**3. Downstream control**

#### **3.1. Particulate matter control**

#### *3.1.1. Cyclone*

*2.3.4. Site activities*

230 Current Air Quality Issues

**2.4. Miscellaneous**

from upstream emissions.

Regulations applied to control air pollutants emitted from site activities include:

**•** Use water as a dust suppressant where applicable

**•** Minimize drop heights to control the fall of materials

using chemical hydrogenation and gasification processes.

spraying mining area; and stabilizing unpaved traffic areas.

**•** Re-vegetate earthworks and exposed areas

**•** Cover, seed, or fence stockpiles to prevent wind whipping

plastic bottle over the material leading to reduce the great amount of generated dust.

**•** If applicable, ensure the concrete crusher or concrete batcher has a permit to operate

**•** Minimize dust-generating activities. For example, when a worker cuts concrete slabs or bricks with a power tool without extraction or suppression, a second worker can pour water from a

Coal, the most abundant solid fuel and widely used for power plant and other industrial activities, is the largest source of air pollutant emissions. Coal combustion produces a signif‐ icant amount of air pollutants such as SOx, heavy metals, and PM. For example, sulfur in coal occurs both as inorganic minerals (mainly pyrite and marcassite) and organic compounds incorporated in the combustible part of coal. The sulfur content can be converted into SOx during the coal combustion. Therefore, reducing the sulfur content in coal before the combus‐ tion processes is a great strategy to reduce SOx emissions from the upstream coal combustion process. Inorganic sulfur in coal can be removed by coal washing and the organic sulfur by

Mining activities can produce significant air pollutants such as heavy metals (in PM form), SOx and NOx. Strategies to reduce air pollutants emitted from mining activities from the upstream process include enclosure or cover mine, mining area, and transfer areas; water

Indoor activities can also be a significant source of air pollution. Strategies to reduce air pollutants emitted from indoor activities include improvement of cooking devices, use of alternative fuels for cooking and reducing the need for fire. Strategies to improve cooking devices include stabilization of stove materials and improvement of stove chimneys, in particular, biomass stoves. Uses of alternative fuels for cooking including charcoal, biogas, liquid petroleum gas, and electricity can significantly reduce air pollutant emissions. For example, the transition from wood to charcoal for cooking can reduce PM10 emissions by more than 80% (although the wider environmental impacts of charcoal production must be consid‐ ered). The need for fire can be reduced based on the use of solar heating or electric devices.

The change of building materials from high-polluting materials such as paint, linoleum, and gypsum to low-polluting materials such as PVC and polyolefin can also control air pollutants

The cyclone is a well-known device used primarily for the collection of medium-sized and coarse particles. The cyclone works by forcing a gaseous suspension downward. The particles move outward by centrifugal force and collide with the outer wall and then slide downward to the bottom of the cyclone. At the bottom of the cyclone, the gas reverses its downward spiral and moves upward in a smaller, inner spiral. The cleaned air exits from the top of the cyclone and the particles are expelled from the bottom of the cyclone through a pipe sealed by a springloaded flapper valve or a rotary valve. The cyclone collector is shown schematically in Figure 4.

**Figure 4.** Schematic diagram of a cyclone.

Cyclones have a wide range of industrial applications in gaseous cleaning and product recovery. They are relatively inexpensive, easy to set up and maintain, and can work at high temperature and pressure. They can be used as a precollector for removing larger particles before next treatment. When well designed, the cyclone can collect particles larger than 10 µm with an efficiency of more than 90%. For smaller particles, however, the well-designed cyclone would have a considerable pressure drop with relatively lower collection efficiency [15]. In addition, the cyclone method cannot be used for removing sticky particles with high moisture content.

#### *3.1.2. Wet scrubber*

A wet scrubber system can be used to control fumes, mists, acid gasses, heavy metals, trace organics, and suspended dusts. An individual wet scrubber can usually be used to control a targeted pollutant. Therefore, a well-designed wet scrubber system often contains two or more single scrubbers leading to a multistage wet scrubber, which affords higher total removal efficiencies than that of a single-stage scrubber [1]. A schematic diagram of a wet scrubber is shown in Figure 5.

