Influence of Potassium and Sodium Salts, Calcium and Phosphorus on Agglomeration Behaviour of Biomass and Quartz Sand under Combustion and Gasification Atmospheres

Abstract

Alkali metals (potassium and sodium) are usually present as inorganic salts or organic-associated elements in biomass and are major contributors to bed agglomeration. This thesis investigated interactions between quartz sand and wood doped with individual alkali salts, together with the effects of Ca and P on such interactions. The interactions behaviours of several agricultural residues and macroalgae with different compositions of alkali salts, Ca and P were assessed. The effect of the reaction atmosphere was also evaluated. All the tests were conducted in a lab-scale, fixed-bed reactor at 900 °C under either a steam gasification (50% v/v steam) or a combustion (5% v/v O2) atmosphere, respectively. Techniques, including scanning electron microscope in combination with energy dispersive spectroscopy (SEM-EDS), X-ray diffraction (XRD), acid digestion, water leaching and Atomic Absorption Spectrophotometer (AAS) analyses, were applied. Significant bed agglomerates were observed for wood doped with alkali carbonate, acetate or sulfate, especially during steam gasification. In contrast, the formation of alkali silicates was much lower for alkali chloride-doped wood, especially during combustion, when only insignificant agglomerates were formed. The reactions between gaseous alkali metals and quartz sand contributed to the agglomerates’ formation, especially for K salts. The formation of alkali silicates in the agglomerates decreased significantly with the increase of Ca content, while molten alkali phosphate ash formed agglomerates via a melting-induced mechanism. The co-existence of Ca and P generated K-Ca phosphates, and the influence of the K-Ca phosphates on the interaction behaviours and the K retention in agglomerates varied significantly with the K salts species. Compared with the combustion atmosphere, steam exacerbated bed agglomeration for the coating-induced mechanism but not for the melting-induced mechanism. Steam affected biomass ash-quartz sand interactions both by increasing the alkali metals’ retention and the fractions of alkali silicates with high melting points in the agglomerates and by facilitating the gas-solid phase reactions. The interaction behaviours of several agricultural residues and macroalgae were consistent with those of individual alkali salt-doped wood. A high P and low Ca content in Oedogonium intermedium (ODN) resulted in the co-existence of both melting-induced and coating-induced mechanisms. Fuel-derived molten alkali silicates formed the wheat straw agglomerates via a melting-induced mechanism. For grape marc, cotton stalk and Derbesia tenuissima (Deb), alkali metals reacted with Si in the quartz sand to form agglomerates, with K-Ca phosphates distributed within these agglomerates. Steam significantly increased the size of the grape marc and Deb agglomerates. Except for Deb during combustion, the size of the agglomerates of the tested biomass increased significantly by increasing the reaction time under both atmospheres. This thesis contributes to the comprehensive understanding of biomass ash-quartz sand interactions for various types of biomass with different inorganic compositions during steam gasification or combustion.