Iron oxide reduction by torrefied microalgae for CO2 capture and abatement in chemical-looping combustion

Microalgae are considered as an advanced renewable feedstock with high photosynthetic efficiency and fast growth rates. Torrefaction enhances the carbon content in microalgae biomass which serves as an ideal reductant in iron oxide reduction enabling CO2 capture and abatement in chemical-looping combustion. Two microalgae species of Chlamydomonas sp. and Chlorella vulgaris were torrefied and evaluated for iron oxide reduction at varying ratios with hematite. Through simultaneous thermogravimetric analysis (TGA) and Fourier transforms infrared spectrometry (FTIR), the reduction behaviors of iron oxides by torrefied microalgae were characterized in terms of TGA curves and evolved gases. The occurrence of reduction was recognized by introducing the reduction degree. The results revealed that the iron oxide performances by the two torrefied microalgae as reducing agents were different from each other. For the blended hematite and the torrefied C. sp, the reduction degree for 1:1 ratio was higher compared to the 2:1 ratio at a temperature range of 300–400 °C then the 2:1 prevailed for temperatures of 400–1200 °C. For the mixed hematite and the torrefied C. vulgaris, the 2:1 ratio was higher compared to the 1:1 ratio at a temperature range of 300–950 °C then switches to 1:1 ratio for temperatures of 950–1200 °C. Using torrefied microalgae as reductants for ironmaking revealed a stepwise iron oxide reduction from hematite to magnetite (Fe2O3 → Fe3O4), magnetite to wustite (Fe3O4 → FeO), wustite to metallic iron (FeO → Fe). Torrefied microalgae yielded a lower onset reduction temperature compared to other carbon sources.