ROLL PRESS BRIQUETTING AND PELLETING

Transcription

1 ROLL PRESS BRIQUETTING AND PELLETING OF CORN STOVER AND SWITCHGRASS N. Kaliyan, R. V. Morey, M. D. White, A. Doering ABSTRACT. Corn stover and switchgrass, potential biomass feedstocks for bioenergy and bioproducts industries, are often harvested during a limited harvest season and stored as bales with bulk densities of about 100 to 200 kg m -3. Because of low bulk density, corn stover and switchgrass are difficult to handle, transport, store, and use in their natural forms. One of the solutions to reduce these problems and the associated costs is to densify these biomass feedstocks into pellets or briquettes. In this study, roll press briquetting characteristics of corn stover and switchgrass were studied using a pilot scale roll press briquetting machine. Almond shaped briquettes 28.7 to 31.3 mm in length were made. Results showed that high durability corn stover and switchgrass briquettes with bulk densities of 480 to 530 kg m -3 could be produced. This corresponds to about a three to five fold increase in bulk densities compared to those of bales. Briquettes produced with the roll press briquetting machine had bulk densities (351 to 527 kg m -3 ), durabilities (39% to 90%), and crushing strengths (28 to 277 N) that were somewhat less than, but in a range comparable to, the pellets (9.6 to 9.8 mm diameter) produced with a conventional ring die pelleting machine. The bulk density, durability, and hardness of the pellets ranged from 528 to 610 kg m -3, 75% to 95%, and 148 to 224 N, respectively. Micro structural studies (chemical composition analyses, scanning electron microscopy imaging, and UV auto fluorescence imaging) on grinds, briquettes, and pellets confirmed that highly dense, strong, and durable briquettes and pellets from corn stover and switchgrass could be produced without adding chemical binders (i.e., additives) by activating (softening) the natural binders such as water soluble carbohydrates, lignin, protein, starch, and fat in the biomass materials by providing moisture and temperature in the range of glass transition of the biomass materials. No steam conditioning was necessary to produce good quality corn stover briquettes. For switchgrass, a grind temperature of about 75 C obtained by steam conditioning was necessary to produce good quality briquettes. Roll press briquetting appears to be a promising low cost, low energy, high capacity densification approach for commercial production of biomass briquettes. Keywords. Biomass, Briquettes, Corn stover, Densification, Pelleting, Pellets, Roll press briquetting, Switchgrass. To reduce dependence on fossil fuels, there is tremendous interest in using biomass materials such as corn stover and switchgrass in the U.S. for producing liquid transportation fuels (ethanol), combined heat and power, chemicals, and bio products (DOE, 2005). In addition to numerous advantages, use of biomass materials in place of fossil fuels would result in low emissions of greenhouse and acid gases (DOE, 2005). In order to make biomass materials available for a variety of applications, challenges with the use of biomass materials in their original forms must be resolved. Because of high moisture content, irregular shape and size, and low bulk density, biomass is very difficult to handle, transport, store, and utilize in its orig Submitted for review in October 2008 as manuscript number FPE 7757; approved for publication by the Food & Process Engineering Institute Division of ASABE in March The authors are Nalladurai Kaliyan, ASABE Member Engineer, Research Associate, Department of Bioproducts and Biosystems Engineering, University of Minnesota, St. Paul, Minnesota; R. Vance Morey, ASABE Fellow, Professor, Department of Bioproducts and Biosystems Engineering, University of Minnesota, St. Paul, Minnesota; Michael D. White, Director of Marketing, Bepex International LLC, Minneapolis, Minnesota; and Alan Doering, Associate Scientist, Agricultural Utilization Research Institute (AURI), Waseca, Minnesota. Corresponding author: R. Vance Morey, Department of Bioproducts and Biosystems Engineering, University of Minnesota, 1390 Eckles Ave., St. Paul, MN 55108; phone: ; fax: ; e mail: rvmorey@umn.edu. inal form. One solution to these problems is densification of biomass materials into pellets or briquettes. Densification can increase the bulk density of biomass materials from an initial bulk density (including baled density) of 40 to 200 kg m -3 to a final bulk density of 600 to 800 kg m -3 (Holley, 1983; Colley et al., 2006). Densification of biomass materials could reduce the costs of transportation, handling, and storage. In addition, because of uniform shape and size, densified products can be more easily handled using existing handling and storage equipment, and can be easily adopted in directcombustion or co firing with coal, gasification, pyrolysis, and in other biomass based conversions. In the U.S., pelleting and roll press briquetting/compaction technologies are widely used for densifying particulate materials for various end uses (Pietsch, 2002). Pelleting technology is commonly used for producing animal feed (Sitkei, 1986), whereas the roll press is used for densifying coals, fertilizers, minerals, and metals (Pietsch, 2002). These two densification approaches (pelleting and roll press briquetting with the configurations shown in fig. 1) would be readily adapted to densifying biomass materials to use for renewable energy applications. In pelleting equipment, the feed material is pressed through open ended cylindrical holes (dies) made in the periphery of a ring (fig. 1a). One to three small rotating rolls push the feed material into the die holes from inside of the ring towards the outside of the ring. The skin friction between the feed particles and the wall of the die resists the free flow of feed, and thus the particles are com Transactions of the ASABE Vol. 52(2): American Society of Agricultural and Biological Engineers ISSN

2 pressed against each other inside the die to form pellets. One or two adjustable knives placed outside the ring cut the pellets into desired lengths. Typical diameter of the pellets may range from 4.8 to 19.0 mm, and the length of the pellets may range from 12.7 to 25.4 mm. In a roll press briquetter/compactor, material is densified by compression between two counter rotating rolls (fig. 1b). Initial densification of the material may occur through compressing the material with a tapered auger in the feed mechanism. The primary purpose of this initial densification phase is to remove air from low bulk density material. The final compaction occurs because of high pressures created as the material flows between the two rolls. The roll surfaces have pockets to form briquettes of desired size and shape when the material passes between the rolls. The briquettes are easily separated and handled after leaving the machine. The densified products are mostly pillow shaped with a size of 10 to 40 mm or larger. Pelleting would produce densified biomass (i.e., pellets) suitable for home, business, and institutional heating where high bulk density and high quality (i.e., high strength and durability) of densified biomass are required. On the other hand, roll press briquetting would produce densified biomass (i.e., briquettes) suitable for applications requiring a large amount of biomass materials such as cellulosic ethanol production or combustion/gasification at ethanol plants where moderate bulk density and moderate quality of densified biomass are sufficient to facilitate the transportation, handling, and storage of biomass with minimum costs. Based on this rationale, densification behaviors of the biomass materials (i.e., corn stover and switchgrass) in pilot scale roll press briquetting and conventional ring die pelleting machines were studied in this research. In addition, in this research, more effort was placed on roll press briquetting than on pelleting because very limited fundamental studies on roll press briquetting of biomass materials are available in the literature (e.g., Köser et al., 1982; Holley, 1983; Dec, 2002). Conversely, several studies on the performance of conventional pelleting of biomass materials have been conducted in the past (e.g., Hill and Pulkinen, 1988; Tabil