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Author: Dr. Lynn Wendt Laboratory Relationship Manager for Idaho National Laboratory

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Bioenergy researchers are always on the lookout for natural materials that can be used as biomass or bioproducts. As biomass processing becomes more efficient, the more economically competitive it becomes, and the easier it is for consumers to make environmentally conscious consumption choices.

Toward making this a reality, an Idaho National Laboratory (INL) research team has identified models that could help scale-up processing to industry scale at an economically low cost.

The Promising Impact of Loblolly Pine Residue on Biorefineries

Loblolly pine, or more specifically the pine residue from commercial tree harvesting in Southern U.S. forests, has the potential for a major impact on bioenergy infrastructure. Its low cost and widespread abundance have made it a promising domestic biomass resource for fuels, chemicals, and advanced manufacturing products. To fully tap into the potential of loblolly pine residue and other biomass more broadly, however, the issue of 'poor flowability' in the refining process must be addressed.

Biomass Should Go with the Flow

Flowability dictates how well granular biomass material moves through feeding and handling equipment during preprocessing and conversion at a biorefinery. High flowability means the material can move swiftly and smoothly through all machinery and processing components. Low flowability, then, means the opposite: machinery can get slowed, clogged, or stopped, lowering the efficiency of the process and thus the productivity.

Typical issues that can affect flowability during feeding and handling include:

feedstock flowability diagrams
  • Bridging, also called arching, which occurs due to a blockage in the feed equipment caused by high cohesive strength or mechanical interlocking of particles.
  • Plugging and jamming in feeding equipment, which occurs due to the material lumping together and impeding the flow.
  • Rat-holing, which happens when a vertical flow channel develops in the feeding equipment of the feedstock supply, resulting in material stagnation and uneven process times.

Any one of these issues can bring a biorefinery to a costly standstill. While flow is not the only challenge, moving ground wood smoothly and consistently within a biorefinery is a key factor in the successful commercialization of biofuel technologies.

INL Researchers Put Together All the Pieces

Companies that design and distribute hoppers and conveying equipment need a complete understanding of the flowability of ground pine residue to ensure their machines match the demands of the process.

In three peer-reviewed journal articles, INL researchers Yidong Xia, Wencheng Jin, and their collaborators—Jonathan Stickel of the National Renewable Energy Laboratory and Jordan Klinger of INL—have made significant strides in characterizing and modeling the flow behavior of ground loblolly pine. By combining physical experiments and model simulations, they have identified a complete set of material flow attributes that must be incorporated into the design of reliable feeding and handling processes.

While right now there is no single tool available for flow modeling of milled biomass, the team’s research, funded by the U.S. Department of Energy Bioenergy Technologies Office, has identified the most applicable aspects of various computational modeling regimes needed to assemble a toolset that can be applied to milled biomass for biorefining. The research team’s recent study used ground pine as their milled biomass resource, but the success in applying the toolset to ground pine indicates that further application can be extended to other biomass such as fir and corn stover.

INL bioenergy research team - 3 men looking at feedstocks

INL bioenergy research team addressing feedstock flowability challenges by modeling and testing. Photo courtesy of INL

Discovery of the Characterization and Modeling of Loblolly Pine Flow Behavior

In their article in Powder Technology 368, “A Density-Dependent Drucker-Prager/Cap Model for Ring Shear Simulation of Ground Loblolly Pine,” Jin et al. detail key limitations of current ground loblolly pine handling methods and identify areas in which future work may have a near-term impact.

Using a modified Drucker-Prager/Cap model—widely used as a pressure-dependent model for dealing with the plastic deformation of soils, rock, concrete, polymers, and other materials—the researchers discovered that with pine residue, the model does not fully capture the effects of compression and shear histories on material shear behavior.

This discovery highlights shortcomings in the traditional assumptions and simplified designs/testing that are currently used for handling equipment.

In their follow-up article in Powder Technology 383, “Flow Characterization of Compressible Biomass Particles using Multiscale Experiments and a Hypoplastic Model,” Jin and team discovered the critical state theory-based hypoplastic model is capable of accurately predicting the flow behavior of ground pine.

Bulk compaction, particle hardness, and friction angle are critical material attributes that dictate the flow behavior of the ground pine. This study provides a potent tool to decipher and resolve material handling upsets using forest products in biorefineries and other energy industries.

Before and after pictures of ground pine.

Before and after pictures of ground pine. Images courtesy of INL

Part 1: The Limitations of Loblolly Pine Feeding and Handling Methods

In a two-part article in ACS Sustainable Chemistry and Engineering, “A Review of Computational Models for the Flow of Milled Biomass,” the team evaluated existing tools that might be applied to loblolly pine flows. Part I of their review focused on the discrete element method (DEM), which has been used for decades to simulate the bulk behavior of materials during processing. DEM can track the motion, and in some cases the deformation, of each particle. It has been widely used for process modeling in engineering applications.

Accurate modeling of a biomass particle system using DEM has its challenges. Unlike other feedstocks that DEM is used for, such as rocks and powders that can be milled into relative uniformity because of their density and flexibility, the material properties of individual biomass particles are essentially non-homogeneous because of their complicated internal and surface structures.  Examples of these biomass particle material properties include:

  • Water content
  • Density
  • Porosity
  • Elastic modulus
  • Surface roughness..

In industrial-scale biomass storage and processing equipment, such as silos and screw-conveyors, there can be millions of particles with their own unique geometries and properties.

Part 2: One Flowability Process Model Does Not Rule Them All

DEM is often used for bulk flow property simulations of granular materials at the laboratory-scale. However, the direct use of DEM for industrial-scale flow simulations is challenging due to its high computational costs.

In Part II of their review, INL researchers proposed that DEM can aid in developing and qualifying continuum-mechanics models. Though these models do not represent particles explicitly, they capture the bulk mechanical behavior using constitutive equations, enabling them to model granular flow from the lab- to industry-scale at a computationally low cost. This is attractive for simulation-aided design of industrial-scale feeders and hoppers for biomass as it addresses the issues of scale up and cost.

Testing Algorithms in the BFNUF’s Process Development Unit

Before these modeling tools can be used to effectively improve the flow of loblolly pine, INL researchers will need to test the detailed numerical algorithms, which describe the complex relationship between the interparticle contact physics in the DEM models and the continuum-mechanics models. The process development unit (PDU) at the Biomass Feedstock National User Facility (BFNUF) at INL will conduct this part of the research as it can test fully integrated industrial-scale preprocessing systems.

The continuous processing and data collection at the PDU will enable the validation of multiscale simulation approaches and will advance numerical capabilities for modeling loblolly pine residue and the granulate flow of other biomass feedstocks. In turn, suitable flow models will enable engineers to design effective and economical biorefineries that produce sustainable biomass-derived products.