Search

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]

Highlights • Volatiles reaction (VR) occurred at 440 °C in 1.5 s, extensive at >600 °C in 4 s. • For the columns and carriers used >50% THF or hexane solubles (HS) reside in column. • With minimal VR, the tars contain mainly HS, more for a lower rank coal tar. • VR reduced HS yield, increased pitch and coke yields, resulting in lighter HS. • VR converts some SIMDIST eluted tar to residue and increases 2-ring aromatics in HS. Abstract In coal pyrolysis, the reaction of volatiles plays an important role in determination of tar yield and composition but has not been studied in detail in the literature. This work studies the volatiles' reaction in pyrolysis of a bituminous coal (SM) and a lignite (HLBE) using a two-stage fixed-bed laboratory reactor with the second-stage temperature of 440–700 °C and the residence time of 1.5–6.9 s. The tars were characterized by solvent extraction with tetrahydrofuran (THF) and hexane, and by simulated distillation (SIMDIST) of the THF soluble matter and high performance liquid chromatography (HPLC) analysis of the hexane soluble matter. It was found that the main component of the tars was oil, which was more in the HLBE tar than in the SM tar. As much as >50 wt% oil and tar retained in the chromatograph columns. The volatiles reacted at temperatures as low as 440 °C and the rate was more pronounced at 600 °C in 4.2 s. The volatiles' reaction reduced the oil yield while increased the pitch yield. The aromatics in the hexane soluble matter were mainly 2-ring compounds and their content increased by the volatiles' reaction. The major tar fractions have boiling points of 200–538 °C and their content varied systematically in the volatiles reaction. The findings are important for design of efficient coal pyrolysis reactors. [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]