and Sokhansanj, 1996; Samson et al., 2000; Jannasch et al., 2004; Colley et al., 2006). OBJECTIVES Kaliyan and Morey (2006) studied the densification characteristics of corn stover and switchgrass using a lab scale piston cylinder uniaxial compression apparatus at a forming pressure of 150 MPa. Compression pressure of 150 MPa or higher can be achieved in commercial roll press briquetting and pelleting machines (Dec, 2002; Kaliyan and Morey, 2009). Kaliyan and Morey (2006) concluded that the particle size obtained from a hammer mill screen size of 3.0 mm (geometric mean particle diameter of 0.56 to 0.66 mm), grind moisture content of 15% (w.b.) for corn stover and 10% (w.b.) for switchgrass, and preheating temperature of 75 C or above would produce strong and high durability briquettes from corn stover and switchgrass. They also concluded that when the moisture content of the corn stover grind was 15% (w.b.), there was no need to preheat the corn stover grind in order to produce high quality corn stover briquettes. For switchgrass, preheating to at least 75 C with a grind moisture content of 10% (w.b.) was required to produce good quality switchgrass briquettes. Evaluation of these optimum densification conditions for corn stover and switchgrass in two Ring-die Roll Knife (a) (b) Pellets Screw feeder Feed Die (hole) Roll with pockets Briquettes Figure 1. Schematics of (a) conventional ring die pelleting and (b) roll press briquetting machines. types of commercial densification machines (i.e., roll press briquetting and pelleting machines) was the overall objective of this study. The specific objectives of this study were to: Determine the roll press briquetting characteristics of corn stover and switchgrass. Determine optimum roll press briquetting conditions (machine variables and biomass variables) for densifying corn stover and switchgrass. Compare performances of the roll press briquetting machine and a conventional pelleting machine at selected densification conditions for corn stover and switchgrass. Assess the binding mechanisms of corn stover and switchgrass by micro structural analyses. The densification of biomass always involves size reduction (i.e., grinding) and may require drying of biomass before compressing of biomass in densification machines. In this research, only the densification process was studied in detail. MATERIALS AND METHODS BIOMASS FEEDSTOCKS Corn stover was purchased from Mat Ag Fiber LLC (Floodwood, Minn.) in October 2006 as coarsely ground (i.e., hammer milled) corn stover. Two particle sizes of corn stover were obtained by grinding the coarse corn stover in an 18.7 kw (25.0 hp) hammer mill (Jacobson Quality Machin 544 TRANSACTIONS OF THE ASABE

3 ery, Minneapolis, Minn.) with two different screens with 2.4 and 4.0 mm (3/32 and 5/32 in.) openings. Switchgrass was harvested as m (4 4 8 ft) square bales in September 2006 from a field in Owatonna, Minnesota. Size reduction of switchgrass was done in two steps. First, switchgrass bales were ground in a 5.6 kw (7.5 hp) chopper (Agri Val, DC Atlas Co., Loyal, Wisc.) to a length of to mm (4 to 5 in.). Then, fine grinding of the chopped switchgrass was done using the hammer mill with two different screens with 2.4 and 4.0 mm (3/32 and 5/32 in.) openings. Moisture content of the biomass grinds was determined using the procedure given in ASABE Standard S358.2 (ASABE Standards, 2006a). The moisture content values reported in this article are on a wet basis (w.b.). Bulk density of the biomass grinds (wet basis) was calculated from the mass of grind that occupied a 250 ml glass container with 80 mm diameter. While measuring the bulk density of the grinds, the glass container was manually filled by slowly discharging the samples to the container from a height of about 100 mm, and the container with the sample was tapped gently five times on a lab bench to remove large voids inside the sample as well as to reduce the sample filling errors. Therefore, the bulk density measurements resulted in tapped bulk density of the grinds. Particle size and particle size distribution of the biomass grinds were determined based on ASABE Standard S319.3 (ASABE Standards, 2006b). Corn stover and switchgrass grind samples were sent to a forage analysis laboratory (Dairy One, Ithaca, N.Y.; to determine the compositions of these two biomass materials using near infrared reflectance (NIR) spectroscopy. The NIR calibration standards developed by Dairy One (Ithaca, N.Y.) based on the Foss model 6500 spectrophotometer (Foss NIR Systems, Inc., Laurel, Md.) with Win ISI II v1.5 AOAC procedures for corn stover and grass hay, respectively, were used to determine the compositions of corn stover and switchgrass. Glass transition temperatures of the corn stover and switchgrass grinds were determined using a differential scanning calorimeter (DSC; Pyris 1, PerkinElmer Life and Analytical Sciences, Shelton, Conn.) (Kaliyan and Morey, 2006). ROLL PRESS BRIQUETTING Roll press briquetting experiments were conducted during January 2007 at Bepex International LLC (Minneapolis, Minn.) using a pilot scale roll press briquetting machine (model CS 25 compactor/briquetter, Bepex International LLC, Minneapolis, Minn.). In the roll press briquetting machine, the rolls were powered by a 3.7 kw (5.0 hp) motor, and the screw feeder was powered by a 2.2 kw (3.0 hp) motor. The diameter and width of the rolls were mm (9.0 in.) and 38.1 mm (1.5 in.), respectively. A constant force of 5.1 kn mm -1 width of rolls (i.e., hydraulic system pressure of 2600 psig) was applied to the rolls during briquetting of corn stover and switchgrass. The pockets on the rolls produced almond shaped briquettes. The dimension of one pocket was mm (1.25 in.) length mm (0.8 in.) width 5.08 mm (0.2 in.) depth. The gap between the rolls was kept constant at about 1.0 mm (0.04 in.). Therefore, the approximate dimension of an almond shaped briquette was mm length mm width mm depth. The densification process involves moisture/steam conditioning the biomass grind, and then roll press briquetting the grind, followed by cooling of the product (briquettes plus fines that leaked through the rolls) to room temperature (about 20 C), and screening the product using a vibratory screen with 5.2 mm openings to separate the fines and briquettes. The cleaned briquettes were used for measuring their properties. The moisture content of the biomass grind was adjusted by mixing a predetermined quantity of distilled water with the grind in a mechanical mixer. Before briquetting, the biomass grinds were left in plastic containers for about 2 to 3 h for moisture equilibration. For the briquetting conditions where the temperature of the grind should be higher than room temperature, the grind was steam conditioned using a Turbulizer (Bepex International LLC, Minneapolis, Minn.) steam conditioner. The Turbulizer was a steam jacketed cylindrical container with mixing paddles at its center. A feeder metered the grind to an airlock. From the airlock, the grind was fed to the inlet of the Turbulizer. In the Turbulizer, paddles moved the grind from the inlet to the outlet while increasing the temperature of the grind to the required levels (within 30 s). Steam at 0.35 to 0.83 MPa g (50 to 120 psig) and 150 C to 178 C was input to the steam jacket of the Turbulizer to indirectly raise the temperature of the grind. The paddles in the Turbulizer were operated at 1200 rpm to obtain a grind temperature of 75 C or at 1650 to 1800 rpm to obtain 100 C or above. In addition, some amount of steam at 0.10 to 0.14 MPa g (15 to 25 psig) was also sprayed directly inside the Turbulizer to avoid any possible moisture loss from the grind during steam conditioning. More details on the moisture conditioning, steam conditioning, and roll press briquetting procedures can be found in Kaliyan and Morey (2007a). Figure 2 shows examples of corn stover and switchgrass briquettes produced. To estimate the throughput (kg h -1 ) of the roll press briquetting machine, the mass of briquettes collected for about 4 to 10 min was weighed. The experimental setup of the pilotscale roll press briquetting machine did not allow accurate measurement of specific energy consumption. PELLETING Pelleting experiments were conducted during February 2007 at the Agricultural