Composition modification of zinc titanate (Zn₂TiO₄) based sorbents for hot coal gas desulfurization

Advanced power generation systems require regenerable sorbents capable of removing sulfur from coal gas to low ppm levels. Zinc titanate (Zn₂TiO₄) based sorbents are currently the leading sorbent for sulfur removal at high temperatures (~700°C). Zinc titanate sorbents are still in need of development to address the problems of zinc vaporization, physical degradation, and low attrition resistance during the many sulfidation and regeneration cycles.

Based on the principles of phase equilibria and crystal chemistry, the approach being investigated in this project to eliminate these problems with zinc titanate sorbents is incorporation of various cations into the Zn₂TiO₄ spinel structure. The study involved synthesis, characterization, and evaluation of the modified Zn₂TiO₄ sorbents at the labscale size. Samples were characterized by XRD and TGA under simulated coal gas and an H₂S gas mixture to 1) determine the solid solubility of five cations (Ni, Cr, Al, Mg, and Cu) in the Zn₂TiO₄ lattice and 2) determine the effect of the cations on the sulfur removal performance of Zn₂TiO₄ sorbents.

The five cations selected were incorporated into the Zn₂TiO₄ lattice in significant amounts, up to ~35 mole percent for Ni, Cr, Cu, and Al and ~25 mole percent for Mg at 1100°C. Based on the high chemical reactivity and durability observed in preliminary screening, Cr-incorporated Zn₂TiO₄ sorbents, with compositions ranging from 3 to 35 mole percent Cr, were selected for further testing and characterization, including XRD, TGA, crush strength, and EDX. No correlation was found between Cr concentration and porosity, crush strength, and weight gain (sulfur removal). Cr-incorporated sorbents reduced Zn losses from 16.4% with 22.2 mole percent to 1.2% loss with 26.1 mole percent Cr.