Eggshell-derived CaO catalyst for efficient conversion of waste cooking oil to fatty acid ethyl esters: characterization and engine performance study

A sustainable waste-to-energy strategy has been developed for the production of fatty acid ethyl esters (FAEE) from waste cooking oil using a heterogeneous CaO catalyst derived from discarded chicken eggshells. The process combines dual waste valorization with enhanced renewable energy production by employing ethanol instead of methanol. Catalyst characterization revealed that a hydration–dehydration treatment generated a highly porous CaO structure, with Ca–O vibrational bands observed at 598 and 545 cm⁻¹, and a maximum basicity of 4.23 mmol g⁻¹ achieved after calcination at 900 °C for 3 h. Optimization using response surface methodology resulted in a maximum FAEE yield of 94.86% at 77 °C, with an ethanol-to-oil molar ratio of 14.65:1 and a catalyst loading of 7.42 wt%. The activation energy was determined to be 94.73 kJ mol⁻¹. The produced FAEE met ASTM D6751 and EN 14,214 specifications, reducing free fatty acid content from 1.07% to 0.11% and exhibiting improved oxidative stability with 500 ppm of tert-butylhydroquinone (TBHQ). Furthermore, engine performance testing of FAEE-diesel blends (B5–B20) demonstrated comparable performance to conventional diesel fuel. At 40% load, brake power increased from 1.95 kW (B0) to 2.00 kW (B20), while brake thermal efficiency decreased slightly from 19.2% to 18.2%, and brake-specific fuel consumption rose from 0.51 to 0.59 kg/kWh. At full load, B20 reduced emissions of CO₂, CO, and particulate matter by 6.7%, 19.0%, and 40.9%, respectively, with a modest increase in NOx emissions. These findings confirm the strong potential of eggshell-derived CaO catalysts for sustainable biodiesel production.