Air pollution engineering

2.1.1 Formation of secondary air pollutants in the atmosphere

The emission of pollutants into the air as a result of various human activities may change the natural dynamics of processes in the atmosphere. On the dry volume base air consists almost entirely of the four gases: molecular nitrogen (N₂, 78 %), oxygen (O₂, 20.94 %), argon (Ar, 0.93 %) and carbon dioxide (CO₂, 0.04 %), while other gases like neon (Ne), helium (He), methane (CH₄), nitrous oxide (N₂O), ozone (O₃), hydroxyl radicals and some other constituents come in traces. Even though some gases such as O₃, N₂O, CH₄ are at trace concentration, they exert profound effects on the environment. Air also contains a variable amount of water vapor (H₂O), with value that varies from 0–4 %, depending on the geographic position, part of the day, weather conditions and other parameters. The Earth’s atmosphere extends to a height of several thousand kilometers and can be divided into several layers: troposphere (0–12 km), stratosphere (12–50 km), mesosphere (50–85 km), thermosphere (85–500 km) and exosphere (>500 km). The most important parts of the atmosphere with respect to air pollution are troposphere and stratosphere (Figure 3).

Figure 2:

Air pollution transport and dispersion [13].

Figure 3:

The most important layers of the atmosphere for the human life [14].

2.1.2 Tropospheric vs stratospheric ozone

Emissions of air pollutants contribute to local, regional (or transboundary) and global air pollution (see Figure 4.).

Figure 4:

Consequences of air pollution – the influence of air pollution on different scales.

When we talk about the local and urban air pollution, the emphasis is mostly on the protection of the human health and preservation of natural biodiversity, protection of the materials and buildings, historical and cultural heritage materials. Transboundary air pollution refers to pollution transported in the atmosphere from one country or region to another and often undergoing chemical transformation in the process. Acidification, eutrophication and ground-level ozone are types of transboundary air pollution, arising from the emissions of the air pollutants such as: NO_x, SO₂, NMVOC, NH₃.

Ozone (O₃) is a secondary air pollutant which is found in two different layers of the atmosphere: troposphere and stratosphere. In the troposphere, the ground-level, tropospheric or “bad” ozone is a secondary air pollutant that damages human health, vegetation and many common materials. It is also a key ingredient of the urban photochemical smog and the third-most important greenhouse gas, after carbon dioxide and methane. The primary sources of the pollutants responsible for formation of photochemical smog are fossil fuel-burning power plants and all types of the internal combustion engines. Formation of tropospheric ozone by chemical reactions between nitrogen oxides (NO_x), carbon monoxide (CO) and hydrocarbons (HCs) in the presence of sunlight is summarized in eq. (1):

NO_x and O₃ interact in the atmosphere by the following reactions:

where hν represents a photon of light of proper wavelength and M denotes any other molecule which must carry away some of the energy released in the reaction (usually N₂ or O₂). Instead of HCs the other terms like non-methane hydrocarbons (NMHC), VOCs and reactive organic gases (ROG) are often used in this context. The major anthropogenic sources of these chemicals are motor vehicle exhaust, industrial emissions and chemical solvents. Nitric oxide (NO) is also an important component of aircraft exhaust, which is formed by oxidation of atmospheric N₂ at the high operating temperatures of the aircraft engine. NO easily react with oxygen in the air to form nitrogen dioxide (NO₂), a foul-smelling brown gas – secondary air pollutant. Some of the NO_x reacts with hydroxyl radicals (OH^·) and produce HNO₃ which fall to the Earth in wet or dry forms, including acid rain, fog or dust that is acidic. Although O₃ is the main cause of the photochemical smog formation, some aldehydes (RCHO), peroxy and hydroperoxy radicals and peroxyacyl nitrates (PANs or RC(O)OONO₂) also contribute to this process through the similar mechanism [15]. PANs are powerful respiratory and eye irritants present in photochemical smog. As secondary air pollutants they are formed from other pollutants by chemical reactions in the atmosphere. For example, formation of the peroxyacetyl nitrate (CH₃COOONO₂) can be described by:

Since PANs dissociate quite slowly in the atmosphere into radicals and NO₂, they are able to transport these unstable compounds far away from the urban and industrial origin.

From a global point of view, the environmental problems which have to be solved or at least minimized involve the stratospheric ozone depletion (the so-called ozone holes), the greenhouse effect as well as the global climate changes and similar emerging environmental problems. In the stratosphere, we find the stratospheric or “good” ozone that protects life on the Earth from the harmful effects of the Sun’s ultraviolet (UV) radiation. About 90 % of the ozone in the Earth’s atmosphere lies in the stratosphere (between 16 and 48 km above the surface of the Earth), where it is a kind of layer (ozone layer) in the stratosphere. Although O₃ is concentrated in the ozone layer more than anywhere else, its concentrations are extremely small, typically only 1 to 10 parts of ozone per 1 million parts of air, compared with about 210,000 parts of oxygen per 1 million parts of air. The concentration of the stratospheric ozone is result of dynamic equilibrium between the chemical processes of its formation and processes of its destruction [16]. The first photochemical mechanism of the stratospheric ozone formation was proposed in 1930 by the famous British scientist Sydney Chapman, who assumed that ozone formation occurs by continuously cyclical processes that begin by photolysis of oxygen in the upper stratosphere. However, it was soon found out that Chapman mechanism predict much higher ozone concentrations than actually measured and it was assumed that ozone is consumed by additional chemical reactions. A large contribution to the understanding of the stratospheric chemistry was given by Paul Crutzen, who won the Nobel Prize for chemistry (in 1995 with Mario José Molina and Frank Sherwood). Paul Crutzen linked nitrogen oxides with the chemistry of stratospheric ozone depletion. He pointed out that emissions of nitrous oxide (N₂O), a stable, long-lived gas produced by soil bacteria or formed as result of the increasing use of fertilizers, could affect the amount of nitric oxide (NO) in the stratosphere. Mario José Molina and Frank Sherwood Rowland predicted the influence of chlorine released from chlorofluorocarbons degradation to stratospheric ozone depletion. It is worth mentioning that the ozone hole over Antarctica was discovered even in 1985, thanks to a British geophysicist Joe Farman.

