Coal

Black or brown rock , often black in color, flammable and combustible , found in ground layers or veins , consisting mainly of carbon , in addition to varying proportions of other elements (mainly hydrogen , sulfur , oxygen , and nitrogen in addition to other elements). ^[1] Coal is a sedimentary rock that forms as layers of rock called seams. Coal is formed when dead plants decompose and become peat , transformed by the high heat and pressure of burial over millions of years. Although coal is mostly formed in stratified sediments, these deposits may be exposed to extremely high temperatures and stresses from igneous intrusions or deformation in the process of formation (i.e. mountain formation processes), which may lead to the development of anthracite and even graphite. ^[2]

Coal is a fossil fuel that has been used throughout history as a source of thermal energy. It was used for heating, and as fuel for locomotives at the beginning of the era of the invention of the steam engine . However, there are huge problems for the environment as a result of its use, represented by the impact of the coal industry on the environment and global warming , since coal is one of the largest sources of abnormal carbon dioxide emissions . (i.e., as a result of human practices). Today, the primary use of this energy worldwide is in the production of electricity . Coke is also used in the production of coke , which is a basic raw material in the iron and steel industry. Other materials produced by the coke production process can be used in the manufacture of medicines, dyes and fertilizers . In the Arab region, Egypt uses coal in the cement industry. ^[3]

Coal-fired power plants generate two-thirds of the electricity consumed in the world, although coal-fired electricity generators emit about 2,000 pounds of carbon dioxide per megawatt-hour, which is about twice what natural gas emits to generate the same energy (estimated at 1,100 pounds). In 1999, the total global emission of carbon dioxide from coal was 8,666 million tons. ^[4] In 2011, the total carbon dioxide emissions from coal were estimated at 14,416 tons. ^[5] Due to the higher efficiency of natural gas in electricity generation, and the market transition in the United States of America to reduce electricity generation with coal and increase it using gas, carbon dioxide emissions there have decreased, and the readings in the first quarter of 2012 were the lowest compared to any reading in the first quarter since 1992. ^[6] In 2013, the head of the United Nations climate agency advised keeping most of the coal stocks in the ground to avoid catastrophic global warming . ^[7]

2009 World Coal Stocks in BTU

Coal extraction in Germany in 1965.

Coal is extracted from the ground either from vertical mines , surface or open pit mines. China has been at the top of the list of coal producers since 1983. ^[8] In 2011, it produced 3,520 million tons of coal, 49.5% of the 7,695 million tons produced globally. In 2011, the United States of America (993 million tons) , India (589), the European Union (576) and Australia (416) were the countries that extracted the most coal. ^[9] In 2010, the largest exporters were Australia (with 328 million tons, 27.1% of the world export volume) and Indonesia (with 316 million tons (26.1%)), ^[10] while the largest importers were Japan with 207 million tons (17.5% of the world export volume). world import of coal), China with 195 million tons (16.6%) and South Korea with 126 million tons (10.7%) ^[11]

Origin of the word

The word originally took the Old English form col, which in turn is supposed to come from "living coal". Cognates in Old Frisian include koli, Middle Dutch köll, Dutch kool, Old High German chole, German Kohle and Old Norse kohl, and in Irish gual is also a cognate via the Indo-European root.

geology

Coal is composed of minerals and water. Fossils and amber may have been present in coal.

Model of the chemical composition of coal

The conversion of dead vegetation into charcoal is called coalescence. At different times in the geological past, the Earth had dense forests in low-lying wetland areas. In these wetlands, the coalification process began when dead plant material was protected from decomposition and oxidation, usually by clay or acidic water, and converted into peat. This trapped the carbon in huge swamps that were eventually buried deeply by sediment. Then, over millions of years, the heat and pressure of deep burial caused a loss of water, methane, and carbon dioxide and an increase in carbon. ^[12] The grade of coal produced depends on the maximum pressure and temperature reached, with lignite (also called "brown coal") produced under relatively mild conditions, and sub-bituminous coal, bituminous coal, or anthracite (also called "hard coal" or "black coal"). ») produced in turn with increasing temperature and pressure. ^[13]

