Coal gasification

Coal gasification is the process of producing coal gas, a type of syngas–a mixture of carbon monoxide (CO), hydrogen (H₂), carbon dioxide (CO₂) and water vapour (H₂O)–from coal. Coal gas, which is a combustible gas, was traditionally used as a source of energy for municipal lighting and heat before the advent of industrial-scale production of natural gas, while the hydrogen obtained from gasification can be used for various purposes such as making ammonia, powering a hydrogen economy, or upgrading fossil fuels. Alternatively, the coal gas (also known as "town gas") can be converted into transportation fuels such as gasoline and diesel through additional treatment via the Fischer-Tropsch process.

History

Main article: History of manufactured gas

In the past, coal was converted to make coal gas, which was piped to customers to burn for illumination, heating, and cooking. 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.

Early history of gas production by carbonization

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Gas lighting in historical center of Wrocław, Poland

The Flemish scientist Jan Baptista van Helmont (1577–1644) discovered that a 'wild spirit' escaped from heated wood and coal, and, thinking that it 'differed little from the chaos of the ancients', he named it gas in his Origins of Medicine (c. 1609). Among several others who carried out similar experiments, were Johann Becker of Munich (c 1681) and about three years later John Clayton of Wigan, England, the latter amusing his friends by lighting, what he called, "Spirit of the Coal". William Murdoch (later known as Murdock) (1754–1839) (partner of James Watt) is reputed to have heated coal in his mother's teapot to produce gas. From this beginning, he discovered new ways of making, purifying and storing gas; illuminating his house at Redruth (or his cottage at Soho) in 1792, the entrance to the Manchester Police Commissioners premises in 1797, the exterior of the factory of Boulton and Watt in Birmingham, England, and a large cotton mill in Salford, Lancashire in 1805.

Professor Jan Pieter Minckeleers lit his lecture room at the University of Louvain in 1783 and Lord Dundonald lit his house at Culross, Scotland, in 1787, the gas being carried in sealed vessels from the local tar works. In France, Philippe le Bon patented a gas fire in 1799 and demonstrated street lighting in 1801. Other demonstrations followed in France and in the United States, but, it is generally recognized that the first commercial gas works was built by the London and Westminster Gas Light and Coke Company in Great Peter Street in 1812 laying wooden pipes to illuminate Westminster Bridge with gas lights on New Year's Eve in 1813. In 1816, Rembrandt Peale and four others established the Gas Light Company of Baltimore, the first manufactured gas company in America. In 1821, natural gas was being used commercially in Fredonia, New York. The first German gas works was built in Hannover in 1825 and by 1870 there were 340 gas works in Germany making town gas from coal, wood, peat and other materials.

Working conditions in the Gas Light and Coke Company's Horseferry Road Works, London, in the 1830s were described by a French visitor, Flora Tristan, in her Promenades Dans Londres:

Two rows of furnaces on each side were fired up; the effect was not unlike the description of Vulcan's forge, except that the Cyclops were animated with a divine spark, whereas the dusky servants of the English furnaces were joyless, silent and benumbed. ... The foreman told me that stokers were selected from among the strongest, but that nevertheless they all became consumptive after seven or eight years of toil and died of pulmonary consumption. That explained the sadness and apathy in the faces and every movement of the hapless men.

The first public piped gas supply was to 13 gas lamps, each with three glass globes along the length of Pall Mall, London in 1807. The credit for this goes to the inventor and entrepreneur Fredrick Winsor and the plumber Thomas Sugg who made and laid the pipes. Digging up streets to lay pipes required legislation and this delayed the development of street lighting and gas for domestic use. Meanwhile William Murdoch and his pupil Samuel Clegg were installing gas lighting in factories and work places, encountering no such impediments.

Early history of gas production by gasification

In the 1850s every small to medium sized town and city had a gas plant to provide for street lighting. Subscribing customers could also have piped lines to their houses. By this era, gas lighting became accepted. Gaslight trickled down to the middle class and later came gas cookers and stoves.

The 1860s were the golden age of coal gas development. Scientists like Kekulé and Perkin cracked the secrets of organic chemistry to reveal how gas is made and its composition. From this came better gas plants and Perkin's purple dyes, such as Mauveine. In the 1850s, processes for making Producer gas and Water gas from coke were developed. Unenriched water gas may be described as Blue water gas (BWG).

Mond gas, developed in the 1850s by Ludwig Mond, was producer gas made from coal instead of coke. It contained ammonia and coal tar and was processed to recover these valuable compounds.

Blue water gas (BWG) burns with a non-luminous flame which makes it unsuitable for lighting purposes. Carburetted Water Gas (CWG), developed in the 1860s, is BWG enriched with gases obtained by spraying oil into a hot retort. It has a higher calorific value and burns with a luminous flame.

