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Air Pollution Engineering
The phenomenon of air pollution involves a sequence of events: the generation of pollutants at and their release from a source; their transport and transformation in and removal from the atmosphere; and their effects on human beings, materials, and ecosystems. Because it is generally either economically infeasible or technically impossible to design processes for absolutely zero emissions of air pollutants, we seek to control the emissions to a level such that effects are either nonexistent or minimized.
We can divide the study of air pollution into three obviously overlapping but somewhat distinct areas:
1. The generation and control of air pollutants at their source. This first area involves everything that occurs before the pollutant is released "up the stack" or "out the tailpipe."
2. The transport, dispersion, chemical transformation in, and removal of species from the atmosphere. This second area thus includes all the chemical and physical processes that take place between the point of emission and ultimate removal from the atmosphere.
3. The effects of air pollutants on human beings, animals, materials, vegetation, crops, and forest and aquatic ecosystems, including the measurement of gaseous and particulate species.
An air pollution control strategy for a region is a specification of the allowable levels of pollutant emissions from sources. To formulate such a strategy it is necessary to be able to estimate the atmospheric fate of the emissions, and thus the ambient concentrations, so that these concentrations can be compared with those considered to give rise to adverse effects. The ultimate mix of control actions and devices employed to achieve the allowable levels might then be decided on an economic basis. Therefore, the formulation of an air pollution control strategy for a region involves a critical feedback from area 3 to area 1. Consequently, all three of the areas above are important in air pollution abatement planning.
A comprehensive treatment of each of these three areas is beyond the scope of a single book, however. The present book is devoted to an in-depth analysis of the generation and control of air pollutants at their source, which we refer to as air pollution engineering.
1.1 AIR POLLUTANTS
Table 1.1 summarizes species classified as air pollutants. By and large our focus in this book is on the major combustion-generated compounds, such as the oxides of nitrogen, sulfur dioxide, carbon monoxide, unburned hydrocarbons, and particulate matter. Table 1.2 provides a list of the most prevalent hydrocarbons identified in ambient air, and Table 1.3 lists potentially toxic atmospheric organic species.
1.1.1 Oxides of Nitrogen
Nitric oxide (NO) and nitrogen dioxide (NO2) are the two most important nitrogen oxide air pollutants. They are frequently lumped together under the designation NOx, although analytical techniques can distinguish clearly between them. Of the two, NO2 is the more toxic and irritating compound.
Nitric oxide is a principal by-product of combustion processes, arising from the high-temperature reaction between N2 and O2 in the combustion air and from the oxidation of organically bound nitrogen in certain fuels such as coal and oil. The oxidation of N2 by the O2 in combustion air occurs primarily through the two reactions
N2 + O -> NO + N
N + O2 -> NO + O
known as the Zeldovich mechanism. The first reaction above has a relatively high activation energy, due to the need to break the strong N2 bond. Because of the high activation energy, the first reaction is the rate-limiting step for NO production, proceeds at a somewhat slower rate than the combustion of the fuel, and is highly temperature sensitive. Nitric oxide formed via this route is referred to as thermal-NOx. The second major mechanism for NO formation in combustion is by the oxidation of organically bound nitrogen in the fuel. For example, number 6 residual fuel oil contains 0.2 to 0.8% by weight bound nitrogen, and coal typically contains 1 to 2%, a portion of which is converted to NOx during combustion. (The remainder is generally converted to N2.) Nitric oxide formed in this manner is referred to as fuel-NOx.
Mobile combustion and fossil-fuel power generation are the two largest anthropogenic sources of NOx. In addition, industrial processes and agricultural operations produce minor quantities. Emissions are generally reported as though the compound being emitted were NO2. This method of presentation serves the purpose of allowing ready comparison of different sources and avoids the difficulty in interpretation associated with different ratios of NO/NO2 being emitted by different sources. Table 1.4 gives NO/NOx ratios of various types of sources. We see that, although NO is the dominant NOx compound emitted by most sources, NO2 fractions from sources do vary somewhat with source type. Once emitted, NO can be oxidized quite effectively to NO2 in the atmosphere through atmospheric reactions, although we will not treat these reactions here. Table 1.5 gives estimated U.S. emissions of NOx in 1976 according to source category. Utility boilers represent about 50% of all stationary source NOx emissions in the United States. As a result, utility boilers have received the greatest attention in past NOx regulatory strategies and are expected to be emphasized in future plans to attain and maintain NOx ambient air quality standards.
1.1.2 Sulfur Oxides
Sulfur dioxide (SO2) is formed from the oxidation of sulfur contained in fuel as well as from certain industrial processes that utilize sulfur-containing compounds. Anthropogenic emissions of SO2 result almost exclusively from stationary point sources. Estimated annual emissions of SO2 in the United States in 1978 are given in Table 1.6. A small fraction of sulfur oxides is emitted as primary sulfates, gaseous sulfur trioxide (SO3), and sulfuric acid (H2SO4). It is estimated that, by volume, over 90% of the total U.S. sulfur oxide emissions are in the form of SO2, with primary sulfates accounting for the other 10%.
