Effect of initial water content on the pyrolysis mechanism of lignocellulosic biomass

Abstract

The initial water content in lignocellulosic biomass significantly influences the pyrolysis process, but the effect of the form of water, especially free water (FW) and bound water (BW), remains unclear. In this study, the pyrolytic characteristics of cellulose (CL), lignin (LG), and rice straw (RS) with different initial water contents were investigated using TG, TG-DSC, TG-MS, and in situ FTIR. The results showed that both FW and BW decreased the maximum thermal decomposition rate and increased the biochar yield. The activation energy of hemicellulose (E-HC) linearly decreased upon increasing the BW content (r = −0.90, p < 0.05). Combined two-dimensional FTIR correlation spectroscopy (2D-FTIR-COS) and TG-MS analyses showed that BW formed hydrogen bonds with O-acetyl groups in HC, accelerating its decomposition and the release of CH₃COOH at lower temperatures. In contrast, a significant positive correlation was found between BW and the activation energy of cellulose (E-CL) due to the hydrogen bond network generated between BW and CL. The 2D-FTIR-COS spectra revealed that the primary sequential water responses of functional groups during RS pyrolysis followed the order: hydroxyl –OH → carboxyl C = O → aliphatic C–H → carbohydrate C–O–C → aromatic rings. This order shows that water promoted biochar formation, especially for biomass with higher lignin contents. The establishment of this relationship between water and pyrolysis products provides an important basis for regulating the water content during biomass pyrolysis.

Graphical Abstract

Keywords

Lignocellulosic biomass / Pyrolysis / Bound water / Free water / Biochar yield

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Funding

National Natural Science Foundation of China(42207286)

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