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

TL;DR

This paper studies how alkali compounds behave in fluidized beds during fuel conversion, focusing on their uptake, release, and speciation using new high-temperature analysis methods.

Contribution

The paper introduces novel methods for real-time alkali speciation and monitoring in fluidized beds using TMSI and TGA techniques.

Findings

Ilmenite showed near-complete uptake of alkali chlorides under reducing conditions.

TMSI analysis revealed that NaCl and KCl are the main alkali species emitted during injections.

Ilmenite releases alkali at high temperatures in both inert and oxidizing conditions.

Abstract

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.

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