Gaseous and Noise Pollutants

Transcription

1 Gaseous and Noise Pollutants Learning Outcome When you complete this module you will be able to: Describe the nature, environmental impacts and control methods for gaseous and noise pollutants in a power plant. Learning Objectives Here is what you will be able to do when you complete each objective: 1. Describe the adverse effects of (and the associated control systems for) carbon monoxide, sulphur oxides and nitrogen oxides. 2. Describe how noise pollution is measured and controlled. 3. Describe typical devices and systems for monitoring gaseous and noise pollutants. 1

2 GASEOUS POLLUTANTS Gaseous pollutants from a power plant result from the burning of common fuels. The gaseous pollutants that a power engineer has to be aware of are: carbon monoxide (CO), sulphur dioxide (SO 2 ), and nitrogen oxides (NO and NO 2 ). The production of these gaseous pollutants is detrimental to the operation and life of a power plant, as well as the environment. For these reasons, a power engineer must be fully aware of the effects of these pollutants, how to detect them, and most of all, how to control their emission into the atmosphere. Carbon Monoxide Carbon Monoxide is a product of incomplete combustion in the boiler furnace. When fuel is burned, carbon reacts with oxygen. If there is enough oxygen, the carbon and the oxygen form carbon dioxide (CO 2 ). If there is a shortage of oxygen, the carbon combines with the oxygen to form carbon monoxide (CO). Carbon monoxide is a serious pollutant as it is a deadly, toxic gas. Besides this, its presence in the stack emissions represents a loss for the operation. Heat energy is lost when the carbon element of fuel burns to carbon monoxide rather than carbon dioxide, because if the carbon element is not completely burned, then most likely the element hydrogen (H 2 ) isn t either. As well, these two gaseous pollutants are very explosive. Should they collect somewhere in the boiler passes and form a stagnant pocket, then it would only take a spark or some source of ignition to cause a furnace explosion. Different conditions in a furnace can create a shortage of oxygen. The most obvious case is that there is not enough air being supplied to the furnace. Another reason is, due to insufficient turbulence, the air and the fuel do not mix well enough for the carbon from the fuel to react completely with the oxygen in the air. Local shortages of oxygen result in the production of carbon monoxide. Also, overloading of the boiler can cause quick combustion, depriving the fuel of the time needed to be completely burned in the hot enclosure of the furnace. The three causes of incomplete combustion mentioned above can all be controlled by the power engineer. Most plants are equipped with automatic flue gas analyzers which continuously monitor the composition of the flue gas that leaves the furnace. Even where there are no flue gas analyzers, a power engineer should, by experience, know how to determine proper combustion from the appearance of the flame, the temperature of the gas at various points in its path, and the appearance of the stack emissions. If necessary, the power engineer should be able to perform an analysis of the flue gas with an Orsat apparatus. 2

3 Sulphur Oxides Sulphur oxides are produced by the burning of sulphur; an element contained in almost all industrial coals and most fuel oils. During the combustion process in a furnace, the sulphur in the fuel is oxidized into sulphur dioxide (SO 2 ) and sulphur trioxide (SO 3 ), with the sulphur dioxide usually equal to at least 95% of the total volume of sulphur oxides. These products are not as toxic as carbon monoxide, but can be injurious to humans and animals as well as vegetation. In the presence of moisture, they form a weak sulphuric acid which irritates skin, corrodes most metals, and disfigures the exterior appearance of most painted surfaces. As part of industrial smog, they cause itchy skin, watering eyes, coughing, and fatigue. Since there are no explicit emission control standards for SO 3, and most of the oxides are SO 2, processes concentrate on the removal of SO 2 from the flue gas. As well, studies have shown that systems designed to reduce the concentration of SO 2 usually remove around 50% of the SO 3. The majority of economical systems designed to remove sulphur oxides from flue gas omissions do not produce a useful product. Sulphur dioxide can be effectively eliminated from the stack emissions by wet lime or limestone scrubbing, double alkali or dilute sulphuric acid systems. None of these systems produces a readily usable product. Regenerable systems are not commonly used only for cleaning up flue gas to meet emission standards. The amount of sulphur in the flue gas is usually too small for economical recovery. This is because the regenerable processes are much more complex and expensive to build and operate. Another major disadvantage is that all regenerable systems use large volumes of toxic or explosive reducing gases such as hydrogen sulphide, carbon monoxide, and hydrogen. They are usually only used for flue gas clean-up if there is a waste disposal problem or if there is enough demand for the end product of elemental sulphur or sulphuric acid to recover the cost of production. Some of these processes are magnesium oxide, catalytic oxidation and aqueous carbonation. Because the regenerable systems are not very common and relatively complex, they will not be discussed any further at this level. The three nonregenerable processes mentioned will be described in more detail. 3

