Catalytic tar cracking over calcium oxide-based bifunctional materials during biomass chemical looping gasification: Experimental and DFT approaches

Abstract

The Ca-looping biomass chemical looping gasification (CaL-BCLG) process employs cyclic-chain reactions of CaO-based sorbents between the gasifier and the combustor to simultaneously enhance H₂ production and enable CO₂ capture, offering broad prospects in the clean energy sector. However, tar-induced carbon deposition on the carrier surface and pipeline blockage significantly impair the stability of CaL-BCLG for hydrogen production. Although SiO₂- or coal gangue (CG, mainly consisting of SiO₂ and Al₂O₃)-modified CCS (calcined carbide slag) sorbents previously developed by our group have shown promising CO₂ capture and hydrogen production performance, their gradual deactivation under heavy-tar conditions remains unavoidable. The underlying interactions between tar and modified sorbents, however, are still poorly understood. In this work, the effect of inert oxide doping on tar cracking performance was systematically evaluated using tar reforming tests, structural characterizations, tar component analysis, and density functional theory–based molecular dynamics simulations. SiO₂ or CG incorporation constructed more stable frameworks and preserved active sites, effectively suppressing sintering and carbon deposition. Thus, CCS-Si2 (doped with 2 wt% SiO₂) and CCS-CG5 (doped with 5 wt% CG) exhibited higher apparent tar reforming performance than pristine CCS under the tested conditions. Basic phases (Ca₂SiO₄, Ca₁₂Al₁₄O₃₃) provided additional active sites that promoted the cracking of acidic oxygenates and the steam reforming of carbon deposits. The ·H and ·OH radicals generated via H₂O ionization were further identified as the dominant species driving tar decomposition on CaO, with ortho-position dehydrogenation serving as the rate-limiting step. Si doping enhanced the catalytic performance by modulating the electronic structure of CCS and optimizing tar adsorption; however, Si-Al interactions can partially diminish the intrinsic cracking activity of CaO sites. These insights elucidate tar–sorbent interaction mechanisms and offer design principles for high-stability CaO-based sorbents enabling efficient hydrogen production and CO₂ capture in CaL-BCLG.