**4. Discussion**

Moisture loss was observed when 2-inch top pine + switchgrass blends that were pelleted at high mositure contents. The loss of moisture varied for the blend ratios tested and for the pelleting process variables, such as the L/D ratio and blend moisture content. There was about 6–10% (w.b.) moisture loss during pelleting, and the loss was largely dependent on the initial moisture content of the blend, and less on the L/D ratio of the pellet die. This observation corroborates with earlier work [21–23] in which corn stover and lodgepole pine, during pelleting at a high moisture content, lost about 6–10% (w.b.), and the loss of moisture was dependent on the initial moisture content of the feedstock. Tumuluru et al. [23] has reasoned that during pelleting, the mositure loss in the biomass is due to: (a) mositure flash-off due to the frictional heat developed in the die; and (b) cooling. This leads to drying most of the pellet surface moisture, resulting in partially dried pellets. Also, it is important to dry the partially dried pellets slowly; otherwise, it can result in case-hardening, making pellets harder outside but trapping moisture inside, which can also result in microbial degradation during storage. Tumuluru [44] indicated that the high moisture pelleting process not only densifies the biomass, but helps to drive some of the moisture from the feedstock. Also, high-moisture pelleting makes drying optional. If, for example, the pellets do not have to be stored for long periods of time and do not require transportation over longer distances, the partially dried pellets can be used as such without any further drying for the biochemical conversion process. This is generally true in biochemical conversion where biomass is rewetted during pretreatment and conversion. Also, in the high moisture pelleting process, the moisture in the biomass is more efficiently managed, which reduces the cost of preprocessing significantly. Lamers et al. [25] indicated a 40% reduction in pellet production costs mainly due to moisture loss during pelleting and drying the high moisture pellets using low-temperature dryers, such as grain or belt dryers, provide cost-savings that are 10 times lower and can operate using low quality heat.

In general, low bulk density is another major limitation of herbaceous biomass and results in issues related to storage, handling, and transportation [16,49,50]. These limitations pose a serious challenge for biomass applications on a commercial scale. The present pelleting study indicates that bulk density increases by almost 3–5 times over the raw material, and the increase in the density is dependent on the process conditions selected. In their studies on biomass blending and densification impacts on the feedstock supply chain and biochemical conversion, Ray et al. [10] concluded that low-density biomass requires more resources for transportation and shipping. In their review on biomass densification systems, Tumuluru et al. [16] suggested that pellet mills, briquette presses, cubers, agglomerators, and tablet presses all help to improve bulk density and produce a consistent product in terms of physical properties (e.g., size, shape, bulk density). Densification of biomass also helps to improve handling and conveyance efficiencies in biomass supply systems and infeed [16]. A big challenge for using biomass blends in biorefineries is feeding and handling. Due to variations in bulk density and particle size distribution, the blends will segregate during storage, handling, and feeding, and can influence feed-handling and conversion-process efficiencies. According to Ray et al. [10], the use of blended and densified feedstocks in conversion pathways instead of conventionally ground biomass from a single source addresses several challenges in the current biomass supply chain, such as transportation, storage, cost, quality, and supply variability. Edmunds et al. [11], Sahoo and Mani [50], and Tumuluru et al. [51] reported that herbaceous biomass, such as switchgrass, has a bulk density in the range of 150–160 kg/m3. Based on the present study, pelleting blends of switchgrass + 2-inch top pine residue increased bulk density values to about 540–580 kg/m3. Also, because the moisture content of the pellets is < 10% (w.b.), they are more aerobically stable during storage.

The present research indicated that both the L/D ratio of the pellet die or compression pressure and blend moisture content influenced the bulk density and durability of the produced pellets. A higher L/D ratio and lower moisture content increased the bulk density for all the blend ratios tested. Studies conducted by Said et al. [52] on rice straw in a flat die pellet mill showed that the durability of the pellets is strongly dependent on the effectiveness of the interparticle bonds created during pelleting. Their studies indicated that higher moisture content (10–17%, w.b.) increased durability, but decreased bulk density values. A similar observation was observed by Serrano et al. [34] on barley straw, where an increase in moisture content increased the length of the pellet and its durability but decreased durability values. Studies conducted by Rhén et al. [53] on the pelleting of woody biomass (*Norway spruce*) at different preheating temperatures and pressure indicated that both preheating temperature and moisture content had a significant effect on the bulk density of the pellets produced. Studies conducted by Jackson et al. [39] and Sarkar et al. [54] also indicated that pelleting corn stover and switchgrass at a higher moisture content of about 20–26% (w.b.) resulted in pellets with a bulk density in the range of 500–600 kg/m3. The research conducted by the earlier researcher and the observations from the present study also seems to corroborate that increasing the moisture content decreases the bulk density of the pellets produced.

