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The reaction of volatiles in a coal pyrolysis reactor determines the yield and quality of tar. This paper studies the yield, group composition and radical concentration of tars obtained from pyrolysis of a subbituminous coal, mainly on the effect of the tars' residence time in the pyrolysis reactor and in post-pyrolysis heating. The latter operation simulates the environment of tars, if they were in a large fast pyrolysis reactor. It is found that an increase in the volatiles' residence time in the pyrolysis reactor reduces the yield and increases the radical concentration of tars. The pitch fraction of the tars is mainly responsible for coke formation which occurs significantly at temperatures higher than 420 °C. The tars contain radicals, mainly in their coke and pitch fractions. The coke concentration in the tars increases with increasing heating time, linearly at 420 and 450 °C but in a sigmoidal pattern at higher temperatures. The coking behavior within 10 min can be expressed by the zero-order or zero-order + autocatalytic kinetics with activation energies of 128–138 kJ/mol. [ABSTRACT FROM AUTHOR]

Coal pyrolysis is regarded by many as a simple method to produce liquid fuels and chemicals and has been studied extensively in the past. However, fast coal pyrolysis technologies intended for higher tar yields and higher productivities, in comparison to the successful slow pyrolysis technologies, had common problems such as poor tar quality and plugging of the volatile products lines. The failing of many technical efforts in solving these problems calls for more fundamental studies, especially on the reaction of volatiles in major pyrolysis reactors. This is because that thermal cleavage of covalent bonds in coal that generates volatiles is a single step and depends mainly on the temperature of the coal, while the reactions of the volatiles involve multiple steps and depend on many factors especially the gas phase temperature that is generally higher than that of the coal due to the large temperature gradient in fast pyrolysis reactors. This article analyzes temperature increases in volatiles’ upon their generation from coal in various pyrolysis reactors and correlates the temperature increases with tar yield and composition. Experimental results on reaction of volatiles are also presented to enlighten the importance of the volatiles’ reactions. [ABSTRACT FROM AUTHOR]