Air Pollution Control
Air Pollution Control 101
One of the favored features of the second edition of the Air Pollution Control Equipment Selection Guide was this introductory chapter. The intent is to provide an overview of the basics of air pollution control and introduce newcomers to some of the terminology that will appear in subsequent chapters. This chapter has been used by some instructors to supplement additional studies particularly in application engineering. For this reason, and others, this chapter with some updates has been included in this 3rd edition.
Having spent more than 40 years in the air pollution control industry, I am still amazed how the basics of air pollution control are misunderstood by so many people.
Our newspapers have numerous articles regarding the need to control toxic or carcinogenic substances, but rarely do you see an article explaining how it is done. In this chapter, we will explore the basics of air pollution control and how the devices work and, in doing so, introduce some of the terminology used in the industry.
It Is Separation Technology
Air pollution control can be generally described as a separation technology. The pollutants, whether they are gaseous, aerosol, or solid particulate, are separated from a carrier gas, which is usually air. We separate these substances because, if we do not, these pollutants may adversely affect our health and that of the environment. Of primary importance is the effect of the pollutants on our respiratory system, where the impact is most noticeable.
Gaseous pollutants are compounds that exist as gas at normal environmental conditions. Usually, normal is defined as ambient conditions. These gases may have, just moments before the release, been in a liquid or even solid form. For the purposes of the air pollution device, however, the state they are in just prior to entering the control device is what is most important.
Aerosols are finely divided solid or liquid particles that are typically under
0.5 pm diameter. They often result from the sudden cooling (condensation) of a gaseous pollutant through partial combustion or through a catalytic effect in the gas phase. In the latter condition, a pollutant in the gas phase may combine to form an aerosol in the presence of, for example, a metal co-pollutant.
Acid aerosols, such as SO,, can form in the presence of vanadium particulate that may be evolved through the combustion of oil-containing vanadium compounds. Solid metals in a furnace can sublime (change phase from solid directly to gaseous) in the heat of an incinerator, then cool sufficiently to form a finely divided aerosol.
Solid particulate can be evolved through combustion or through common processing operations such as grinding, roasting, drying, calcining, coating, or metallizing.
Whatever the state of the pollutant is, the function of the air pollution control device is to separate that pollutant from the carrier gas so that our respiratory system does not have to.
Our respiratory system is our natural separation system. Figure 1.1 depicts major portions of the human respiratory system. Large particles are removed
Respiratory system diagram. (From Marshall, James, The Air Ne Live lit, Coward, McCann, and
Geoghegan, New York, 1968.) in the larger openings of the upper respiratory area, smaller particles are removed in the more restricted bronchial area, and the tiniest particles are (hopefully) removed in the tiny alveolar sacs of the lungs.
Air pollution control truly mimics Mother Nature in its separation function. In general, low-energy input wet-type (those using water as the scrubbing medium) gas-cleaning devices remove large particles, higher-energy devices remove smaller particles, and even higher energy (or special technology) devices remove the finest pollutants. In order of decreasing pollutant size, it goes like this:
The larger the particle, or liquid droplet for that matter, the easier it is to separate from the carrier gas.
These characteristics were codified into a helpful chart known as the Frank chart, shown in Figure 1.2. It was named after its creator, an engineer at American Air Filter. This chart shows common particulate sizes and general types of collection mechanisms and devices used for their control. The pollutants are grouped by their settling characteristics. Larger particles (above about 2 pm aerodynamic diameter) generally follow Stokes' law regarding settling velocities. Below about 2 pm, a correction factor (Cunningham's correction factor) is needed to adjust Stokes for the longer settling times for these particles.
In general, particles greater than 20 pm aerodynamic diameter can be controlled using low-energy wet-type devices. Subsequent chapters will explore these devices in detail. These are knockout chambers (traps or settling chambers), cyclone collectors, mechanically aided wet scrubbers, eductors, fluidized-bed scrubbers, spray scrubbers, impactor scrubbers, and Venturi scrubbers (low energy).
For particles 5 pm aerodynamic diameter and above, the Venturi scrubbers (moderate energy) are the most common devices in use. Some vendors have improved the performance of low-energy devices sufficiently to span the gap between those capable of removing 20+ and 5+ pm pollutants. Some mechanically aided wet scrubbers also bridge this gap at higher energy input. For lower concentrations of particles in this size range, enhanced scrubbers such as air/steam atomized spray scrubbers and some proprietary designs are used.
