Coal chemistry

Typical chemical structural units of coal

Coal chemistry (also carbon chemistry or carbonate ion chemistry ) refers to a subsection of the coal upgrading , in which from coal supplied to the valuable substances contained therein use a material economic or in gaseous or liquid fuels to be transferred. The processes of coal chemistry include degassing processes such as carbonisation and coking , gasification processes and processes for coal liquefaction .

history

The first beginnings of coal chemistry go back to the 16th century. At that time, large amounts of charcoal were needed to smelt iron ore . Since the production required large amounts of slowly renewable wood , the use of hard coal was studied. The content of liquid and solid secondary components made the use of pure coal difficult. The coal was first treated like wood in charcoal piles to make coke .

Around 1684, John Clayton discovered that coal gas was combustible. He described his discovery in the Royal Society's Philosophical Transactions in 1735.

At the beginning of the 18th century, Abraham Darby developed the process of coking hard coal in a coke oven . With the invention of closed ovens in 1756, it was possible to decisively improve the production of coke.

In 1785 , Jan Piter Minckelers lit a lecture hall at the University of Leuven with gas made from coal. The highly viscous coal tar was soon used for sealing in shipbuilding. At the beginning of the 19th century, the first industrial structures for the products of coal chemistry were formed. In Freiberg / Saxony the first street lamp was operated with gas in 1811, and the first gas works was opened in London in 1812. By 1860 there were already 320 gas works in Germany. The gas was used for lighting, but also increasingly for energy supply, as a raw material for the chemical industry and also in the household.

The by-product coke was obtained in large quantities. The gasification of coke with air and steam produced water gas that was mixed with the coke oven gases. With the increasing demand of the chemical industry for gases containing hydrogen and carbon monoxide, several large-scale gasification processes have been developed in Germany. Winkler developed fluidized bed gasification at BASF around 1926, and Lurgi pressure gasification was developed in 1930. Twenty years later, the entrained flow carburetor followed in Koppers-Totzek.

In both world wars , it was of great importance in Germany to be able to use local coal as a raw material for chemical products and to produce fuels such as gasoline and diesel . In the GDR this was continued on a large scale at the Leuna site until reunification in 1990 .

Before the Second World War , coal refining was the standard process for extracting organic chemical raw materials. Their importance declined sharply with the advent of the cheaper and easier-to-process raw material oil . After the scarcity and price increase of crude oil ( oil crisis ) since the 1970s, coal chemistry is gaining importance again.

At present, the Sasol company in Sasolburg in South Africa operates plants for coal gasification and Fischer-Tropsch synthesis for fuels and basic products.

Process of coal chemistry

Coal chemistry can be divided into degassing, gasification and liquefaction processes. When solid fuels are heated in the absence of air, they are decomposed, splitting off liquid and gaseous products and a solid residue, the coke . The most frequently used degassing process is the coking of hard coal.

Coking

In a narrower sense, coking is the high-temperature coking of hard coal , which ends at temperatures of up to 1150 ° C. Depending on the process, the main product of coking is either the coke that is produced, which is primarily used in iron extraction , metallurgy and foundry technology, or the coke oven gases. The proportion of coke oven gases released and their composition depend on the coal used and the process conditions. Younger coals are advantageous for gas generation.

The coking can be divided into several temperature ranges. The pre-degassing takes place up to temperatures of approx. 350 ° C. The processes are initially of a physical nature and consist in the drying of the coal and the desorption of nitrogen , oxygen and methane . From around 250 ° C, the thermal breakdown begins with the release of light paraffins and aromatics .

In the temperature range from 350 ° C to 425 ° C, the central degassing takes place, during which further paraffins and aromatics are released. In addition, the charcoal becomes plastic and puffs up.

In the following main degassing process, the bulk of the gas is obtained, with naphthenes , paraffins, aromatics and phenols being released. The coal solidifies again and the formation of semi-coke begins. From around 850 ° C, the hydrocarbons break down into their components carbon and hydrogen . In the post-degassing up to 1000 ° C, ammonia is released and the alkenes polymerize to aromatics. After separating ammonia and tar and benzene, the gas is returned to the process as fuel gas .

