Algal biomass: occurrence of the main inorganic elements and simulation of ash interactions with bed material

The work presented here provides greater insight into the occurrence of the main inorganic elements in algae and how this varies for different species of algae and production methods. It also provides valuable insights into the behavior of algal ashes during combustion, particularly in regard to their interaction with quartz bed material, which is of direct relevance to agglomeration in fluidized bed reactors. A diverse range of algal species was selected for this work using a species of marine microalgae (Tetraselmis sp.) and marine macroalgae (Derbesia tenuissima) and a species of freshwater macroalgae (Oedogonium sp.). Two samples of Oedogonium were tested. One sample was cultivated using a standard nutrient addition regime, and the other was starved of essential nutrients at the completion of the culture cycle. Samples of algae were subject to chemical fractionation and the mode of occurrence of the main inorganic elements was inferred from their leaching behavior. Both the relative proportions of the main inorganic elements in the algae and their mode of occurrence are largely dependent on the culturing environment and harvesting history. The leaching behavior of the inorganic elements indicates a high level of inorganic, water-soluble, alkali salts in all of the tested algae. In order to simulate ash interactions with bed particles during fluidized bed combustion, pellets consisting of algal ashes mixed with quartz particles were heated in a muffle furnace at 850 degrees C in air. Scanning electron microscopy and energy dispersive spectroscopy analyses were performed on cross sections of the resultant samples. Melted ash coatings had formed on the surfaces of the quartz particles. In all cases there was no evidence of chemical reactions between the ashes and quartz particles. Melt-induced agglomeration appeared to be the principal agglomeration mechanism. The ashes from the two marine algae both formed salt-type melts. The ashes from the Oedogonium sample, which had been starved of essential nutrients during cultivation, formed a silicate-type melt. The resultant ash coating consisted of melted, Ca-rich, K-silicate. The Oedogonium sample grown under a standard nutrient addition regime had higher levels of P in its inorganic matter. The increased P content appeared to have a significant influence on the composition and adhesive behavior of the formed melt. The melt formed from the ashes of the P-rich Oedogonium sample had the greatest affinity for the quartz particles and the strongest tendency to form bridges. The resultant ash coating consisted of melted alkali silicate with discrete dispersions of alkali phosphate rich in Ca and Mg.