Air Pollution Control – Control of Particulate Emissions
Air Pollution Control – Control of Particulate Emissions
Steel plant has several metallurgical processes which take place at high temperatures. Further many of these processes handle raw materials some of them are in the form of fines. Hence all these processes are prone to emit pollutant gases and particulate matter into the atmosphere. This, in fact, affects the quality of air around the plant. In order to improve and protect the quality of air, different pollution control devices are used for the control of the emissions.
Earlier for several years, pollution control equipments were used only for those processes where the pollutants amounts were very high or they were toxic in nature. These equipments were also earlier used where they had some recovery value. But today, with the environment regulations becoming more and more stringent and with the increasing concerns of the society regarding the environment, it has become necessary for the steel industry to look into its emissions and install equipments in all the areas to reduce the emissions to minimum possible levels. The emission control equipments are basically of two types (i) particulate emission control equipments and (ii) gaseous emission control equipment. This article describes the particulate emission control systems.
Particulate matter (PM) regulations adopted over the last thirty years have gradually shifted from regulating the coarse mode particles which comprised total suspended particle (TSP) to regulating the very small particles in the PM10 and PM2.5 size ranges. This shift has occurred primarily since the health effects research data indicated that small particles are most closely related to the adverse health effects
The regulation of particulate matter emissions dates back to the early stages of the industrial revolution. Even in the 1600s, people could see the relationship between particulate matter emissions and problems such as solids deposition, fabric soiling, material corrosion, and building discolouration. As technology and public awareness expanded, it became apparent that particulate matter emissions also contributed to certain types of lung disease and related illnesses.
In the late 1940s, several types of particulate matter control systems advanced from relatively rudimentary designs to forms which resemble present day, high efficiency systems. For example, electrostatic precipitators (ESPs) advanced from one-field, tubular units for acid mist control to one- field and two-field plate-type precipitators. Venturi scrubbers also began to be used for particulate matter control. These control systems were installed primarily to minimize the nuisance and problems created by the dust.
Particulate matter can be divided into two categories namely (i) primary particulate matter, and (ii) secondary particulate matter. Primary particulate matter is the material emitted directly in to the atmosphere. These emissions have been the focus of all particulate matter control actions prior to 1997. Primary particulate matter can consist of particles less than 0.1 micrometer to more than 100 micrometers. However, most of the primary particulate matter falls in the coarse category.
With the promulgation of the PM2.5 standard aimed at fine and ultra-fine particles, there is increasing attention concerning secondary particulate matter. This is particulate matter which forms in the atmosphere due to the reactions of gaseous precursors. Secondary formation processes can result in the formation of new particles or the addition of particulate material to pre-existing particles. The gases most commonly associated with secondary particulate matter formation include sulphur dioxide, nitrogen oxides, ammonia, and volatile organic compounds (VOCs). Most of these gaseous precursors are emitted from anthropogenic sources; however, biogenic sources also contribute some nitrogen oxides, ammonia, and volatile organic compounds.
Secondary particulate matter can be further subdivided into two categories namely (i) secondary particulate matter formed from condensed vapours emitted from anthropogenic and biogenic sources, and (ii) secondary particulate matter formed due to atmospheric reactions of gaseous precursors. VOCs and sulphuric acid are two common examples of emissions which can condense to form secondary particulate matter. These materials pass through particulate matter control systems, including high efficiency devices, due to their vapour form in the stationary source gas stream. However, the vapour phase material can, under some conditions, potentially condense in the ambient air to form particles measured by ambient sampling systems. The relative importance of condensable particulate matter is just beginning to be evaluated.