Among the factors involved in the inoculation process, temperature is more important than pressure or burial time. Bituminous coal can form at temperatures as low as 35 to 80 °C (95 to 176 °F) while anthracite requires a minimum temperature of 180 to 245 °C (356 to 473 °F). ^[14]

Although coal is known for most geological periods, 90% of all coal seams were deposited in the Carboniferous and Permian periods, which account for only 2% of Earth's geological history. ^[15] Ironically, this was during the late Pleistocene ice age, the time of global glaciation. However, the decrease in global sea level associated with the ice has exposed the previously submerged continental shelf, and has added wide river deltas resulting from increased erosion due to lower base level. These large areas of wetlands provided ideal conditions for coal formation. The rapid formation of coal ended with the Coal Gap and the Permian-Triassic extinction event, where coal is scarce. ^[16]

Favorable geography alone does not explain the extensive carbonate coal seams. Other factors that contributed to the rapid deposition rate of the coal were high oxygen levels, in excess of 30%, which led to severe wildfires and the formation of coal that was indigestible by decomposing organisms. high levels of carbon dioxide that promoted plant growth; and the nature of carboniferous forests, which included lycophytes whose specific growth meant that carbon was not restricted to the heartwood of living trees for extended periods. ^[17]

One theory suggested that about 360 million years ago, some plants developed the ability to produce lignin, a complex polymer that makes cellulose stems stiffer and more woody. The ability to produce lignin led to the evolution of the first trees. But bacteria and fungi did not immediately evolve the ability to decompose lignin, so the wood did not fully decompose but became buried under the sediment, eventually turning into charcoal. About 300 million years ago, mushrooms and other fungi evolved this ability, ending a major coal-forming period in Earth's history. ^[18] However, a 2016 study largely disproved this idea, finding extensive evidence of lignin degradation during the Carboniferous, and that shifts in lignin abundance had no effect on coal formation. They suggested that climatic and tectonic factors were a more plausible explanation.

Charcoal is known from the Precambrian strata, which preceded terrestrial plants. This charcoal is presumed to have originated from the remains of algae. ^[19]

Sometimes coal seams (also known as coal seams) are interwoven with other deposits in the cyclothem. Cyclothems are thought to have their origin in glacial cycles that produced fluctuations in sea level , which alternately exposed and then submerged large areas of the continental shelf. ^[19]

Coalition chemistry [ edit ]

Modern peat is mostly lignin. The component of cellulose and hemicellulose ranges from 5% to 40%. Various other organic compounds, such as waxes and compounds containing nitrogen and sulfur, are also present. ^[20] Lignins are polymers of monolignols, a family of alcohols that share a benzene ring with the side chain of an allyl alcohol. These are crosslinked by the carbohydrate chains to form lignin, which has a general formula approximating (C ₃₁ H ₃₄ O ₁₁ ) _n . Cellulose is a polymer of glucose with the approximate formula (C ₆ H ₁₀ O ₅ ) n. ^[21] By weight, lignin is composed of approximately 54% carbon, 6% hydrogen, and 30% oxygen, while cellulose has a weight composition of approximately 44% carbon, 6% hydrogen, and 49% oxygen. Bituminous coal is composed of approximately 84.4% carbon, 5.4 % hydrogen, 6.7% oxygen, 1.7% nitrogen, and 1.8% sulfur, on a weight basis. This means that the chemical processes during the fortification process must remove most of the oxygen and much of the hydrogen, leaving the carbon behind, a process called carbonation. ^[18]

The carbonation process is primarily done by dehydration , decarboxylation, and methanation. Dehydration removes water molecules from mature char through such reactions ^[22].

2 R – OH → R – O – R + H ₂ O

2 R-CH2-O-CH2-R → R-CH = CH-R + H ₂ O

Decarboxylation removes carbon dioxide from mature coal and is achieved by a like reaction

RCOOH → RH + CO ₂

While the process of methane removal proceeds through a reaction such as

2 R-CH ₃ → R-CH ₂ -R + CH ₄

In each of these formulas, R represents the rest of the cellulose or lignin molecule to which the reacting groups are attached.

