Coal

Coal is a readily combustible black or brownish-black sedimentary rock normally occurring in rock strata in layers or veins called coal beds. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure. Coal is composed primarily of carbon along with variable quantities of other elements, chiefly sulphur, hydrogen, oxygen and nitrogen. Coal begins as layers of plant matter accumulate at the bottom of a body of water. For the process to continue the plant matter must be protected from biodegradation and oxidization, usually by mud or acidic water. The wide shallow seas of the Carboniferous period provided such conditions. This trapped atmospheric carbon in the ground in immense peat bogs that eventually were covered over and deeply buried by sediments under which they metamorphosed into coal. Over time, the chemical and physical properties of the plant remains (believed to mainly have been fern-like species antedating more modern plant and tree species) were changed by geological action to create a solid material. Coal, a fossil fuel, is the largest source of energy for the generation of electricity worldwide, as well as one of the largest worldwide anthropogenic sources of carbon dioxide emissions. Gross carbon dioxide emissions from coal usage are slightly more than those from petroleum and about double the amount from natural gas.The EIA reports the following emissions in million metric tons of carbon dioxide:

• Nat gas: 5,840
• Petroleum: 10,995
• Coal: 11,357

For 2005 as the official energy statistics of the US Government. link Coal is extracted from the ground by mining, either underground or in open pits.

Types

toward the end of the Carboniferous period; the light blue represents shallow seas where many of today's coal deposits are found, as opposed to deeper waters which gave rise to oil-bearing rocks derived from marine species. The ice caps were known to be very large, lowering sea levels extensively by locking up oceanic waters into solid ice, though how large the ice caps became is a matter of debate.]] As geological processes apply pressure to dead biotic material over time, under suitable conditions it is transformed successively into

• Peat, considered to be a precursor of coal, has industrial importance as a fuel in some regions, for example, Ireland and Finland. In its dehydrated form, peat is a highly effective absorbent for fuel and oil spills on land and water
• Lignite, also referred to as brown coal, is the lowest rank of coal and used almost exclusively as fuel for electric power generation. Jet is a compact form of lignite that is sometimes polished and has been used as an ornamental stone since the Iron Age
• Sub-bituminous coal, whose properties range from those of lignite to those of bituminous coal are used primarily as fuel for steam-electric power generation. Additionally, it is an important source of light aromatic hydrocarbons for the chemical synthesis industry.
• Bituminous coal, dense mineral, black but sometimes dark brown, often with well-defined bands of bright and dull material, used primarily as fuel in steam-electric power generation, with substantial quantities also used for heat and power applications in manufacturing and to make coke
• Steam coal is a grade between bituminous coal and anthracite, once widely used as a fuel for steam locomotives. In this specialized use it is sometimes known as sea-coal in the U.S. Funk and Wagnalls, quoted in Small steam coal (dry small steam nuts or DSSN) was used as a fuel for domestic water heating
• Anthracite, the highest rank; a harder, glossy, black coal used primarily for residential and commercial space heating. It may be divided further into metamorphically altered bituminous coal and petrified oil, as from the deposits in Pennsylvania
• Graphite, technically the highest rank, but difficult to ignite and is not so commonly used as fuel: it is mostly used in pencils and, when powdered, as a lubricant.

The classification of coal is generally based on the content of volatiles. However, the exact classification varies between countries. According to the German classification, coal is classified as follows:Eberhard Lindner; Chemie für Ingenieure; Lindner Verlag Karlsruhe, S. 258 The middle six grades in the table represent a progressive transition from the English-language sub-bituminous to bituminous coal, while the last class is an approximate equivalent to anthracite, but more inclusive (the U.S. anthracite has < 6% volatiles). Cannel coal (sometimes called "candle coal"), is a variety of fine-grained, high-rank coal with significant hydrogen content. It consists primarily of " exinite" macerals, now termed "liptinite".

