Energy
- 1. Energy (Ch-2) Elements of Mechanical Engineering.(B.E. 1ST SEM ) Computer engg. Sigma Group Of Institute Ajwa Nimeta Road, Bakrol,Waghodia,Vadodara,Gujarat 390019 Prepared by: 1.HEMIN PATEL (15co35) (150500107025) 2.YASH JAIN (15co33) 3. PATEL AHMED (15co34) Guided by : SUCHIT SHAH (Asst. Professor)
- 2. Sources of Energy ▪ The various sources of energy can be listed as follows: 1. Fossil Fuels 2. Stored or flowing water (Hydel Energy) 3. Nuclear Fuels (Nuclear Energy) 4. Sun (Solar Energy) 5.Wind (Wind Energy) 6. Rise and fall of tides (Tidal Energy) 7. Geothermal Energy 8. Biomass and bio-fuels
- 3. ▪ Renewable energy is generally defined as energy that comes from resources which are naturally replenished on a human timescale, such as sunlight, wind, rain, tides, waves, and geothermal heat. ▪ Non-renewable energy comes from sources that will run out or will not be replenished in our lifetimes—or even in many, many lifetimes. Most non-renewable energy sources are fossil fuels: coal, petroleum, and natural gas. Carbon is the main element in fossil fuels.
- 4. Energy from fossil fuel ▪ FUEL is defined as ” a substance composed mainly of carbon and hydrogen which produced a large amount of heat while burning with oxygen.”.The main combustible elements of each fuel are carbon,hydrogen,compounds of hydrocarbons and small amount of other sunstance,such as Sulphur,oxygen,nitrogen etc. ▪ The combustion of fuel is one of the most important sourcesof energy utilized for driving prime movers.The combustion of fuel is the process of chemical combustion of carbon, hydrogen and Sulphur with oxygen which comes from air. When the fuel is burnt in presence of O2 (air).it produces heat and flue gases. this heat is utilizing for heating purpose or for produce mechanical energy with help of prime movers .
- 5. Classification of fuel • Solid fuel : Primary (natural) : 1. wood 2. Anthracite coal 3. peat 4. Bituminous coal Secondary (artificial) : 1.coke 2. Char coal 3. Briquettes coal • liquid fuel : Primary (natural) : 1. Petroleum Secondary (artificial) : diesel, gasoline, kerosene, LPG, coal tar, naptha, ethanol • Gaseous fuel : Primary (natural) : 1. Natural gas Secondary (artificial) : hydrogen, propane, coal gas, water gas, blast furnace gas, coke oven gas, CNG
- 6. Solid fuels Primary (natural) : 1.Wood : Wood fuel (or fuelwood) is a fuel, such as firewood, charcoal, chips, sheets, pellets, and sawdust. The particular form used depends upon factors such as source, quantity, quality and application. In many areas, wood is the most easily available form of fuel, requiring notools in the case of picking up dead wood, or few tools, although as in any industry, specialized tools, such as skidders and hydraulic wood splitters, have been developed to mechanize production. Sawmill waste and construction industry by-products also include various forms of lumber tailings. 2.Peat : Peat, an organic fuel consisting of spongy material formed by the partial decomposition of organic matter, primarily plant material, in wetlandssuch as swamps, muskegs, bogs, fens, and moors.The development of peat is favoured by warm, moist climatic conditions; however, peat can develop even in cold regions such as Siberia, Canada, and Scandinavia. Peat is only a minor contributor to the world energy supply
- 7. Wood ,peat Bituminous coal, Anthracite coal
- 8. 3. Bituminous coal : Bituminous coal is an organic sedimentary rock formed by diagenetic and sub metamorphic compression of peat bog material. Its primary constituents are macerals: vitrinite, and liptinite.The carbon content of bituminous coal is around 60-80%; the rest is composed of water, air, hydrogen, and sulphur, which have not been driven off from the macerals. Bank density is approximately 1346 kg/m³ (84 lb/ft³). Bulk density typically runs to 833 kg/m³ (52 lb/ft³).The heat content of bituminous coal ranges from 24 to 35 MJ/kg (21 million to 30 million BTU per short ton) on a moist, mineral-matter-free basis. 4.Anthracite coal : Anthracite is the most metamorphosed type of coal (but still represents low-grade metamorphism), in which the carbon content is between 92.1% and 98%.The term is applied to those varieties of coal which do not give off tarry or other hydrocarbon vapours when heated below their point of ignition.Anthracite ignites with difficulty and burns with a short, blue, and smokeless flame.