4 1. Lime/Limestone Scrubbing In the past, lime/limestone systems have been used to remove sulphur at the combustion stage in the furnace or from the flue gas with a wet scrubber. In the late sixties and early seventies, some systems used the method at the combustion stage where pulverized limestone was injected into the furnace with the pulverized coal to react with the sulphur in the combustion chamber. Because of problems with scaling and plugging of boiler convection surfaces, most of these combustion stage type systems have been changed to some sort of wet flue gas scrubbing system and are no longer built. The wet scrubbing process has several advantages over the dry process, including higher efficiency of SO 2 removal, less boiler operation interference, and usually a lower operating cost for large power plants. The wet process also requires less limestone for a much higher percentage of SO 2 removal. Because less limestone is required, less material handling facilities and less solid waste disposal is required. Fig. 1 shows a flow diagram for a lime/limestone wet scrubbing process. The flue gas enters the scrubber where it comes in contact with a limestone slurry. The slurry absorbs the sulphur dioxide which reacts with the limestone and precipitates out as calcium salts in the effluent hold tank. Fresh limestone is added to replace the spent absorbent and the slurry is recirculated through the scrubber. The absorbent slurry usually contains about 15% dissolved solids. To control the build up of solids, there is a bleed line from the scrubber hold tank which feeds a clarification tank. Figure 1 Lime/Limestone Scrubbing Process (Courtesy of Combustion Engineering) 4

5 2. Double Alkali Systems Double alkali systems are similar to the lime/limestone systems in that limestone is consumed and a waste product of calcium sulphite or calcium sulphate is produced. Fig. 2 shows a process diagram for a double alkali system. In this method, an alkali solution such as sodium sulphite is circulated through the scrubber to absorb the SO 2. It leaves the scrubber and goes to a reactor where it is combined with a lime or limestone solution. The reaction with the lime produces a precipitate of calcium solids. Figure 2 Double Alkali System Process (Courtesy of Combustion Engineering) The calcium solids are separated from the liquid in conventional separation equipment such as the thickener shown in the system in Fig. 2. The solids are disposed and the clear liquid is replenished with sodium and recirculated through the system. This system has a major advantage over the lime/limestone method because the absorbent sodium solution contains no suspended solids which tend to create scaling and plugging of interior scrubber parts. 5

6 3. Dilute Sulphuric Acid The dilute sulphuric acid method uses a dilute solution of sulphuric acid in water to absorb the SO 2. The absorbed SO 2 is then oxidized to sulphuric acid using an iron oxidization catalyst. Some of the sulphuric acid is recirculated through the absorber and some of it goes to a crystallizer where limestone is added and gypsum is formed. Fig. 3 shows a dilute sulphuric acid process diagram. The disadvantages of this system are a high initial cost and corrosion of system parts due to the acidity of the absorbent solution. Figure 3 Dilute Sulphuric Acid Process (Courtesy of Combustion Engineering) Nitrogen Oxides In recent years, nitrogen monoxide and nitrogen dioxide have been shown to react with sunlight in a complicated reaction to form a photochemical smog. As well, in the presence of other hydrocarbons, they may form cyanides which are lethal poisons. Their presence in industrial emissions is now closely regulated and power plants must ensure that emissions do not exceed the permitted level. 6

7 The nitrogen from these oxides originates from the atmospheric nitrogen and from the nitrogen contained in all fossil fuels. The reactions during the burning process that create nitrogen oxides (NO x ) are hard to prevent and they will occur under ideal combustion conditions. There are two practical means of controlling emissions of nitrogen oxides from power plants. Control may be accomplished either by minimizing their formation, which is mostly dependent on high temperature and the availability of oxygen, or by removing the oxides from the flue gas. It is difficult to control NO x production in power plants because the conditions favorable for high NO x production are created by combustion practices developed to increase power plant operating efficiency and to control other air pollutants. Boilers are ideally designed to have high flame temperature and use excess air to ensure complete combustion. This controls the amount of the undesirable byproducts such as smoke and carbon monoxide, but the high temperature and increased oxygen encourage high NO x formation. Large coal-fired power plants can use firing system modifications to reduce the initial excess air in the combustion process. The reduction in air decreases the amount of oxygen available to form NO x. These modifications include reduced excess air percentage, staged combustion, the use of overfire air, tangential firing, and gas recirculation. If formation of NO x is not sufficiently controlled during firing then some form of flue gas scrubbing must be used to bring their concentration down to an acceptable level. Dry processes using a reduction reaction with ammonia are most common. However, at the present time, modifications in combustion techniques and furnace design appear to be the most effective means for reduction of nitrogen oxide emissions from power plant operations. NOISE POLLUTANTS The effects of industrial noise on workers and the surrounding community have been the subject of much research over the past twenty years. The results of this research have shown that excessive noise can cause irreversible hearing losses. Some of the research even suggests that noise may also be the cause of other health problems and research is continuing in this area. The effects of noise on the communities near the source are mostly sleep interference and annoyance; depending on the frequency, intensity, and duration of the sound. For example, high frequency tones are perceived as louder than lower frequency tones at the same decibel level. Intermittent or impulse noises are often more annoying than a constant noise. 7