Dehydration and decarboxylation occur early in the cementation process, while the methane process only begins after the coal has already reached bitumen status. ^[23] The effect of decarboxylation is to reduce the percentage of oxygen, while the removal of methane reduces the percentage of hydrogen. Dehydration does both, and also reduces carbon backbone saturation (increasing the number of double bonds between carbons).

As carbonation continues, aliphatic compounds (compounds of carbon that feature chains of carbon atoms) are replaced by aromatic compounds (compounds of carbon that feature rings of carbon atoms) and aromatic rings begin to combine into many aromatic compounds (compounds of carbon that feature rings of carbon atoms). ^[24] The structure is increasingly similar to graphene , the structural component of graphite.

Chemical changes are accompanied by physical changes, such as a decrease in the average pore size. The mineral elements (organic particles) of lignite are composed of humminite, which is earthy in appearance. As the coal matures to semi-bituminous coal, the huminite begins to be replaced by the glassy (shiny) vitrinitite. ^[25] The maturation of bituminous coal is characterized by the conversion of part of the coal into bitumen, a gelatinous substance rich in hydrocarbons. Maturation to anthracite is characterized by its removal (from methane removal) and an increased tendency of anthracite to separate by conchoidal fracture, similar to the way thick glass is fractured. ^[26]

Species

Coal

As geological processes put pressure on dead biological material over time, under suitable conditions, its metamorphic degree or order increases sequentially to:

• peat , precursor of coal
• Lignite , or brown coal, the lowest rank of coal, and the most harmful to health ^[27] , is used almost exclusively as a fuel for electric power generation
• Semi-bituminous coal, whose properties range from those of lignite to those of bituminous coal, is primarily used as a fuel for steam and electric power generation.
• Coal is a dense sedimentary rock, usually black, but sometimes dark brown, often with well-defined bands of bright and dull material. It is mainly used as a fuel in steam and electric power generation and for making coke . Known as steam coal in the United Kingdom, it was historically used to raise steam in steam locomotives and ships
• Anthracite , the highest rank of coal, is a harder, more lustrous black coal used primarily for heating residential and commercial spaces.
• Graphite is difficult to ignite and is not commonly used as a fuel; It is most commonly used in pencils, or as a lubricant.

Channel coal (sometimes called "candle coal") is a variety of fine-grained, fine-quality coal with a large hydrogen content, which consists primarily of leptinite.

There are many international standards for coal. ^[28] The classification of coal is generally based on the volatile matter content. But the most important distinction is between thermal coal (also known as steam coal), which is burned to generate electricity by steam; Metallurgical coal (also known as coke), which is burned at a high temperature to make steel .

Hilt's law is a geological observation that (within a small area) the deeper the coal, the higher its rank (or grade). Applied if the temperature gradient is fully vertical; However, the transformation may cause lateral changes in rank, regardless of depth. For example, some of the coal seams in the Madrid, New Mexico coalfield were partially converted to anthracite by igneous sill contact metamorphism while the remaining layers remained as bituminous coal. ^[29]

An Alaska Railroad transporting coal on its way to Anchorage in 1917

The oldest known use is from the Shenyang region of China where by 4000 BC Neolithic people began carving ornaments from black lignite. ^[30] Coal from the Fushun mine in northeastern China for copper smelting from 1000 BC. Marco Polo, an Italian who traveled to China in the thirteenth century, described coals as "black stones ... which burn like logs", and said that coals were so plentiful, that people could take three hot baths a week. ^[31] In Europe, the oldest reference to the use of coal as a fuel is from the geological treatise on stones (Lap 16) by the Greek scholar Theophrastus (c. 371-287 BC): ^[32]

Among the materials excavated because they are useful, those known as embers (coal) are made of earth, and once set on fire, they burn like coal. They are found in Liguria... and in Elis when one approaches Olympia by the mountain road; And they are used by those who work in metallurgy. -

