Sulfur Oxides Contents Characteristics Lesson Material Practice Problems Formation Mechanisms Lesson Material Practice Problems Control Techniques Introduction Absorption Adsorption Alternative Fuels Practice Problems Sulfur Oxides Characteristics Contents Lesson Material Practice Problems Objective Describe the basic characteristics of sulfur dioxide and sulfuric acid Lesson Material Sulfur oxides include sulfur dioxide (SO2), sulfur trioxide (SO3), and sulfuric acid (H2SO4) The major sources of sulfur dioxide are shown in Figure Combustion of fossil fuels for generation of electric power is clearly the primary contributor of sulfur dioxide emissions Industrial processes, such as nonferrous metal smelting, also contribute to sulfur dioxide emissions Sulfur dioxide is a colorless gas, which is moderately soluble in water and aqueous liquids It is formed primarily during the combustion of sulfur-containing fuel or waste Once released to the atmosphere, sulfur dioxide reacts slowly to form sulfuric acid (H 2SO4), inorganic sulfate compounds, and organic sulfate compounds Some of the sulfur dioxide in high temperature processes is oxidized to form sulfur trioxide as shown below Sulfur trioxide remains in the vapor state while the combustion gases are very hot As the gases cool, sulfur trioxide adds a water molecule and forms sulfuric acid as indicated by the reaction below Below 500 to 600°F, most of the sulfur trioxide, which is extremely hygroscopic, reacts with water molecules to form sulfuric acid Sulfuric acid is a strong acid In addition to the sources mentioned above, sulfuric acid can also be released from plants that manufacture batteries Sulfuric acid vapor in moderate concentrations (2 to ppm) is very beneficial to electrostatic precipitators because it adsorbs onto particle surfaces and creates a moderate resistivity High concentrations can be detrimental to precipitator performance High sulfuric acid levels can also cause significant corrosion problems for precipitators, fabric filters, and other control devices The temperature of flue gases should be kept well above the dew point for sulfuric acid to prevent condensation on ductwork surfaces and components in the air pollution control system Sulfur Oxides Formation Mechanisms Contents Lesson Material Practice Problems Objective Describe the basic mechanisms contributing to the formation of sulfur dioxide and sulfuric acid Lesson Material Sulfur dioxide and sulfuric acid are formed during the combustion of fuel or waste that contains sulfur compounds Sulfur oxides can also be released from chemical reactors and sulfuric acid plants The sulfur in the fuel or waste being fired enters the combustion process in a variety of chemical forms including but not limited to inorganic sulfates, organic sulfur compounds, and pyrites A small fraction of the fuel or waste sulfur (usually less than five percent) remains in the bottom ash leaving the combustion processes The remaining 95+ percent is converted to sulfur dioxide, which remains in the gaseous form throughout the combustion system A small fraction of the sulfur dioxide generated in the combustion zone is oxidized further to form sulfur trioxide The reaction mechanisms that could contribute to the formation of this pollutant are not entirely known; however, they probably include the following: • Free radical reaction of sulfur dioxide with atomic oxygen in the high temperature zones • Catalytic oxidation of sulfur dioxide on the surfaces of particles entrained in the gas stream • Thermal reactions between sulfur dioxide and other inorganic gases generated during combustion The concentration of sulfur trioxide generated during combustion varies widely from unit to unit for reasons that have not been determined entirely; however, sulfur trioxide concentrations are generally related directly to the concentration of sulfur in the fuel and the concentration of oxygen in the combustion zone The sulfur trioxide concentrations are usually 0.5 to percent of the sulfur dioxide concentration Sulfur trioxide quickly converts to sulfuric acid upon cooling in the gas stream or atmosphere Sulfur Oxides Control Techniques Contents Introduction Absorption Adsorption Alternative Fuels Practice Problems Objective Describe the basic techniques and systems used to control sulfur dioxide and sulfuric acid emissions Introduction Air pollution control systems for sulfur dioxide removal are large and sophisticated Sulfur dioxide is controlled by three different techniques: absorption, adsorption, and the use of low-sulfur fuels The control systems used for sulfur dioxide are usually not designed to remove sulfuric acid The sulfuric acid concentrations are usually below the levels where it is economically