Approximate Range of Coal Sulfur Content (%)


Pounds of Sulfur




per Million Btu

Bituminous Coal


Low sulfur

<0.4 to 0.6

<0.5 to 0.8

<0.3 to 0.5

Medium sulfur

0.61 to 1.67

0.8 to 2.2

0.5 to 1.3

High sulfur

1.68 to >2.50

2.2 to >3.3

1.3 to >1.9

Source: U.S. Energy Information Administration, U.S. Coal Reserves: 1997 Update, U.S. Department of Energy, Office of Coal, Nuclear, Electric and Alternate Fuels, U.S. Government Printing Office, Washington, D.C., February 1999.

Source: U.S. Energy Information Administration, U.S. Coal Reserves: 1997 Update, U.S. Department of Energy, Office of Coal, Nuclear, Electric and Alternate Fuels, U.S. Government Printing Office, Washington, D.C., February 1999.

Low-sulfur coals are also imported to the United States, specifically to coastal areas such as Florida or the Eastern seaboard. Similar to the replacement of high-sulfur coals with non-coal fuels in power generation units, this practice of importing lower sulfur coals to the United States for sulfur dioxide compliance also needs to be questioned from the standpoint of energy security.

Coal Cleaning Coal preparation, or beneficiation, is a series of operations that remove mineral matter (i.e., ash) from coal. Preparation relies on different mechanical operations (not discussed in detail here) to perform the separation, such as size reduction, size classification, cleaning, dewatering and drying, waste disposal, and pollution control. Coal preparation processes, which are physical processes, are designed mainly to provide ash removal, energy enhancement, and product standardization [8]. Sulfur reduction is achieved because the ash material removed contains pyritic sulfur. Coal cleaning is used for moderate sulfur dioxide emissions control, as physical coal cleaning is not effective in removing organically bound sulfur. Chemical coal cleaning processes are being developed to remove the organic sulfur; however, these are not used on a commercial scale. An added benefit of coal cleaning is that several trace elements, including antimony, arsenic, cobalt, mercury, and selenium, are generally associated with pyritic sulfur in raw coal and they, too, are reduced through the cleaning process. As the inert material is removed, the volatile matter content, fixed carbon content, and heating value increase, thereby producing a higher quality coal. The moisture content, a result of residual water from the cleaning process, can also increase, which lowers the heating value, but this reduction is usually minimal and has little impact on coal quality. Coal cleaning does add additional cost to the coal price; however, among the several benefits of reducing the ash content are lower sulfur content; less ash to be disposed of; lower transportation costs, as more carbon and less ash is transported (coal cleaning is usually done at the mine and not the power plant); and increases in power plant peaking capacity, rated capacity, and availability [10]. Developing circumstances are making coal cleaning more economical and a potential sulfur control technology and include [8]:

• Higher coal prices and transportation costs;

• Diminishing coal quality because of less selective mining techniques;

• The need to increase availability and capacity factors at existing boilers;

• More stringent air quality standards;

• Lower costs for improving fuel quality versus investing in extra pollution control equipment.

Wet Flue Gas Desulfurization Wet scrubbers are the most common FGD method currently in use (or under development); they include a variety of processes and involve the use of many sorbents and are manufactured by a large number of companies. The sorbents used by wet scrubbers include calcium-, magnesium-, potassium-, or sodium-based sorbents, ammonia, or seawater. Currently, no commercial potassium-based scrubbers are in use, and only a limited number of ammonia or seawater systems are in use or being demonstrated. The calcium-based scrubbers are by far the most popular, and this technology is discussed in this section along with the use of sodium- and magnesium-based sorbents.

Limestone- and Lime-Based Scrubbers Wet scrubbing with limestone or lime is the most popular commercial FGD system. The inherent simplicity, the availability of an inexpensive sorbent (limestone), production of a usable by-product (gypsum), reliability, availability, and the high removal efficiencies obtained (which can be as high as 99%) are the main reasons for the popularity of this system. Capital costs are typically higher than other technologies, such as sorbent injection systems; however, the technology is known for its low operating costs as the sorbent is widely available and the system is cost effective.