- 9. Secondary (artificial) : 1.coke : Coke is a fuel with few impurities and a high carbon content, usually made from coal. It is the solid carbonaceous material derived fromdestructive distillation of low-ash, low- sulfur bituminous coal.Cokes made from coal are grey, hard, and porous.While coke can be formed naturally, the commonly used form is man-made.The form known as petroleum coke, or pet coke, is derived from oil refinery coker units or other cracking processes. 2. Char coal : Charcoal is a light, black residue, consisting of carbon and any remaining ash, obtained by removing water and other volatile constituents from animal and vegetation substances. Charcoal is usually produced by slow pyrolysis, the heating of wood or other substances in the absence of oxygen (see char and biochar). 3. Briquettes coal : A briquette (or briquet) is a compressed block of coal dust or other combustible biomass material such as charcoal, sawdust, wood chips,peat, or paper used for fuel and kindling to start a fire.
- 10. Liquid fuel Petroleum (crude oil) : It consists of hydrocarbons of various molecular weights and other organic compounds.The name petroleum covers both naturally occurring unprocessed crude oil and petroleum products that are made up of refined crude oil.A fossil fuel, petroleum is formed when large quantities of dead organisms, usuallyzooplankton and algae, are buried underneath sedimentary rock and subjected to intense heat and pressure. Petrol : Gasoline also known as petrol outside of North America, is a transparent, petroleum-derived liquid that is used primarily as a fuel in internal combustion engines. It consists mostly of organic compounds obtained by the fractional distillation of petroleum, enhanced with a variety of additives. Kerosene : Regardless of crude oil source or processing history, kerosene's major components are branched and straight chain alkanes and naphthenes, which normally account for at least 70% by volume. Aromatic hydrocarbons in this boiling range, such as alkyl benzenes and alkylnaphthalenes do not normally exceed 25% by volume of kerosene streams. Olefins are usually not present at more than 5% by volume.The flash point of kerosene is between 37 and 65 °C (100 and 150 °F), and its autoignition temperature is 220 °C (428 °F).The pour point of kerosene depends on grade, with commercial aviation fuel standardized at −47 °C (−53 °F).
- 12. Advantage and disadvantage of liquid fuel over solid fuel: • Handling of liquid fuel is easy and they require less storage space • Liquid fuels can be fired easily and maximum temperature is attained in time as compared to solid fuels. •The solid fuels containing higher of moisture burn with great difficulty. •The solid fuels leave a large quantity of ash after burning and then disposal of ash becomes a problem.Where as the liquid fuels as very little ash after burning. •The combustion of liquid fuel is uniform therefor the change in load can be easily met by controlling the flow of fluid. Disadvantage: •They are costly as compared to solid fuels •They require special type of burners • In cold climate the oil stored in tanks is to be heated in order to avoid the stoppage of flow.
- 13. Gaseous Fuels 1.Natural Gas : Natural gas is a fossil fuel formed when layers of decomposing plant and animal matter are exposed to intense heat and pressure over thousands of years.The energy that the plants originally obtained from the sun is stored in the form of chemical bonds in natural gas. 2.Coal Gas : Coal gas is a flammable gaseous fuel made from coal and supplied to the user via a piped distribution system.Town gas is a more general term referring to manufactured gaseous fuels produced for sale to consumers and municipalities. 3.Water Gas : Water gas is a synthesis gas, containing carbon monoxide and hydrogen. It is a useful product but requires careful handling due to its flammability and the risk of carbon monoxide poisoning. The gas is made by passing steam over a red-hot carbon fuel such as coke: H2O + C → H2 + CO (ΔH = +131 kJ/mol)
- 14. 4. Blast Furnace Gas : Blast furnace gas (BFG) is a by-product of blast furnaces that is generated when the iron ore is reduced with coke to metallic iron. It has a very low heating value, about 93 BTU/cubic foot, because it consists of about 60 percent nitrogen, 18-20% carbon dioxide, which are not flammable. 5.Sewer Gas : Sewer gas is a complex mixture of toxic and nontoxic gases produced and collected in sewage systems by the decomposition of organic household or industrial wastes, typical components of sewage. Sewer gases may include hydrogen sulfide, ammonia, methane, carbon monoxide, sulfur dioxide, and nitrogen oxides. 6.Coke-oven Gas : Around 25 to 30% of the weight of the coal charged to coke ovens is driven off as effluent gases rich in volatile matter and moisture. After drying this raw gas and separating tar, light oil and sulphur fractions (which have values themselves), coke oven gas (COG) is obtained.
