Solid and Liquid Pollutants

Transcription

1 Solid and Liquid Pollutants Learning Outcome When you complete this module you will be able to: Discuss the nature, environmental impacts and control methods for solid and liquid pollutants in a power plant. Learning Objectives Here is what you will be able to do when you complete each objective: 1. Describe the construction and operation of various types of mechanical precipitators. 2. Describe the construction and operation of electrostatic precipitators. 3. Describe how flyash is removed from a steam generator. 4. Describe the construction and operation of equipment used to control liquid thermal pollution. 1

2 INTRODUCTION Thermal coal has varying amounts of ash, dependent upon the quality of coal burned at any particular plant. When the coal is pulverized and burned in a modern coal-fired boiler, a large percentage of this ash is carried in the flue gas stream as a very fine solid material, and if not collected, escapes the plant through the stack, resulting in an environmentally unacceptable emission. Flyash is a very fine powdery material which can vary in size from submicron to relatively coarse material. At most plants, the major portion of the flyash is less than 30 microns in diameter. MECHANICAL COLLECTORS Baffle-type flyash collectors separate the flyash from the flue gases by projecting the particles out of the gas stream when the gases make an abrupt change in direction. Usually the baffles are arranged in rows so that the gas stream is divided into a series of narrow ribbons. This type of collector can be designed to operate on natural draft installations or induced draft fans when the available draft is limited. The collection efficiency is best on the larger, heavier particles. AI_3_0_1.mov A AI_3_0_2.mov A Figure 1 Combination Flyash Collector and Induced Draft Fan 2

3 Mechanical Centrifugal Collectors These devices achieve particulate removal by centrifugal, inertial, and gravitational forces developed in a vortex separator. The dust-laden gas is admitted either tangentially or axially over whirl vanes (Fig. 2) to create a high velocity in the cylindrical portion of the device. AI3_fig2.gif Figure 2 Cyclone Separator Operation Particles are subjected to an outward centrifugal force and an oppositely directed viscous drag. The balance between these two forces determines whether a particle will move to the wall or be carried into the vortex sink and be passed on to the clean-gas outlet tube. The collecting force developed is more effective on larger particles. The centrifugal collector is relatively simple in design, construction, and operation, having no moving parts, and is therefore a relatively inexpensive collector both in initial cost and operating cost. This mechanical collector is not suitable for high efficiency collection of fine particulates such as in the case of a coal-fired utility boiler. It has the added disadvantage of a relatively high pressure drop which results in excessive horsepower consumption of the induced draft fan motor. 3

4 Fig. 3 illustrates centrifugal collector arrangements. Figure 3 Centrifugal Collectors (Courtesy of Western Precipitation Corp.) Bag Filters Fabric filters operate by trapping or impinging the dust on the fine filters which make up the fabric. Cloth filters are usually tubular, and a number of bags are enclosed in a large chamber. As the collection of fine flyash and dust continues, the dust particles adhere to the fabric surface. The fabric filter obtains its maximum efficiency during this period of dust buildup. After a fixed operating period, the bags are cleaned by a rapper system. It simply shakes the bags, and the dust or flyash fall to a hopper as shown in Fig. 4. 4

5 AI_3_0_3.bmp G Figure 4 Bag House Filters Fig. 5 illustrates the principle of operation of a bag filter. Immediately after cleaning, the filtering efficiency is reduced until the buildup of collected ash takes place. The fabric filter (baghouse) can be applied in any process area where dry collection is desired and where the temperature and humidity of the gases to be handled do not impose limitations. For particulate matter, efficiencies above 99% can be achieved with fabric filters. AI_3_0_4.mov A Figure 5 Fabric Filter (Courtesy Flakt Canada Ltd.) 5

6 Wet Collectors or Scrubbers Wet scrubbers remove dust from a gas stream by collecting it with a suitable liquid. Fig. 6 illustrates a spray type wet scrubber. AI3_fig6.gif AI_3_0_6.mov A Figure 6 Cyclonic Spray Scrubber Wet scrubbers operate by passing the gas stream through a sprayed mist. Deflectors may be added to provide an impinging surface, and the particulate matter is removed by the liquid stream. Fig. 7 illustrates the principle of operation of a Venturi scrubber. AI3_fig7.gif Figure 7 Venturi Scrubber Operation 6

7 Unlike other mechanical particulate collectors, the wet scrubbers simultaneously remove dust and gaseous pollutants. Wet scrubbers are efficient on particles 1 to 5 microns in diameter. However, they have a serious drawback in cases where the combustion gases contain sulphur dioxide (SO 2 ). Sulphur dioxide in water forms a weak acid which is corrosive to steel, an irritant to human skin, and a soil pollutant. Since in this case the discharge water from the spray type scrubber is acidic, it requires a safe disposal site. The scrubbing water may be treated with lime or dolomite to control water ph, resulting in higher operating and maintenance costs. Fig. 8 illustrates a Venturi type wet scrubber. AI3_fig8.gif Figure 8 Venturi Scrubber 7