**Figure 5.** Schematic diagram of a wet scrubber chamber.

The wet scrubber system works based on direct interaction between the adsorbent liquid and the particles. The adsorbent liquid is usually water; however, several chemicals are also added to water to increase the adsorption ability of the liquid phase with the particles. Based on the interaction between the particles and the liquid phase, the particles can diffuse out of the gas phase and be absorbed in the liquid phase and the particle-loaded air can be cleaned. The absorption of particles into the liquid phase can be both physical and chemical absorption, depending on the particle and liquid phase and gaseous properties. The particle removal efficiency depends on:


#### *3.1.3. Electrostatic precipitators*

targeted pollutant. Therefore, a well-designed wet scrubber system often contains two or more single scrubbers leading to a multistage wet scrubber, which affords higher total removal efficiencies than that of a single-stage scrubber [1]. A schematic diagram of a wet scrubber is

The wet scrubber system works based on direct interaction between the adsorbent liquid and the particles. The adsorbent liquid is usually water; however, several chemicals are also added to water to increase the adsorption ability of the liquid phase with the particles. Based on the interaction between the particles and the liquid phase, the particles can diffuse out of the gas phase and be absorbed in the liquid phase and the particle-loaded air can be cleaned. The absorption of particles into the liquid phase can be both physical and chemical absorption, depending on the particle and liquid phase and gaseous properties. The particle removal

shown in Figure 5.

232 Current Air Quality Issues

**Figure 5.** Schematic diagram of a wet scrubber chamber.

**•** The solubility of the pollutant in the chosen scrubbing liquor

**•** Pollutant concentration in the gas phase being treated

**•** Flow rate of the gas and liquid phases

**•** Gas–liquid phase contact surfaces

efficiency depends on:

Electrostatic precipitation (ESP), which is one of the most popular and efficient particle control systems in the United States, is defined as a particle control method that uses electrical forces to move the particles out of the flowing gas stream and onto collector plates [16]. The ESP processes include:


The particles are charged when the particles in the air stream pass through a corona, a region of gaseous ions flow. In the corona, the ions bombard the surface of the particles leading to charging particles. When these charged particles pass through the surface of the collecting electrodes, oppositely charged plates, they are trapped on the collected electrodes by the electrostatic field. The charged particles are accelerated toward the collecting electrodes by Coulomb forces, but inertial and viscous forces can resist the motion. When the plates (electrodes) collected a certain particle amount, the collected particles must be removed from the plates to prevent their re-entrainment into the gas stream. The plates could be knocked to let the collected layer of particles to slide down into a hopper from which they are evacuated [16]. The plates could also be continuously washed with water to remove the collected particles. A schematic diagram of an electrostatic precipitator is shown in Figure 6.

The principal difference between the ESP and other scrubbing methods are that in the ESP, the separation forces are electrical and are applied directly to the particles or droplets themselves while in others the separation forces are usually applied indirectly through the contaminated air system [17]. Therefore, the ESP could remove small particles or liquid droplets at a high efficiency with low energy consumption or low cost and small pressure drop through the gas cleaning system [17]. ESPs are built in either single-stage or two-stage versions. In the singlestage precipitator, the ionization and collection of particles or liquid droplets are achieved in a single stage and the corona discharge and precipitating field extend over the full length of the device. In the two-stage precipitator, the ionization of particles or liquid droplets is carried out in the first stage confined to the region around the corona discharge wires, followed by the particle collection in the second stage which provides an electrostatic field whereby the previously charged particles migrate onto the surface of the collecting electrodes [2].

**Figure 6.** Schematic diagram of an electrostatic precipitator.

#### *3.1.4. Fabric filtration*

Fabric filtration is a well known and accepted physical technique in which a gas stream containing mainly solids passes through a porous fabric medium which retains the solids. This process may operate in a batch or semicontinuous mode for removing the retained solid particles from the filter medium. Filtration systems may also be designed to operate in a continuous manner.

In air fabric filtration, the contaminated gas flows into and passes through a number of filter bags placed in parallel, leaving the solid particles retained by the fabric filter. The fabric filter can be classified into two basic groups, depending on the fabric properties: felt and woven. Felt media are normally used in high-energy cleaning systems, while woven media are used in low-energy systems. Felt fabrics are tighter in construction and they can be considered to be more of a true filter medium and should be kept as clean as possible to perform satisfactorily as a filter. The woven fabric is merely a site upon which the true filtering occurs as the dust layer builds up, through which the actual filtering take place.