- 15. Advantages of Gaseous fuels 1.Less Harmful than Coal or Oil: As compared to petroleum or coal, natural gas causes less damage to the environment. It is made up of methane and results in less carbon emissions. In fact emissions of carbon dioxide are 45% lesser than other conventional fuels and 30% less than oil. 2. Easy Storage andTransport: Natural gas is easier to preserve than other fuels. It can be stored and transported through pipelines, small storage units, cylinders or tankers on land and sea. 3. Residential Use: Natural gas can be piped into houses for heating and cooking purposes and running a variety of appliances. Where there are no pipes, it can be supplied in small tanks. 4.Vehicle Fuel: Natural gas can be used as a fuel for vehicles (cars, trucks, jet engines). It is a cleaner, cheaper fuel than diesel or gasoline. 5. Burns Cleaner: Natural gas burns cleaner without leaving any smell, ash or smoke. 6. Precision in Kitchen: Natural gas is the best fuel to power kitchens because of its control, reliability and precision. A gas flame provides for precise temperature control and variety of heat settings allowing shift from hot to cold or vice versa, with the turning of the knob. 7. Industrial use: Natural gas is used for producing hydrogen, ammonia for fertilizers and some paints and plastics.
- 16. Disadvantages of Gaseous fuels 1.Toxic and Flammable: Leaks of natural gas are tremendously dangerous. Such leaks may cause explosions or fire. When inhaled, the gas is highly toxic.The main danger is that it is odorless and leaks cannot be detected unless some odorant has been added to the gas. It is for this reason that LPG (residentially used gas) is suffused with odorants, that in the event of a leak, detection is easy and appropriate actions can be taken. In the case of an underground leak, we are helpless as odorant becomes weaker and the gas leak goes undetected. 2. Damage to Environment: Burning of natural gas also releases carbon dioxide, carbon monoxide and other carbon compounds which are greenhouse gases that cause global warming and climate change. Even though it is cleaner than oil or coal as far as its by products are concerned, leakage of natural gas can be have serious consequences as methane is more toxic than carbon dioxide. 3. Complex Processing: For use as fuel, except for methane, all other constituents of natural gas have to be extracted. Processing results in many byproducts: hydrocarbons (propane, ethane etc.), sulfur, water, helium, nitrogen, and carbon dioxide. 4. Non-Renewable: Like all fossil fuels, natural gas though found in abundance is non-renewable and hence likely to be exhausted at some point of time. It is not a long term solution to our energy problems. 5. Expensive Installation: The infrastructure for natural gas production and distribution is fairly expensive.This includes separate plumbing systems and specialized tanks.
- 17. LPG(Liquefied petroleum gas) Liquefied petroleum gas or liquid petroleum gas (LPG or LP gas), also referred to as simply propane or butane, are flammable mixtures of hydrocarbon gases used as fuel in heating appliances, cooking equipment, and vehicles. It is increasingly used as an aerosol propellant and a refrigerant replacing chlorofluorocarbons in an effort to reduce damage to the ozone layer.When specifically used as a vehicle fuel it is often referred to as auto gas. Varieties of LPG bought and sold include mixes that are primarily propane (C3H8), primarily butane (C4H10) and, most commonly, mixes including both propane and butane. In the northern hemisphere winter, the mixes contain more propane, while in summer, they contain more butane. In the United States, primarily two grades of LPG are sold: commercial propane and HD-5.These specifications are published by the Gas Processors Association (GPA) and the American Society ofTesting and Materials (ASTM).Propane/butane blends are also listed in these specifications.
- 18. • LPG is an economical fuel source. • It burns readily and gives off instant heat.The flame is visible and its size is easily controllable to meet your heating needs. • LPG burns very efficiently, with very low combustion emissions and does not create black smoke. It does not leave messy soot so your cooking vessels can be cleaned easily. • It is easy to store an LPG cylinder. It is clean and takes up very little space in kitchen. • Requesting for a replacement cylinder is easy. Simply call your nearest authorised Esso LPG distributor for fast, efficient and free home delivery service.
