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Gaseous and Noise Pollutants
Learning Outcome
When you complete this chapter 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.
- Describe the adverse effects of (and the associated control systems for) carbon monoxide, sulphur oxides, and nitrogen oxides
- Describe how noise pollution is measured and controlled.
- Describe typical devices and systems for monitoring gaseous and noise pollutants.
Objective One
When you complete this objective you will be able to… Describe the adverse effects of (and the associated control systems for) carbon monoxide, sulphur oxides, and nitrogen oxides
Learning Material
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.
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.
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)
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.
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. 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
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. 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 / Ao) where A is the measured quantity, and Ao 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, Lw, can be expressed as:
Lw = 10 log(W / Wo) where W is the sound power, in watts (W), of the measured sound and Wo is a reference sound power, generally 10-12^ W. Table 1 gives decibels for some common noises and their corresponding sound pressure level. Table 1 Sound Pressures and Sound Pressure Levels Sound Pressure (Pa)
SPL
(dB ref 0.00002 Pa) Source (long time average) Distance (m) 200 140 Threshold of pain 130 Pneumatic clipper 2 20 120 Threshold of discomfort 110 Automobile horn 6 2 100 New York subway train Inside 90 Motor bus Inside 0.2 80 Traffic on street corner 70 Conversational speech 1 0.02 60 Typical business office Inside 50 Quiet residence Inside 0.002 40 Library Inside 30 Whisper 1. 0.0002 20 10 0.00002 0 Threshold of hearing The most important thing to realize from these definitions is that because decibels represent a logarithmic scale, sound levels givein decibels do not add directly together
= 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.
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.
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:
- Reduce the sound at the source.
- Modify the sound wave path to the location.
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.
Objective Three
When you complete this objective you will be able to… Describe typical devices and systems for monitoring gaseous and noise pollutants.
Learning Material
Pollution Monitoring
In this chapter, 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 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.