According to the Chapman mechanism the ozone is produced by the photolysis of oxygen molecules into oxygen atoms, shown in eq. (6), followed by the reaction of one oxygen atom with an oxygen molecule, eq. (7):

The third molecule, M is needed to remove the excess energy and can be any other molecule in the atmosphere (usually N₂ or O₂). The overall reaction eq. (8) indicates that ozone and oxygen atoms are produced from oxygen molecules and light (hν). Ozone can be also destroyed by photodissociation of ozone in the presence of ultraviolet radiation (UV), eq. (9) or by reaction with an oxygen atom, eq. (10):

However, reaction given by eq. (9) produces an oxygen atom and oxygen molecule, which can participate in the regeneration of oxygen molecule and the recombination of ozone as in eq. (7). The above reaction is the predominant mechanism by which ozone performs the function of shielding the Earth from harmful UV light. The formation of O₃ is balanced by its dissociation through several reactions and one of them is:

There are many other ways in which ozone can be destroyed besides by the reaction given by eq. (10). Paul Crutzen assumed in the early 1970s that ozone can be catalytically destroyed in a cycle involving NO_x (x = 1, 2). He proposed the following catalytic cycle:

The overall reaction is also given by the eq. (10), indicating that cycle of ozone destruction is only catalyzed by NO_x, because the NO consumed in reaction (11) is regenerated in reaction (12) and the NO₂ produced by reaction (11) is consumed in reaction (12). Obviously, this cycle can be repeat as long as NO_x is available for the reaction. NO_x in the stratosphere originate from natural and anthropogenic sources. A natural source of NO_x is the oxidation of N₂O released by bacteria in soil and oceanic microorganisms. An anthropogenic source of N₂O is fertilization. N₂O reacts with highly energized oxygen atoms which results in formation of NO molecules. Some other molecules can also participate in the chemistry of the stratospheric ozone destruction, such as chlorine or bromine molecule, eqs (13)–(15)

with the same overall reaction given by eq. (10). According to some estimation one chlorine atom can participate in the degradation of 10⁴–10⁶ ozone molecules. The main source of chlorine molecules is CFCs, known by the DuPont brand name freons. Many CFCs, like CF₂Cl₂ (Freon-12, CFC-12) and CFCl₃ (Freon 11, CFC-11), are chemically stable molecules in the troposphere, but they drift into the upper atmosphere where their chlorine components destroy ozone. In the past, CFCs were used as inert, non-toxic and easily liquefied chemicals in refrigeration, air-conditioning, as solvents, as well as aerosol propellants. Because of their recognized contribution to the depletion of stratospheric ozone, after the signing of the Montreal protocol in 1987, they are replaced with other compounds such as HFCs. HFCs are “safer” than CFCs for the ozone layer, because they react in the troposphere. Bromine species are 50–100 times more destructive for ozone than chlorine species [17]. The main source of bromine in the stratosphere is methyl bromide. It is emitted naturally by oceanic biological activity and from anthropogenic activities, such as soil fumigation and biomass burning. The well-known anthropogenic bromine sources are halons used in fire extinguishing systems, agriculture, dry cleaning and other applications. Fortunately, production of halons is currently being regulated for the same reasons like CFCs. Hydroxyl radicals (OH^·) can also catalyze the dissociation of ozone in the stratosphere; however, this kind of reaction is estimated to account for the degradation of only ca. 15 % of the stratospheric ozone. Many of the molecules released on Earth do not reach the stratosphere because they are soluble in water and return to the surface in precipitation or are broken down by chemical reactions in the troposphere.

2.1.3 Secondary particulate maters

PM can be directly emitted to the atmosphere (primary PMs) and formed in the atmosphere (secondary PMs). Primary PM originates from both natural and anthropogenic sources. Natural sources of PMs include sea salt, naturally suspended dust, pollen, volcanic ash. Anthropogenic sources of PMs, such as dust or soot (black carbon), include fuel combustion in thermal power generation, waste incineration, domestic heating for households and fuel combustion for vehicles, as well as vehicle (tyre and brake), road wear and other types of anthropogenic dust. The main precursor gases for secondary PMs are SO₂, NO_X, NH₃ and VOCs. The largest sources of NH₃ emissions are agricultural activities and NH₃-based fertilizer applications. Other sources of NH₃ emissions include some industrial processes, biomass burning (including forest fires) and to a lesser extent fossil fuel combustion and volatilization from soils and oceans. The result of the atmospheric reactions in which participate NH₃, SO₂ and NO_X is formation of ammonium (NH₄⁺), sulfate (SO₄^–2) and nitrate (NO₃^–) compounds, which form new particles or condense onto existing ones and form secondary inorganic aerosols. Some VOCs also contribute to formation of the secondary organic aerosols.

