Comparison of the thermochemical conversion of spruce wood and tomato plant residues: characterization of the pyrolysis process through the coupling of TGA-based kinetic analyses and pilot-scale auger reactor tests

The present work compares the pyrolysis of spruce wood (SW) and tomato plant residues (TPR), primarily composed of stems and leaves. To that end, thermogravimetric analyses (TGAs) were first performed to evaluate kinetic parameters accounting for the pyrolysis behavior of both these feedstocks. Measurements were carried out using 4 heating rates: 5, 10, 15, and 30 K min−1. The results obtained were then processed by means of 3 isoconversional methods (namely Kissinger–Akahira–Sunose (KAS), Flynn–Wall–Ozawa (OFW), and Friedman). Based on the obtained rate constant parameters, the variation of the conversion degree of the fuels as a function of the temperature was simulated considering different reaction models (e.g., order-based, diffusion, geometrical, nucleation, and power law). The results obtained showed that the three isoconversional approaches allowed to properly reproduce the TGA results provided order-based models are selected. SW and TPR were, moreover, shown to exhibit quite distinct pyrolysis behaviors due to their widely varying biopolymer and mineral contents. Specifically, it was observed that the conversion of TPR is fostered as compared to that of SW, for temperatures below ⁓650 K (as corroborated by the lower activation energies $${E}_{\text{a}}$$ E a inferred in the case of TPR), contrary to what is observed for higher temperatures for which the decomposition of SW is promoted ( $${E}_{\text{a}}$$ E a values becoming lower for SW than for TPR). A series of experiments were then conducted in an auger reactor by setting two reaction temperatures (400 and 600 °C) and two solid residence times (75 and 125 s) to assess the influence of these key operating parameters on the properties of obtained products. The elemental composition and mineral contents of the collected biochar were characterized ex situ, while the syngas composition (CO, CO2, H2, CH4, and CnHm) was monitored online during the tests. Regarding the bio-oils, their elemental composition, water content, pH, density, higher heating value, and sooting propensity were determined, while Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) was used to gain insights into their oligomeric composition. Overall, the results revealed the pyrolysis temperature to be the main factor influencing the product yields for both feedstocks, with the solid residence time only exhibiting a minor effect. TPR pyrolysis produced higher biochar and aqueous phase bio-oil yields, likely due to the high ash content and the catalytic effect of inherent alkali and alkaline earth metals (AAEMs) promoting dehydration reactions. TPR biochars exhibit a high ash content and an O/C ratio < 0.2 at 400 °C, hence suggesting a strong potential for long-term carbon sequestration. The oily phases from TPR showed a high water content (> 80%), a basic pH (8.8–9.4), and a lower sooting tendency as compared to the SW oily phases. FT-ICR MS analyses revealed that TPR bio-oils are characterized by lower molecular weights and fewer oxygenated organic species than the SW ones, across all ionization modes, reflecting their distinct biochemical composition and the catalytic effect of AAEMs. TPR syngas contained higher concentrations of CO2 and H2, likely due to its higher AAEM content, which promotes the water–gas shift reaction, while SW syngas contained more CO and CH4.