Particles are collected on the fabric surface through four mechanisms including:


The particles were captured on the filter leading to formation of a dust cake on the filter. The formation of the dust cake could increase the resistance to gas flow. Therefore, the filter containing the dust cake must be frequently cleaned.

Fabric filters are extremely efficient solid removal devices and operate at nearly 100% effi‐ ciency. The efficiency depends on several factors including:

**•** Particle properties

the particle collection in the second stage which provides an electrostatic field whereby the

Fabric filtration is a well known and accepted physical technique in which a gas stream containing mainly solids passes through a porous fabric medium which retains the solids. This process may operate in a batch or semicontinuous mode for removing the retained solid particles from the filter medium. Filtration systems may also be designed to operate in a

In air fabric filtration, the contaminated gas flows into and passes through a number of filter bags placed in parallel, leaving the solid particles retained by the fabric filter. The fabric filter can be classified into two basic groups, depending on the fabric properties: felt and woven. Felt media are normally used in high-energy cleaning systems, while woven media are used in low-energy systems. Felt fabrics are tighter in construction and they can be considered to be more of a true filter medium and should be kept as clean as possible to perform satisfactorily as a filter. The woven fabric is merely a site upon which the true filtering occurs as the dust

**•** Inertial collection – the fibers, which are placed perpendicular to the gas flow direction, could

previously charged particles migrate onto the surface of the collecting electrodes [2].

**Figure 6.** Schematic diagram of an electrostatic precipitator.

layer builds up, through which the actual filtering take place.

**•** Interception – particles are trapped in the filter matrix.

Particles are collected on the fabric surface through four mechanisms including:

collect the particles in the stream without changing gas flow direction.

*3.1.4. Fabric filtration*

234 Current Air Quality Issues

continuous manner.

	- **◦** Surface depth: Shallow surfaces form a sealant dust cake sooner than napped surfaces do.
	- **◦** Weave thickness: Fabrics with high permeability, when clean, show lower efficiencies.
	- **◦** Electrostatics: Particles, fabrics, and gas can all be influenced electrostatically and proper combination.
	- **◦** Residual weight: The heavier the residual loading, the sooner the filter is apt to seal over.
	- **◦** Residual particle size: The smaller the base particles, the smaller (and fewer) are the particles likely to escape.
	- **◦** Humidity: With some dusts and fabrics, 60% relative humidity is much more effective than 20% relative humidity. Increased humidity or moisture level can be a frequent cause of clogging pores of the filter medium and increasing filter pressure drop.
	- **◦** Velocity: Increased velocity usually lowers the efficiency, but this can be reversed depending on the collection mechanisms, for example, impaction and infusion.
	- **◦** Pressure: Probably not a factor, except that an increase in pressure after the dust cake has been formed can fracture the filter medium and greatly reduce efficiency until the cake reseals.
	- **◦** Cleaning: Without frequent or periodical cleaning, the air filtration system cannot be operated.

The advantages and disadvantages of methods to control particulate matter including cyclone, wet scrubber, ESP, and fabric filtration are summarized in Table 2.


**Table 2.** Advantages and disadvantages of particulate control methods

### **3.2. Gaseous pollutants control**

#### *3.2.1. Adsorption*

**Advantages Disadvantages**

are smaller than 10 mm

⋅ Corrosion problems

⋅ High capital cost

resistivity

⋅ High pressure drop problems

⋅ Relatively high maintenance costs

⋅ Could not use for sticky materials

⋅ Relatively low efficiencies for collection particles which

⋅ Treatment issue concerning with water disposal/effluent

⋅ Solid buildup problems at the wet–dry interface

⋅ High sensitivity to fluctuations in gas stream ⋅ Problems with particles, with extremely high or low

⋅ Require highly trained maintenance personnel

⋅ Shortened fabric life at elevated temperatures and in the

⋅ Difficult to operate at high temperature ⋅ Need for fabric treatment after removal process

presence of acid or alkaline particulate or gas ⋅ Respiratory protection requirement for fabric

