Fossil Fuel PDF

## Chapter 88 ### Fossil Fuel In developed world, fossil fuels are in the top of the list of energy resources. Fossils fuels are formed from plant and animal remains buried in the earth for millions of years. The fossil fuels provide 85-90% of the energy demand of the present industrialized world....

## Chapter 88 ### Fossil Fuel In developed world, fossil fuels are in the top of the list of energy resources. Fossils fuels are formed from plant and animal remains buried in the earth for millions of years. The fossil fuels provide 85-90% of the energy demand of the present industrialized world. These fossil fuels include- - Coal - Oil & Natural Gas ### A. Coal Coal has been the basis of industrial revolution. Its importance as a source of energy has declined after the introduction of mineral oil and natural gas. In India coal constitutes the mainstay of power generation in India. It constitutes about 70% of total commercial energy consumed in the country. The power sector and industries account for 94% of the total consumption. Due to its high utility as a source of energy and as a raw material for a large number of industries, it is often called black gold. Coal is found in seams in sedimentary rocks. Its major quality is its combustibility and volatileness. Most of the coal has been formed during the carboniferous period in geological history owing to the submergence of natural vegetation and as a result of combined effects of microbial action, pressure and heat over a considerable time period. This process is commonly called 'coalification'. The process takes millions of years. The greater the depth of the deposit the more mature is the coal. It is composed mostly of carbon ($50–98$ per cent), hydrogen (3-13 per cent) and oxygen, and smaller amounts of nitrogen, sulphur and other elements. It also contains water and particles of other inorganic matter. When burnt, coal releases energy as heat which has a variety of uses. ### Coal bearing strata of India The coal bearing strata of India are geologically classified into two main categories- 1. **Gondwana Coal**: This coal is about 250 million years old. It accounts for 98 percent of the total reserves and 99 percent of the production of coal in India. It is the store house of superior quality coal. It includes cooking as well as non- cooking and non- bituminous as well as sub-bituminous coal. Anthracite is generally not found in the Gondwana fields. 2. **Tertiary Coal**: It is about 15-60 million years old and is found mainly in the extra peninsula. ### Classification of Coal: As per the International Coal Classification of the Economic Commission for Europe (UNECE) coal can be divided into two broad categories: - **Hard coal** – Coal of gross calorific value not less than $5700$ kcal/kg (23.9 GJ/t) on an ash-free but moist basis and with a mean random reflectance of vitrinite of at least 0.6. - **Brown coal** – Non-agglomerating coal with a gross calorific value less than $5700$ kcal/kg (23.9 GJ/t) containing more than 31% volatile matter on a dry mineral matter free basis. It should be stressed that the above classification system is based on the inherent qualities of the coal in question and not on the final use of the coal. In this way the classification system attempts to be objective and simple to apply. 1. **Hard Coal** - Anthracite - Bituminous coal - Coking coal - Other bituminous coal 2. **Brown coal** - Sub-bituminous coal 3. **Lignite** Based on its carbon content coal can be of following types: 1. **Anthracite** - It is dense, hard rock with zed black colour and metallic lusture. It contains very low Sulphur (S) and high Carbon (C) about 86-98%. It burns slowly with pale blue flame and lowest smoke. - This is the highest-grade coal with calorific value of 7791kcal/kg - It contains 86-98% of carbon. - Volatile matter is low being just 2-14%. - It is not found in India. It mainly occurs in UK & USA. - It is used in industrial furnaces, graphite electrodes and in metallurgical processes. 