Module 6: Air Pollutants and Control Techniques - Review Exercises | Basic Concepts in Environmental Sciences | APTI

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Instructions:

Answer these questions on a sheet of paper and check your responses against those provided below.

Important:

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#1

What is the most important characteristic of particles with respect to their physical behavior?

Particle size
Particle density
Particle composition
Particle shape

Answer: a. Particle size

The most important characteristic of particulate matter with respect to its behavior is particle size.

To review material, see Module 6 lesson on Particulate Matter (Characteristics).

#2

What particle characteristic influences the resistivity of particulate matter in a gas stream treated by an electrostatic precipitator?

Particle density
Particle size
Particle composition
Particle surface area

Answer: c. Particle composition

Particle size, particle surface area, and particle composition are all related to the electrostatic attraction of particles. However, particle composition is the only one of these parameters that influences the resistivity of the particulate matter.

To review material, see Module 6 lesson on Particulate Matter [Control Techniques (see section on Electrostatic Precipitators)].

#3

Which of the following is not a colorless gas?

Sulfur dioxide (SO₂)
Nitric oxide (NO)
Ozone (O₃)
None of the above

Answer: d. None of the above

Sulfur dioxide, nitric oxide, and ozone are all colorless gases.

To review material, see Module 6 lessons on Sulfur Oxides (Characteristics), Nitrogen Oxides (Characteristics), and Ozone (Characteristics).

#4

The emissions of which pollutant are most likely to increase when the combustion zone temperature of a boiler increases?

Nitrogen oxides
Carbon monoxide
Volatile organic compounds
Dioxins and furan compounds

Answer: a. Nitrogen oxides

The emissions of nitrogen oxides are most likely to increase when the combustion zone temperature of a boiler increases. Higher combustion zone temperatures favor the complete oxidation of carbon-containing compounds to carbon dioxide and water. Therefore, emissions of carbon monoxide and volatile organic compounds would be expected to decrease at higher temperatures. Likewise, dioxins and furans are destroyed above temperatures of approximately 1400°F.

To review material, see Module 6 lessons on Nitrogen Oxides (Formation Mechanisms), Nitrogen Oxides (Control Techniques), Incompletely Oxidized Compounds (Formation Mechanisms), and Dioxins and Furans (Formation Mechanisms).

#5

The control room operators at a municipal waste plant want to lower the emissions of incompletely oxidized organic compounds leaving the incinerator. What step should they take?

Increase the moisture levels in the combustion zone.
Decrease the excess air levels.
Adjust the fuel-to-air distribution to increase the combustion temperature.
Decrease the load to the incinerator.

Answer: c. Adjust the fuel-to-air distribution to increase the combustion temperature.

Good combustion practices are the primary way to minimize the emission of partially oxidized organic compounds (and carbon monoxide). Maintaining high enough temperatures and supplying an appropriate amount of oxygen in the combustion zone is necessary to allow oxidation reactions to go to completion.

To review material, see Module 6 lesson on Incompletely Oxidized Compounds (Formation Mechanisms).

#6

The burning of fuel containing trace amounts of chlorine can emit which pollutant(s)?

Dioxins and furans
Hydrogen chloride
All of the above
None of the above

Answer: c. All of the above

The formation mechanisms for dioxin and furan compounds depend on the availability of chlorinated precursor compounds in the fuel and/or waste being burned and the appropriate gas temperature conditions (400 - 1000°F). The concentration of HCl formed during combustion is directly related to the chloride concentration in the fuel being burned.

To review material, see Module 6 lessons on Dioxins and Furans (Formation Mechanisms) and Halogens (Formation Mechanisms).

#7

A plant is planning to install an SNCR system to control emissions. Which pollutant will this control system help reduce?

Carbon monoxide
PM₁₀ particulate matter
Volatile organic compounds
Ozone

Answer: d. Ozone

A selective non-catalytic reduction (SCNR) system reduces emissions of nitrogen oxides, which are one of the precursor compounds involved in the photochemical formation of ground level ozone.

To review material, see Module 6 lessons on Nitrogen Oxides (Control Techniques) and Ozone (Formation Mechanisms).

#8

A testing company has determined that a facility has exceeded the emission limits of sulfur dioxide, hydrogen chloride, and hydrogen fluoride. Which of the following is the most appropriate add-on control device to reduce these three pollutants?

SNCR
Low NO_x burners
Thermal oxidizer
Spray-dryer-type dry scrubber

Answer: d. Spray-dryer-type dry scrubber

Spray-dryer-type dry scrubbers, which spray an alkaline slurry into the hot gas stream, are useful in reducing emissions of acid gases such as sulfur dioxide, hydrogen chloride, and hydrogen fluoride. Alkali wet scrubbers can also be used to achieve the same goal.

Selective non-catalytic reduction (SNCR) and low NO_x burners are control techniques for nitrogen oxides. Thermal oxidizers are used to destroy VOCs.

To review material, see Module 6 lessons on Sulfur Oxides (Control Techniques) and Halogens (Control Techniques).

#9

Particles that are collected with the greatest difficulty in wet scrubbers are in the ____________________ size range.

1 to 10 micrometer diameter
0.5 to 1 micrometer diameter
0.2 to 0.5 micrometer diameter
< 0.2 micrometer diameter

Answer: c. 0.2 to 0.5 micrometer diameter

To review material, see Module 6 lesson on Particulate Matter (Control Techniques).

#10

Specific information about the flue gas is provided below. The plant environmental manager wants to know the most appropriate particulate control device to use.