Depending on the target product, a distinction is made between coking and gas generating furnaces. Coking ovens use a large part of the gas produced to supply their own energy, whereas gas-generating ovens are heated with lean gases from other processes.

Smoldering

Lignite , lignin , peat and oil shale are carbonized at 450 to 600 ° C. Products are smoldering coke as well as smoldering tars and oils. The resulting coke is richer in hydrogen than the high-temperature coke.

Carbohydrate

→ Main article : Carbohydrates

Friedrich Bergius

In 1913 Friedrich Bergius first succeeded in attempts to convert coal into an oil by hydrogenation in an autoclave at temperatures of 400 to 500 ° C and hydrogen pressures of 100 to 200 bar. In 1925, Matthias Pier succeeded in optimizing the Bergius process by using catalysts such as tin oxalate and iron salts. As early as 1931, a test facility with a capacity of 100,000 tons per year was put into operation in the Leuna works . In the years that followed, up to the end of World War II, twelve hydrogenation plants with a capacity of 4 million tons were built in Germany.

After the end of the war, the processes were uneconomical due to the easy availability of oil . Only in the wake of the oil crisis were two small test facilities built again in Germany.

Coal gasification

→ Main article : coal gasification

The coal gasification processes are carried out as fixed bed , fluidized bed or entrained flow gasification . The processes differ in the grain size of the coal used, the size of which contributes significantly to the overall costs of the process. In the fixed bed process, coarse-grained coal with typical grain sizes of 6 to 40 mm can be used. The fluidized bed process requires grain sizes in the range from 1 to 8 mm, the flue dust process requires grain sizes of less than 0.1 mm. In addition to the grain size, the chemical properties of the coals used, such as the ash content and baking capacity, are important.

Principle of the Winkler generator

Coal gasification is carried out as an autothermal process with water vapor and air or oxygen. The energy for the endothermic water gas reaction is supplied by burning part of the coal. Lime or basic alkali salts are added to the coal to reduce the sulfur content in the gas . The first step in carbon chemistry is the water gas reaction: For this purpose, coal heated to incandescence is gassed with steam and converted into carbon monoxide and hydrogen .

${\displaystyle \mathrm {C\ +\ H_{2}O\ \rightleftharpoons \ CO\ +\ H_{2}} \qquad \Delta H_{R}=+131\ \mathrm {kJ/mol} \,}$

Different coal gasification techniques are used depending on the type of coal used. One of the oldest processes is the fluidized bed gasification developed by Fritz Winkler in the Winkler generator . The process is suitable for lignite and is carried out in the temperature range from 850 to 1100 ° C.

In the Lurgi pressurized gas process , coal or lignite is gasified in counterflow at increased pressure . The temperatures must be below 1000 ° C in order to avoid liquefaction of the ash . The process is carried out either continuously or in a cyclical process, with air and water being applied alternately. This process produces large amounts of degassing products such as naphtha, oils, tar and phenols as by-products.

In the Koppers-Totzek process , which is suitable for all types of coal, the coal is gasified in cocurrent at high temperatures of 1500 to 1600 ° C. The coal must be ground very finely for this. Due to the high temperatures, no methane or higher hydrocarbons are produced. With the Koppers-Totzek gasifier, the coal is fed into the gasifier together with the air or oxygen and water vapor. The use of air leads to lean gases that are used for heating purposes. Pure oxygen is used to generate synthesis gas . The dwell times in the Koppers-Totzek process are short compared to the other processes. Some of the ash is produced as slag or is discharged from the gasifier with the raw gas.

Acetylene production

Acetylene is obtained from calcium carbide by hydrolysis . Industrial calcium carbide production began in Switzerland, Norway and Germany (Rheinfelden) at the end of the 19th century.

To produce calcium carbide, coal is reacted with quicklime in an electric furnace at temperatures above 2000 ° C. The resulting carbide is converted into acetylene and calcium hydroxide by reaction with water .

Follow-up chemistry

Degassing products

In addition to coke, the raw materials obtained from the degassing process are mainly acetylene , benzene and naphthenes. Acetylene can be used as a fuel gas for the synthesis of vinyl chloride by the addition of HCl. Naphthenes are mainly used as raw materials for dyes , tannins , decalin and wood preservatives . The use of naphthalene for the production of phthalic anhydride has been almost completely replaced by the catalytic oxidation of o- xylene .