⋅ Medium pressure-drop requirements

⋅ Require high maintenance costs

⋅ Explosion problems

replacement

⋅ Require large space for installation ⋅ Produce ozone as by-product

Cyclone

⋅ Low capital cost

236 Current Air Quality Issues

⋅ Require small spaces

⋅ No secondary production ⋅ Require small space

Wet scrubber

gas streams ⋅ Low capital cost

risk

⋅ Simple and insignificant maintenance problems

⋅ Operation to collect both gases and sticky particles ⋅ Operation at high-temperature as well as high-humidity

⋅ Operation with flammable and explosive dust with little

⋅ Very high collection efficiencies of coarse as well as fine

⋅ Operation capability at high temperatures as well as high

⋅ Very high collection efficiencies of coarse as well as fine

⋅ Relative insensitivity to gas stream fluctuations and large

**Table 2.** Advantages and disadvantages of particulate control methods

⋅ Simple maintenance, flammable dust collection ⋅ High collection efficiency of submicron smoke and

Source: Bounicore and Davis 1992 [18]

⋅ Ability to operate at high temperature

⋅ High effective to collect fine particles

particulates with low energy consumption

⋅ Relatively low operation and maintenance costs

Electrostatic precipitation (ESP)

⋅ Collection dry dust ⋅ Low pressure drop

pressure or under vacuum ⋅ High collect capacity

changes in inlet dust loadings ⋅ Recirculation of filter outlet air

⋅ No corrosion issues

gaseous contaminants ⋅ Many application types ⋅ Simple operation

Fabric filtration

particulates

Adsorption is the phenomenon via which molecules of a fluid adhere to the surface of a solid material (adsorbent). Gas adsorption is used for industrial applications such as odor control, recovery of volatile solvents such as benzene, toluene, and chloroflurocarbon, and drying of process gas streams. During this process, the molecules or particles (adsorbate) in airstream gases and liquids can be selectively removed or captured despite being at low concentrations. There are two distinct adsorption mechanisms: physisorption and chemisorption. Physisorp‐ tion or physical adsorption, also called van der Waals adsorption, involves a weak bonding of gas molecules with the adsorbent. The bond energy is similar to the attraction forces between molecules in the stream. The adsorption process is exothermic and the heat of adsorption is slightly higher than the heat of the vaporization of the adsorbate. The forces holding the adsorbate to the adsorbent are easily overcome by either the application of heat or the reduction of pressure, which are methods that can be used to regenerate the adsorbent. Chemisorption or chemical adsorption involves an actual chemical bonding by reaction of the adsorbate with the adsorbent, leading to new chemical bonds such as covalent bonding generated at the adsorbent surface.

When a stream comes into contact with an adsorbent, one or several components of the stream are adsorbed by the adsorbent. At all adsorbent interfaces, adsorption can occur, but often at a low level unless the adsorbent is highly porous and possesses fine capillaries. For an effective solid adsorbent, it should have a large surface-to-volume ratio, and a preferential affinity for the individual component of concern. The adsorption occurs by a series of steps. In the first step, the adsorbate diffuses from the stream to the external surface of the adsorbent. In the second step, the adsorbate molecule migrates from the relatively small area of the external surface to the pores within each adsorbent. The bulk of the adsorption occurs in these pores because of the majority of available surface area. In the final step, the adsorbate adheres to the surface in the pores of the adsorbent [19].

Most industrial adsorbents could be divided into three classes including:


Silica gel, which is usually prepared by the reaction between sodium silicate and acetic acid, is a chemically inert, nontoxic, polar, and dimensionally stable amorphous form of SiO2. It is used for the drying of processed air and the adsorption of polar hydrocarbons from natural gas.

Zeolites are natural or synthetic crystalline aluminosilicates, which have a repeating pore network and release water at high temperature. Zeolites are applied in the drying of processed air, CO2 removal from natural gas, CO removal from reforming gas, air separation, catalytic cracking, and catalytic synthesis and reforming.

Activated carbon is a highly porous, amorphous solid consisting of microcrystallines with a graphite lattice, usually prepared in small pellets or a powder. It is nonpolar and cheap. Activated carbon is used for the adsorption of organic substances and nonpolar adsorbate. Activated carbon is also usually used for waste gas (and waste water) treatment. It is the most widely used adsorbent because its chemical and physical properties such as surface groups, pore size distribution, and surface area can be tuned as required. Its usefulness also derives from its large microspore (and sometimes mesoporous) volume and the resulting high surface area.

