Combustion Kinetics of Natural Polymers in Lemon-Tree Pruning Wood at High Heating Rates: Criado Master Plots and Šesták–Berggren Modelling

Lemon-tree pruning wood is a lignocellulosic natural-polymer solid whose cell wall is built from cellulose, hemicellulose and lignin. This study evaluates the use of this polymer matrix as a solid biofuel by combining physicochemical characterization with non-isothermal oxidative conversion at 20–80 K min−1. Lemon-tree branch (LTB) from orchards in Nubaria, Egypt, was analyzed for proximate and ultimate composition, Van Soest fiber fractions, HHV, FTIR, XRD and SEM–EDX. Thermogravimetric combustion was carried out in air at 20, 40, 60 and 80 K min−1. Apparent activation energies were obtained by five isoconversional methods (FR, FWO, KAS, STK, VY) and by Kissinger analysis of the two dry-fuel DTG peaks. Reaction models were screened by Criado master plots, a single-event Šesták–Berggren fit and Coats–Redfern. LTB is a moderately crystalline xylem (cellulose 52.11 wt%, CrI 52.6%, lignin 17.11 wt%) with moisture 7.10 wt%, volatile matter 77.80 wt%, fixed carbon 19.90 wt%, ash 2.30 wt% and HHV 18.50 MJ kg−1. Nitrogen and sulfur are 0.55 and 0.18 wt%. Combustion proceeds in three stages: Stage 1 moisture release, Stage 2 holocellulose-dominated oxidative devolatilization (65–68 wt% loss, Tp1 ≈ 600–640 K) and Stage 3 char oxidation (≈25 wt% loss, Tp2 ≈ 710–730 K). Mean isoconversional Ea on α = 0.1–0.7 is 98–115 kJ mol−1 and rises to ~212 kJ mol−1 at α = 0.8. Criado plots follow F2/F3/D3 up to α ≈ 0.5; a Šesták–Berggren function with m = 0.174 and n = 2.3 describes the Stage 2 window (R2 = 0.890) as an empirical fit only. Coats–Redfern selects F2 for Stage 2. ΔH tracks Ea, ΔG is positive, and ΔS is negative on Stage 2. LTB is a low-ash woody pruning fuel suitable for pellets or co-firing if Stage 3 is completed above ~750 K.