feasible or environmentally necessary to install control systems Absorption Absorption processes use the solubility of sulfur dioxide in aqueous solutions to remove it from the gas stream Once sulfur dioxide has dissolved in solution to form sulfurous acid (H 2SO3), it reacts with oxidizers to form inorganic sulfites ( ) and sulfates ( ) This process prevents the dissolved sulfur dioxide from diffusing out of solution and being re-emitted For more information about the process of absorption, see the lesson on Gas Absorption in Module The most common type of sulfur dioxide absorber is the limestone wet scrubber An example flowchart is shown in Figure Limestone is the alkali most often used to react with the dissolved sulfur dioxide Limestone slurry is sprayed into the sulfur dioxide-containing gas stream The chemical reactions in the recirculating limestone slurry and reaction products must be carefully controlled in order to maintain the desired sulfur dioxide removal efficiency and to prevent operating problems Wet scrubbers used for sulfur dioxide control usually operate at liquid pH levels between to to maintain high efficiency removal Typical removal efficiencies for sulfur dioxide in wet scrubbers range from 80 to 95% #1 Which of the following types of wet scrubbers has the best design for sulfur dioxide removal? a Venturi scrubber b Impingement plate scrubber c Spray tower Answer: c Spray tower Spray towers scrubbers have the best design for sulfur dioxide removal They have a spray mechanism for introducing the slurry into the gas stream Also, their simple, open design presents fewer opportunities for the alkaline slurry to plug parts The residence time is also adequate for sulfur dioxide to be absorbed into the slurry droplets Venturis are not a good choice in this situation because the residence time is too short In sulfur dioxide removal, there must be adequate contact between the pollutant gas and the alkaline slurry for at least a minimal length of time While venturi scrubbers can be used to remove both particulate and gaseous pollutants, they are primarily designed to maximize the collection of particles by creating a large difference in velocity between particles and liquid droplets in the gas stream Venturi scrubbers also have large pressure drops, so they are more expensive to operate than spray towers Impingement plate scrubbers are not good candidates in this situation because the alkaline slurry would plug the small holes in the impingement plates The wet scrubber (absorber) vessels not efficiently remove particulate matter smaller than approximately micrometers However, as in the case with low-efficiency particulate wet scrubbers, the particulate removal efficiency increases rapidly with particle size above micrometers Usually, a moderate-to-high efficiency particulate control system is used upstream from the sulfur dioxide absorber to reduce the particulate matter emissions in the less than micrometer size range These upstream collectors also reduce the quantity of particulate matter that is captured in the absorber The evaporation of water that occurs in wet scrubber vessels can keep gas temperatures relatively cold, in the range of 110 to 140°F These gas temperatures are well below the typical operating temperatures of other air pollution control systems used on sources that generate sulfur dioxide emissions Another type of absorption system is called a spray atomizer dry scrubber (see Figure 2), which belongs to a group of scrubbers called spray-dryer-type dry scrubbers In this case, an alkaline slurry is sprayed into the hot gas stream at a point upstream from the particulate control device As the slurry droplets are evaporating, sulfur dioxide absorbs into the droplet and reacts with the dissolved and suspended alkaline material Large spray dryer chambers are used to ensure that all of the slurry droplets evaporate to dryness prior to going to a high efficiency particulate control system The term "dry scrubber" refers to the condition of the dried particles approaching the particulate control system Fabric filters or electrostatic precipitators are often used for high efficiency particulate control The system shown in Figure has a fabric filter Spray-dryer-type absorption systems have efficiencies that are similar to those for wet-scrubber-type absorption systems These generate a waste stream that is dry and, therefore, easier to handle than the sludge generated in a wet scrubber However, the equipment used to atomize the alkaline slurry is complicated and can require considerably more maintenance than the wet scrubber systems Spraydryer-type absorption systems operate at higher gas temperatures than wet scrubbers and are less effective for the removal of other