Effects of feedstock particle size and moisture content on the density and mechanical durability of camphor leaf pellet fuel

Abstract: Thermo-briquetting of biomass fuels has been a commonly-used forming process in recent years. Biomass fuels can be prepared by mechanical pressurization and heating, due to the high thermal efficiency and bulk density. Among them, the hot pressing of biomass pellets cannot fully meet the large-scale production so far. This study aims to explore the effects of moisture content and particle size on the density and mechanical durability of the formed fuel in the briquetting environment using hot pressing. Taking the camphor leaf as the raw material, the specific mechanism was also proposed to determine the correlation between the moisture content and particle size in the press molding, together with the density and mechanical durability of the formed particles. The main research was summarized as follows. Firstly, the theoretical basis of the particle pressing was explained using scanning electron microscopy (SEM) images. Secondly, a single-factor optimization was made to clarify the effects of raw material moisture content and particle size on the density and mechanical durability of the formed fuel during the hot pressing of camphor leaves under limited temperature and pressure. Finally, an analysis was conducted to determine the correlation between density and mechanical durability. The results showed that: 1) A spreading state was observed in the cross-section leaves of camphor leaf molding pellets, indicating a lamellar structure of sheets. The solid-bridge structure of hot-pressed leaves with particle diameters of 0.6-1 mm presented denser connections, more complex structures, and smaller gaps, compared with 1-2 mm leaves. 2) The fuel density of camphor leaf pellets was positively correlated with mechanical durability. Specifically, the mechanical durability increased significantly, as the density increased. 3) The moisture content and pellet size dominated the density and mechanical durability of hot-pressed pellets for the camphor leaf raw materials. The increase in particle size greatly contributed to molding performance, particularly in the maximum and minimum moisture content. Furthermore, the density of the formed pellets increased and then decreased, whereas, the mechanical durability continued to decrease, as the moisture content increased from 3%-18%. By contrast, there was an outstanding increase in the density and mechanical durability of the formed fuel, as the particle size decreased from 1-2 mm to less than 0.6mm. Moreover, there was a more dense overall distribution of the pellets under the decreased moisture. An optimal combination was achieved under the holding time of 60 s: the moisture content was 3%-9%, indicating each Mesh size of the raw material production of particles with a more ideal density and mechanical durability. Once the particle size of raw material was less than 0.6 mm, the pellets were better formed with moisture content in the range of 3%-15%. Alternatively, better density and mechanical durability were achieved in the raw material with a particle size of 0.6-2 mm and moisture content of 3%-6%. The orthogonal test verified that there was a significant interaction between the moisture content and particle size on the density and mechanical durability. The optimal parameters were determined for the pressing process of camphor leaf pellet fuel. Among them, the optimal parameters for the molding density process were the water content of 6%, and the particle size of 0.6-1 mm, whereas, those for the mechanical durability were the water content of 3%, and the particle size of 0-0.6 mm, under the conditions of ambient temperature 120 ℃, pressure 110.14 MPa, and holding time 60 s.