Microwave vacuum catalytic co-pyrolysis of coconut shell and millet residues: parameters optimization and high-quality biofuel production

Abstract

Waste-to-energy offers a promising solution to address energy shortages while simultaneously reducing environmental pollution. The present study aimed to enhance the biofuel production by microwave vacuum co-pyrolysis of high-lignin biomass, i.e., coconut shells (CS), and high-hemicellulose biomass, i.e., millet residues (MR). Their complementary compositions enhance synergistic effects, improving product quality and process efficiency compared to individual pyrolysis. Also, bentonite was utilized as a low-cost catalyst, and the impact of vacuum and N₂ pyrolysis environments was compared. Firstly, multi-factor optimization, specifically co-feeding ratio, catalyst ratio, and pyrolysis environment, was performed to maximize bio-oil yield and its heating value using response surface methodology. Then, the intensive effect of the study variables on the quantity and quality of pyrolytic products has been investigated and assessed. The optimal pyrolysis condition was found to be a 1:3 C S:MR co-feeding ratio, 15 % catalyst ratio, and under vacuum pyrolysis environment. The results revealed that the bentonite catalyst promotes heating by increasing the heating rate and reaction temperature by 33 % and 26 % respectively, compared to the non-catalytic condition. Also, CS produces a high biochar yield, while MR results in high bio-oil and gas yields. Mixing CS/MR in a ratio of 1:3 and using bentonite enhanced the quality of the products under vacuum environment. Whereas higher hydrocarbon (27.5 %) bio-oil, low-ash (7 %) and high heating value (22.7 MJ/kg) biochar, and H₂-rich gas have been achieved. This promoted the net energy recovery, reaching a maximum value of 77 %.