*1.3. Densification*

The low bulk density of biomass, which is typically in the range of 150–200 kg/m<sup>3</sup> for woody biomass [19] and 80–100 kg/m<sup>3</sup> for herbaceous biomass [16], limits its application at the commercial scale. The low bulk densities of biomass make biomass material difficult to store, transport, and interface with biorefinery infeed systems [20]. In general, high-bulk and low-energy-density biomass results in difficulty in feeding the biomass and reduces conversion efficiencies. Densification of biomass helps to overcome this limitation. According to Tumuluru et al. [16], the densification process is critical for producing a feedstock material suitable as a commodity product. Densification helps to overcome the physical properties variability issues, such as moisture, particle size distribution, and density. Densified biomass has improved handling and conveyance efficiencies throughout the supply system and biorefinery infeed, and improved feedstock uniformity and density.

Common biomass densification systems have been adapted from other highly efficient processing industries like feed, food, and pharmacy, and include: (1) a pellet mill; (2) a cuber; (3) a briquette press; (4) a screw extruder; (5) a tabletizer; and (6) an agglomerator [16,20]. The major challenge in biomass densification using the pelleting process is drying of the biomass to about 10–12% (w.b.) moisture content using conventional drying systems, such as a rotary dryer [21–23]. In their study on the validation of advanced feedstock supply systems, Searcy et al. [24] indicated that one of the major limitations biorefineries face in using high-moisture woody and herbaceous biomass for biofuels production is high preprocessing (i.e., for size reduction, drying, and densification) costs. Figure 1 indicates the different unit operations in a conventional pellet production process [25].

**Figure 1.** Various unit operation in the conventional pelleting process [25].

Techno-economic analysis indicated efficient moisture managemen<sup>t</sup> is critical for reducing the preprocessing costs of biomass [26]. According to Pirraglia et al. [27], Sakkampang and Wongwuttanasatian [28], and Yancey et al. [8], the drying of biomass using rotary dryers is a significant energy-consuming unit operation in the pelleting process. According to Tumuluru [23], drying biomass from 10–30% (w.b.) for pelleting takes about 65% energy, whereas pelleting itself only requires about 8–9%, as shown in Figure 2 [23]. Another major limitation with high-temperature biomass drying for biofuels production is the emission of volatile organic emissions.

According to Granström [29] and Johansson and Rasmuson [30], woody biomass drying using a high-temperature rotary dryer results in the emission of volatiles and extractives that are suitable neither for human health nor the environment. When released into the environment, these emissions form photo-oxidants, which are dangerous if inhaled by humans and can also damage forests and the plant canopy.

**Figure 2.** Energy consumption of various unit operations in pelleting of woody biomass [23].