3.1.1 The primary or preventative approach to the air pollution control

An efficient air pollution control requires a complete knowledge of the air contaminants and the source of their emission. It is desirable to prevent the formation of pollutants whenever is possible. Changing or elimination of process steps that are basic sources of pollution is often one of the possible primary or preventative methods available for control of emissions of air pollutants discharged by the industries. Examples of such control measures to meet the emission standards are based on consideration of the source reduction opportunities, careful planning and process optimization to minimalize pollutants and carrier fluid generation at the source and include: operational changes, minimizing the volumetric flow rate of the stream to be treated, substitution of raw materials or fuels, use of efficient engines and more efficient burners design, delocalization of the plant, process equipment modification or replacement of old process equipment and other types of the process-oriented specific measures and control strategies. A modern nuclear power plants appears to be relatively pollution free solutions, compared to the more familiar fossil fuel-fired plants, which emit a lot of pollutants (carbon oxides, nitrogen oxides, sulfur dioxide, hydrocarbons and fly ash). However, waste and spent-fuel disposal problems may limit the apparent advantages[20, 21]. The biggest improvement in the air quality in most cities of the United States and European Union was achieved by replacement of coal with natural gas, by switching vehicles from gasoline to unleaded gasoline, compressed natural gas and other more environmentally friendly fuels, by addition of oxygenated compounds to motor fuels, by using of low-sulfur fuels, etc. [15]. There are also additional preventative ways to reduce air pollution including proper planning and location of the industrial areas, development of new technologies and legal regulations as well as using of administrative controls. The government plays a very important role in prevention of all kind of environmental pollution through government regulations that forced industries to reduce pollutant discharge and to promote new developments in technology.

3.1.2 The secondary approach to the air pollution control

The secondary air pollution control approach is relied on recovery or abatement of contaminants from the waste gas streams and application of the end-of-pipe treatment devices. This implies installation of control device (or equipment) between the source of pollutant generation and its release to the atmosphere. In that case control may consist of either removal/degradation of the pollutant or conversion to a less polluting form or recovery of economically valuable waste products. These solutions are focused on the specific air quality objectives or emission limits and primarily directed on the well-defined (or controllable) sources of emissions. The end-of-pipe treatment devices can be applied only for ducted emissions, which means that collection hoods and a ventilation system are required upstream of the end-of-pipe abatement system. Abatement is the term used for all technical and mechanical engineering devices, methods and technologies which may help in reducing quantity of pollutants emitted into atmosphere. Such devices are divided into two categories: devices (or equipment) for reducing particulates and devices for reducing emissions of gaseous and vaporous pollutants, although some of devices can be also used for simultaneous removal of the both groups of pollutants. In many cases, heating or cooling of the gaseous effluent is required before it enters the appropriate control device.

The environmental engineers must be aware of the gas laws, thermodynamic properties and all reactions ensuring a satisfactory design of the control device. The key design issues for air pollution control equipment are usually based on consideration: the process conditions, the physico-chemical and other important properties of pollutants and the added value of the emitted compound. The process conditions include: the total gas flow rate (or velocity) and volume of the waste gas to be treated, temperature limitations, allowable pressure drop, degree of variability depending on the operating and process conditions (i.e. variation in the pollutant concentration, gas flow rate, temperature), power/energy requirements, removal efficiency requirements, etc. Besides nature of the air pollutant and its concentration (including minimum and maximum values) typical properties of pollutants usually considered during design of air pollution control system are: solubility, corrosivity, flammability, toxicity or reactivity (if pollutants are gaseous) and size range and distribution, particle shape, agglomeration tendencies, corrosivity, abrasivity, hygroscopic tendencies, stickiness, flammability, toxicity, conductivity, electrical resistivity or reactivity (if pollutants are in the form of particulates). The added value of emitted product is also very important and can affect the selection of adequate air pollution control equipment or the chose between: the recovery techniques (e. g. fabric filter, cyclone, adsorption, condensation) and the abatement or destruction techniques (e. g. thermal or catalytic oxidation, chemical reduction, biofiltration). Typical air pollutants for which recovery technique is economically feasible are those which concentrations in the waste stream are high enough, e. g. VOCs from solvent vapors and vapors of low-boiling compounds, NH₃ (to recycle in the production process), SO₂ (converted into H₂SO₄, sulfur or gypsum), dust which contains higher amounts of solid raw products or end products [10]. Some of the mentioned recovery techniques can be used to minimize emissions of odors which also belong to the highly volatile compounds.

3.1.3 The integral approach (or process-integrated techniques)

In order to achieve all the stringent criteria of the air protection it is often necessary to combine different engineering approaches and methods of control, i.e. an integrated approach and comprehensive solution of the environmental pollution related to certain human activity. Some waste gas techniques (like scrubbing, adsorption, electrostatic precipitation) require further downstream treatment of the waste water or solid waste generated during the process of air purification. Unfortunately, the cost of the waste disposal can be a significant part of the total cost of the air pollution control.

Engineers working in air pollution control have many responsibilities and one of them is the design of air pollution equipment in order to achieve emission standards and the requirements of legal regulations. The key challenges in using appropriate equipment for air pollution control are [10]:

1. quantity and concentrations of pollutants in air streams (including physical and chemical properties of the effluent from the emission source),

2. available space and equipment location,

3. contribution of air pollution control system to wastewater and land pollution as well as disposal of waste recovered,

4. economy (costs of investment and installation of the process equipment, costs of maintenance) and

5. safety.

3.3.1 Mechanical collectors

In the following text three types of control devices will be considered, including gravity settlers and cyclone separators, which belong to the mechanical collectors and electrostatic precipitators (ESPs). All these devices are function of driving the particles to a solid wall, where they adhere to each other to form agglomerates that can be removed from the collection device and disposed of. Therefore, these devices are sometimes called the wall collection devices. Instead of driving the particles to a wall, filters and scrubbers divide the flow into smaller parts in order to collect the particles. For this reason they are called the dividing collection devices [15].

3.3.1.1 Gravity settler

An gravity settler is a long chamber through which the polluted gases pass very slowly (less than about 0.5 ms⁻¹ for good results), allowing particles enough time to settle out at the bottom of the chamber (in the hopper or collector) under the action of gravity (Figure 6). The chamber is cleaned manually to dispose the waste. Gravity settlers are more efficient in the laminar flow conditions. Under intense turbulent flow conditions deposition of particles is difficult and therefore the efficiency is lower. The separation effect is more pronounced by reducing the gas stream velocity by means of baffles and similar design elements (e. g. horizontal trays or shelves). Gravity settler is the simplest control device which is efficient only for large particles (>50 μm) and therefore it is useful for processing of very “dirty” gases (from cement industry, metallurgical processes, etc.). Even with horizontal trays the minimum particle size which can be removed is about 10 μm. The pressure drop through the gravity settler is very small and consists mostly of entrance and exit losses. Separated particles might be contaminated with toxic or hazardous contents which need to be considered for further treatment or disposal. Gravity settler is usually used as pre-cleaner, e. g. as preliminary step in various filter systems and scrubbers [10].