The range of particle sizes formed in a process is largely dependent on the types of particle formation mechanisms present. It is possible to estimate the general size range simply by recognizing which of these is important in the process being evaluated. The most important particle formation mechanisms in the air pollution sources include (i) physical attrition / mechanical dispersion, (ii) combustion particle burnout, (iii) homogeneous condensation, (iv) heterogeneous nucleation, and (v) droplet evaporation
Physical attrition occurs when two surfaces rub together. Further when the fuel particles are injected into the hot furnace area of the combustion process, most of the organic compounds are vapourized and oxidized in the gas stream. The fuel particles get smaller as the volatile matter leaves. The fuel particles are quickly reduced to only the incombustible matter (ash) and slow burning char composed of organic compounds. Eventually, most of the char also burns, leaving primarily the incombustible material. As oxidation progresses, the fuel particles, which started as 100 mm -1,000 mm particles, are reduced to ash and char particles which are primarily in the range of 1 mm to 10 mm. This mechanism for particle formation can be termed combustion fuel burnout.
Homogeneous nucleation and heterogeneous nucleation involve the conversion of vapour phase materials to a particulate matter form. Homogeneous nucleation is the formation of new particles composed almost entirely of the vapour phase material. Heterogeneous nucleation is the accumulation of material on the surfaces of particles which have formed due to other mechanisms. In both the cases, the vapour-containing gas streams are required to cool to the temperature at which nucleation can occur.
Some air pollution control systems use solids-containing water recycled from wet scrubbers to cool the gas streams. This practice inadvertently creates another particle formation mechanism which is very similar to fuel burnout. The water streams are atomized during injection into the hot gas streams. As these small droplets evaporate to dryness, the suspended and dissolved solids are released as small particles. The particle size range created by this mechanism has not been extensively studied. However, it probably creates particles which range in size from 0.1 mm to 2.0 mm.
Air pollution control systems apply forces to particles in order to remove them from the gas stream. The forces are basically the ‘tools’ which can be used for particulate collection. All of these collection mechanism forces are strongly dependent on particle size. The forces which are applied are (i) gravity settling, (ii) inertial impaction and interception, (iii) particle Brownian motion, (iv) electrostatic attraction, (v) thermophoresis, and (vi) diffusiophoresis.
Applying one or more of these forces, such as electrostatic force or inertial force, accelerates the particle in a direction where it can be collected. The extent to which the particle is accelerated is indicated by the equation ‘F = Mp x Ap’, where F is the force on the particle in gram centimeter per square second, Mp is the mass of the particle in grams, and Ap is the acceleration of the particle in cm/sq sec. Air pollution control devices are designed to apply the maximum possible force on the particles in the gas stream. The more the particle (or agglomerated mass of particles) is accelerated, the more effective and economical can be the air pollution control device.
There are three fundamental steps which are involved in the collection of particulate matter in high efficiency particulate control systems such as fabric filters and electrostatic precipitators. These are (i) initial capture of particles on vertical surfaces, (ii) gravity settling of solids into the hopper, and (iii) removal of solids from the hopper. The particle collection mechanisms control the effectiveness of the first two steps, that is, initial capture of incoming particles and gravity settling of collected solids. Particle size distribution is important in each of these steps.
All particulate emission control equipments collects particulate matter by mechanisms involving an applied force. Various particulate equipments are settling chambers, cyclones, bag filters and electrostatic precipitators. The mechanisms of dust removal in these equipments and the applied force are given in Fig 1.
Fig 1 Mechanisms of removal of dust
Settling chambers was one of the first devices used to control the particulate emissions. However, it is very rarely used today since its effectiveness in collecting particles is very low. The collection force in settling chamber is gravity. Large particles moving slow enough in a gas stream can be overcome by gravity and gets collected in the settling chamber.
The unit is constructed as a long horizontal box with an inlet, chamber, outlet, and dust collection hoppers. The velocity of the particle laden gas stream is reduced in the chamber. All the particles in the gas stream are subject to the force of gravity. At the reduced gas velocity in the chamber, the larger particles (larger than 40 micrometers) are overcome and fall into hoppers. It is mainly used as a pre-cleaner for other particulate emission control devices to remove very large particles. Fig 2 gives a simple gravity settling chamber design.
Fig 2 A simple gravity settling chamber design
Cyclones are simple mechanical devices which are normally used to remove relatively large particles from gas streams. They are used as pre-cleaner for more sophisticated air pollution control equipment such as electrostatic precipitators or bag filters. Cyclones are more efficient than settling chambers.