Stationary fuel combustion (primarily utility and industrial) and industrial processes (primarily smelting) are the main SO2 sources. Stationary fuel combustion includes all boilers, heaters, and furnaces found in utilities, industry, and commercial/ institutional and residential establishments. Coal combustion has traditionally been the largest stationary fuel combustion source, although industrial and residential coal use has declined. Increased coal use by electric utilities, however, has offset this decrease. SO2emissions from electric utilities account for more than half of the U.S. total. A more detailed breakdown of U.S. sulfur oxide emissions in 1978 is given in Table 1.7.
1.1.3 Organic Compounds
Tables 1.2 and 1.3 list a number of airborne organic compounds. Organic air pollutants are sometimes divided according to volatile organic compounds (VOCs) and particulate organic compounds (POCs), although there are some species that will actually be distributed between the gaseous and particulate phases. The emission of unburned or partially burned fuel from combustion processes and escape of organic vapors from industrial operations are the major anthropogenic sources of organic air pollutants.
A major source of airborne organic compounds is the emissions from motor vehicles. Motor vehicle emissions consist of unburned fuel, in the form of organic compounds; oxides of nitrogen, in the form primarily of nitric oxide; carbon monoxide; and particulate matter. Since motor vehicle emissions vary with driving mode (idle, accelerate, decelerate, cruise), to obtain a single representative emission figure for a vehicle, it is run through a so-called driving cycle in which different driving modes are attained for prescribed periods. The driving cycle is carried out in the laboratory on a device called a dynamometer that offers the same resistance to the engine as actual road driving.
Three different driving cycles have been employed in emissions testing: the Federal Test Procedure (FTP), a cycle reflecting a mix of low and high speeds; the New York City Cycle (NYCC), a low-speed cycle to represent city driving; and the Crowded Urban Expressway (CUE) cycle, representative of high-speed driving. The average cycle speeds of the three cycles are: FTP—19.56 mi/hr (31.5 km h-1); NYCC—7.07 mi/hr (11.4 km h-1); CUE—34.79 mi/hr (56.0 km h-1). Emissions of all pollutants are generally larger for the lower-speed cycles.
1.1.4 Particulate Matter
Particulate matter refers to everything emitted in the form of a condensed (liquid or solid) phase. Table 1.7 gives the total estimated U.S. particulate matter emissions in 1978, and Table 1.8 presents a summary of the chemical characteristics of uncontrolled particulate emissions from typical air pollution sources.
In utility and industrial use, coal and, to a lesser extent, oil combustion contribute most of the particulate (and sulfur oxides) emissions. Coal is a slow-burning fuel with a relatively high ash (incombustible inorganic) contents. Coal combustion particles consist primarily of carbon, silica (SiO2), alumina (A12O3), and iron oxide (FeO and Fe2O3). In contrast to coal, oil is a fast-burning, low-ash fuel. The low ash content results in formation of less particulate matter, but the sizes of particles formed in oil combustion are generally smaller than those of particles from coal combustion. Oil combustion particulate matter contains cadmium, cobalt, copper, nickel, and vanadium.
Major industrial process sources of particulate matter include the metals, mineral products, petroleum, and chemicals industries. Iron and steel and primary smelting operations are the most significant emission sources in the metals industry. The iron and steel industry involves coke, iron, and steel production, each of which is a source of particulate emissions. The primary metals industry includes the smelting of copper, lead, and zinc, along with aluminum production. Sulfur in unprocessed ores is converted to SO2 during smelting, with a relatively small portion emitted as particulate sulfate and sulfuric acid. Emissions from the mineral products industry result from the production of portland cement, asphalt, crushed rock, lime, glass, gypsum, brick, fiberglass, phosphate rock, and potash. The particles emitted from crushing, screening, conveying, grinding, and loading operations tend to be larger than 15 µm.
1.2 AIR POLLUTION LEGISLATION IN THE UNITED STATES
The 1970 Clean Air Act Amendments was a major piece of legislation that in many respects first put teeth into air pollution control in the United States. A major goal of the Act was to achieve clean air by 1975. The Act required the Environmental Protection Agency (EPA) to establish National Ambient Air Quality Standards (NAAQS)—both primary standards (to protect public health) and secondary standards (to protect public welfare). The Act also required states to submit State Implementation Plans (SIPs) for attaining and maintaining the national primary standards within three years.
Automobile emissions were arbitrarily set at a 90% reduction from the 1970 (for CO and hydrocarbons) or 1971 (for NOx) model year emissions to be achieved by 1975 (or 1976 for NOx). Since there was no proven way to achieve these goals when the law was enacted, the industry was in effect forced to develop new technology to meet the standards by a certain deadline. This has been called "technology-forcing legislation." Emissions standards were to be written by the EPA for certain new industrial plants. These New Source Performance Standards (NSPS) represented national standards that were to be implemented and enforced by each state.
