LAUSR

Abstract The interactions between ash and quartz sand as the bed material were studied for two species of macroalgae, Oedogonium intermedium (a freshwater species) and Derbesia tenuissima (a marine species). The interaction tests were performed in a lab-scale, fixed-bed reactor under a steam gasification (50% v/v steam) or a combustion (5% v/v O2) atmosphere at 900 °C for different reaction times (10 min to 6 h). For both macroalgae, some of the Na and K are found to interact with other elements, such as Mg, Si, P, Ca and Fe to form ash particles. For ODN, these low melting-point phosphorus-rich ash particles are present in sufficient quantities to adhere to the quartz sand and form agglomerates via a melting-induced mechanism. In addition, some of the Na and K react with Si from the quartz sand to form a coating layer, which promotes a coating-induced mechanism. Both mechanisms contribute to a similar extent for ODN; while the coating-induced mechanism dominates for Deb, which includes the diffusion of some Ca into the coating layer. For Deb, the steam gasification atmosphere significantly affects the size of the agglomerates, the mass fractions of K and Na retained in the agglomerates and the formation of various types of silicates in the agglomerates, when compared with those under the combustion atmosphere. However, the influence of the steam gasification atmosphere on both the morphology and the elemental composition of the ODN agglomerates is less obvious. For Deb, a high content of S is detected for the individual separate ash particles within the agglomerates during combustion, whereas negligible S is found during steam gasification.