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Module 6: Air Pollutants and Control Techniques - Sulfur Oxides - Practice Problems
- Instructions:
- Answer these questions on a sheet of paper and check your responses against those provided below.
-
#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?
- Sulfur dioxide is a yellow gas.
- Sulfur dioxide is not soluble in water and aqueous liquids.
- Sulfur dioxide at high temperatures can oxidize to form sulfur
trioxide.
- a and b, only
- a, b, and c
- Sulfur dioxide is a yellow gas.
-
- 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?
- Sulfuric acid is a strong acid.
- In moderate concentrations, sulfuric acid is beneficial to fabric
filtration performance.
- Sulfuric acid is extremely hygroscopic.
- a and b, only
- a, b, and c
- Sulfuric acid is a strong acid.
-
- 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
- Instructions:
- Answer these questions on a sheet of paper and check your responses against those provided below.
-
#1
- In fossil-fuel-fired boilers, what fraction of the total sulfur entering with the fuel is usually converted to gaseous sulfur dioxide?
-
- 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 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 acid concentrations are also in the range of 0.5 to 2 percent of sulfur dioxide levels.
Practice Problems
Control Techniques
- Instructions:
- Answer these questions on a sheet of paper and check your responses against those provided below.
-
#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
- 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.
- Dry-injection-type dry scrubbers
- Limestone wet scrubbers
- Spray atomizer dry scrubber
- Dry-injection-type dry scrubbers
-
- 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.
- Dry-injection-type dry scrubbers
- Limestone wet scrubbers
- Spray atomizer dry scrubbers
- Dry-injection-type dry scrubbers
-
- 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 re-emitted 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.
- Dry-injection-type dry scrubbers
- Limestone wet scrubbers
- Spray atomizer dry scrubbers
- Dry-injection-type 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.
- The system must achieve a high sulfur dioxide removal efficiency.
- The client insists that the system must generate a waste stream
that is easy to handle.
- 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.
- The system must achieve a high sulfur dioxide removal efficiency.
-
- 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.
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#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.
-
#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.