Mechanical devices use the inertia of the particles for collection. The particulate-laden gas stream is forced to spin in a cyclonic manner. The mass of the particles causes them to move toward the outside of the vortex. Most of the large-diameter particles enter a hopper below the cyclonic tubes while the gas stream turns and exits the tube.
There are two main types of cyclones namely (i) large-diameter cyclones, and (ii) small-diameter multi-cyclones. Large-diameter cyclones are normally 300 mm to 2 m in diameter. The small-diameter multi-cyclones normally have diameters between 80 mm and 300 mm.
The gas stream enters the cyclone tangentially and creates a weak vortex of spinning gas in the cyclone body. Large-diameter particles move toward the cyclone body wall and then settle into the hopper of the cyclone. The cleaned gas turns and exits the cyclone. Large-diameter cyclones are used to collect particles ranging in diameters from 1.5 mm to more than 150 mm.
Collection forces used for particle collection in a cyclone are centrifugal and gravitational forces. The shape or curvature of the cyclone causes the gas stream to rotate in a spiral motion. Larger particles move toward the outside of the wall by virtue of their momentum. The particles lose kinetic energy there and are separated from the gas stream. These particles are then overcome by gravitational force and falls down to get collected. Inlet of the cyclones is designed to change the flow pattern of the incoming gas from straight flow into a circular pattern to form the vortex.
In case of multi-cyclone, axial cyclones are used in parallel. In axial cyclones, the gas enters from the top and is directed into a vortex pattern by the vanes attached to the centre tube. In multi-cyclone dirty gas enters uniformly through all the individual cyclones. Fig 3 gives cyclone devices.
Fig 3 Cyclone devices
Bag filters use a filter material such as nylon or wool to remove particles from the dust laden gases. The particles are retained on the fabric material. While the clean gas passes through the material. The collected particles are then removed from the fabric filter by a cleaning mechanism which is either the mechanical shaking or by use of air blast. The removed particles are stored in a collection hopper. Various fiber materials used in bag filters along with their properties are given in Tab 1.
|Tab 1 Typical fabrics used for bags|
|Fiber||Maximum temperatures||Acid resistance||Alkali resistance||Flex abrasion resistance|
|deg C||deg C|
|Cotton||82||107||Poor||Very good||Very good|
|Polypropylene||88||93||Good to excellent||Very good||Excellent|
|Nylon||93-107||121||Poor to fair||Good to Excellent||Excellent|
|Orlon||116||127||Good to excellent||Fair to good||Good|
|Nomex||204||218||Poor to good||Good to Excellent||Excellent|
|Teflon||204-232||250||Excellent except poor to Fluorine||Excellent except poor to tri-fluoride, chlorine and molten alkaline metals||Fair to good|
|Fiber glass||260||288||Fair to good||Fair to good||Fair|
In bag filters, three separate forces namely impaction, direct interception, and diffusion are responsible for particle removal from gases and their collection. Impaction occurs when the particle is so large that it cannot follow the gas stream and hits or impacts on the fiber of the bag filter and get separated from the gas stream. In case of direct impaction, the particle follow the gas stream around the fibers till a collision occur due to the distance between the particle centre and the fiber being less than the particle radius. Diffusion occurs because of very small particles undergo Brownian motion throughout the gas volume. Very small particles become affected by collision of molecules in the gas stream. These randomly moving particles diffuse through the gas to impact on the fiber and get collected.