Utilization Research Institute (AURI), Waseca, Minnesota, using a pilot scale conventional ring die pelleting machine (CPM Master model , California Pellet Mill (CPM) Co., San Francisco, Cal.). A 29.8 kw (40.0 hp) motor powered the two corrugated rolls and the ring die in the pelleting chamber, and a 3.7 kw (5.0 hp) motor powered a screw conveyor below the feed hopper, which directed the biomass grind to the pelleting chamber. The ring die was operated at a constant speed of 232 rpm. Before pelleting corn stover and switchgrass, the ring die was cleaned to remove rust and any previous plugging in the holes of the ring die. To achieve effective cleaning of the ring die, a mixture of oat grain, fine sand, and vegetable oil was extruded through the ring die for about 2 to 3 min. This start up procedure also preheated the ring die. In addition, the same mixture was used to clean the ring die whenever there was a change in the pelleting condition, such as pelleting switchgrass after pelleting corn stover. The pelleting process involves moisture conditioning the biomass grind, and then feeding the moisture conditioned grind to the pelleting mill to make pellets with a die diameter of 9.5 mm (3/8 in.). The moisture content of the biomass grinds was adjusted by mixing predetermined quantities of water with the biomass grinds in a mechanical mixer. No Vol. 52(2):

4 (a) (a) (b) Figure 2. (a) Corn stover and (b) switchgrass briquettes produced. steam conditioning was done either for corn stover or switch grass grind for any of the pelleting tests. Hot pellets were spread in a thin layer on a screen with 6.4 mm (1/4 in.) diame ter round holes, which is less than the pellet mill die diameter, to naturally cool the pellets to room temperature (about 12.8 C). After cooling, the screen was mechanically shaken to collect fines less than 6.4 mm, and the cleaned pellets were used for measuring their properties. Kaliyan and Morey (2007a) describe the procedure involved for the pelleting of corn stover and switchgrass. Figure 3 shows examples of the corn stover and switchgrass pellets produced. To calculate the throughput (kg h-1) of the pelleting ma chine, the mass of pellets collected for about 1.5 to 2.0 min was measured. During pelleting, the current input to the over all operation of the pelleting machine was recorded using an ammeter to estimate the specific energy consumption for the pelleting process. The specific energy consumption per unit mass of pellets produced (MJ t-1) was estimated by subtract ing the energy consumption for running the empty pelleting machine (i.e., no load condition) from the energy consump tion during biomass pelleting (Kaliyan and Morey, 2007a). MACHINE AND BIOMASS FEEDSTOCK VARIABLES STUDIED The ranges of biomass variables, such as particle size, moisture content, and preheating temperature, that could pro duce good quality densified products in the pilot scale roll press briquetting and pelleting machines were selected based on the results from the laboratory scale densification studies on corn stover and switchgrass (Kaliyan and Morey, 2006). 546 (b) Figure 3. (a) Corn stover and (b) switchgrass pellets produced. For roll press briquetting experiments, the effects of two particle sizes (grinds obtained from hammer mill screen sizes of 2.4 and 4.0 mm), two moisture levels (10% and 15% w.b.), and three preheating (i.e., steam conditioning) temperatures (room temperature, 75 C, and 100 C) on briquetting of both corn stover and switchgrass were studied. Although we planned to study the effect of two moisture contents (10% and 15% w.b.), the moisture contents of the moisture conditioned or steam conditioned grinds deviated from these two values and ranged from 7% to 20% (w.b.). In addition, it was diffi cult to achieve the other experimental variables at exact pre set levels. During briquetting, the roll speed and screw feeder speed had to be adjusted simultaneously to match (i.e., syn chronize) them for each briquetting condition to produce consistent briquettes. If the speed of the screw feeder was too fast or the speed of the rolls was too slow, the biomass grinds plugged the hopper just above the rolls, and thus no briquettes were formed. We then needed to stop the roll press and re move the plugged materials before further operation. If the speed of the rolls was too fast, biomass grinds came out through the rolls without forming briquettes. For the roll press briquetting machine used in this study, the lowest me chanically achievable roll speed was 4 rpm. Therefore, the speed of the rolls was controlled using a frequency controller to achieve roll speeds of less than 4 rpm. The speeds of the TRANSACTIONS OF THE ASABE

5 rolls and screw feeder were variables, and these speeds were recorded for each briquetting condition. To compare the densification performance of the roll press briquetting machine with the conventional ring die pelleting machine, pelleting experiments were conducted for selected conditions. For pelleting experiments, the effects of particle size (grinds obtained from 2.4 and 4.0 mm hammer mill screen sizes for corn stover, and grinds obtained only from 2.4 mm hammer mill screen size for switchgrass), and two die (i.e., hole in the ring die) length to diameter (L/D) ratios (5.3 and 6.0) were studied. The diameter of the die was kept constant at 9.5 mm (3/8 in.) for all of the pelleting tests. Preliminary pelleting trials showed that at a moisture content of 15% (w.b.) or below, consistent pellets were not formed; pellets appeared to be soft with weak particle particle bonding. Therefore, a constant moisture content of about 20% (w.b.) was used for both corn stover and switchgrass for all of the pelleting tests. In addition, no steam conditioning was used for any of the pelleting tests. No external binding agents (i.e., additives) were used for any of the briquetting or pelleting experiments. PROPERTIES OF BRIQUETTES AND PELLETS Properties of briquettes and pellets were measured immediately after forming, and after one week of storage at room temperature (about 23 C) in zip lock plastic bags to allow more curing of particle particle bonds (Raghavan and Conkle, 1991). The properties measured were bulk density, durability, crushing strength or hardness, individual briquette and pellet dimensions, and moisture content of the briquettes and pellets. The bulk density and durability are properties of a bulk sample of briquettes or pellets, whereas the crushing strength or hardness is a property of individual briquettes or pellets. Bulk density of the briquettes and pellets (wet basis) was measured from the mass of briquettes or pellets occupying a 1.0 L glass container with 125 mm diameter. While measuring the bulk density of briquettes or pellets, the glass container was manually filled by slowly discharging the samples to the container from a height of about 100 mm, and the container with the sample was tapped gently five times on a lab bench to remove large voids inside the sample as well as to reduce the sample filling errors. Therefore, the bulk density measurements resulted in tapped bulk density of the briquettes or pellets. The moisture content of the briquettes and pellets was quantified based on ASABE Standard S269.4 (ASABE Standards, 2006c). Individual briquette or pellet dimensions were measured using a digital Vernier caliper. Durability of briquettes and pellets was measured using the tumbling can method given in ASABE Standard S269.4 (ASABE Standards, 2006c). About 500 g of briquettes or pellets were tumbled in a tumbler at 50 rpm for 10 min. Then, the percentage of original weight of briquettes or pellets retained on a 3.2 mm (1/8 in.) screen was calculated as the durability. The screen size of 3.2 mm, used for removing the fines from the good quality briquettes or pellets after tumbling, was based on the recommendation by the U.S. Pellet Fuels Institute (PFI, Arlington, Va.; g). The U.S. Pellet Fuels Institute (Arlington, Va.) classifies particles that are less than 3.2 mm as fines in packaged biomass pellets used for heating purposes. Crushing strength of the briquettes (along length and width) was measured using an Instron universal testing machine (model 4206, Instron Corp., Canton, Mass.) at a compression speed of 2.54 mm min -1. Pellet hardness (along