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BioResources
Chen, Z., Yu, G., Yuan, X., Wang, Q., and Kan, J. (2015). "Improving the conventional pelletization process to save energy during biomass densification," BioRes. 10(4), 6576-6585.

Abstract

A pellet mill is currently the most frequently used method for producing pellets using either a ring die or a flat die. In the densification process, a great amount of energy is required to avoid spring-back and to overcome the friction between the material and the channel surface of the die. However, extra energy is unnecessarily consumed because of friction between the roller and densified material and the pressure between the roller and die, where there are no opening channels. The aim of this work was to attempt to eliminate a portion of the frictional and compaction energy consumption based on an improved method of densification using a ring die. An upgraded pellet mill was designed and manufactured with rams on its roller. When the die and the roller rotate in a fixed transmission ratio, the rams precisely press raw material into opening channels on the die. Experimental tests on its feasibility were carried out. The results showed that the pellet mill, with this improvement, worked without wear on the surface of either the ring die or the roller; furthermore, the density and mechanical durability of pellets were the same as those produced using the traditional method.


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Improving the Conventional Pelletization Process to Save Energy during Biomass Densification

Zhongjia Chen, Guosheng Yu,* Xiangyue Yuan, Qingyu Wang, and Jiangmin Kan

A pellet mill is currently the most frequently used method for producing pellets using either a ring die or a flat die. In the densification process, a great amount of energy is required to avoid spring-back and to overcome the friction between the material and the channel surface of the die. However, extra energy is unnecessarily consumed because of friction between the roller and densified material and the pressure between the roller and die, where there are no opening channels. The aim of this work was to attempt to eliminate a portion of the frictional and compaction energy consumption based on an improved method of densification using a ring die. An upgraded pellet mill was designed and manufactured with rams on its roller. When the die and the roller rotate in a fixed transmission ratio, the rams precisely press raw material into opening channels on the die. Experimental tests on its feasibility were carried out. The results showed that the pellet mill, with this improvement, worked without wear on the surface of either the ring die or the roller; furthermore, the density and mechanical durability of pellets were the same as those produced using the traditional method.

Keywords: Biomass; Densification; Pelletization process; Energy consumption; Upgraded ring die mill

Contact information: School of Technology, Beijing Forestry University, Beijing, China, 100083;

* Corresponding author: sgyzh@bjfu.edu.cn

INTRODUCTION

Biomass is the third most widely used energy source in the world. It is free of sulfur, renewable, and can significantly reduce net carbon emissions. However, its low bulk density limits its utilization. The bulk density is normally under 100 kg/m3 (Adapa et al. 2002; Larsson et al. 2008) for agricultural residues and under 400 kg/m3 (Sokhansanj and Fenton 2006) for forest residues. If the low bulk density biomass is not densified, its transportation cost will be very high and this kind of transportation is not efficient, because the distances between biomass production sites to industrial and residential locations are usually very long. For the convenience of logistics and utilization, biomass materials can be densified into briquettes, pellets, or logs (Zhang et al. 1999; Sultana and Kumar 2012).

In general, there are several densification processes, which include baling, pelletization, extrusion, and briquetting (Tumuluru et al. 2010; Zhang et al. 2014). These operations are carried out using a bailer, pelletizer, screw press, piston press, or roller press. Pelletization and briquetting are the two most commonly used processes for densification (Stelte et al. 2012; Zhang et al. 2014).

A briquette extruder is used for making biomass briquettes or logs (Zhang et al. 1999; Wu et al. 2014). Raw material is put into a hopper and is dropped in front of a piston by gravity or by a screw. With the reciprocating motion of the piston, the briquette is squeezed out through an open end die intermittently, as shown in Fig. 1.

Fig. 1. Working process of briquette extruder

Compared to a briquetting machine, a ring die mill is more commonly used in China because of its high productivity (Zhao et al. 2007; Zhang et al. 2014). Both the machine and the pellet production technology are mature, and the productivity is higher compared with other biomass machines (Zhang et al. 2014). The ring die mill in general has a ring die with cylindrical opening channels and rotational roller(s) that will press the raw material into and through the channels, as shown in Fig. 2. The friction between the channel surface of the die and the raw material builds up the pressure, and this pressure is the key factor in the densification. The densification process requires large amounts of energy to press material into the opening channel to overcome spring-back and friction on the surface between material and cylindrical channels. As the roller passes over the channel under various pressures, a small layer of the pellet will be squeezed out of the channel. The pelletization process is considered continuous (Zhang et al. 2014), because the roller rotates at a high speed.

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Fig. 2. Working process of ring die mill

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Fig. 3. Compression of particles

In addition to the essential energy required to compress material, extra energy is wasted by the compaction of raw material at locations where there is no channel, and by the friction among the roller, compact material, and ring die, as shown in the red circle in Fig. 3. Research shows that the friction generates heat and helps the raw material to melt and flow easily (Nielsen et al. 2009). However, extra energy consumption not only generates heat, but it also leads to wear and deformation on both the roller and the ring die, no matter how hard the steel parts are that are used, as shown in Figs. 4 and 5.

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Fig. 4. Roller failure with wear