Treatment & Control of Air Pollutants

Transcription

1 Treatment & Control of Air Pollutants Air Quality Standards Air quality standards are provided by many groups and organisations such as: National Environment Protection Council Standards WHO US EPA NSW EPA standards Standards Australia Chapter 5 These have been discussed in detail in other modules. All are used to some extent in air quality monitoring in NSW, but those provided by the NSW EPA and Standards Australia are the most significant. Controlling Emission of Particles Particulate matter leads to the most obvious forms of air pollution therefore it is the form that receives the most effort in pollution reduction measures. Many devices and practices that vary greatly in cost and effectiveness are used throughout industry and society to reduce the number of particles that are poured into the atmosphere. The more important of these are discussed in the following chapter. Process Modifications Sometimes it is possible to reduce the level of particulate pollutants by simply changing a procedure or a part of the manufacturing process. Probably the simplest of these is fuel substitution. From the middle ages to the mid-twentieth century most of the homes in Britain had wood or coal fires as their main heating source. These were an enormous source of particulate pollution, and probably greatly contributed to the countries smog problems. The British Government realised this and banned the use of coal and wood in home heating forcing all private citizens to change their heating over to oil or gas. This dramatically reduced the level of particulate pollutants in the atmosphere in subsequent years. Other process modifications such as encapsulation and wet operation can also greatly reduce the amount of fugitive particles emanating from a potential pollutant source. Gravitational Settling and Cyclones This is the simplest way of controlling particulate emissions. This includes several systems. One of the simplest is the gravitational settling chamber, where particles are allowed to drop out of the air by sedimentation in a still environment. This method is not

2 so common, but separation by centrifugation is probably the most common form of particulate removal. Here the gas is spun rapidly, which causes the heavier particulate matter to collect on the outside of the separator by centrifugal force, where it is collected and removed. This device is called a cyclone separator, and is demonstrated in Figure 5.1. cleaned gas outlet dirty gas inlet particulate outlet Figure 5.1 A typical cyclone separator Filtration Systems Collection of particles by filtration systems such as fibre bags or beds of small particles are becoming increasingly popular due to their ease of use and low cost. Fabric filters are the most common of these devices. These are made of material that allows gas molecules to pass through, but prevents the much larger particulate matter from passing. The bags are normally open tubes, manufactured from material with small pores and many fibrous hairs that stick out from the main membrane. The small particulates adhere to these hairs, which causes them to coagulate into larger particles. The bags are periodically shaken to remove the accumulated particles, which fall down into a collector and are removed. Fibre bags are commonly used for the control of particulate emissions with very high dust loadings and smaller particles.

3 In the typical baghouse unit (figure 5.2) waste gas enters from the side and flows downward toward a hopper, where the flow is reversed upward into the array of bags. As the gas changes direction, large particles are removed by inertial separation and collected in the hopper. As gas passes through the tubular bags, dust is collected on the inside of the bag surface and the filtered gas is discharged to the atmosphere. Fabric filtration has a collection efficiency of >99%, making it the most efficient method of particulate removal. Limitations of the method include high initial costs, flammability problems with some dusts, large size requirements for the baghouse unit, limitation on flue gas temperatures to <300 C, and possible compromises on efficiency with large changes in gas moisture content. Figure 5.2 Filtration bag for collection of particulate matter Wet Scrubbers These devices consist of spray systems where fine water droplets are sprayed at high velocity at right angles to the emerging gas. Most of the particles in the gas stream are scavenged by the water droplets, which fall and are collected along with the particles. One example of a wet scrubber is the open spray tower (see Figure 5.3). In these devices a scrubbing liquid is sprayed downward through low-pressure nozzles. The dust-laden waste gas enters from the side and moves downward toward the liquid pool at the bottom of the tower. Large particles are removed by impingement on the liquid surface. The waste air then changes direction and moves in the opposite direction to the flow of the scrubbing liquid and is discharged at the top. Dust particles are captured by the falling droplets. The liquid-particle agglomerate is collected in the liquid pool at the bottom of the tower.

