Asgnmnt thermo

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Asgnmnt thermo

  1. 1. 1.0 INTRODUCTION “Acid Rain,” or more precisely acid precipitation, is the word used to describe rainfall that has a pH level of less than 5.6. Acid rain is formed when oxides of nitrogen and sulfite combine with moisture in the atmosphere to make nitric acid and sulfuric acids. These acids can be carried away far from its origin. The two primary sources of acid rain are sulfur dioxide (SO2), and oxides of nitrogen (NOx). Any precipitation with a pH level less than 5.6 is considered to be acid rainfall. The difference between regular precipitation and acid precipitation is the pH level. pH is a symbol indicating how acidic or basic a solution is in ratios of relative concentration of hydrogen ions in a solution. Not only does the acidicity of acid precipitation depend on emission levels, but also on the chemical mixtures in which sulfur dioxide and nitrogen oxides interact in the atmosphere each having varying degrees of success. Acid rain is rain consisting of water droplets that are unusually acidic because of atmospheric pollution most notably the excessive amounts of sulfur and nitrogen released by cars and industrial processes. Acid rain is also called acid deposition because this term includes other forms of acidic precipitation such as snow. Acidic deposition occurs in two ways which is wet and dry. Wet deposition is any form of precipitation that removes acids from the atmosphere and deposits them on the Earth’s surface. Dry deposition polluting particles and gases stick to the ground via dust and smoke in the absence of precipitation. This form of deposition is dangerous however because precipitation can eventually wash pollutants into streams, lakes, and rivers. Acidity itself is determined based on the pH level of the water droplets. PH is the scale measuring the amount of acid in the water and liquid. The pH scale ranges from 0 to 14 with lower pH being more acidic while a high pH is alkaline; seven is neutral. Normal rain water is slightly acidic and has a pH range of 5.3-6.0. Acid deposition is 1
  2. 2. anything below that scale. It is also important to note that the pH scale is logarithmic and each whole number on the scale represents a 10-fold change. 1.1FORMATION OF AN ACID RAIN Natural unpolluted rain is not pure water, it is a dilute solution of carbonic acid, which forms when atmospheric carbon dioxide dissolves in water. This acid dissociates in water to release only enough H+ ions to lower the pH of precipitation from 7 to about 5.6; thus, the acidic precipitation which arises from man’s pollution of the atmosphere has a pH below 5.6. (A pH of 7 represents neutrality. Each unit decrease of pH corresponds to a ten-fold increase in acidity.) Acid rain is formed when pollutant compounds, primarily the oxides of sulphur and nitrogen, react with oxygen and moisture in complex reactions in the atmosphere to form acids. Although the details of the chemical reactions which take place in the atmosphere are not completely understood, some of the oxides of sulphur and nitrogen are converted to sulphuric and nitric acids, respectively. These are strong acids and they dissociate completely in water releasing hydrogen ions to solution; thus, they can lower the pH of precipitation significantly. One of the most acidic rainfalls yet recorded fell in Scotland in 1974 and was measured at 2.4 on the pH scale -- roughly the pH of vinegar (dilute acetic acid) and over one thousand times as acidic as natural rain. Formation of Reactants Burning of Fossil Fuels: S(in compounds) + O2(g) -> SO2(g) Burning of Zinc Sulfide: 2ZnS(s) + 3O2(g) - > 2ZnO(s) + 2SO2(g) The other major acidic oxide that contributes to the formation of acid rain is nitrogen dioxide. Nitric oxide is formed in high localised temperatures created by lightning strikes and naturally reacts in the atmosphere to produce nitrogen dioxide. Nitrogen dioxide is also produced in the high temperatures of combustion chambers of power stations and motor vehicles. Chemical Equations for the Formation: Formation of Nitrogen Dioxide: N2(g) + 2O2(g) -> 2NO2(g) 2
  3. 3. Both sulfur dioxide and nitrogen dioxide are acidic oxides and react with water to form acids. Sulfur dioxide reacts with water to form sulfurous acid. SO2(g) + H2O(l) -> H2SO3(aq) Substances in the upper atmosphere then catalyse the reaction between sulfurous acid and oxygen to form sulfuric acid. 2H2SO3(aq) + O2(g) -> 2H2SO4(aq) Similarly, nitrogen dioxide reacts with water to form a mixture of nitric acid and nitrous acid. 2NO2(g) + H2O(l) -> HNO3(aq) + HNO2(aq) Substances in the atmosphere then catalyse the reaction between nitrous acid and oxygen causing the formation of more nitric acid. 2HNO2(aq) + O2(g) -> 2HNO3(aq) Both sulfuric acid and nitric acid are soluble in water and are the major acids present in acid rain. As this forms and falls onto the Earth's surface, these strong acids are also brought to the surface causing harmful effects on the built and the natural environment. 