- 19. CNG(Compressed natural gas) Compressed natural gas (CNG) (methane stored at high pressure) can be used in place of gasoline (petrol), Diesel fuel andpropane/LPG. CNG combustion produces fewer undesirable gases than the fuels mentioned above. It is safer than other fuels in the event of a spill, because natural gas is lighter than air and disperses quickly when released. CNG may be found above oil deposits, or may be collected from landfills or wastewater treatment plants where it is known as biogas. CNG is made by compressing natural gas (which is mainly composed of methane, CH4), to less than 1 percent of the volume it occupies at standard atmospheric pressure. It is stored and distributed in hard containers at a pressure of 20–25 MPa (2,900–3,600 psi), usually in cylindrical or spherical shapes. CNG is used in traditional gasoline/internal combustion engine automobiles that have been modified or in vehicles which were manufactured for CNG use, either alone ('dedicated'), with a segregated gasoline system to extend range (dual fuel) or in conjunction with another fuel such as diesel (bi-fuel). Natural gas vehicles are increasingly used in Iran, especially Pakistan, the Asia-Pacific region, Indian capital of Delhi, and other large cities like Ahmedabad, Mumbai, Kolkata—as well as cities such as Lucknow, Kanpur, etc. Its use is also increasing in South America, Europe and North America because of rising gasoline prices. In response to high fuel prices and environmental concerns, CNG is starting to be used also in tuk-tuks and pickup trucks, transit and school buses, and trains.
- 20. • Natural gas is significantly less expensive than gasoline.The cost of natural gas can go as low as $0.64 a GGE or Gasoline Gallon Equivalent. In some areas, natural gas costs as low as a third of the cost of gasoline, on average. • CNG is more eco-friendly than gasoline. Natural gas produces far fewer harmful emissions and hydrocarbons than gasoline. • Using CNG makes the engine cleaner and more efficient. Unlike gasoline,CNG does not produce harmful carbon deposits when combusted.This results to a cleaner and more efficient engine as well as longer lasting spark plugs and catalytic converters. Oil changes are also minimized because of carbon deposits that contaminate the oil is eliminated. • CNG is abundant in the US.The country has extensive natural gas resources and a well- established network of pipelines. Switching toCNG can help ease the country’s dependence to foreign oil whose price continue to fluctuate. • CNG makes the engine run quieter. Because of the higher octane levels of natural gas, the engine runs quieter resulting to minimized engine nose.
- 21. Nuclear Fuel Nuclear fuel is a material that can be 'burned' by nuclear fission or fusion to derive nuclear energy. Nuclear fuel can refer to the fuel itself, or to physical objects (for example bundles composed of fuel rods) composed of the fuel material, mixed with structural, neutron-moderating, or neutron-reflecting materials. Most nuclear fuels contain heavy fissile elements that are capable of nuclear fission.When these fuels are struck by neutrons, they are in turn capable of emitting neutrons when they break apart.This makes possible a self- sustaining chain reaction that releases energy with a controlled rate in a nuclear reactor or with a very rapid uncontrolled rate in a nuclear weapon. The most common fissile nuclear fuels are uranium-235 (235U) and plutonium-239 (239Pu).The actions of mining, refining, purifying, using, and ultimately disposing of nuclear fuel together make up the nuclear fuel cycle. Not all types of nuclear fuels create power from nuclear fission. Plutonium-238 and some other elements are used to produce small amounts of nuclear power by radioactive decay in radioisotope thermoelectric generators and other types of atomic batteries. Also, light nuclides such as tritium (3H) can be used as fuel for nuclear fusion.
