Alkali Uptake, Release, and Speciation in Fluidized Beds Using Oxygen Carriers

Recent advancements in combustion-related alkali chemistry have been increasingly driven by the adoption of CO₂-neutral fuels, such as bioderived materials and waste, which often contain high amounts of alkali compounds. While alkali compounds may have catalytic effects on, e.g., fuel conversion and tar cracking, they also contribute to fluidized bed agglomeration, ash deposition, and corrosion. A thorough understanding of alkali uptake, release, and emission control is therefore crucial for scaling up and commercializing advanced fuel conversion technologies. This study presents recently developed methods for high-temperature alkali analysis, including (1) a temperature-modulated surface ionization (TMSI) technique for real-time alkali speciation, (2) a laboratory-scale reactor enabling continuous alkali vapor injection into fluidized beds with real-time monitoring of exhaust alkali emissions, and (3) a TMSI-thermogravimetric analysis (TGA) method for monitoring real-time alkali release and mass loss. The summarized results provide valuable insights into high-temperature alkali chemistry processes and their interaction with different oxygen carriers. Oxygen carriers of calcium manganite, manganese oxide, and ilmenite exhibit varying alkali uptake efficiencies based on reactor gas conditions. Ilmenite showed near-complete alkali absorption (>90% uptake of alkali chlorides), particularly in reducing conditions. Alkali speciation analysis revealed that NaCl and KCl were the main alkali species emitted during NaCl and KCl injections, with a similar trend for alkali sulfates. Ilmenite previously used as an oxygen carrier industrially releases alkali at high temperatures in both inert and oxidizing conditions. Furthermore, the TMSI method was applied to study alkali emissions during biomass pyrolysis, where KOH dominated emissions during low-temperature pyrolysis, while both KOH and NaOH were emitted from the remaining char and ash. This real-time characterization of sodium and potassium compounds offers new opportunities to optimize solid fuel conversion processes for fuels such as low-grade biomass, waste, and coal.