Synthesis gas chemistry

Depending on the ratio of the two components carbon monoxide and hydrogen, the resulting gas mixture can be used in various processes: on the one hand, processes such as the Fischer-Tropsch synthesis , which supplies a broad base of hydrocarbons, or specific carbon monoxide hydrogenations, which selectively produce individual products such as methanol production . By reacting these products with further synthesis gas or other basic chemicals such as ammonia , a wide range of intermediate products such as acetic anhydride , acetaldehyde , glycols , methyl acetate , acetic acid and many other components can be produced.

Fischer-Tropsch reaction

→ Main article : Fischer-Tropsch synthesis

The Fischer-Tropsch reaction is a synthesis reaction of CO / H ₂ -mixtures of iron - or cobalt - catalysts for paraffins , olefins and alcohols . The reaction takes place at atmospheric pressure, higher pressures shift the equilibrium towards alkenes. The reaction is carried out at a temperature of 160 ° C - 200 ° C. The most important reaction equations are:

${\displaystyle \mathrm {n\,CO+(2n+1)\,H_{2}\rightarrow C_{n}H_{2n+2}+n\,H_{2}O} }$( Alkanes )

${\displaystyle \mathrm {n\,CO+(2n)\,H_{2}\rightarrow C_{n}H_{2n}+n\,H_{2}O} }$ (Alkenes)

${\displaystyle \mathrm {n\,CO+(2n)\,H_{2}\rightarrow C_{n}H_{2n+1}OH+(n-1)\,H_{2}O} }$( Alcohols )

The alkanes can optionally be further processed in conventional refinery processes. A certain cut of the resulting alcohols can be used as raw material for the production of surfactants.

Methanol production and by-products

→ Main article : Methanol production

Today, methanol is produced on an industrial scale from synthesis gas, a carbon monoxide-hydrogen mixture in a ratio of 1 to 2 in the low or medium pressure process. The resulting raw methanol is partially contaminated with by-products.

${\displaystyle \mathrm {CO+2\,H_{2}\rightarrow CH_{3}OH} }$

Methanol has a rich subsequent chemistry. A wide variety of secondary products can be produced through conversion with carbon monoxide and carbon monoxide-hydrogen mixtures. For example, methanol can be converted into acetic acid by reacting with carbon monoxide.

${\displaystyle \mathrm {CH_{3}OH+CO\rightarrow CH_{3}COOH} }$

Acetic acid can be converted to methyl acetate by esterification with methanol, which in turn can react with carbon monoxide to form acetic anhydride.

${\displaystyle \mathrm {CH_{3}COOH+CH_{3}OH\rightarrow CH_{3}COOCH_{3}+H_{2}O} }$

${\displaystyle \mathrm {CH_{3}COOCH_{3}+CO\rightarrow CH_{3}COOOCCH_{3}} }$

Methanol can be converted into dimethyl ether by dehydration on silica-aluminum catalysts.

${\displaystyle \mathrm {2\ CH_{3}OH\rightarrow CH_{3}OCH_{3}+H_{2}O} }$

By choosing suitable process conditions and catalysts such as ZSM-5, methanol can be converted into a wide range of petrochemicals and energy sources in processes such as methanol-to-gasoline (MtG), methanol-to-olefins (MtO) or methanol-to-aromatics (MtA) convert.

Ethylidene diacetate can be produced by hydrocarbonylation of acetic anhydride and methyl acetate with carbon monoxide and hydrogen, which can be converted into vinyl acetate with the release of acetic acid by acid catalysis at around 170 ° C.

${\displaystyle \mathrm {CH_{3}COOOCCH_{3}+CH_{3}COOCH_{3}+CO+H_{2}\rightarrow CH_{3}C(OCOCH_{3})_{2}+CH_{3}COOH} }$

${\displaystyle \mathrm {CH_{3}C(OCOCH_{3})_{2}\rightarrow CH_{2}CHOCOCH_{3}+CH_{3}COOH} }$

Ethylene glycol

Ethylene glycol can be produced directly from a carbon monoxide-hydrogen mixture over rhodium catalysts at high pressures.