8 ELECTROSTATIC PRECIPITATORS Electrostatic precipitators (Fig. 9) produce an electric charge on the particles to be collected and then propel the charged particles by electrostatic forces to the collecting electrodes. AI_3_0_5.bmp G Figure 9 Electrostatic Precipitator 8

9 The precipitator operation involves four basic steps: (Fig. 10) 1. An intense electrostatic field is maintained between the discharge electrode and the collecting electrodes. 2. Particles entrained in a gas become electrically charged when subjected to a strong electrostatic field. 3. The negatively charged particles, still in the presence of an electrostatic field, are attracted to the positively charged (grounded) collecting electrodes. 4. The collected dust is knocked off the electrodes, by rapping, into storage hoppers. AI3_fig10.gif (a) (b) Figure 10 Principle of Electrostatic Precipitator Operation 9

10 Figure 11 illustrates the details of an electrostatic precipitator s basic components. Figure 11 Electrostatic Precipitator Precipitators operate at 80-99% efficiency, resulting in a high percentage of particulate removal from the flue gases from coal-fired boilers. SOLID WASTE DISPOSAL In coal-fired power plants, the flyash removal has been accomplished by either mechanical or electrical collectors, or a combination of both to ensure that no atmospheric pollution results. The ash removal system is either hydraulic or pneumatic. A pneumatic transfer disposal system is shown in Fig

11 Figure 12 Pneumatic Flyash Removal System In the pneumatic system, flyash from the dust collectors drops into hoppers, and a high velocity air stream picks up the ash and carries it to an ash storage bin for disposal. The most effective method of disposal is to bury the flyash under a suitable landfill. At some plants, flyash is a saleable product to the concrete industry and therefore the equipment is more than just a pollution control device. LIQUID THERMAL POLLUTANTS One of the main factors to be taken into consideration when choosing the location of a power plant is the availability of a sufficient and suitable supply of cooling water. The condensing steam turbine requires considerable quantities of cooling water for use in the main condensers. After the water travels through the condensers and other related equipment, it is discharged back to its source at an elevated temperature. This constitutes thermal pollution. 11

12 According to Dalton s Law, the gases in the atmosphere exert pressures which are in proportion to their volume ratios. It is now evident that the apparently insignificant rise in water temperature raises the pressure exerted by water vapor above the water s free surface. This water vapor pressure reduces the partial pressure of oxygen. Henry s law states that the amount of any gas that water can dissolve depends on the partial pressure the gas exerts on the free surface of the water. Thus the amount of oxygen dissolved in the water is reduced. This is called the deaeration principal. Such reductions in the percentage of oxygen can have lethal effects for many kinds of water life. This has been proven both by experiment and by actual observations in the vicinity of power plant effluent discharge. Of course, certain tropical forms of water life may survive in a water temperature as high as 35 C (Persian Gulf in August). Yet water life on the North American continent is hardly adaptable to such a thermal environment. Especially in a narrow river, the power plant effluent in moderate quantities may represent a deadly asphyxiation snare to the fish that may pass through it. Cooling Ponds Cooling ponds are an effective means to combat thermal pollution. If cooling water can be supplied in unlimited quantities and there is also sufficient flat space around the power plant, then a cooling pond system might be the answer. Figure 13 Cooling Pond System The arrangement shown in Fig. 13 employs two ponds in series, although only one might be used at a time. When both ponds are used in series, then valves A, C, F, and G are open, while B, D, and E are closed. High temperature water enters through A and its temperature is recorded at thermometer a. It then goes through the sprays of the main pond. There it is discharged in a fine spray above the surface of the pond. 12

13 The droplets coming into contact with the atmospheric air not only cool down, but they also have a chance to enrich themselves in oxygen. A barrier as shown retains the water in the pond for a maximum length of time. The water now exits through valve C, its temperature is recorded at b and it enters the suction of the auxiliary pump. Again the spraying process is repeated in the auxiliary pond and the temperature is recorded at c. The cooled water is discharged into the river or lake. Cooling Towers Where the supply of cooling water is limited a cooling tower is used to allow recycling of the cooling water. The principle of cooling remains the same whether cooling ponds or cooling towers are employed; that is, the heat must be given up to the atmosphere. Cooling towers are classified according to the method of passing the air over the water to be cooled. In all types, the water supply is introduced at or near the top and it falls by gravity over the fill into the water reservoir at the bottom. The fill consists of some arrangement of splash bars, generally constructed of redwood or cypress, or cement asbestos and designed to cause the falling water to be broken into droplets or to run across the boards in a film; the object being to present the maximum water surface area to the cooling air. 1. Natural Draft Cooling Towers a. Open Type The open or atmospheric type has walls constructed of wooden louvres or slats laid horizontally along the length of the walls and angled so that the air enters the tower in a downward direction. This reduces the tendency to lift the fine water spray out of the top of the tower and gives a better distribution of cooling air across the whole cross-section. The movement of air is dependent upon natural convection currents. 13

