Boiler pollution control

India Boiler dot Com

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CHAPTER - 19

Boiler Pollution Control Introduction: Undesirable contamination of environment is termed ‗Pollution‘. In all industrial processes, there are some byproducts that possess any commercial value and are harmful to humans and/or cattle beyond a certain concentration. When such byproducts are disposed freely in the environment ‗Pollution‘ is caused. Uncontrolled disposal, into environment, of these byproducts constitutes Pollution of Air and Water. In early times adverse effects of such Pollution were not known. Hence it was taken for granted that Pollution is a necessary evil associated with industrialization. However, as more and more knowledge is being gathered and Pollutants and their effects are being studied and recorded, scientific norms for making disposal of polluting byproducts are evolving. Norms are being established about the degree of pollution that can be tolerated by everyone harmlessly. Rules and Regulations have been made in almost all countries for this purpose. In India too there are Acts of Law and Rules & Regulations for control of Pollution. Certain standards have also been laid down for most potential forms of Pollution nuisance. Failure to comply constitutes a breach of the law with the possibility of fines or even injunctions restricting production. In the following have been discussed the sources of pollution associated with boilers and the steps to abate them. Persons engaged in the design operation and maintenance of industrial plant should aim at not only meeting the emission norms but also minimizing the nuisance. Boilers can cause Pollution of Atmosphere through emissions, Water through effluents and Noise. Sources of such pollution are shown in diagram below.


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Atmospheric Pollution Atmospheric pollution can be caused by a) Dark smoke, b) Ash particles as Suspended Matters or Particulate, c) Oxides of Sulphur, d) Acidic smut emission from oil fired boilers, e) Oxides of nitrogen and f) CO as described below. Dark smoke: Dark smoke causes blackening of buildings and by excluding sunlight it is detrimental to health and adversely affects the growing of crops. It is due to the presence of particles of carbonaceous matter because of bad combustion, inefficient maintenance and cleaning of heating surfaces, faulty design and installation of firing equipments, overloading of plants, insufficient air supply, improper draught, air leakages from the brickwork and other openings and inadequate size of stack. With modern plant and skilled operators it can be avoided considerably. The colour of smoke is measured by comparing it with a Ringlemann chart. Ash particles or Particulates: In pulverised coal firing 85% ash appear as fly ash and in others it is 25%. The term fly ash is used when fine ash and slag particles are carried over to the exit end of the plant. Cleaning and washing coal is considered as one of the means to reduce pollution. Flyash from the combustion process is collected using one of four major technologies: a. electrostatic precipitator (ESP), b. fabric filters (or baghouse), c. mechanical collectors and d. wet scrubbers. With today‘s removal requirements in excess of 99.9%, modern ESPs and fabric filters dominate fly ash collection. Mechanical collectors are still used for specialty applications as a preliminary collection device, especially where fly ash recycle is part of the combustion process, but they are almost always followed by an ESP or fabric filter for final particulate control. Wet scrubbers are no longer used for primary particulate collection because of their high energy requirements for the desired removal efficiencies. In addition to efficient grit collection the use of high stacks distributes the residual ash over a wide area. The dust can travel over a distance of 70 miles in a light wind from a 300 ft. stack. Oxides of Sulphur: Sulphur in the fuel burns off to liberate SO2 which forms SO3 in the final stage of flame burning when there is an exigency of atomic oxygen. SO3 is also produced from SO2 on the surface of the superheater deposits that act as a catalyst at elevated temperatures. SO3 reacts with the atmospheric moisture to form an aerosol of sulphuric acid which rains down as acid rain. Historically, SOx pollution has been controlled by either dispersion or reduction. Dispersion involves the utilization of a tall stack, which enables the release of pollutants high above the ground and over any surrounding buildings, mountains, or hills, in order to limit ground level SOx emissions. Today, dispersion alone is not enough to meet more stringent SOx emission requirements; reduction methods must also be employed.


