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The goal of the current study is to investigate the thermal degradation of palm fronds (PF), olive leaves (OL), and wheat straw (WS) through pyrolysis and calculate their kinetic data using TG-DTG and DTA approaches. The kinetic parameters were assessed using isoconversional techniques like the Ozawa-Flynn-Wall (OFW) and Kissinger–Akahira–Sunose (KAS) methods, as well as model-fitting techniques like the integral method, which employs various diffusion and reaction order models. Using kinetics data models, typical parameters for pyrolysis and thermodynamics were estimated. For PF, OL, and WS, the values of activation energy (E) from the integral method ranged between 8.82 and 167.13, 23.06 and 149.20, and 11.01 and 156.27, respectively, for diffusion models. On the other hand, the values of (E) ranged between 22.3 and 117.49, 51.69 and 92.88, and 23.48 and 125.97, respectively, for reaction-order models. The average activation energies (E) calculated by using PF, OL, and WS samples are 91.9, 69.1, and 65.2, respectively, for the OFW method and 87.5, 101.8, and 63.4, respectively, for the KAS method. The results demonstrated that the integral method provided values of (E) that were almost identical to those produced by the KAS and OFW methods. In the same range of (α), results showed that reaction order models yielded greater frequency factor values than diffusion models, demonstrating how simpler and quicker pyrolysis is. The values of ($${\Delta \mathrm{G}}_{av}$$ Δ G av ) demonstrated the acceptability of these materials for pyrolysis, and for the OFW and KAS techniques, the sequence of the degradation process was OL > WS > PF. The calculated ($${\Delta \mathrm{G}}_{av}$$ Δ G av ) showed that more heat energies are required for OL, PF, and WS to dissociate the reagent bonds, which agrees with the (E) values derived from the OFW model. Graphical Abstract

Nowadays, thermal energy production from solid fuels as biomass materials focused on the densified biofuels to overcome the disadvantages of raw biomass as irregular shape, low bulk density, and high moisture content. The quality of biomass pellets is sensitive to the pelletization conditions. Consequently, in this work the impact of the operating parameters as applied pressure, die temperature, and moisture content on the physical properties of solid and hollow pellets as pellet length (Lp), normal volume expansion (Vexp, n), initial and relaxed pellet densities (ρp, i and ρp, rel), moisture content of pellet (Mp), water absorption rate (k), and absorption volume expansion (Vexp, ab) of rice straw (RS) pellets was studied. Furthermore, the synergic effect of these parameters on the pellet properties was evaluated at the optimum conditions. The operating parameters were optimized using a multi-objective optimization approach of response surface method (RSM) based on the predicted significant models that describe the physical properties of the pellets. The optimization results showed that high-quality pellets could be produced at a pressure of 72.76 MPa, temperature of 110°C, and moisture of 7.23% for solid pellets and 81.5 MPa, 109°C, and 7.72% for hollow pellets. Significant individual and interaction effects of all independent parameters on the pellets properties were investigated using statistical analysis results, main plot curves, and 2D interaction contour plots. New four significant empirical dimensionless relationships that correlate the properties of pellets were proposed for calculating ρp, rel, Mp, k, and Vexp, ab directly based on other physical and mechanical properties and indirectly based on the operating parameters through the predicted models for different properties. These relationships can be generalized to any type of pellets that are produced under various operating conditions. Hollow pellets are recommended being used in the industrial sector due to their enhanced heat transfer which resulted from the high surface to volume ratio besides their ease of production at normal operating condition similar to that required for solid pellets.

The combustion behavior, chemical kinetics, and thermodynamic parameters of Poinciana cover and Poinciana seeds were investigated using TGA at different heating rates in the air atmosphere. Kinetic characteristic parameters were estimated by adopting five traditional and parallel kinetic models besides the DTA technique. Functional group properties using FTIR were presented and discussed. The physicochemical characteristics of these materials assured their bioenergy potential for conversion into valuable, sustainable, and renewable energy resources. The activation energy values calculated from DTA measurements for Poinciana cover and seeds were found to be in good agreement with the kinetic data obtained by parallel methods using TGA data. The significant variations of the values of the pre-exponential factor of the Poinciana cover confirm the presence of a complex multi-step reaction during its combustion. DTG profiles of the Poinciana cover showed one sharp peak with a high mass loss rate value in a short time required high energy and resulted in high activation energy values. Also, the high ash content of this material resisted the oxygen diffusion and delayed the ignition which in turn played a key role in raising the activation energy values. The higher volatile content of Poinciana seeds and its high IR absorbance helped in lowering the activation energy values obtained from various models. Advanced ignition and delayed burnout give the Poinciana seeds superiority over Poinciana cover as a new alternative source of energy. The Gibbs free energy (ΔG) values of Poinciana seed material are higher than those of Poinciana cover either for the total conversion process of the volatiles and char conversion. This result reflects the bioenergy potential of Poinciana seeds through oxidation. The resulted high entropy change ( $$\Delta S$$ ) values for the poinciana cover material ensure that this material is far from its thermodynamic equilibrium and its reactivity is high.