2. **Bituminous coal** - It is a medium-rank coal used for gasification, industrial coking and heat raising and residential heat raising. - It contains 69-86% Carbon by weight. - It is also high-grade coal next to anthracite. - It has a calorific value in the range of 7217 kcal/kg - Volatile matter is low and can be removed by heating in the absence of air when it gets converted into coke. - Bituminous coal that can be used in the production of a coke capable of supporting a blast furnace charge is known as coking coal. Other bituminous coal, not included under coking coal, is also commonly known as thermal coal. This also includes recovered slurries, middling and other low-grade, higher-rank coal products not further classified by type. 3. **Lignite** - It is very soft coal contain up to 17% water by weight and causes more pollution than other coals. - It is inferior quality, low grade coal. - It has laminar (banded) structure with woody, fibrous, brownish black appearance, resembling wood, hence the name. In Latin, Lignum means wood. - Its calorific value is 3346 kcal/kg - It has a moisture content of about 30-50% before exposure to air. - Lignite when exposed to air gets oxidized. - Due to high moisture, rapid oxidation and low heating value. - It is not economical to transport it to distant places. - It is burned in pit head power plants. 4. **Peat** - Peat is a solid fuel with highest moisture content. - It is not fully matured form as it is partially decomposed, so it is not true coal. - Its heating value is much less and is lower than that of wood. - It has about 90% moisture. - It is used as a low-grade fuel. | % weight | Anthracite | Bituminous | Sub-Bituminous | Lignite | |---|---|---|---|---| | Moisture | < 15% | 2–15% | 10–45% | 30–60% | | Fixed Carbon | 85–98% | 45–85% | 35–45% | 25–35% | | Ash | ≤ 10% | ≤ 10% | ≤ 10% | 10-50% | | Sulphur | 0.6-0.8% | 0.7-4.0% | <2% | 0.4-1.0% | | Chlorine (ppm) | 340±40ppm | 340 ± ppm | 120 ± 20ppm |120 ± 20ppm | * The rank of coal from most to least Carbon content: Anthracite > Bituminous > Sub bituminous > Lignite * The rank of coal from most to least Sulfur content: Bituminous> Anthracite > Sub bituminous > Lignite * The rank of coal from most to least Moisture content: Lignite > Sub bituminous > Bituminous>Anthracite ### World Coal Production in MT | S.N | Country | Coal production in MT | |---|---|---| | 1. | China | 3942 | | 2. | India | 767 | | 3. | Indonesia | 550 | | 4. | United States | 544 | | 5. | Australia | 544 | | 6. | Russia | 399 | | 7. | South Africa | 248 | | 8. | Germany | 107 | | 9. | Poland | 100 | | 10. | Kazakhstan | 70 | ### Coal Reserves in India | S.N | Name of state | Reserves in billion tonne | |---|---|---| | 1. | Jharkhand | 86.2 | | 2. | Odisha | 84.8 | | 3. | Chattisgarh | 73.4 | | 4. | West Bengal | 33.0 | | 5. | Madhya Pradesh | 30.2 | | 6. | Telangana | 22.48 | | 7. | Maharstra | 122.9 | ### Classification of Coal in India In India coal is broadly classified into two types – Coking and Non-Coking. The former constitutes only a small part of the total coal resources of the country. These two are further subdivided as follows on the basis of certain physical and chemical parameter as per the requirement of the industry. 1. **Coking Coal**: Coking coal, when heated in the absence of air, form coherent beads, free from volatiles, with strong and porous mass, called coke. Coking coal has coking properties and is mainly used in steel making and metallurgical industries. 2. **Semi Coking Coal**: Semi Coking Coal, when heated in the absence of air, form coherent beads not strong enough to be directly fed into the blast furnace. Such coal is blended with coking coal in adequate proportion to make coke. Clearly, Semi Coking Coal has comparatively less coking properties than coking coal. It is mainly used as blendable coal in steel making, merchant coke manufacturing and other metallurgical industries. 3. **Non-Coking Coal**: Non-Coking Coal does not have coking properties and is mainly used for power generation. It is also used for cement, fertilizer, glass, ceramic, paper, chemical and brick manufacturing, and for other heating purposes. 4. **Washed Coal**: Processing of coal through water separation mechanism to improve the quality of coal by removing denser material (rocks) and high ash produces washed coal which has less ash, higher moisture, bettersizing, better consistency, less abrasive, etc. The washed coking coal is used in manufacturing of hard coke for steel making. Washed non-coking coal is used mainly for power generation but is also used by cement, sponge iron and other industrial plants. 