Alkali Uptake and Release from Oxygen Carriers in Chemical Looping Applications: Development and Application of Reactor Systems and Measurement Techniques

Abstract

Chemical looping combustion (CLC) of biomass is a heat and power generation technology with minimal associated costs for carbon capture, potentially resulting in negative CO2 emissions. The CLC technology utilizes fluidized beds of oxygen carrier (OC) particles to separate CO2 from the combustion air. The high content of potassium and sodium compounds in biomass fuels may cause detrimental problems during thermal conversion, including agglomeration, fouling and corrosion, while also enhancing conversion processes due to their catalytic abilities. Further knowledge about processes involving these alkali metals, including their uptake and release from OC materials and the control of alkali emission, is critical for the upscaling and commercialization of biomass CLC.

The aim of this thesis is to improve the understanding of interactions between alkali compounds and OCs under conditions representative of biomass CLC. A novel technique based on temperature modulated surface ionization was developed to determine the contributions of alkali chlorides, hydroxides, and sulfates to the flux from different reactors. A novel laboratory-scale reactor was developed, facilitating continuous alkali vapor injection to a fluidized bed while monitoring the concentrations of alkali and gas in the reactor exhaust. An additional method was developed to monitor the real-time alkali release and mass loss from small, fixed bed samples, including OC particles and solid biomass.

The type of OC material is observed to play a crucial role in alkali uptake, where fluidized beds of the promising CLC materials: calcium manganite, manganese oxide, and ilmenite, exhibiting varying levels of efficiency depending on the specific gas conditions present. Ilmenite showed near complete absorption of the injected alkali, especially during reducing conditions, making it a promising option to limit alkali emissions. The alkali speciation analysis revealed that NaCl and KCl were the predominant alkali species emitted during NaCl and KCl injection, and a similar pattern was observed for alkali sulfates. Alkali hydroxide injections resulted in highly efficient alkali uptake with emissions dominated by alkali hydroxides and chlorides. The study highlights the balance between alkali absorption efficiency and fuel conversion and oxidizing efficiency of the OC materials. While ilmenite demonstrated excellent alkali uptake, manganese oxide and calcium manganite exhibited superior fuel conversion and oxidizing efficiency. In addition, ilmenite previously used in an industrial process releases alkali in both inert and oxidizing environments at high temperatures. The described development and application of new methods are concluded to open new possibilities to understand and optimize biomass CLC.