Acid Rain from Burning Fossil Fuels (in Power Plants and Other Causes):

Burning fossil fuel is considered as the major cause of acid rain. Fossil fuels such as coal, oil, and natural gas are burned as they give off heat energy, which act as fuel for cars, airplanes, to provide energy for power plants to produce electricity, or to blast ore for steel manufacturing. Fossil fuels are combusted in order to produce an endothermic reaction, where the released energy is essential in making electricity or powering vehicles. Depending on the type of fossil fuel, compounds such as Sulfur Dioxide (SO2) and Nitrogen Oxides (NOX), or Hydrogen Sulfide gas (H2S) that originate from fuels containing sulfur compounds (coal, crude oil) are released into the atmosphere, where they react with water, oxygen, and other chemicals to form sulfuric acid and nitric acid (H2SO4 and HNO3). These acidic compounds can mix with rain clouds, where they could be sent back down as ‘acid’ rain.Consequences made by Acid Rain:Acid rain is bad for several types of environments, especially aquatic ones including freshwater lakes, streams, and ponds, as acid rain makes the water more acidic, and thus making the water absorb toxic substances such as aluminum from the soil. Aquatic wildlife will not be able to tolerate too much acidity, in this case when the usual pH level of a lake decreases from the natural pH level of 6-8, to a pH level falling below 6. For example, freshwater bass, when affected by acid rain, may not be able to survive if the pH level of its habitat is at or below 5.5.  The insoluble aluminum hydroxide from the soil is made soluble due to acid deposition on neutral soil, reacting with the aluminum through hydrolysis:H2SO4 + Al(OH)3 ? Al2(SO4)3 + H2CO3. The reaction produces soluble aluminum sulfate and carbonic acid.[helpful source]This asphyxiates fishes through excessive mucus formation, essentially clogging their gills. Acid rain can also lower the amount of pH a body of water has (decreasing pH), which results in more fish dying and an increase in the amount of plankton and moss. Lower pH levels on both water and soil reduce the amount of soil bacteria, a bacteria that breaks down decaying matter in exchange for nutrients such as calcium, magnesium, phosphate, etc. Lack of soil bacteria means less nutrients for trees to grow healthy, as well as lack of calcium within fish bones and eggs, making fish unhealthy and their eggs very prone to damage and death.Acid Rain doesn’t negatively affect humans, although the pollutants that cause the production of acid rain are quite harmful to humans. Pollutants such as carbon monoxide can cause headaches, dizziness, vomiting, and nausea, or even death if found in high concentrations. (source)Process:Fossil fuels such as coal and oil are combusted within the furnace of a power plant, where the heat of both the reaction and the surface causes the water, flowing inside pipes within the furnace, to turn into steam. The steam is then sent to a turbine in high pressures which spins the turbine, which in turn spins the rotor of a generator connected to the turbine, making electricity. Metal processing factories also follow a similar process. After the fossil fuels have been combusted, coal or oil gets converted into carbon dioxide, as coal is mostly made of carbon, whereas oil is a hydrocarbon, made mostly of hydrogens and carbons.  Since both coal and oil contain mostly carbon and some sulfur atoms, the combustion reaction occurs as: C + O2 ? CO2, and S + O2 ? SO2. Sometimes water vapour (H2O (g)) is formed due to the H+  ions in hydrocarbons. After the combustion of coal/oil and the formation of CO2 gas, the CO2 and NO2 fly out through the flues of a power plant or a metal processing plant, where the SO2 gas has a photochemical reaction with oxygen to form sulfur trioxide, and eventually dissolved into rainwater through hydrolysis of the SO3 gas, forming sulfuric acid (H2SO4), possibly making the next rainfall an acid rain. The CO2 gas will act as a greenhouse gas. Both these gases can be very harmful for our environment, and for every living creature, and so there are ways to reduce pollution and prevent damage. A similar process is observed in metal processing factories, where coal is combusted and undergoes the same routine of turning into CO2 and SO2. In this case, sulfide ores can help increase the production of SO2 gas being made.(source)Treatment of harmful flue gases to protect the environment:In power stations, the released sulfur dioxide gas after combustion of fossil fuels are then treated before it is released into the atmosphere. A way that sulfur dioxide is treated through a method called ‘wet scrubbing’, which is a type of ‘flue-gas desulfurization’ process. A wet scrubber is a device used to treat harmful flue gases by mixing them with a ‘scrubbing liquid’. The sulfur dioxide gas (acidic gas) gets sprayed with a slurry of limestone (or lime, basic due to CaCO3) causing a neutralization reaction, since acid SO2 gas needs alkaline for it to be removed. Examples of wet-scrubbing methods are:Spraying the sulfur dioxide gas with seawater droplets that contain sodium carbonate. A reaction occurs where sodium bisulfite (NaHSO3) and carbon dioxide (CO2) are produced. NaHSO3 is used to preserve wine and prevent oxidation.Reaction: 2SO2 (g) + Na2CO3 (aq) + H2O (l) ? 2NaHSO3 (s) + CO2 (g)Spraying the sulfur dioxide gas with limestone slurry (CaCO3), where the gas and the slurry react, forming carbon dioxide gas and Calcium Sulfate (CaSO4), which are both clean and  beneficial products. CaSO4 can be added with water and oxygen to form gypsum, the main ingredient for wall plaster.Reaction: SO2 (g) + CaCO3 (s)  ?  