Early uses as fuel

The earliest reference to the use of coal as fuel is from the geological treatise On stones (Lap. 16) by the Greek scientist Theophrastus (c. 371–287 BC): Outcrop coal was used in Britain during the Bronze Age (3000–2000 BC), where it has been detected as forming part of the composition of funeral pyres.Britannica 2004: Coal mining: ancient use of outcropping coal and Civilisation in China" by Peter J Golas and Joseph Needham. p. 186-191. Cambridge University Press 1999. ISBN 052158005, 9780521580007. Google Books In Roman Britain, with the exception of two modern fields, "the Romans were exploiting coals in all the major coalfields in England and Wales by the end of the second century AD". Evidence of trade in coal (dated to about AD 200) has been found at the inland port of Heronbridge, near Chester, and in the Fenlands of East Anglia, where coal from the Midlands was transported via the Car Dyke for use in drying grain.Salway, Peter (2001): A History of Roman Britain. Oxford University Press Coal cinders have been found in the hearths of villas and military forts, particularly in Northumberland, dated to around AD 400. In the west of England contemporary writers described the wonder of a permanent brazier of coal on the altar of Minerva at Aquae Sulis (modern day Bath) although in fact easily accessible surface coal from what became the Somerset coalfield was in common use in quite lowly dwellings locally.Forbes, R J (1966): Studies in Ancient Technology. Brill Academic Publishers, Boston Evidence of coal's use for iron-working in the city during the Roman period has been found.{{cite book | last = Cunliffe | first = Barry W. | authorlink = Barry Cunliffe | coauthors = | title = Roman Bath Discovered | publisher = Routledge | year = 1984 | location = London | pages = 14–15; 194 | url = | doi = | id = | isbn = 0710201966}} In Eschweiler, Rhineland, deposits of bituminous coal were used by the Romans for the smelting of iron ore. There is no evidence that the product was of great importance in Britain before the High Middle Ages, after about AD 1000. Mineral coal came to be referred to as "seacoal," probably because it came to many places in eastern England, including London, by sea, but the origin of the term seems to predate this, being used for coal having fallen from the exposed coal seams on cliffs above the shore or washed out of underwater coal seam outcrops. (See Industrial processes below for modern uses of the term.) These easily accessible sources had largely become exhausted (or could not meet the growing demand) by the 13th century, when underground mining from shafts or adits was developed. In 1257-59 coal from Newcastle was shipped to London for the smiths and lime-burners building Westminster Abbey. In London there is still a Seacoal Lane and a Newcastle Lane (from the coal-shipping city of Newcastle) where in the seventeenth century coal was unloaded at wharves along the River Fleet. An alternative name was "pitcoal," because it came from mines. It was, however, the development of the Industrial Revolution that led to the large-scale use of coal, as the steam engine took over from the water wheel. In ancient China, coal was used as fuel by the 4th century AD, but there was little extensive use until the 11th century.Read, Thomas T. (1939–40): "The Earliest Industrial Use of Coal", Transactions of the Newcomen Society, Vol. 20, p. 119

Uses today

Coal as fuel

Coal is primarily used as a solid fuel to produce electricity and heat through combustion. World coal consumption was about 6,743,786,000 short tons in 2006 World coal consupmption 1980-2006 October 2008 EIA statistics and is expected to increase 48% to 9.98 billion short tons by 2030.EIA, World Energy Projections Plus (2009) China produced 2.38 billion tons in 2006. India produced about 447.3 million tons in 2006. 68.7% of China's electricity comes from coal. The USA consumes about 14% of the world total, using 90% of it for generation of electricity.http://www.eia.doe.gov/cneaf/coal/page/special/feature.html When coal is used for electricity generation, it is usually pulverized and then combusted (burned) in a furnace with a boiler. The furnace heat converts boiler water to steam, which is then used to spin turbines which turn generators and create electricity. The thermodynamic efficiency of this process has been improved over time. Simple cycle steam turbines have topped out with some of the most advanced reaching about 35% thermodynamic efficiency for the entire process. Increasing the combustion temperature can boost this efficiency even further. Old coal power plants, especially "grandfathered" plants, are significantly less efficient and produce higher levels of waste heat. About 40% of the world's electricity comes from coal,http://www.worldcoal.org/pages/content/index.asp?PageID=188 and approximately 49% of the United States electricity comes from coal.http://www.eia.doe.gov/cneaf/electricity/epa/figes1.htmlThe emergence of the supercritical turbine concept envisions running a boiler at extremely high temperatures and pressures with projected efficiencies of 46%, with further theorized increases in temperature and pressure perhaps resulting in even higher efficiencies. Other efficient ways to use coal are combined heat and power cogeneration and an MHD topping cycle. Approximately 40% of the world electricity production uses coal. The total known deposits recoverable by current technologies, including highly polluting, low energy content types of coal (i.e., lignite, bituminous), is sufficient for many years. However, consumption is increasing and maximal production could be reached within decades (see World Coal Reserves, below). A more energy-efficient way of using coal for electricity production would be via solid-oxide fuel cells or molten-carbonate fuel cells (or any oxygen ion transport based fuel cells that do not discriminate between fuels, as long as they consume oxygen), which would be able to get 60%–85% combined efficiency (direct electricity + waste heat steam turbine). Currently these fuel cell technologies can only process gaseous fuels, and they are also sensitive to sulphur poisoning, issues which would first have to be worked out before large-scale commercial success is possible with coal. As far as gaseous fuels go, one idea is pulverized coal in a gas carrier, such as nitrogen. Another option is coal gasification with water, which may lower fuel cell voltage by introducing oxygen to the fuel side of the electrolyte, but may also greatly simplify carbon sequestration. However, this technology has been criticised as being inefficient, slow, risky and costly, while doing nothing about total emissions from mining, processing and combustion. Clean coal facts and figures page by Rising Tide Newcastle Another efficient and clean way of coal combustion in a form of coal-water slurry fuel (CWS) was well-developed in Russia (since the Soviet Union time). CWS significantly reduces emissions saving the heating value of coal.