pollutants in the gas stream such as condensable particulate matter The choice between a wet-scrubber absorption system and a spray-dryer absorption system depends primarily on site-specific costs The options available for environmentally sound disposal of the waste products are also an important consideration in selecting the type of system for a specific application Both types of systems are capable of providing high efficiency sulfur dioxide removal Adsorption Sulfur dioxide can be collected by adsorption systems In this type of control system, a dry alkaline powder is injected into the gas stream Sulfur dioxide adsorbs to the surface of the alkaline particles and reacts to form compounds that cannot be re-emitted to the gas stream Hydrated lime (calcium hydroxide) is the most commonly used alkali However, a variety of alkalis can be used effectively A flowchart for a dry-injection-type dry scrubber (adsorber) is shown in Figure A dry-injection-type dry scrubber can be used on smaller systems as opposed to using the larger, more complicated spray-dryer-type dry scrubber However, the dry injection system is slightly less efficient, and requires more alkali per unit of sulfur dioxide (or other acid gas) collected Accordingly, the waste disposal requirements and costs are higher for adsorption systems than absorption systems Alternative Fuels Other techniques used for limiting the emissions of sulfur dioxide are simply to switch to fuels that have less sulfur or to convert to synthetic (processed) fuels that have low sulfur levels The sulfur dioxide emission rate is directly related to the sulfur levels in coal, oil, and synthetic fuels However, not all boilers can use these types of fuels Each type of boiler has a number of very specific and important fuel characteristic requirements and not all low sulfur fuels meet these fuel-burning characteristics Sulfur Oxides Practice Problems Characteristics #1 What is the primary source of sulfur dioxide emissions? Answer: Combustion of sulfur-containing fossil fuels for generation of electric power is the primary source of sulfur dioxide emissions #2 Which of the following statements about sulfur dioxide is true? a Sulfur dioxide is a yellow gas b Sulfur dioxide is not soluble in water and aqueous liquids c Sulfur dioxide at high temperatures can oxidize to form sulfur trioxide d a and b, only e a, b, and c Answer: c Sulfur dioxide at high temperatures can oxidize to form sulfur trioxide Sulfur dioxide is a colorless gas Its moderate solubility in water and aqueous liquids is the basis for using wet scrubbers as one effective control technique for this gas (This is discussed in greater depth later in this Module.) #3 Which of the following statements about sulfuric acid is true? 10 a Sulfuric acid is a strong acid b In moderate concentrations, sulfuric acid is beneficial to fabric filtration performance c Sulfuric acid is extremely hygroscopic d a and b, only e a, b, and c Answer: a Sulfuric acid is a strong acid In moderate concentrations, sulfuric acid is beneficial to electrostatic precipitator performance (not fabric filters) because this acid adsorbs onto particle surfaces and gives them moderate resistivity Sulfur trioxide, not sulfuric acid, is extremely hygroscopic In fact, sulfuric acid forms when sulfur trioxide reacts with a molecule of water #4 Describe some ways in which sulfuric acid can become present in ambient air Answer: Sulfuric acid can become present in ambient air in a number of different ways First, a small amount of sulfuric acid can be emitted from the stacks of fossil-fuel-fired power plants as a result of the oxidation of sulfur dioxide to sulfur trioxide in the combustion chamber When the sulfur trioxide cools to temperatures below approximately 500°F, it reacts with available water molecules to form sulfuric acid Most of the sulfuric acid in ambient air forms after sulfur dioxide is emitted into the atmosphere In the atmosphere, sulfur dioxide reacts slowly to form sulfuric acid as well as some organic and inorganic sulfate compounds Sulfuric acid can also be released from plants that manufacture batteries Practice Problems Formation Mechanisms #1 In fossil-fuel-fired boilers, what fraction of the total sulfur entering with the fuel is usually converted to gaseous sulfur dioxide? 