For particles below 5 pm aerodynamic diameter, higher-energy input devices are typically used, or techniques are applied to enlarge these particles to make them easier to capture. Such designs are Venturi scrubbers (high energy), air/steam atomized spray scrubbers, condensation scrubbers, and
The "Frank" chart (American Air Filter).
combination devices. If the inlet loading (concentration) is less than approximately 1-2 grs/dscf (grains per dry standard cubic foot), electrostatic forces can be sometimes applied. These include wet electrostatic precipitators and electrostatic scrubbers.
For dry-type separation devices, such as fabric filter collectors (baghouses) and electrostatic precipitators, the energy input is constant regardless of the particle size. Even among these designs, however, increases in energy input yield increases in the collection of finer pollutants. Baghouses are often precoated with a fine material to reduce the permeability of the collecting filter cake and improve fine particulate capture. This cake adds to the pressure drop, which mandates, in turn, an increase in energy input. Precipitators are often increased in field size to remove finer particulate thereby requiring greater power input. These dry devices, in general, use less total power input than equivalent wet devices when removing particulate.
Wet Collection of Particulate
Wet scrubbers exhibit an increase in total energy input as the target particle size decreases because of the capture technique used.
How is particulate removed in a wet scrubber?
Studies of particle settling rates and motion kinetics have shown that particles greater than approximately 2-5 pm behave inertially and smaller particles tend to behave more like gases. For the former, if you could throw a particle like a baseball, it would follow a given trajectory (perhaps curve or slide but generally follow a given path). Particles less than approximately 2 pm diameter tend to be influenced by gas molecules, temperature and density gradients, and other subtle forces and do not follow predictable trajectories. If you could throw one of these particles, it might turn and hit you in the face. These are the "givens" in the wet scrubber design equation.
Nearly all wet separation devices use the same three capture mechanisms:
Basically, wet scrubbers remove particulate by shooting the particulate at target droplets of liquid.
Figure 1.3 shows a target droplet being impacted by a particle. The particle has sufficient inertia to follow a predicted course into the droplet. Once inside the droplet, the combined particle/droplet size is aerodynamically
Impaction (Bionomic Industries, Inc.).
much larger, and therefore the separation task becomes easier. Now simply separate the droplet from the gas stream (more on that later) and the particle(s) are removed.
Figure 1.4 shows a particle, perhaps a bit smaller, moving along the gas streamlines being intercepted at the droplet surface. The particle in this case comes close enough to the droplet surface that it is attracted to that surface and is combined with the droplet. Again, once the particles are intercepted, the bigger droplets are easier to remove.
Figure 1.5 depicts an even tinier particle that is so small that it bounces around in the moving air stream buffeted by water and gas molecules. In this case, the particle diffuses over to the droplet and, by chance, is absorbed into
Interception (Bionomic Industries, Inc.).
Diffusion (Bionomic Industries, Inc.).
the droplet. Obviously, to increase the chances of capture by diffusion, one needs to increase the number of droplets per unit volume. This decreases the distance the particle must travel and reduces the chances that it might miss a droplet. Experience has shown that the smaller is the target droplet and the closer the droplet is to an adjacent droplet, the greater is the percentage particulate capture. To make greater quantities of smaller droplets requires increased energy input to shear or form the liquid into tiny target droplets. This is evident in common garden hose spray. The higher the velocity out of the nozzle, the finer the spray.
Once the particulate is into the droplet, Mother Nature tends to help us. Luckily, water droplets generally tend to agglomerate and increase in size upon contact. If we spin, impact, or compress the droplets together, they combine to form even easier-to-remove droplets.
In Figure 1.6, we see a Venturi scrubber (left) connected to a typical cyclonic-type separator. This device separates the droplets using centrifugal force. The centrifugal force pushes the droplets toward the vessel wall, where they form a compressed film, agglomerate, accumulate, and drain out of the air stream by gravity.
Sometimes chevron-type droplet eliminators are used. These place a waveform in the path of the droplet (Figure 1.7). The same process occurs. The droplets build up, drain, and carry their particulate cargo out of the gas stream.
Other forces can also be used to separate fine particulate. If we saturate the gas stream with water vapor and then cool the gas stream, the water vapor will condense on the particulate to form water drops. This same event occurs every day in the form of rainstorms. If it were not for the fact that water vapor condenses on micron and submicron particulate during cleansing rainstorms, we would all suffocate. Condensation scrubbing is the manmade version of the rainstorm.
Venturi scrubber and cyclonic separator.
Chevron droplet eliminator (Munters Corp.l.