Abstract

Biomass gasification represents the most flexible, energy efficient, and “green” source for production of low-to-medium energy fuel gases, synthesis gas, electricity, and hydrogen for fuel cell applications. Abundant availability (220 billion dry tons) and sustainability makes biomass an important alternative for decreasing the current dependence on petroleum products [1]. The main difficulty in direct use of biomass gasification gas for producing chemicals or electricity is the presence of impurities such as ammonia, tars, and hydrogen sulfide. The typical composition of these contaminants produced during gasification is: 1-150 g/Nm³ of tars, 1000-14000 ppmv of NH₃, and 20-200 ppmv of H₂S [1]. Tars pose a number of problems in downstream processing and need to be cracked to linear hydrocarbons. Dolomites are the most studied catalyst for tar removal. Other conventional catalysts for tar cracking include acid catalysts, basic catalysts, Ni and Fe based catalysts. The current paper reports effect of Pt promotion on behavior of tungstated zirconia (WZ) and ultra-stable Y zeolites (USY) for toluene (model compound for tars) cracking. The kinetic study of toluene cracking on platinated tungstated zirconia (PtWZ) and platinated ultra-stable Y zeolite (Pt-USY) in the absence and presence of H₂ and CO is also reported. The catalysts were characterized by BET, XRD, chemisorption, TPD, ion-exchange titrations, and high resolution transmission electron microscopy (HRTEM). Toluene cracking reaction was carried out at temperatures ranging from 600 – 800°C and at 1 atm. The inlet concentration of toluene was 2500 ppm with other gases being 10% H₂, 15% CO and balance He. The time-on-stream (TOS) experiments at 700°C showed that on PtWZ, toluene was cracked to benzene and methane only. PtWZ was more active initially and displayed more selectivity towards CH₄. But after 5 min TOS, the catalyst showed a rapid partial deactivation and selectivity towards CH₄ dropped, possibly due to coke formation. The role played by Pt, WO₃ and ZrO₂ and behavior of PtWZ and Pt-USY in the presence of gasification gases (H₂, CO, CO₂, and NH₃) will be discussed.  

1. Torres, W, Pansare, S. S., Goodwin, J. G., Jr., Catal. Rev., in press.

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E-mail address: james.goodwin@ces.clemson.edu