11 Answer: Approximately 95% Approximately five percent of the sulfur contained in fuel or waste remains in the bottom ash The remaining 95 percent or more is converted to gaseous sulfur dioxide #2 In fossil-fuel-fired boilers, sulfuric acid vapor concentrations are approximately what percentage of the sulfur dioxide concentrations? Answer: 0.5 to 2% Sulfur trioxide concentrations are usually 0.5 to percent of sulfur dioxide concentrations Since sulfur trioxide quickly converts to sulfuric acid upon cooling to temperatures below approximately 500 to 600°F, sulfuric acid concentrations are also in the range of 0.5 to percent of sulfur dioxide levels Practice Problems Control Techniques #1 What type of control system is used for the control of sulfuric acid vapor generated in combustion processes? Answer: None The concentrations of sulfuric acid vapor are usually too low to economically justify a separate control system #2 12 Why are alkalis used in wet scrubbing systems to remove sulfur dioxide? Answer: Alkalis help maintain high efficiency sulfur dioxide scrubbing by converting the dissolved sulfur dioxide to a form that cannot escape the liquid phase #3 Which of the scrubbers listed below use the process of absorption to control sulfur dioxide emissions? Select all that apply a Dry-injection-type dry scrubbers b Limestone wet scrubbers c Spray atomizer dry scrubber Answer: b Limestone wet scrubbers c Spray atomizer dry scrubber Both limestone wet scrubbers and spray atomizer dry scrubbers use the process of absorption to control sulfur dioxide emissions In limestone wet scrubbers, sulfur dioxide gas is absorbed into an aqueous solution containing an alkaline substance (limestone) Once the sulfur dioxide is absorbed in solution, it reacts with oxidizers to form inorganic sulfites and sulfates This action prevents sulfur dioxide from diffusing out of solution and being re-emitted In spray atomizer dry scrubbers, an alkaline slurry is sprayed into the hot gas stream As the slurry droplets are evaporating, sulfur dioxide absorbs into the droplet and reacts with the dissolved and suspended alkaline material #4 Which of the scrubbers listed below use the process of adsorption to control sulfur dioxide emissions? Select all that apply a Dry-injection-type dry scrubbers b Limestone wet scrubbers c Spray atomizer dry scrubbers 13 Answer: a Dry-injection-type dry scrubbers In these control devices, sulfur dioxide adsorbs to the surface of the alkaline powder, which is injected into the gas stream The sulfur dioxide then reacts to form compounds that cannot be reemitted into the gas stream #5 Which of the following sulfur dioxide control devices require a particulate control device downstream from them? Select all that apply a Dry-injection-type dry scrubbers b Limestone wet scrubbers c Spray atomizer dry scrubbers Answer: a Dry-injection-type dry scrubbers c Spray atomizer dry scrubbers Both dry-injection-type dry scrubbers and spray atomizer dry scrubbers produce particulate matter that must be removed downstream from the device In dry-injection-type systems, a dry alkaline powder is actually introduced into the gas stream In spray atomizer dry scrubbers, the alkaline slurry droplets evaporate to dryness With wet scrubbers, the waste stream produced is a liquid rather than particulate matter Limestone wet scrubbing systems usually have a moderate-to-high efficiency particulate matter control device upstream from them #6 Suppose you are an air quality consultant with expertise in the control of sulfur dioxide emissions Based on the following criteria would you recommend a limestone wet scrubber or a spray atomizer dry scrubber to decrease sulfur dioxide emissions? In some cases, both types of systems may meet the criterion equally well i The system must achieve a high sulfur dioxide removal efficiency 14 ii The client insists that the system must generate a waste stream that is easy to handle iii The facility has an existing ESP in the process to control particulate matter Due to space limitations, the control device must be placed upstream from the ESP The client wishes to keep the flue gas temperature as high as possible in the ESP Answer: i Neither type would be preferred Both limestone wet scrubbers and a spray atomizer dry scrubbers have approximately the same removal efficiency of 80 to 95% Answer: ii Spray atomizer dry scrubber Spray atomizer dry scrubbers generate a waste stream that is dry and is therefore easier to handle than the sludge produced by wet scrubbers Answer: iii Spray atomizer dry scrubber A spray atomizer dry scrubber appears to be the appropriate choice The existing ESP will capture the dry particulate from the spray atomizer scrubber The gas temperature will be lowered more by the wet scrubber than by the spray atomizer dry scrubber #7 What are the advantages and disadvantages of dry-injection-type dry scrubbers compared to other sulfur dioxide control devices? Answer: Advantage: They work effectively on smaller systems Dry-injection-type dry scrubbers are more costly to operate on larger systems because they have higher alkaline requirements Disadvantages: Dry injection-type dry scrubbers are slightly