadmium oxide, cadmium chloride, and arsenic trioxide can also volatilize in the hot incinerator. Once the gas stream passes through the heat exchange equipment (i.e. waste heat boiler) used to produce steam, the organic vapors and metal vapors can condense homogeneously or heterogeneously. Generally, the metals and metal compounds reach their dew point first and begin to nucleate in relatively hot zones of the unit as shown in Figure 5.
The organic vapors begin to condense in areas downstream from the process where the gas temperatures are cooler. These particles must then be collected in the downstream air pollution control systems. Homogeneous and heterogeneous nucleation generally create particles that are very small, often between 0.1 and 1.0 micrometer.
Heterogeneous nucleation facilitates a phenomenon called enrichment of particles in the submicrometer size range. The elemental metals and metal compounds volatilized during high temperature operations (fossil fuel combustion, incineration, industrial furnaces, and metallurgical processes) nucleate preferentially as small particles or on the very small particles produced by these processes. Consequently, very small particles have more potentially toxic materials than the very large particles leaving the processes. Heterogeneous nucleation contributes to the formation of particle distributions that have quite different chemical compositions in different size ranges.
Another consequence of particle formation by heterogeneous nucleation is that a greater variety of chemical reactions may occur in the gas stream than would otherwise happen. During heterogeneous nucleation, small quantities of metals are deposited on the surfaces of many small particles. In this form, the metals are available to participate in catalytic reactions with gases or other vapor phase materials that are continuing to nucleate. Accordingly, heterogeneous nucleation increases the types of chemical reactions that can occur as the particles travel in the gas stream from the process source and through the air pollution control device.
Some air pollution control systems use solids-containing water recycled from wet scrubbers to cool the gas streams. This practice inadvertently creates another particle formation mechanism that is very similar to fuel burnout. The water streams are atomized during injection into the hot gas streams. As these small droplets evaporate to dryness, the suspended and dissolved solids are released as small particles. The particle size range created by this mechanism has not been extensively studied. However, it probably creates particles that range in size from 0.1 to 20 micrometers. All of these particles must then be collected in the downstream air pollution control systems.
Knowing the size range of particles is important in air pollution control because the collection efficiency of different types of particulate control equipment is heavily dependent on particle size. Figure 6 summarizes the expected particle size range produced by the formation mechanisms discussed in this lesson. In most industrial processes, more than one particle formation mechanism is at work. Thus, industrially generated particles usually cover a broad size range.
Practice Problems
Particle Formation
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Instructions:
- Complete the Practice Problems before proceeding to the next lesson. Click on the button below.