5. **Middlings and Rejects**: In the process of coal washing, apart from Clean Coal we also gettwo by-products, namely, Middlings and Rejects. Clean coal has low density whereas rejects have high density. Middlings have intermediate density. Rejects contain high ash, mineral impurities, fraction of raw coal feed, etc. and are used for Fluidized Bed Combustion (FBC) Boilers for power generation, road repairs, briquette (domestic fuel) making, land filling, etc. Middlings are fraction of raw coal feed having values of classificatory parameters between that of clan coals and rejects. It is used for power generation. It is also used by domestic fuel plants, brick manufacturing units, cement plants, industrial plants, etc. 6. **Hard Coke**: Solid product obtained from carbonization of coal, used mainly in the iron & steel industry. ### Coal Mining The two basic types of coal mines are surface and subsurface (underground) mines. The type of mine chosen depends on surface contours and on the location of the coal bed relative to the surface. If the coal bed is within 30 m (100 ft) or so of the surface, surface mining is usually done. In one type of surface mining, strip mining, a trench is dug to extract the coal, which is scraped out of the ground and loaded into railroad cars or trucks. Then a new trench is dug parallel to the old one, and the overburden from the new trench is put into the old trench, creating a spoil bank, a hill of loose rock. Digging the trenches involves using bulldozers, giant power shovels, and wheel excavators to remove the ground covering the coal seam. Mountain top removal is a type of surface mining which is very effective in obtaining coal as it levels the top of the mountains. When the coal is deeper in the ground or runs deep into the ground from an outcrop on a hillside, underground mining is preferred. Surface mining has several advantages over subsurface mining: It is usually less expensive and safer for miners, and it generally allows a more complete removal of coal from the ground. However, surface mining disrupts the land much more extensively than subsurface mining and has the potential to cause several serious environmental problems. ### Environmental impacts of the coal mining and its burning Coal mining, especially surface mining, has substantial effects on the environment. Surface coal mines are usually left as large open pits or trenches. Acid and toxic mineral drainage from such mines, along with the removal of topsoil, which was buried or washed away by erosion, prevents most plants from naturally recolonizing the land. Streams are polluted with sediment and acid mine drainage, produced when rainwater seeps through iron sulfide minerals exposed in mine wastes. Dangerous landslides occur on hills unstable from the lack of vegetation. We can restore surface-mined land to prevent such degradation and to make the land productive for other purposes, although restoration is expensive and technically challenging. One of the most land-destructive types of surface mining is mountaintop removal. It takes enormous chunks out of a mountain, eventually removing the entire mountaintop to reach the coal located below. The valleys and streams between the mountains are gone as well, filled with debris from the mountaintops. Hazards from underground mining include mine shaft collapses (cave-ins), explosions, fires, and respiratory illnesses, especially the well-known black lung disease, which is related to exposure to coal dust, which has killed or disabled many miners over the years. Some of the environmental problems associated with underground mining include the following: 1. Acid mine drainage and waste piles have polluted thousands of kilometers of streams. 2. Land subsidence can occur over mines. Vertical subsidence occurs when the ground above coal mine tunnels collapses, often leaving a crater-shaped pit at the surface. 3. Coal fires in underground mines, either naturally caused or deliberately set, may belch smoke and hazardous fumes, causing people in the vicinity to suffer from a variety of respiratory diseases. Burning coal can affect air and water quality, and impacts range from local (for example, sooty fallout) to global (climate change and ocean acidification). Coal burning generally contributes more air pollutants (including CO2) than does burning either oil or natural gas to generate the same amount of useful energy. Coal often contains mercury that is released into the atmosphere during combustion. This mercury moves readily from the atmosphere to water and land, where it accumulates and harms humans as well as wildlife. Much bituminous coal contains sulfur and nitrogen that, when burned, are released into the atmosphere as sulfur oxides (SO2 and SO3) and nitrogen oxides (NO, NO2, and N2O). Sulfur oxides and the nitrogen oxides NO and NO2 form acids when they react with water. These reactions result in acid deposition. The combustion of coal is responsible for acid deposition, which is particularly prevalent downwind from coal-burning electric power plants. Normal rain is slightly acidic (pH 5.6), but in some areas acid precipitation has a pH of 2.1, equivalent to that of lemon juice. Acidification of lakes and streams has resulted in