Abstract
Dry-bed adsorptive desulfurization of biomass-based syngas with low to medium sulfur content using ZnO was investigated as an alternative to the conventional wet scrubbing processes. The technical feasibility of ZnO-based desulfurization was studied in laboratory-scale H2S breakthrough experiments. The experiments were set up to utilize realistic H2S concentrations from gasification and therefore long breakthrough times. Experiments were performed in a steam-rich model biosyngas in varying conditions. The long-term breakthrough experiments showed apparent ZnO utilization rates between 10 and 50% in the tested conditions, indicating intraparticle mass-transfer resistances partly due to space velocity and particle size constraints as well as the most likely product-layer resistances as evidenced by the large spent adsorbent surface area decrease. An empirical deactivation model to estimate full breakthrough curves was fitted to the laboratory-scale experimental data. Breakthrough experiment in tar-rich syngas was also performed with the conclusion that ZnO performance is not significantly affected by hydrocarbons despite carbon deposition on the particle surfaces.
Highlights
With the increasing need for renewable fuel and energy sources, gasification of woody and herbaceous biomass is of high interest
Equilibrium calculations for the main sulfidation reaction and H2S reactions with syngas components to form COS were performed with equilibrium constant data from HSC 8 software
The equilibrium calculations suggest that lower temperatures are preferred for deep H2S removal and that ZnO has the potential for deep H2S removal in biosyngas
Summary
With the increasing need for renewable fuel and energy sources, gasification of woody and herbaceous biomass is of high interest. Large-scale processes have high investment costs and create problems with procurement of biomass feedstock. Decentralized small-scale fuel and chemical production seems an attractive alternative approach, but this in turn further increases the need for lowcost purification solutions. Hydrogen sulfide needs to be removed to prevent downstream catalyst poisoning and to protect equipment from corrosion.[2] Biomass contains usually under 0.1 mass % sulfur, that is mostly converted to H2S in gasification conditions.[3] Raw biosyngas, depending on the biomass feedstock, contains tens to hundreds of ppmv H2S.4. Methanol synthesis catalysts (Cu-based) in general have slightly higher poison tolerance than FT catalysts.[7]
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