Release of Cl, S, P, K, and Na during thermal conversion of algal biomass

Abstract

The release of volatile inorganic elements during thermal conversion of algal biomass may cause operational problems in industrial reactors such as fouling, deposition, corrosion, and bed agglomeration. The release of Cl, S, P, K, and Na during the thermal conversion of algal biomass has been experimentally investigated in this paper. A freshwater macroalga (Oedogonium sp.), a freshwater polyculture of microalgae, and a marine microalga (Tetraselmis sp.) were used as feedstocks to assess the influence of different species and culturing environments on the release of the inorganic elements. Char and ash samples were prepared in a laboratory-scale fixed-bed reactor under isothermal conditions ranging from 500 to 1100 °C, under pyrolysis, combustion, and gasification atmospheres. The release of the inorganic elements was quantified by mass balances based on elemental analyses of the char and ash residues. Differences in the release of Cl, S, K, and Na were significant between the marine alga and the freshwater algae but were only minor between the freshwater microalgae and the freshwater macroalga. In the freshwater algae, the majority of the total Cl was released at low temperatures, below 500 °C. The majority of the fuel-S was also released at low temperatures. The remaining S was released with increasing temperature during combustion and gasification but was partially retained in the char during pyrolysis. Retentions of K and Na in the char and ash residues were both relatively high. Only 20–35% of the total K and 35–50% of the total Na in the freshwater algae had been released by 1100 °C. In the marine alga, only around 20% of the total Cl was released below 500 °C with the majority being released above 850 °C. The fraction of S released at low temperatures was also lower when compared to the freshwater algae. Potassium and Na were more or less completely released from the marine alga by 1100 °C under pyrolysis and combustion conditions and by 1000 °C under gasification conditions. The release behavior of P was similar for all of the tested algae. Phosphorus release commenced around 850 °C under pyrolysis, combustion, and gasification atmospheres, and by 1100 °C, 40–70% of the total P had been released from the algae. Select ash residues were analyzed using X-ray diffraction and scanning electron microscopy, coupled with energy-dispersive spectroscopy, in order to gain insights into the release mechanisms. A two-step mechanism has been proposed for S release. The alkali metals and Cl were released proportionally from the marine alga but disproportionately from the freshwater algae. It is expected that K, Na, and Cl were released from the marine alga primarily by sublimation of alkali chlorides. Different mechanisms were responsible for the release of these elements from the freshwater algae. It has been suggested that Cl is dissociated from the alkali metals and then released as HCl vapor, and that K may be released by volatilization of melted K-phosphates. Sodium appeared to be released by similar mechanisms to K above 700 °C. The mechanisms governing the release of P are not entirely clear and require further investigation.