the decline of aquatic animal populations and is linked to some of the forest decline documented worldwide. Although it is relatively easy to identify and measure pollutants such as sulfur oxides in the atmosphere, it is more difficult to trace their exact origins. Air currents transport and disperse air pollutants, which are often altered as they react chemically with other pollutants in the air. Even so, some nations clearly suffer the damage of acid deposition caused by air pollutants produced in other countries, and as a result acid deposition is an international issue. In our county discoloration of Taj Mahal is a prominent example of such pollution. ### Making coal a cleaner fuel We often hear the term clean coal. However, at its cleanest, coal still has many environmental downsides. Scrubbers, or desulfurization systems, can remove sulfur from a power plant's exhaust. As polluted air passes through a scrubber, chemicals in the scrubber react with the pollution and cause it to precipitate (settle) out. Modern scrubbers can remove 98% of the sulfur and 99% of the particulate matter in smokestacks. Desulfurization systems are expensive; they cost about 10% to 15% of the construction costs of a coal-fired electric power plant. In lime scrubbers, a chemical spray of water and lime neutralizes acidic gases such as sulfur dioxide, which remain behind as a calcium sulfate sludge that becomes a disposal problem. A large power plant may produce enough sludge annually to cover 2.6 km² (1 mi²) of land 0.3 m (1 ft) deep. Although many power plants currently dispose of the sludge in landfills, some have found markets for the material. In resource recovery, the sludge is treated as a marketable product rather than as a polluted emission. Some utilities sell calcium sulfate from scrubber sludge to wallboard manufacturers. (Wallboard is traditionally manufactured from gypsum, a mineral composed of calcium sulfate.) Other companies use fly ash, the ash from the chimney flúes, to make a lightweight concrete that could substitute for wood in the building industry. Some farmers apply calcium sulfate sludge as a soil conditioner. Plants grow better because calcium sulfate neutralizes acids in some soils and increases the water-holding capacity of the soil. (The calcium sulfate acts like a sponge.) Several technologies burn coal in ways that minimize sulfur oxides and nitrogen oxide releases. These include fluidized-bed combustion and coal gasification and liquefaction. However, these technologies have little impact on reducing CO2 emissions. Fluidized-bed combustion takes place at a lower temperature than regular coal burning, and fewer nitrogen oxides are produced. (Higher temperatures cause atmospheric nitrogen and oxygen to combine, forming nitrogen oxides.) Because the sulfur in coal reacts with the calcium in limestone to form calcium sulfate, which then precipitates out, sulfur is removed from the coal during the burning process, so scrubbers are not needed to remove it after combustion. Fluidized-bed combustion is more efficient than traditional coal burning that is, it produces more heat from a given amount of coal—and therefore reduces CO2 emissions per unit of electricity produced. If improvements of this technology were developed and adopted widely by coal-burning power plants, fluidized-bed combustion could significantly reduce the amount of CO2 released into the atmosphere. Pressurized fluidized- bed combustion is being developed as a way to reduce CO2 as well as nitrogen and sulfur oxides. By operating fluidized-bed combustion under high pressure, complete combustion of coal occurs at low temperatures. Sulfur emissions are removed as calcium sulfate, and few nitrogen oxides form because of the low temperatures. Pressurized fluidized-bed combustion is more expensive than regular fluidized-bed combustion because it requires a costly pressurized vessel. ### Coal-bed methane The processes responsible for the formation of coal include partial decomposition of plants buried by sediments that slowly convert the organic material to coal. This process also releases a lot of methane (natural gas) that is stored within the coal. The methane is actually stored on the surfaces of the organic matter in the coal, and because coal has many large internal surfaces, the amount of methane for a given volume of rock is something like seven times more than could be stored in gas reservoirs associated with petroleum. Extraction of coalbed methane is usually from one of two sources: 1. drilling vertically into a coal seam (making use of pre-existing fracture patterns); or more likely 2. directional