In a limestone/lime wet scrubber, the flue gas is scrubbed with a 5 to 15% (by weight) slurry of calcium sulfite/sulfate salts along with calcium hydroxide (Ca(OH2)) or limestone (CaCO3). Calcium hydroxide is formed by slaking lime (CaO) in water according to the reaction:

In the limestone and lime wet scrubbers, the slurry containing the sulfite/ sulfate salts and the newly added limestone or calcium hydroxide is pumped to a spray tower absorber and sprayed into it. The sulfur dioxide is absorbed into the droplets of slurry and a series of reactions occur in the slurry. The reactions between the calcium and the absorbed sulfur dioxide create the compounds calcium sulfite hemihydrate (CaCO3 ■ fHfO) and calcium sulfate dihydrate (CaSO4 ■ 2H2O). Both of these compounds have low solubility in water and precipitate from the solution. This enhances the absorption of sulfur dioxide and further dissolution of the limestone or hydrated lime.

The reactions occurring in the scrubbers are complex. The simplified overall reaction for a limestone scrubber is:

SOf(g) + CaCO3(s) + iH2O(I) CaSO3 ■ fHfOS) + COf(g) (6-10)

and the reaction for a lime scrubber is:

SOf(g) + Ca(OH)2(s) + HfO(i) CaSO3 ■ 1HfO(s) + fHfOtf) (6-11)

The calcium sulfite hemihydrate can be converted to the calcium sulfate dihydrate with the addition of oxygen by the reaction:

CaSO3 ■ 1H2O(s) + fHfO(l) + fOf(g) ^ CaSO4 ■ 2HfO(s) (6-12)

FIGURE 6-3. Limestone scrubber system with forced oxidation.

The actual reactions that occur, however, are much more complex and include a combination of gas-liquid, solid-liquid, and liquid-liquid ionic reactions. In the limestone scrubber, the following reactions describe the process [11]. In the gas-liquid contact zone of the absorber (see Figure 6-3 for a typical schematic diagram of a limestone scrubber system), sulfur dioxide dissolves into the aqueous state:

and is hydrolyzed to form ions of hydrogen and bisulfite:

The limestone dissolves in the absorber liquid forming ions of calcium and bicarbonate:

CaCO3(s) + H+ «—> Ca++ + HCO- (6-15) which is followed by acid-base neutralization:

HCO- + H+ «—> CO2(I) + H2O(I) (6-16) stripping of the CO2 from the slurry:

CO2(l) ^ CO2(g) (6-17) and dissolution of the calcium sulfite hemihydrate:

In the reaction tank of a scrubber system, the solid limestone is dissolved into the aqueous state (Reaction (6-15)), acid-base neutralization occurs (Reaction (6-16)), the CO2 is stripped out (Reaction (6-17)), and the calcium sulfite hemihydrate is precipitated by the reaction:

Ca++ + HSO- + ±H2O(I) ^ CaSOs ■ ±H2O(s) + H+ (6-19)

The dissolution of the calcium sulfite in the gas-liquid contact zone in the absorber is necessary in order to minimize scaling of the calcium sulfite hemihydrate in the absorber [6]. The equilibrium pH for calcium sulfite is ~6.3 at a CO2 partial pressure of 0.12 atm, which is the typical concentration of CO2 in flue gas. Typically, the pH is maintained below this level to keep the calcium sulfite hemihydrate from dissolving (i.e., keep Reaction (6-18) from proceeding to the right). The slurry returning from the absorber to the reaction tank can have a pH as low as 3.5, which is increased to 5.2 to 6.2 by the addition of freshly prepared limestone slurry to the tank [6]. The pH in the reaction tank must be maintained at a pH that is less than the equilibrium pH of calcium carbonate in water, which is 7.8 at 77°F.

The reaction equations for the lime scrubber are similar to those for the limestone scrubber, with the exception that the following reactions are substituted for Reactions (6-15) and (6-16), respectively [11]:

Limestone with Forced Oxidation (LSFO) Limestone scrubbing with forced oxidation (LSFO) is one of the most popular systems in the commercial market. A limestone slurry is used in an open spray tower with in situ oxidation to remove SO2 and form a gypsum sludge. The major advantages of this process, relative to a conventional limestone FGD system (where the product is calcium sulfite rather than calcium sulfate (gypsum)), are easier dewatering of the sludge, more economical disposal of the scrubber product solids, and decreased scaling on the tower walls. LSFO is capable of greater than 90% SO2 removal [12].

In the LSFO system, the hot flue gas exits the particulate control device, usually an ESP, and enters a spray tower where it comes into contact with a sprayed dilute limestone slurry. The SO2 in the flue gas reacts with the limestone in the slurry via the reactions listed earlier to form the calcium sulfite hemihydrate. Compressed air is bubbled through the slurry, which causes this sulfite to be naturally oxidized and hydrated to form calcium sulfate dihydrate. The calcium sulfate can be first dewatered using a thickener or hydrocyclones then further dewatered using a rotary drum filter. The gypsum is then transported to a landfill for disposal. The formation of the calcium sulfate crystals in a recirculation tank slurry also helps to reduce the chance of scaling.