${\displaystyle \mathrm {2\ CO+3\ H_{2}\rightarrow HOCH_{2}CH_{2}OH} }$

acetylene

Acetylene has an extensive subsequent chemistry. It can be converted to benzene or cyclooctatetraene by cyclization over metal catalysts . The Reppe chemistry deals with the reaction of acetylene to a number of interesting derivatives such as acrylic acid .

Analytical method

The analysis methods for the coal used as well as the various products and by-products are varied and mostly described in DIN standards.

A number of analytical methods are used to assess the suitability of the coal for the various processes. An important parameter is the content of water, ash and volatile matter. These are determined in the so-called immediate analysis. Other important parameters of the charcoal are the dilatomer test, the degree of expansion, the baking ability number and the driving pressure.

The resulting coke is examined for its auto-ignition temperature, drum strength and porosity, among other things.

The ash melting behavior makes a qualitative statement about the slagging problems to be expected.

literature

• Karl-Heinz Schmidt, Ingo Romey, Fritz Mensch: Coal, oil, natural gas: chemistry and technology. Vogel Verlag, Würzburg 1981, ISBN 3-8023-0684-8 , ISBN 978-3-8023-0684-6 .
• Walter Buschmann : Coke, gas, coal chemistry. History and representational tradition of coal refinement. Klartext Verlag, Essen 1993, ISBN 3-88474-028-8 , ISBN 978-3-88474-028-6 .

Web links

• Coal chemistry archive
• Fischer-Tropsch Archives ; Extensive collection of documents on the Fischer-Tropsch trial
• Benjamin Steininger: A cornucopia of the 20th century In: “Project 100 Years of the Present” (publisher: House of World Cultures ), November 29, 2017: “The culture and media theorist Benjamin Steininger from the Beauty of Oil group explains the merging of the Coal has been in the petrochemical industry since the 1920s and outlines its far-reaching consequences from World War II to the present day. "

Individual evidence

1. ^ John Clayton: An Experiment concerning the Spirit of Coals, Being Part of a Letter to the Hon. Rob. Boyle, Esq; from the Late Rev. John Clayton, DD Communicated by the Right Rev. Father in God Robert Lord Bishop of Corke to the Right Hon. John Earl of Egmont, FRS In: Philosophical Transactions . tape 41 , no. 452-461 , 1739, pp. 59–61 , doi : 10.1098 / rstl.1739.0006 (free full text).
2. ^ Walter T. Layton: The Discoverer of Gas Lighting: Notes on the Life and Work of the Rev. John Clayton, DD, 1657-1725. London 1926.
3. ↑ Coking technology and iron smelting. ( Memento of the original from January 6, 2014 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. 
4. ↑ General information about gas.
5. ↑ Herbert Diehl, Josef Jessenberger: Ways to Coal Refinement Part I: The coal gasification . In: Physics in Our Time . tape 14 , no. 5 , 1983, pp. 146–156 , doi : 10.1002 / piuz.19830140505 .
6. ↑ ^{a b c d} Coal, petroleum, natural gas: chemistry and technology ; by Karl-Heinz Schmidt, Ingo Romey, Fritz Mensch, 256 pages, Vogel Verlag (1981), ISBN 3-8023-0684-8 .
7. ↑ Availability and supply of energy resources ( Memento of May 12, 2013 in the Internet Archive ) (PDF; 62 kB), short report by the Energy Resources Working Group of the BMWi, Department III.
8. ↑ World power oil. In: Spiegel Online. December 24, 1973, from Spiegel.de, accessed on February 11, 2017.
9. ↑ Energy - three times the amount. In: Spiegel Online. February 4, 1980, from Spiegel.de, accessed on February 11, 2017.
10. ↑ E. Rebhan (Ed.): Energy handbook: Production, conversion and use of energy . Springer, Berlin / Heidelberg 2002, ISBN 978-3-540-41259-5 , p. 113 ( limited preview in Google Book search).
11. ^ Fritz Brandt: Fuels and combustion calculation. Vulkan-Verlag GmbH, 1999, ISBN 978-3-8027-5801-0 ( limited preview in Google book search).