14 b. Closed Type The natural draft type of tower is often built with closed sides which are carried above the level of the water entry; this type is known as the closed or chimney type. Air passing over the water to be cooled absorbs sensible heat, increasing its temperature and vapor content. The air s density is reduced, and the surrounding heavier atmosphere forces it upward and out of the tower. The flow of air through the tower varies according to the difference in densities between the ambient air and the air leaving the heat transfer surfaces. AI3_fig14.gif Figure 14 Natural Draft Cooling Tower One style of the natural draft tower in common use is the Hyperbolic Tower. This type is built of reinforced concrete in sizes up to m 3 /h, with dimensions of about 60 m base diameter and 90 m high. The air inlet, water distribution, and fill are similar to a mechanical draft tower, and the great majority of the height is purely chimney. These towers are used in large, central electricity generating stations. 14

15 2. Mechanical Draft Cooling Towers These towers are constructed using either forced draft or induced draft and have several advantages over the natural draft type. They are smaller for equivalent duty and therefore need less ground area and use less pumping power. They are not dependent upon weather conditions to aid convection and so they give a more constant performance over all seasons of the year. Against these advantages must be set the extra complication of the fan and the power it absorbs. a. Induced Draft The induced draft method is the most widely used at the present time. As illustrated in Fig. 15, the induced draft cooling tower has a suitable enclosure that contains the spray nozzles, baffles, and an induced draft fan at the top. Hot water is sprayed in the upper part of the tower and hits an arrangement of drift eliminators and baffles from which it slowly drips into the collecting basin. As it drips, it comes in contact with the atmospheric air. Part of the water evaporates, and as it does, the temperature of the dripping water is lowered. As the warmed air rises higher, assisted by the fan at the top, it accelerates towards the exit, creating a vacuum in the lower part of the tower. This vacuum is filled by a cool air current that rushes in from the bottom. In this way, the dripping water is always in contact with a cooler draft of air, and consequently the cooling effect is more evenly distributed across all sections of the tower, than with a natural draft tower. AI3_fig15.gif Figure 15 Induced Draft Cooling Tower 15

16 There is little chance of the fan being subjected to icing because it is in the path of the warm discharge air, and noise from the fan is at a minimum because of its location. Air enters the tower through a very large louvre section, thus decreasing frost tendency in winter. b. Forced Draft The forced draft method includes a fan placed at the bottom of the tower to draw air from the surrounding atmosphere and force it upwards across the fill, counterflow to the falling water. This has the advantage that the fan is more accessible during operation; however, in colder climates frost may form on the fan blades and suction dampers as cold air is handled. This type is also prone to recycling the vapor laden air from the tower discharge, thus reducing its ability to cool the water. AI3_fig16.gif Figure 16 Forced Draft Tower c. Dry Tower Another type of cooling tower used in some instances where the cooling water supply is very restricted is the dry tower. The cooling water passes through finned tubes placed in the tower in banks and cooled by air currents produced by mechanical or natural means. By this method, the water lost by evaporation and drift is eliminated and the makeup required by the system is only that caused by joint leakage, etc. 16

17 Despite their advantages, cooling towers under certain circumstances may actually contribute to the problem of pollution. This can happen when their number or size is such that the relative humidity over an adjacent community is raised. For example, on a hot windless day, the air can support a considerable pressure of water vapor. This oxygen displacing vapor makes the air feel heavy and breathing may be difficult. Another pollution consideration is water vapor leaving the tower that may come in contact with the effluent from the boiler stacks. If the latter happens to contain sulphur dioxide, then a weak acid may result. This acid falling as fog or morning dew can attack and disfigure the appearance of buildings, cause itchiness of human skin, watering eyes, coughing, etc. Persons suffering from respiratory diseases may feel quite uncomfortable in such a polluted environment. Thus the size and number of cooling towers must be considered carefully before the towers are erected as they may, on one hand, eliminate one kind of pollution (thermal) but create others. Cooling towers, may cause water pollution when the recycled water is discharged partly or totally. Such water is usually laden with chemicals to inhibit metal corrosion, wood decay, and scale formation in the heat exchanger and tower itself. Such water, although suitable for industrial use, may not be compatible with the requirements for aquatic life growth. Usually the suppliers of water treatment chemicals are obligated under law to also provide suitable chemicals to neutralize the water about to be discarded, thus avoiding the adverse effects it may have upon the aquatic life. 17

18 References and Reference Material For more information on this topic, the following are recommended: 1. Babcock & Wilcox. Steam / its generation and use. 39th ed. New York: Babcock & Wilcox; Graham, Frank D. Power Plant Engineers Guide. Indianapolis, IN: Bobbs- Merrill; Salisbury, J. Kenneth. Kent s Mechanical Engineer s Handbook - Power. 12th ed. New York: John Wiley & Sons. 4. Singer, Joseph G., ed. Combustion - Fossil Power Systems. 3rd ed. Windsor, CT: Combustion Engineering Inc.;