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Methods of SOx reduction include switching to low sulfur fuel, desulphurizing the fuel, and utilizing a flue gas desulphurization (FGD) system. Fuel desulphurization, which primarily applies to coal, involves removing sulfur from the fuel prior to burning. Flue gas desulphurization involves the utilization of scrubbers to remove SOx emissions from the flue gases. Flue gas desulphurization systems are classified as either non-regenerable or regenerable. Non-regenerable FGD systems, the most common type, result in a waste product that requires proper disposal. Regenerable FGD converts the waste by-product into a marketable product, such as sulfur or sulfuric acid. SOx emission reductions of 90-95% can be achieved through FGD. Fuel desulphurization and FGD are primarily used for reducing SOx emissions for large utility boilers. Generally the technology cannot be cost justified on industrial boilers. For users of industrial boilers, utilizing low sulfur fuels is the most cost effective method of SOx reduction. Because SOx emissions primarily depend on the sulfur content of the fuel, burning fuels containing a minimal amount of sulfur (distillate oil) can achieve SOx reductions, without the need to install and maintain expensive equipment. Acidic smuts from oil fired boilers The burning of high sulphur content fuel oil can give appreciable quantities of sulphuric acid which together wilt particles of unburnt carbon can cause acidic soot to fall in an area of about 1 mile radius. These smuts are very destructive to car finishes, clothing, etc., although the total quantity is very low compared with coal burning. Improved boiler operating technique and the use of chemical additives to limit sulphuric acid formation has reduced the extent of the problem. Oxides of nitrogen (NOX): Oxides of nitrogen are produced by combustion of fuels. Nitrogen dioxide is a highly dangerous air pollutant. NO + NO2 are produced in the high-temperature zones of the flame. NOX may undergo a photochemical reaction with the hydrocarbons in the atmosphere in the presence of sunlight to release some toxic substances in the air. The concentration of NOX in combustion gases largely depends on the combustion technique in the furnace. Since the bulk of NOX produced in the combustion process comes from chemical reaction between aerial nitrogen and oxygen in the high temperature combustion zone (above 1600oC), the chief means of limiting NOX generation is to lower the temperature in the combustion zone. NOx control technologies: NOx controls can be classified into two types: post combustion methods and combustion control techniques. Post combustion methods address NOx emissions after formation while combustion control techniques prevent the formation of NOx during the combustion process. Post combustion methods tend to be more expensive than combustion control techniques and generally are not used on boilers with inputs of less than 100 MMBtu/hr. Following is a list of different NOx control methods. Post combustion control methods include: Selective Non-Catalytic Reduction Selective Catalytic Reduction

Combustion control techniques include: Low Excess Air Firing Low Nitrogen Fuel Oil


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Burner Modifications Water/Steam Injection Flue Gas Recirculation

Each method results in a different degree of NOx control. For example, when firing natural gas, low excess air firing typically reduces NOx by 10%, flue gas recirculation by 75%, and selective catalytic reduction by 90% Post combustion control methods: Selective Non-catalytic Reduction Selective non-catalytic reduction involves the injection of a NOx reducing agent, such as ammonia or urea, into the boiler exhaust gases at a temperature of approximately 1400-1600 °F. The ammonia or urea breaks down the NOx in the exhaust gases into water and atmospheric nitrogen. Selective non-catalytic reduction reduces NOx up to 70%. However, the technology is extremely difficult to apply to industrial boilers that modulate or cycle frequently. This is because the ammonia (or urea) must be injected in the flue gases at a specific flue gas temperature. And, in industrial boilers that modulate or cycle frequently, the location of the exhaust gases at the specified temperature is constantly changing. Thus, it is not feasible to apply selective non-catalytic reduction to industrial boilers that have high turndown capabilities and modulate or cycle frequently. Selective Catalytic Reduction Selective catalytic reduction involves the injection of ammonia in the boiler exhaust gases in the presence of a catalyst. The catalyst allows the ammonia to reduce NOx levels at lower exhaust temperatures than selective non-catalytic reduction. Unlike selective non-catalytic reduction, where the exhaust gases must be approximately 1400-1600 °F, selective catalytic reduction can be utilized where exhaust gasses are between 500 °F and 1200 °F, depending on the catalyst used. Selective catalytic reduction can result in NOx reductions up to 90%. However, it is costly to use and can rarely be cost justified on boilers with inputs less than 100 MMBtu/hr. Choosing the best NOx technology for the job: What effect does NOx control technology ultimately have on a boiler's performance? Certain NOx controls can worsen boiler performance while other controls can appreciably improve performance. Aspects of the boiler performance that could be affected include turndown, capacity, efficiency, excess air, and CO emissions. Failure to take into account all of the boiler operating parameters can lead to increased operating and maintenance costs, loss of efficiency, elevated CO levels, and shortening of the boiler's life. The following section discusses each of the operating parameters of a boiler and how they are related to NOx control technologies. Turndown: Choosing a low NOx technology that sacrifices turndown can have many adverse effects on the boiler. When selecting NOx controls, the boiler should have a turndown capability of at least 4:1 or more, in order to reduce operating costs and the number of on/off cycles. A boiler utilizing a standard burner with a 4:1 turndown can cycle as frequently as 12 times per hour or 288 times a day because the boiler must begin to cycle at inputs below 25% capacity.


