The direct solid-solid reaction between coal char and iron-based oxygen carrier and its contribution to solid-fueled chemical looping combustion

Highlights: • Direct reaction between coal char and iron-based OC particles investigated in the dynamic and static contact regimes. • Determines reaction kinetics and carbon conversion of the direct solid-solid reaction. • Operational temperature and the use of different OCs significantly affect the reaction kinetics. • In-situ gasification and solid-solid reaction in competition with each other in steam-gasified CLC. • Methods developed to determine the contribution of solid-solid reaction in CLC. - Abstracts: Chemical looping combustion (CLC) is an advanced technology developed to achieve highly efficient fuel combustion with in-situ CO₂ capture. In this process, metal oxide particles are used as an oxygen carrier (OC) to transport lattice oxygen for fuel combustion. In this process, a stream of CO₂ and steam is produced by successful separation of atmospheric N₂ and the gaseous product of combustion. In CLC of solid fuel, metal oxide particles are physically mixed and react with solid fuel at high temperature using gasification enhancer, such as steam, or CO₂. A full understanding of the reaction mechanism between the OC and solid fuel is vital for OC development and the fuel reactor design. Several reactions may be involved in solid-fueled CLC when an iron-based OC is used, including (1) solid fuel devolatilization/gasification, (2) OC reduction with intermediate syngas, (3) the solid-solid reaction between OC and solid fuel via direct contact, and (4) the homogeneous water-gas shift reaction. The former two reactions have been extensively studied in recent years. This study focuses on the third reaction, the solid-solid reaction, which occurs thermodynamically at typical operational temperatures of CLC. The direct solid-solid reaction between coal char and two iron-based OCs via random particle collision in a fluidization bed regime was investigated and focuses on the reaction kinetics and the carbon conversion at different temperature. The contribution of the solid-solid reaction to the global carbon conversion was estimated for steam-gasified CLC at different temperature. The solid-solid reaction via static contact in a thermal-gravimetric analyzer (TGA) was also tested to evaluate the role of different OCs and to better understand the reaction mechanism between the two solid particles.