Module 6: Air Pollutants and Control Techniques - Particulate Matter - Practice Problems | 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.

#1

As of December 1999, the U.S. EPA has promulgated National Ambient Air Quality Standards (NAAQS) for which type of particulate matter? Select all that apply.

PM_2.5
PM₁₀
Total filterable particulate matter
Condensable particulate matter

Answer:

PM_2.5
PM₁₀

In 1988, the U.S. EPA revised the NAAQS for particulate matter from applying to "total filterable particulate matter" to filterable particulate matter less than or equal to 10 micrometers in size (PM₁₀). In 1997, EPA promulgated a new NAAQS for particulate matter less than 2.5 micrometers because these small particles may be related to significantly adverse health effects. The NAAQS for PM_2.5 was remanded by a District of Columbia court in May of 1999 and is under litigation as of the writing of these modules (December 1999).

#2

List five important characteristics of particulate matter that relate to health effects and the choice of an appropriate control technique.

Answer: Particle size, chemical composition, resistivity, stickiness, and explosiveness.

#3

The adverse health effects usually associated with small particles (less than 10 micrometers) can be attributed to:

Their ability to penetrate the upper respiratory tract
Their chemical composition
Their ability to remain airborne for a long time
a and b, only
a, b, and c

Answer: e. a, b, and c

The adverse health effects usually associated with small particles (less than 10 micrometers) can be attributed to their ability to penetrate the upper respiratory tract, their chemical composition, and their ability to remain airborne for a long time.

#4

True or False? Particles within the PM₁₀ size category tend to have similar chemical compositions.

True
False

Answer: b. False

The composition of particles that are less than 2.5 micrometers in diameter (PM_2.5) is considerably different than the composition of larger particulate matter (even particles in the size range from 2.5 to 10 micrometers).

#5

True or False? Particles in the PM_2.5 size category can penetrate the upper respiratory tract; while particles larger than this are generally considered to be nonrespirable.

True
False

Answer: b. False

In general, particles with diameters of 10 micrometers or less can penetrate the upper respiratory tract and are considered respirable.

Instructions:

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

#1

Describe the following four basic mechanisms of particle formation.

Physical attrition
Combustion particle burnout
Homogeneous and heterogeneous nucleation
Droplet evaporation

Answer: i.

Particles form by physical attrition when two surfaces rub together. The composition and density of the particles formed are identical to the parent material and range in size from less than 10 micrometers to approximately 1,000 micrometers.

Answer: ii.

Particles form by combustion particle burnout when fuel particles are injected into the hot furnace area of a combustion process. During combustion, most of the organic compounds in the fuel vaporize and oxidize in the gas stream, leaving ash and char particles. Particles formed by this mechanism are usually in the 1- to 100-micrometer size range. (Some of this material was covered in the lesson on Particle Formation in Module 3.)

Answer: iii.

Particles form by homogeneous and heterogeneous nucleation when gas streams containing vapors cool to their dew point. The vapor phase materials are converted to a particulate form. Homogeneous nucleation is the formation of new particles composed almost entirely of vapor phase material. Heterogeneous nucleation is the accumulation of material on the surfaces of existing particles. (Some of this material was covered in the lesson on Particle Formation in Module 3.)

Answer: iv.

Particles form by droplet evaporation when air pollution control systems use solids-containing water, which has been recycled from wet scrubbers, to cool the gas stream. The water streams are atomized during injection into the hot gas streams. As these small droplets evaporate to dryness, the suspended and dissolved solids are released as small particles.

Instructions:

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

#1

What is (are) the primary particulate collection mechanism(s) operating in each of the following control devices? Briefly explain how each mechanism works and discuss the implications for particle removal.

Gravity settling chambers
Mechanical collectors
Wet scrubbers
Electrostatic precipitators
Fabric filters

Answer: i.

Gravity settling chambers use gravitational force (gravity settling) to remove particles from the gas stream. Therefore, these control devices can effectively remove only very large particles (greater than approximately 75 micrometers).

Answer: ii.

Mechanical collectors use particle inertia for collection. They force the particle-laden gas stream to spin in a cyclonic manner, which makes the particles, due to their mass, move toward the outside of the vortex and then settle in the hopper. Mechanical collectors are generally only efficient for particles greater than 5 micrometers.

Answer: iii.

