Behaviour and Interaction of Calcium and Potassium during Catalytic Gasification
Date
2019-04-29
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Abstract
Gasification is a technique for the conversion of carbon sources such as biomass into syngas. Catalysts reduce the necessary gasification temperature and increase the reaction rate. Potassium is more active than calcium, but calcium addition has been shown to promote the rate of potassium-catalyzed gasification. The mechanism by which this promotion occurs was not well-understood. This thesis used switchgrass and biosolids as carbon feeds, as well as ash-free carbon black to isolate the effects of potassium and calcium. Calcium was found to promote potassium both indirectly and directly. Indirectly, calcium served as a sacrificial species, reacting with aluminosilicates, limiting the sites on which potassium could deactivate by forming catalytically inactive potassium aluminosilicates. X-ray diffraction demonstrated calcium aluminosilicate formation. Directly, calcium carbonate formed a low-melting eutectic phase with potassium carbonate above 820 °C, demonstrated by differential scanning calorimetry and scanning electron microscopy. The eutectic increased the diffusivity of potassium, facilitating the movement of potassium between active carbon sites, increasing the gasification rate below 40% conversion. Above 50% conversion, the eutectic phase inhibited the gasification rate by hindering CO2 diffusion to the carbon. Activation energy calculations showed that diffusion became the rate-determining step at higher conversions. Two mixing methods, hand-mixing and ball-milling, were compared as to their effect on potassium- and calcium-catalyzed gasification. The diffusivity of potassium was high enough that increased dispersion by ball-milling with carbon black did not increase the reaction rate compared to ball-milling the components separately. Ball-milling calcium and carbon black together greatly increased the gasification rate when compared to ball-milling the components separately. The lower diffusivity of calcium compared to potassium explained its lower activity. As the reaction progressed, calcium sintered and hindered CO2 access to the carbon, which reduced the reaction rate even below uncatalyzed carbon black. Reducing the calcium particle size prior to gasification minimized inhibition in addition to increasing catalytic surface area. The experimental results helped to better understand the individual catalytic behaviours of calcium and potassium during gasification, as well as their interactions. The results will help in the design of gasifiers for carbon feeds containing potassium and calcium.
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Keywords
gasification, catalysis
Citation
Arnold, R. A. (2019). Behaviour and Interaction of Calcium and Potassium during Catalytic Gasification (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.