1 Introduction
CONTENTS
1.1 Combustion
1.2 History of Air Pollution
1.3 Thermal Oxidation’s Wide Applicability
1.4 Air Pollutant Emissions in the United States
1.5 Industrial Sources of Air Pollution
The control of emissions of volatile organic compounds (VOC) became a very prominent environmental issue with the passage of the 1990 Clean Air Act Amendments (CAA). While environmental regulations were in existence long before that time, they focused on controlling the concentrations of six “priority pollutants” in the ambient air. These pollutant species were ozone, nitrogen oxides, carbon monoxide, lead, particulates, and sulfur dioxide. The 1990 CAA amendments changed the focus to control of emissions of a set of specific chemical compounds called volatile organic compounds (VOC).
No single technology has played as important a role in the control of VOC emissions as thermal oxidation. Its prominence is a result of its ability to destroy VOCs in a one-step process while producing innocuous by-products (for the most part). VOCs are not normally classified as hazardous waste and thus thermal oxidizers are not normally subject to the more stringent design and operating requirements imposed on hazardous waste incinerators.
1.1 Combustion
Thermal oxidation is a combustion process. Combustion is defined as the burning of gases, liquids, or solids, in which the substance is oxidized, evolving heat and often light. Thermal oxidizers are distinguished from incinerators by virtue of the fact that they generally only treat vapors or relatively pure liquids. Thus, the combustion products usually do not contain particulates, dioxins, or heavy metals, the species that have made incinerators a somewhat controversial treatment device. As described in this book, the term “thermal oxidation” is synonymous with “afterburner.”
Other technologies are available for treatment of volatile organic compounds (VOC). However, thermal oxidizers are favored in most circumstances because of their reliability, the fact that usually no further treatment is required (not always true, as will be explained in Chapter 13), and because of their ability to achieve high VOC destruction efficiencies. The prevalence of combustion systems such as boilers, furnaces, fired heaters, and burners in a myriad of manufacturing and production facilities has aided the acceptance of thermal oxidizers by operators. Most other treatment technologies require further treatment of the VOCs once they are removed from the gas stream. An example is carbon adsorption. Here, the waste gas containing the VOC is forced through a bed of activated carbon. The VOCs are adsorbed into the pores of the carbon and clean gas exhausts from the opposite end. The drawback lies in the fact that the VOC has not been destroyed but transferred from the waste gas to the carbon adsorbent. The carbon bed must be regenerated. This can either be done on-site or by interchanging spent carbon canisters with fresh canisters. While feasible and a common practice in many applications, it adds to the complexity of the operation. Again, thermal oxidation is generally a one-step process obviating the need for further processing.
1.2 History of Air Pollution
While regulation of air emissions has been a relatively recent trend in contemporary history, the generation of noxious fumes from industrial processes has been around for hundreds of years. In the 14th century, a man was tortured in England for emitting a “pestilent odor” to the atmosphere from burning coal. Fumes from coal burning became so bad in the late 1550s that the English parliament passed a law forbidding the burning of coal in London when parliament was in session. The first air pollution episodes in the U.S. were also from burning coal. Chicago adopted a smoke control ordinance in 1881. In those days, the responsibility for controlling air emissions rested with state and city governments. However, pollution controls laws did not become prevalent in the U.S. until after World War II. These were usually enacted after the effects of air pollution were undeniable.
One of the first recorded public health episodes resulting from air pollution occurred in the small industrial town of Donora, PA in 1948. Steel, zinc, and sulfuric acid plants poured noxious gases into the air on a daily basis. Topographically, Donora is located in a valley. Usually winds dispersed these gases over great distances. However, in a 5-day period between October 26th and 30th, 1948, the noxious gases accumulated in the valley to such an extent that thousands of people became ill and 20 died.
A similar incident occurred in Poza Rica, Mexico in 1950. A natural gas recovery and “sweetening” process was starting up. Sweetening consists of removing the hydrogen sulfide that is a normal constituent of natural gas. Due to an equipment malfunction, a large quantity of hydrogen sulfide escaped into the atmosphere at the same time that a thermal inversion enveloped the area. There were 320 people hospitalized and 22 deaths. A thermal inversion also caused a buildup of noxious gases in London in December of 1952. At least 4000 deaths were attributed to the polluted air.
