1. Introduction
According to the United States (U.S.) Department of Energy–Office of Energy Efficiency and Renewable Energy (DOE–EERE), agricultural crop and forest residues, grasses, and woody energy crops that are grown specifically for their energy, together with algae, industrial wastes, sorted municipal solid waste, urban wood waste, and food waste, are considered to be biomass, which can be used for the production of fuels and chemicals [
1]. Liquid fuels produced from biomass can also act as a supplement to petroleum-based liquid transportation fuels, such as gasoline and diesel [
1]. In addition, biomass can be used to produce valuable chemicals and electricity. According to the DOE Billion-Ton report [
2], there are more than a billion tons of biomass produced annually in the U.S., which could sustainably be accessed for continuous biofuels production. A 2005 study conducted by both the U.S. Department of Agriculture (USDA)/DOE indicated that both woody and herbaceous perennial bioenergy crops should be considered for bioenergy and bioproducts production [
3].
The southeastern United States is projected to supply almost 50% of the 16 billion gallons of advanced cellulosic biofuels mandated by the Renewable Fuels Standard [
4]. Assuming typical yields of 80 gallons of fuel per dry ton and 4–8 dry tons/acre, this demand translates to roughly 200 million tons of biomass produced on 25 million acres [
5]. This target is considered to be readily achievable, playing to the region’s strength of lignocellulosic biomass production [
5]. An analysis conducted by the DOE Logistics for Enhanced Attributes Feedstock (LEAF) project estimates that by using the Billion-Ton report as a base-case scenario, primary sources of available biomass are residues generated from forest-products industry and agricultural residues. However, in future, a diverse portfolio of forest, agriculture biomass and second generation bioenergy crops such as miscanthus, switchgrass, and energy cane are going to play a major role in biofuels production. One of the major cost contributors in bioenergy production is feedstock. To make bioenergy a reality, reducing feedstock cost and improving feedstock quality are important for commercial-scale implementation of biomass conversion technologies for fuels and chemical production.
1.1. Biomass Variability
Due to the diversity in the supply of available biomass, biorefineries are expected to receive varied feedstocks with different physical properties and chemical compositions. Biomass obtained from different regions, weather patterns, harvesting, handling and storage conditions, and crop varieties increases this variability factor even further. To address these variability issues, biorefineries need either to re-engineer their processes for each feedstock or to design systems with extreme tolerance, which can have a significant impact on the overall cost. One of the major limitations in using biomass at large scale is variability in the physical and chemical properties of the biomass and the seasonal and geographic availability of the biomass [
6]. The variability in biomass physical and chemical properties limit its commercial-scale applications [
7]. For example, biomass moisture variability influences grinder throughput and particle size distribution, which in turn causes inconsistent mass and heat transfer in conversion. Particle size variability creates feed-handling and conversion issues. For example, larger particle sizes (such as chips and coarsely ground herbaceous biomass) plug bins and augers and do not fully cook in digesters, thereby plugging downstream equipment. Fine particles influence ash composition, thus causing fire, explosion, and health hazards, plugging of weep holes in digesters, and creating inconsistent mass and heat transfer during biochemical and thermochemical conversion.
1.2. Blending
Blending is a common method that mixes different types of biomass to improve their physical properties and chemical composition. In their studies on the pelleting of woody and herbaceous biomass blended feedstocks, Yancey et al. [
8] indicated that blending helps to reduce physical-property and chemical-composition variability in various biomass sources while producing a consistent feedstock. For example, different grades of coals are blended to reduce their sulfur and nitrogen content. Various high-ash biomass sources are blended with low-ash biomass sources for biopower generation. In the agricultural industry, grains are blended to adjust their moisture content. In the feed industry, ingredients are blended to maintain the nutrient content of the feed [
9]. Ray et al. [
10] suggested that biomass blending helps to overcome cost and quality limitations of biomass for biofuels production, while Edmunds et al. [
11] suggested that the blending of different biomass sources helps to improve feedstock specifications. For thermochemical conversion, attributes of interest include carbon content, total ash, and specific minerals, density, and moisture content.