The Clean Air Act Amendments of 1977 incorporated a number of modifications and additions to the 1970 Act, although it retained the basic philosophy of federal management with state implementation. In this Act, the EPA was required to review and update, as necessary, air quality criteria and regulations as of January 1, 1980 and at five-year intervals thereafter. A new aspect was included for "prevention of significant deterioration" (PSD) of air quality in regions cleaner than the NAAQS. Prior to the 1977 Amendments it was theoretically possible to locate air pollution sources in such regions and pollute clean air up to the limits of the ambient standards. However, the Act defined class 1 (pristine) areas, class 2 (almost all other areas), and class 3 (industrialized) areas. Under the PSD provisions, the ambient concentrations of pollutants will be allowed to rise very little in class 1 areas, by specified amounts in class 2 areas, and by larger amounts in class 3 areas.
The 1977 Amendments also addressed the issue of nonattainment areas: those areas of the country that were already in violation of one or more of the NAAQS. The law appeared to prohibit any more emissions whatsoever and thus seemed as if it would prevent any further growth in industry or commerce in these areas. However, subsequent interpretations by EPA led to a policy known as emissions offset that allowed a new source to be constructed in a nonattainment area provided that its emissions were offset by simultaneous reductions in emissions from existing sources.
Emissions standards for automobiles were delayed, and the standard for NOx was permanently relaxed from the original goals of the 1970 Act. CO and hydrocarbon standards were set at a 90% reduction from the 1970 model year to 3.4 g/mi for CO and 0.41 g/mi for hydrocarbons to be achieved by the 1981 model year. The required NOx reduction was relaxed to 1 g/mi by the 1982 model year, representing a reduction from about 5.5 g/mi in 1970. Standards were also proposed for heavy-duty vehicles such as trucks and buses.
Two types of air pollution standards emerged from the legislation. The first type is ambient air quality standards, those that deal with concentrations of pollutants in the outdoor atmosphere. The second type is source performance standards, those that apply to emissions of pollutants from specific sources. Ambient air quality standards are always expressed in concentrations such as micrograms per cubic meter or parts per million; whereas source performance standards are written in terms of mass emissions per unit of time or unit of production, such as grams per minute or kilograms of pollutant per ton of product.
Table 1.9 presents the current National Ambient Air Quality Standards. Some states, such as California, have set their own standards, some of which are stricter than those listed in the table. New Source Performance Standards (NSPS) are expressed as mass emission rates for specific pollutants from specific sources. These standards are generally derived from field tests at a number of industrial plants. A separate category of standards for emissions from point sources has been created for hazardous air pollutants, such as beryllium, mercury, vinyl chloride, benzene, and asbestos.
The particulate matter entry in Table 1.9 requires some explanation. After a periodic review of the National Ambient Air Quality Standards and a revision of the Health and Welfare Criteria as required in the 1977 Clean Air Act Amendments, the EPA proposed in 1987 the following relative to the particulate matter standard:
1. That total suspended particulate matter (TSP) as an indicator for particulate matter be replaced for both the primary standards, that is, the annual geometric mean and the 24-hour average, by a new indicator that includes only those particles with an aerodynamic diameter smaller than or equal to a nominal 10 µm (PM10)
2. That the level of the 24-hour primary standard be 150 µg m-3 and the deterministic form of the standard be replaced with a statistical form that permits one expected exceedance of the standard level per year
3. That the level of the annual primary standard be 50 µg m-3, expressed as an expected annual arithmetic mean
EPA also proposed in the Federal Register to revise its regulations governing State Implementation Plans to account for revisions to the NAAQS for TSP and PM10. Under the Act, each state must adopt and submit an SIP that provides for attainment and maintenance of the new or revised standards within nine months after the promulgation of an NAAQS. The revision authorizes the EPA Administrator to extend the deadline for up to 18 months as necessary.
Table 1.10 gives some selected New Source Performance Standards. The uncontrolled emission rates for a variety of processes can be estimated from the data available in the EPA publication generally referred to as AP-42, "Compilation of Air Pollutant Emission Factors" (U.S. Environmental Protection Agency, 1977).
1.3 ATMOSPHERIC CONCENTRATION UNITS
We note from Table 1.8 that two concentration units that are commonly used in reporting atmospheric species abundance are µg m-3 and parts per million by volume (ppm). Parts per million by volume is just
ci/c × 106
where ci and c are moles per volume of species i and air, respectively, at pressure p and temperature T. Note that in spite of the widespread reference to it as a concentration, parts per million by volume is not really a concentration but a dimensionless volume fraction.
Excerpted from Fundamentals of Air Pollution Engineering by Richard C. Flagan, John H. Seinfeld. Copyright © 2012 Richard C. Flagan and John H. Seinfeld. Excerpted by permission of Dover Publications, Inc..
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