FIGURE 6-1. Emission rates of sulfur dioxide, nitrogen oxides, and particulate matter from coal-fired power plants for the period 1970 to the present. (From DOE, National Energy Technology Laboratory Accomplishments FY 2002, Office of Fossil Energy, U.S. Department of Energy, Washington, D.C., August 2003.)

Average rate of pollutant emissions Coal use for power from U.S. coal-fired power plants generation in the U.S.

Average rate of pollutant emissions Coal use for power from U.S. coal-fired power plants generation in the U.S.

■1970 I_11997 I_12005 (projected)

FIGURE 6-2. Past, present, and future emission rates of sulfur dioxide, nitrogen oxides, and particulate matter from coal-fired power plants. (From DOE, National Energy Technology Laboratory Accomplishments FY 2002, Office of Fossil Energy, U.S. Department of Energy, Washington, D.C., August 2003.)

anthropogenic emissions, fossil fuel combustion accounted for approximately 63% (i.e., 10 million short tons). Sulfur dioxide emissions have decreased 33 and 31% for the periods from 1983 to 2002 and from 1993 to 2002, respectively [1]. Reductions in SO2 emissions and concentrations since 1990 are primarily due to controls implemented under the U.S. Environmental Protection Agency's (EPA's) Acid Rain Program beginning in 1995. As of 2000, 192 coal-fired electric generators were equipped with scrubbers and provided a total of nearly 90,000 MW generating capacity [3]. It must be noted, however, that there is variability in reported generating capacity under SO2 control among the various agencies, so the quantity ranges from -90,000 to 102,000 MW [3-5]. The chemistry of sulfur dioxide formation is reviewed in this section, followed by technologies used to control SO2 emissions. Control technologies will focus on commercially available and commercially used systems. Industry deployment of the SO2 removal process worldwide is discussed as are the economics of flue gas desulfurization.

Chemistry of Sulfur Oxide (SO2/SO3) Formation

Sulfur in coal occurs in three forms: as pyrite, organically bound to the coal, or as sulfates. The sulfates represent a very small fraction of the total sulfur while pyritic and organically-bound sulfur comprise the majority. The distribution between pyritic and organic sulfur is variable with up to approximately 40% of the sulfur being pyritic. During combustion, the pyritic and organically bound sulfur are oxidized to sulfur dioxide with a small amount of sulfur trioxide (SO3) being formed. The SO2/SO3 ratio is typically 40:1 to 80:1 [6].

The overall reaction for the formation of sulfur dioxide is:

and the overall reaction for the formation of sulfur trioxide is:

It is proposed that sulfur monoxide, SO, is formed early in the reaction zone from sulfur-containing molecules and is an important intermediate product [6]. The major SO2 formation reactions are believed to be:

with the highly reactive O and H atoms possibly entering the reaction scheme later.

The reactions involving SO3 are reversible. The major formation reaction for SO3 is the three-body process:

where M is a third body that is an energy absorber [6]. The major steps for removal of SO3 are thought to be the following:

Sulfur Dioxide Control

Methods to control sulfur dioxide emissions from coal-fired power plants include switching to a lower sulfur fuel, cleaning the coal to remove the sulfur-bearing components such as pyrite, or installing flue gas desulfuriza-tion systems. In the past, building tall stacks to disperse the pollutants was a control method; however, this practice is no longer an alternative, as tall stacks do not remove the pollutants; they only dilute the concentrations to reduce the ground-level emissions to acceptable levels.

When fuel switching or coal cleaning is not an option, flue gas desulfu-rization (FGD) is selected to control sulfur dioxide emissions from coal-fired power plants (except for fluidized-bed combustion systems, which are discussed later in this chapter). FGD has been in commercial use since the early 1970s and has become the most widely used technique to control sulfur dioxide emissions next to the firing of low-sulfur coal. Many FGD systems are currently in use and others are under development. This section summarizes the worldwide application of FGD systems with an emphasis on the United States. FGD processes are generally classified as wet scrubbers or dry scrubbers but can also be categorized as follows [4]:

• Dry (sorbent) injection processes;

• Regenerable processes;

• Circulating fluid-bed and moving-bed scrubbers;

• Combined SO2/NO x removal systems.

Based on the nature of the waste/by-product generated, a commercially available throwaway FGD technology may be categorized as wet or dry. A wet FGD process produces a slurry waste or a salable slurry by-product. A dry FGD process application results in a solid waste, the transport and disposal of which is easier compared to the waste/by-product from wet FGD applications. Regenerable FGD processes produce a concentrated SO2

SO3 + O SO2 + O2 SO3 + H SO2 + OH SO3 + M SO2 + O + M

by-product, usually sulfuric acid or elemental sulfur. The recent focus on mercury removal in FGD systems is discussed later in this chapter.

Worldwide Deployment of FGD Systems Post-combustion control of sulfur dioxide emissions from pulverized coal combustion began in the early 1970s in the United States and Japan. Western Europe followed in the 1980s. In the 1990s, the application of FGD became more widespread, and countries in Central and Eastern Europe and Asia, for example, have installed FGD systems. Table 6-1 lists various control technologies and the amount of electricity generation that is being controlled in countries throughout the world [4]. According to Soud [4], as of 1999, 680 FGD systems were installed in 27 countries, and 140 systems are currently under construction or planned in nine countries.

Worldwide, approximately 30,000 MW of generating capacity were controlled in 1980 compared to no controlled capacity in 1970. Controlled generating capacity subsequently increased to ~ 130,000 MW in 1990 and -230,000 MW in 2000. In the United States, controlled capacity rose from zero in 1970 to 25,000 MW in 1980, to -75,000 MW in 1990, and to -100,000 MW in 2000. FGD systems were installed (as of 1999) to control sulfur dioxide emissions from over 229,000 MW of generating capacity worldwide. Of these systems, -87% consisted of wet FGD technology and 11% dry FGD technology, with the balance consisting of regenerable technology [5]. Of the worldwide capacity controlled with FGD technology, -44% is in the United States alone, as shown in Table 6-2. In the United States, -100,000 MW of capacity were equipped with FGD technology in 1999. Of these FGD systems, approximately 83, 14, and 3% consisted of wet FGD, dry FGD, and regenerable technology, respectively. Worldwide, out of the 668 units equipped with FGD in 1999, 522 were equipped with wet FGD, 124 with dry FGD, and 22 with regenerable FGD.

Of the U.S. wet FGD technology population today, 69% are limestone processes [5]. Abroad, limestone processes comprise as much as 93% of the total wet FGD technology installed. Of the worldwide capacity equipped with dry FGD technology, 74% use spray drying processes. This compares with 80% for spray drying processes in the United States. A summary of the FGD systems in the United States, by process, is given in Table 6-3 for 1989 (actual capacity) and 2010 (projected capacity). The three primary processes are throwaway-product systems, including the two wet scrubbing systems using limestone and lime where a synthetic gypsum (CaSO4) is produced. A lack of commercial markets for the gypsum results in this material being disposed of rather than utilized. Characteristics of these processes are provided in the next section. A variety of FGD processes exist, and the selection of a system is dependent upon site-specific consideration, economics, and other criteria. Elliot [8] provides a ranking of various FGD processes used in the United States in Tables 6-4 and 6-5 where cost, performance,

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