Structure-Reactivity- and Modelling-Relationships during Thermal Annealing in Biomass Entrained-Flow Gasification: The Effect of Temperature and Residence Time

Biomass entrained-flow gasification enables the sustainable production of chemicals and liquid fuels. For reliable and accurate gasifier operation and design, the analysis of biomass char reactivity represents one of the key studies. Therefore, the annealing-induced char reactivity loss needs to be investigated for biogenic feedstocks from microscopic to reactor level. In the present work, the influence of pyrolysis temperature and particle residence time on solid digestate char reactivity and structure is studied. Several char types are prepared in a pressurized entrained-flow reactor between 1200 °C and 1600 °C with varying residence times between 0.4–2.4 s. Isothermal char reactivity in O₂ and CO₂ atmosphere is measured by TGA and reactivities are determined. Strong char deactivation between 1200 °C and 1400 °C resulted from severe heat treatment, especially toward the CO₂ reaction. At 1600 °C, the influence of residence time becomes less relevant since all measured reactivities are comparably low. Experimental data are fitted to a coal deactivation model and verified for biogenic residues with very good agreement. The results are further corroborated by char structure analysis using FT-IR, XRD, Raman spectroscopy, SEM-EDX and ETV-ICP-OES. The decrease in char reactivity is mainly attributed to graphitization and the formation of aromatic ring structures. Char deactivation is further promoted by the loss of catalytic mineral matter and by high concentrations of inhibiting elements like phosphorus.