Slurry coals were used in Britain during the Bronze Age (3000–2000 BC), where they formed part of burial pyres. In Roman Britain, with the exception of two modern fields, "the Romans were exploiting coal in all major coalfields in England and Wales by the end of the second century AD". Evidence of trade in coal, dating to about AD 200, has been found at the Roman settlement of Heronbridge, near Chester . In Fenland, where coal from the Midlands was transported across the Car Dyke for use in drying grain. ^[33] Charcoal ash has been found in the hearths of Roman villas and fortresses, especially in Northumberland, dating back to about A.D. 400. In the west of England, modern writers have described the wonder of the enduring brazier of coal on the altar of Minerva at Aqui Solis (modern Bath ), Although the easily accessible surface coal of what became the Somerset coalfield was commonly used in very low dwellings locally. ^[34] Evidence has been found of the use of coal in ironworks in the city during the Roman period. In Eschweiler, Rhineland, the Romans used coal deposits to smelt iron ore. ^[35]

Workers at the Umbilin coal field in Sumatra circa 1860-1900

There is no evidence that product miniaturization was of much importance in Britain before about AD 1000, or in the High Middle Ages. ^[36] Coal was referred to as "sea coal" in the thirteenth century. The wharf at which the materials arrived in London was known as sea coal, so it was specified in a charter granted by King Henry III in 1253. Initially, the name was given because a lot of coal was found on the beach, having fallen from the exposed seams onto The cliffs above or that coal outcrops were washed under water, but by the time of Henry VIII it was understood to derive from the manner in which he was transported to London by sea. ^[37]

These easily accessible sources became largely depleted (or could not meet the growing demand) by the 13th century, when underground extraction by shaft mining was developed. An alternative name was "petcoal" because it came from the mines. The development of the Industrial Revolution led to the widespread use of coal, as the steam engine took over the water wheel. In 1700, five-sixths of the world's coal was mined in Britain. Britain would have run out of suitable sites for watermills by the 1830s if coal had not been available as an energy source. ^[38] In 1947 there were about 750,000 miners working in Britain ^[39] but the last deep-sea coal mine in the UK closed in 2015. ^[40]

The grade between bituminous coal and anthracite was formerly known as "steam coal" as it was widely used as fuel for steam locomotives. In this specialized use, it is sometimes known as "sea coal" in the United States. Small "steam coal", also called dry small steam nuts (or DSSN), as fuel for domestic water heating. ^[41]

Coal played an important role in industry in the nineteenth and twentieth centuries. The predecessor of the European Union, the European Coal and Steel Community, was based on trade in this commodity.

Coal continues to reach beaches around the world from the natural erosion of exposed coal seams and wind spills from cargo ships. Many homes in such areas collect this coal as an important, and sometimes major, source of home heating fuel. ^[42]

emission intensity

Emissions intensity is the greenhouse gases emitted over the life of the generator per unit of electricity generated. The emissions intensity of coal power plants is high, emitting about 1,000 grams of CO2-equivalent per kilowatt-hour generated, while natural gas has an average emissions intensity at about 500 grams of CO2-equivalent per kilowatt-hour. The intensity of coal emissions varies with type and generator technology and exceeds 1200 grams per kilowatt-hour in some countries. ^[43]

energy density _ _

Coal has an energy density of about 24 megajoules per kilogram (about 6.7 kilowatt-hours per kilogram). For a coal power plant with 40% efficiency, it would take 325 kg (717 lb) of coal to power a 100-watt light bulb for one year. ^[44]

27.6% of global energy was provided by coal in 2017, and Asia used nearly three-quarters of it. ^[45]

chemistry

formation

Coal composition is reported as either rough analysis (moisture, volatiles, fixed carbon, and ash) or final analysis (ash, carbon, hydrogen, nitrogen, oxygen, and sulfur). There is no "volatile matter" per se (except for some adsorbed methane ) but rather specifies the volatile compounds that are produced and released by heating the coal. Typical bituminous coal may have a final analysis on a dry, ash-free basis of 84.4% carbon, 5.4% hydrogen, 6.7% oxygen, 1.7% nitrogen , and 1.8% sulfur, on a weight basis. ^[46]