4 Wet scrubbers such as spray towers are limited to low gas flows to prevent liquid drop entrainment in the scrubbed gas stream. This scrubber attains relatively low efficiencies (80-90%) and is usually employed as a pre-cleaner to remove particles larger than 5µm. Other forms of wet scrubbers such as the venturi scrubber are used if greater collection efficiencies are required. These are also more effective at collecting fine particles. Scrubbers are relatively inexpensive to set up, but have high maintenance costs due to corrosion by acidic flue gases and abrasion of fine particles. Figure 5.3 A spray type wet scrubber 1 Electrostatic Precipitators These pass the dirty gas through a series of fine wires (called coronas). These wires support a DC current and are either positively or negatively charged. They work by relying on the fact that many suspended particles (particularly in smoke) are charged and will be attracted to the oppositely charged wire, causing them to coalesce and fall out by precipitation. Alternatively the corona produces negative ions that cause the particles in the gas stream to become negatively charged, and therefore attracted to the positive terminal where they also coalesce and fall into a collection hopper.

5 The collection efficiency of these devices is greatly affected by the size of the precipitation field and the gas velocity. Large precipitators and low gas flow rates give better results. Figure 5.4 shows a typical electrostatic precipitator. soot free gas escape soot laden smoke inlet charged electrodes earth point removal of soot particles Figure 5.4 A typical electrostatic precipitator Control of Particulate Fugitive Emissions As is the case with most fugitive emissions, modifications to process and procedures generally provide the greatest improvement in decreasing the level of fugitive particulate emissions. Their are some simple steps however which can greatly decrease the amount of emissions when process errors occur. The use of ventilation and collection hoods over parts of the process reduces emissions by sucking particulate matter into filters that are located in the hood. A simple example of this is the common kitchen range hood. Other methods such as wet suppression and chemical or physical stabilisation are also used. Wet suppression involves spraying the work or process at regular intervals in order to wet any particulate matter. Provided the area is not too hot this means that the particles are adsorbed onto water, increasing their mass and/or adhesion to the ground and preventing them from entering the atmosphere. Control of Gaseous Pollutants

6 Even though the health effects of gaseous pollutants have been known for a long time, public concern has in the past always been focused on particulate pollutants due to their visibility. Gaseous pollutants were largely ignored because they were invisible. As a consequence the methods for control of gaseous pollutants is nowhere near as advanced, as is that for particulate pollutants. Additionally in particulate control, specific techniques can be utilised for a variety of dust collection problems and sources. The major consideration in selecting such equipment is the performance required. The application of control technology for gaseous pollutants cannot be as Generically applied. Gas cleaning equipment must be developed/designed to control specific gaseous pollutants or pollutant categories that vary considerably in their chemical and physical properties. The control of gaseous pollutants is an inherently more complex technological problem. It is also an inherently more expensive one, as both capital and operating costs for control equipment are often high. Major physical and/or chemical principles used in removing gaseous contaminants from effluent gas streams include combustion, adsorption, and absorption. These principles and their applications are described below. Process Modifications As was the case with particulate pollutants some of the simplest and least expensive methods for the control of gaseous pollutants are the most obvious. Modifications to manufacturing processes can often lead to substantial reductions in gas emissions and pollutants. As was the case with particulates fuel substitution can greatly reduce pollution levels. For example the use of low sulfur coal, or fuel oils in place of cheaper coal can greatly reduce the amount of SO 2 emissions at the source. Another example of a process modification is the maintenance of correct combustion ratio for fuel and air in a furnace. Ensuring the oxygen concentration stays high (through air injection processes) not only increases the heat output, but also ensures that combustion is complete and CO 2 is generated rather than the far more dangerous CO. This type of source control is always the best approach wherever possible. Combustion or Incineration Combustion involves a series of complex chemical reactions in which oxygen is combined with organic molecules, to form CO 2 and H 2 O. Although combustion releases considerable quantities of heat energy, the process requires a flame or high temperature for its initiation. The use of combustion processes for control of effluent gases is commonly referred to as incineration or afterburning. The term afterburning is applicable when the treatment process is located downstream of a primary combustion process. Incineration is often applied to effluent streams containing combustible gases in which the volume flow rate of gas is large and the concentration of the combustible contaminant is relatively low. Incineration can be used to eliminate; malodourants such as mercaptans and H 2 S