Figure 6:

Schematic of gravity settler (inspired by [25]).

The mathematical analysis of the gravity settlers is very easy and with some exceptions it appears in similar form for cyclones and electrostatic precipitators. The separation efficiencies for laminar and turbulent settling chambers are given by the following equations [23]:

(28)ηlaminar=vtLuH=gdp2ρp18μLuH

(29)ηturbulent=1−exp⁡(−ηlaminar)

where v_t is the particle settling velocity (m s⁻¹) (Stokes low), L is chamber length (m), H is chamber height (m) and u is the average horizontal gas velocity in the chamber:

(30)u=QWH

In the eq. (30) Q is the volumetric air flow rate (m³s⁻¹).

3.3.1.2 Cyclone

Cyclones are the most common centrifugal or inertial separators. Their efficiency is much better compared to gravity settlers and yields about 90–99 % for particles with diameter between 5 and 10 μm [10, 22]. At modest velocities and common diameters the centrifugal forces acting on particles can be two orders of magnitude larger than the gravity forces [15]. The cyclone consists of a vertically placed cylinder which has an inverted cone attached to its base. The gas stream containing particulates enters tangentially at the inlet point to the cylinder. The velocity of this inlet gas stream is then transformed into a confined vortex (called “external vortex”), from which centrifugal forces tend to drive the larger suspended particles to the walls of the cyclone. Due to the reduction of the flow cross section, the increase in pressure and the higher concentration of solid particles in cone part, a part of “external” vortex turn into “inner” directed upward at the bottom of the cone part. The clean gas (also with the smallest particles) is removed from a central cylindrical opening at the top (called “apex”), while the dust particles are collected at the bottom in a storage hopper by gravity. Schematic of a cyclone separator and the flow field inside a cyclone is given in Figure 7.

Figure 7:

Schematic flow diagram of a standard centrifugal separator – cyclone (inspired by [25]).

Cyclones are sized on the basis of centrifugal force, F_c

(31)Fc=mvc2r=mωcr

where m is mass, v_c is velocity, r is radius and ω_c is angular velocity for rotational motion of particle inside the cyclone. Using a centrifugal equivalent of Stokes’ law, i.e. after substitution of the gravitational force by the centrifugal force in eq. (20) it is possible to calculate the settling velocity of particle for the centrifugal settling [15]:

(32)vt=vc2rdp2ρp18μ

The cyclone diameter is important design parameter that has the highest effect on the cyclone separation efficiency. For a given pressure drop, smaller the diameter, greater is the efficiency, because centrifugal action also increases with decreasing radius of rotation. The efficiency of a cyclone can be increased by the use of a large number (up to seven thousand) or a battery of smaller cyclones (with a diameter ranging between 0.005 m and 0.3 m) operating either in parallel or serial. This kind of cyclone arrangements with a common inlet chamber, outlet plenum and collection system is known as multiple cyclones or multi-clones. The separation efficiency will also increase with increasing particle size, particle density, inlet particle loading, cyclone body length and ratio of body diameter to gas outlet tube diameter [10, 15, 26]. However, as efficiency increases the operating costs also increase because of the higher pressure drops. Common problems related to the work of the cyclone are erosion, corrosion and a dust cake builds up on the cyclone walls, especially if the dust is hygroscopic [26]. Cyclones are usually designed by set of procedures. More information on design considerations can be found out in the literature [15, 23, 26].

3.3.2 Electrostatic precipitator

An electrostatic precipitator (ESP) is a separation devices like gravity settler or cyclone, but it use electrostatic force to move particles entrained within a waste gas stream to the walls of the collector plates. It is very useful for the removal of fine dusts (between 1 and 10 μm, but mostly less than 5 μm) from different kind of waste gases with very high efficiency (approaching even 100 %) [10, 26]. The principle behind all electrostatic precipitators is to give electrostatic charge to particles in a given gas stream and then pass the particles through an electrostatic field that drives them to a collecting electrode of opposite charge. The overall process of dust separation in the ESP consists of the following steps: generation of electric field, generation of electric charges, transfer of electric charge to a dust particle, movement of a charged dust particle in an electric field to the collection electrode, adhesion of the charged dust particle to the surface of the collection electrode and mechanical removal of the dust layer from the collection electrode (by rapping, vibration or washing) to a hopper.

The electrostatic precipitators require maintenance of a high potential difference (20–100 kV, but mostly 40–60 kV) between the two electrodes: discharging electrode and collecting electrode. The applied voltage depends on the distance between electrodes. The high voltage between electrodes results in a strong electric field and electric discharge of ions from the discharging electrode called the corona. Besides on process conditions (flow rate, composition, temperature, dew point) and properties of particles such as concentration, size distribution, hygroscopy and tendency to agglomerate, the efficiency of ESP significantly depends on the particle resistivity. The particle resistivity is a measure of the resistance of the dust layer to the passage of a current. For efficient work of ESP, the resistivity should be between 10⁷ and 10¹⁰ ohm-cm [10, 23]. Operation at the lower or higher resistivity leads to a decrease in removal efficiency. To improve efficiency it is possible to perform chemical conditioning of the flue gases (with moisture, by addition SO₃ gas, sodium or ammonium salts) or to work at the lower operating temperature (with caution because of the possible condensation and corrosion) [15].

