Pyrolysis behavior and kinetic mechanisms of garden waste plant leaves

Garden wastes have posed significant challenges to waste management and environmental sustainability. One type of thermochemical conversion, the pyrolysis, can transform biomass into biochar, bio-oil, and syngas. This study aimed to explore the high-value utilization of garden wastes using pyrolysis. Three typical garden plant leaves—coniferous leaves (Pinus sylvestris Linn. var. mongolica Litv.), broad leaves (Amygdalus triloba (Lindl.) Ricker), and herbaceous plants (Setaria viridis (L.) P. Beauv.)—were systematically investigated using thermogravimetric analysis (TGA). A kinetic model was established to determine the effects of leaf type, particle size (0.053-0.250 mm), and heating rate (10–20 °C/min) on the pyrolysis process. The TGA experiments were conducted under a nitrogen atmosphere for an inert environment, with the temperature ranging from room temperature to 800 °C. The heating rates were varied at 10, 15, and 20°C/min. The particle sizes of the samples were classified into three ranges (0.053-0.15, 0.15-0.25, and 0.053-0.25 mm). Their influence on the pyrolysis efficiency and residual amounts was examined after experiments. The results show that the pyrolysis processes of the three types of leaves all exhibited four-stage characteristics, namely the dehydration, slight weight loss, rapid pyrolysis, and carbonization stages. The optimal pyrolysis conditions were determined for the coniferous leaves, broad-leaved leaves, and herbaceous plants. In coniferous leaves, the optimal particle size and heating rate were 0.053-0.15 mm and 10 °C/min, respectively. In broad-leaved leaves, the optimal conditions were 0.15-0.25 mm and 10 °C/min. In herbaceous plants, the optimal conditions were 0.15-0.25 mm and 15 °C/min. The final residues were ranked in descending order of the coniferous leaves, herbaceous plants, and broad-leaved leaves under these optimal conditions. The three-element analyses showed that there were some differences in the contents of cellulose, hemicellulose, and lignin in the leaves. Woody plants with a higher lignin content shared the higher thermal stability and thus a higher residue amount. In contrast, the herbaceous plants with a lower lignin content and a higher cellulose content were more easily decomposed, indicating the lower residue amount. The pyrolysis kinetic analysis was conducted using the Coats - Redfern integral method. The reaction activation energies of the coniferous, broad-leaved, and herbaceous leaves were 15 837.41, 12 271.75, and 8 676.48 kJ·mol^-1, respectively. The pyrolysis difficulty levels were ranked in descending order of the coniferous leaves, broad-leaved leaves, and herbaceous leaves, which was consistent with the thermogravimetric analysis curves. This finding can provide the key process parameters for the directional pyrolysis conversion of garden wastes. It is also significant to optimize the biomass char preparation and the resource utilization of garden wastes.