#### *3.2.2. Absorption*

Absorption is a physical or chemical process in which atoms, molecules, or ions enter some bulk phase – gas, liquid, or solid material. As compared to the adsorption process, in which the molecules are adhered on the surface of the adsorbent, the absorption process takes place when the volume takes up molecules.

Gas absorption is the removal of one or more pollutants from a contaminated gas stream when the gas stream passes through a gas–liquid interface and ultimate dispersion in the liquid. Absorption is a process that may be chemical (reactive) or physical (nonreactive). Physical absorption is formed based on the interaction of two phases of matter including a liquid absorbs a gas or a solid absorbs a liquid. When a liquid solvent absorbs a part or all of a gas mixture, the gas mass could move into the liquid volume. The mass transfer could take place at the interface between the gas and the liquid. The mass transfer rate depends on both the liquid and the gas properties. The solubility of gases, the pressure and the temperature are the main factors affecting to this type of absorption. In addition, the absorption rate also depends on the surface area of the interface and its duration in time. When a solid absorbs a part or all of a liquid mixture, the liquid mass could move into the solid volume. The mass transfer could take place at the interface between the liquid and the solid. The mass transfer rate depends on both the solid and the liquid properties. Chemical absorption or reactive absorption is a chemical reaction between the absorbed and the absorbing substances. Sometimes, it is combined with physical absorption. This type of absorption depends upon the stoichiometry of the reaction and the concentration of its reactants.

Gas absorption is usually carried out in packed towers. The contaminated gas stream enters the bottom of the column and passes upward through a wetted packed bed. The absorbing liquid enters from the top of the column and is distributed over the column packing. The column packing may have one or more commercially available geometric shapes designed to maximize the gas–liquid contact and minimize the gas–phase pressure drop [20]. The require‐ ments for the packing column include high wetted area per unit volume, minimal weight, sufficient chemical resistance, low liquid holdup, and low pressure drop.

#### *3.2.3. Condensation*

Condensation is a separation process to convert one or more volatile components of a vapor mixture to a liquid through saturation process. Any volatile components can be converted to liquids by sufficiently lowering their temperature and increasing their pressure. The most common process is reducing the temperature of the vapor because increasing the vapor pressure is expensive. The condensation process is primarily used to remove VOCs from gas streams prior to other control methods, but sometimes it can be used alone to reduce emissions from high-VOCs concentration gas streams [2, 21].

The simple and relatively inexpensive condenser uses water or air to cool and condense the vapor stream to the liquid. Since these devices are not required to reach or capable of reaching low temperature, high removal efficiencies of most vapor pollutants cannot be obtained unless the vapor will condense at high temperature. That is why condensers are typically used as a pretreatment device. They can be used together with adsorption, absorption, and incinerators to reduce the gas volume to be treated by other expensive methods.

A typical condenser device includes condenser, refrigeration system, storage tanks, and pumps. The condensation process includes:


Activated carbon is a highly porous, amorphous solid consisting of microcrystallines with a graphite lattice, usually prepared in small pellets or a powder. It is nonpolar and cheap. Activated carbon is used for the adsorption of organic substances and nonpolar adsorbate. Activated carbon is also usually used for waste gas (and waste water) treatment. It is the most widely used adsorbent because its chemical and physical properties such as surface groups, pore size distribution, and surface area can be tuned as required. Its usefulness also derives from its large microspore (and sometimes mesoporous) volume and the resulting high surface

Absorption is a physical or chemical process in which atoms, molecules, or ions enter some bulk phase – gas, liquid, or solid material. As compared to the adsorption process, in which the molecules are adhered on the surface of the adsorbent, the absorption process takes place

Gas absorption is the removal of one or more pollutants from a contaminated gas stream when the gas stream passes through a gas–liquid interface and ultimate dispersion in the liquid. Absorption is a process that may be chemical (reactive) or physical (nonreactive). Physical absorption is formed based on the interaction of two phases of matter including a liquid absorbs a gas or a solid absorbs a liquid. When a liquid solvent absorbs a part or all of a gas mixture, the gas mass could move into the liquid volume. The mass transfer could take place at the interface between the gas and the liquid. The mass transfer rate depends on both the liquid and the gas properties. The solubility of gases, the pressure and the temperature are the main factors affecting to this type of absorption. In addition, the absorption rate also depends on the surface area of the interface and its duration in time. When a solid absorbs a part or all of a liquid mixture, the liquid mass could move into the solid volume. The mass transfer could take place at the interface between the liquid and the solid. The mass transfer rate depends on both the solid and the liquid properties. Chemical absorption or reactive absorption is a chemical reaction between the absorbed and the absorbing substances. Sometimes, it is combined with physical absorption. This type of absorption depends upon the stoichiometry