7 organic aerosols and visible plumes such as those produced by coffee roaster and enamel bake ovens combustible gases produced by refineries, and solvent vapours produced by a variety of industrial processes. There are three types of combustion systems commonly utilised for pollution control, These include; direct flame, thermal, and catalytic incineration systems. The use of any of these for air pollution control requires a knowledge of the flammability range of the pollutant gas-air mixture. The concentration of combustible contaminants and oxygen relative to the flammability range may determine the type of incineration system used and requirements for supplemental fuel and/or air. Thermal incineration is used in a large number of industrial processes to reduce gaseous emissions such as enamel baking ovens and coke ovens. Due to the need for fuel and/or catalysts the cost of this type of pollution control is high. Adsorption Some contaminant gases can be removed from effluents by their physical adsorption to solid surfaces. The solid collecting media are called adsorbents, the collected gases or vapours, the adsorbate. The adherence of adsorbate to adsorbents results from the intermolecular attractive (Van der Waals) forces between them. Adsorption is reversible; the adsorbate can be removed from the adsorbent by increasing temperature or lowering pressure. Because of this reversibility, adsorption is widely used for solvent recovery in dry cleaning, metal degreasing operations, surface coating, and rayon, plastic, and rubber processing. The application of adsorption technology has received only limited. use in solving ambient air pollution problems with its main use involved in the reduction of odour. Adsorbents that have a very large surface area to volume ratio such as activated carbon are the preferred agents for this type of gaseous pollutant control. Efficiencies may be as high as 99% if the operating parameters are optimised. Absorption Control systems that employ liquid media to remove gases are called scrubbers. Scrubbers remove gases by chemical absorption in a medium that may be a liquid or a liquid-solid slurry. Because of its low cost, water is the most commonly used scrubbing medium. Additives, however, are commonly employed to increase chemical reactivity and absorption capacity. For example, lime or ethanolamine is added to scrubbing media for the control of acidic gases.

8 Scrubbers must be designed to provide maximum contact between the gas phase and the scrubbing medium. Significant gas-media contact can be achieved by employing mixing mechanisms such as spraying, atomisation, and agitation. Gas-media contact can also be enhanced by the use of column/tower packing materials. These are usually ceramic and made in a variety of configurations designed to provide a large amount of surface area The most widely used scrubbing system has been the fixed bed packed tower (see Figure 5.5). The scrubbing medium is introduced at the top and flows downward, against the flow of the effluent gas. The efficiency of absorption is affected by the height of the packed tower since the longer the gas path through the scrubber, the greater the chance of the gas being absorbed. Dry Scrubbing Systems This process is used to remove large amounts of SO x from flue gases. This dry flue gas desulfurisation is achieved by using a dry alkaline absorbent (usually lime or sodium carbonate) into contact with the flue gases. These absorbents can also be applied on a spray, where the heat vapourises the liquid, and the mixture of flyash, sulfates and dry absorbent is collected and removed by standard dust collectors. Dry scrubbers have several advantages over wet scrubbers. These include: they do not suffer from scaling or residue build up they do not require elaborate sludge handling systems for waste materials they require less maintenance as there is less corrosion they use up to 50% less power, and they use less water.

