Chemistry of Tar Formation and Maturation in the Thermochemical Conversion of Biomass

An understanding of the molecular details of tar formation and maturation in thermochemical processes is fundamental to the development of gasification and gas cleaning systems for high efficiency applications, such as in internal combustion engines or in gas turbines. Tars are functionally defined as the condensible organic fraction from gasifier effluents, and hence, are generally considered tobe aromatic in nature. This definition does not allow a distinction between classes of compounds which originate under different reaction regimes, such as the primary pyrolysis products, which may be in the gasifier effluent because of low temperature operation or due to process upsets, and high molecular weight polynuclear aromatic hydrocarbons, which are produced under higher reaction severityand are the precursors of particulate matter formation. This paper describes the effect of time, temperature, and oxygen on product composition and maturation of tars through distinct classes of products. The effect of oxygen at temperatures of 600 deg. C to 700 deg. C is shown to accelerate the destruction of primary pyrolysis products but has no significant effect on benzene.