Coke and its uses for iron smelting

Coke is taken from a railroad in Ohio, USA

Coke is a solid carbon residue derived from coke (low ash, low sulfur bituminous coal, also known as metallurgical coal), which is used in the manufacture of steel and other iron products. ^[47] Coke is made from coke by baking it in an oven without oxygen at temperatures up to 1000°C, eliminating volatile components and fusing the fixed carbon and residual ash together. Metallurgical coke is used as a fuel and as a reducing agent in the smelting of pig iron in a blast furnace. ^[48] The carbon monoxide produced by its combustion reduces hematite (iron oxide) to iron.

Waste carbon dioxide is also produced with pig iron, which is very rich in dissolved carbon, so it must be processed again to make steel.

The amount of coke is low in the ash, because sulfur and phosphorous do not combine with this metal. The coke must be strong enough to withstand the weight of the ballast in the blast furnace ^[47] , which is why coke is so important for making steel using conventional methods. Coal coke is grey, hard, porous and has a heating value of 29.6 MJ/kg. Some processes for the coke industry produce byproducts, including coal tar, ammonia, light oils, and coal gas.

Petroleum coke is the solid residue obtained in petroleum refining, which is similar to coke but contains too many impurities to be useful in metallurgical applications.

Use in foundry components

Finely ground bituminous coal, known in this application as marine coal, is a component of foundry sand. While the molten metal is in the mold, the charcoal burns slowly, gassing less when pressurized, and thus preventing the metal from penetrating the pores of the sand. It is also present in "mold wash", which is a paste or liquid with the same function applied to the mold prior to casting. ^[49] Sea coal can be mixed with the clay lining (the “body”) used in the bottom of the cupola oven. When heated, the charcoal decomposes and the body becomes somewhat brittle, making it easier to open holes to extract the molten metal. ^[50]

Coke substitutes

Steel scrap can be recycled in the electric arc furnace; An alternative to making iron by smelting is direct reduced iron, where any carbonaceous fuel can be used to make sponge or granulated iron. Carbon dioxide emissions are considered to reduce hydrogen and a reducing agent , biomass or waste can be used as a carbon source. ^[51] Historically, coal was used as a substitute for coke in blast furnaces, where the resulting iron is known as coal iron.

gasification process

Coal gasification is used, as part of an integrated gasification cycle (IGCC) coal-fired power plant, to produce syngas, a mixture of carbon monoxide (CO) and hydrogen gas (H ₂ ) to fire gas turbines to produce electricity. The increased gas can also be converted into transportation fuels, such as gasoline and diesel, through the Fischer-Tropsch process. Alternatively, syngas can be converted to methanol, which can be blended into fuel directly or converted into gasoline via the methanol-to-gasoline process. Gasification with Fischer-Tropsch technology by Sasol A South African chemical company that manufactures chemicals and motor fuels from coal. ^[52]

During gasification, the coal is mixed with oxygen and vapor while also being heated and pressurized. During the reaction, oxygen and water molecules oxidize the charcoal into carbon monoxide (CO), while also releasing hydrogen gas (H ₂ ). This was done in underground coal mines, and also to make city gas that was taken to customers to be burned for lighting, heating, and cooking.