Bag filters have normally large numbers of cylindrical fiber bags which hangs vertically in the bag filter. When dust layers have built upto a sufficient thickness, the bags are cleaned, causing the dust particles to fall into a collection hopper. Bags can be cleaned by a number of methods. The three most common methods are shaking, reverse air cleaning, and the pulse jet. In the mechanical shaking, the bags are gently shaked by a drive system for the removal of the deposited dust. In the reverse air cleaning mechanism, the bag filter compartment is backwashed with a low pressure flow of air. Dust is removed by merely allowing the bags to collapse. Pulse jet cleaning mechanism is the most popular mechanism. It uses a high pressure jet of air to remove the dust from the bag. The dust cake is removed from the bag by a blast of compressed air injected into the top of the bag tube. The blast of the high pressure air stops the normal flow of air through the filter. The air develops into a shock wave which causes the bag to flex or expand as the shock wave travels down the bag tube. As the cake flexes, the cake fracture and the deposited particles fall from the bag. The shock wave travels down and back up the tube in around 0.5 seconds. The blast of the compressed air is to be strong enough to travel the length of the bag and shatter or crack the dust cake.
The bag of bag filters has a life and needs replacement when this life is over. Three conditions affect the life of the bags adversely. These are abrasion, high temperature, and chemical attack.
One of the important variables in the design of a bag filter is air to cloth ratio. Very high air to cloth ratio results into excessive pressure drops, reduced collection efficiency, bags becoming caked solidly with dust, and rapid bag deterioration. Fig 4 shows pulse jet bag house filter.
Fig 4 Pulse jet bag house filter
Electrostatic precipitator is used to collect particles with diameters in size range 0.1 micro-meters to 10 micro-meters. It uses non-uniform, 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. Its collection efficiency is high and sometimes exceeds even 99 %. ESPs can handle large exhaust gas volumes at temperature range of 175 deg C to 700 deg C.
There are three main styles of electrostatic precipitators namely (i) negatively charged dry precipitators, (ii) negatively charged wetted-wall precipitators, and (ii) positively charged two-stage precipitators. The negatively charged dry precipitators are the type most frequently used on large applications. Wetted-wall precipitators (sometimes called wet precipitators) are frequently used to collect mist and / or solid materials which are moderately sticky. The positively charged two-stage precipitators are used only for the removal of mists. Fig 5 shows electrostatic precipitator and its concept.
Fig 5 Electrostatic precipitator
The particle collection mechanism involved in an ESP is electrostatic force. The dust particles suspended in flue gas are charged as they pass through the ESP. A high voltage, pulsating, direct current is applied to an electrode system consisting of a small diameter discharge electrode and a collection electrode. The discharge electrode is normally negatively charged. The collection electrode is normally grounded. The applied voltage is increased until it produced a corona discharge which can be seen as a luminous blue glow around the discharge electrode. The corona causes the gas molecules to ionize. The negative gas ions which are produced migrate towards the grounded collection electrode. The negative gas ions bombard the particles suspended in the flue gas stream and impart a negative charge to them. Negatively charged particles then migrate to the collection electrode and are collected.
The ESP has six essential elements. These are (i) discharge electrode, (ii) collection electrode, (iii) electrical system, (iv) rapper, (v) hopper, and (vi) shell. The discharge electrode is normally a small diameter metal wire. This electrode is used to ionize the gas which charges the dust particles, and to create a strong electrical field. The collection electrode is either a flat plate or tube with an opposite charge to that of the discharge elctrode. This electrode collects the charged particles. The electrical system consists of high voltage components used to control the strength of the electrical field between the discharge and collection electrodes. The rapper imparts a vibration or the shock to the electrodes for removing the collected dust. Rappers remove dust which has accumulated on both collection electrodes and discharge electrodes. Hoppers are at the bottom of the ESPs and are used to collect and temporary store the dust removed during the rapping process. Shell encloses the electrodes and supports the entire ESP.
The resistivity of dust particles drastically affects the collection efficieency of ESP. Resistivity describes the resistance of the collected dust layer to the flow of electrical current. Particles which have high resistivity are more difficult to collect than those having normal resistivity. High resistivity can be reduced by adjusting the temperature and the moisture content of the flue gas flowing into the ESP. However while adjusting the temperature and moisture, it is to be ensured that flue gas temperature is above the dew point otherwise there is corrosion of the plates.
One of the important parameter in ESP design is the specific collection area which is defined as the ratio of the collection surface area to the gas flow rate into the ESP. Increasing the surface area for a given flue gas flow rate normally increases the collection efficiency of the precipitator.