Experimental Bench-Scale Study of Residual Biomass Syngas Desulfurization Using ZnO-Based Adsorbents

Dry-bed adsorptive desulfurization of biomass-based syngas with a low- to medium sulfur content using ZnO wasstudied as an alternative to conventional wet-scrubbing processes for a small- to medium-scale biomass-to-liquid process concept.Following laboratory-scale long-term H2S breakthrough experiments in a previous study, desulfurization tests were scaled-up tobench-scale with actual bio-syngas to verify the lab-scale results under more realistic process conditions. A desulfurization unit wasconstructed and connected to a steam-blown atmospheric pilot-scale fluidized bed gasifier. Two successful 70+ h test campaignswere conducted with H2S removal below the breakthrough limit using full-sized ZnO adsorbent particles. Scanning electronmicroscopy (SEM), energy-dispersive X-ray spectroscopy elemental analysis, and Brunauer−Emmett−Teller (BET) surface areacharacterization of the fresh and spent adsorbent pellets were performed. SEM micrographs displayed the outward enlarging particlesize in the sulfided layer. Characterization showed significant core−shell sulfidation behavior with a few hundred micron-thicksulfided layer leaving the majority of ZnO unutilized. Adsorbents lost most of their porosity in use, which was evident from BETsurface area results. Simultaneous COS removal was found possible by the hydrolysis reaction to H2S. Furthermore, evidence ofminor chlorine adsorption was found, thus highlighting the need for a dedicated HCl removal step upstream of desulfurization