Unit 5

Types of Electrostatic Precipitators• An electrostatic precipitator (ESP) uses nonuniform, high-voltage fields

to apply large electrical charges to particles moving through the field. • The charged particles move toward an oppositely charged collection

surface, where they accumulate.• There are three main styles of electrostatic precipitators: • (1) negatively charged dry precipitators - The negatively charged dry

precipitators are the type most frequently used on large applications such as coal-fired boilers, cement kilns, and kraft pulp mills

• 2) negatively charged wetted-wall precipitators - Wetted-wall precipitators (sometimes called wet precipitators) are often used to collect mist and/or solid material that is moderately sticky

• (3) positively charged two-stage precipitators - The positively charged two-stage precipitators are used only for the removal of mists.

• Essentially all of these units are divided into a number of separately energized areas that are termed fields .

• Most precipitators have between three and ten fields in series along the gas flow path.

• On large units, the precipitators are divided into a number of separate, parallel chambers, each of which has an equal number of fields in series.

• There is a solid partition or physical separation between the 2 to 8 chambers that are present on the large systems

Arrangement of fields and chambers in ESP

Advantages and Disadvantages of ESPs

• Electrostatic precipitators can have very high efficiencies due to the strong electrical forces applied to the small particles.

• These types of collectors can be used when the gas stream is not explosive and does not contain entrained droplets or other sticky material.

Resistivity

• The composition of the particulate matter is very important because it influences the electrical conductivity within the dust layers on the collection plate.

• Resistivity, an important concept associated with electrostatic precipitators, is a measure of the ability of the particulate matter to conduct electricity and is expressed in units of ohm-cm.

• As the resistivity increases, the ability of the particulate matter to conduct electricity decreases.

• Precipitators can be designed to work in any resistivity range; however, they usually work best when the resistivity is in the moderate range (108 to 1010 ohms-cm).

Fabric filters• Fabric filters collect particulate matter on the surfaces of filter

bags. • Most of the particles are captured by inertial

impaction, interception, Brownian diffusion, and sieving on already collected particles that have formed a dust layer on the bags.

• The fabric material itself can capture particles that have penetrated through the dust layers.

• Electrostatic attraction may also contribute to particle capture in the dust layer and in the fabric itself.

• Due to the multiple mechanisms of particle capture possible, fabric filters can be highly efficient for the entire particle size range of interest in air pollution control.

Types of Fabric Filters• Reverse-air-type fabric filter, is used mainly for large industrial

sources. • In this type of unit, the particle-laden gas stream enters from the

bottom and passes into the inside of the bags. • The dust cake accumulates on the inside surfaces of the bags. • Filtered gas passes through the bags and is exhausted from the unit.• When cleaning is necessary, dampers are used to isolate a

compartment of bags from the inlet gas flow. • Then, some of the filtered gas passes in the reverse direction (from

the outside of the bag to the inside) in order to remove some of the dust cake.

• The gas used for reverse air cleaning is re-filtered and released.

Pulse jet fabric filter• In Pulse jet fabric filter the bags are supported on metal wire cages

that are suspended from the top of the unit.

• Particulate-laden gas flows around the outside of the bags, and a dust cake accumulates on the exterior surfaces.

• When cleaning is needed, a very-short-duration pulse of compressed air is injected at the top inside part of each bag in the row of bags being cleaned.

• The compressed air pulse generates a pressure wave that moves down each bag and, in the process, dislodges some of the dust cake from the bag.

Advantages and Disadvantages of Fabric Filters

• Fabric filters are used in a wide variety of applications where high efficiency particulate collection is needed.

• The control efficiencies usually range from 99% to greater than 99.5% depending on the characteristics of the particulate matter and the fabric filter design.

• Fabric filters can be very efficient at collecting particles in the entire size range of interest in air pollution control.

• The performance of fabric filters is usually independent of the chemical composition of the particulate matter.

• However, they are not used when the gas stream generated by the process equipment includes corrosive materials that could chemically attack the filter media.

• Fabric filters are also not used when there are sticky or wet particles in the gas stream. • These materials accumulate on the filter media surface and block gas movement.• Fabric filters must be designed carefully if there are potentially combustible or explosive

particulate matter, gases, or vapors in the gas stream being treated.• If these conditions are severe, alternative control techniques, such as wet scrubbers, are

often used.