According to Ray et al. [
10], Kenney et al. [
12], and Thompson et al. [
13], biomass blending helps to overcome challenges associated with feedstock quality, variability, supply, and cost. The major advantages of biomass blending are: (1) an increase in potential biomass supply for a given biorefinery area; (2) feedstock cost reduction; and (3) improvement in biomass flow and pelleting characteristics [
8,
14]. Recently, the blending of different sources of lignocellulosic biomass to produce feedstocks for thermochemical conversion has gained importance. For example, Mahadevan et al. [
15] reported that blending switchgrass and southern pinewood resulted in bio-oils with low acidity and viscosity, but higher water content. The major challenges of blending biomass from various sources are these variabilities in biomass physical properties in terms of particle size, moisture, and density. These feedstock variability parameters can result in issues related to feeding, handling, transport, and storage.
According to Tumuluru [
6], biomass pretreatment and preprocessing can help to overcome variability issues. Mechanical (e.g., size reduction, densification), chemical (e.g., ammonia fiber expansion, acid, alkali, ionic), and thermal (e.g., torrefaction, hydrothermal liquefaction) preprocessing and pretreatment help to address biomass variability in physical properties and chemical composition. In addition, Tumuluru [
6,
7], Tumuluru et al. [
16,
17], and Tumuluru and Yancey [
18] suggested that mechanical preprocessing and thermal pretreatment of biomass helps to improve biomass physical properties (such as particle size distribution and bulk density), chemical properties (such as proximate and ultimate composition), and energy property (such as calorific value).
1.3. Densification
The low bulk density of biomass, which is typically in the range of 150–200 kg/m
3 for woody biomass [
19] and 80–100 kg/m
3 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,
22,
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].
Techno-economic analysis indicated efficient moisture management 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.
1.3.1. Pelleting Process Variables
The pelleting process is influenced by various process variables, such as feedstock moisture, content particle size, residence time, length-to-diameter (L/D) ratio, preheating temperature, steam conditioning, and pellet die diameter [
16,
20]. Temperature is another important variable that can impact the quality of the produced pellets. Temperature influences the glass transition temperature of the biomass components and helps in particle bonding during pelleting. Pelleting studies conducted by Stelte et al. [
31] indicated pelleting pressure decreased significantly at 100 °C for woody biomass. Compression pressure, which is dependent on the L/D ratio of the pellet die, also influences the quality of the produced pellets. Under steady state pellet production, the compression pressure equals to the extrusion pressure. If the compression is lower than the extrusion pressure, it generates back pressure and blocks the die. According to Holm et al. [
32], pelleting pressure is dependent on the friction coefficient, the compression ratio, and the Poisson ratio. Their model does not take moisture content, particle size, and preheating temperature into account, but the Poisson ratio and friction coefficient for given biomass is dependent on the biomass type, temperature, and moisture content.
Feedstock composition is another important variable that impacts pellet quality and the consumption of energy during the process. In general, the degradation of hemicellulose produces adhesive products that result in natural bonding. Additionally, lignin in the biomass helps to form the solid bridges above the glass transition temperature and helps to form densified products. Van Dam et al. [
33] noted that lignin above 140 °C acts as a binder and helps to form more durable pellets. Tumuluru et al. [
16], Pradhan et al. [
20], Serrano et al. [
34], Mani et al. [
35], Shaw et al. [
36], Carone et al. [
37], and Puig-Arnavat et al. [
38] have all indicated that moisture plays a major role in the quality of the produced pellets. Most of the pelleting work reported in the literature is in the 10–15% (w.b.) moisture content. Higher moisture in the feedstock results in bulk density and durability loss of the produced pellets. Typically, moisture content necessary for pelleting depends on feedstock type. In general, grasses and crop residues need high moisture for pelleting as compared to woody biomass. Jackson et al. [
39] noted that the pelleting of switchgrass, miscanthus, and wheat straw at 10% (w.b.) in a flat die pellet mill resulted in no pellet formation. These same authors also found that pellets with grasses formed when the moisture content was greater than 20% (w.b.). Understanding the impact of feedstock moisture content on pellet quality is very important to optimize the pelleting process in terms of pellet quality and cost.
