Thermo-catalytic transformation of indigenous agricultural waste into chemically diverse bio-oil: process refined through response surface optimization

Agriculture-centric economies worldwide including India grapple with the challenge of sustainable management of agro-residues. Thermochemical transformation of the agro-residues is one of the most prominent technologies adapted globally for their upcycling. This manuscript presents one such rigorously investigated alternative for the efficient transformation of agro-residues into potential products. The current study presents optimized processes of catalytic pyrolysis (CP) and catalytic co-pyrolysis (CCP) of the rice straw (RS) using metal salt catalysts (CuCl₂, MgCl₂). These processes were optimized by a response surface mechanism (RSM) following a Box-Behnken design (BBD). The non-catalytic pyrolysis (NCP) was also optimized by selecting N₂ flow rate, process temperature, and reaction time as the independent parameters. The maximum bio-oil formation of 22.93% was recorded at 4 mL/min of N₂ flow, 723.15 K, and 45 min. Reaction temperature, time, and catalyst loading were discerned as the key parameters for the optimization of CP and CCP. The CuCl₂-catalyzed pyrolysis (CP-CuCl₂) resulted in a maximum bio-oil yield of 46.08% while it was 48% for MgCl₂ (CP-MgCl₂). Inclusion of LDPE as co-feed to the RS in equal mass proportions is catalyzed by MgCl₂ (CCP: MgCl₂ + LDPE), resulting in the highest bio-oil formation with a 62.40% yield. The optimum conditions obtained are 723.15 K, 45 min, and 10% catalyst loading. Quadratic models were developed to correlate the process parameters with the yield of bio-oil. ANOVA confirmed the statistical significance of these models with F-test, p-values, along with R² values. Integration of a catalytic component into the pyrolytic approach transformed RS into an enriched bio-oil with optimized process conditions and enhanced yield.