Chemicals from tar processing

Abstract

—Coal structure models indicate that aromatic and hetero-aromatic monomer units are already preformed in the macromolecular structure of coal. High-temperature pyrolysis liberates these aromatic monomer substances as condensable tar vapours—a discovery of the 17th century. The beginning of the 19th century saw the start of industrial processing of such tars from coal with the production of metallurgical coke and town gas by means of coal carbonization, the tars being obtained as by-products. After the invention of synthetic dyestuffs in the second half of the 19th century, the three basic aromatic hydrocarbons benzene, naphthalene and anthracene discovered in coal tar formed the main raw materials basis for the rising organic chemical industry in Western Europe. Up to now, coal tar remains an inevitable by-product in the production of metallurgical coke, which is still used for the reduction of more than 98% of the mined iron ores to metallic iron in blast furnaces. On a world-wide basis, the yield of coal tar linked with coke production is approx. 16 million t/a. The modern tar refining technology for recovering primary aromatic chemicals and other tar products is a combination of 6 unit operations: distillation, crystallization, extraction, catalytic polymerization, thermal polymerization, and pyrolysis. These tar refining processes supply thermoplastic indene-coumarone resins for adhesives and plastics, phenols for duroplastics and plant protecting agents, nitrogen heterocyclics for plant protecting agents and pharmaceuticals, condensed aromatic hydrocarbons for the synthesis of textile dyestuffs and pigments, fully aromatic creosote oils as impregnating oils for wood preservation and as feedstocks for the manufacture of carbon black fillers and pigment black for automobile tyres, other rubber compounds, printing inks and varnishes, and finally electrode pitches and pitch coke for carbon anodes used in aluminium smelters and graphite electrodes for electric steelmaking furnaces. In addition to the high-temperature carbonization process, tars are also obtained from low-temperature carbonization and low-temperature gasification of hard coal and lignite as well as from carbonization of wood and hydropyrolysis of lignin. Such low-temperature tars have a lower content of aromatic hydrocarbons but very much more phenol and its homologues as compared with high-temperature coal tars. Up to the year 2000, an increasing quantity of tar will be available as raw material to the chemical industry, since on the one hand iron and steel production based on blast-furnace operations using coke will be further expanded; and on the other hand, coal and perhaps wood conversion processes still under development to obtain pollution-free fuel gases, power station fuels and synthesis gases will generate further quantities of tar as a by-product when increasingly applied on an industrial scale.