Research on efficient tar conversion driven by oxygen carriers in pickling sludge: Phenol, anthracene, and naphthalene as biomass model compounds

Biomass chemical looping processing faces challenges such as low reaction efficiency and severe carbon deposition due to tar generation. The development of highly efficient and stable oxygen carriers is crucial for achieving clean conversion of tar. The pickling sludge, as a bulk industrial solid waste containing iron-chromium spinel and CaF 2 , exhibits both catalytic activity and oxygen transfer capability. In this study, phenol, anthracene and naphthalene were used as model tar compounds to compare the conversion characteristics of pickling sludge (PS), metal-doped oxygen carrier (MD) and Fe 2 O 3 (FE), and to reveal their catalytic mechanisms by combining pore structure, surface element distribution and kinetic analysis of the reaction. The results showed that PS significantly enhanced tar conversion (phenol＞97 %, naphthalene 77.86 %, anthracene 74.71 %) due to its high specific surface area (40.25 m²/g) with mesopore-dominated pore structure (67.48 %), and the cumene fraction was reduced by more than 40 % compared to that of FE, which was attributed to the synergistic effect of the stabilized active sites of Fe-Cr spinel and CaF 2 porous structure. The increase in temperature (800–950 °C) promotes the migration of lattice oxygen, leading to an improvement in the conversion rates of anthracene and naphthalene by 10.20 % and 5.85 %, respectively. Increasing the oxygen carrier-to-fuel ratio (φ=0.75–1.5) optimizes oxygen supply, enhancing the conversion efficiency. However, excessive oxygen carrier (φ=1.5) results in a reduced contact area, leading to a decrease in lattice oxygen utilization efficiency. A detailed analysis of the tar fractions was conducted, revealing that elevated temperatures and heightened levels of oxygen significantly enhance the formation of oxygen-containing groups, such as ketones and esters. Conversely, anthracene has been observed to accumulate substantial carbon due to its polycyclic structure, a property that facilitates the formation of stable intermediates. ● Pickling sludge as an oxygen carrier is a low-cost industrial solid waste. ● The article compares the conversion properties of three oxygen carriers to tars. ● The article analyzes the results in terms of reaction kinetics. ● The article analyzes the synergistic effect of Fe-Cr spinel with CaF₂.