The absorbing reagent, limestone, is normally fed to the open spray tower in an aqueous slurry at a molar feed rate of 1.1 mol of CaCO3 per mol of SO2 removed. This process is capable of removing more than 90% of the SO2 present in the inlet flue gas. The advantages of LSFO systems include [12]:

• The scaling potential is lower on lower internal surfaces due to the presence of gypsum seed crystals and reduced calcium sulfate saturation levels; this in turn provides greater reliability of the system;

• The gypsum product is filtered more easily than the calcium sulfite (CaSO3) produced with conventional limestone systems;

• The chemical oxygen demand is lower in the final disposed product;

• The final product can be safely and easily disposed of in a landfill;

• The forced oxidation allows greater limestone utilization than in conventional systems;

• Costs of the raw material (limestone) used as an absorbent are lower;

• LSFO is an easier retrofit than natural oxidation systems because the process uses smaller dewatering equipment.

A disadvantage of this system is the high energy demand due to the relatively higher liquid-to-gas ratio necessary to achieve the required SO2 removal efficiencies.

Limestone with Forced Oxidation Producing a Wallboard Gypsum By-Product In the limestone/wallboard (LS/WB) gypsum FGD process, a limestone slurry is used in an open spray tower to remove SO2 from the flue gas. The flue gas enters the spray tower where the SO2 reacts with the CaCO3 in the slurry to form calcium sulfite. The calcium sulfite is then oxidized to calcium sulfate in the absorber recirculation tank. The calcium sulfate produced with this process is of a higher quality so that it may be used in wallboard manufacture.

There are a few differences with this process in order to achieve a higher quality gypsum. The LS/WB system uses horizontal belt filters to produce a drier product and provides sufficient cake washing to remove residual chlorides. Because the by-product is a higher quality, the use of the product handling system is replaced with by-product conveying and temporary storage equipment. Sulfuric acid addition is used in systems with an external oxidation tank. The acid is used to control the pH of the slurry and neutralizes unreacted CaCO3.

The limestone feed rate in this process is 1.05 mol CaCO3 per mol of SO2 removed, which is slightly lower than the feed rate for the LSFO system [12]. Other advantages of this process are that the disposal area is kept to a minimum because most of the by-product is reusable. The gypsum can be sold to cement plants and agricultural users. Also, SO2 removal is slightly enhanced because of the high sulfite-to-sulfate conversion.

There are some disadvantages to this process. Few full-scale operating systems actually produce quality gypsum in the United States. To produce quality gypsum, specific process control and tight operator attention are constantly needed to ensure that chemical impurities do not lead to off-specification gypsum. Another disadvantage is the inability to use cooling tower blowdown as system make-up water due to chloride limits in the gypsum by-product.

Limestone with Inhibited Oxidation In the limestone with inhibited oxidation process, the hot flue gas exits the particulate control device and enters an open spray tower, where it comes into contact with a dilute CaCO3 slurry. This slurry contains thiosulfate (Na2S2O3), which inhibits natural oxidation of the calcium sulfite. The calcium sulfite is formed from the reaction with SO2 in the flue gas and the CaCO3 slurry. The slurry absorbs the SO2, then drains down to a recirculation tank below the tower. By inhibiting natural oxidation of the sulfite, gypsum scaling on process equipment is reduced along with gypsum relative saturation, which is reduced below 1.0. Thiosulfate is either added directly as Na2S2O3 to the feed tank or is generated in situ by the addition of emulsified sulfur. In some cases, thiosulfate has the ability to increase the dissolution of the calcium carbonate and enlarge the size of the sulfite crystals to improve solids dewatering [12]. This process is capable of removing more than 90% of the SO2 in the flue gas. The calcium sulfite slurry product is thickened, stabilized with fly ash and lime, and then sent to a landfill. The calcium carbonate feed rate is 1.10 mol Ca per mol of SO2 removed. The effectiveness of thiosulfate is site specific because the amount of thiosulfate required to inhibit oxidation strongly depends on the chemistry and operating conditions of each FGD system. Variables such as saturation temperature, dissolved magnesium, chlorides, flue gas inlet SO2 andO2 concentrations, and slurry pH affect the thiosulfate effectiveness [12].