Gas absorption is usually carried out in packed towers. The contaminated gas stream enters the bottom of the column and passes upward through a wetted packed bed. The absorbing liquid enters from the top of the column and is distributed over the column packing. The column packing may have one or more commercially available geometric shapes designed to maximize the gas–liquid contact and minimize the gas–phase pressure drop [20]. The require‐ ments for the packing column include high wetted area per unit volume, minimal weight,

Condensation is a separation process to convert one or more volatile components of a vapor mixture to a liquid through saturation process. Any volatile components can be converted to

sufficient chemical resistance, low liquid holdup, and low pressure drop.

area.

*3.2.2. Absorption*

238 Current Air Quality Issues

*3.2.3. Condensation*

when the volume takes up molecules.

of the reaction and the concentration of its reactants.


A condensing system usually contains either a contact condenser or a surface condenser. Contact condensing systems cool the contaminated gas stream by spraying ambient or chilled liquid directly into the gas stream. A packed column is usually used to maximize the surface area and contact time. The direct mixing of the coolant and contaminant necessitates separation or extraction before coolant reuse. This separation process may lead to a disposal problem or secondary emissions. Contact condensers usually remove more contaminated air as a result of greater condensate dilution. In the surface condensing systems, the coolant does not mix with the gas stream, but flows on one side of a tube or plate in the surface condensing systems. The condensing vapor contacts the other side, forms a film on the cooled surface, and drains into a collection vessel for storage, reuse, or disposal. Surface condensers require less water and generate 10–20 times less condensation than contact condensers do.

The advantages of the condensation method include lower installation cost, little required auxiliary equipment, and less maintenance requirement. However, the remaining disadvan‐ tages of the method include problems of water disposal, low efficiencies, and the need for further treatment.

#### *3.2.4. Incineration*

Incineration or thermal oxidation is a broadly used method to control air pollutants such as VOCs, using oxidation at high temperature. Incineration is considered as an ultimate disposal technique in which VOCs are converted to carbon dioxide, water, and other inorganic gases. The two popular incineration methods are thermal incineration and catalytic incineration.

In thermal incineration, organic compounds in the contaminated gas are burned or oxidized at a high temperature with air in the presence of oxygen [22]. The thermal oxidizer involves specifying a temperature of operation along with a desired residence time and then optimum sizing the device to achieve the desired residence time and temperature with proper flow velocity. Selection of the proper piece of equipment depends on the mode of operation, oxygen content, and concentration of the organic gases. They are very important when trying to minimize the overall cost of the incineration and reduce the volume of the gas stream to be treated as much as possible. Depending on the types of heat recovery, incinerators can be classified into two categories: recuperative and regenerative. The recuperative incinerator uses a shell and tube heat exchanger to transfer the heat generated by the incinerator to the preheat of the feed stream. The recuperative incinerator can recover about 70% of the waste heat from the exhaust gases [21]. The regenerative incinerator includes a flame-based combustion chamber that connects two or three fixed beds containing ceramic or other inert packing. The input gas enters over these beds where it is preheated before passing into the combustion chamber and being burned. Then, the hot flue gases pass through the packed beds where the heat generated during incineration is recovered and stored. The packed beds keep the heat during one cycle and release it as the beds preheat the input organic gases in the second cycle. This regenerative incinerator method can recover up to 95% of the energy from the flue gas [23].

