Environmental impact assessment of sustainable methyl stearate (biodiesel) synthesis employing fly ash supported tin oxide catalyst

Abstract

Management of Fly ash (FA), a thermal power plant waste, is a major global issue since a sizeable fraction of fly ash's annual generation is not effectively valorised. FA is a heterogeneous mix of crystalline and amorphous phases containing significant amounts of aluminium and silicon elements alongside high surface energy, making FA an economically suitable catalyst support framework. Accordingly, the development of a low-cost, recyclable fly ash-supported tin oxide solid acid catalyst has been investigated for esterification of stearic acid (SA) with methanol to produce methyl stearate (MS); which is regarded as biodiesel and is presently being blended with petro-diesel as a cleaner fuel substitute. The characterisations of the prepared SnO₂-FA catalyst (SFC) have been performed through TGA, XRD, BET-BJH and FESEM-EDS analyses. The optimal process conditions (assessed through response surface methodology (RSM)) viz. 475.06 °C calcination temperature, 3.39:1 weight ratio of SnCl₄.5H₂O: FA and 73.16 °C esterification temperature rendered a significant 85.734% SA conversion. The optimal mesoporous SFC comprising SnO₂ active phase possessed 11 m²/g specific surface area (much greater than that of the support material, FA: 0.60 m²/g); 0.0109 cc/g pore volume and 2.9 nm modal pore size. Important fuel properties of the optimally produced MS conformed to the ASTM biodiesel (B100) standards. The overall environmental sustainability of the process assessed through the openLCA platform (ecoinvent database 3.8) revealed lower environmental impacts of the developed process. The LCA study divulges the fossil depletion potential and the global warming potential of the overall process to be 4.34 kg oil Eq. and 4.03 kg CO₂-Eq. respectively. The present study could establish a green and effective FA valorisation avenue through a sustainable methyl stearate (biodiesel) production process.