Improvement of the pellet quality and fuel characteristics of agricultural residues through mild hydrothermal treatment

Abstract

Agricultural residues such as straw and grasses lack lignin and other natural binders, which make them difficult to produce durable pellets. Hydrothermal treatment modifies the complex hemicellulose, cellulose and lignin structure, which holds promise for making durable pellets from crop residues. However, most hydrothermal treatment applies mild temperature (180−260 °C), which is an energy-intensive process. In this research, mild hydrothermal treatment was applied to pine woodchip, wheat straw, timothy hay, and canola straw prior to pelletization. The mild hydrothermal treatment consisted of exposing the ground biomass to 135℃ for 90 min in an autoclave unit. The results show that mild hydrothermal treatment is a promising way to improve agricultural pellet quality and fuel properties. After treatment, the durability and hardness of all pellets increased, especially for canola straw (from 2.21 N/mm2 to 4.67 N/mm2 and 83.42%–95.41% respectively). The mass density of woodchip, wheat straw, timothy hay and canola pellets increased by 2.89 %, 5.85 %, 3.69 %, and 9.21 %. and the volumetric energy density increased by 8.04 %, 7.46 %, 8.34 % and 12.59 %, respectively. The elevated devolatilization index and decreased activation energy show volatile matters of all pellets are easier to release during pyrolysis. Thus, mild hydrothermal treatment is a clean and economical method for improving agricultural pellets quality and the heat applications. It has the potential to be applied in industrial pellet manufacture.