The carburetted water gas process was improved by Thaddeus S. C. Lowe in 1875. The gas oil was fixed into the BWG via thermocracking in the carburettor and superheater of the CWG generating set. CWG was the dominant technology in the USA from the 1880s until the 1950s, replacing coal gasification. CWG has a CV of 20 MJ/m³ i.e. slightly more than half that of natural gas.

Development of the gas industry in the UK

The advent of incandescent gas lighting in factories, homes and in the streets, replacing oil lamps and candles with steady clear light, almost matching daylight in its colour, turned night into day for many—making night shift work possible in industries where light was all important—in spinning, weaving and making up garments etc. The social significance of this change is difficult for generations brought up with lighting after dark available at the touch of a switch to appreciate. Not only was industrial production accelerated, but streets were made safe, social intercourse facilitated and reading and writing made more widespread. Gas works were built in almost every town, main streets were brightly illuminated and gas was piped in the streets to the majority of urban households. The invention of the gas meter and the pre-payment meter in the late 1880s played an important role in selling town gas to domestic and commercial customers.

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1934 gas cooker in England

The education and training of the large workforce, the attempts to standardise manufacturing and commercial practices and the moderating of commercial rivalry between supply companies prompted the founding of associations of gas managers, first in Scotland in 1861. A British Association of Gas Managers was formed in 1863 in Manchester and this, after a turbulent history, became the foundation of the Institute of Gas Engineers (IGE). In 1903, the reconstructed Institution of Civil Engineers (ICE) initiated courses for students of gas manufacture in the City and Guilds of London Institute. The IGE was granted the Royal Charter in 1929. Universities were slow to respond to the needs of the industry and it was not until 1908 that the first Professorship of Coal Gas and Fuel Industries was founded at the University of Leeds. In 1926, the Gas Light and Coke Company opened Watson House adjacent to Nine Elms Gas Works. At first, this was a scientific laboratory. Later it included a centre for training apprentices but its major contribution to the industry was its gas appliance testing facilities, which were made available to the whole industry, including gas appliance manufacturers. Using this facility, the industry established not only safety but also performance standards for both the manufacture of gas appliances and their servicing in customers' homes and commercial premises.

During World War I, the gas industry's by-products, phenol, toluene and ammonia and sulphurous compounds were valuable ingredients for explosives. Much coal for the gas works was shipped by sea and was vulnerable to enemy attack. The gas industry was a large employer of clerks, mainly male before the war. But the advent of the typewriter and the female typist made another important social change that was, unlike the employment of women in war-time industry, to have long-lasting effects.

The inter-war years were marked by the development of the continuous vertical retort which replaced many of the batch fed horizontal retorts. There were improvements in storage, especially the waterless gas holder, and distribution with the advent of 2–4 inch steel pipes to convey gas at up to 50 psi (340 kPa) as feeder mains to the traditional cast iron pipes working at an average of 2–3 inches water gauge (500–750 Pa). Benzole as a vehicle fuel and coal tar as the main feedstock for the emerging organic chemical industry provided the gas industry with substantial revenues. Petroleum supplanted coal tar as the primary feedstock of the organic chemical industry after World War II and the loss of this market contributed to the economic problems of the gas industry after the war.

A wide variety of appliances and uses for gas developed over the years. Gas fires, gas cookers, refrigerators, washing machines, hand irons, pokers for fire lighting, gas-heated baths, remotely controlled clusters of gas lights, gas engines of various types and, in later years, gas warm air and hot water central heating and air conditioning, all of which made immense contributions to the improvement of the quality of life in cities and towns world wide. The evolution of electric lighting made available from public supply extinguished the gas light, except where colour matching was practised as in haberdashery shops.

Modern coal gasification

The Great Plains Synfuels Plant has been operating in Beulah, North Dakota since 1984. It produces synthetic natural gas from lignite.

During the 2011 session of the Illinois legislature proposals to provide financial support for state-of-the-art coal gasification plants in Chicago and Southern Illinois were considered. The bills require Illinois utilities to purchase gas at fixed rates from the plants for 30 years. The Chicago plant to be built by Chicago Clean Energy, a subsidiary of Leucadia National Corporation, is budgeted to cost $3 billion. It would be located in an existing industrial area on the Southeast Side on Burley Avenue near 116th Street. In addition to coal the plant would use coke, an oil refinery byproduct, as feed stock. Carbon dioxide produced during the project would be sequestered.The bill to build the Chicago plant was passed by the legislature but vetoed by the Illinois governor Pat Quinn who cited cost issues. Due to uncertainty about natural gas supplies and prices alternative financing is doubtful. Another plant, Indiana Gasification, LLC also a Leucadia National Corporation subsidiary and with a similar business plan, is proposed for Rockport, Indiana where the state has agreed to purchase gas for 30 years at a fixed price.