Thiosulfate has been shown to increase limestone utilization when added to the system. This occurs because the thiosulfate reduces the gypsum relative saturation level, which in turn reduces the level of calcium dissolved in the liquor. The dissolution rate is increased by lowering the calcium concentration in the slurry. Thiosulfate also improves the dewatering characteristics of the sulfite product. By preventing the high concentrations of sulfate, the thiosulfate allows the calcium sulfite to form larger, single crystals. This increases the settling velocity of the crystals and improves the filtering characteristics, which results in a higher solid content of dewatered product.

There are a few disadvantages of the process. The thiosulfate/sulfur reagent requires additional process equipment and storage facilities. Also, the reagent can cause corrosion of many stainless steels under scrubber conditions. Another disadvantage is that the thiosulfate is fairly temperature dependent, thus requiring the system to operate within a particular temperature range.

Magnesium-Enhanced Lime In the magnesium-enhanced lime (MagLime) process, the hot flue gas exits the particulate control device and enters a spray tower, where it comes into contact with a magnesium sulfite/lime slurry. Magnesium lime, such as thiosorbic lime (which contains 4-8% MgO), is fed to the open spray tower in an aqueous slurry at a molar feed rate of 1.1 mol CaO per mol of SO2 removed. The SO2 is absorbed by the reaction with magnesium sulfite, forming magnesium bisulfite. This occurs through the following reactions [12]:

The magnesium sulfite absorbs the H+ ion and increases the HSO- concentration in Reaction (6-23). This allows the scrubber liquor to absorb more of the SO2. The absorbed SO2 reacts with hydrated lime to form solidphase calcium sulfite. The magnesium sulfite is reformed by the following reactions:

Ca(OH)2(s) + 2HSO- + Mg++ Ca++SO-- + 2H2O(l) + MgSO3(s)

Inside the absorber, some magnesium sulfite present in the solution is oxidized to sulfate. This sulfite reacts with the lime to form calcium sulfate solids. Calcium sulfite and sulfate solids are the main products of the MagLime process. The calcium sulfite sludge is dewatered using thickener and vacuum filter systems then fixated using fly ash and lime prior to disposal in a lined landfill. The magnesium remains dissolved in the liquid phase.

Some advantages of the MagLime process compared to the LSFO process include [12]:

• High SO2 removal efficiency at low liquid-to-gas ratios;

• Lower gas-side pressure drop due to lower liquid-to-gas ratios;

• Reduced potential for scaling, which improves reliability of the system;

• Lower power consumption due to a lower slurry recycle rate;

• Lower capital investment due to smaller reagent handling equipment and no oxidation air compressor;

• Reduction in freshwater use because the process water may be recycled for the mist eliminator wash.

The three major disadvantages of the process are the expense of the lime reagent compared to the limestone, the use of fresh water for lime slaking, and the difficult dewatering characteristics of the calcium sulfite/sulfate sludge. The sulfite can be oxidized to produce gypsum, but this requires extensive equipment and process control.

Limestone with Dibasic Acid The dibasic acid enhanced limestone process is very similar to the LSFO process. The hot flue gas exits the particulate control device and enters a spray tower, where it comes into contact with a diluted limestone slurry. The SO2 in the flue gas reacts with the limestone and water to form hydrated calcium sulfite:

SO2(g) + CaCO3(s) + 1H2O(I) CaSO3 ■ 2^O(s) + CO2(g) (6-10)

This equation is rate limited by the absorption of SO2 into the scrubbing liquor:

The dissolved SO2 ions then react with the calcium ions to form calcium sulfite. The hydrogen ions in solution are partly responsible for reforming the SO2.

After absorbing the SO2, the slurry drains from the tower to a recirculation tank. Here, the calcium sulfite is oxidized to calcium sulfate dihydrate using oxygen:

CaSO3 ■ 1H2O(s) + 2H2O(l) + ±O2(g) ^ CaSO4 ■ 2H2O(s) (6-12)

Dibasic acid acts as a buffer by absorbing free hydrogen ions formed by Reaction (6-14), which shifts the reaction to the right to form more sulfite ions, thus removing more SO2. Alkaline limestone is added to replace the buffering capabilities of the acid; therefore, there is no net consumption of the dibasic acid during SO2 absorption. The limestone dissolution rate is increased by increasing the SO2 removal efficiency at a low slurry pH. This results in a lower reagent consumption due to an increase in calcium carbonate availability in the recirculation tank.