drilling along a coal seam In some cases the coals may be fractured to improve flow rates; the well is then pumped to remove water and lower the pressure within the seam to allow release of methane. However, coal-bed methane presents several environmental concerns, including (1) disposal of large volumes of water produced when the methane is recovered and (2) migration of methane, which may contaminate groundwater or migrate into residential areas. A major environmental benefit of burning coal-bed methane, as well as methane from other sources, is that its combustion produces a lot less carbon dioxide than does the burning of coal or petroleum. Furthermore, production of methane gas prior to mining coal reduces the amount of methane that would be released into the atmosphere. Both methane and carbon dioxide are strong greenhouse gases that contribute to global warming. However, because methane produces a lot less carbon dioxide, it is considered one of the main transitional fuels from fossil fuels to alternative energy sources. ### Black Shale (tight) Natural Gas Shale gas is methane found in rocks deep below the earth's surface which had previously been considered too impermeable ('tight') to allow for economic recovery. ### B. Oil and Natural Gas: Although coal was the most important energy source in the world during the early 1900s, oil and natural gas became increasingly important, particularly after the 1930s. This change occurred largely because oil and natural gas are more versatile, easier to transport, and cleaner burning than coal. Globally in 2015, oil and natural gas provided 67% of the world's energy. In comparison, other major energy sources included coal (30%), hydroelectric power (7%), and nuclear power (4%). Most geologists accept the hypothesis that crude oil (petroleum) and natural gas are derived from organic materials (mostly plants) that were buried with marine or lake sediments in what are known as depositional basins. Oil and gas are found primarily along geologically young tectonic belts at plate boundaries, where large depositional basins are more likely to occur. However, there are exceptions, such as in, the Gulf of Mexico, and the North Sea, where oil has been discovered in depositional basins far from active plate boundaries. The source material, or source rock, for oil and gas is fine-grained (less than 1/16 mm, or 0.0025 in., in diameter), organic-rich sediment buried to a depth of at least 500 m (1,640 ft), where it is subjected to increased heat and pressure. The elevated temperature and pressure initiate the chemical transformation of the sediment's organic material into oil and gas. The pressure compresses the sediment; this, along with the elevated temperature in the source rock, initiates the upward migration of the oil and gas, which are relatively light, to a lower-pressure environment (known as the reservoir rock). The reservoir rock is coarser-grained and relatively porous (it has more and larger spaces between the grains). Sandstone and porous limestone, which have a relatively high proportion (about 30%) of empty space in which to store oil and gas, are common reservoir rocks. The liquid component of petroleum is crude oil. Crude oils vary widely in composition, but all contain a mix of hydrocarbons. Typically, crude oil is about 85% carbon by weight, and most of the rest is hydrogen. Sulfur, oxygen, and nitrogen are also present in significant quantities. Crude oil is refined to produce a variety of products, ranging from the heaviest oils for industrial boilers to fuel oil used in home heating, diesel oil that powers most trucks and some cars, jet aircraft fuel, and gasoline for our cars. | | Crude Oil | | | |---|---|---|---| | Distillation column | Gas 20° C | | | | | 150°C | Gasoline | | | | 200°C | Kerosene | | | | 300°C | Diesel oil | | | | 370°C | Fuel oil | | | | 400°C | | | | | Furnace | | | | | | Lubricating oil, paraffin wax, asphalt | | Simplified diagram of the fractional distillation process used in oil refining, showing temperatures at which different products condense out of the distillation column. The main refining process involves fractional distillation, in which crude oil is first heated to vaporize it. The vapor then rises through a vertical column, where its temperature decreases with height. Heavier components condense out at higher temperatures, so they're removed near the bottom of the column. The lighter components rise higher before condensing, with gasoline near the top. Gaseous fuels such as propane (C3 H8, widely used for heating and cooking where natural gas isn't available) then exit from the