diameter) was measured using a hand operated Dillon Quantrol basic force gauge (model BFG 500 N, Itin Scale Co., Inc., Brooklyn, N.Y.). The crushing strength test for briquettes is equivalent to the hardness test for pellets because compressive stress is applied to fail the briquettes and the pellets (Raghavan and Conkle, 1991). Angle of repose of briquettes and pellets (about 250 g sample) on a galvanized steel surface was measured using a tilted plane angle of repose apparatus (Mohsenin, 1986). Scanning electron microscopy (SEM) (Hitachi S3500N) images were taken for corn stover and switchgrass grinds, and for cross sections (i.e., fractured surfaces) of the briquettes and pellets. The SEM images were analyzed to understand the binding mechanisms of corn stover and switchgrass. Ultraviolet (UV) auto fluorescence images (Olympus IX70 inverted microscope; UV excitation at nm, dichroic mirror at 410 nm, emission at nm) of corn stover and switchgrass grinds, and of cross sections (i.e., fractured surfaces) of the briquettes and pellets, were obtained to identify the natural binders. According to Rost (1995), the color interpretation of UV auto fluorescence is: deep red for chloroplasts; green or yellow green for protein compounds; brilliant blue or bluish white for lignin, cutin, suberin, or phenolic acids such as ferulic acid; and whitish fluorescence for cutin (cuticle). In addition, pure carbohydrates (cellulose, hemicellulose, and starch) and lipid/fat molecules do not fluoresce (Rost, 1995). STATISTICAL ANALYSES All statistical analyses were performed using SPSS 16.0 for Windows (SPSS, Inc., Chicago, Ill.) at 5% significance level. We first performed paired samples t tests on data for bulk density or durability of briquettes/pellets taken immediately after forming and after one week of storage at room temperature to determine if there were significant differences. If there were no significant differences, then we pooled the data from the results at these two times in order to increase the sample size for additional statistical analyses. The results of the paired samples t tests for briquettes showed that the bulk density and durability of the briquettes measured after one week of storage were significantly higher than the values measured immediately after forming (P < 0.05). Therefore, pooling of the data measured at two different times was not appropriate for briquettes, and hence further statistical analyses could not be done for the briquettes. The briquetting results measured after one week of storage are discussed based on the trends observed for the effects of particle size, moisture content, and steam conditioning temperature. The results of the paired samples t tests for pellets showed that the bulk density and durability of the pellets measured after one week of storage were not significantly different from the values measured immediately after forming (P > 0.05). This indicates that the change in pellet mass and/or volume and the effect of curing occurred during the one week storage period were negligible. Therefore, the data on bulk density or durability of pellets measured immediately after forming and after one week of storage at room temperature were pooled to conduct independent samples t tests to investigate the effect of die L/D ratio (5.3 and 6.0) for corn stover and switchgrass, and the effect of particle size (grinds ob Vol. 52(2):

6 tained from 2.4 and 4.0 mm hammer mill screen sizes) for corn stover. RESULTS AND DISCUSSION CHARACTERISTICS OF BIOMASS FEEDSTOCKS Currently, corn stover and switchgrass are collected from the field as bales with bulk density of 100 to 200 kg m -3 (Shinners et al., 2003, 2006). Compared to the bulk density of the bales, the increase in the bulk density of the biomass feedstocks due to the size reduction (i.e., chopping and then hammer milling) was small (table 1). Table 2 gives the compositions of the corn stover and switchgrass grinds used for this study. In addition, in table 2, the composition values provided by Mani et al. (2006) and DOE (2007) are compared. The constituents such as lignin, crude protein, starch, crude fat, and water soluble carbohydrates, are natural binders in the biomass materials. These natural binders can be activated (softened or melted locally) either by high moisture or heat or steam to use their binding functionality (Kaliyan and Morey, 2009). Lignin and hemicellulose were found to be amorphous thermoplastic materials that would undergo plastic deformation at temperatures in the range of their glass transition (i.e., softening temperatures) (Back and Salmen, 1982). The glass transition occurs in corn stover and switchgrass from 50 C to 113 C (Kaliyan and Morey, 2006). The mean glass transition temperature for both corn stover and switchgrass is 75 C (Kaliyan and Morey, 2006). The ash contents (i.e., mineral contents) in corn stover (11.2% d.b.) and switchgrass (5.0% d.b.) show their relative abrasiveness to densification equipment when there is high friction/shear during densification, such as in pelleting. The higher the mineral content, the higher the abrasion (Kaliyan and Morey, 2009). ROLL PRESS BRIQUETTING The paired samples t tests for briquettes showed that the bulk density and durability of the briquettes measured after one week of storage were significantly higher (by 31.1 kg m -3 in bulk density and 3.0% points in durability) than the values measured immediately after forming (P < 0.05). This may be due to the change in the briquette mass and/or volume (possibly due to the drying and shrinkage of briquettes) and curing of bonds during the one week storage period. Thus, the briquette properties measured after one week of storage were used to evaluate the effects of particle size, moisture content, and steam conditioning temperature (table 3). The briquette properties measured immediately after forming the briquettes can be found in Kaliyan and Morey (2007a). Effect of Roll Press Briquetting Machine Variables The biomass briquettes produced were of almond shape with length of 28.7 to 31.3 mm. The combination of screw feeder speed and roll speed that created consistent briquettes was different for each briquetting condition; however, the differences in the respective speeds were small (table 3). The roll speed and screw feeder speed obtained from this study could serve as starting values for making briquettes in commercial scale roll press briquetting machines, although these speeds can be adjusted on the go. For briquetting cases without steam conditioning, the temperature of the corn stover and switchgrass briquettes ranged from 51 C to 63 C due to the frictional heating of the grinds during briquetting (table 3). Because this temperature range is well within the range of glass transition temperature of corn stover and switchgrass (i.e., 50 C to 113 C), the natural binders in the biomass materials would have been activated to some extent to help produce durable particle particle bonding. Throughput of the roll press briquetting machine ranged from 16.8 to 32.6 kg of cleaned briquettes per hour for corn stover, and 9.6 to 30.6 kg of cleaned briquettes per hour for Table 1. Properties of corn stover and switchgrass grinds used for the roll press briquetting and pelleting experiments. Biomass Feedstock Hammer Mill Screen Used for Grinding [a] Initial Grind Moisture Content [b] (% w.b.) (n = 3) Particle Size of Grinds [c] (mm) (n = 3) Bulk Density of Grinds [b] (kg m 3 ) (n = 3) Corn Stover 2.4 mm (3/32 in.) 7.3 ± ± ± mm (5/32 in.) 8.5 ± ± ±7.8 Switchgrass 2.4 mm (3/32 in.) 9.2 ± ± ± mm (5/32 in.) 10.4 ± ± ±1.2 [a] Diameter of the holes in the screen. [b] Mean ± standard deviation. [c] Geometric mean particle diameter by mass (mm) ± geometric standard deviation of particle diameter by mass (mm). Component This Study (% of dry matter) Table 2. Compositions of corn stover and switchgrass grinds. Corn Stover Mani et al. (2006) (% of dry matter) DOE (2007) (range, % of mass) This Study (% of dry matter) Switchgrass Mani et al. (2006) (% of dry matter) DOE (2007) (range, % of mass) Cellulose Hemicellulose Lignin 8.8 [a] 3.1 [a] [a] 9.2 [a] 7.4 [a] [a] Crude protein NA [b] NA Starch 0.4 NA NA 1.0 NA NA Crude fat NA NA Water soluble carbohydrates 7.9 NA NA 2.2 NA NA Ash [a] Lignin values measured for the biomass materials used in this study and in Mani et al. (2006) were acid insoluble lignin contents, whereas the lignin contents obtained from DOE (2007) were total lignin in the biomass materials. [b] NA = data not available. 548 TRANSACTIONS OF THE ASABE

