CaO-based High-temperature CO2 Sorbents

Abstract

Calcium oxide (CaO)-based materials were recognized as promising high temperature CO2 sorbents and have often been studied since the 1950s in the field of coal gasification and more recently in the field of hydrogen production. Interest in these materials is associated with their high theoretical sorption capacity (below 650 °C) and low cost, and also the exothermic character of the carbonation reaction. CaO is used in several industries and more particularly in the cleaning of flue gases, in biomass gasification and in sorption enhanced steam reforming (SESR) processes. It permits the reduction of CO emissions, the promotion of steam reforming and water gas shift reactions, and separation of CO2 to produce a H2-rich gas. The impurities present in natural calcite have led researchers to be inventive when producing CaO-based sorbents. Their weakness lies in the sintering that occurs during the regeneration step at higher temperatures (>800 °C) and leads to a large and continued decrease of their sorption capacity. This phenomenon was extensively studied and simulated by several models (mainly based on grain models and random pore approaches) depending on the conditions of their use as CO2 sorbents. The improvement of sintering resistance can rely on these studies to propose some solutions to limit CaO-based sorbent decay, which is inevitable in multi-cyclic processes. This can include the CaO precursor choice, the sorbent pre-treatment, the improvement of the synthesis method associated with the change of morphology, structure, and surface, and the synthesis of CaO-based mixed oxides. Attrition resistance increases, degraded sorbents’ reactivation, and the influence of sulfur compounds are also to be studied depending on the use conditions.