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.