7 Table 3. Properties of corn stover and switchgrass briquettes measured after one week of storage at room temperature (about 23 C). Roll Press Briquetting Conditions (mean ±SD, if given) Properties of Briquettes (mean ±SD, if given) Moisture Content of Grind (% w.b.) (n = 3) Temperature of Grind ( C) (n = 3) Roll Speed (rpm) Screw Feeder Speed (rpm) Fines Leakage through Rolls [a] Throughput of Cleaned Briquettes (kg h 1 ) Temp. of Briquettes ( C) [b] Moisture Content (% w.b.) (n = 3) Bulk Density (kg m 3 ) Durability (%) Crushing Strength (N) (n = 3) Along Along Length Width Corn stover grind with particle size of 0.34 mm ±0.29 (hammer mill screen size = 2.4 mm (3/32 in.)) 7.3 ± ± % ± ± ± ±0.0 [c] 20.3 ± % ± ± ± ± ±5.9 [d] % 32.6 > ± ± ±22.8 Corn stover grind with particle size of 0.36 mm ±0.35 (hammer mill screen size = 4.0 mm (5/32 in.)) 16.6 ±0.6 [c] 18.0 ± % ± ± ± ± ±0.6 [d] % 20.5 > ± ± ±25.6 Switchgrass grind with particle size of 0.49 mm ±0.38 (hammer mill screen size = 2.4 mm (3/32 in.)) 9.2 ± ± % ± NA [e] NA 19.8 ±0.5 [c] 19.0 ± % ± ± ± ± ±7.6 [d] % ± ± ± ± ±10.7 [d] % 26.9 > ± ± ±62.5 Switchgrass grind with particle size of 0.59 mm ±0.43 (hammer mill screen size = 4.0 mm (5/32 in.)) 10.4 ± ± % NA ± NA NA 10.0 ± ±0.0 [d] % NA ± ± ±6.3 [a] Fines leakage through rolls (%) = (kg of fines 100) / (kg of fines + kg of cleaned briquettes). The size of fines is <5.2 mm. [b] Temperature of the briquettes measured immediately after forming. For cases with steam conditioning, temperature of the briquettes was less than the temperature of the grinds because of faster cooling of briquettes exiting the roll press briquetting machine. [c] The grind was moisture conditioned by adding distilled water to increase the moisture content of the grind before briquetting (no steam conditioning was involved). [d] The grind was steam conditioned to increase the temperature of the grind before briquetting. The corresponding moisture content of the grind was measured after steam conditioning (but before briquetting) to include the effect of steam conditioning on the initial moisture content of the grind. [e] NA = data not available. For these briquetting conditions, consistent briquette shape was not formed although the switchgrass particles agglomerated. switchgrass (table 3). The throughput was mostly influenced by the roll speed rather than the screw feeder speed. In general, the faster the roll speed, the higher the throughput of briquettes. For corn stover, 4.2% to 20.7% of the product from the roll press briquetting machine was fines (<5.2 mm) with bulk densities of 231 to 357 kg m -3 (table 3). For switchgrass, 10.7% to 51.2% of the product was fines (<5.2 mm) with bulk densities of 252 to 392 kg m -3 (table 3). It was found that when the percentage of fines through the rolls was lower, the durability and crushing strength of the briquettes were higher. During commercial scale production of briquettes, the fines can be recycled to produce good quality briquettes. Effect of Biomass Feedstock Variables No previous studies on roll press briquetting of corn stover and switchgrass were found in the literature. For roll press briquetting of corn stover without steam conditioning, increasing the particle size from 0.34 mm to 0.36 mm increased the bulk density, durability, and crushing strength of the briquettes; however, the differences between the briquette properties at these two particle sizes were small because of the small difference between the two particle sizes tested (table 3). For briquetting of switchgrass, with and without steam conditioning, increasing the particle size from 0.49 mm to 0.59 mm decreased the bulk density and durability of the briquettes (table 3). Therefore, for large scale roll press briquetting of corn stover and switchgrass, particle sizes with geometric mean particle diameters of 0.36 mm (i.e., hammer mill screen size of 4.0 mm) and 0.49 mm (i.e., hammer mill screen size of 2.4 mm), respectively, could be used to produce good quality briquettes. For roll press briquetting without steam conditioning of corn stover and switchgrass, increasing the moisture content of the grinds decreased the bulk density of the briquettes, but increased the durability of briquettes (table 3). The lower bulk density at the higher moisture content of the grind may have been due to loss of mass due to the evaporation of water from the briquettes, and due to the possible expansion of the briquettes after leaving the roll press briquetting machine. The higher briquette durability at the higher moisture content may have been due to the activation of the water soluble carbohydrates in the biomass materials. Furthermore, when briquetted without steam conditioning, at the higher moisture content, corn stover resulted in higher durability briquettes (durability of 87% at a moisture content of about 15% w.b.) than switchgrass (durability of 50% at a moisture content of about 20% w.b.). This may be due to the presence of larger amounts of water soluble carbohydrates (i.e., natural binding components) in corn stover (7.9% d.b.) than in switchgrass (2.2% d.b.). For the roll press briquetting of corn stover with a particle size of 0.34 mm or 0.36 mm, increasing the temperature of the grind from room temperature (i.e., C) to C by steam conditioning increased the durability and crushing strength of the briquettes, but decreased the bulk density of the briquettes (table 3). In addition to activating the water soluble carbohydrates, steam conditioning of corn stover grind appeared to activate the binding functionalities of the additional natural binding components such as lignin, protein, starch, and fat, which need a grind temperature close to the glass transition temperature (i.e., 75 C) for complete activation. This improved binding effect due to the elevated temperature of the grind was reflected in increased durability and crushing strength values of the corn stover briquettes (table 3). However, the results suggest that if the corn stover grind moisture content is 15% (w.b.) or above, then steam conditioning to increase the temperature of the grind Vol. 52(2):

8 to about 75 C is not necessary in order to produce strong and durable corn stover briquettes. With no steam conditioning (i.e., briquetting at room temperature), a consistent briquette shape was not formed during roll press briquetting of switchgrass. However, the switchgrass grind was agglomerated, and the agglomerated product had durability of 39% to 40% (table 3). Steam conditioning the switchgrass grind to a temperature of 68 C or 85 C resulted in higher values of bulk density (410 to 420 kg m -3 ), durability (58% to 70%), and crushing strength (53 to 171 N) of briquettes compared to cases without steam conditioning for both particle sizes. The improved strength and durability of the switchgrass briquettes for the steam conditioning cases was probably due to the activation of the natural binding components in the switchgrass. Steam conditioning of switchgrass grind to 109 C resulted in lower durability of briquettes than those values measured at 68 C (table 3). This may be due to the loss of moisture from the switchgrass when heating the grind to more than 100 C. Because there was not enough moisture (at least 10% w.b.) available to soften the natural binding components that require water for activation (softening), the net natural binding effect could have been reduced at the elevated temperature of 109 C. The maximum switchgrass briquette durability obtained in this study was 70%. We believe that higher switchgrass briquette durability can be obtained in a commercial scale roll press briquetting machine. Kaliyan and Morey (2007b) studied several strategies to improve the durability of switchgrass briquettes. They found that mixing of either 20% (wt.) corn stover or 5% (wt.) lime at preheating temperatures of 75 C to 100 C could increase the switchgrass briquette durability to >80%. Addition of 20% (wt.) corn stover would involve no additional cost for briquetting of switchgrass if the price of corn stover