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With each cycle, pre- and post-purge air flow removes heat from the boiler and sends it out the stack. The energy loss can be reduced by using a high turndown burner (10:1), which keeps the boiler on even at low firing rates. Every time the boiler cycles off, before it comes back on, it must go through a specific start-up sequence for safety assurance. It takes between one to two minutes to get the boiler back on line. If there is a sudden load demand, the response cannot be accelerated. Keeping the boiler on line assures a quick response to load changes. Frequent cycling also deteriorates the boiler components. The need for maintenance increases, the chance of component failure increases, and boiler downtime increases. So, when selecting NOx control, always consider the burners turndown capability. Capacity: When selecting the best NOx control, capacity and turndown should be considered together because some NOx control technologies require boiler derating in order to achieve guaranteed NOx reductions. For example, flame shaping (primarily enlarging the flame to produce a lower flame temperature - thus lower NOx levels) can require boiler derating, because the shaped flame could impinge on the furnace walls at higher firing rates. However, the boiler's capacity requirement is typically determined by the maximum load in the steam/hot water system. Therefore, the boiler may be oversized for the typical load conditions that may occur. If the boiler is oversized, its ability to handle minimum loads without cycling is limited. Therefore, when selecting the most appropriate NOx control, capacity and turndown should be considered together for proper boiler selection and to meet overall system load requirements. Efficiency: Some low NOx controls reduce emissions by lowering flame temperature, particularly in boilers with inputs less than 100 MMBtu/hr. Reducing the flame temperature decreases the radiant heat transfer from the flame and could lower boiler efficiency. The efficiency loss due to the lower flame temperatures can be partially offset by utilizing external components, such as an economizer. Or, the offset technique can be inherent in the NOx design. One technology that offsets the efficiency loss due to lower flame temperatures in a firetube boiler is flue gas recirculation. Although the loss of radiant heat transfer could result in an efficiency loss, the recirculated flue gases increase the mass flow through the boiler - thus the convective heat transfer in the tube passes increases. The increase in convective heat transfer compensates for losses in radiant heat transfer, with no net efficiency loss. When considering NOx control technology, remember, it is not necessary to sacrifice efficiency for NOx reductions. Excess Air A boiler's excess air supply provides for safe operation above stoichiometric conditions. A typical burner is usually set up with 10-20% excess air (2-4% O2). NOx controls that require higher excess air levels can result in fuel being used to heat the air rather than transferring it to usable energy. Thus, increased stack losses and reduced boiler efficiency occur. NOx controls that require reduced excess air levels can result in an oxygen deficient flame and increased levels of carbon monoxide or unburned hydrocarbons. It is best to select a NOx control technology that has little effect on excess air. Carbon Monoxide (CO) Emissions: High flame temperatures and intimate air/fuel mixing are essential for low CO emissions. Some NOx control technologies used on industrial and commercial boilers


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reduce NOx levels by lowering flame temperatures by modifying air/fuel mixing patterns. The lower flame temperature and decreased mixing intensity can result in higher CO levels. An induced flue gas recirculation package can lower NOx levels by reducing flame temperature without increasing CO levels. CO levels remain constant or are lowered because the flue gas is introduced into the flame in early stages of combustion and the air fuel mixing is intensified. Intensified mixing offsets the decrease in flame temperature and results in CO levels that are lower than achieved without FGR. But, the level of CO depends on the burner design. Not all flue gas recirculation applications result in lower CO levels. Total Performance: Selecting the best low NOx control package should be made with total boiler performance in mind. Consider the application. Investigate all of the characteristics of the control technology and the effects of the technology on the boiler's performance. A NOx control technology that results in the greatest NOx reduction is not necessarily the best for the application or the best for high turndown, adequate capacity, high efficiency, sufficient excess air, or lower CO. The newer low NOx technologies provide NOx reductions without affecting total boiler performance. Carbon monoxide: Carbon monoxide is a pollutant that is readily absorbed in the body and can impair the oxygen-carrying capacity of the hemoglobin. Impairment of the body's hemoglobin results in less oxygen to the brain, heart, and tissues. Even short-term over exposure to carbon monoxide can be critical, or fatal, to people with heart and lung diseases. It may also cause headaches and dizziness in healthy people. During combustion, carbon in the fuel oxidizes through a series of reactions to form carbon dioxide (CO2). However, 100 percent conversion of carbon to CO2 is rarely achieved in practice and some carbon only oxidizes to the intermediate step, carbon monoxide. Older boilers generally have higher levels of CO than new equipment because CO has only recently become a concern and older burners were not designed to achieve low CO levels. In today's equipment, high levels of carbon monoxide emissions primarily result from incomplete combustion due to poor burner design or firing conditions (for example, an improper air-to-fuel ratio) or possibly a leaky furnace. Through proper burner maintenance, inspections, operation, or by upgrading equipment or utilizing an oxygen control package, the formation of carbon monoxide can be controlled at an acceptable level. Water pollution: Large boilers in thermal power station and of process industries contribute to water pollution by way of discharging into the water basin the: a. boiler blowdown b. SO2-scrubber waste c. Cooling waters that mainly causes thermal pollution d. Waste waters from waste treatment plants and demineralising units e. Waste waters contaminated with petroleum products f. Waste waters from hydraulic ash-disposal system.

These discharged waste waters carry a rich load of harmful impurities, viz heavier metal cations, organic substances and coarse-dispersed solids besides dissolved salts. The toxic substances added to the water basin from boiler plants may adversely affect the hydrobionts-all living organisms inhabiting the water basin. At higher concentrations they will simply perish while at lower concentrations they may suffer