9 Figure 5.5 A fixed bed tower absorption scrubber 1 Control of Gaseous and Liquid Fugitive Emissions Most effort in reducing the level of gaseous pollutants involves reduction of the pollutant at the source which is most often a stack or emission valve. Much of the polluted gaseous material released into the atmosphere does not come through deliberate emission, but rather fugitive emission. As is the case with all fugitive emissions, control is normally associated with improved maintenance and procedures. Routine checking of small sources such as seals, pumps and valves for leaks normally completely eliminates them as fugitive emission sources. Large sources such as solvent baths generally need to be enclosed or at least adequately ventilated with appropriate fume hoods or other ventilation techniques that fed the exhaust gas through appropriate treatment systems. Wet suppression methods are also used to scrub effluent gases from fugitive emissions such as those used in the analytical chemistry laboratory in Block E at Hunter Institute of Technology. Odour Control The human nose is a very sensitive organ. This means that it will detect many gaseous pollutants that are odorous at very low concentrations. Hence it is often necessary to control the level of substances which although not toxic or directly harmful to health, smell repulsive. This is the case with many sulfur containing organic materials. The main approaches used to remove odorous materials from gas emission include wet scrubbing, charcoal filtration and incineration. Wet scrubbing is useful for the removal of many water-soluble materials. Here the pollutant is trapped in mist, absorbed and then collected in a waste stream in exactly the same fashion as with removal of other undesirable gaseous material. An alternative method relies on the fact that many of the materials which cause strong odours are strongly adsorbed onto materials with high surface areas (this is the case with most organic materials). Hence a charcoal filter (or a series of filters) can be used to remove odour. Often for strong odours, or in the case when there are higher concentrations of odorous material in the emission gas, both processes will be utilised with the effluent gas stream being passed through a charcoal filter on the way to a scrubber. Charcoal filters are generally not placed in stacks as they are combustible hence they are used in lower temperature environments.

10 The incineration process used to remove other undesirable air pollutants is also very effective at removing those substances that cause bad odours. The substances are generally combusted into other inorganic odourless gases. Control of Vehicle Emissions There has been considerable effort in reducing the amount of gaseous materials which emanate from motor vehicles due to the considerable number which are used world wide hence making these a great contributor to air pollution in general. Motor vehicle emissions produce large amounts of unburnt hydrocarbons, carbon monoxide and oxides of nitrogen. Older vehicles also produce large amounts of lead, but there are now very few of these on the load and so these will not be considered here in any detail. Reduction in motor vehicle exhaust emissions generally involves simple procedures such as maintaining the correct tuning for the engine, or more complex processes such as the use of catalytic converters. When a hydrocarbon fuel is burnt in air there is an optimum ratio of fuel to air which produces maximum power and minimum unburnt fuel. The carburettor or fuel injections system is responsible for maintaining the correct fuel to air ratio. As with all mechanical devices it comes out of adjustment with time and so needs regular tuning to maintain optimum performance. If the vehicle is not tuned it wastes fuel which is voided with other exhaust gases, and produces much higher levels of CO particularly if the engine is running too fuel rich (insufficient oxidant). Most modern motor vehicles that run on gasoline use a catalytic converter to help the conversion of unburnt hydrocarbons and CO into CO 2 and water. These catalytic converters use platinum and palladium attached to some form of ceramic material. They have an extremely high surface area (in hundreds of square meters) which allows the catalytic materials to come into contact with the exhaust gases, oxidising them to CO 2 and water vapour. Platinum and palladium are used because they are inert to the sulfur materials also found in exhaust gas streams, but they are rendered inactive if they come into contact with leaded fuels a process which is referred to as catalyst poisoning.

11 Figure 5.6 A typical catalytic converter (from Godish 1 ) One other major pollution product from motor vehicles is NO x. Unfortunately all the measures which decrease CO and hydrocarbon emissions, increase NO x emissions. Measures such as changing engine spark plug timing and reduction of compression ratios have allowed NO x emissions to be lowered without greatly increasing other pollutant emissions. References