1.3.2. High-Moisture Pelleting Process
Idaho National Laboratory (INL) has developed a high-moisture pelleting process that can help to reduce pellet-production costs significantly. Tumuluru [
21,
22,
23], Tumuluru et al. [
40], Bonner et al. [
41], and Hoover et al. [
42] have all successfully tested this process on both woody and herbaceous biomass, as well as chemically pretreated biomass, in both pilot and commercial-scale pelleting systems. In this process, the biomass is pelleted at a higher moisture content of > 15% (w.b.), while the high moisture pellets that are produced are further dried in low-temperature and low-cost dryers, such as grain or belt dryers. That is, this pelleting process replaces a rotary dryer in the front end with a grain or belt dryer at the back end of the pelleting process.
In this process, the biomass loses some moisture during pelleting due to frictional heat developed in the pellet die. Roughly 5–10% (w.b.) moisture loss in the biomass is seen during high-moisture pelleting. Also, the amount of moisture lost from the biomass depends on the initial moisture content of the feedstock. Lamers et al. [
25] and Tumuluru et al. [
26] indicated that by performing this process, a 40% reduction in pellet production costs could be realized as compared to the conventional method followed by industry. Even though the high moisture pelleting process is relatively new, there are some studies in the published literature where researchers have used this process to densify wood, agricultural crop residue, straws, grasses, and the compost [
20,
21,
39,
43,
44].
1.4. Objectives
The overall goal of the project is to develop and demonstrate a state-of-the-art biomass merchandising and processing depot to identify and reduce sources of variation along the supply chain of two high-impact biomass species (i.e., pine and switchgrass), and to develop practices that manage biomass variability to deliver a consistent feedstock optimized for performance in a specific conversion-technology platform [
5]. One way to manage the moisture and particle size variability is to densify the biomass. Many refineries are not ready to densify biomass, and the cost is a prohibitive factor. In this paper, new pelleting concepts, such as high-moisture pelleting, were tested on woody and herbaceous biomass blends, and the ability of this technology to efficiently manage the moisture in the blends was demonstrated.
Most of the pelleting work completed by earlier researchers has been focused on either woody or herbaceous biomass feedstocks. In addition, most of the reported literature on pelleting has been on the single pellet press. Harun and Afzal [
45] worked on pelleting of agricultural and woody biomass blends in a single pellet press. These authors studied the effect of blending spruce and pine with reed canary grass, timothy hay, and switchgrass. Tumuluru et al. [
9] and Harun and Afzal [
45] indicated that pelleting agricultural biomass alone does not result in good pellet quality in terms of durability, which could be due to low lignin content in the agricultural biomass. In addition, low lignin content in the agricultural biomass results in higher energy consumption during the pelleting process.
The data on the pelleting blends of woody and herbaceous biomass at high moisture content (≥ 20%, w.b.) in a continuous flat die pellet mill are not available. Experimental data on how high moisture content, the compression (L/D) ratio in the pellet die, and the blend ratio of pine and switchgrass impact pellet quality and energy consumption of the pelleting process are also not available. The present study aims to understand the pelleting characteristics of pine + switchgrass blends at high moisture contents. The specific objectives of the present study are to: (1) understand how the L/D ratio in the pellet die in a flat die pellet mill and blend moisture content in the range of 20–30% (w.b.) impact the quality of pellets produced using the blends of 2-inch top pine residue + switchgrass at different ratios (i.e., 25:75; 50:50; and 75:25); (2) develop response surface models and surface plots to understand the interactive effect of process variables on pellet quality and the specific energy consumption (SEC) of the process; and (3) optimize the response surface models to identify the process conditions that can minimize the pellet moisture content and maximize bulk density and the durability of blend pellets.
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,
22,
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/m
3. Based on the present study, pelleting blends of switchgrass + 2-inch top pine residue increased bulk density values to about 540–580 kg/m
3. 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/m
3. 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
3 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 agent 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.