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from reduced metabolism and growth rate, abnormal change in mutagenesis and reproductive capacity. The load of impurities discharged into water basins can be decreased in two ways: 1. by purifying the waste waters 2. by reducing the quantitative discharge of impurities from particular technological process. Boiler blowdown: This waste stream results from periodic purging of the impurities that become concentrated in steam boiler systems. These pollutants include metals such as copper, iron and nickel, as well as chemicals added to prevent scaling and corrosion of steam generator components. Thermal pollution: The heat released by cooling water (open-circuit) from the turbine condensers of super thermal power stations is enormous. This creates a high zone of elevated temperature in the water basin, reduces its dissolved oxygen thereby impairs the growth and development of aquatic life. The zone of elevated temperature in a water basin can be reduced by allowing the inflow of hot discharge water into the basin through: a. open type spillways with

i. transverse and side weir bulkheads ii. distributing grill

b. submerged jet-type spillways Waste waters from water treatment plants and DM units: Waste water of water treatment plants (WTP) contains slime, coarse dispersed solid, organic substances, magnesium hydroxide, calcium carbonate and salts of iron and aluminum. The composition and concentration of various impurities in waste water depends on the quality of raw water and the methods adopted for water treatment. In DM-unit, regeneration of H-cation exchanger and OH-anion exchanger is done by using H2SO4 and NAOH solutions and as a consequence, the disposed waste becomes respectively acidic and alkaline in nature. Purification: The waste water of WTP and discharge from DM-unit may be disposed as follows: a. transferring the waste water into the hydraulic ash handling system of coal fired

boiler units b. neutralizing the waste water (pH>9) of WTP with acid wastes of the DM-unit. c. Subjecting the waste to slime separation, i.e. slime dewatering in drum type

vacuum filters and recycling the clarified water for washing of mechanical filters.

Reducing waste water discharge of WTP and DM-units: The amount of impurities discharged into the water basin by waste waters from the WTP and DM-unit can be diminished by adopting techniques that will minimise the use of reagents and water for water treatment and regeneration purposes. The quantity of water used for regeneration of mechanical filters can be drastically cut down by increasing the filtering capacity of mechanical filters. By using expanded clay instead of quartz sand in mechanical filters it is possible to use less water for regeneration by a factor 2.5-3.5. The flowrate of wastes from the DM-unit can be effectively reduced by adopting the process of: a. continuous ion exchange


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b. stepwise counter-current ion-exchange c. thermal regeneration (instead of chemical regeneration) of ion exchanges.

The colloidal impurities of waste water can be precipitated down by electrocoagulator using either Fe or Al anodes. Stripping waste water of its dissolved salts can be effectively accomplished by a physical process known as reverse osmosis. In this process, the waste water with dissolved salts is forced through a semi-permeable membrane at a pressure, exceeding the osmotic pressure of the solution. The membrane allows only water and a small fraction of salts as ions to pass with the effect that the filtrate contains an appreciably smaller quantum of dissolved impurities. Reverse osmosis technique has drastically cut down the consumption of reagents used for water treatment with the effect that the concentration of impurities in discharged waste waters sharply declines. The line up of reverse osmosis plant upstream of DM-units can slash down the discharge of salt solution by 50% and the cost of desalted water by 25%. Desalination of waste waters can be successfully carried out by another technique known as electrodialysis. The apparatus consists of an assembly of parallel cation and anion exchange membranes flanked by a stainless steel cathode and platinum coated steel anode. Electrodialysis substantially reduces the use of reagents for the waste water treatment and consequently the amount of salt discharged to waste waters is also diminished. Waste waters contaminated with petroleum products: The petroleum products like lube oils, fuel oils, kerosene. etc. find their way to the water basin in an emulsified, colloidal or dissolved state. They are particularly dangerous for water basins. The maximum allowable concentration of petroleum products in the water basin is 0.5 mg/kg of water. They form films on the water surface, inhibit the natural aeration of water and thereby inflict serious harm to aquatic life. Purification: The oil contaminated waste water is charged to oil traps that separate out efficiently the course oil particles of size 80-100 μm or more. The clarified water is then fed into the flotator where finer oil particles are separated from water at a high rate under pressure flotation. The purified water is then filtered through a mechanical filter. The former consists of double layer packing of quartz sand and anthracite. The carbon filter consists of bed of activated carbon to adsorb oily suspensions. The final effluent water of this plant is 95% free from oil. Waste water of hydraulic ash disposal system: In a hydraulic ash disposal system water is allowed to act upon the ash, a part of which dissolves and the rest forms a pulp (suspension) which is pumped to ash settling ponds where the course impurities settle down and the clarified water can either be discharged directly into a water basin or recycled for reuse. The composition and concentration of impurities transferred from ash to water depends largely upon the chemical composition of ash which may vary appreciably. SiO3: 10-70%; Al2O3: 10-40%; Fe2O3: 2-30%; CaO: 2-70%; MgO: 0-10%; K2O + Na2O: 0-10%. It may also contain traces amount of compounds of vanadium, germanium, arsenic, mercury, beryllium and fluorine. Decontamination of ash disposal water: The high flowrate and high concentration of impurities in the waste water of a hydraulic ash disposal system prevent purification


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of the entire bulk of the ash disposal system. What can be achieved is decontamination of toxic impurities to a safe level and the principal processes involved are as follows: 1. Precipitation as well as co-precipitation of impurities 2. Sorption of impurities 3. Oxidation reduction followed by precipitation.