The most important challenge in consideration of ESP design is calculation of collection efficiency or collection area. The collection efficiency and/or the collection area of an ESP can be estimated using several equations and one of them is known as the Deutch-Anderson equation [16, 26]:

where A is the effective collecting plate area of the precipitator (m²), ω is the drift velocity or the particle migration in an electric force field (m s⁻¹) and Q is volumetric gas flow through the precipitator (m³ s⁻¹). More accurate estimates of collection efficiency can be obtained by modifying the eq. (32) or by decreasing the calculation of collection efficiency by a factor of k. The resulting Matts–Ohnfeldt equation is given by [15, 26]:

where k is constant (usually 0.4 to 0.6). The particle migration velocity is the speed at which a particle, once charged, migrates toward the grounded collection electrode. Variables affecting ω are particle size, the strength of the electric field and the viscosity of the gas. The theoretical drift velocity of a spherical particle (or particle settling velocity in ESP) subjected to constant gas flows and electrostatic fields in an ESP is given by

C is the Cunningham correction factor, μ is gas viscosity (kg m⁻¹ s⁻¹), ε₀ is permittivity of free space (8.85 10⁻¹² C V⁻¹ m⁻¹), ε is dielectric constant for the particle relative to free space and E is average field strength (V m⁻¹). The drift velocity is also related to the corona power, P_c (W):

where k is an adjustable constant. The drift velocities are usually in the range 0.03 and 0.2 m s⁻¹ [27]. After substitution of eq. (32) into the Deutsch-Anderson equation, eq. (32) the collection efficiency can be calculated as

Based on the shape of the collection electrode and depending on the mode of operation there are several types of ESPs: dry and wet wire-plate ESPs as well as dry and wet wire-pipe (or tubular) ESPs [10]. The discharge electrodes are commonly in the form of a wires (made from noncorrosive materials like tungsten or alloys of steel and copper), because of small radius of curvature. In the wire-plate ESP the high voltage electrodes are long wires (discharge electrodes) that are weighted and hang between the plates (collection electrodes). In this case the waste gas flows horizontally and parallel to vertical plates of ESP. Plates are usually 1–2 m wide, 3–6 m high and plates are at distance between 15 and 35 cm [10]. In the wire-pipe type precipitator the discharge electrodes are in the form of wires and the collecting electrodes consist of parallel pipes which may be round, square and octagonal. The pipe electrodes are 2–5 m high and about 30 cm (or less) in diameter. The space between the electrodes is in the range 8–20 cm. In dry ESPs the PM is removed from the collecting electrodes by rapping, while in the wet precipitator water or any other fluid can be used for removal of the solid particulates. Wet precipitators are more efficient, but dry type plate precipitators are mostly used in the practice. The most important fields of their application are: cement factories, steel plants, chemical industry, petroleum industry.

3.3.3 Fabric filters

Fabric filtration is a well-known physical technique in which a gas stream containing dry particles passes mostly through a porous fabric medium, which retains the solids. During this process, the contaminated gas flows into and passes through a number of filter bags placed in parallel, leaving the solid particles retained by the fabrics. The air pollution control equipment where this process takes place is called bag house. The fabric is a filter medium; however it is also a support for the layer of dust (dust cake or filter cake) that accumulates on it. This dust layer can significantly increase the efficiency of filtering, especially for very small particles. However, these filter bags must be periodically cleaned, because with the increase in the thickness of the filter cake also increases the pressure drop or the air resistance through the filter. The filter bags are usually cleaned by rapping, shaking or vibration as well as by the reverse air flow, causing falling of the filter cake into the hopper. Generally, the economy of filtration depends on separation efficiency, pressure drop and lifetime. According to some opinions, fabric filters belong to the most effective dust removal devices. Therefore, they are widely used in industrial application (steel industry, foundry, cement industry, power stations, etc.), although depending on the applied filter medium, they may be quite sensitive to high temperatures and humidity.

According to the chemical composition filter medium can be natural or man-made fibers, plastic, ceramic, glass or metals. The choice of fabrics is determined by the following conditions: temperature, gas composition and/or presence of acids or alkalis in a gas streams, gas flow rate as well as size and shape of dust particles and its concentration. The filter medium can be fibrous (e. g. cloth), granular (e. g. sand), a rigid solid (e. g. screen) and a mat (e. g. a field pad). It can be made in different forms, i.e. in the shape of a bag (the most common form), tube or sheet with a number of the individual filter units housed together in a bag house as well as in the form of bed or fluidized bed. Generally, filters can be divided into the following types: fabric or cloth filters (surface filters) and fibrous or deep bed filters (depth filters). Surface filters usually compete with cyclones and ESPs for treatment of gas stream contaminated with particulates. In the cloth filters, the filter is in the form of tubular bags, while in the case of deep-bed filters, a fibrous material (cotton, wool, cellulose, etc.) has the role of separator and the collection of particulates takes place in the interstices of the bed. The number of filter bags may vary between a few hundred and a few thousand. The diameter of a bag filter ranges from 0.1 to 0.4 m and the height may be up to 10 m. The velocities at which a gas stream is passed through the bags are very low 0.4–1 m min⁻¹ [26]. The total pressure drop through a filter bag, ΔP (N m⁻²) is given as the sum of the pressure drop owing to the fabric, ΔP_f and the drop owing to the caked or adhered particles, ΔP_p [22]:

where u is superficial filtering velocity (m min⁻¹), μ is gas viscosity (kg m⁻¹s⁻¹), x_f and x_p are thicknesses of the filter and the particulate layer (m), K_f and K_p are permeabilities of the filter and the particulate layer (m²) (values of K_f and K_p must be determined experimentally for each type of fabric and type of dust removed) and 60 is conversion factor (s min⁻¹). The superficial filtering velocity, u is also known as the air/cloth ratio and can be determined as the ratio of the volumetric gas flow rate, Q (m³ min⁻¹) and the cloth area, A (m²). Excessive air/cloth ratio lowers particulate removal efficiency.