Particulate Wet Scrubbers

• VenturisImpingement and Sieve PlatesSpray TowersMechanically AidedCondensation GrowthPacked BedsEjectorMobile BedCaternary GridFroth TowerOriented Fiber PadWetted Mist Eliminators

Venturi Scrubbers

• Particulate matter, which accelerates as it enters the throat, is driven into the slow moving, large water droplets that are introduced near the high velocity point at the inlet of the venturi throat.

• The adjustable dampers in the unit illustrated are used to adjust the open cross-sectional area and thereby affect the speed of the particles entrained in the inlet gas stream.

Impingement Plate Scrubbers

Spray Tower Scrubbers

Scrubber Operating Principles• All particulate wet scrubber designs utilize particle and/or droplet inertia as the

fundamental force to transfer particles from the gas stream to the liquid stream. • Within the scrubber, particle-laden air is forced to contact the liquid droplets,

sheets of liquid on a packing material, or jets of liquid from a plate. • Particles with too much inertia impact on the water droplet, water sheet, or

water jet instead of passing around the "target" with the gas stream.• The ability of a particulate wet scrubber to remove particles depends on two or

more of the following variables:• The size (aerodynamic diameter) of the particle

• The velocity of the particle

• The velocity of the droplet, sheet, or jet

Collection Efficiency of Wet Scrubbers

• All of the particulate wet scrubbers in commercial use depend on inertial impaction.

• However, the velocities of the particle-laden gas stream and the liquid targets vary substantially.

• Accordingly, there are substantial differences in the ability of particulate wet scrubbers to collect particles less than approximately 5 micrometers.

• If a significant portion of the particulate matter mass is composed of particles less than 5 micrometers, care is needed to select the type of scrubber that is effective in this size range.

• It should be noted that some types of wet scrubbers have limited capability to remove particles in the less than 0.3-micrometer range.

• Methods of particle collection in this very small size range take advantage of these particles' tendencies to diffuse slowly due to their interactions with gas molecules (Brownian diffusion).

• In other words, these particles are so small that their movement is influenced by collisions with individual molecules in the gas stream.

Advantages and Disadvantages of Scrubbers

• Many types of particulate wet scrubbers can provide high efficiency control of particulate matter.

• One of the main advantages of particulate wet scrubbers is that they are often able to simultaneously collect particulate matter and gaseous pollutants.

• Also, wet scrubbers can often be used on sources that have potentially explosive gases or particulate matter.

• They are compact and can often be retrofitted into existing plants with very limited space.

• One of the main disadvantages of particulate wet scrubbers is that they require make-up water to replace the water vaporized into the gas stream and lost to purge liquid and sludge removed from the scrubber system.

• Wet scrubbers generate a waste stream that must be treated properly.

Combustion Odours and their control.

• Development close to the site is to be excluded.• A reasonable "buffer zone" around the area sources has to

be determined. • The actual size of this zone will depend upon a number of

factors, including the size of the area from which odours emanate, the intensity of the odours being emitted, the duration and frequency of the odour emissions, the actual process being undertaken, the topography of the site, the weather conditions that prevails at the site.

• Green belt development in the buffer zone may help at least partially to obfuscate the odour.

• Ensuring that the operation is carried out under the best management practice• Best management practices (BMP) will vary according to the industry producing the

odour. • However, for all new developments, BMPs will start with the site selection and the

building of the facilities.• Nozzles, sprayers and atomizers that spray ultra-fine particles of water or chemicals

can be used along the boundary lines of area sources to suppress odours.• Rotary atomizer is one such technique widely recommended for adoption for effective

control of odour in case of area sources. • The Atomizer uses centrifugal action by a spinning inner mesh to force droplets on to

an outer mesh which "cuts" the water into atoms • The rotary atomizer produces millions of microscopic droplets of water -- up to 238

billion from a single litre droplets that are thinner than a human hair and a fine spray which covers up to 30 linear metres.

• This creates a fine mist, which is more effective with minimal use of water and electricity.

• There are a large number of chemicals and proprietary products that claim to reduce odour when they are applied to area sources.

• Atmospheric odours that are contained in a restricted area can be oxidized by atomization of the chlorine dioxide.

• Odour from sources such as holding ponds, lagoons, and sewage pre or post treatment effluent can be controlled by atomized spray of chlorine dioxide.

