Rabu, 20 April 2011

COAL COMBUSTION IN THE COAL-FIRED POWER STATION



Chapter I : INTRODUCTION

1.         History of Coal Formation

Coal is a fossil fuel created from the remains of plants that lived and died about 100 – 400 million years ago when parts of the earth were covered with huge swampy forests. Coal is classified as a non renewable energy source because it takes millions of years to form.
The energy we get from coal today comes from the energy that plants absorbed from the sun millions of years ago. All living plants store solar energy through a process known as photosynthesis. When plants die, this energy is usually released as the plants decay. Under conditions favorable to coal formation, however, the decay process is interrupted, preventing the release of the stored solar energy. The energy is locked into the coal.

Millions of years ago, dead plant matter fell into swampy water and over the years, a thick layer of dead plants lay decaying at the bottom of the swamps. Over time, the surface and climate of the earth changed, and more water and dirt washed in, halting the decay process.
The weight of the top layers of water and dirt packed down the lower layers of plant matter. Under heat and pressure, this plant matter underwent chemical and physical changes, pushing out oxygen and leaving rich hydrocarbon deposits. What once had been plants gradually turned into coal.



Coal is combustible material consisting primarily of the element carbon, but with low percentages of solid, liquid, and gaseous hydrocarbons and other materials, such as compounds of nitrogen and sulfur. Coal is usually classified into the sub-groups known as anthracite, bituminous, lignite, and peat. The physical, chemical, and other properties of coal vary considerably from sample to sample.


2.         How the Coal is mined

There are two ways to remove coal from the ground – surface and underground mining. Surface mining is used when a coal seam is relatively close to the surface, usually within 200 feet. The first step in surface mining is to remove and store the soil and rock covering the coal, called the overburden. Workers use a variety of equipment – draglines, power shovels, bulldozers, and front-end loaders – to expose the coal seam for mining.

After surface mining, workers replace the overburden, grade it, cover it with topsoil, and fertilize and seed the area. This land reclamation is required by law and helps restore the biological balance of the area and prevent erosion. The land can then be used for croplands, wildlife habitats, recreation, or as sites for commercial development.



Underground (or deep) mining is used when the coal seam is buried several hundred feet below the surface. In underground mining, workers and machinery go down a vertical shaft or a slanted tunnel called a slope to remove the coal. Mine shafts may sink as deep as 1,000 feet. One method of underground mining is called room-and-pillar mining. With this method, much of the coal must be left behind to support the mine’s roofs and walls. Sometimes as much as half the coal is left behind in large column formations to keep the mine for collapsing.
A more efficient and safer underground mining method, called longwall mining, uses a specially shielded machine that allows a mined-out area to collapse in a controlled manner. This method is called long wall mining because huge blocks of coal up to several hundred feet wide can be removed.


3.         Types of Coal

Coal is classified into four main types, depending on the amount of carbon, oxygen, and hydrogen present. The higher the carbon content, the more energy the coal contains

Lignite is the lowest rank of coal, with a heating value of 4,000 – 8,300 Btu per pound. Lignite is crumbly and has high moisture content.

Sub-bituminous coal typically contains less heating value and more moisture than bituminous coal (8,300 – 13,000 Btu per pound). It contains 35 – 45% carbon.

Bituminous coal was formed by further addition of heat and pressure on lignite during coal formation process. Bituminous coal looks smooth and sometimes shiny, contain 11,000-15,500 Btu per pound, between 45 – 86% carbon.

Anthracite was created where additional pressure combined with very high temperature inside the earth. It is deep black and looks almost metallic due to its glossy surface, contain around 15,000 Btu per pound energy and 86 – 97% carbon.
























































