Applied Engineering in Agriculture Vol. 24(3): 351‐358 E 2008 American Society of Agricultural and Biological Engineers ISSN 0883-8542 351 BULK DENSITY OF WET AND DRY WHEAT STRAW AND SWITCHGRASS P ARTICLES P. S. Lam, S. Sokhansanj, X. Bi, C. J. Lim, L. J. Naimi, M. Hoque, S. Mani, A. R. Womac, X. P. Ye, S. Narayan ABSTRACT. Bulk density is a major physical property in designing the logistic system for biomass handling. The size, shape, moisture content, individual particle density, and surface characteristics are few factors affecting the bulk density. This research investigates the effects of true particle lengths ranging from 6 to 50 mm and moisture contents ranging from 8% to 60% wet basis (wb) on the bulk density of wheat straw and switchgrass. Three types of particle densities of straw and switchgrass measured were: a hollow particle density assuming a hollow cylindrical geometry, a solid particle density assuming a solid cylindrical geometry, and a particle density measured using a gas pycnometer at a gas pressure of 40 kPa. The bulk density of both loose‐fill and packed‐fill biomass samples was examined. The calculated wet and dry bulk density ranged from 24 to 111 kg m ‐3 for straw and from 49 to 266 kg m ‐3 for switchgrass. The corresponding tapped bulk density ranged from 34 to 130 kg m ‐3 for straw and 68 to 323 kg m ‐3 for switchgrass. The increase in bulk density due to tapping the container was from 10% for short 6‐mm particles to more than 50% for long 50‐mm particles. An equation relating the bulk density of stems as a function of moisture content, dry bulk density, and particle size was developed. After the validation of this bulk density equation, the relationship would be highly useful in designing the logistics system for large‐scale transport of biomass to a biorefinery. The bulk density and particle density data of uniform particles would be important, if straw and switchgrass is used for pulping and paper making. Keywords. Bulk density, Particle density, Straw, Switchgrass, Packing, Porosity, Fibrous biomass. ulk density is an important characteristic of biomass that influences directly the cost of feedstock delivered to a biorefinery and storage cost (Sokhansanj and Fenton, 2006). It also impacts storage requirements, the sizing of the material handling system and how the material behaves during subsequent thermo‐chemical and biological processes. (McKendry, 2002). The engineering design and operation of transport equipment, storages, and conversion processes Submitted for review in November 2007 as manuscript number FPE 7262; approved for publication by Food & Process Engineering Institute Division of ASABE in April 2008. Presented at the 2007 ASABE Annual Meeting as Paper No. 076058. The authors are Pak Sui Lam, ASABE Member, Graduate Research Associate, Department of Chemical and Biological Engineering, University of British Columbia, Vancouver, British Columbia, Canada; Shahab Sokhansanj, ASABE Member, Adjunct professor, Department of Chemical and Biological Engineering, University of British Columbia, Vancouver, British Columbia, Canada and Distinguished Research Scientist Oak Ridge National Laboratory, Environmental Sciences Division, Oak Ridge, Tennessee; Xiaotao Bi, Professor, C. Jim Lim, Professor, Ladan J. Naimi, ASABE Member, Graduate Research Associate, Mozammel Hoque, ASABE Member, Posdoctoral Associate Department of Chemical and Biological Engineering, University of British Columbia, Vancouver, British Columbia, Canada,; Sudhagar Mani, Assistant Professor, Faculty of Engineering, Driftmier Engineering Center, University of Georgia, Athens Georgia; Alvin Ray Womac, ASABE Member, Professor, X. Philip Ye, Assistant Professor, Department of Biosystems Engineering and Soil Science, The University of Tennessee, Knoxville, Tennessee; Sundar Narayan, Professor, Lambton College, Sarnia, Ontario, Canada. Corresponding author: Pak Sui Lam, Department of Chemical and Biological Engineering, University of British Columbia, Vancouver, British Columbia, Canada, V6T1Z3; phone: 604‐827‐3413; fax: 604‐802‐6003; e‐mail: plam@chml.ubc.ca. depends on bulk density and flow characteristics of feedstock (Woodcock and Mason, 1987). A recent study by Ryu et al. (2006) investigated the effect of bulk density on the combustion characteristics of biomass. They found that the ignition front speed was inversely proportional to bulk density, while the burning rate tends to decrease linearly. As a physical property, bulk density (r b ) depends on material composition (c), particle shape (f) and size (l, d), orientation of particles (s), specific density of individual particles (r p ), particle size distribution (PSD), moisture content (mc), and applied axial pressure (p) (Peleg, 1983; Lang et al., 1993; Sokhansanj and Lang, 1996). ) p , mc , PSD , s , , d , l , , c ( f p b = ò f ò (1) where l and d are length and diameter of particles. The variables in the right side of equation 1 are not all independent of each other. For example, moisture content has an effect on particle density and surface characteristics. Similarly, particle size and distribution depends on upstream processes such as chopping, drying, grinding, sieving, etc. Length (l) and diameter (d) define a shape factor (f) for the particle. Other factors such as surface characteristics also affect the bulk density and flow characteristics of biomass. The agricultural biomass particles are usually cylindrical or disk shape depending upon the part of the plant they originate. The long particles with aspect ratio (l/d) of greater than one are classified as cylindrical while the short particles (l/d<1) are classified as disk. The packing behavior of these non‐spherical shapes usually starts from studying the mono‐sized particles (Zou and Yu, 1996a; Zhang et al., 2006) and then gradually moves to the particle mixtures with B