Ceramic and metal filters also belong to the surface filters. There are more resistant at much higher temperature than fabric filters (up to 1000 °C in the case of metal filters) and also they have higher chemical resistance. The process of filtering and removal of particles from the contaminated gas is comparable to that on fabric filters and only significant difference is in the filtering material. There are some special designs applicable for removal of compounds such as HCl, NO_x, SO_x and dioxins, but the process of filtering in these cases involve the presence of catalysts and the injection of some reagents [10].

Deep-bed filters can be filled with granular materials as well as with fibers. They are usually composed of mats of wood, cellulose, glass or iron fibers. The most important filed of their application is in air conditioning, heating and ventilating systems as well as for the cleaning under the conditions of light dust loads. The advantage of a granular bed over a fiber filter bed can be the greater structural strength of a packed bed. Deep-bed fiber filters have the potential to remove the particles that are too small to be removed efficiently by any other type of surface filters. The low capital costs of small fiber filters make them very attractive for small gas streams (e. g. household furnace filters, air filters for automobiles).

3.3.4 Wet dust scrubbers

Wet collectors or scrubbers have several important advantages in comparison to other air pollution control devices. The most important advantages of scrubbers are: low initial cost, moderately high collection efficiency for small particles and applicability for the high temperature installations. They can handle large volume of gases and collect dust particulates like flammable, sticky and explosive dusts with a little risk. Additionally, wet scrubbing (or absorption) is process which can be used for the removal of gaseous pollutants (such as SO₂, NH₃, H₂S, hydrogen halides, VOCs) as well as for the removal of mists, fumes, heavy metals and suspended dusts [28]. They also provide cooling hot gases and neutralization of corrosive gases and dust. Based on the main purpose of their using, scrubbers can be divided on the wet gas scrubbers and the wet dust scrubbers. Special configurations of the so called dry scrubbers belong to the additional group scrubbers, which are very efficient for the elimination of the problematic liquid waste stream, acid and odorous gases by means of a dry sorbent injection (mostly alkaline material) into a gas stream. The dry scrubber is usually followed by a bag house to clean the effluent to acceptable levels.

Wet dust scrubbers are devices that remove PM by direct contact of the dirty gas stream with liquid drops (mostly water) (Figure 8). Scrubbing is usually carried out by intensively mixing of the incoming gas stream with water in combination with the removal of particles by means of centrifugal force. 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. By replacement of water with other aqueous solutions (lime, potassium carbonate or slurry of MgO) it is possible to perform simultaneous removal of gaseous and solid pollutants from a gas streams. Different types of scrubbers can be found on the market. Most of them can be grouped according to the contacting mechanism with liquid. The most popular scrubber designs which vary in terms of both function and performance are: spray tower scrubber, Venturi scrubbers, cyclone scrubbers, packed scrubbers and mechanically aided scrubber. According to energy requirements they can be classified as low-, moderate- and high-energy scrubber units. The simpler types of scrubbers with low energy requirements are efficient in collecting particles above 5–10 µm in diameter, while the more efficient, high energy input scrubbers are favorable for separation of very small particles (1–2 µm). Energy requirements are related to the pressure drop across the scrubber or to the level of contacting power. Obviously, the disadvantages of scrubbers are previously mentioned high power consumption for higher efficiency, moderate to high maintenance costs (corrosion and abrasion) as well as disposal of waste sludge which can be very expensive.

Figure 8:

Spray tower with counter-current flow (inspired by [29]).

A key parameter in the design of scrubbers is the liquid-to-gas ratio (L/G). This ratio is determined by the solubility of the gas pollutants, concentrations of pollutants and particulates in the gas stream and the mass transfer properties of the scrubber. The increase in the (L/G) increases the separation efficiency of the system, so finding the optimum ratio is very important for the balancing of performance with operation costs. Besides energy needs and requirements for performance efficiency, the other important parameters for the selection of scrubber for a particular application include compatibility of materials used in the construction of the scrubber with the chemical (acidity, reactivity), physical properties of the gas stream (abrasivity), optimal operating temperature, need for addition reagents, etc.

In the spray tower scrubber the washing liquid is sprayed by spray nozzles, fast-spinning nebulizer disk or rotating sprays, creating a large contact surface for the drops and the inlet gas. Droplet size is controlled to optimize particle-liquid contact and to provide easy separation of droplets from the air stream. Very high (L/G) are required (>3 l m⁻³) to capture fine particles. Power consumption for spray tower scrubber is low and they operate with a low pressure drop. Spray tower are capable for removal of particles <8 µm with 90 % efficiency [22]. Cyclone spray scrubbers combine the capture techniques of cyclones and spray towers. Gas streams enter into the device tangentially at high speeds. The high speeds in combination with the centrifugal force promote droplets separation, allowing the use of a smaller droplet size, which also increases collection efficiency (95 % for particles >5 µm). They are more efficient than spray towers and have lower liquid requirements, but require more power, due to higher pressure drop. They are preferred over spray towers for gas streams with heavier particulate loads. Venturi wet scrubbers are applied for removal of fine particulate from corrosive, high temperature, hazardous or volatile gas streams. These are scrubbers with a Venturi-shaped chamber including converging and diverging sections. Water is injected at low pressure into the throat of the Venturi, through which the gas stream passes at high velocities. The energy from the gas atomizes the liquid, allowing particles and pollutants to be entrained in droplets. Venturi jet scrubbers use a modified design in which liquid at high velocity is jetted (injected) into the throat of a Venturi, rather than the gas stream. Venturi scrubbers have very high separation efficiencies for particulate pollution (>98 % for particles >0.5 µm) and are simple to install and maintain. However, they require large pressure drops leading to higher power requirements than other scrubber designs. Other often used designs involve packed scrubbers and mechanically aided scrubber. Packed scrubbers are occasionally used for particulate control, but they are restricted to separation of fine and/or soluble particulates, aerosol and mists, because of possible blockage of nozzles and plugging of bed. In line with this, they are limited to applications with low particle content (<0.5 g m⁻³). Scrubber can also use mechanically enhanced gas-liquid contact. However, the main disadvantage of mechanical scrubber is high maintenance cost.