Until the twentieth century chemists knew very little about the composition and molecular structure of the different kinds of coal, and as late as the 1920s they still believed that coal consisted of carbon mixed with hydrogen-containing impurities. Their two methods of analyzing or separating coal into its components, destructive distillation (heating out of contact with air) and solvent extraction (reacting with different organic solvents such as tetralin), showed only that coal contained significant carbon, and smaller percentages of the elements hydrogen, oxygen, nitrogen, and sulfur. Inorganic compounds such as aluminum and silicon oxides constitute the ash. Distillation produced tar, water, and gases. Hydrogen was the chief component of the gases liberated, although ammonia, carbon monoxide and dioxide gases, benzene and other hydrocarbon vapors were present. (The composition of a bituminous coal by percentage is roughly: carbon [C], 75–90; hydrogen [H], 4.5–5.5; nitrogen [N], 1–1.5; sulfur [S], 1–2; oxygen[O], 5–20; ash, 2–10; and moisture, 1–10.) Beginning in 1910, research teams under the direction of Richard Wheeler at the Imperial College of Science and Technology in London, Friedrich Bergius (1884–1949) in Mannheim, and Franz Fischer (1877–1938) in Mülheim made important contributions that indicated the presence of benzenoid (benzenelike) compounds in coal. But confirmation of coal's benzenoid structure came only in 1925, as a result of the coal extraction and oxidation studies of William Bone (1890–1938) and his research team at Imperial College. The benzene tri-, tetra-, and other higher carboxylic acids they obtained as oxidation products indicated a preponderance of aromatic structures with three-, four-, and five-fused benzene rings, and other structures with a single benzene ring. The simplest structures consisted of eight or ten carbon atoms, the fused-ring structures contained fifteen or twenty carbon atoms.


Figure 1. An example of the structure of coal.


Classification and Uses of Coal

European and American researchers in the nineteenth and early twentieth centuries proposed several coal classification systems. The earliest, published in Paris in 1837 by Henri-Victor Regnault (1810–1878), classifies types of coal according to their proximate analysis (determination of component substances, by percentage), that is, by their percentages of moisture, combustible matter, fixed carbon, and ash. It is still favored, in modified form, by many American coal scientists. Another widely adopted system, introduced in 1919 by the British scientist Marie Stopes (1880–1958), classifies types of coal according to their macroscopic constituents: clarain (ordinary bright coal), vitrain (glossy black coal), durain (dull rough coal), and fusain, also called mineral charcoal (soft powdery coal). Still another system is based on ultimate analysis (determination of component chemical elements, by percentage), classifying types of coal according to their percentages of fixed carbon, hydrogen, oxygen, and nitrogen, exclusive of dry ash and sulfur. (Regnault had also introduced ultimate analysis in his 1837 paper.) The British coal scientist Clarence A. Seyler developed this system in 1899–1900 and greatly expanded it to include large numbers of British and European coals. Finally, in 1929, with no universal classification system, a group of sixty American and Canadian coal scientists working under guidelines established by the American Standards Association (ASA) and the American Society for Testing Materials (ASTM) developed a classification that became the standard in 1936. It has remained unrevised since 1938.
The ASA–ASTM system established four coal classes or ranks—anthracite, bituminous, subbituminous, and lignite—based on fixed-carbon content and heating value measured in British thermal units per pound (Btu/lb). Anthracite, a hard black coal that burns with little flame and smoke, has the highest fixed-carbon content, 86–98 percent, and a heating value of 13,500–15,600 Btu/lb (equivalent to 14.2–16.5 million joules/lb [1 Btu=1,054.6 joules, the energy emitted by a burning wooden match]). It provides fuel for commercial and home heating, for electrical generation, and for the iron, steel, and other industries. Bituminous (low, medium, and high volatile) coal, a soft coal that produces smoke and ash when burned, has a 46–86 percent fixed-carbon content and a heating value of 11,000–15,000 Btu/lb (11.6–15.8 million joules/lb). It is the most abundant economically recoverable coal globally and the main fuel burned in steam turbine-powered electric generating plants. Some bituminous coals, known as metallurgical or coking coals, have properties that make them suitable for conversion to coke used in steelmaking.

Coal is one of the world's most abundant sources of energy.
Subbituminous coal has a 46–60 percent fixed-carbon content and a heating value of 8,300–13,000 Btu/lb (8.8–13.7 million joules/lb). The fourth class, lignite, a soft brownish-black coal, also has a 46–60 percent fixed-carbon content, but the lowest heating value, 5,500–8,300 Btu/lb (5.8–8.8 million joules/lb). Electrical generation is the main use of both classes. In addition to producing heat and generating electricity, coal is an important source of raw materials for manufacturing. Its destructive distillation (carbonization) produces hydrocarbon gases and coal tar, from which chemists have synthesized drugs, dyes, plastics, solvents, and numerous other organic chemicals. High pressure coal hydrogenation or liquefaction and the indirect liquefaction of coal using Fischer–Tropsch syntheses are also potential sources of clean-burning liquid fuels and lubricants.