1.2ACID DEPOSITION Acid deposition, also called acid rain, is rain or gases that have been polluted by high amounts of chemicals and acids in the atmosphere. It can result from decaying plants and animals or natural cataclysms, such as volcanoes, but the major cause of acid rain is the releasing of chemicals by humans. The main gases that lead to acid rain are sulfur dioxide and nitrogen dioxide. When they come into contact with water and oxygen they turn into acids. Acid Deposition can be in the form of precipitation, which is called wet deposition, or it could be in the form of gases and microscopic particles floating the air, which is called dry deposition. Scientists can measure how much acid is in rain or a body of water by using the pH scale. There are 14 numbers on it, ranging from 0 through 14. If a lake has a low pH, that tells us that there is a high amount of acid in the lake. If a lake has a pH 8 or above, it is alkaline, which means there is not a lot of acid in it. When a body of water 3
  4. 4. has a pH of 7, it is neutral, since it is in the middle. New York State's rain pH level is between 4 and 4.5. That is 30 times more acidic than the normal level. One of the central sources of sulfur dioxide and nitrogen oxide come from power plants. When power plants generate electricity, they are burning the fossil fuel, coal. Coal is sometimes dubbed as the dirty fuel source because when it is burned, it lets out sulfur, nitrogen, and other gases. The more coal we use, the more sulfur and nitrogen we are admitting into our atmosphere. Fumes and emissions from cars and other vehicles are also another source of sulfur dioxide and nitrogen oxide. 1.3 Environmental Effects of Acid Rain More is known about the effects of acid rain on some sectors of the environment than on others. A good deal is known about the effects of acidification on aquatic ecosystems but much less is known about the effects on terrestrial ecosystems, agricultural crops, or human health. i) Aquatic Ecosystems Aquatic organisms vary greatly in their ability to tolerate fluctuations in the acidity of their environment. Some species are very sensitive to acidification and, as the pH of lakes, rivers and groundwaters decreases, the least-tolerant species disappear first, followed by less sensitive species as the pH continues to drop. Studies have shown that the number and diversity of fish species decrease in lakes when the pH drops below 6.0. Other organisms such as algal become less diverse in lakes as the pH drops below 6.0 and the survival of rooted plants is generally diminished in acidified lakes, while the growth of benthic (bottomgrowing) mosses and attached algae is usually enhanced. As the pH falls, the number of invertebrates in the water column and in the sediments decreases, the rate of decomposition of organic matter decreases, and fungi begin to replace bacteria as the dominant decomposer organisms. These developments can lead to a reduction in nutrient cycling in a lake, and this in turn can result in reduced productivity. 4
  5. 5. The loss of species from an ecosystem reduces its diversity and may make the whole community progressively more unstable. Thus, even an acidresistant species may be lost, if its natural prey is acid-sensitive and disappears from the environment.Limited information now suggests that aquatic biological communities can recover fairly quickly from acidification (in years rather than decades) once the level of acid loading is reduced. The artificial "liming" of acidified waters has had some success in decreasing acidification and reestablishing some fish populations. However, the procedure is expensive and liming does not precisely reverse the acidification process. ii) Terrestrial Ecosystems Terrestrial ecosystems are inherently extremely complex; so many factors influence the growth and development of land-based ecosystems that it is difficult to isolate and characterize the effects of acid rain alone. Some facts are known, however. Acid rain can: damage foliage; accelerate the erosion of the waxy covering of leaves which may lead to the loss of water or which may reduce a plant’s ability to resist the attack of disease-causing organisms; inhibit the germination of seeds and the growth of seedlings; decrease the respiration of organisms living in the soil, which may in turn affect the availability of some nutrients; increase the leaching of nutrient ions from the soil; and enhance the solubilization of aluminum in the soil, which can have negative effects on biological processes. On the other hand, it is not known to what extent the illeffects listed above might be counterbalanced by the nutrient input which could be derived from the sulphur and (especially) the nitrogen compounds which are found in acidic precipitation. Acidic precipitation has not yet been shown to damage agricultural crops directly, but air pollution in general does inhibit the growth of some commercial species. A related issue concerns the damaging effect of ozone on sensitive agricultural crops, which is now well documented. Ozone is a major component of photochemical smog, of which nitrogen dioxide is a precursor. Thus, a reduction in NOx emissions could have the two-fold beneficial effect of reducing both acid rain and ozone pollution. 5