• To reduce odour, chemicals have to be applied over very large area, the cost of materials and labors would be very high.

• The large quantity of these compounds required could themselves cause pollution.

• The spray/ atomizer techniques are used to conceal odours also from building and fugitive sources.

• Dispersion method is the simplest of the methods that can be adopted for odour abatement.

• This is nothing but to release odorous gases from tall stack. It results in normal dispersion in the atmosphere and consequent decrease in ground-level concentration below the threshold value.

• Dispersal by stacks requires careful consideration of the location & meteorological parameters, etc. In general, dispersion of odour emissions via chimneys is not a recommended method.

Control of odour from gas streams• Mist filtration• Thermal oxidation/ Incineration• Catalytic oxidation• Biofiltration• Adsorption• Wet scrubbing/Absorption• Chemical treatment• Irradiation• The choice of the technology is often influenced by the following factors:• The volume of gas (or vapor) being produced and its flow rate• The chemical composition of the mixture causing the odour• The temperature• The water content of the stream

Mist filtration

• While gases cause most odour, problems may also result from aerosols in the fumes.

• Odorous air streams frequently contain high concentration of moisture.

• If these vapour discharge can be cooled to less than 40° C, a substantial quantity of the water vapour will be condensed and so reduce the volume of gases to be incinerated.

• Mist filters can be used for this purpose. Mist filters can also remove solids and liquids from gas stream; if the odour is caused by these particles, then it will result in odour reduction.

Thermal oxidation/ Incineration

• Thermal oxidation/ incineration is the oxidation of the odour into carbon dioxide and water by the combustion of the odour with fuel and air.

• The reaction takes place at temperatures ranging from 750oC to 850oC. • This is generally above the auto-ignition temperature of most solvents and other VOCs

and is a reflection of the heat required to maintain the reaction at dilute concentrations with additional process heat losses.

• In this regime, the destruction efficiency is almost 100%, assuming adequate oxygen supply. In some cases, other compounds may be formed depending on the mixture of fuel and air used, the flame temperature and the composition of the odour.

• These compounds may include carbon monoxide, oxide of nitrogen and sulfur oxides. • Thermal oxidizer is a refractory-lined furnace fitted with one or more burners. • The furnace consists of two chambers- mixing chamber and combustion chamber.

There are 3 types of thermal oxidizer:– Direct-fired thermal oxidizer– Recuperative thermal oxidizer– Regenerative thermal oxidizer

Direct-fired thermal oxidizer• It is effectively a combustion chamber with a burner and the appropriate

control system. • The exhaust from a direct-fired unit is typically at the combustion temperature

with no primary or secondary heat recovery. • This is used where heat recovery is not required (e.g. when fuel for the burner

is free or very cheap).• In many cases the fuel cost of heating a process stream to the combustion

temperature leads to the inclusion of some sort of heat recovery mechanism.• Where the level of VOC is significant, then the heat release from the VOC can

be recovered to improve the cost effectiveness of the system. • Both recuperative and regenerative thermal oxidizer technologies include heat

recovery systems to recover heat as a utility for other energy requirements.

Recuperative systems

• They are basic thermal oxidisers with built-in primary shell and tube heat exchangers.

• A primary heat exchanger can recover up to 70% of the heat input by the burner or released during the oxidation process by heating up the inlet stream thus reducing the required burner load to maintain the required oxidation temperature (typically 750oC-800oC).

• These are a simple, cost effective, means of destroying VOC where the inlet concentration is relatively high or particularly where heat can be usefully recovered for other processes.

Regenerative thermal oxidizer (RTO

• It is the most often used type of thermal oxidizer because of its robust performance and its ability to operate at high thermal efficiency.

• The RTO utilizes beds of ceramic media to provide the thermal efficiency. • Two or more beds are used in a controlled cycle and alternatively operate

to heat incoming air and to cool exit air. The unit can operate at thermal efficiencies of between 80% and 98% and can handle most types of fume.

• This means that where an exhaust stream contains a significant level of VOC, then auto-thermal burning (without the use of burners) is possible.

• At lower concentrations also, the RTO often provides cost-effective operation because of its very high thermal efficiency.

Catalytic oxidation

• Catalytic oxidation reaction can be forced to proceed at much lower temperatures (e.g. 200oC) in the presence of a catalyst

• Thus, the advantage of this process over thermal oxidation is the reduction in required energy input.