Chapter II: COAL TRANSPORTATION & STOCK PILE MANAGEMENT

Chapter III: COAL COMBUSTION

By far the most important property of coal is that it combusts. When the pure carbon and hydrocarbons found in coal burn completely only two products are formed, carbon dioxide and water. During this chemical reaction, a relatively large amount of energy is released. The release of heat when coal is burned explains the fact that the material has long been used by humans as a source of energy.
The complete combustion of carbon and hydrocarbons described above rarely occurs in nature. If the temperature is not high enough or sufficient oxygen is not provided to the fuel, combustion of these materials is usually incomplete. During the incomplete combustion of carbon and hydrocarbons, other products besides carbon dioxide and water are formed, primarily carbon monoxide,hydrogen, and other forms of pure carbon, such as soot. During the combustion of coal, minor constituents are also oxidized. Sulfur is converted to sulfur dioxide and sulfur trioxide, and nitrogen compounds are converted to nitrogen oxides. The incomplete combustion of coal and the combustion of these minor constituents results in a number of environmental problems. For example, soot formed during incomplete combustion may settle out of the air. Carbon monoxide formed during incomplete combustion is toxic gas and may cause illness or death in humans and other animals. Oxides of sulfur and nitrogen react with water vapor in the atmosphere and then are precipitated out as acid rain. Acid rain is thought to be responsible for the destruction of certain forms of plant and animal (especially fish) life.
In addition to these compounds, coal often contains a few percent of mineral matter: quartz, calcite, or perhaps clay minerals. These do not readily combust and so become part of the ash. The ash then either escapes into the atmosphere or is left in the combustion vessel and must be discarded. Sometimes coal ash also contains significant amounts of lead, barium, arsenic, or other compounds. Whether air borne or in bulk, coal ash can therefore be a serious environmental hazard.

Environmental Concerns

The major disadvantage of using coal as a fuel or raw material is its potential to pollute the environment in both production and consumption. This is the reason why many coal-producing countries, such as the United States, have long had laws that regulate coal mining and set minimum standards for both surface and underground mining. Coal production requires mining in either surface (strip) or underground mines. Surface mining leaves pits upon coal removal, and to prevent soil erosion and an unsightly environment, operators must reclaim the land, that is, fill in the pits and replant the soil. Acid mine water is the environmental problem associated with underground mining. Water that seeps into the mines, sometimes flooding them, and atmospheric oxygen react with pyrite (iron sulfide) in the coal, producing acid mine water. When pumped out of the mine and into nearby rivers, streams, or lakes, the mine water acidifies them. Neutralizing the mine water with lime and allowing it to settle, thus reducing the presence of iron pyrite before its release, controls the acid drainage.
Coal combustion emits sulfur dioxide and nitrogen oxides, both of which cause acid rain. Several methods will remove or reduce the amount of sulfur present in many coals or prevent its release into the atmosphere. Washing the coal before combustion removes pyritic sulfur (sulfur combined with iron or other elements). Burning the coal in an advanced-design burner known as a fluidized bed combustor, in which limestone added to coal combines with sulfur in the combustion process, prevents sulfur dioxide from forming. Scrubbing the smoke released in the combustion removes the sulfur dioxide before it passes into the atmosphere. In a scrubber, spraying limestone and water into the smoke enables the limestone to absorb sulfur dioxide and remove it in the form of a wet sludge. Improved clean coal technologies inject dry limestone into the pipes leading from the plant's boiler and remove sulfur dioxide as a dry powder (CaSO3) rather than a wet sludge. Scrubbing does not remove nitrogen oxides, but coal washing and fluidized bed combustors that operate at a lower temperature than older plant boilers reduce the amount of nitrogen oxides produced and hence the amount emitted.
Clean coal technologies and coal-to-liquid conversion processes have led to cleaner burning coals and synthetic liquid fuels, but acid rain remains a serious problem despite society's recognition of its damaging effects since 1852. Global warming resulting from the emission of the greenhouse gases, carbon dioxide, methane, and chlorofluorocarbons, is another coal combustion problem that industry and government have largely ignored since 1896, but it can no longer be avoided without serious long-term consequences.








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