Characterization of products derived from the high temperature flash pyrolysis of microalgae and rice hulls

Abstract

Flash heating and high temperature processing of two biomass feed materials is presented as a sustainable thermochemical technique to leverage the entire carbon content of the starting material for various end-uses. Microalgae and rice hulls were flash pyrolyzed in a drop-tube reactor to form exclusively gas and solid products in the temperature range 1573–1873 K at gas residence times of 3–5 s. Product gas yields and compositions approached thermodynamic equilibrium predictions with increasing temperature, which were used in calculations for a conceptual solarthermal processing framework. The derived carbonaceous residue was further characterized for composition (atomic mass and crystalline structures), morphology (surface area, pore volume, surface roughness), and reactivity (via CO2 thermogravimetry). Solid residues generated under flash conditions tended to become nearly indistinguishable, becoming spherical with high macroporosities and similar apparent reactivities. These results were contrasted with slow-pyrolyzed chars of the same starting materials, whose characteristics were markedly opposite for microalgae and rice hulls, further evidencing the importance of char particle thermal history. Results from solarthermal operation show a high calorific value syngas, especially for microalgae at 1873 K. The applicability of carbonaceous residue in a particular downstream use requires further characterization and study.