In catalytic incineration, the organic compounds in the contaminated gas are converted into carbon dioxide and water by using a catalyst that facilitates incineration at low temperature. Thus, the requirement incineration temperature can be decreased by hundreds of degrees. Therefore, the application of catalyst incineration can save a large amount of energy to heat up the gas stream containing pollutants for combustion. The contaminated gases are heated by a small auxiliary burner, and then the gases passed through the catalyst bed. The space requirement for operation of catalytic incineration is much smaller than that of thermal incineration. Thanks to the catalytic activity, the degree of oxidation of the pollutants is greatly increased compared with that in the incineration system without any catalyst. The catalyst activity refers to the degree of the chemical reaction rate. The catalyst can also be selective with higher activity for some compounds. Such activity and selectivity enable a lower operating temperature while still achieving the desired destruction efficiencies. In air pollution control, the catalyst is usually a noble metal (Pd, Pt, Cr, Mn, Cu, Co, and Ni) deposited on an alumina support in a configuration to minimize the pressure drop, which is often critical for incinerator designs [23, 24].

#### *3.2.5. Biological system*

The biological system for controlling air pollutants such as VOCs and odor uses microbes or microorganisms, immobilized on a biologically active solid support, to treat the gas pollutants. The principle of the method is that the gaseous pollutants are used by the microbes as a food or energy source and thus destroyed and converted into innocuous metabolic end products such as carbon dioxide and water. The processes via which microbes destroy or convert pollutants contain:


The process requires careful attention to design and operation in order to ensure firstly good contact between the contaminated gases and the microbes contained on the solid support; and secondly, that the microbe population is sustained and maintained in a healthy state. The key concerns in the design and operation of a biological system to control air pollutants contain [25]:


*3.2.4. Incineration*

240 Current Air Quality Issues

designs [23, 24].

*3.2.5. Biological system*

Incineration or thermal oxidation is a broadly used method to control air pollutants such as VOCs, using oxidation at high temperature. Incineration is considered as an ultimate disposal technique in which VOCs are converted to carbon dioxide, water, and other inorganic gases. The two popular incineration methods are thermal incineration and catalytic incineration. In thermal incineration, organic compounds in the contaminated gas are burned or oxidized at a high temperature with air in the presence of oxygen [22]. The thermal oxidizer involves specifying a temperature of operation along with a desired residence time and then optimum sizing the device to achieve the desired residence time and temperature with proper flow velocity. Selection of the proper piece of equipment depends on the mode of operation, oxygen content, and concentration of the organic gases. They are very important when trying to minimize the overall cost of the incineration and reduce the volume of the gas stream to be treated as much as possible. Depending on the types of heat recovery, incinerators can be classified into two categories: recuperative and regenerative. The recuperative incinerator uses a shell and tube heat exchanger to transfer the heat generated by the incinerator to the preheat of the feed stream. The recuperative incinerator can recover about 70% of the waste heat from the exhaust gases [21]. The regenerative incinerator includes a flame-based combustion chamber that connects two or three fixed beds containing ceramic or other inert packing. The input gas enters over these beds where it is preheated before passing into the combustion chamber and being burned. Then, the hot flue gases pass through the packed beds where the heat generated during incineration is recovered and stored. The packed beds keep the heat during one cycle and release it as the beds preheat the input organic gases in the second cycle. This regenerative incinerator method can recover up to 95% of the energy from the flue gas [23]. In catalytic incineration, the organic compounds in the contaminated gas are converted into carbon dioxide and water by using a catalyst that facilitates incineration at low temperature. Thus, the requirement incineration temperature can be decreased by hundreds of degrees. Therefore, the application of catalyst incineration can save a large amount of energy to heat up the gas stream containing pollutants for combustion. The contaminated gases are heated by a small auxiliary burner, and then the gases passed through the catalyst bed. The space requirement for operation of catalytic incineration is much smaller than that of thermal incineration. Thanks to the catalytic activity, the degree of oxidation of the pollutants is greatly increased compared with that in the incineration system without any catalyst. The catalyst activity refers to the degree of the chemical reaction rate. The catalyst can also be selective with higher activity for some compounds. Such activity and selectivity enable a lower operating temperature while still achieving the desired destruction efficiencies. In air pollution control, the catalyst is usually a noble metal (Pd, Pt, Cr, Mn, Cu, Co, and Ni) deposited on an alumina support in a configuration to minimize the pressure drop, which is often critical for incinerator

The biological system for controlling air pollutants such as VOCs and odor uses microbes or microorganisms, immobilized on a biologically active solid support, to treat the gas pollutants.