top of the column. Additional steps, including so-called catalytic cracking, break up large hydrocarbon molecules to increase yields of lighter components, especially gasoline. After their separation, the fuels may undergo further refinement to remove undesirable substances such as sulfur or to have additives mixed in for improved combustion. Figure above diagrams the fractional distillation process. The energy content of refined fuels varies somewhat, but a rough figure is about 10755.3 Kcal/kg for any product derived from crude oil. Refining is itself an energy intensive process, with some 7% of the total U.S. energy consumption going to run oil refineries. ### Petrol vs Diesel Both petrol and diesel are obtained during fractional distillation of petroleum. Petrol is produced at temperature between 35 degrees to 200 degrees while diesel is produced at a boiling point of 250-350 degrees. After distillation, in order to use these by products as commercially acceptable petrol and diesel, some blending with other elements has to be done. Petrol is produced first in this process as it is produced at a lower temperature than diesel. Diesel is composed of about 75% saturated hydrocarbons (primarily paraffins including n, iso, and cycloparaffins), and 25% aromatic hydrocarbons (including naphthalenes and alkylbenzenes). The average chemical formula for common diesel fuel is C12H23, ranging from approx. C10H20 to C15H28. Petrol consists of hydrocarbons with between 5 and 12 carbon atoms per molecule but then it is blended for various uses. Overall a typical petrol sample is predominantly a mixture of paraffins (alkanes), naphthenes (cycloalkanes), aromatics and olefins (alkenes). The ratios vary based on a variety of factors. The calorific value of diesel fuel is roughly 10755.3 Kcal/kg, slightly lower than petrol which is 10946.463 Kcal/kg. However, diesel fuel is denser than petrol and contains about 15% more energy by volume (roughly 8819.31Kcal/litre compared to 8054.49 Kcal/litre). ### Cetane Rating: Cetane rating is a measure of the ignition quality of a fuel. The ease at which diesel fuel ignites, and the manner in which it burns, influences engine starting and combustion roughness. Pure cetane is a colourless liquid hydrocarbon with excellent ignition qualities and is rated at 100. The higher the cetane rating, the shorter the lag time between the time the fuel enters the combustion chamber and the time it begins to burn. A good quality diesel fuel with a high cetane rating has a lag time of approximately 0.001 seconds. Cetane rating requirements depend on the engine size, design, load and atmospheric conditions. For example, engines operating at higher altitudes or lower temperature demand a higher cetane fuel to start and operate correctly. Typical cetane ratings for No. 2 diesel would be 46 – 48. No. 1 diesel is usually about 51 - 53. | | Octane | |---|---| | | 100 | | | 90 | | SLOW BURNING | 80 | | | 70 | | | | | | | | | | | | 30 | | | 40 | | | 50 | | CETANE | | | **FAST BURNING** | | About half of the "Other" category in above pie diagram is turned into road-building products, lubricants, and petrochemical feedstocks. These feedstocks go into making a vast array of products everything from plastics to medicines, ink to contact lenses, insecticides to toothpaste, perfumes to fertilizers, lipstick to paint, false teeth to food preservatives. When oil grows scarce, it will become increasingly valuable as a raw material for manufacturing, giving us all the more incentive to look elsewhere for our energy. ### Natural Gas At normal temperature and pressure, the contents of commercial natural gas are mainly methane (CH4) ethane (C2H6) and varying amounts of propane (C3H8) and butane (C4H10). An average composition of natural gas indicates methane- 83.0%, ethane-7.2%, propane-2.3%, butane-1.0%, N2-5.8%, CO2- 0.2% etc. There may be traces of helium, oxygen, hydrogen and other substances. The main impurities are N2, CO2and H2S. If H2S is more than 10 gm/m³, it is removed commercially and converted to elemental sulphur by Claus's process. If concentration of H2S is less, it is removed by the process called 'sweetening'. Natural gas containing H2S is called 'SOUR GAS'. It has an unpleasant odour and H2S dissolved in water follows a mild acid which is corrosive to pipes and valves. Some sources of natural gas contain helium up to 8% also. As such, natural gas is the main source of helium. Propane and butane are separated from the natural gas, stored in pressurized tanks as a liquid called liquefied petroleum gas, and used primarily as fuel for heating and cooking in rural