  6. 6. iii) Human Health Although direct effects of acid rain on human health have yet to be unambiguously demonstrated, some health authorities feel it may be injurious to some people. However, evidence that sulphate air pollution affects human health is now widely accepted by medical authorities. Extremely small particulates, formed in the atmosphere by the oxidation of sulphur dioxide, are capable of penetrating deeply into the human respiratory system. Acidic particulates can cause chronic bronchitis or emphysema, with the resultant difficulty in breathing leading to increased strain and, possibly, eventually to heart disease. Oxides of nitrogen can suppress the action of pulmonary scavenger cells whose function it is to purify the lungs by removing insoluble particulates. This effect could also lead to increased susceptibility to respiratory ailments. iv) Man-Made Structures Acidic precipitation can contribute to the processes of materials erosion. Thus, buildings, roads, paint, sculptures and other man-made structures can be aesthetically and functionally damaged. The question is, what portion of the observed corrosion and deterioration is due to the effect of acid rain? At the present time, no useful estimate can be made. The most that can be said, in advance of the extensive research needed in this area, is that air pollution has an effect on materials deterioration and acid deposition is one component of that complex situation. 1.4ENGINEERING SOLUTIONS  CONTROL METHODS FOR SO2 1. GAS ABSORPTION & STRIPPING: This is the standard method for removing any component from a gas stream, but it is only good as long as we can find a liquid solvent in which the gaseous component we want to remove is much more soluble than the other components of the gas stream. 6
  7. 7. The overall process is very straightforward. The feed gas enters an absorber (a vertical column) where the gas stream flow up and the liquid stream flows down. Normally, a packing material is used inside the column in order to provide a larger surface area for the liquid and gases to come in to contact. Once the gas has been stripped of the target component, it is released into the atmosphere or used for some other purpose. The liquid solvent which now contains the element we wanted to remove, passes to a stripper. Inside the stripper, the solubility of the gas in the solvent is greatly reduced. This allows the gas to come out of the solution. Later, this gas is cooled and sent to storage for future use an d the stripped solvent is returned to the absorber. 2. LIMESTONE WET SCRUBBERS: This method is used primarily with sources who produce SO2 through coal or oil burning. In the limestone wet scrubber, the solid ash particulates are first removed. Then, the flue gas flows to a tower where it travels countercurrent to a scrubbing slurry comprised of water and limestone particles. To increase the efficiency of scrubbing, some designs use a packing amterial with a very high open area in the tower, a spraying and mist eliminator system, or custom-made bubbler designs. Inside the tower, the SO2 dissolves in the slurry and reacts with the limestone to produce CO2 and solid CaSO3. The CO2 enters the gas stream, while the CaSO3 is oxidized to CaSO4. This oxidation can occur in variety of places. For example, it is partly accomplished by the excess oxygen already present in the flue gases. It can also be done in an effluent holding tank or an additional oxidizing vessel. Once the process is complete, the slurry is recirculated from the holding tank. An additional stream is sent to a settler and filter to remove solids. Finally , freshly ground limestone is added to an effluent hold tank. The scrubber will operate near the adiabatic saturation temperature of the entering flue gas. The cleaned gas is usually heated to restore plume buoyancy and to prevent acid corrosion of duct work. Then, the gas is released to the stack. The water in the waste slurry is reduced by thickening. The filter 7