and switchgrass is about the same. PELLETING The paired samples t tests for pellets showed that the bulk density and durability of the pellets measured after one week of storage were not significantly different from the values measured immediately after forming (P > 0.05). This suggests that the change in pellet mass and/or volume and the effect of curing that occurred during the one week storage period were negligible. Therefore, the data for the bulk density and durability of the pellets measured immediately after forming and after one week of storage were pooled for statistical analysis (table 4). Effect of Pelleting Machine Variables A review by Kaliyan and Morey (2009) found that the following pellet machine variables influence the strength and durability of the pellets: length to diameter (L/D) ratio of the die, speed of the ring die, gap between the rolls and ring die, the specific energy input to the pellet mill, and steam conditioning/high shear processing before pelleting, such as feed conditioning in an expander. In this study, only the effect of the L/D ratio of the die (5.3 and 6.0) was tested, and other pellet machine variables were kept constant. In addition, in this study, a constant die diameter of 9.5 mm was used to pellet corn stover and switchgrass. This resulted in pellets with diameter of 9.6 to 9.8 mm, length of 20.7 to 24.0 mm, and individual pellet density of 1029 to 1111 kg m -3. With a die diameter of 9.5 mm, switchgrass pellets with durability of 75% to 86% were produced in this study (table 4). Colley et al. (2006) found that a die diameter of 4.76 mm produced switchgrass pellets with durability of 78% to 97%. In addition, for pelleting alfalfa, a die diameter of 6.1 to 6.4 mm was found to produce good quality pellets (i.e., pellet durability of up to 80%) (Hill and Pulkinen, 1988; Tabil and Sokhansanj, 1996). In this study, we used a larger die diameter than has been used in the past for pelleting biomass materials. Selection of a larger die diameter would result in higher throughput of pellets, and thus a die diameter of 9.5 mm could be used for commercial production of corn stover and Moisture Content of Grind (% w.b.) (n = 3) Table 4. Properties of corn stover and switchgrass pellets. Pelleting Conditions (mean ±SD, if given) Properties of Pellets (mean ±SD, if given) Temp. of Grind ( C) Die L/D Ratio [a] Fines Output along with Pellets [b] Throughput of Cleaned Pellets (kg h 1 ) Temp. of Pellets [c] ( C) Moisture Content (% w.b.) [d] (n = 3) Bulk Density (kg m 3 ) [e] (n = 4) Durability (%) [e] (n = 4) Hardness (along diameter) (N) [d] (n = 10) Corn stover grind with particle size of 0.34 mm ±0.29 (hammer mill screen size = 2.4 mm (3/32 in.)) [f] 19.8 ± % ± ± ± ± ± % ± ± ± ±99.3 Corn stover grind with particle size of 0.36 mm ±0.35 (hammer mill screen size = 4.0 mm (5/32 in.)) [g] 21.8 ± % ± ± ± ±52.4 Switchgrass grind with particle size of 0.49 mm ±0.38 (hammer mill screen size = 2.4 mm (3/32 in.)) [h] 20.8 ± % ± ± ± ± ± % ± ± ± ±61.6 [a] L = length of the holes in the ring die; D = diameter of the holes in the ring die (D = 9.5 mm). [b] Fines output along with pellets (%) = (kg of fines 100) / (kg of fines + kg of cleaned pellets). The size of fines is <6.4 mm. [c] Temperature of pellets measured immediately after forming ( C). [d] Moisture content of pellets and pellet hardness were measured after one week of storage of pellets at room temperature (about 23 C). [e] Bulk density and durability of pellets are average values for the pooled data measured immediately after forming and after one week of storage of pellets at room temperature (about 23 C). [f] For corn stover with particle size of 0.34 mm, the die L/D ratio of 5.3 versus 6.0 produced significantly different bulk densities (P < 0.05), but not durabilities (P > 0.05). [g] At the die L/D ratio of 6.0, the corn stover grind with particle size of 0.34 mm versus 0.36 mm resulted in significantly different bulk densities (P < 0.05), but not durabilities (P > 0.05). [h] For switchgrass with particle size of 0.49 mm, the die L/D ratio of 5.3 versus 6.0 produced significantly different bulk densities and durabilities (P < 0.05). 550 TRANSACTIONS OF THE ASABE

9 switchgrass pellets. In general, increasing the L/D ratio of the die from 5.3 to 6.0 increased the temperature of the pellets, bulk density of the pellets, durability of the pellets, and hardness of the pellets (table 4). For the corn stover grind with particle size of 0.34 mm, the die L/D ratio of 6.0 resulted in 55.3 kg m -3 higher bulk density of pellets (P < 0.05), but similar pellet durability (P > 0.05), than for the die L/D ratio of 5.3. At the die L/D ratio of 6.0, the corn stover grind with particle size of 0.34 mm resulted in 62.3 kg m -3 higher bulk density of pellets (P < 0.05), but similar pellet durability (P > 0.05), than for the corn stover grind with particle size of 0.36 mm. For the switchgrass grind with particle size of 0.49 mm, the die L/D ratio of 6.0 resulted in 42.0 kg m -3 higher bulk density and 10.3% points higher durability of pellets than for the die L/D ratio of 5.3 (P < 0.05). Therefore, the L/D ratio of the die of 6.0 could be chosen for commercial scale pelleting of corn stover and switchgrass. A peripheral ring die speed of 4 to 5 m s -1 has been suggested to expel a large volume of air during pelleting low bulk density feed (Heinemans, 1991; Tabil and Sokhansanj, 1996; Kaliyan and Morey, 2009). Tabil and Sokhansanj (1996) produced highly durable alfalfa pellets at a ring die speed of 250 rpm (2.8 m s -1 ). In this study, the speed of the ring die used was 232 rpm, which is equivalent to a peripheral ringdie speed of 4.3 to 4.4 m s -1. With this speed of the ring die (i.e., 232 rpm), highly durable pellets were produced from corn stover (pellet durability of up to 95%) and switchgrass (pellet durability of up to 86%). Although no steam conditioning was used for pelleting corn stover and switchgrass, the temperature of the pellets was measured at 67 C to 81 C due to the frictional heating of the grinds during pelleting (table 4). This suggests that the increase in temperature of the grinds during pelleting could have created temperatures in the range of glass transition of the biomass grinds, and thus the natural binding components in the biomass materials would have been activated. Throughput of the pelleting machine ranged from 163 to 226 kg of cleaned pellets per hour for corn stover, and from 86 to 176 kg of cleaned pellets per hour for switchgrass (table 4). In addition, 1.1% to 4.4% of fines (<6.4 mm) along with pellets were collected as products (table 4). The fines were produced mainly by the action of the knife while cutting the pellets, the abrasion of pellets in the collection container, and handling immediately after production before sufficient curing could take place. In commercial pellet production, fines can be recycled to make good quality pellets. Effect of Biomass Feedstock Variables No previous pelleting study on corn stover was found in the literature; however, Samson et al. (2000), Jannasch et al. (2004), and Colley et al. (2006) studied the pelleting of switchgrass. Samson et al. (2000) and Jannasch et al. (2004) produced switchgrass pellets with bulk density of 593 to 641 kg m -3 and hardness of >30 as measured by the Pfizer tablet hardness tester. They found that switchgrass pellets created large quantities of fines (about 43% of initial feedstock dry matter) during the process of handling, screening, and shaking before bagging of pellets; however, no pellet durability data were reported. Colley et al. (2006) reported that switchgrass grind with a moisture content of about 20% (w.b.) produced high quality pellets. Similarly, in this study, a grind moisture content of about 20% (w.b.) resulted in good quality corn stover and switchgrass pellets. In addition, Colley et al. (2006) manufactured switchgrass pellets (die diameter = 4.76 mm) with bulk density of 687 kg m -3, durability of 96%, and pellet hardness of 27 N at a pellet temperature of 85 C and pellet moisture content of 11% (w.b.). Likewise, in this study, switchgrass pellets (die diameter = 9.5 mm) with bulk density of 570 kg m -3, durability of 86%, and pellet hardness of 216 N at a pellet temperature of 81 C and pellet moisture content of 