Removal of impurities by forming sparingly soluble precipitates or by adsorption on the surface of the solid phase separated in the water mass during the course of the chemical treatment of ash disposal water is the most popular one Zero liquid discharge: Utilities are looking for waste water treatment techniques to retrieve almost entire gamut of their effluents as a high quality distillate for reuse and to convert the contaminants of waste steam into manageable solid wastes for disposal. The most popular among these techniques is the ZERO LIQUID DISCHARGE (ZLD) system that can recover 90-99% of the waste water as a high quality distillate suitable for reuse in the boiler makeup or the cooling tower while simultaneously producing a clean salt for byproduct sale with only a minor volume of contaminants. Pollution Control Acts In India Environmental Laws 1. The water (Prevention and Control of Pollution) Act, 1974; 2. The water (Prevention and Control of Pollution) Cess Act, 1977; 3. The Air (Prevention and Control of Pollution) Act, 1981; 4. The Environment (Protection) Act, 1986; I. Water (Prevention and Control of Pollution) Act, 1974: The water (Prevention and Control of Pollution) Act, 1974 provides for the prevention and control of water pollution and the maintaining or re-storing of wholesomeness of water. Under the scheme of the Act, the relevant provisions, casting obligations on persons may be referred under sections - 24, 25/26 and 31 of the Act. Section - 24, prohibits on the use of stream or well for the disposal of polluting matters, which is in excess of the standards laid down by the Board. In other words, people are under obligation to observe the standards laid down by the Board, in the matter of use of stream or well by way of disposal of the polluting matters, determined in accordance with the Board's Standard. Section - 25/26, restricts establishment or use of new or existing outlets or discharge without prior consent from the Board. In other words, person are under obligation to apply for consent, before they are taking steps to establish any industry or are bringing into use any new outlet or are continuing with the existing Outlet for the discharge of sewage or trade effluent. Section - 31, cast obligation on any industry, operation or process to furnish information to the Sate Board, including other agencies, about accidental discharge of any poisonous matter into a stream or well or sewer on land. Failure to carry out the aforesaid obligations attracts penal provisions under sections 43, 44 and 42 respectively. II. The water (Prevention and Control of Pollution) Cess Act, 1977: The water (Prevention and Control of Pollution) Cess Act, 1977 provides for the levy and collection of Cess on water consumed by persons carrying on specified industry and by local authorities, with a view to augmenting the resource of Central and State


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Board's for the prevention and Control of water pollution, constituted under the Water (Prevention and Control of Pollution) Act, 1974. The relevant provisions casting obligation under this Act may be referred under sections 3,4 and 5. Section - 3 casts liability on every person carrying on any specified industry under Schedule I of the Act, and also on every authority to pay a Cess for the purpose of the Water (Prevention and Control of Pollution) Act, 1974 and utilization of water there under. Section - 4, requires every person carrying on any specified industry and every local authority to affix meters of prescribed standard, so as to measure the quantity of water consumed by them. Section - 5, requires the said persons to furnish returns in the prescribed format, showing the quantity of water consumed in the previous month. Failure to carry out the obligations and liability, as aforesaid, attracts the penal provision under section 14 of the Act. III. Air (Prevention and Control of Pollution) Act,1981: The Air (Prevention and Control of Pollution) Act, 1981 provides for the prevention, control and abatement of air pollution. The liability/obligations imposed on the concerned persons under the scheme of this Act may be referred under the provisions of sections 21,22 and 23. Section - 21, similar to the provision under section 25/26 of the Water Act, 1974, puts obligation by way of restriction on any person on the establishment or operation of any industrial plant in an air pollution control area, without obtaining prior consent from the concerned Board. Section - 22 is also comparable to section 24 of the Water Act, 1974. It requires any person carrying on any industrial plant to allow discharge or emission only within the prescribed standard. Section - 23 is also comparable to section 31 of the Water Act, 1974, where under any person, carrying on an industrial plant, shall furnish information to the State Board or other agencies, in case due to any accident or other unforeseen act or event emission has occurred in excess of the standards laid down by the Board. In the event of failure to carry out ones obligations or liabilities, as aforesaid, penal provision under Section - 37 of the Act is attracted. IV. Environment (Protection) Act, 1986; It provides for the protection and improvement of environment and the matters connected therewith. This legislative piece was brought into existence by the Parliament, as Umbrella Act, which intended to cover all the specific and general provisions left by the earlier enactments. The instant Act, being broader in approach, has broader catch also in creating liabilities and obligations on the Nation. The obligations created under this Act may be broadly referred under sections 7,8,and 9. Section - 7 puts obligation on every person carrying on any industry or operation to allow emission or discharge only within the standards prescribed by the Central Government. Section - 8 requires any person handling hazardous substance to handle them in accordance with such procedure and safe - guards as has been prescribed by the Central Government by the following Rules . These Rules, further break obligations and liabilities on certain persons to carry out, as discussed below separately :- (A.) The Hazardous Wastes (Management and Handling) Rules, 1989:


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These rules apply to hazardous wastes, as specified in its Schedule but shall not apply to discharge or emissions covered under the the Water Act, 1974 and the Air Act, 1981; shall not apply to wastes arising out of the operation from ships, beyond 5 Kilometers in the sea; and shall also not apply to Radio-Active Wastes covered under the Atomic Energy Act, 1962. These rule create liability on all such persons, who are handling or causing to be handled hazardous wastes specified in its schedule. Rule - 4 creates responsibilities on the occupiers, who generate hazardous waste listed in column - II of the schedule in quantities equal to or exceeding the limit given in column - III of the said Schedule, to take all proper steps during handling and disposal of such waste without creating any adverse effect. He is also required to give specified information by the State Board to the operator of a facility intending to get his hazardous Waste treated. Under Rule 5, every occupier generating hazardous wastes and having facility for collection, treatment, storage and disposal of such wastes, is required to obtain authorization for handling such hazardous wastes from State Pollution Control Board. Similarly, any person, who intends to or is operating a facility for the collection, reception, treatment, transport, storage and disposal of hazardous wastes, shall have to obtain authorization from the State Board. Under Rule 10, the occupier or the operator of a facility shall be under obligation to report immediately to the state Board about any accident occurring at the facility or on a hazardous wastes site. Water Pollution:

THERMAL POWER PLANTS

Parameters Maximum limiting concentration milligram per liter (Except for pH and temperature)

1. CONDENSER COOLING WATERS ( ONCE THROUGH COOLING SYSTEM )

pH 6.5 to 8.5

Temperature Not more than 50 C higher than the intake water temperature.

Free available chlorine 0.5

II. BOILER BLOWDOWNS

Suspended Solids 100

Oil & Grease 10

Copper ( total ) 1.0

Iron ( total ) 1.0

III. COOLING TOWER BLOW DOWN

Free available chlorine 0.5


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Zinc 1.0

Chromium ( Total ) 0.2

Phosphate 5.0

IV. ASH POND EFFLUENT

PH 6.5 - 8.5

Suspended Solids 100

Oil and grease 20

Standards prescribed for emissions of Air Pollutants under Air (Prevention & Control of Pollution) Act, 1981

1. (a) STACK EMISSION STANDARDS FOR BOILER PLANTS

Steam Generating

Required particular matter

Capacity A B

Area upto 5 Km from the periphery of class I and class II Town

other than "A"

less than 2 ton/ht

800 mg/NM3 1200 mg/NM3

2 ton to 10 ton/hr

500 mg/NM3 1000 mg/NM3

above 15 ton/hr

150 mg/NM3 150 mg/NM3

All emissions normalized to 12% carbon dioxide

(1)) STANDARDS FOR STACK HEIGHT FOR BOILER PLANTS

Steam Generating Capacity Stack Heights

1 More than 2 ton/hr. 9 meters or 2.5 times the height of neighboring building whichever is more

2.more than 2 ton/hr to5 ton/hr.

12 meters


15 meters


18 meters

5.more than 15 ton/hr to 20 ton/hr.

21meters

6.more than 20 ton/hr to 25 ton/hr.

24 meters

7.more than 25 ton/hr to 30 27 meters


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ton/hr.

8.more than 30 ton/hr 30 meters or using the formula H = 14(Q)0.3 where Q SO2 emission in kg/hr.

For higher KVA rating, stack height H (in meter) shall be worked out according to the formula H=h+0.2 Where h=height of the building in meters where the generator set is installed. For the industries which install the facilities for removal of particulate or gaseous emissions to adhere to the limits prescribed, the stack height can be relaxed as under: b) H= 14 (Q)0.3 where Qg is the gaseous emissions in kg/hr. c) H=74 (Qp is the quantity of particular matter in tonnes /hr minimum stack height in all cases shall be 9 meters or as calculated from relevant formula whichever is more.

2. STACK EMISSION STANDARD FOR FURNACES

1) cupola Furnace Capacity of the furnace

Particular matter i) Less than 3 tonnes /hr=450 mg/NM3

ii) 3 tons /hr and above = 150 mg/NM3

2) Are Furnace

Particular matter All sizes = 150 mg/NM3

3) Induction Furnace


4) Reheating (Reverberatory Furnace


3. EMISSION STANDARDS FOR THERMAL POWER PLANT

A) STANDARDS FOR PARTICULAR MATTER EMISSIONS

i) Less than 210 MW

150 mg/NM3 350mg/NM3

ii)210 MW & above

150 mg/NM3 350mg/NM3

B) STANDARDS FOR SULPHUR DIOXIDE EMISSIONS

i) So2 when emitted shall not exceed 600 ng/i

of energy produced

ii) Control through stack Height

Boiler size Stack Height

Less than 210 MW H=14 (Q)0.3

210 MW to less than 500 MW 220 meters


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500 MW and more 275 meters

Where Q = SO2 , emission in kg/hr. H = Stack height in meters.


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Electrostatic Precipitators Introduction: Particulate matter is one of the industrial air pollution problems that must be controlled. Electrostatic Precipitators are one of the most frequently used devices for collection of fine ash particles from flue gases of boilers. Theory of Precipitation Every particle either has or can be given a charge—positive or negative. If we impart a negative charge to all the particles in a gas stream and then set up a grounded plate having a positive charge, the negatively charged particle would migrate to the grounded collection plate and be captured. The particles would quickly collect on the plate, creating a dust layer. The dust layer would accumulate until it is removed, which can be done by rapping the plate. Charging, collecting, and removing—that's the basic idea of an ESP, Basic Principle of ESP: Electrostatic Precipitator functions on the principle of attraction between particles having opposite charge. For this purpose, Flue Gas is made to pass through a succession of chambers (called ‗field‘) made of steel ducting material with vertical plates suspended from top such that they do not obstruct the flow of Flue Gases but horizontally divide the flow in a large number of parallel paths.