Coking and use of coke

plant, Wales]] Coke is a solid carbonaceous residue derived from low-ash, low-sulphur bituminous coal from which the volatile constituents are driven off by baking in an oven without oxygen at temperatures as high as 1,000 °C (1,832 °F) so that the fixed carbon and residual ash are fused together. Metallurgical coke is used as a fuel and as a reducing agent in smelting iron ore in a blast furnace. The product is too rich in dissolved carbon, and must be treated further to make steel. The coke must be strong enough to resist the weight of overburden in the blast furnace, which is why coking coal is so important in making steel by the conventional route. However, the alternative route to is direct reduced iron, where any carbonaceous fuel can be used to make sponge or pelletised iron. Coke from coal is grey, hard, and porous and has a heating value of 24.8 million Btu/ton (29.6 MJ/kg). Some cokemaking processes produce valuable by-products that include coal tar, ammonia, light oils, and " coal gas". Petroleum coke is the solid residue obtained in oil refining, which resembles coke but contains too many impurities to be useful in metallurgical applications.

Ethanol production

The reaction of coal and natural gas was used by a German manufacturer for Buna rubber: Chemische Werke Huls, at Marl, Germany, and AVCO Corp in the US. Consequently several references had described both Huls Arc Process and AVCO rotating arc reactor.Ullmann's Encyclopedia of Industrial Chemistry, Acetylene, 5th Ed., Vch Pub, 1987Encyclopedia of Chemical Technology, Kirk and Othmer, 4th Ed., Acetylene, Wiley-Interscience, 2004 ISBN 978-0-471-48522-3 Both reactors are of cylindrical shape and have a rotating electric arc. The cathode is at the cylinder axis, while the anode is on the circumference. As methane gas provided the highest yield, then it is forced with coal powder into a vortex passing through the electric arc for few milliseconds. Huls Arc ProcessKlaus Weissermel et al, Industrial Organic Chemistry, Science, 2003, ISBN 3-527-30578-5 produced a mixture of acetylene and ethylene gases. The reaction conditions can be varied to determine the needed product. Increasing the Specific Energy Requirement (SER) favors acetylene production, and lower SER is for ethylene: Enthalpy Change for Ethylene:Perry's Chemical Engineers' Handbook, 6th Ed., Robert Perry and Don Green, McGraw Hill, Section 3, 1984 = 127.34 kJ/mol, while for acetylene: = 301.4 kJ/mol. As a consequence, recent production processes are using conventional heating instead of electric arc. Hydration of ethylene gas producing ethanol is the most important process for ethanol production. Vapor phase process is the preferred oneEncyclopedia of Chemical Technology, Kirk and Othmer, Ethanol, 4th Ed., Wiley-Interscience, 2004 in which ethylene and steam pass over a catalyst. One of the most accepted catalysts is diatomite impregnated with phosphoric acid.

Gasification

Coal gasification can be used to produce syngas, a mixture of carbon monoxide (CO) and hydrogen (H2) gas. This syngas can then be converted into transportation fuels like gasoline and diesel through the Fischer-Tropsch process. This technology is currently used by the Sasol chemical company of South Africa to make gasoline from coal and natural gas. Alternatively, the hydrogen obtained from gasification can be used for various purposes such as powering a hydrogen economy, making ammonia, or upgrading fossil fuels. During gasification, the coal is mixed with oxygen and steam ( water vapor) while also being heated and pressurized. During the reaction, oxygen and water molecules oxidize the coal into carbon monoxide (CO) while also releasing hydrogen (H2) gas. This process has been conducted in both underground coal mines and in coal refineries. (Coal) + O2 + H2O → H2 + CO If the refiner wants to produce gasoline, the syngas is collected at this state and routed into a Fischer-Tropsch reaction. If hydrogen is the desired end-product, however, the syngas is fed into the water gas shift reaction where more hydrogen is liberated. CO + H2O → CO2 + H2 High prices of oil and natural gas are leading to increased interest in "BTU Conversion" technologies such as gasification, methanation and liquefaction. The Synthetic Fuels Corporation was a U.S. government-funded corporation established in 1980 to create a market for alternatives to imported fossil fuels (such as coal gasification). The corporation was discontinued in 1985. In the past, coal was converted to make coal gas, which was piped to customers to burn for illumination, heating, and cooking. At present, the safer natural gas is used instead.

Liquefaction

Coal can also be converted into liquid fuels such as gasoline or diesel by several different processes. In the direct liquefaction processes, the coal is either hydrogenated or carbonized. Hydrogenation processes are the Bergius process,Robert Haul: Friedrich Bergius (1884-1949), p. 62 in 'Chemie in unserer Zeit', VCH-Verlagsgesellschaft mbH, 19. Jahrgang, April 1985, Weinheim, Germany the SRC-I and SRC-II (Solvent Refined Coal) processes and the NUS Corporation hydrogenation process.