Exploring the pyrolysis of Agave species as a novel bioenergy source: Thermo-kinetics, modeling, and product composition insights

Abstract

The Agave economic chain generates a significant waste, which has potential for sustainable bioenergy through pyrolysis. However, the diversity and heterogeneity of chemical composition of Agave species may pose challenges. This study investigates the impact of species heterogeneity on the pyrolysis of three Agave species (Agave sisalana, Agave tequilana, and Agave wercklei), aiming to correlate pyrolysis and biomass properties. Solid characterization and Py-GC/MS were used to understand Agave physicochemical characteristics and organic group distribution in volatiles, respectively. A multi-step model and advanced numerical methods were employed for the kinetic study. The physicochemical characteristics showed similar values but a distinct distribution of inorganic compounds, predominantly composed of alkali and alkaline earth metals (6–11 %_w.b.), potentially influencing the organic groups’ distribution in the volatiles. High relative areas of aliphatic components (13–28 % at 773 K and 16–36 % at 873 K) and low quantity of acidic groups (<2 %) could be attributed to the catalytic deoxygenation promoted by alkali and alkaline earth metals. These findings are significant for future application of Agave in bio-oil production by pyrolysis, as commercial biomasses often yield a high content of oxygenated and acid groups. For the kinetic study, six decomposition profiles were identified in the pyrolysis, encompassing the decomposition of extractives, saponins, lignocellulose, and oxalate salts. The similarity in profiles resulted in approximately equivalent kinetic parameter values and mechanisms among the species. The average values of E_a ranged from 71 to 324 kJ mol⁻¹, k₀ values varied between 10⁷-10²², and the reaction mechanisms included n-order and Avrami-Erofeyev types. The validity of the parameters was verified through curve reconstruction. The inorganic composition was chosen as the parameter related to pyrolysis characteristics, as it was the only parameter that differed significantly among the species. Based on the data, normalization and the proposed model demonstrated satisfactory values of R² (>0.9251), QOF (>94 %), and MSE (<2.73 ×10⁻³). This underscores the model's potential to describe decomposition profiles solely based on knowledge of inorganic composition, regardless of selected Agave species.