Zinc oxide sorbents for the removal of hydrogen sulfide from Syngas

Several pure zinc oxide materials, prepared with different methods (combustion synthesis and a modified version of the citrates method) and calcined at different temperatures, were investigated as low-temperature desulfurizing sorbents from gaseous streams (syngas, in particular). Comparative tests with a commercial sorbent were also carried out. The sulfidation performance was investigated in a fixed-bed reactor in terms of breakthrough curves at 250 degreesC. Fresh and sulfided samples were characterized by X-ray diffraction, scanning electron microscopy-energy dispersion spectroscopy, BET, pore volume, pore size, and pore size distribution analyses. The ZnO sorbent prepared by the citrated method calcined at 400 degreesC showed the most durable effectiveness in reducing sulfur from 100 to less than 1 ppm: its breakthrough time is about 9 h measured at a space velocity of about 10(5) h(-1). Its regenerability was evaluated by subsequent sulfidation-thermal regeneration cycles. A numerical model was also developed and validated on the experimental data of the ZnO sorbent calcined at 400 degreesC; a good agreement was obtained. The internal mass-transfer resistance resulted in the rate-limiting step of the process. Sulfur sorption was found to be confined to the external layers of the pellets (because it was difficult for H(2)S to reach the adsorbent core), and a maximization of internal open porosity to improve the system performance was pointed out as the main route for further developments and improvements.