The dibasic acid process offers some advantages compared to the LSFO process [12]:

• Increased SO2 removal efficiency;

• Reduced liquid-to-gas ratio and the potential to decrease the reagent feed rate, which lowers capital and operating costs for the limestone grinding equipment, slurry handling, and landfill requirements;

• Reduced scaling because of the low pH and reduced gypsum relative saturation levels;

• Increased system reliability by reducing the maintenance requirements and increasing the flexibility of the system.

Disadvantages of the process include:

• More process capital is needed for the dibasic acid feed equipment;

• There is the potential for corrosion and erosion due to the low system pH;

• Odorous by-products are produced by the dibasic acid degradation; although the SO2 absorption reactions do not consume the dibasic acid, the acid does degrade by carboxylic oxidation into many short chain molecules, one of which is valeric acid, which has a musty odor;

• Control problems may be caused by foaming in the recirculation and oxidation tanks due to the presence of the dibasic acid.

Sodium-Based Scrubbers Wet sodium-based systems have been in commercial operation since the 1970s. These systems can achieve high SO2 removal efficiencies while burning coals with medium to high sulfur content. A disadvantage of these systems, however, is the production of a waste sludge that requires disposal.

Lime Dual Alkali In the lime dual alkali process, the hot flue gas exits the particulate control device and enters an open spray tower where the gas comes into contact with a sodium sulfite (Na2SO3) solution that is sprayed into the tower [12]. An initial charge of sodium carbonate (Na2CO3) reacts directly with the SO2 to form sodium sulfite and CO2. The sulfite then reacts with more SO2 and water to form sodium bisulfite (NaHSO3). Some of the sodium sulfite is oxidized by excess oxygen in the flue gas to form sodium sulfate (Na2SO4). This does not react with SO2 and cannot be reformed by the addition of lime to form calcium sulfate. The above process is described by the following reactions:

along with the minor reaction:

The calcium sulfites and sulfates are reformed in a separate regeneration tank and are formed by mixing the soluble sodium salts (bisulfate and sulfate) with slaked lime. The calcium sulfites and sulfates precipitate from the solution in the regeneration tank. The scrubber liquor then has a pH of 6 to 7 and consists of sodium sulfite, sodium bisulfite, sodium sulfate, sodium hydroxide, sodium carbonate, and sodium bicarbonate [12].

The lime dual alkali process has several advantages over the LSFO process, including the following [12]:

• The system has a higher availability because there is less potential for scaling and plugging of the soluble absorption reagents and reaction products;

• Corrosion and erosion are prevented with the use of a relatively high pH solution;

• Maintenance labor and material requirements are lower because of the high reliability of the system;

• The main recirculation pumps are smaller because the absorber liquid/gas feed rate is less;

• Power consumption is lower due to the smaller pump requirements;

• There is no process blowdown water discharge stream;

• The highly reactive alkaline compounds in the absorbing solution allow for better turndown and load following capabilities.

There are two main disadvantages of the process compared to the LSFO system: The sodium carbonate reagent is more expensive than limestone, and the sludge must be disposed of in a lined landfill because of sodium contamination of the calcium sulfite/sulfate sludge.

Regenerative Processes Regenerative FGD processes regenerate the alkaline reagent and convert the SO2 to a usable chemical by-product. Two commercially accepted processes are discussed in this section. The Wellman-Lord process is the most highly demonstrated regenerative technology in the world, while the regenerative magnesia scrubbing process is in commercial service in the United States. Other processes have undergone demonstrations, are used on a limited basis, or are currently under development and include ammonia-based scrubbing, an aqueous carbonate process, and the citrate process.

Wellman-Lord Process The Wellman-Lord process uses sodium sulfite to absorb SO2, which is then regenerated to release a concentrated stream of SO2. Most of the sodium sulfite is converted to sodium bisulfite by reaction with SO2 as in the dual alkali process. Some of the sodium sulfite is oxidized to sodium sulfate. Prescrubbing of the flue gases is necessary to saturate and cool the flue gas to about 130°F. This removes chlorides and remaining fly ash and prevents excessive evaporation in the absorber. A schematic of the system is shown in Figure 6-4 [8]. The basic absorption reaction for the Wellman-Lord process is:

The sodium sulfite is regenerated in an evaporator-crystallizer through the application of heat. A concentrated SO2 stream (i.e., 90%) is produced at the

Venturi Prescrubber Flue-Gas Blower yl

Makeup Water


To Ash Pond



To Stack


To Ash Pond


Feed Dissolving Tank

C00ler Vent Scrubber


Condensate Strippers

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