Industrial sources were not the only source of air pollutants. The popularity of the automobile produced more air emissions than industrial sources in some areas. One of these areas was Los Angeles, CA. Health effects stemming from air contaminants began to appear in Los Angeles after World War II. Industry was initially blamed for producing a brown haze that became known as smog. It consisted of a mixture of nitrogen oxides (NOx), sulfuric acid mist from the condensation of sulfur dioxide, and particulates. But even after emissions from industrial sources were reduced, the smog persisted. Eye and skin irritation and plant damage could not be attributed to industrial pollutants alone. It was then discovered that automobiles were a major factor in the generation of this smog.
1.3 Thermal Oxidation’s Wide Applicability
The first major national legislation enacted to control air emissions was the Clean Air Act, passed in 1955. It has been amended several times since. However, the most dramatic amendments were added in 1990. The resulting regulations serve as the basis for reduction of millions of pounds of VOCs emissions to the atmosphere annually. The most effective technology for reaching the mandated emission reduction is thermal oxidation.
By definition, VOCs are “organic.” While there are more than 100 natural and man-made chemical elements in total, organic compounds consist only of the elements carbon (C), hydrogen (H), nitrogen (N), oxygen (0), sulfur (S), and chlorine (Cl). Other elements are sometimes present, but over 99% of the organic compounds in existence are comprised of these six elements. However, not all organic compounds are volatile. “Volatile” refers to the tendency of a compound to evaporate. In many cases, compounds classified as VOCs are gases at standard temperature and pressure. Solid organic compounds are usually not volatile. An example of an organic compound that is not volatile is glucose (sugar). Generally, organic compounds with high molecular weights (>200) are not volatile. Neither are viscous liquids.
Table 1.1 Passenger Car Tail-Pipe Emissions (grams emitted per kilometer driven)
Not all air emissions can be treated using thermal oxidation. Inorganic particulate emissions are one category for which thermal oxidation is ineffective. This is unfortunate since emissions of fine particulate matter (< 2.5 microns in size) has been identified as a major health concern. Nor can thermal oxidation be used to treat emissions from nonindustrial sources. Automobiles fit this category. While we normally think of automobiles as emitting carbon monoxide and nitrogen oxides, they are also a source of VOC emissions. Table 1.1 shows VOC emissions from passenger cars tested in 1995. Values shown represent grams emitted per kilometer driven. A comparison is made between three different car models that include catalytic converters.
Emission of air pollutants is certainly not a phenomenon of the U.S. alone. However, on a per capita basis, emissions in the U.S. generally exceed the emissions from European countries and Japan. When compared on a per capita basis, U.S. emissions are approximately 15 times greater than Japanese emissions and 10 times greater than West German and Swedish emissions.
1.4 Air Pollutant Emissions in the United States
All industrial sources in the U.S. must report their annual air emissions to the US Environmental Protection Agency (EPA). The EPA compiles these data in a database called the Toxics Release Inventory (TRI). In 1993, a total of 23,000 facilities reported emissions data to the EPA. In total, approximately 2.8 billion pounds of toxics substances were released to the atmosphere that year. The top ten chemicals released and the quantities released are shown in Table 1.2. Of these ten, thermal oxidation technology could be used to treat all but hydrochloric acid and chlorine. Releases by industry category are shown for 1994 in Table 1.3.
Table 1.2 Top 10 Substances Released to the Air (1993)
Control of VOC emissions will increasingly rely on thermal, regenerative, and catalytic oxidizers as environmental regulations become more stringent. The market for this equipment is projected to exceed $2.1 billion by the year 2000.1 It is estimated that 25% of these orders will be placed by the chemicals industry. The forest products and electronics industries are also expected to account for much of the demand.
Even though air emissions releases have declined in the U.S. from 3.75 billion pounds in 1988 to 2.168 billion pounds in 1995, the demand for thermal oxidation systems is growing, due to increasingly stringent environmental regulations.
Table 1.3 Air Emission Releases by Industry Category
1.5 Industrial Sources of Air Pollution
VOCs are generated by a wide variety of industries in a wide variety of operations. A sampling of those industries is shown in Table 1.4. This abbreviated list demonstrates that the need for thermal oxidation systems is almost universal. Thus, a working knowledge of their design and operation is beneficial to mechanical engineers, chemical engineers, process engineers, environmental engineers, environmental regulators, and environmentalists.
Table 1.4 Industries Requiring Thermal Oxidation Systems
