Formation of light gases and aromatic species during the rapid pyrolysis of coal

Abstract

Light gases, long chain aliphatic species and aromatic hydrocarbons produced by the rapid pyrolysis of a sub-bituminous coal in a fluid-bed reactor have been determined as a function of temperature in the range 400–1050°C for residence times of the order of 1 s. The species released from the coal at the lowest temperatures (400–600°C) were predominantly large, complex molecules, with the exception of small quantities of aliphatic material that had been physically trapped in the microporous structure of the coal. At temperatures above 550–600°C long-chain polymethylene groups decomposed to form light olefins, predominantly ethylene and propylene. A significant proportion of the polymethylene groups appear to have been chemically bound to aromatic nuclei. Coincident with the polymethylene decomposition, yields of simple one- and two-ringed aromatic species increased significantly above 600°C. The more stable of these, benzene and naphthalene, continued to increase to 1050°C but the alkyl substituted species began to decrease at 800–850°C. Phenolic compounds also increased in yield between 600–800°C but decomposed at higher temperatures by decarbonylation to produce CO. At the highest temperatures studied, polycyclic aromatic hydrocarbons became increasingly important components of the tar, demonstrating that it is possible to form these species solely from coal pyrolysis reactions. Significant yields of phenylacetylene, styrene and indene, identified as important intermediates in the formation of polycyclic aromatic hydrocarbons in previous flame studies, were also observed in tars produced above 700°C.