Process

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Scheme of a Lurgi gasifier

During gasification, the coal is blown through with oxygen and steam (water vapor) while also being heated (and in some cases pressurized). If the coal is heated by external heat sources the process is called "allothermal", while "autothermal" process assumes heating of the coal via exothermal chemical reactions occurring inside the gasifier itself. It is essential that the oxidizer supplied is insufficient for complete oxidizing (combustion) of the fuel. During the reactions mentioned, oxygen and water molecules oxidize the coal and produce a gaseous mixture of carbon dioxide (CO₂), carbon monoxide (CO), water vapour (H₂O), and molecular hydrogen (H₂). (Some by-products like tar, phenols, etc. are also possible end products, depending on the specific gasification technology utilized.) This process has been conducted in-situ within natural coal seams (referred to as underground coal gasification) and in coal refineries. The desired end product is usually syngas (i.e., a combination of H₂ + CO), but the produced coal gas may also be further refined to produce additional quantities of H₂:

3C (i.e., coal) + O₂ + H₂O → H₂ + 3CO

If the refiner wants to produce alkanes (i.e., hydrocarbons present in natural gas, gasoline, and diesel fuel), the coal gas is collected at this state and routed to a Fischer-Tropsch reactor. If, however, hydrogen is the desired end-product, the coal gas (primarily the CO product) undergoes the water gas shift reaction where more hydrogen is produced by additional reaction with water vapor:

CO + H₂O → CO₂ + H₂

Although other technologies for coal gasification currently exist, all employ, in general, the same chemical processes. For low-grade coals (i.e., "brown coals") which contain significant amounts of water, there are technologies in which no steam is required during the reaction, with coal (carbon) and oxygen being the only reactants. As well, some coal gasification technologies do not require high pressures. Some utilize pulverized coal as fuel while others work with relatively large fractions of coal. Gasification technologies also vary in the way the blowing is supplied.

"Direct blowing" assumes the coal and the oxidizer being supplied towards each other from the opposite sides of the reactor channel. In this case the oxidizer passes through coke and (more likely) ashes to the reaction zone where it interacts with coal. The hot gas produced then passes fresh fuel and heats it while absorbing some products of thermal destruction of the fuel, such as tars and phenols. Thus, the gas requires significant refining before being used in the Fischer-Tropsch reaction. Products of the refinement are highly toxic and require special facilities for their utilization. As a result, the plant utilizing the described technologies has to be very large to be economically efficient. One of such plants called SASOL is situated in the Republic of South Africa (RSA). It was built due to embargo applied to the country preventing it from importing oil and natural gas. RSA is rich in Bituminous coal and Anthracite and was able to arrange the use of the well known high pressure "Lurgi" gasification process developed in Germany in the first half of 20-th century.

"Reversed blowing" (as compared to the previous type described which was invented first) assumes the coal and the oxidizer being supplied from the same side of the reactor. In this case there is no chemical interaction between coal and oxidizer before the reaction zone. The gas produced in the reaction zone passes solid products of gasification (coke and ashes), and CO₂ and H₂O contained in the gas are additionally chemically restored to CO and H₂. As compared to the "direct blowing" technology, no toxic by-products are present in the gas: those are disabled in the reaction zone. This type of gasification has been developed in the first half of 20-th century, along with the "direct blowing", but the rate of gas production in it is significantly lower than that in "direct blowing" and there were no further efforts of developing the "reversed blowing" processes until 1980-s when a Soviet research facility KATEKNIIUgol' (R&D Institute for developing Kansk-Achinsk coal field) began R&D activities to produce the technology now known as "TERMOKOKS-S"[1] process. The reason for reviving the interest to this type of gasification process is that it is ecologically clean and able to produce two types of useful products (simultaneously or separately): gas (either combustible or syngas) and middle-temperature coke. The former may be used as a fuel for gas boilers and diesel-generators or as syngas for producing gasoline, etc., the latter - as a technological fuel in metallurgy, as a chemical absorbent or as raw material for household fuel briquettes. Combustion of the product gas in gas boilers is ecologically cleaner than combustion of initial coal. Thus, a plant utilizing gasification technology with the "reversed blowing" is able to produce two valuable products of which one has relatively zero production cost since the latter is covered by competitive market price of the other. As the Soviet Union and its KATEKNIIUgol' ceased to exist, the technology was adopted by the individual scientists who originally developed it and is now being further researched in Russia and commercially distributed worldwide. Industrial plants utilizing it are now known to function in Ulaan-Baatar (Mongolia) and Krasnoyarsk (Russia).

Underground coal gasification

Main article: Underground coal gasification

Underground coal gasification is an industrial in-situ gasification process, which is carried out in non-mined coal seams using injection of oxidants, and bringing the product gas to surface through production wells drilled from the surface. The product gas could to be used as a chemical feedstock or as fuel for power generation. The technique can be applied to resources that are otherwise not economical to extract and also offers an alternative to conventional coal mining methods for some resources. Compare to the traditional coal mining and gasification, the UCG has less environmental and social impact.