The biological technology is most suitable for high volumetric flow rate air streams containing low pollutant concentrations. The two most common biological systems are biofilter and bioscrubber. Biofilter is a biological system which uses an organic or synthetic media to host and nourish the microorganism without the requirement for an aqueous flush system. Bioscrubber is a biological system which uses an inorganic or synthetic media to provide a structural base for physically hosting the microorganisms requiring a continuous water flush or an intermittent containing carbon nutrient that supports the microorganism. The use of a biological system to control air pollutant offers several advantages including effective removal of compounds, little or no by-product pollutants, uncomplicated installations, and low costs. However, the method retains several disadvantages including a reduced suitability to high concentration streams, large area requirement for installation, need for careful attention to moisture control, and the possibility of becoming clogged by particulate matter or biomass growth.

#### *3.2.6. Application of photocatalyst*

An alternative technology, which offers a number of advantages over the above-mentioned technologies, for controlling organic air pollutants is to use photocatalysts. The use of a photocatalyst supports the operation of a low- or room-temperature photocatalytic oxidation process that can degrade a broad range of organic contaminants into innocuous final products such as CO2 and H2O without significant energy input. When the photocatalyst, for example, TiO2, is irradiated with ultraviolet (UV) radiation that has energy higher than the band gap energy of the TiO2, an electron could be excited from the valence band to the conduction band of TiO2, leaving holes behind on the TiO2 surface. The leaved holes could react with surround‐ ing H2O to produce hydroxyl radicals (\* OH), while the excited electrons could react with O2 to produce superoxide radical anions (*o*<sup>2</sup> − ). These oxy radical species can participate in the oxidation reaction to destroy many organic contaminants (CxHyOz) completely [26]. The photocatalytic process mainly follows the following reactions:

$$\begin{aligned} \mathrm{TiO}\_{2} + h\nu &\rightarrow e^{-} + h^{+} \\ h^{+} + H\_{2}\mathrm{O} &\rightarrow H^{+} + \mathrm{^{\cdot}OH} \\ e^{-} + \mathrm{O}\_{2} &\rightarrow \mathrm{O}\_{2}^{-} \\ \mathrm{O}\_{2}^{-} + e^{-} + 2H^{+} &\rightarrow H\_{2}\mathrm{O}\_{2} \\ H\_{2}\mathrm{O}\_{2} &\rightarrow 2\,^{\cdot}OH \\ \mathrm{^{\cdot}OH} + \mathrm{C}\_{x}H\_{y}\mathrm{O}\_{z} &\rightarrow \mathrm{x}\mathrm{CO}\_{2} + yH\_{2}\mathrm{O} + ... \end{aligned}$$

The three common reactor types designed to use a photocatalyst for air purification purposes are the honeycomb monolith, fluidized-bed, and annular reactors [27]. A honeycomb monolith reactor contains a certain number of channels, each of which typically has an internal dimen‐ sion of the order of 1 mm. The cross-sectional shapes of the channels are square or circular. The photocatalyst is coated onto the walls of channels in a very thin wash coat. Fluidized-bed reactors are designed to treat a high gas feed rates directly passing through the catalyst bed. Based on reactor design, the solid photocatalyst could directly contact with the UV irradiation as well as gaseous reactants. The fluidized-bed reactors generally consisted of two concentric cylinders, which form an annular region with a certain gap. The photocatalyst is deposited onto the interior wall of the outer cylinder. The light source is usually located at the center. The thickness of the deposited photocatalyst film is sufficiently thin ensuring that all of the photocatalyst could be illuminated by UV irradiation [28].

The applications of the photocatalyst for photocatalytic oxidation processes to reduce air pollutants have been considered as alternatives to conventional air pollution control technol‐ ogies. However, they have yet to overcome the problems of low energy efficiency and poor cost competitiveness. Therefore, numerous methods for modifying photocatalysts have been developed and investigated to accelerate the photo-conversion, enable the absorption of visible light, or alter the reaction mechanism to control the products and intermediates [29]. In this regard, metals or nonmetals were used as doping agents to implant or coprecipitate on the surface or in the lattice of TiO2. Electron donors or hole scavengers have been added to such photocatalytic systems. In addition, another semiconductor was integrated with TiO2 to establish a suitable two-semiconductor system [29]. The modifications not only change the mechanism and kinetics of the photocatalytic processes under UV irradiation but also enhance the photocatalytic activities of the photocatalyst, thereby enabling the photocatalytic oxidation processes to proceed even under visible light [30].
