Combustion performance of solid waste-based pellet fuel from fiber feedstock waste residues in the pulping process

Agricultural waste grasses can be reused to alleviate the shortage pressure of fiber feedstock in the pulping and papermaking industry. The pellet fuel can also be prepared with industrial solid wastes, such as fiber feedstock waste residues and pulping liquid wastes. Furthermore, sustainable and clean energy sources are required to realize resource utilization, with the ever-increasing shortage of fossil energy and the growing environmental issues. In this study, the combustion performance of the waste residues was evaluated on the different fiber feedstock from the pulping process. The waste residue-based pellet fuel of fiber feedstock was also prepared by compression molding. The granular fuel was characterized by thermogravimetry, scanning electron microscopy (SEM), X-ray diffractometer (XRD), and thermal analyzation-infrared gas generation combined instrument. A systematic investigation was made to explore the effect of pulping waste liquid on the combustion performance of pellet fuel. The results showed that the combustion performance of waste residue-based pellet fuel from grass fiber feedstock was slightly worse than that from wood fiber feedstock. The reason was that the grass fiber feedstock with a higher content of non-combustible impurities and lower fixed carbon content. There were relatively low combustion performance and short combustion time with a comprehensive combustion index of 0.92×10^-7 %²/(min²·℃³). The grass waste residue-based pellet fuels were agglomerated lime-ash and blocked after combustion, which was different from wood waste residue-based pellet fuel. The alkali metal and silicon element in the waste residue-based pellet fuel from grass fiber feedstock were reacted at high temperatures, and then produced the slagging metal silicate particles ash containing SiO₂, Na₂Si₂O₃, and K₆Fe₂O₅ metal salt oxides. The high content of non-combustible impurities of grass waste residue-based pellet fuel led to the incomplete combustion of fixed carbon, which increased the CO gas composition in the combustion emission. In order to improve the combustion performance of wheat straw waste residue-based granular fuels, the mixture of pulping waste liquid/wheat straw waste residue with a solid content of 10.28% was compressed to prepare pellet fuel. The pulping waste liquid had high output after the pulping process, including a lot of organic matter, such as lignin and hemicellulose. Moreover, the lignin and sugars can be expected to serve as the natural adhesive in the granular fuels, in order to improve the combustion performance and mechanical strength of grass waste residue-based granular fuels. Furthermore, the pulping waste liquid was added to bind the solid waste for stability. The comprehensive combustion index and combustion activation energy of granular fuels from the mixture of pulping waste liquid/wheat straw waste residue was 2.50×10^-7 %²/(min²·℃³) and 59.88 kJ/mol, respectively. Correspondingly, the combustion index increased by 171.74%, whereas, the combustion activation energy decreased by 22.72%, compared with the pure wheat straw waste residue-based pellet fuel. The combustion performance was outstandingly improved in the wheat straw waste residue-based pellet fuel. The finding can provide the theoretical reference for the high-value utilization of agricultural waste plant fiber and resource utilization of solid wastes in pulping and papermaking.