Evolution of Aromatic Tar Composition in Relation to Methane and Ethylene from Biomass Pyrolysis-Gasification

Abstract

Tar reduction and monitoring is the major stake for gasification processes. Pyrolysis is the precursor mechanism of the gasification of solid fuels and tar production. The evolution of gas and tar composition produced from wood chips pyrolysis was investigated in a tubular reactor as a function of its wall temperature (700–1000 °C, with gas mean residence times of 1.1–2.7 s). High thermal severities lead to the “gasification” regime, promoting gas production from tar conversion. Tar (benzene, toluene, o- and m-xylenes, phenol, indene, o-, m-, and p-cresols, naphthalene, 1- and 2-methylnaphthalenes, acenaphthylene, and phenanthrene) were quantified by gas chromatography/mass spectrometry (GC/MS) analysis using deuterated internal standards. Closed mass balances were obtained. A simplified chemical scheme of secondary tar conversion is proposed. Under the investigated range of thermal severity, CH4 production is mainly controlled by aromatic tar demethylation. Linear relations were observed between the molar production of benzene and CH4 and between all quantified tars and C2H4 (ethene). CH4 and C2H4 could thus be analyzed by direct online methods and used as indicators of the tar content and speciation for gasifier monitoring. The validity of these relations depends upon the thermal conditions of reactors and biomass composition. These relations could be suitable for dual-fluidized-bed gasifiers because H2O has very few chemical effects on hydrocarbon thermal conversion.