By-products

The by-products of coal gas manufacture included coke, coal tar, sulfur and ammonia; all useful products. Dyes, medicines, including sulfa drugs, saccharin and many organic compounds are therefore derived from coal gas.

Coke is used as a smokeless fuel and for the manufacture of water gas and producer gas. Coal tar was subjected to fractional distillation to recover various products, including

• tar, for roads
• benzole, a motor fuel
• creosote, a wood preservative
• phenol, used in the manufacture of plastics
• cresols, disinfectants

Sulfur is used in the manufacture of sulfuric acid and ammonia is used in the manufacture of fertilisers.

Environmental effects

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Gasometer at West Ham, United Kingdom.

From its original development until the wide-scale adoption of natural gas, more than 50,000 manufactured gas plants were in existence in the United States alone. The process of manufacturing gas usually produced a number of by-products that contaminated the soil and groundwater in and around the manufacturing plant, so many former town gas plants are a serious environmental concern, and cleanup and remediation costs are often high. Manufactured gas plants (MGPs) were typically sited near or adjacent to waterways that were used to transport in coal and for the discharge of wastewater contaminated with tar, ammonia and/or drip oils, as well as outright waste tars and tar-water emulsions.

In the earliest days of MGP operations, coal tar was considered a waste and often disposed into the environment in and around the plant locations. While uses for coal tar developed by the late-19th century, the market for tar varied and plants that could not sell tar at a given time could store tar for future use, attempt to burn it as fuel for the boilers, or dump the tar as waste. Commonly, waste tars were disposed of in old gas holders, adits or even mine shafts (if present). Over time, the waste tars degrade with phenols, benzene (and other mono-aromatics – BTEX) and polycyclic aromatic hydrocarbons released as pollutant plumes that can escape into the surrounding environment. Other wastes included "blue billy", which is a ferroferricyanide compound—the blue colour is from Prussian blue, which was commercially used as a dye. Blue billy is typically a granular material and was sometimes sold locally with the strap line "guaranteed weed free drives". The presence of blue billy can give gas works waste a characteristic musty/bitter almonds or marzipan smell which is associated with cyanide gas.

The shift to the CWG process initially resulted in a reduced output of water gas tar as compared to the volume of coal tars. The advent of automobiles reduced the availability of naphtha for carburetion oil, as that fraction was desirable as motor fuel. MGPs that shifted to heavier grades of oil often experienced problems with the production of tar-water emulsions, which were difficult, time consuming, and costly to break. (The cause of tar-water emulsions is complex and was related to several factors, including free carbon in the carburetion oil and the substitution of bituminous coal as a feedstock instead of coke.) The production of large volumes of tar-water emulsions quickly filled up available storage capacity at MGPs and plant management often dumped the emulsions in pits, from which they may or may not have been later reclaimed. Even if the emulsions were reclaimed, the environmental damage from placing tars in unlined pits remained. The dumping of emulsions (and other tarry residues such as tar sludges, tank bottoms, and off-spec tars) into the soil and waters around MGPs is a significant factor in the pollution found at FMGPs today.

Commonly associated with former manufactured gas plants (known as "FMGPs" in environmental remediation) are contaminants including:

• BTEX
  • Diffused out from deposits of coal/gas tars
  • Leaks of carburetting oil/light oil
  • Leaks from drip pots, that collected condensible hydrocarbons from the gas
• Coal tar waste/sludge
  • Typically found in sumps of gas holders/decanting ponds.
  • Coal tar sludge has no resale value and so was always dumped.
• Volatile organic compounds
• Polycyclic aromatic hydrocarbons (PAHs)
  • Present in coal tar, gas tar, and pitch at significant concentrations.
• Heavy metals
  • Leaded solder for gas mains, lead piping, coal ashes.
• Cyanide
  • Purifier waste has large amounts of complex ferrocyanides in it.
• Lampblack
  • Only found where crude oil was used as gasification feedstock.
• Tar emulsions

Coal tar and coal tar sludges are frequently denser than water and are present in the environment as a dense non-aqueous phase liquid.

In the UK, former gasworks have commonly been developed over for residential and other uses (including the Millennium Dome), being seen as prime developable land in the confines of city boundaries. Situations such as these are now lead to problems associated with planning and the Contaminated Land Regime and have recently been debated in the House of Commons.

The more modern coal gasification processes (circa 1970 to 2006) also have environmental problems requiring various available technologies for mitigation.

External links

• "Practical Experience Gained During the First Twenty Years of Operation of the Great Plains Gasification Plant and Implications for Future Projects" (PDF-3.1MB), DOE’s Office of Fossil Energy, May 2006.ca:Gasificació del carbó