CaSO3 (s) + CO2 (g) ;(source) There’s also calcium oxide, which is used to scrub sulfur dioxide with oxygen, also forming Calcium Sulfite. Reaction: CaO (s) + SO2 (g) ? CaSO3 (s)Both lime and limestone scrubbing methods will have calcium sulfite (CaSO3) react with oxygen and water to finally form gypsum: 2CaSO3 (aq) + 4H2O + O2 (g) ? 2CaSO4 ? 4H2O (s) (source)Another way to mitigate coal pollution is a process called Carbon Capture and Storage (CCS).(source)The process of capturing harmful CO2 gas and permanently storing it in an underground site. The gas is captured through the use of a carbon dioxide scrubber device, which uses various amines that react and ‘capture’ CO2 gases. The CO2 gas is captured through ‘adsorption’, where the carbon dioxide molecules stick to a surface, and in this case, amines are used to attract and stick CO2 gas molecules to the surface, or the ‘adsorbent’. It is currently under development. Some examples of CCS methods are:Pre-combustion or Gasification:The process of removing the carbon from the coal before combustion through oxidation in steam and oxygen, otherwise known as ‘gasification’ (source). The products formed can be stored and used as fuel for power plants or cars. Water is added to then make the CO2 to be liquefied and stored.Oxidation reaction: Coal + O2 (g) ? CO(g) + H2 (g)   &   CO(g) + H2O(l) ? CO2 (g) + H2 (g)Post-combustion:The process of removing CO2 gas after the combustion of coal, much like the flue-gas desulfurization process described previously. This time, ammonia (an amine) is reacted with CO2 flue gases, then blasted by steam or exposed to moist air, to form CO2 for storage. An example of the formation of ammonium carbonate from passing CO2 gas through ammonia:Reaction: CO2 (g) + NH3 (aq) + H2O(l) ? NH4HCO3 (aq) (source)Oxycombustion:The process of blowing extra oxygen into a power plant’s furnace to achieve complete combustion between the coal and oxygen, producing less CO2 and less chemicals mixed after combustion, such as NOx or SOx pollutants. Since the CO2 is now mostly separated, it is easier to capture it.(source)Electrostatic Precipitators (ESP), another form of coal pollution mitigation, acts similar to CCS. Since smoke is produced alongside CO2, SO2, and unburnt fuel after the combustion of fossil fuels, ESP is a method that prevents smoke particles from being released. This is done by giving the smoke particles a negative charge as they fly up a flue, through a negatively charged metal grid (the cathode). Eventually these negatively charged smoke particles will be attracted by positively charged plates that act as an anode. The electrostatic force between the charged particles and the grounded plates will cause the particles to bunch up through cohesion. The particles, either coal ash or  are then knocked off from the plate into a container by a process called rapping, or placed in hoppers for later disposal or recycling, like the gypsum by-product from wet scrubbing. Compared to CCS which is a very recent technology, currently under heavy research and not being widely implemented as of yet, ESP was invented in 1907 and has been used ever since as a way to prevent harmful smoke particles from escaping into the atmosphere.(source) (source 2) (source 3)Case Study (Real NZ Facts):The Huntly Power Station, located in Huntly, Waikato and operated by Genesis Energy, is a fossil fuel power plant that uses coal to produce electricity. The power station is located beside the Waikato river as the boiled water from the power station is disposed there. Since water is boiled in power stations, the hot water being dumped into the river can kill organisms in the river, although the station is designed to prevent that through cooling the water beforehand. They use imported Indonesian coal and New Zealand mined coal. Both sources extract bituminous hard coking coal, a type of coal that is essential for steel production because of its coking ability, but mainly used as a fuel source for power plants like Huntly. Bituminous coal has a high sulfur content, a major cause of acid rain. The power station is currently making use of Electrostatic Precipitator technology, but not other methods such as CCS. (source – HPS) (source – No CCS, Page 3) (evidence of ESP in Huntly)The Electrostatic Precipitator (ESP), invented in 1907, is made to collect charged smoke particles by the use of electrodes, which can filter out harmful air. Since the Huntly Power Station uses bituminous coal, a type of coal with a high sulfur content, this reduces the electrical resistivity in ESPs, making ESPs more effective at capturing pollutants. Sulfur content in coal is inversely proportional to the electrical resistivity of its ash when combusted. ESP prevents thick smoke to billow out of flues, preventing pollution and acid rain, despite New Zealand having no acid rain at all. Any how, the Huntly power station’s ESP method does still prevent SO2 deposition which can negatively affect vegetation and cities.ESP equipment can be expensive, can use up a lot of energy, thus increasing the overall capital cost, but this is done to prevent further pollution in an already clean and green country.Sources and Links (Used): –  Causes of Acid Rain – Fossil Fuels –  New Zealand Energy (Oil, Gas, Coal) – Combustion – Sulfur Dioxide – Why Acid Rain is Harmful – Nitrogen Oxide to Nitric Acid – Aluminum + Water – Acid rain affecting Fish – Aluminum to Plants and Wildlife – Aluminum to Trout fish; Asphyxiation – Sulfur Dioxide + Limestone – What is Acid Rain? – Reducing Acid Rain and effectsWikipedia articles and Youtube videos (linked above)

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