Biomass to biofuel densification of coconut fibers: kinetic triplet and thermodynamic evaluation

Abstract

This paper aims to investigate coconut fiber’s thermal behavior and evaluate its potential energy production through kinetic and thermodynamic studies, as well as the potential to produce solid biofuels (briquettes). Structural chemical analysis, proximate analysis, and higher heating value characterized coconut fiber. The thermogravimetric experiments were carried out in an inert atmosphere (N2), varying the heating rates at 5, 10, 15, and 20 K min−1. The kinetic triplet was determined using isoconversional methods and master plot methodology. The pre-exponential factor, enthalpy, entropy, and Gibbs free energy parameters were calculated. The briquettes were made by different particle sizes: mixed particles (without granulometric classification); particles between 0.35 mm and 0.25 mm, and particles lower than 0.25 mm. The coconut fiber briquettes were produced in a compaction system at a temperature of 393.15 K under 15 MPa pressure for 20 min. This study also determined the apparent density, the resistance to diametral compression, and the energy density for coconut fiber briquettes. The pyrolysis reaction was modeled considering the reaction mechanism of as the three-dimensional Jader equation, with global activation energy 129.8 kJ mol−1 and global pre-exponential factor 2.68 × 107 s−1. The enthalpy and entropy values have shown considerable variations due to the conversion, suggesting that the pyrolysis of coconut fiber involves complex reaction mechanisms. The briquetting process enhanced the coconut fibers, and the results have shown that the lower particle size (particles ≤ 0.25 mm) presented better physical–mechanical properties and energy density. It is concluded that coconut fiber has the potential to be turned into biofuels from the thermochemical processes and may be enhanced by the densification process.