areas. Methane is used to heat residential and commercial buildings, to generate electricity in power plants, and for a variety of purposes in the organic chemistry industry. It is estimated that every barrel of crude oil is associated with fixed amount of gas i.e. 170 m³/barrel. Transportation of gas is a big problem. Use of natural gas is increasing in three main areas - generation of electricity, transportation, and commercial cooling. One example of a systems approach is cogeneration, in which natural gas is used to produce both electricity and steam; the heat of the exhaust gases provides the energy to make steam for water and space or industrial heating. Cogeneration systems that use natural gas provide relatively clean and efficient electricity. Natural gas as a fuel for trucks, buses, and automobiles offers significant environmental advantages over gasoline or diesel: Natural gas vehicles emit up to 93% fewer hydrocarbons, 90% less carbon monoxide, 90% fewer toxic emissions, and almost no soot. Engines that use natural gas are essentially the same as those that burn gasoline. As a fuel, natural gas can be cheaper than gasoline: Individuals can install equipment to compress natural gas in their homes. Natural gas efficiently fuels residential and commercial air-cooling systems. One example is the use of natural gas in a desiccant-based (air-drying) cooling system, which is ideal for supermarkets, where humidity control is as important as temperature control. Restaurants are also important users of natural gas-powered desiccant-based cooling systems. The main disadvantage of natural gas is that deposits are often located far from where the energy is used. Because it is a gas and is less dense than a liquid, natural gas costs four times more to transport through pipelines than crude oil. To transport natural gas over long distances, it is first compressed to form liquefied natural gas (LNG), then carried on specially constructed refrigerated ships. After LNG arrives at its destination, it is returned to the gaseous state at regasification plants before being piped to where it will be used. ### Exploration for oil and natural gas: Geologic exploration is continually under way in search of new oil and natural gas deposits, usually found together under one or more layers of rock. Oil and natural gas deposits are usually discovered indirectly by the detection of structural traps. Plate tectonic movements sometimes cause the upward folding of sedimentary rock strata (layers). Sometimes the strata that arch upward include both porous and impermeable rock. If impermeable layers overlie porous layers, any oil or natural gas present from a source rock such as shale may work its way up through the porous rock to accumulate under the impermeable layer. Many important oil and natural gas deposits (for example, oil deposits known to exist in the Gulf of Mexico) are found in association with salt domes, underground columns of salt. Salt domes develop when extensive salt deposits form at Earth's surface because of the evaporation of water. All surface water contains dissolved salts. The salts dissolved in ocean water are so concentrated they can be tasted, but even fresh water contains some dissolved material. If a body of water lacks a passage to the ocean, as an inland lake often does, the salt concentration in the water gradually increases. If such a lake were to dry up, a massive salt deposit would remain. Layers of sediment may eventually cover such deposits and convert to sedimentary rock after millions of years. The rock layers settle, and the salt deposit, which is less dense than rock, rises in a column-a salt dome. The ascending salt dome, together with the rock layers that buckle over it, provides a trap for oil or natural gas. Geologists use a variety of techniques to identify structural traps that might contain oil or natural gas. One method is to drill test holes in the surface and obtain rock samples. Another method is to produce an explosion at the surface and measure the echoes of sound waves that bounce off rock layers under the surface. These data are interpreted to determine whether structural traps are present. However, many structural traps do not contain oil or natural gas. Three-dimensional seismology produces maps of oil field area and depth, enabling geologists to have a higher rate of success when drilling. Another new technology that improves oil recovery is horizontal drilling. Traditional oil wells are vertical and cannot veer off to follow the contours of underground formations that contain oil. Wells dug with horizontal drilling follow contours, and they