• Catalytic systems are therefore more favourable where auto-thermal operation is not practical and heat cannot be economically used elsewhere

Biofiltration •

This method is becoming an acceptable and successful way of reducing odours from biological process.

• Biofiltration is a natural process that occurs in the soil that has been adopted for commercial use.

• Bio-filters contain micro–organisms that break down VOC’s and oxidize inorganic gases and vapors into non–malodorous compounds such as water and CO2.

• The bacteria grow on inert supports, allowing intimate contact between the odorous gases and the bacteria.

• The process is self-sustaining. Bio-filters constructed of various materials including compost, straw, wood chips, peat, soil, and other inexpensive biologically active materials.

Adsorption

A method that is suitable for controlling odorous substances, even at low concentrations, is adsorption on to activated carbon. • For effectivity, the contaminated air stream must be free from dusts and particulates that

might clog the carbon particles. • Regeneration of carbon for re-use will produce either waste water, which will require

further treatment before disposal, or a concentrated vapour stream, which can be incinerated more cheaply than the original air stream.

• There are also systems that use activated alumina impregnated with potassium permanganate for adsorption. The alumina absorbs the odorous substances so that the permanganate can oxidize them, usually to carbon dioxide, water, nitrogen and sulfur dioxide, depending on their composition.

• The alumina bed is replaced progressively as the permanganate is exhausted. • This has an advantage over carbon because no further treatment is needed; this may

offset the cost of alumina.

Wet scrubbing/ Absorption

• Wet scrubbing of gases to remove odour involve either absorption in a suitable solvent or chemical treatment with a suitable reagent. It is important that hot, moist streams are cooled before they contact scrubbing solutions.

• If this is not done the scrubbing solution will be heated and become less efficient, the scrubbing medium will become diluted from condensation of water vapour.

Wet scrubbing or absorption systems can be either ventury systems or packed tower systems.

• Venturi systems are co-current scrubbers that accelerate the gas stream into a high density liquor spray.

• The aqueous droplets then impinge or impact at high relative velocity with solids in the gas stream. The resulting conglomerated particle is then separated from the gas stream in a disengagement tower by virtue of inertial forces.

• The high density spray also provides reasonable mass transfer to the absorption of gaseous contaminants.

Packed Towers• They are typically counter current scrubbers that utilise high surface area media as a

contact zone for the gas stream with suitable scrubbing liquor. • The media facilitates high efficiency mass transfer to provide >99.9% removal of

gaseous contaminants.

When the odour is caused by the presence of unsaturated organic compounds, it may be necessary to use an oxidizing agent such as chlorine, diluted sulfuric acid and sodium hydroxide to treat odour.

Absorption is applicable when the odorous gases are soluble or emulsifiable in a liquid or react chemically in solution.

• Wet scrubbing is a useful process to handle acid gas streams, ammonia or streams with solids that might foul other equipment.

• It has been suggested that liquid scrubbing becomes economically attractive compared to incineration and adsorption on activated carbon when the volume of odorous gas to be treated is greater than 5000 cubic meters per hour.

Chemical treatment• Injecting controlled quantities of chemicals such as chlorine or ozone into process-gas stream can

control odour.• Similarly, unlike various other "odour control" treatments, chlorine dioxide will destroy the odour

at source. • Chlorine dioxide is several times more effective than chlorine and other commonly used

treatments, and will not form hazardous by products, such as chlorinated organic, which can cause more problems than the original odour itself.

• Odours arising from water bodies can generally be eliminated by adding the chlorine dioxide solution directly to the odoriferous fluid.

• The first action of chlorine dioxide is to rapidly oxidize the vapor gases dissolved in the fluid to their oxide form.

• As the dissolved gases are oxidized and the amount of chlorine dioxide will increase, next action of chlorine dioxide is the oxidation of small molecular material (micro-organisms), and, as the amount of chlorine dioxide will further increase, the larger molecules and compounds are oxidized.

• Due to this versatility, chlorine dioxide can be used in all aspects of the odour control process, from air scrubbers and wastewater treatment with stabilized chlorine dioxide solutions.

Irradiation

Ultra-violet irradiation can be used to control of odour. Here, the action is probably due to ozone formation or bactericidal effect.