(2024) Applied Energy_Fabrication of red mud-carbon composite from extremophilic microalgae and its utilisation in biodiesel production

Choi D.; Kim M.; Kim S.; Lee D.; Tsang Y.F.; Park W.-K.; Kwon E.E.

(Elsevier Ltd) Applied Energy ISSN: 3062619 Vol.372 Issue. Article No.123837 DOI: 10.1016/j.apenergy.2024.123837

Superior carbon fixation rate of microalgae compared to terrestrial biomass offers a significant opportunity to build a reliable carbon supply chain for carbon source. Extremophilic microalgae, Galdieria sulphuraria (G. sulphuraria), is further underscored by its exceptional adaptability to harsh environments such as wastewater. This study proposed a strategy to efficiently utilize carbon in G. sulphuraria and to valorise metallic industrial waste, particularly red mud (RM), through co-pyrolysis. To improve the eco-friendliness, CO₂ was used as a reactive gas medium in pyrolysis system. Use of CO₂ in single-stage pyrolysis of G. sulphuraria leads to the enhanced formation of CO through gas-phase reactions (GPRs) of it with volatile matter (VM). Specifically, CO₂ partially oxidize VM while reduce into CO. From single-stage co-pyrolysis of G. sulphuraria with RM, RM accelerated the reaction kinetics for GPRs between CO₂ and VM, leading to the increased formation of syngas from 27.5 to 37.6 mmol compared to N₂ environment. From multi-stage co-pyrolysis, an additional heat source (700 °C) facilitated the GPRs induced by CO₂, increasing evolution syngas from 37.6 to 73.8 mmol compared to single-stage co-pyrolysis under CO₂ environment. RM‑carbon composites (RMCs) from co-pyrolysis of G. sulphuraria with RM under N₂/CO₂ environments was utilized as a catalyst in thermally induced transesterification. Both RMCs exhibited superior performance over commercial silica. In specific, both RMCs resulted in higher biodiesel yields of >95.0 wt% at 300 °C, compared to commercial silica (biodiesel yield of 16.6 wt% at corresponding reaction temperature). Such performance of RMCs in thermally induced transesterification could be improved by the modification of their surface properties but its related study was not conducted in this stage. A series of experimental findings propose that co-pyrolysis of G. sulphuraria and RM creates opportunities to maximise syngas production and fabricate red mud‑carbon composites to enhance biodiesel conversion efficiency. © 2023

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean Government (MSIT) (Grant No. NRF-2023R1A2C3003011).