Fabrication of red mud-carbon composite from extremophilic microalgae and its utilisation in biodiesel production

Abstract

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.