3.4.1 Absorption

Absorption (also called gas absorption, gas scrubbing or gas washing) is the standard method of removing any compounds from a gas stream, which can be used if is possible to find a liquid solvent in which the gaseous pollutant is more soluble than the other compounds in the gas stream [10, 25]. The species transferred to the liquid phase are referred to as solutes or absorbate. This is probably one of the most important techniques in the control of gaseous pollutant emission, although the most important disadvantage of this process is the generation of waste water. The most important fields of application this technique include control of SO₂ and NO_x from combustion sources, removal and recovery of NH₃ in fertilizer production, removal of HF from glass furnace exhaust, control of odorous gases from rendering plants, recovery of water-soluble solvents such as acetone and methyl alcohol, etc. Absorption takes place in the absorption unit (absorber or scrubber) and basic principle of this process is the selective mass transfer of gaseous substance from the gas to liquid phase, owing to the preferential solubility of a gaseous component in the liquid. During this process the substance can be only dissolved in the liquid phase (physical absorption) or may react with the liquid or with a specific substance contained in the liquid (chemical absorption). As mentioned previously, water is commonly used liquid or absorbent. However, gases of very little solubility such as SO_x, H₂S and NO_x, can be absorbed readily in an alkaline solution such as NaOH [10]. Thus, when water is used as the solvent, it may contain added species, such as acids, alkaline, oxidants or reducing reagents to react with the gas being absorbed and to enhance its solubility. Absorption is favorable process for the recovery of products or for purification of a gas streams that have high concentration of organic compounds, otherwise the process is not economically acceptable. Generally, absorber can achieve separation efficiency higher than 95 %.

Basic principles of the process operation are described previously (see Section 3.3.4). The packed column absorber/scrubber is most commonly used technical device for separation of gaseous pollutants. The packed absorbers/scrubbers are usually filled with an inert material such as plastic or ceramic with the aim to increase the gas-liquid interface. The adsorbent can be in a wide variety of physical forms such as pellets in a thick bed, small beads in a fluidized bed or fibers pressed onto a flat surface. There are many types of packing and some of the commercially used are Raschig rings, Berl saddles, Pall rings and Intalox saddles. Generally, it is required for the packing to have large surface area per unit volume and low pressure drop of gas through it. Bubble caps and sieve trays can be also used in the interior of the absorption/scrubbing device to enhance counter-current contact between the liquid and gas stream. The main elements of the absorber design are: material balance to determine a liquid circulation rate, calculation of a packed absorber high or number of plates (depending on the type of absorber selected for the process) and determination of the absorber diameter sufficient to handle the required liquid and gas flow rates. The process opposite to the gas absorption is the stripping (Figure 9). After the solubility equilibrium is achieved during the process of absorption, the liquid which contains most of the gaseous compound removed from the waste gas, passes to the stripper – another device which works at much higher temperature and/or at much lower pressure than the absorber. Because of this the solubility of the gas in liquid is reduced, the gas comes out from the liquid and after the cooling it can be sent to storage. At the same time, the stripped or recovered liquid can be sent back to the absorber unit.

Figure 9:

Absorption-stripping system.

3.4.2 Adsorption

Adsorption is another type of equilibrium separation process; in this case it involves a gas-solid equilibrium. Adsorption means the attachment or the binding of gaseous and vaporous molecules to the surface of a solid adsorbent or in the pores or interstices of adsorbent material. The adsorbing solids are usually called adsorbent, while the gas which is adsorbed is referred to as the adsorptive (Figure 10).

Figure 10:

Simple illustration of gas adsorption (adopted from [23]).

Adsorption may be physical or chemical (chemisorption), depending on the strength of molecules binding to a surface of adsorbent. Physical adsorption (reversible process) is used primarily for the control of organic compounds, while chemical adsorption (irreversible process) is frequently used for the control of acid gases (hydrogen chloride, hydrogen fluoride, hydrogen sulfide) and mercury vapor. Depending on the flow rate of the polluted air, the adsorption process involves one or several adsorbers that operate in parallel. Adsorption is an exothermic process, because in the adsorption process the molecule loses kinetic energy. Contrary, the reverse process of adsorption is desorption and this process is endothermic. Conventional adsorption plant usually consists of two devices: one for the adsorption cycle and another for desorption, because the adsorbent must be regenerated after adsorption in order to be available for another process. Adsorption is mostly used to capture, concentrate and recovery a pollutant that is present in dilute form in waste stream. This means that adsorption may result in the recovery of economically valuable final product or in disposal of pollutants after previous treatment if there is a need. Separation efficiency of 95–99 % can be achieved using the adsorption process based on activated carbon as adsorbent [26].

The most often used adsorbent are activated carbon, silica gel, alumina, molecular sieve, polymer-based materials and zeolites. The common types of adsorbent equipment are thin bed absorbers and deep bed adsorbers. As the names of devices imply, the thin bed adsorber consists of a thin layer of adsorbed solid, which is preferable if adsorption is very quick process, while another type of adsorber is usually based on a deep bed of adsorbent to adsorb contaminants from the air that is heavily polluted.

The adsorbers usually work in the temperature range between 30 and 60 °C. Besides on temperature, the rate of adsorption is depended on the concentration of the adsorptive in the gas phase, the surface area and pore volume of the adsorbent and other properties of adsorbent (e. g. porosity, adsorption capacity, selectivity) and adsorptive. Regeneration of the adsorbent is usually carried out by increasing the temperature, by means of water vapor desorption at temperatures of 100 °C (and higher), by contact with a hot inert gas or by decreasing the total pressure (under vacuum). Reactivation of adsorbent in the case of chemisorption can be performed at the high temperatures (ca. 700 °C, depending on adsorbent nature). Design and analysis of adsorption system involves heat and mass transfer, fluid dynamics, process control and chemical analysis. More information on design of adsorbers can be found in [22, 23].

