Waste-to-energy: Kinetic and carbonation-decarbonation studies of solar calcined eggshells and their application as a catalyst in the pyrolysis of brewer’s spent grain

Brewer's spent grain (BSG) was subjected to pyrolysis using calcium oxide (CaO) derived from eggshells calcined via concentrated solar energy (Fresnel lens), offering an environmentally benign and low-carbon route for catalyst synthesis. A kinetic study using the Kissinger-Akahira-Sunose (KAS) and Flynn-Wall-Ozawa (FWO) methods determined the activation energy ranging from 284 kJ/mol for BSG to 176 kJ/mol for BSG/EG, indicating that CaO reduces the energy barrier for thermal decomposition. CO₂ capture performance of calcined eggshells was evaluated through multiple carbonation-regeneration cycles, showing reduced sorption capacity, mainly due to the sintering effect, yet maintaining acceptable cyclic stability for reuse. Despite having lower efficiency than commercial CaO, eggshell-derived material represents a sustainable and cost-effective strategy for CO₂ mitigation and catalytic applications. Central composite design (CCD) was used to evaluate the influence of pyrolysis temperature and catalyst loading on vapor composition. Highest hydrocarbon yields (∼50 %) were obtained at temperatures of 750 and 791 °C, using catalyst mass fractions of 74 % and 50 %, respectively. Lower relative yields occurred at 501 °C or the lowest catalyst loading amount (16 %). Catalytic tests at 550 °C produced similar hydrocarbon profiles to non-catalytic tests at 750 °C, indicating that the catalyst reduced the activation energy of the reaction. Increasing temperature and catalyst loading promoted deoxygenation, reducing multi-oxygenated compounds and enriching mono-oxygenated species, upgrading vapor quality. Overall, integrating solar-driven calcination with catalytic pyrolysis establishes a closed-loop waste-to-energy route that converts BSG into hydrocarbon-rich products while recycling eggshell waste into multifunctional CaO, advancing the circular economy and sustainable energy generation.