James Dooley 2017-04-27 01:52:21
The bane of biomass feedstock engineers Poor feedstock flowability was identified by the U.S. Department of Energy’s Bioenergy Technologies Office and by the USDA as one of the critical issues limiting success in the emerging bioeconomy. Traditional analytical and laboratory flowability metrics are proving to be of little use for designers of hoppers, feeders, mixers, and other essential processing equipment. Even with advanced modeling tools, the performance of biomass hoppers and feeders is rarely successful on the first try. The best designers and builders have decades of experience and a junkyard full of lessons learned. Major scale-up and start-up problems at cellulosic biorefineries and biomass processing facilities are being traced to feedstocks with poor flowability. Augers stall, drive motors burn out, hoppers bridge, mixers plug—the litany goes on and on. Many of the agricultural and biological engineers who are designing facilities for the advanced bioeconomy have a background in grain handling, and they dream of a time when biomass feedstocks might flow like corn, wheat, rice, or marbles. Unfortunately, ground corn stalks and shredded poplar trees behave more like matted cat fur than marbles. It’s a running joke in my company that we have never designed and built a biomass feed hopper that worked without many artisanal modifications and iterations. If we build a hopper, we have to add a platform for a human assistant, who uses a big stick to keep things flowing. There are two solutions to the problem of flowability. One is to preprocess fibrous biomass materials into particle forms that have better flow properties and less propensity to interlock. The other solution is to quantify the physical properties of available biomass feedstocks in engineering science terms and develop better analytical models to enable design of equipment and handling systems that actually work. This article is a summary of current research in biomass flowability. Making biomass flowable ASABE member David Lanning at Forest Concepts is leading an effort funded by the Bioenergy Technologies Office of the U.S. Department of Energy to design biomass comminution and screening equipment that produces flowable feedstocks. The Crumbler® rotary shear machine, which received a 2016 AE50 Award, operates in tandem with optimized screening to produce feedstocks that have low aspect ratios, uniform cross-sections, and low compressibility—all of which improve flowability. This machine system and knife mills from other manufacturers are gaining market share. However, although new comminution methods can improve flowability, the best materials produced by these innovations are still marginally flowable compared to corn and small grains. Idaho National Laboratory (INL) has been working on technologies that blend biomass materials into pelletized “uniform format feedstocks” that can be handled like conventional fuel pellets. This approach deals with flowability at the biomass source and at depots close to the source. Biorefineries can then use material handling technologies from the established wood pellet industry. Although the INL approach greatly reduces downstream issues, those who convert raw biomass and wood chips to densified pellets still face the flowability problem. ASABE Fellow Shahab Sokhansanj at the University of British Columbia and team members at the Oak Ridge National Laboratory in Tennessee are studying the interactions of biomass grinding and flowability characteristics. In particular, they seek to understand the relationships between particle size, shape, and flow properties. ASABE member Amit Kumar and associates at the University of Alberta in Edmonton have been developing workable methods for transporting biomass feedstocks in pipelines using water slurry. Pipeline transport over long distances presents unique challenges that are associated with feedstock physical properties. The same mixing, slurry stability, compressive dewatering, and feedstock issues are present within the short-distance piping, pumping, and mixing unit operations of biorefineries. Flowability testing and modeling Research engineers around the world are extending their understanding of flowability from related work with minerals, soil, powders, and grain to the problems of biomass feedstocks. They are finding that some flowability metrics, such as the Hausner index, angle of repose, and aspect ratio, correlate well with the observed relative flowability of biomass. Laboratory equipment for quantifying internal shear properties, hopper friction angles, and the like is being applied with variable success to biomass. However, to go beyond saying that one material flows well while another flows poorly, much new research is needed. This will enable engineering of biomass handling and processing equipment that is not negatively impacted by feedstocks that do not flow well. Researchers at the Department of Energy’s INL Biomass Feedstock National User Facility (BFNUF; https://bfnuf.inl.gov) in eastern Idaho use the facility’s characterization laboratory to develop high-quality data on biomass physical properties that affect feedstock handling. Physical characterization includes methods to measure particle density, bulk density, compressibility, elasticity, unconfined yield strength, effective angle of internal friction, wall friction angle, and permeability. Researchers determine the 3-D size and shape distributions of particles, as well as the storage modulus and loss modulus of slurries and pastes, using automated digital imaging. INL also includes the Process Demonstration Unit (PDU), a full-size, fully integrated feedstock preprocessing system. The PDU allows BFNUF scientists to empirically measure the feeding and handling behavior of particulate solids using a suite of custom augers and hoppers. Researchers can test the flowability of biomass materials using a specialized hopper with a continuously adjustable outlet that makes it possible to characterize materials in-line and in real-time for quality assurance and control. Empirical research and experiments with the PDU help engineers get closer to success in materials handling on their first try with specific materials of interest. Álvaro Ramírez-Gómez at the Technical University of Madrid in Spain is developing engineering data on how the physical and mechanical properties of biomass affect flowability, the design of storage facilities, and the like. Physical and mechanical properties are determined at the particle level for use in numerical models based on the discrete element method and at the system-ofparticles level for use in numerical models based on the finite element method. These numerical models allow researchers and engineers to study the flow patterns and pressures that develop in storage facilities. Properties determined at the particle level include Young’s modulus using a texture analyzer, the true density, the particle-particle restitution coefficient, and the particle-wall coefficient of friction, among others. System-level