In wet scrubbers, particle and/or droplet inertia is the fundamental force used to transfer particles from the gas stream to the liquid stream. Depending on the wet scrubber design, liquid droplets, sheets of liquid on packing material, or liquid jets serve as impaction targets for particles moving in the gas stream. Particles are captured by the processes of inertial impaction and interception (discussed in the lesson on Collection Mechanisms in Module 3) due to the difference in velocities between the particles and the target droplets. Particle capture efficiency is dependent on the type of wet scrubber system. However, wet scrubbers generally have at least 95% collection efficiency for particles greater than 3 micrometers.

Answer: iv.

Electrostatic precipitators use nonuniform, high-voltage fields to apply electrical charges to particles moving through the field. Particles become saturated with charge and then move to a collection plate having the opposite charge. The efficiency of ESPs depends on the resistivity and size of the particles. Resistivity is dependent on the gas stream temperature and the chemical composition of the gas stream and the particles. Particulate matter with moderate resistivity is usually collected with the greatest efficiency.

Answer: v.

Fabric filters use a variety of collection mechanisms to remove particulate matter from the gas stream: inertial impaction, interception, Brownian diffusion, sieving, and electrostatic attraction. Both the fabric surface and the accumulated dust layer on the fabric surface act as filtering media.

The fabric surface and dust layer can serve as a barrier to particles travelling in the gas stream. Fibers and particles in the dust layer may trap relatively large particles as they try to pass with the gas stream through the small openings in the media. The processes of inertial impaction and interception are operational here. With Brownian diffusion, collisions between gas molecules and very small particles in the gas stream impart a random movement to the particles, increasing the likelihood that the filtering media will capture them. Sieving refers to the process whereby a dust cake, accumulated on the fabric surface, acts as a filter for collecting particles. Electrostatic attraction can enhance collection rates when particles have the opposite electrostatic charge from the filtering media. Due to the multiple particle collection mechanisms that may be operating within filtration systems, these control devices can be highly efficient for the entire particle size range of interest in air pollution control.

#2

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

Answer: 0.1 to 0.5 micrometer in diameter

#3

As a general rule, electrostatic precipitators work best when the resistivity is:

Low
Moderate
High

Answer: b. Moderate

As a general rule, electrostatic precipitators work best when the resistivity is moderate.

#4

A proposed source will generate particulate matter that is dry and has particle sizes larger than 10 micrometers. Is a small-diameter multi-cyclone collector appropriate for this type of source?

Yes
No

Answer: a. Yes

A small-diameter multi-cyclone collector is appropriate for the conditions described in the question. They can collect particles having diameters down to 5 micrometers and are applicable when the particulate matter generated is not sticky or wet. If either of these conditions exist, a particulate wet scrubber would be a more appropriate choice.

#5

A gas stream generated by an industrial process contains particulate matter that is not sticky or wet and is primarily in the 1 to 5 micrometer (aerodynamic diameter) size range. Neither the particulate matter nor any of the gases in the gas stream are explosive. A very high collection efficiency is required. The plant also does not want to generate any wastewater. What type(s) of particulate control devices is (are) appropriate for this type of source?

Answer:

Fabric filters and electrostatic precipitators could be appropriate control devices in this situation. They are good choices when the particulate matter is not sticky or wet and when high efficiency is required. More information about the chemical composition of the gas stream and the resistivity of the particulate matter would be necessary. Fabric filters would not be a good choice if the gas stream contains corrosive chemicals that could attack the filter media. If the particles have high resistivity, an electrostatic precipitator may not be a good choice. Wet scrubbers are not a good choice because they generate wastewater. Also, they are generally less efficient for particles in this size range.

#6

A gas stream generated by a newly designed industrial process will have particulate matter that is primarily less than 1 micrometer (aerodynamic diameter). In addition to the particulate matter, the gas stream will contain 15% moisture and 2,000 ppm sulfur dioxide. On an intermittent basis, CO concentrations near the lower explosive limit will be present. What types of particulate control devices would be appropriate for this type of new source?

Answer:

Considering that explosive conditions will occur on an intermittent basis, a particulate wet scrubber is the only available choice.

Module 6: Air Pollutants and Control Techniques - Particulate Matter - Practice Problems

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Practice Problems

Formation Mechanisms

Practice Problems

Control Techniques

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