11% (w.b.) were produced (table 4). The main difference between this study and Colley et al. (2006) is that Colley et al. (2006) steam conditioned the switchgrass grind to raise the temperature of the grind to 75 C before pelleting in a lab scale, low power pelleting machine (1.5 kw), whereas in this study, steam conditioning was not done before pelleting the switchgrass in a pilot scale, high power pelleting machine (29.8 kw). The results suggest that, considering the cost of grinding, corn stover grind with a geometric mean diameter of 0.36 mm (i.e., hammer mill screen size of 4.0 mm) could be chosen for producing highquality corn stover pellets in commercial scale pelleting machines (table 4). For switchgrass, results from this study and from Colley et al. (2006) showed that switchgrass grind with geometric mean diameters of 0.49 mm to 0.87 mm (i.e., hammer mill screen sizes of 2.4 to 3.2 mm) could be used to produce highly durable switchgrass pellets in commercial scale pelleting machines. COMPARISON OF ROLL PRESS BRIQUETTING WITH PELLETING The briquettes produced were of almond shape with a maximum size of 31.3 mm length 23.3 mm width 17.9 mm depth, and the briquettes had a maximum of 5.0 mm flashings. The pellets made were of cylindrical shape with a maximum size of 9.8 mm diameter 24.0 mm length. Higher bulk density, durability, and strength values were obtained for pellets than for briquettes. The briquettes had bulk densities from a minimum of 1.8 to 2.6 times to a maximum of 3.5 to 5.3 times the bulk density of the bales (bulk density of the bales is 100 to 200 kg m -3 ). The pellets had bulk densities from a minimum of 2.6 to 3.1 times to a maximum of 5.3 to 6.1 times the bulk density of the bales. The non uniform shape of the briquettes may have contributed to lower bulk density of briquettes than that of pellets. In addition, during the durability test of briquettes, the loss of mass occurred due to the breakage of flashings around the almond shape of the briquettes, but the almond shape was approximately preserved. Thus, selection of a more uniform shape for the briquette pockets on the rolls could result in higher bulk density and higher durability briquettes. With no steam conditioning, the temperature of the briquettes was 51 C to 63 C, and the temperature of the pellets was 67 C to 81 C. This rise in temperature of the products was purely due to the heating provided by the friction/shear between the biomass particles, and between the biomass particles and the machine. In this study, we were unable to measure the specific energy consumed by the roll press briquetting machine. The specific energy consumed for the pelleting process was 0.95% to 1.31% of the energy in corn stover (189 to 262 MJ t -1 ) and 2.37% to 2.43% of the energy in switchgrass (403 to 414 MJ t -1 ). The above specific energy values did not include the energy consumed for running the empty (i.e., no load) pelleting machine (Kaliyan and Morey, 2007a). The power consumption for running the empty pel Vol. 52(2):

10 leting machine was about 18 kw, which was equal to an aver age of 344 MJ t-1 for corn stover and 554 MJ t-1 for switchgrass. The energy content of the corn stover and switchgrass was 20 and 17 MJ kg-1, respectively (Lemus et al., 2002; Pordesimo et al., 2005). Komarek (1963) reported that the specific energy required for roll press briquetting of organic solids (i.e., biomass mate rials) was 16 to 32 MJ t-1. Drzymala (1993) reported that large scale roll press briquetting (up to 60 t h-1) of ores, metal chips, and coals required specific energy of 20 to 60 MJ t-1. Dec (2002) found that roll press briquetting machines con sumed about 18 to 36 MJ t-1 at optimum briquetting condi tions for sodium chloride, quick lime, and municipal sludge. Samson et al. (2000) reported that pelleting alfalfa, switch grass, and straw required specific energy of 108, 162, and 300 MJ t-1, respectively, for pellet mill operation. Jannasch et al. (2004) found that specific energy consumption by a pel leting mill was 268 MJ t-1 for pelleting switchgrass in a com mercial pelleting plant (2 t h-1). Thus, specific energy consumption is likely to be less for both briquettes and pellets produced in commercial scale systems optimized for pro duction. Since roll press briquetting relies primarily on nor mal compression with a small amount of friction, while ring die pelleting requires some normal compression and rel atively high friction/shear (fig. 1), it is likely that lower spe cific energy consumption may be possible in briquetting compared to pelleting in commercial scale systems opti mized for production. The angle of repose values for corn stover and switchgrass briquettes ranged from 21 to 24. The angle of repose values (a1) Corn stover grind (particle size = 0.34 mm) before briquetting or pelleting (no coatings of natural binders on most of the particles). for corn stover and switchgrass pellets ranged from 19 to 21. For comparison, the angle of repose of shelled corn ranges from 16 to 27 (Mohsenin, 1986). The angle of re pose values provide an indication of flow ability of the bri quettes and pellets (Mohsenin, 1986). The moisture content of the corn stover and switchgrass briquettes ranged from 6% to 14% (w.b.) (table 3). The moisture content of the corn stov er and switchgrass pellets ranged from 11% to 15% (w.b.) (table 4). The storage stability of the briquettes and pellets is determined by their moisture contents (Maier et al., 1992; Colley et al., 2006). Future work is required to determine the safe storage moisture contents for briquettes and pellets made from corn stover and switchgrass. BINDING MECHANISMS OF CORN STOVER AND SWITCHGRASS According to Rumpf (1962) and Pietsch (2002), bonding between particles can take place through a solid bridge or through inter particle attraction forces when there is no solid bridge. Solid bridges are developed due to partial melting of components, crystallization of soluble substances, chemical reaction, hardening of binders, adsorption layers due to high ly viscous binders (adhesion and cohesion forces due to the binder layer between particles), and mechanical interlocking of particles. When there is no solid bridge between particles, but the particles are brought together, short range attraction forces such as molecular (valence forces, for instance, due to free chemical bonds; hydrogen bridges; and van der Waals' forces), electrostatic, and magnetic forces can cause the par ticles to adhere to each other. (b1) Cross-section of a corn stover briquette made with a particle size of 0.34 mm, grind moisture content of 15% (w.b.), and no steam conditioning (with coatings of natural binders). (a2) Switchgrass grind (particle size = 0.49 mm) (b2) Cross-section of a switchgrass briquette made with a particle size of 0.49 mm, before briquetting or pelleting (some natural grind moisture content of 12% (w.b.), binders are expressed on the particles and steam conditioning to 685C due to size reduction). (with coatings of natural binders). (c1) Cross-section of a corn stover pellet made with a particle size of 0.34 mm, grind moisture content of 19% (w.b.), die L/D ratio of 6, and no steam conditioning (with coatings of natural binders). (c2) Cross-section of a switchgrass pellet made with a particle size of 0.49 mm, grind moisture content of 20% (w.b.), die L/D ratio of 6, and no steam conditioning (with coatings of natural binders). Figure 4. Scanning electron microscopy (SEM) images of corn stover and switchgrass grinds, briquettes, and pellets (magnification at 600 ). The SEM images of the briquettes and pellets show that the particles are coated with natural binders, which are pressed out of the biomass cells during briquetting or pelleting. 552 TRANSACTIONS OF THE ASABE

