Abstract
Gasification of biomass and waste produces tar as an undesired byproduct. Previous studies have shown that dolomite is an effective catalyst for the reduction of the tar content in the gas. The result, compared to thermal cracking, is that the remainder of unconverted tar consists mainly of lighter aromatics, BTX (Benzene, Toluene, Xylene) and naphthalene instead of polyaromatics.
A gasification process developed by Studsvik, CFBG (Circulating Fluidized Bed Gasifier), has made tar reduction greater than 95% possible by utilizing catalytic cracking with dolomite. In order to understand the mechanism of the decomposition of the tar, further fundamental studies has been performed in a laboratory reactor. Naphthalene was choosen as the model compound, since it is the most abundant condensable product after cracking the tarry fuel gas with dolomite at 800-900°C.
Efforts have been made to determine if hydrogen abstraction from the naphthalene molecule is the rate determining step in the cracking mechanism. The kinetic isotope effect was used as a tool for this investigation. There was no distinct difference between the conversion efficiency of deuterated and normal naphthalene. Neither were there any correlation with degree of conversion and amount of exchanged hydrogen/deuterium atoms when naphthalene was cracked in the presence of deuterium oxide vapour. Hence the rate determining step in the catalytic cracking of naphthalene is not the abstraction of hydrogen.
Studies made on the effect of surronding gaseous species shows that carbon dioxide and moderate amounts of water vapour enhance the cracking efficiency, whereas hydrogen is a strong inhibitor.
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Aldén, H., Björkman, E., Carlsson, M., Waldheim, L. (1993). Catalytic Cracking of Naphthalene on Dolomite. In: Bridgwater, A.V. (eds) Advances in Thermochemical Biomass Conversion. Springer, Dordrecht. https://doi.org/10.1007/978-94-011-1336-6_17
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