less efficient at controlling sulfur dioxide emissions They require more alkali per unit of acid gas collected 15 #8 To reduce pollution, plant personnel may switch to low-sulfur fuel for boiler operations What factor should they consider when making this decision? Answer: The sulfur dioxide emission rate is directly related to the sulfur levels in coal, oil, and synthetic (processed) fuels Switching to low-sulfur fuels, such as low-sulfur coal, or converting to synthetic fuels that have low-sulfur levels could be a good way for the facility to restrict its production of sulfur dioxide in boilers However, the facility should make sure that the new fuel meets the specific fuel burning requirements for that particular type of boiler 16 [...]... result of the oxidation of sulfur dioxide to sulfur trioxide in the combustion chamber When the sulfur trioxide cools to temperatures below approximately 500°F, it reacts with available water molecules to form sulfuric acid Most of the sulfuric acid in ambient air forms after sulfur dioxide is emitted into the atmosphere In the atmosphere, sulfur dioxide reacts slowly to form sulfuric acid as well as... gaseous sulfur dioxide #2 In fossil-fuel-fired boilers, sulfuric acid vapor concentrations are approximately what percentage of the sulfur dioxide concentrations? Answer: 0.5 to 2% Sulfur trioxide concentrations are usually 0.5 to 2 percent of sulfur dioxide concentrations Since sulfur trioxide quickly converts to sulfuric acid upon cooling to temperatures below approximately 500 to 600°F, sulfuric... and gives them moderate resistivity Sulfur trioxide, not sulfuric acid, is extremely hygroscopic In fact, sulfuric acid forms when sulfur trioxide reacts with a molecule of water #4 Describe some ways in which sulfuric acid can become present in ambient air Answer: Sulfuric acid can become present in ambient air in a number of different ways First, a small amount of sulfuric acid can be emitted from... pollution, plant personnel may switch to low -sulfur fuel for boiler operations What factor should they consider when making this decision? Answer: The sulfur dioxide emission rate is directly related to the sulfur levels in coal, oil, and synthetic (processed) fuels Switching to low -sulfur fuels, such as low -sulfur coal, or converting to synthetic fuels that have low -sulfur levels could be a good way for...a Sulfuric acid is a strong acid b In moderate concentrations, sulfuric acid is beneficial to fabric filtration performance c Sulfuric acid is extremely hygroscopic d a and b, only e a, b, and c Answer: a Sulfuric acid is a strong acid In moderate concentrations, sulfuric acid is beneficial to electrostatic precipitator performance... of 0.5 to 2 percent of sulfur dioxide levels Practice Problems Control Techniques #1 What type of control system is used for the control of sulfuric acid vapor generated in combustion processes? Answer: None The concentrations of sulfuric acid vapor are usually too low to economically justify a separate control system #2 12 Why are alkalis used in wet scrubbing systems to remove sulfur dioxide? Answer:... atomizer dry scrubbers use the process of absorption to control sulfur dioxide emissions In limestone wet scrubbers, sulfur dioxide gas is absorbed into an aqueous solution containing an alkaline substance (limestone) Once the sulfur dioxide is absorbed in solution, it reacts with oxidizers to form inorganic sulfites and sulfates This action prevents sulfur dioxide from diffusing out of solution and being... some organic and inorganic sulfate compounds Sulfuric acid can also be released from plants that manufacture batteries Practice Problems Formation Mechanisms #1 In fossil-fuel-fired boilers, what fraction of the total sulfur entering with the fuel is usually converted to gaseous sulfur dioxide? 11 Answer: Approximately 95% Approximately five percent of the sulfur contained in fuel or waste remains in... evaporating, sulfur dioxide absorbs into the droplet and reacts with the dissolved and suspended alkaline material #4 Which of the scrubbers listed below use the process of adsorption to control sulfur dioxide emissions? Select all that apply a Dry-injection-type dry scrubbers b Limestone wet scrubbers c Spray atomizer dry scrubbers 13 Answer: a Dry-injection-type dry scrubbers In these control devices, sulfur. .. are an air quality consultant with expertise in the control of sulfur dioxide emissions Based on the following criteria would you recommend a limestone wet scrubber or a spray atomizer dry scrubber to decrease sulfur dioxide emissions? In some cases, both types of systems may meet the criterion equally well i The system must achieve a high sulfur dioxide removal efficiency 14 ii The client insists that