generally yield three to five times as much oil as vertical wells. ### Reserves of oil and natural gas: Although oil and natural gas deposits exist on every continent, their distribution is uneven, and a large portion of total oil deposits are clustered relatively close together. Enormous oil fields containing more than half of the world's total estimated reserves are situated in the Persian Gulf region, which includes Iran, Iraq, Kuwait, Oman, Qatar, Saudi Arabia, Syria, the United Arab Emirates, and Yemen. In addition, major oil fields are known to exist in Venezuela, Mexico, Russia, Kazakhstan, Libya, and the United States (in Alaska and the Gulf of Mexico). More than 40% of the world's proved recoverable reserves of natural gas are located in two countries, Russia and Iran. The United States has more deposits of natural gas than Western Europe, and use of natural gas is more common in North America than in Western Europe. Canada and the United States also extract coal bed methane, a form of natural gas associated with coal deposits. Large oil deposits probably exist under the continental shelves, the relatively flat underwater areas that surround continents, and in deep water areas adjacent to the continental shelves. Despite problems such as storms at sea and the potential for major oil spills, many countries engage in offshore drilling for this oil. New technologies, such as platforms the size of football fields, enable oil companies to drill down several thousand feet for oil, making seafloor oil fields once considered inaccessible open for tapping. Continental shelves off the coasts of western Africa and Brazil are also promising. The oil industry is currently developing remote-controlled robots that can install and maintain underwater equipment and pipelines. Environmentalists generally oppose opening the outer continental shelves for oil and natural gas exploration because of the threat a major oil spill would pose to marine and coastal environments. Coastal industries, including fishing and tourism, also oppose oil and natural gas exploration in these areas. In India the potential of oil bearing areas are located in States like Assam, Tripura, Manipur, West Bengal, Punjab, Himanchal Pradesh, Gujarat (Kutchh) and Eastern and Western Coastal Areas in Tamilnadu, Kerala, Andhra Pradesh and in Continental shelf adjoining areas. Oil refineries in India are in Traumbay, Mathura, Cochin, Vishakhapatnam, Mumbai, Guwahati, Chennai, Digboi (Assam). Despite adequate oil supplies for the near future, in the long term we will need other resources. Some experts think that global oil production has already reached. Peak Oil, the point at which the oil is being withdrawn at the highest possible rate. About 80% of current production comes from oil fields discovered before 1973, and most of these fields have started to decline in production. These analysts say the world must move quickly to develop alternative energy sources because the global demand for energy will only continue to increase even as production declines. Industry analysts are generally more optimistic. They think that improving technology will allow us to extract more oil out of old oil fields. (Currently, about 60% is left because it is too expensive to remove using current technology.) New technologies may help us obtain oil from fields formerly unreachable (such as beneath deep-ocean waters). Improved technology may allow us to produce oil from natural gas, coal, and synfuels. Even so, the most optimistic predictions are for Peak Oil to occur at around 2035. Natural gas is more plentiful than oil. Experts estimate that readily recoverable reserves of natural gas, if converted into a liquid fuel, would be equivalent to between 500 billion and 770 billion barrels of crude oil, enough to keep production rising for at least 10 years after conventional supplies of petroleum have begun to decline. However, if the global use of natural gas continues to increase as it has in recent years, then its supply horizon will be shorter than current projections predict. In India we have approximately 250 billion m³ gas reserves. ### Environmental Impacts of Oil and Natural Gas Two sets of environmental problems are associated with the use of oil and natural gas: the problems that result from burning the fuels (combustion) and the problems involved in obtaining them (production and transport). As with coal, the burning of oil and natural gas produces CO2. As CO2 accumulates in the atmosphere, it insulates the planet, preventing heat from radiating back into space. The global climate is warming more rapidly now than it did during