The flue gas contains an explosive gas, particles with aerodynamic diameters greater than 0.5 micrometer and moderate resistivity, and no sulfuric acid.

Mechanical collector Fabric filter Wet scrubber Electrostatic precipitator

The flue gas consists of non-explosive gases, a moderate concentration of sulfuric acid, and particles greater than 0.5 micrometer. The plant manager requests that there be no wastewater to treat.

Mechanical collector Biological oxidation Wet scrubber Electrostatic precipitator

Flue gas consists of an explosive gas, particles greater than 5 micrometers with moderate resistivity, and no sulfuric acid. The plant environmental manager would like to minimize operating costs.

Mechanical collector Fabric filter Wet scrubber Electrostatic precipitator

Flue gas contains particles with aerodynamic diameters greater than 0.1 micrometer and high resistivity. The flue gas composition (except for the presence of particles) is identical to ambient air and therefore contains no sulfuric acid. The plant manager requests a control device that is highly efficient for the entire particle size range with no wastewater.

Mechanical collector Fabric filter Wet scrubber Electrostatic precipitator

Answer: i. c. Wet scrubber

The explosive nature of the flue gas narrows the control device choices to wet scrubbers and mechanical collectors. Electrostatic precipitators would not be a good choice here due to the risk of electrical sparking in the precipitator fields, which could lead to ignition of the flue gas. Mechanical collectors are used when particle size distributions are relatively large (approximately 5 micrometers and greater). Wet scrubbers are the best choice in this situation because some particles appear to be in the submicrometer size range. Fabric filters do not appear to be an appropriate selection due to the explosive gas. Even though fabric filters can be designed to accommodate certain explosive gas streams, a wet scrubber is the more suitable choice considering the given information. See Figure 1 (General Applicability of Particulate Control Systems).

Answer: ii. d. Electrostatic precipitator

Electrostatic precipitators are a good choice here because the gas is non-explosive and the control process creates no wastewater. ESPs have very high collection efficiencies for particles larger than 0.5 micrometer. Precipitators work best when particles have moderate resistivity. The presence of sulfuric acid vapor in moderate concentrations (2 to 8 ppm) in the gas stream actually can improve the performance of electrostatic precipitators because it adsorbs onto particle surfaces and creates a moderate resistivity. See Figure 1 (General Applicability of Particulate Control Systems).

Answer: iii. a. Mechanical collector

Due to the explosive nature of the gas stream and the relatively large size of the particulate matter, either wet scrubbers or mechanical collectors would be possible options. Mechanical collectors are the more economical choice. Mechanical collectors have a lower purchase cost and a lower operating cost than wet scrubbers. A liquid feed stream and a wastewater stream would be necessary with the wet scrubber. See Figure 1 (General Applicability of Particulate Control Systems).

Answer: iv. b. Fabric filter

Fabric filters are the best choice here because they have high collection efficiencies across the entire particle size range, including particles in the difficult-to-collect range. The high efficiency for the entire particle size range of interest for fabric filters is due to the combined effect of inertial impaction and interception, Brownian diffusion, and sieving. Also, fabric filters meet the qualification that no wastewater be produced. Electrostatic precipitators would not be a good choice because the particles have high resistivity and no sulfuric acid is present to lower the resistivity to a moderate level. Wet scrubbers would not be a good choice here because they create wastewater.

To review material, see Module 6 lessons on Particulate Matter (Control Techniques), Sulfur Oxides (Characteristics), and Module 3 lesson on Collection Mechanisms.

#11

The plant manager also needs to know the most appropriate control device to reduce VOCs in a separate process of the facility. The following information is known.

The flue gas exits a small-diameter pipe at a slow velocity and contains a high concentration of a variety of VOCs.

Thermal incinerator Catalytic incinerator Carbon bed Condensation or refrigeration

The flue gas only contains the pollutants acetone and methylene chloride. The gas flow rate and the level of the VOC concentrations is moderate. The plant manager wants to collect and sell these solvents.

Thermal oxidizer Catalytic oxidizer Carbon bed adsorber Condensation or refrigeration

The flue gas has moderate flow with a variety of VOCs and particulate matter. The VOCs in the gas stream have a lower molecular weight (16 to 32 gm/gm mole). Due to process constraints, the gas exiting the control device cannot exceed 1100°F.

Thermal oxidizer Catalytic oxidizer Carbon bed adsorber Condensation or refrigeration

Answer: i. d. Condensation or refrigeration

Condensation or refrigeration systems are usually used on sources with high concentrations of VOCs and low gas flow rates.

Answer: ii. c. Carbon bed adsorber

Thermal and catalytic oxidizers will destroy the VOCs in the gas stream. Condensation and refrigeration would be an appropriate method of recovery at a lower flow rate and a higher concentration. A carbon bed adsorber is the best choice in this situation to recover the two VOC compounds in the gas stream.

See Figure 2 (General Applicability of VOC Control Systems for High Concentration Sources.)

Answer: iii. b. Catalytic oxidizer

A carbon bed adsorber would not be the best choice due to the particulate matter in the gas stream and the low molecular weights of the VOC compounds present. (Adsorption systems are usually limited to sources generating VOCs having a molecular weight greater than 50 and less than approximately 200.) Particulate matter can also reduce the efficiency of condensation and refrigeration control equipment. Thermal oxidizers operate at temperatures of approximately 1000 to 2000°F and therefore may be too high for this application. Catalytic oxidizers destroy VOCs in the temperature range of 500 to 1000°F.

To review material, see Module 6 lesson on Volatile Organic Compounds (Control Techniques).

Module 6: Air Pollutants and Control Techniques - Review Exercises

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