Fate of Polycyclic Aromatic Hydrocarbons during Tertiary Tar Formation in Steam Gasification of Biomass

Artikel i vetenskaplig tidskrift, 2018

This work investigates the fate of polycyclic aromatic hydrocarbons (PAH) in relation to the process severity in the steam- and H2-containing reaction environment of steam gasification of biomass. The focus is on the regimen of tertiary tar formation during the gasification in a fluidized bed gasifier; the tertiary tar is tar that is predominantly aromatic compounds. The process severity reflects the following operating conditions: temperature, gas residence time in the reactor, and contact time between the gas phase and catalytic bed material. The conducted experiments employed a raw gas upgrading process downstream of the gasifier. A mature tar-containing raw gas produced in the Chalmers 2–4-MWth dual fluidized bed biomass gasifier was upgraded in a bench-scale, bubbling fluidized bed reactor, in which inert silica sand and a naturally occurring ilmenite catalyst were used as the bed materials. The obtained results show that, following the increased process severity, the growth of PAH can either enhance or suppress. For the growth of PAH being suppressed, it is required that the process severity is sufficient to convert steam and H2 into the reactive hydrogen intermediates that prevent the combination of the carbon-containing species. To ensure this, the application of silica sand as a bed material requires an operating temperature of 850 °C and a gas residence time of >11.5 s, while the use of ilmenite requires an operating temperature of 800 °C and a gas residence time of >3.4 s, together with a gas–solid contact time of about 0.7 s. In particular, the results obtained for ilmenite encourage the use of naturally occurring catalysts in fluidized bed gasifiers, despite the fact that their catalytic activities are lower than that of synthetic catalysts.