Air pollution basically refers to the foreign materials found in the air. These foreign materials can occur naturally or can be manmade. Naturally occurring eruption normally happens during volcano eruption where lava is spewed onto land, poisonous gases with ash, sulfur dioxide (SO2) and Hydrogen sulfide, and emit particles are spewed into the atmosphere. Manmade air pollution is normally in high concentration in densely populated areas.
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Human can cause at least one contaminant that include vapor, dust, smoke, fumes, odor, gas or mist in quantities of duration, and characteristics into the atmosphere, resulting to injurious property, plant, or human or unreasonably obstructs of comfortable enjoyment of property and life (Wang, Pereira & Hung, 2004). Human activities contribute the highest percentage of the air pollution in the world. This prompted environmentalists to consider investigating on technologies that can be used to control air pollution while still achieving our daily operation goals. This paper reviews various technologies used today in air pollution control.
Technologies Used Today in Air Pollution Control
Flares refer to open flames employed for waste gases disposal during emergencies and normal operations. Flares describe a process of open combustion where oxygen to the flame is supplied by the surrounding air, and they can be operated either at elevated positions or at the ground level. Flares operated at elevated level employ steam injection to enhance combustion by augmenting turbulence or mixing and pulling in extra combustion air. Effectively operated flares can attain destruction efficiencies of about 98%. The flare technology is characteristically employed when the waste gases heating value cannot be economically recovered due to uncertain or intermittent flow or when the recovered product’s value is low. In some instances,, flares are run together with recovery systems of baseload gas. Flares address emergency and process upset gas release which the baseload system is not structured to recover.
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There exist various forms of flare which include pressure head, air assisted, and steam assisted. Distinctive flare operation can be categorized as endothermic or fired, non-smokeless, and smokeless. In a smokeless operation, flares employ outside source of momentum to offer enough gas-air turbulence and mixing for complete combustion. Smokeless flaring is needed for the damage of organics which are heavier than methane. Non-smokeless function is employed for organic or other steams vapor which readily burn without producing smoke. Endothermic or fired flaring needs extra energy so as to guarantee complete waste streams oxidation for instance in case of waste streams of ammonia and tail gas of sulfur (Wang, Pereira & Hung, 2004).
Thermal oxidation is an extensively employed method of air pollution control. This technique involves the oxidation of organic vapors at high temperatures. Incineration which involves both catalytic oxidation and thermal oxidation is regarded as an ultimate method of disposal whereby organic compounds in a stream of waste gas are not collected and instead they are converted to water, and carbon dioxide among other inorganic gases.
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In thermal oxidation, a high temperature of more than 1000oF is used to heat contaminant-laden waste gas, where the organic chemicals are burned with air in oxygen presence. The main advantage of thermal oxidation is that almost any gaseous organic stream can be cleanly and safely incinerated, as long as suitable design, maintenance, engineering, operation, and installation are done. The technique is also able to achieve high; 99% and above, destruction efficiency with extensive range of emission streams (Hao, Wu & Wang, 2005).
Catalytic oxidation also known as catalytic
incineration refers to the oxidation process which changes organic compounds to
water and carbon dioxide with the aid of a catalyst. In catalytic oxidation,
the flame-founded incineration concept is changed by a catalyst addition to
enhance the oxidation reaction, permitting minimized reaction temperature
and/or faster reaction. A faster reaction needs a smaller vessel, therefore
lowering capital, enhancing requirements for reduced auxiliary fuel, and low
operating temperatures, and thus, lowering operating costs. Catalytic
incineration is highly appropriate for emission streams treatment. This
emission streams normally has low volatile organic compounds (VOCs)
concentration. Catalytic oxidation might permit an operation that is more cost
effective compared to processes of thermal incineration. However, this
technology is not highly embraced compared to thermal oxidation since it has a
higher sensitivity to process conditions and pollutant characteristics.
Operating and design considerations are thus critical to catalytic oxidation
applications in air pollution control (Wang, Pereira & Hung, 2004).
Fabric filtration refers to a physical
process of separation where in a liquid or gas containing solid passes via a
medium of porous fabric where solids are retained. The process might operate in
a semi-continuous or batch mode, with periodic elimination of the solids
retained from the filter medium. Filtration systems might as well be structured
to work in a continuous way. Similar to other filtration methods, the
filtration bulk is performed by accumulating solid cake. Significantly, the
first filter cake layer must be created as the start of the filtration
operation. Fabric filtration controls environmental pollutants in liquid or
gaseous streams effectively. In the control systems of air pollution, fabric
filtration eliminates gaseous emission dry particles. It also lowers air
disposal problems (Wu, Ma & Zhang, 2018).
Wet and Dry Scrubbing
The scrubbing process is a single operation where in at least one gas stream components are absorbed into an absorbent selectively. There are two forms of scrubbing which include wet and dry scrubbing. Water is normally used as the most common absorbent liquor in wet scrubbing. Semidry slurry or dry powder is also applied as possible absorbents in dry scrubbing based on needs of a particular situation. Scrubbing is mostly experienced when flue gas is being treated to manage odors, acid gases, trace organics, particulates and heavy metals. In most cases a scrubbing system is comprised of more than one scrubber series. This is done to ensure an individual stage of a scrubber can use an absorbent unique to a targeted pollutants or pollutant. Higher total efficiencies of removal are frequently probable in a multistage system of scrubber compared to what could have happened with a single-stage scrubber. A good example of this is mostly found in the industry of rendering where both hydrogen sulfide and ammonia are produced in normal operations.
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A scrubber employing an acid-founded absorbent liquid is utilized to eliminate ammonia from the air. The physical principles in structuring a wet scrubber is to utilize an absorption liquid that gives a high pollutant solubility in the gas steam to be treated. The second principle is maximizing liquid-gas contact surfaces. Once the two conditions are adhered to, the pollutant will easily be absorbed into the liquid phase after diffuse out of the gas phase. Pollutant absorption in a gas phase into the liquid phase in contact with it theoretically happens when the liquid with below equilibrium pollutant concentration. This implies that pollutant found in the gas phase have to contain some level of solubility in liquid (Schnelle & Brown, 2002).
Condensation refers to the process of
separation where one or more vapor mixture volatile components are separated
the remaining vapors via saturation and then a change of phase. The change of
phase from gas to liquid can be attained either by increasing system pressure
at a particular temperature, or the system temperature might be lowered at a
particular pressure. Condensation happens when a volatile partial pressure
component applied on the vapor-phase goes past that of the liquid-phase
component. The unit operations are condensers basically employed to eliminate
volatile organic compound from streams of gas before applying other controls
that include absorbers or incinerators, though can in some cases be employed
alone to lower emission from the gas streams of high-VOC-concentration. A
distinctive condensation system contains the pumps, condenser, storage tanks,
and refrigeration system (Tang, Bi, Mortimer & Pan, 2019).
Biofiltration is a biochemical process that
involves immobilized, microorganisms on a solid biologically active support,
for an airstream chemical treatment. Biofilters have been utilized for
abatement of volatile organic compound (VOC), odor-causing compounds
mitigation, and can be used together with other technologies of treatment.
Biofilters have been a progressively popular option as a treatment choice due
to their low cost of operation, and comparatively low cost of capital in
contrast to other technologies. It works under the ground that the airstream
partition contaminants into a layer of aqueous on the bioavailable solid
support, and them degraded through the presence of microbial community. An
organic compound complete metabolism results to water and carbon dioxide, which
are them taken out of the biofilter. Generally, conventional biofilters is
regarded as the most fruitful in applications containing relatively low
contaminants concentration and low flow rates (Wang, Pereira & Hung, 2004).
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