Advancing Calcium Looping Reaction Using Salt-promoted CaO Materials for Thermochemical Energy Storage

Abstract

Calcium looping process is a chemical looping process involving reversible gas-solid carbonation-calcination reactions among CaO, CO2, and CaCO3. Thermochemical energy storage (TCES) via calcium looping is a promising option for the thermal energy storage in concentrated solar power (CSP) generation, due to its distinct advantages of high energy storage density at high operation temperatures. However, the continuous deactivation of CaO through carbonation-calcination reactions degrades its energy storage ability. In this dissertation, novel salt-promoted CaO materials are proposed to address the deactivation issue through a new material modification strategy. Throughout the chapters, effects, mechanisms, and applications of salt-promotion method for synthetic CaO were investigated to advance calcium looping reactions. In chapter 1, backgrounds of calcium looping process were briefly introduced. The mechanisms related to deactivation phenomena and currently reported modification methods for CaO materials were reviewed based on the underlying mechanisms. Then, the research objectives of this dissertation is stated, that a new concept of CaO modification method is proposed to promote carbonation reaction by addition of alkali and alkaline earth metal chloride salts to enhance the mass-transfer of reactants. In chapter 2, the effects of CaCl2 as an additive to CaO materials were investigated to address the mass-transfer issue. A variety of CaO-based materials with CaCl2 were synthesized, characterized, and examined their reaction patterns. The results showed that the addition of CaCl2 promoted the carbonation reaction, and CaO-based materials with CaCl2 and MgO achieved high reversibility with enhanced durability. In chapter 3, The promotion mechanism of chloride salt additives was investigated by applying eutectic chloride salt (ECS) to CaO-based materials. The effects of the ECS additives were investigated in terms of carbonation temperature and melting point of the salts. The salt-promoted carbonation mechanism was proposed that molten ECS dissolves the CaO from a Ca–O grid, and provides an additional pathway for diffusion of free CaO ion pairs to CO2 adsorbed on the particle surface. In chapter 4, the applicability of salt-promoted CaO materials was investigated. The low optical absorbance and deactivation phenomena were simultaneously addressed by using salt-promoted CaO materials with transition metals additives. The results revealed that the salt-promotion method induced synergistic enhancements in optical absorbance and long-term energy storage, achieving extraordinary performances. Based on such findings, new CaO modification strategy was proposed that synergistic uses of the salt-promoted CaO and other methods can advance the calcium looping reactions.