properties include bulk density, angle of repose, angle of internal friction, etc., depending on the nature of the biomass material. Full-scale tests are also carried out in a test facility that includes three silos with different eccentric hoppers for understanding and validating the load distribution of stored materials during filling and discharge. Dr. Ramírez-Gómez is a member of a European flowability working group that includes five universities. The Bio4Flow consortium collaborates on feedstock handling research for biorefineries (www.bio4flow.com). Another Bio4Flow consortium member, the Wolfson Centre for Bulk Solids at the University of Greenwich (www.gre.ac.uk/engsci/research/groups/wolfsoncentre/home), has world-renowned laboratory facilities for quantifying the flow properties of biomass materials. Current research includes developing methods to measure the tensile strength of clumps and bulk fiber solids. Tensile failure of particles and small clumps from larger bulk biomass at discharge openings may lead to better engineering designs for hoppers and feeders. In the private sector, Jenike & Johanson (http://jenike.com/) is a world leader in the design of hoppers and biomass feedstock handling systems that work (unlike our own company’s hoppers that often don’t!). Jenike has perfected the application of the discrete element method to design hoppers, chutes, augers, and the like for biomass materials. They also have largescale physical modeling capabilities and the most comprehensive and multi-continent infrastructure of test laboratories for flow properties. Jenike & Johanson is not alone in developing models for the flow of particulate materials. The ASABE Technical Library currently lists more than 80 documents related to the use of discrete element modeling for various materials, including biomass feedstocks. The bioprocess engineering laboratory at Auburn University, led by ASABE Fellow Oladiran Fasina, conducts studies and quantifies the physical properties that are needed in the design and selection of equipment and systems for handling, processing, storage, and transport of biomass feedstocks. Some of these properties include bulk density, tap density, compressibility, particle size and size distribution, fluidization, flowability, cohesion, angle of internal friction, and angle of wall friction. Unlike most other researchers, Dr. Fasina is exploring the effects of moisture content and particle shape on these properties because (1) biomass feedstocks are biological and therefore they exchange moisture with the environment, and (2) particles of ground biomass feedstocks are non-uniform in size and shape. This multi-sized nature of ground biomass particles makes it difficult to measure the flowability of biomass feedstocks with shear-type testers, which are typically used for quantifying the flow properties of bulk materials. The Auburn lab is gaining recognition for its finding that compressibility testing is a better predictor of flow properties of multi-sized and non-spherical materials, such as biomass feedstocks. At West Virginia University, ASABE member Kaushlendra Singh has first-hand experience with the difficulties of feeding woody biomass and poultry litter into gasifiers. To him, statistical size descriptors are insufficient. He has found that just one or two long, thin pieces in an entire load of fuel can clog the feeders and jam the augers—a piece as big as a new pencil can pass through a 9 mm screen opening. With poultry litter, the problem can be particle cohesion due to chemical bonding or surface moisture. Dr. Singh is currently studying the flowability and friability of biochar products that easily break down to dust during handling and spreading with conventional equipment. Poor flowability is not just a problem for those who handle biomass such as corn stalks, bagasse, and wood fiber. Fiber-rich fractions from milling grains for corn ethanol biorefineries have flowability issues as well. ASABE Fellow Kasiviswanathan Muthukumarappan (aka Muthu) at South Dakota State University and his colleague ASABE member Kurt Rosenstrater at Iowa State University are developing engineering data for a corn milling co-product: distillers dried grains with solubles (DDGS). The flow of DDGS often becomes restricted by caking and bridging, which occur during transport and storage. These issues probably result from a number of factors, including storage moisture, temperature, relative humidity, particle size, time, and temperature variations. The flow properties of DDGS (including cohesion, effective angle of friction, internal angle of friction, yield locus, flow function, major consolidating stress, and unconfined yield strength) have been studied using the Jenike shear tester. In addition, the Carr powder tester has been explored to measure various flow properties of DDGS. A simple yet robust model was developed by combining the important flow properties obtained from conventional Carr and Jenike tests using dimensional analysis and response surface modeling. However, the current model was based on DDGS from just one ethanol plant. Actual DDGS flow properties will be different for each plant due to the host of factors mentioned above. In addition to exploring physical properties to explain flowability, Dr. Muthu is looking into the surface chemistry and potential adhesion properties of DDGS. Staining of DDGS particles indicated a higher amount of surface layer protein compared with carbohydrate thickness in DDGS particles that had a lower flow function index (which indicated potential flow issues). The glass transition and sticky point temperatures of DDGS have also been analyzed. Stickiness of DDGS increased with an increase in moisture content, indicating flow problems resulting from moisture. A step toward understanding DDGS flow using intelligence-based modeling tools was attempted. Neural network modeling was successful in predicting the behavior of key flow properties of DDGS as a function of multiple environmental and storage variables. Hope flows The good news that this summary delivers to ASABE members and other engineers who design facilities and equipment for the new bioeconomy is that a bunch of really smart people are working on the flowability problem in all corners of the globe. Some are developing equipment that comminutes fibrous biomass raw materials into feedstocks with improved flow properties. Others are improving our understanding of the physical, mechanical, and surface chemistry properties that affect the flowability of biomass feedstocks. All of this work comes together to enable advanced modeling and mathematical solutions that inform the design of hoppers, mixers, piping, and material handling systems. Someday soon, our engineering team at Forest Concepts may even design a hopper and feeder system that actually works on the first try. ASABE Fellow and Past President Jim Dooley, Chief Technology Officer, Forest Concepts, LLC, Auburn, Wash., USA, firstname.lastname@example.org.
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