- 22. Water Energy Hydraulic energy pertains to the power related to pressurized fluid, typically hydraulic fluid, used to accomplish machine motion.The pressure can be relatively static (such as reservoirs) or in motion though tubing or hoses.A good percentage of industrial equipment subject to OSHA 1910.147 will have at least one hydraulic Energy Source.To do useful work, industrial equipment often converts hydraulic energy into other forms of kinetic or Potential Energy. it is a clean energy source, without waste products and is easy to store.Also the water stored in reservoirs situated at altitude permits the regulation of the flow of the river. the construction of hydroelectric plants is expensive and needs large networks of power cables. Reservoirs also mean the loss of productive soil and fauna due to the flooding of their habitat.They also cause a decrease in the flow of the rivers and streams below the dam and alter the quality of the waters.The following animation shows how hydraulic energy is stored and exploited
- 24. Solar Energy Solar energy is radiant light and heat from the Sun harnessed using a range of ever-evolving technologies such as solar heating, photovoltaics, solar thermal energy, solar architecture and artificial photosynthesis. It is an important source of renewable energy and its technologies are broadly characterized as either passive solar or active solar depending on the way they capture and distribute solar energy or convert it into solar power. Active solar techniques include the use of photovoltaic systems, concentrated solar power and solar water heating to harness the energy. Passive solar techniques include orienting a building to the Sun, selecting materials with favourable thermal mass or light dispersing properties, and designing spaces that naturally circulate air. The large magnitude of solar energy available makes it a highly appealing source of electricity.The United Nations Development Programme in its 2000 World EnergyAssessment found that the annual potential of solar energy was 1,575–49,387 exajoules (EJ).This is several times larger than the total world energy consumption, which was 559.8 EJ in 2012
- 25. Wind Energy Wind is a form of solar energy.Winds are caused by the uneven heating of the atmosphere by the sun, the irregularities of the earth's surface, and rotation of the earth.Wind flow patterns are modified by the earth's terrain, bodies of water, and vegetative cover.This wind flow, or motion energy, when "harvested" by modern wind turbines, can be used to generate electricity. HowWind Power Is Generated? The terms "wind energy" or "wind power" describe the process by which the wind is used to generate mechanical power or electricity.Wind turbines convert the kinetic energy in the wind into mechanical power.This mechanical power can be used for specific tasks (such as grinding grain or pumping water) or a generator can convert this mechanical power into electricity to power homes, businesses, schools, and the like. Wind turbines, like aircraft propeller blades, turn in the moving air and power an electric generator that supplies an electric current. Simply stated, a wind turbine is the opposite of a fan. Instead of using electricity to make wind, like a fan, wind turbines use wind to make electricity.The wind turns the blades, which spin a shaft, which connects to a generator and makes electricity.
- 26. Wind and Solar Energy
- 27. Bio-fuel A biofuel is a fuel that is produced through contemporary biological processes, such as agriculture and anaerobic digestion, rather than a fuel produced by geological processes such as those involved in the formation of fossil fuels, such as coal and petroleum, from prehistoric biological matter. Biofuels can be derived directly from plants, or indirectly from agricultural, commercial, domestic, and/or industrial wastes. Renewable biofuels generally involve contemporary carbon fixation, such as those that occur in plants or microalgae through the process of photosynthesis.Other renewable biofuels are made through the use or conversion of biomass(referring to recently living organisms, most often referring to plants or plant-derived materials).This biomass can be converted to convenient energy containing substances in three different ways: thermal conversion, chemical conversion, and biochemical conversion.This biomass conversion can result in fuel in solid, liquid, or gas form.This new biomass can also be used directly for biofuels.
- 28. Advantages • Source material: Whereas oil is a limited resource that comes from specific materials, biofuels can be manufactured from a wide range of materials including crop waste, manure, and other byproducts.This makes it an efficient step in recycling. • Renewability: It takes a very long time for fossil fuels to be produced, but biofuels are much more easily renewable as new crops are grown and waste material is collected. • Security: Biofuels can be produced locally, which decreases the nation's dependence upon foreign energy. By reducing dependence on foreign fuel sources, countries can protect the integrity of their energy resources and make them safe from outside influences. • Economic stimulation: Because biofuels are produced locally, biofuel manufacturing plants can employ hundreds or thousands of workers, creating new jobs in rural areas. Biofuel production will also increase the demand for suitable biofuel crops, providing economic stimulation to the agriculture industry. • Lower carbon emissions:When biofuels are burned, they produce significantly less carbon output and fewer toxins, making them a safer alternative to preserve atmospheric quality and lower air pollution.
- 29. Global Warming GlobalWarming is the increase of Earth's average surface temperature due to effect of greenhouse gases, such as carbon dioxide emissions from burning fossil fuels or from deforestation, which trap heat that would otherwise escape from Earth.This is a type of greenhouse effect. Earth's climate is mostly influenced by the first 6 miles or so of the atmosphere which contains most of the matter making up the atmosphere. This is really a very thin layer if you think about it. In the bookThe End of Nature, author Bill McKibbin tells of walking three miles to from his cabin in the Adirondack's to buy food. Afterwards, he realized that on this short journey he had traveled a distance equal to that of the layer of the atmosphere where almost all the action of our climate is contained. In fact, if you were to view Earth from space, the principle part of the atmosphere would only be about as thick as the skin on an onion! Realizing this makes it more plausible to suppose that human beings can change the climate.A look at the amount of greenhouse gases we are spewing into the atmosphere (see below), makes it even more plausible.