Currently, the major challenge to use pelleted biomass in biorefining operations is the cost. In this study, the high moisture pelleting process that was tested helps to significantly reduce pelleting costs. Also, this process helps to produce pellets with different bulk density and durability values. According to Tumuluru [21], if pellets are transported by a truck, which is a weight-limited system, very high bulk densities are not needed to fill the truck. Based on maximum weight and volume of the truck, densified products with a bulk density in the range of 350–400 kg/m<sup>3</sup> can fill the truck to capacity. Also if the pellets are transported to shorter distances they do not neet to meet the durability standards set for long-distance transportation. Tumuluru [21] suggested that the cost of pellet production using conventional method cannot be completely offset by saving in the transportation costs especially if the transportation distances are less than 200–300 miles. One way to make pelleting an economically viable technology for the biorefineries is by reducing the cost. The high-moisture pelleting tested in this study can make pelleting more cost-effective. Also, the cost savings achieved in terms of storage, handling, and feeding due to the use of pellets are not quantified throughly, it they are quantified it can make pelleting a more favorable operation for biorefineries. Another major advantage of blending woody with herbaceous biomass is that it improves the chemical composition. Woody biomass has a higher carbon content and is lower in ash, while the herbaceous biomass is lower in carbon content and higher in ash. Blending woody with herbaceous biomass can help to overcome herbaceous biomass feedstock specification limitations and make them meet specifications required for thermochemical conversion in terms of calorific value, volatiles, oxygen, hydrogen, nitrogen, chlorine, sulfur, nitrogen, and ash content [9].

In general, the lignin in the biomass is considered a natural binding agen<sup>t</sup> and plays an important role in the densification process. In the present study, increasing the pine content in the blend to 75% increased durability values and reduced the specific energy consumption. In his studies on the pelleting of woody and herbaceous biomass at high moisture content, Tumuluru [44] indicated that higher lignin content in woody biomass increased the bulk and durability values of the pellets. In addition, the same study also indicated that energy sorghum resulted in low-quality pellets in terms of their density and durability. According to Tumuluru et al. [9], grasses with lower lignin content are difficult to pelletize and consume higher pelleting energy, in addition to producing low-quality pellets in terms of their density and durability. However, the same authors indicated that blending straws and grasses with woody biomass, which has higher lignin and lower ash content, could help to improve pellet properties and reduce pelleting energy consumption. In their studies on the chemical and mechanical propeties of agricultural and woody biomass, Harun and Afzal [45] indicated that higher percentages of woody biomass in the blend of pine and switchgrass increased the pellet strength and durability values. This present research corroborates this observation and proves that blending pine with switchgrass does indeed help to produce a good quality of pellet in terms of durability and reduce specific energy consumption.

Edmunds et al. [11] indicated that switchgrass has about 21% lignin content, while 2-inch top pine residue has about 37.5% on an as-received dry-weight basis. The previous research published on pelleting of grasses indicated that grasses take more energy to pellet as well as they do not make a good pellet due to its low lignin content and needle-shaped particles. Pine and switchgrass blending studies conducted by Edmunds et al. [11] indicated that significant improvement in terms of lignin content and particle size distribution could be achieved. These improvements in terms of physical properties

and biochemical composition, especially lignin, can help to produce good quality pellets at lower energy consumption. The blending of these types of biomass not only helps to modify their chemical composition but often improve their pelleting characteristics as well, due to better interlocking ability and flowability of the biomass in the pellet die. This observation was corroborated by the present study, where increasing the pine percentage to 75% in the blend improved the durability of the pellets. Also, the energy consumption of the pelleting process was lower when the pine percentage was higher in the blends tested. The improvements in bulk density and durability and lower energy consumption for the pine and switchgrass blend pellets tested can be due to improved chemical composition and particle size distribution, which might have resulted in better flow characteristics in the pellet die.

Many researchers have indicated that particle size distribution has a significant impact on the quality of the produced pellets [16,55,56]. It is critical to manage the particle size to produce the right quality of densified products at a lower specific energy consumption. The blending of woody and herbaceous biomass helps to alter particle size distribution and can make feedstock suitable for different densification systems. In general, a pellet mill requires smaller particles as the contact area between the particles plays a major role in creating necessary bonding between the particles. The common bonding mechanism during pelleting are: (1) particle bonding due to interfacial forces and capillary pressures; and (2) solid bridges that are formed due to chemical reactions, sintering, solidification, hardening of the binder, hardening of the melted substances, or crystallization of the dissolved materials results in agglomeration of biomass particles [16]. In addition, according to MacBain [55] and Payne [56], finely ground materials are suitable for pelleting because they have higher surface area to absorb steam during conditioning and can result in higher-starch gelatinization and increased particle binding. The same authors have also suggested that a certain ratio of fines—medium and coarse particles—are necessary to improve pellet quality and reduce pelleting energy consumption. Based on the present study, blending of pine and switchgrass at different ratios might have influenced particle size distributions, positively impacted pellet quality (i.e., bulk density and durability), and reduced the overall specific energy consumption of the pelleting process. Future work on the pelleting of woody and herbaceous biomass blends should be focused on testing the process in a ring die pellet mill at both the pilot and commercial scales; understanding how the chemical composition and energy properties changes with respect to moisture content and pelleting process variables, such as the L/D ratio; and understanding the effect of grind size on the quality of the pellets and energy consumption of the process.