8 Public awareness is increasing and the requirements for noise limitation by commercial operations have also become greater. Power plants can be very noisy. In the past, some hearing loss by a plant worker was mostly taken for granted. Now employers can be liable for compensation for hearing loss and occupational health regulations on noise have become more specific and demanding. Sound Waves Sound travels as waves through the air in the same way that waves travel through water. The waves are formed by vibrating bodies or air turbulence which causes a variation in air pressure. This variation is passed via the air molecules through the air and represents an energy transfer or flow through the medium (air). Because it is a form of energy, sound can be expressed as having specific power and intensity levels. Sound waves have varying frequencies and wavelengths and they can be reflected, deflected, and absorbed. Absorption of sound occurs when a sound wave strikes a nonrigid barrier. The energy of the wave moves the fibers on the barrier s surface. Internal friction in the barrier opposes this movement and the energy of the air is converted to heat energy within the barrier. Reflection and deflection of waves occur when the sound strikes a rigid barrier. Very little of the energy of the wave is absorbed by the barrier and only the direction of the wave is changed. Sound Measurement Variations in sound, which are heard by the human ear, are caused by the different pressures and frequencies of the sound waves. A human ear is usually capable of hearing sounds which represent pressures from 2 x 10-5 Pa to 200 Pa. Because of this large range, the concept of a decibel was created to compress this range of sound levels into a more meaningful scale. A decibel (db), in mathematical terms is: decibel = 10 log (A / A o ) where A is the measured quantity, and A o is a fixed reference quantity. A decibel can represent a sound power level, a sound intensity level, or a sound pressure level. For example, the sound power level, L w, can be expressed as: 8

9 L w = 10 log (W / W o ) where W is the sound power, in watts (W), of the measured sound and W o is a reference sound power, generally W. Table 1 gives decibels for some common noises and their corresponding sound pressure level. Sound Pressure (Pa) SPL (db ref Pa) Source (long time average) Distance (m) Threshold of pain 130 Pneumatic clipper Threshold of discomfort 110 Automobile horn New York subway train Inside 90 Motor bus Inside Typical business office Inside 50 Quiet residence Inside Library Inside 30 Whisper Threshold of hearing Table 1 Sound Pressures and Sound Pressure Levels The most important thing to realize from these definitions is that because decibels represent a logarithmic scale, sound levels given in decibels do not add directly together. 9

10 For example, a man stands at a point and hears noise from two distinct sources, S 1 and S 2 The level of sound power from each source, L s, equals 80 decibels. This does not mean that the total level of sound power, L T, received by the man is 160 decibels. In fact the sound power level his point is 83 decibels, which can be shown by the following calculations: Let the reference level, W o equal watts (W). Then L s = 10 log (W s / W o ) 80 db = 10 log (W s / ) 8 = log (W s / ) log -1 (8) = W s / = W s / Solving for W s : W s = 10 8 x = 10-4 W (watts) Because there are two sources, W s, the level of the power intensity at the point is 2W s then: L s = 10 log (2 x 10-4 / ) = 10 log (2 x 10 8 ) = 10 x 8.3 = 83 db From this calculation, it is easily seen that doubling the sound intensity at point A is represented as only a shift of 3 decibels. Therefore, it is important to remember that decibels levels from multiple sources cannot be simply added together. Decibels from two or more sources requires a conversion of each level back to its antilogarithm before addition can occur. 10

11 Sources of Noise There are numerous sources of noise in a plant. These sources include machinery, furnaces, air movement (fans and compressors) and structural vibrations transferred from moving parts. Fig. 4 shows some common sources of noise, their levels and their effects on people. Figure 4 Noise Pollution Graph Furnaces, fans, reciprocating compressors, and pulverizers generally produce low frequency noise while gas or steam passing through vents and valves produce high frequency noise. High frequency noise is generally perceived as louder and more annoying than low frequency noise. 11

