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

Area of Science:

• Chemical Engineering
• Environmental Science
• Materials Science

Background:

• Biomass gasification produces syngas, a crucial intermediate for biomass-to-liquid (BTL) processes.
• Conventional wet-scrubbing for syngas desulfurization faces challenges in small- to medium-scale applications.
• Dry-bed adsorption using zinc oxide (ZnO) presents a promising alternative for sulfur removal.

Purpose of the Study:

• To evaluate the efficacy of ZnO in dry-bed adsorptive desulfurization of biomass-based syngas at a bench-scale.
• To verify laboratory-scale hydrogen sulfide (H2S) breakthrough findings under realistic process conditions.
• To characterize the performance and physical changes of ZnO adsorbent during prolonged operation.

Main Methods:

• Scaled-up desulfurization tests using actual bio-syngas in a connected desulfurization unit and pilot-scale fluidized bed gasifier.
• Conducted two 70+ hour test campaigns to assess H2S removal efficiency and breakthrough.
• Utilized Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray Spectroscopy (EDS), and Brunauer-Emmett-Teller (BET) analysis for adsorbent characterization.

Main Results:

• Achieved sustained H2S removal below the breakthrough limit throughout the test campaigns using ZnO particles.
• Observed core-shell sulfidation behavior in SEM and EDS analysis, indicating limited utilization of the ZnO core.
• Demonstrated significant loss of adsorbent porosity (BET analysis) and simultaneous carbonyl sulfide (COS) removal via hydrolysis.

Conclusions:

• Dry-bed ZnO desulfurization is effective for low- to medium-sulfur biomass syngas in small- to medium-scale BTL concepts.
• Adsorbent utilization is limited by core-shell sulfidation and porosity loss, suggesting potential for adsorbent optimization.
• The process also removes COS, but upstream HCl removal is necessary due to minor chlorine adsorption.