11 In the roll press briquetting and pelleting machines, bio mass particles could have experienced pressures of 100 to 200 MPa (Dec, 2002; Kaliyan and Morey, 2009). Because of the application of high pressures, particles were brought close together, causing inter particle attraction forces, and the natural binding components in the corn stover and switch grass were squeezed out of the cells, which made solid bridges between the particles. After cooling, these solid bridges hardened (i.e., curing process). This caused the bri quettes and pellets to become strong and durable. This postu late on the binding mechanisms was verified through SEM and UV auto fluorescence images. Figures 4a1 and 4a2 show that most of the corn stover par ticles appear to be bare without any sign of coating of natural binding components; however, on the switchgrass particles, expression of some natural binding components (due to size reduction) can be observed. The SEM images of the bri quettes and pellets show that the particles are covered with a layer of natural binders (fig. 4). When viewed with light mi croscopy (Nikon SMZ 1500), these coatings appeared as glassy/white sugar like coatings on the particles, and high ac cumulation of these binding components was observed at the junctions of particles (Kaliyan and Morey, 2008). The UV auto fluorescence images of the briquettes and pellets indi cate that the natural binders that were coated on the particles in the briquettes or pellets were primarily lignin and protein compounds (fig. 5). The molecules that did not fluoresce (i.e., black color) could be carbohydrates (cellulose, hemi cellulose, simple sugars, and starch) and fat (fig. 5). The UV auto fluorescence images of the shiny appearance of pellets confirmed that lignin was heavily coated on the outer surface of the pellets (Kaliyan and Morey, 2008). Because lignin is hydrophobic in nature, the lignin coating on the outer sur faces of the briquettes/pellets can make them water resistant (Anglés et al., 2001). The SEM images of briquettes and pellets shown in fig ure 4 are for the briquetting or pelleting conditions that re sulted in the highest durability and strength values. The SEM images of briquettes and pellets made under conditions that resulted in lower durability and strength values also appeared to have similar coatings of natural binding components (Kali yan and Morey, 2008). Therefore, the SEM images can only explain the squeezing of natural binders from the particles due to the high pressure, but not the differences in the durabil ity and strength values. The differences in the durability and strength values can be explained by the extent of activation of the expressed natural binders through moisture or temperature or both. The amount of moisture required de- (a1) Corn stover grind before briquetting or pelleting (particle size = 0.34 mm). (b1) Cross-section of a corn stover briquette made with a particle size of 0.34 mm, grind moisture content of 15% (w.b.), and no steam conditioning. (c1) Cross-section of a corn stover pellet made with a particle size of 0.34 mm, grind moisture content of 19% (w.b.), die L/D ratio of 6, and no steam conditioning. (a2) Switchgrass grind before briquetting or pelleting (particle size = 0.49 mm). (b2) Cross-section of a switchgrass briquette made with a particle size of 0.49 mm, grind moisture content of 12% (w.b.), and steam conditioning to 685C. (c2) Cross-section of a switchgrass pellet made with a particle size of 0.49 mm, grind moisture content of 20% (w.b.), die L/D ratio of 6, and no steam conditioning. Figure 5. UV auto fluorescence images of corn stover and switchgrass grinds, briquettes, and pellets (magnification at 145 ). Kaliyan (2008) also docu mented these images in color. Auto fluorescence color interpretation (Rost, 1995): green or yellow green for protein compounds; brilliant blue or bluish white for lignin, cutin, suberin, or phenolic acids such as ferulic acid; and whitish fluorescence for cutin (cuticle). Pure carbohydrates (cellulose, hemicellulose, and starch) and lipid/fat molecules do not fluoresce (Rost, 1995). The UV auto fluorescence images for briquettes and pellets indicate that the natural binders that are coated on the particles in the briquettes and pellets are primarily lignin and protein compounds. Vol. 52(2):

12 pends on the amount of natural binder (e.g., water soluble carbohydrates) available that requires moisture for activation. For example, as discussed previously, at high moistures (15% to 20% w.b.), corn stover resulted in stronger and more durable briquettes than switchgrass because of the presence of a higher amount of water soluble carbohydrates than switchgrass. Glass transition temperature is the minimum temperature required for activation of natural binders. The glass transition in corn stover and switchgrass starts at 50 C (Kaliyan and Morey, 2006). Therefore, in addition to extracting the natural binding components from the particles (due to the application of high pressures by the densification machines), the natural binding components should be fully activated to use their binding functionality by providing enough moisture and a temperature in the range of the glass transition for the biomass materials. SUMMARY AND CONCLUSIONS In this research, pilot scale roll press briquetting and pelleting behaviors of corn stover and switchgrass were studied. The optimum machine variables (screw feeder and roll speeds in the roll press briquetting machine, and length to diameter (L/D) ratio of the die in the pelleting machine) and biomass feedstock variables (particle size, moisture content, and steam conditioning temperature) that could produce strong and durable briquettes and pellets in commercial scale briquetting and pelleting machines were determined. The briquettes made were of almond shape with a maximum size of 31.3 mm length 23.3 mm width 17.9 mm depth, and the briquettes had a maximum of 5.0 mm flashings. The pellets made were of cylindrical shape with a maximum size of 9.8 mm diameter 24.0 mm length. Corn stover briquettes had bulk density of 422 to 481 kg m -3, durability of 67% to 90%, and crushing strength of 88 to 277 N, whereas corn stover pellets had bulk density of 548 to 610 kg m -3, durability of 94% to 95%, and hardness of 197 to 224 N. In addition, switchgrass briquettes had bulk density of 351 to 527 kg m -3, durability of 39% to 70%, and crushing strength of 28 to 171 N, whereas switchgrass pellets had bulk density of 528 to 570 kg m -3, durability of 75% to 86%, and hardness of 148 to 216 N. The bulk densities of corn stover and switchgrass briquettes ranged from 1.8 to 5.3 times the bulk density of bales (bulk density of bales is 100 to 200 kg m -3 ). The bulk densities of corn stover and switchgrass pellets ranged from 2.6 to 6.1 times the bulk density of bales. This study proved that high quality briquettes and pellets could be produced from corn stover and switchgrass without adding chemical binders (i.e., additives) by activating (softening) the natural binding components, such as water soluble carbohydrates, lignin, protein, starch, and fat, in the corn stover and switchgrass through moisture and temperature. In addition, this study showed that if the temperature rise of the biomass grinds due to frictional heating in the briquetting and pelleting machines is in the range of glass transition (i.e., softening) temperature of the biomass materials (i.e., >75 C), then strong and durable briquettes and pellets could be produced without steam conditioning. Briquettes produced with the roll press briquetting machine had bulk densities, durabilities, and crushing strengths that were somewhat less than, but in a range comparable to, the pellets produced with the conventional pelleting machine. Future work is required to compare the cost and energy involved for commercial scale briquetting and pelleting of corn stover and switchgrass. Moreover, roll press briquetting appears to be a promising low cost, low energy, highcapacity approach for densifying corn stover and switchgrass for renewable energy applications. ACKNOWLEDGEMENTS The authors wish to acknowledge the Initiative for Renewable Energy and the Environment (IREE), University of Minnesota, for providing support for this study; Bepex International LLC, Minneapolis, Minnesota, for providing access to their facility and assistance with the roll press briquetting study; and the Agricultural Utilization Research Institute (AURI), Waseca, Minnesota, for providing access to their facility and assistance with the size reduction of biomass feedstocks and pelleting study. In addition, the authors would like to thank Mr. Matthew De Kam (University of Minnesota) for his assistance with this study. REFERENCES Anglés, M. N., F. 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