3C + O ₂ + H ₂ O → H ₂ + 3CO

A graphic showing the Fischer-Tropsch reaction

If a refiner wants to produce gasoline, syngas is directed to the Fischer-Tropsch reaction. This is known as indirect coal liquefaction. However, if hydrogen is the desired end product, syngas is fed into the water gas conversion reaction, where more hydrogen is liberated:

Carbon monoxide + H ₂ O → CO ₂ + H ₂

liquefied

Diagram showing the coal liquefaction process

Coal can be converted directly into a synthetic fuel equivalent to gasoline or diesel by hydrogenation or carbonization. ^[53] Coal liquefaction emits more carbon dioxide than producing liquid fuels from crude oil. Mixing in biomass and using CCS would emit slightly less than the oil process but at a high cost. ^[54] The state-owned China Energy Investment Company operates the coal liquefaction plant and plans to build two more. ^[55]

Coal liquefaction may also indicate cargo hazards when shipping coal. ^[56]

Chemical production

Chemicals have been produced from coal since the 1950s. Coal can be used as a feedstock in the production of a wide range of chemical fertilizers and other chemical products. The main route for these products was the gasification of coal to produce syngas. Primary chemicals that are produced directly from syngas include methanol, hydrogen and carbon monoxide, which are the chemical building blocks from which a whole host of derivative chemicals are synthesized, including olefins, acetic acid, ammonia, urea and others. The versatility of syngas as a precursor to primary chemicals and high-value derivative products offers the option of using coal to produce a wide range of commodities. However, in the 21st century, the use of coal bed methane has become more important. ^[57]

Because the list of chemical products that can be made by gasifying coal can also be used in general feedstocks derived from natural gas and petroleum, the chemical industry tends to use whatever feedstock is most cost effective. Therefore, the interest in using coal tended to increase due to high oil and natural gas prices and during periods of high global economic growth which may have put a strain on oil and gas production. ^[58]

Electricity generation

pre-combustion treatment

Refined coal is the product of coal upgrading technology that removes moisture and some contaminants from lower quality coals such as semi-bitumen and lignite (brown). It is a form of pre-combustion treatment and process for coal that changes the properties of the coal before it is burned. Thermal efficiency improvements can be achieved through improved pre-drying (particularly relevant to high-moisture fuels such as lignite or biomass). ^[59] The goals of pre-combustion coal technologies are to increase efficiency and reduce emissions when burning coal. Pre-combustion technology can sometimes be used as a complement to post-combustion technologies to control emissions from coal-fired boilers.

Power plant

Coal that is burned as a solid fuel in coal power plants to generate electricity is called thermal coal. Coal is also used to produce very high temperatures through combustion. Premature deaths from air pollution have been estimated at 200 per gigawatt-year, but it may be higher around power plants where scrubbers are not used or lower if far from cities. ^[60] Efforts around the world to reduce coal use have led some regions to switch to natural gas and electricity from low-carbon sources.

When coal is used to generate electricity, it is usually crushed and then burned in a furnace with a boiler. ^[61] The heat of the furnace turns the boiler water into steam, which is then used to spin turbines that turn generators and generate electricity. ^[62] The thermal efficiency of this process ranges between 25% and 50% depending on pre-combustion treatment, turbine technology (eg supercritical steam generator) and age of the generator. ^[63]

A few gasification integrated combined cycle (IGCC) power plants have been built, which burn coal more efficiently. Instead of pulverizing coal and burning it directly as fuel in boilers generating steam, the coal is gasified to create growing gas, which is burned in a gas turbine to produce electricity (just like natural gas is burned in a turbine). Hot exhaust gases from the turbine are used to raise steam in a heat recovery steam generator which drives an auxiliary steam turbine. The overall efficiency of the plant when used to provide combined heat and power can reach 94%. ^[64] IGCC power plants emit less local pollution than conventional pulverized coal plants; However, carbon capture and storage technology after gasification and before combustion has thus far proven too expensive to use with coal. ^[65] Other methods use coal as a coal-water slurry fuel (CWS), which was developed in the Soviet Union, or in the MHD cycle. However, it is not widely used due to lack of profit.

In 2017, 38% of the world's electricity came from coal, the same percentage as 30 years ago. ^[66] In 2018, the global installed capacity was 2 TW (of which 1 TW was in China) which accounted for 30% of the total electricity generation capacity. The most dependent major country is South Africa, with more than 80% of electricity generated from coal.

The maximum use of coal was reached in 2013. ^[67] In 2018 the average capacity factor of a coal-fired power plant was 51%, i.e. it runs for about half of the available operating hours.

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