- 30. Global Warming Impacts 1.Rising Seas--- inundation of fresh water marshlands (the everglades), low-lying cities, and islands with seawater. 2.Changes in rainfall patterns --- droughts and fires in some areas, flooding in other areas. See the section above on the recent droughts, for example! 3.Increased likelihood of extreme events--- such as flooding, hurricanes, etc. 4.Melting of the ice caps --- loss of habitat near the poles. Polar bears are now thought to be greatly endangered by the shortening of their feeding season due to dwindling ice packs. 5.Melting glaciers - significant melting of old glaciers is already observed. 6.Widespread vanishing of animal populations --- following widespread habitat loss. 7.Spread of disease --- migration of diseases such as malaria to new, now warmer, regions. 8.Loss of Plankton due to warming seas ---The enormous (900 mile long) Aleution island ecosystems of orcas (killer whales), sea lions, sea otters, sea urchins, kelp beds, and fish populations, appears to have collapsed due to loss of plankton, leading to loss of sea lions, leading orcas to eat too many sea otters, leading to urchin explosions, leading to loss of kelp beds and their associated fish populations.
- 31. Ozone Depletion Ozone depletion describes two distinct but related phenomena observed since the late 1970s: a steady decline of about 4% in the total volume of ozone in Earth's stratosphere (the ozone layer), and a much larger springtime decrease in stratospheric ozone around Earth's polar regions.The latter phenomenon is referred to as the ozone hole. In addition to these well-known stratospheric phenomena, there are also springtime polar tropospheric ozone depletion events. The details of polar ozone hole formation differ from that of mid- latitude thinning but the most important process in both is catalytic destruction of ozone by atomic halogens.The main source of these halogen atoms in the stratosphere is photodissociation of man- made halocarbon refrigerants, solvents, propellants, and foam-blowing agents (CFCs, HCFCs, freons, halons).These compounds are transported into the stratosphere by winds after being emitted at the surface.Both types of ozone depletion were observed to increase as emissions of halocarbons increased.
- 32. Ozone Cycle overview Three forms (or allotropes) of oxygen are involved in the ozone-oxygen cycle: oxygen atoms (O or atomic oxygen), oxygen gas (O2 or diatomic oxygen), and ozone gas (O3 or triatomic oxygen). Ozone is formed in the stratosphere when oxygen molecules photodissociate after intaking an ultraviolet photon whose wavelength is shorter than 240 nm.This converts a single O2 into two atomic oxygen radicals.The atomic oxygen radicals then combine with separate O2 molecules to create two O3 molecules.These ozone molecules absorb UV light between 310 and 200 nm, following which ozone splits into a molecule of O2 and an oxygen atom.The oxygen atom then joins up with an oxygen molecule to regenerate ozone.This is a continuing process that terminates when an oxygen atom "recombines" with an ozone molecule to make two O 2 molecules.2 O3 → 3 O2 The overall amount of ozone in the stratosphere is determined by a balance between photochemical production and recombination. Ozone can be destroyed by a number of free radical catalysts, the most important of which are the hydroxyl radical (OH·), nitric oxide radical (NO·), chlorine atom (Cl·) and bromine atom (Br·).The dot is a common notation to indicate that all of these species have an unpaired electron and are thus extremely reactive. All of these have both natural and man-made sources; at the present time, most of the OH· and NO· in the stratosphere is of natural origin, but human activity has dramatically increased the levels of chlorine and bromine.
- 34. These elements are found in certain stable organic compounds,especially chlorofluorocarbons (CFCs), which may find their way to the stratosphere without being destroyed in the troposphere due to their low reactivity. Once in the stratosphere, the Cl and Br atoms are liberated from the parent compounds by the action of ultraviolet light, e.g. CFCl3 + electromagnetic radiation → Cl· + ·CFCl2 The Cl and Br atoms can then destroy ozone molecules through a variety of catalytic cycles. In the simplest example of such a cycle,a chlorine atom reacts with an ozone molecule, taking an oxygen atom with it (forming ClO) and leaving a normal oxygen molecule.The chlorine monoxide (i.e., the ClO) can react with a second molecule of ozone (i.e., O3) to yield another chlorine atom and two molecules of oxygen. The chemical shorthand for these gas-phase reactions is: • Cl· + O3 → ClO + O2:The chlorine atom changes an ozone molecule to ordinary oxygen • ClO + O3 → Cl· + 2 O2:The ClO from the previous reaction destroys a second ozone molecule and recreates the original chlorine atom, which can repeat the first reaction and continue to destroy ozone.