Reaction kinetics and mechanisms for carbon-negative chemical looping gasification of biomass coupled with CO2 splitting

Chemical looping gasification of biomass coupled with CO2 splitting (CLGCS) was proposed to produce highquality syngas from biomass and split CO2 into CO by using Fe/Ni mixed oxygen carrier without extra catalyst and excessive energy consumption, providing a carbon negative emission technology for the cascade utilization of biomass and greenhouse gas. In this work, the reaction kinetic and the activation energy were investigated based on the non-model kinetic model and the lattice oxygen migration mechanism was revealed by the valence evolution of oxygen carriers. The chemical looping gasification (CLG) involved four stages based on the activation energy distribution, which are dehydration, pyrolysis, gas reforming, and char reacting. Ni provided an active site in pyrolysis stage, resulting in a rapid decline activation energy. The higher energy barrier of 341.38 kJ/mol needed to be overcome in char reacting due to the solid-solid reaction. The reaction activation energy exhibited a rising trend as the oxygen vacancies decrease in CO2 splitting stage (CS), where the internal lattice oxygen transfer resistance increases. The lattice oxygen O2- dissociated into chemisorbed oxygen O2-/O-, which was further converted into molecular O2 on the surface of the carrier. Ni2+ from Ni-Fe is preferentially converted to Ni-O in the reduction reaction, and then reduced to Ni0. The Ni/Fe intermetallic synergies were also confirmed by the difference in Ni-Ni and Ni-O electron binding energies of Ni2p. The transfer mechanism for CLG was expressed as: O2-(lattice) - e O2 -/O-(chemical)- e O2(phy)-e O2(g). The lattice oxygen migration displayed an opposite trend for CO2 splitting reaction,where Ni played a catalytic activation role, facilitating the transfer of lattice oxygen rather than participating in the reaction.