Typically the space between two successive plates is 600 mm. These plates are called ‗Collector Electrodes or Collector Plates‘. Between these Collector Plates are rods or helical coil shaped components, which are electrically insulated from the Collector Plates and are suspended through insulating bushings from top of the chamber of field. These components are called ‗Emitting Electrodes‘. The Emitting Electrodes are negatively charged with the help of a system of High Voltage Transformer and Rectifiers. Typically the Voltage at which the Emitting Electrodes are charged is of the order of 25 to 125 kilo Volts dc. As the flue gas containing ash particles passes between the Collector Plates, the ash particles get negatively charged by the Emitting Electrodes and as a result they are attracted towards the Collector Plates and get deposited on them.


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From time to time the Collector Plates are rapped with the help of a hammer arrangement due to which the collected ash on the Collector Plates falls down into ash collecting hoppers situated below the chamber. From the above description it becomes clear that the slower the flow of flue gas through the arrangement of Emitting Electrodes and Collector Plates the higher the ESP collection efficiency. Alternatively, the larger the area of Collector Plates the larger the collection efficiency of the ESP. Particle Removal Dust that has accumulated to a certain thickness on the collection electrode is removed by a process called rapping. Rapping is a process whereby deposited, dry particles are dislodged from the collection plates by sending mechanical impulses, or vibrations, to the plates. Precipitator plates are rapped periodically while maintaining the continuous flue-gas cleaning process. In other words, the plates are rapped while the ESP is on-line; the gas flow continues through the precipitator and the applied voltage remains constant. Plates are rapped when the accumulated dust layer is relatively thick (0.08 to 1.27 cm or 0.03 to 0.5 in.). This allows the dust layer to fall off the plates as large aggregate sheets and helps eliminate dust re-entrainment. Most precipitators have adjustable rappers so that rapper intensity and frequency can be changed according to the dust concentration in the flue gas. Installations where the dust concentration is heavy require more frequent rapping. Dislodged dust falls from the plates into the hopper. The hopper is a single collection bin with sides sloping approximately 50 to 70° to allow dust to flow freely from the top of the hopper to the discharge opening. Dust should be removed as soon as possible to avoid (dust) packing. Packed dust is very difficult to remove. Most hoppers are emptied by some type of discharge device and then transported by a conveyor. Cold-side ESPs have been used for over 50 years with industrial and utility boilers, where the flue gas temperature is relatively low (less than 204°C or 400°F). Cold-side ESPs generally use plates to collect charged particles. Because these ESPs are


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operated at lower temperatures than hot-side ESPs, the volume of flue gas that is handled is less. Therefore, the overall size of the unit is smaller, making it less costly. The location of this ESP is between the last heat exchanger (generally Air Pre-heater) and the ID fan/ chimney as shown below.

ESP size is often measured in terms of Specific Collection Area (SCA). The Specific Collection Area (SCA) has units, of square feet per 1,000 actual cubic feet per minute (acfm) of flue gas flow. The SCA for the required performance can be determined by using the Deutsch-Anderson equation, which relates the collection efficiency (E) to the unit gas flow rate, the particulate‘s effective migration velocity and the collection surface area: 1 - E = e (-wA/V) or A = [ ln (1/(1-E)]. V/w Where, E = ESP removal efficiency, % = 100 [(Inlet dust loading - Outlet dust loading)/(Inlet dust loading)] w = effective migration velocity, ft/min or (m/s) A = Collection surface area, ft2 or (m2) V = gas flow, ft3/min or (m3/s) Because of the assumption about an effective migration velocity to make use of this sizing equation, the empirical nature of ESP design is obvious.


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Filter systems Principle of operation, layout Filter systems are the main alternative for ESP systems discussed in the previous

section. Gases (and also liquids) are separated from dispersed particles by passing it

through a fabric or ceramic filter ―medium‖ with a large surface area. Particles

that are not able to penetrate the medium will be retained on its surface, forming

the so- called ―filter cake‖. Generally this filter cake is equally important to

the actual filtration process as the medium. Filter systems offer very high collection efficiencies of typically above 99%, over

rather large size ranges. Operating mostly in the same temperature range (120 –

200oC) they have the advantage over ESPs that the electric resistivity of the particles

does not play any role, making them competitive for high-resistivity ashes. A

disadvantage when compared with an ESP is the larger pressure drop and the

allowable gas velocity: typically the face velocity (= gas flow/filter surface, unit: m/s)

also referred to as ―air-to-cloth‖ (A/C) ratio is in the range 0.5 - 5 cm/s. Hundreds

or more than a thousand typically cylindrical or tubular filter bags of fabric materials

are collected in a ―baghouse‖ in which the filtration process is confined, see Figure

below.

Alternatively, more rigid ―candle‖ filter elements can be used, depending on the filter

medium choice which depends on temperature, gas and particle properties and unit

size.

Bag house filter systems based on inside out (left) and outside-in (right) filtration


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Figure above shows the two possible modes of operation for baghouse fi lters.

