Investigating the interaction between ilmenite and zinc for chemical looping

Abstract

The iron and titanium oxide ilmenite is a benchmark oxygen carrier for chemical looping combustion (CLC) and oxygen carrier-aided combustion (OCAC). Both of them are combustion technologies for biomass and waste fuels with lower emissions and low costs for carbon capture. Here, the interaction between the ash component zinc and oxygen carrier ilmenite is studied in a two-staged vertical tube reactor. Three types of ilmenites─Norwegian rock ilmenite, synthesized ilmenite, and ilmenite extracted after 200 h of OCAC in a full-scale fluidized bed unit─were exposed to gas-phase Zn and ZnCl2. Following the exposure, samples were analyzed concerning morphology, chemical distribution, composition, and crystalline phases. The observations were complemented with thermodynamic equilibrium calculations. It is observed that the iron-rich layer formed on the external surface of rock ilmenite after activation promotes the reaction with both gaseous zinc compounds, with zinc ferrite formed in the external Fe-rich layer. In contrast, ilmenite with no segregation of Fe and Ti showed to interact less with zinc species. Metallic Zn penetrated the particles, while the interaction depth was shallow with ZnCl2 for all investigated ilmenite oxygen carriers. The gaseous conditions, particle ash layer composition, and iron availability are shown to play an important role in the interaction between zinc compounds and ilmenite particles. Based on these results, interaction mechanisms for Zn and ZnCl2 are proposed. This interaction could have environmental implications for the toxicity of ash streams from waste combustion in addition to possibilities for Zn recycling.