3.4.3 Condensation

Condensation is a separation process used to convert one or more volatile components of a vapor mixture to a liquid through saturation process. Condensation can be accomplished by reducing the temperature or by increasing the pressure. The first approach is more often used in the practice, because an increase in the vapor pressure can be expensive [30]. Condensers are typically used as a pre-treatment device to reduce the volume of the effluent gas before treatment by other devices. They can be also used for the reducing emissions from the high concentration gas streams to recovery of economically valuable products in a waste stream or to remove corrosive components and components that can damage other part of the system. Two most important representatives of condensers are contact condensers and surface condensers. In contact condenser the contaminated gas stream comes into contact with cold liquid by spraying ambient or chilled liquid directly into the gas stream. The direct mixing of the coolant and contaminant causes separation or extraction before coolant reuse. This additional separation process may lead to a disposal problem or secondary emissions. In the surface condensers, the coolant does not mix with the gas stream (which is advantage comparing to contact condensers), but contacts a cooled surface in which cooled liquid or gas is circulated. Surface condensers require less water and generate 10–20 times less condensation than contact condensers do. Removal efficiency of condensers range from 50 % to 95 %, depending on theirs design and application.

3.4.4 Chemical waste gas treatment

Chemical waste gas treatment methods involve process of incineration (thermal oxidation and catalytic oxidation) and flaring, in which an organic pollutant can be converted to CO₂, water and some inorganic gases. Incineration (or combustion) refers to the rapid oxidation of pollutants through the combination of oxygen from the air with supplementary combustible fuel in the presence of heat. The chemical conversion of pollutants (mostly gaseous but also solid and liquid particles) contained in waste gases is the most efficient process in the air pollution control which results in the complete degradation of almost all pollutants, independent on their chemical properties. Equipment for control waste gases by incineration can be categorized as: thermal oxidizers or afterburners, catalytic oxidizers and direct combustor or flare.

The thermal oxidizer involves specifying a temperature (650–825 °C) of operation along with a desired residence time (0.5–1.0 s) and then optimum sizing the device to achieve the desired residence time and temperature with proper flow velocity. Efficiency of thermal oxidation can be higher than 99 % when operated correctly. However, if the process of ignition is not complete, some pollutants can be released into the atmosphere, which is undesirable. In the catalytic oxidation the presence of catalyst is responsible for achievement of high reaction rate at much lower temperatures (315–485 °C), which results in considerable savings in the fuel costs. The space requirement for operation of the catalytic oxidation is also much smaller. However, the costs of the catalytic systems are higher than those of the thermal oxidation systems, because of expensive catalysts and associates systems. Anyway, application of catalytic afterburners is favorable because of possibility to avoid formation of the thermal NO_x, dioxins and other undesirable products. Thermal NO_x are formed at the high temperatures (>1300 °C) due to complex reaction of oxygen and nitrogen with high energetic free radicals (O, N, OH, H and hydrocarbons that have lost one or more hydrogen). The catalysts are usually based of combination of a noble and other metals (Pd, Pt, Cr, Mn, Cu, Co, and Ni) deposited on an alumina monolithic support to minimize the pressure drop, which is one of the critical parameters of the incinerator designs. Limitation of the catalytic systems is catalyst deactivation, due to the presence of contaminants, attrition or catalyst sintering. Catalytic systems are suited for a gas stream with low VOC content. Depending on the types of heat recovery, both thermal and catalytic incinerators can be classified into two categories: recuperative and regenerative. The recuperative incinerator can recover about 70 % of the waste heat from the waste gases, while the regenerative incinerator can recover up to 95 % of the energy. Flare gas systems are used in a wide variety of applications and are generally used to burn off excess gas, usually hydrocarbons. A flare gas system may contain open flame flares or combustors (flame enclosed) with small differences between them. A flare can be used to control almost any emission stream containing VOCs [23, 28]

3.4.5 Biological waste gas treatment

In the biological control systems gaseous pollutants are converted into less harmful or harmless substances by microorganisms. Almost all kind of organic (hydrocarbons, chlorinated hydrocarbons, ketones, esters, aldehydes, odors) and inorganic pollutants (NH₃, NO_x, H₂S, CS₂) can be treated by microorganisms. There are two groups of microorganisms that may be used for this purpose: autotrophic bacteria which are particularly suitable for the conversion of inorganic substances and heterotrophic bacteria which are well suited for the conversion of organic pollutants. The microorganisms oxidize the pollutants and produce cell substrate, CO₂, water and inorganic salts. This process takes place in the presence of water and it is possible only for the very low pollutant concentration. Biological systems are especially suitable for degradation of VOCs, odors and toxic compounds. Obviously, they are similar to biological systems used for treatment of the wastewater and can be divided into three categories: biofilters, biotrickling filters and bioscrubbers (Table 3).

Biofilms of microorganisms are grown on porous media in biofilters and biotrickling filter, while a contaminated gas stream is passed through a porous packed bed at a low velocity. The main difference between those two systems is that liquid phase is stationary in a biofilter and moves through the porous media in a biotrickling system. Bioscrubbing combines wet gas scrubbing (absorption) and biodegradation. The scrubbing water with suspended microorganisms is recirculated through the reactor and the absorbed pollutants are degraded in aeration sludge tank. Bioscrubbing is suitable for removing of readily biodegradable compounds, such as NH₃, amines, hydrocarbons, H₂S and odorous pollutants. Biofiltration mainly use compost as the porous media is cheapest and thus widely used biological waste gas treatment for the removal of low concentration of pollutants that are easily soluble in water [10, 22].

Readers interested in the additional information on design and scale-up principles and operational guidance regarding to the control devices for cleaning gaseous pollutants can refer to available literature [5, 10, 13, 15, 23, 31].