Inside-out filtration implies that the gas passes through the filter from the

inside. This ―blows up‖ the bag filters to their maximum volume and produces

the cake on the inside of the bag. Outside-in operation involves that the

gas enters the filter from the outside surface where the cake builds up accordingly.

In this case a support structure is needed to keep the filters in their shape. The

pro‘s and contra‘s of the two options depend mainly on the mechanical properties

of the filter medium and the method that is used to clean the filter after a certain

pressure drop has been reached (see next section). During this filter cleaning

stage the cake is to detach from the filter medium and is collected in a hopper

which usually comprises the bottom part of the filter unit.

Early stages of dust cake build-up (left) and filtration through an established dust

cake The actual filtration process must be distinguished from filtration on a clean filter and

the early stages of filtration until a filter cake has built-up, as shown in Figure

5.34. Filtration efficiency is at its lowest for a clean filter element and the earliest

stages of filtration may result in bad filter performance over a longer filter period.

Often a pre- coat and pre-heat procedure is used that prevents that the filter

medium from acid condensation and from becoming ―blinded‖ by the finest particles

present in the gas to be cleaned.



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Filter types and filter cleaning methods

Three types of fabric filter systems can be distinguished, based on the filter cleaning method that is used: 1) reverse air cleaning, 2) pulse-jet cleaning and 3) shake/deflate systems, the principles of which are shown in Figure below.

Baghouse cleaning methods: reverse-air (top), pulse-jet (centre) and

shake/deflate (bottom) Reverse-gas and shake/deflate methods operate off-line, i.e. the dusty gas stream must be temporarily interrupted or by- passed. The pulse-jet method operates on-line, cleaning a few bags at a time while the rest of the filter bags continue filtration, and is most suitable for outside- in filter systems. Depending on the duration of the pulse that is required high pressure (3 ~ 7 bar over-pressure), intermediate pressure (1 ~ 2 bar over- pressure) or low pressure (0.5 ~ 0.7 bar over-pressure) pulses can be applied. Reverse gas systems are



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found in inside- out systems, using cleaned gas from another filter unit. Low frequency sound helps removing the cake from the filter. Shake/deflate systems are based on a shaking force exposed by a mechanical system in combination with reverse air.

A shake/deflate baghouse filter

The forces that are exerted on the particles that actually removes them from the filter are inertial forces in shake/deflate systems, viscous drag forces in reverse flow systems and a combination of these two in pulse-jet systems. The filter velocity (or air to cloth ratio) for reverse air systems is ~ 1 cm/s, for pulse-jet systems 1.5 ~ 2 cm/s and for shake/deflate systems 3 ~4 cm/s, giving a comparable pressure drop. Dust cake loads vary from 1~2.5 kg/m² for shake/deflate systems and 2.5~7.5 kg/m² for reverse air systems to 5~10 kg/m² for pulse-jet filters. A typical filter bag has a length of 5~10 m, and a diameter of 0.2~0.3 m, giving a surface of 3~10 m² per bag. Pulse-jet units operate with somewhat smaller bags Filtration efficiency, pressure drop Particles of different size are removed by different physical mechanisms in a baghouse filter and rigid barrier filters. As shown in Figure below, which shows the flow around a filter fibre, five m e c h a n i s m s c a n b e distinguished. The largest particles experience a gravity force that determines their t r a j e c t o r i e s . S e c o n d l y , somewhat smaller particles will be removed by internal impaction, not being able to follow the trajectory of the gas. These particles may also be come in contact with the fibre collector by a third mechanism: the streamlines of the gas flow are contracting when passing the fibre which leads to interception of the particle. The finest particles are removed by a fourth mechanism, which is diffusion as a result of Brownian motion. A fifth mechanism may be effective when electrostatic forces are generated between



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the particles and the collector. This can be accomplished by an electric field across the filter in combination with a particle charging process.

Particle capture mechanisms in fabric filtration

Which collection mechanism finally will be the most effective depends on particle size and mass, velocity, density and viscosity of the gas, electrostatic forces and the filter used. Moreover, the different mechanisms are not independent but operate simultaneously. The highest removal efficiencies are obtained for the large particles at high gas

velocities and for the finest particles at low velocities. The removal

efficiency of the intermediate size range of 0.2 to 2 µm, roughly, depends

much more on the particle size/collector diameter ratio, and shows a minimum

in the size versus efficiency curve shown in Figure below. This minimum can be

shifted t o f i n e r p a r t i c l e s i z e s b y higher gas velocities; it can be

alleviated by electrostatic forces.

Typical filter efficiency as function of particle size



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Examples

Q.1 What are the elements in flue gases which pollute the atmosphere? How they can be controlled?

Q.2 What are the measures for controlling the air pollution? Q.3 Enumerate objectionable ingredients in flue gases with respect to air

pollution control. Q.4 What causes heavy black smoke when fuel oil is burnt? Q.5 What is the main constituent in fuel to be considered for chimney height

calculation? What is the formula for calculating chimney height?

Q.6 What are the Boiler flue gas pollution control equipments and measuring devices? Explain function and working principal of bag filter and ESP with sketch. Mention limits of boiler flue gas pollutant discharged through chimney as per pollution control board.

Boiler pollution control

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