12 Noise Control Efforts to control noise are generally aimed at lowering the sound intensity at a given location. The minimum noticeable noise reduction is approximately ten percent on the decibel scale. This means a reduction in noise of 5- to 10-fold is probably necessary to be worthwhile and noticeable. There are three ways to reduce noise at a given location: 1. Reduce the sound at the source. 2. Modify the sound wave path to the location. 3. Protect the receiver with a barrier device; i.e., ear plugs or muff-type ear defenders. Noises are often controlled by a combination of these methods. Reducing the sound at the source can be achieved by vibration isolation mounts and vibration damping equipment for machinery, and by reducing speed of operation and tip velocities in fans. To be cost effective, most of these modifications must be considered and included at the design phase. After the design stage, modification of the sound path to the receiver is often used. Enclosures are a form of sound path modification. They are designed using special materials which reflect sound back to the source, and in some cases, absorb the sound as well. Soundproof materials absorb sound waves by trapping them in the material so only a very small wave leaves the side opposite from where it entered. The material also reflects some of the sound back in the direction it came from. The best materials for sound absorption are generally porous and relatively easy to deform. Lead, loose sand, brick, and almost all heat insulants (glass, wool, polyurethane, cork) are good sound absorbers. Fig. 5 illustrates how sound wave paths are changed by a mainly reflective barrier and an absorbent barrier. Figure 5 Sound Reflectors and Absorbers 12

13 Fig. 6 shows two types of noise enclosures: mufflers and lined ducts. Both of these enclosures absorb and reflect the sound. Inlet silencers on forced draft fans are also a form of enclosure. Figure 6 Sound Path Modifiers Sometimes it is necessary to enclose an entire piece of equipment. If such an enclosure is necessary, it must be leak tight. A hole 1 / 1000 of the total wall area at the enclosure would leak enough sound to make the enclosure noneffective. For high frequency noises, the best results are obtained by a double structure with a sound absorbing material in between the two structures. Another more obvious form of sound path modification is creating a buffer zone around the source. By doubling the receiver s distance from the source, the sound pressure that they receive can be halved. In many situations in a plant, it is very difficult to use sound absorption or path modification techniques. In this case personal protection, such as earplugs or muffs must be used to reduce noise to the receiver and prevent damage to hearing. 13

14 POLLUTION MONITORING In this module, pollutants from power plants and their adverse effects were discussed. As time goes on, and as the government and the public demand stricter measures on pollution, the challenge of environmental control will continue to grow. Many pollutants cannot be totally eliminated; only their concentration can be controlled. In order to effectively fight environmental pollution in any form, plant outputs must be continually monitored. This monitoring also serves as an indicator of how well the existing equipment is functioning. Gaseous Emission Monitoring For the power plant, the boiler stack is the main source of atmospheric pollution. Ventilation and exhaust stacks also contribute. Whatever the case, a clear picture of what is going out from the stacks is of vital importance for effective pollution control. Flue gas analysis proceeds in a similar manner as the Orsat gas analyzer. The student may recall that this device traps a sample of flue gas and cools it to ambient temperature. The gas is then exposed to absorbent solutions which react with the different gases in the sample. As each gas is absorbed its partial pressure is eliminated and an amount of water takes its place. The volume of the gas, as a percentage of the total volume, can then be measured. A similar method is also applied in measuring pollutants such as sulphur dioxide and nitrous oxides. The apparatus may be hand-operated exactly as the Orsat analyzer or it may form an automatic sampling train. The sampling train shown in Fig. 7, may, through a selector switch, sample the flue gas in the stack alternatively with the ambient air at a remote location. The pump operates continuously to fill the storage box with the gas being tested and move the sample to be tested through the sampling train. Figure 7 Automatic Sampling and Analyzing Train 14

15 The position in Fig. 7 shows the flue gas being tested. Valves A and D are open while B and C are closed. When the storing box is full, valves A and D close. The sample is cooled by circulating cooling water. When the temperature is correct, valves B and C open and the sample is pumped through the series and analyzer trains #1, #2, #3, #4, #5, and #6. Each analyzer contains a reagent designed to absorb or react with a distinct pollutant and the amount of that gas is determined. Results from the analyzers (electric or pneumatic) are sent to recorders and indicators in the control room. Alarms may be installed to go off if a preset allowable limit of any gas is exceeded. The continuous monitoring of the plant emissions is vital. As well as ensuring that the emissions are kept at an acceptable level, the results from the monitoring can be used to see how well existing equipment is functioning. Continual monitoring also provides records of plant emissions. Noise Monitoring As discussed in noise pollution, sound levels are measured in decibels without distinguishing between frequencies. However, with people, higher frequency sounds at a given decibel level are perceived as louder than lower frequency sounds at the same level. Sound measurement instruments (sound meters) contain weighting networks which take into account frequency and adjust the readout of the meter to correspond to how loud a person would perceive the sound to be. There are three weighting networks, A, B, and C. When noise is expressed in decibels there may be an extra letter after the db, such as dba. This letter, A, B, or C indicates, which, if any weighting network was used to measure the noise. Fig. 8 shows a hand-held level meter, which is rated as Type 1, 2, 3, or S in accordance with noise legislation. Noise laws or regulations will specify which of these types of sound meters and which weighting network must be used to investigate noise levels for conformance to the regulations. 15

16 Figure 8 Sound Level Meter (Courtesy of B and K Instruments, Inc. and General Radio Co.) 16