  8. 8. cake is usually mixed with dry fly ash from the plant to further reduce the water in the waste stream. The ultimate destination for the final waste stream is a landfill. Some problems with limestone scrubbers are corrosion of stacks and duct work due to chemical content of exhaust gases, solid deposition, scaling and plugging caused by calcium sulfate, and plugging of mist eliminator equipment. Alternative wet systems include: using quicklime as an alternative to limestone in wet throwaway processes (i.e. processes that use a reagent only once and then throw it away) and double alkali systems which avoids the solid deposition, scaling and plugging problems caused by limestone. 3. DRY SYSTEMS: Overall, dry systems have fewer corrosion and scaling problems associated with them. Typically, dry systems inject dry alkaline particles into the gas stream, where they react with the gas to remove SO2. The particles containing the SO2 are then collected in the particle collection device that is used to collect fly ash. Dry systems eliminate problems with disposal of wet sludge associated with wet scrubbers, while increasing the amount of dry solids to be disposed of. Sine it is generally more difficult to dispose of the sludge, dry systems are often considered a good alternative. 4. WET/DRY SYSTEMS: These systems are a combination of the feature of both wet and dry systems. The type of wet-dry system that is most often used is the spray dryer. Spray dryers are mainly used in process industries (i.e. industries that produce products like dried milk, instant coffee, laundry detergents, etc.) In theses spray dryers, water containing dissolved or suspended solids is dispersed as droplets into a hot gas stream. The gas stream, usually contaminated with SO2 enters the chamber either through the side or the top and generally exits through the bottom. The temperature of the gas is much greater than that of the water so the water droplets will evaporate quickly. The particles that are formed from the evaporation process are dry before they reach the walls or the bottom of the reactor. Therefore, the particles for a fine powder that is relatively easy to be removed. Generally, the powder is cooled before being removed. 8
  9. 9. One benefit of this process is that since we are able to control the size of the water droplets and the concentration in the feed stream, we are able to control the size of the particles that are formed. In the end, all of these specifications give us a powder with a size distribution that could not be obtained any other way.  CONTROL METHODS FOR Nox CONTROL TECHNIQUES FOR NOx REDUCTION There are only two way to reduce NOx emissions: •Modifying combustion processes to prevent NOx formation •Treating combustion gases after flame to convert NOx to N2 COMBUSTION MODIFICATION : This is the most widely used approach to NOx control. Combustion modification involves mixing part of the combustion air with the fuel and burning as much of the fuel that the air will allow. Then, some of the heat from the flames is transferred to whatever is being heated. Next, the remaining air is added and combustion is finished. This is known as two-stage combustion or reburning One of the major advantages to this technique is that it is cheap. The disadvantages are that it requires a larger firebox without a higher combustion rate. Also, it is difficult to get complete burning of the fuel in the second stage. Therefore, the amount of unburned fuel and/or carbon monoxide in the exhaust gas increases. POST-FLAME TREATMENT Many of these processes require the addition of a reducing agent to the combustion gas stream to take oxygen away from NO. In automobile engines, a platinum-rhodium catalyst is used. The reaction is: 2NO + 2CO + p-r catalyst-----------> N2 + 2CO2 On the other hand, for power plants and other large furnaces, there are many choices of reducing agents. However, the most popular is ammonia. The desired conversion reaction is: 6NO + 4NH3 -------------> 5N2 + 6H2O 9
  10. 10. However, there is always some oxygen present. This oxygen causes reactions like the following: 4NO + 4NH3 +O2 ---------> 4N2 + 6H2O If the above reaction occurs, the NO2 is reduced by the following reaction: 2NO2 + 4NH3 +O2 ---------> 3N2 + 6H2O All of these reactions are expensive to carry out. They can occur either over a zeolite catalyst or in a gas stream in a part of a furnace where the temperature is between 1600 and 1800 degrees Fahrenheit. If the temperature is greater than 1800 degrees, the NO content increases rather than decreases, which exactly what we DON'T want. The dominant reaction is: NH3+O2 ---------> NO + 3/2H2O 1.5 CONCLUSION “Acid rain”, or more precisely acid precipitation, is a broad term referring to a mixture of wet and dry deposition from the atmosphere containing a pH level of less than 5.6 that is unusually acidic. Wet deposition refers to acidic rain, for and snow while dry deposition refers to the acid chemicals become dust or smoke and falls to the ground, buildings, cars and trees then washed away rainstorms, leading to acidic mixture. This acid precipitation is caused by emissions of sulphur dioxide and nitrogen oxide, which react with the water molecules in the atmosphere to produce acids. Acid rain causes the acidification of lakes and streams and accelerates the decay of building materials and paints, buildings, statues and sculptures. It is best to reduce the use of any related to the emission of sulphur and nitrogen to control the acidic level of acid rain. 10

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