Microscale Mechanical Characterization for Macroscale Quality Assessment of Densified Biomass Assemblies
Open Access
- Author:
- Karamchandani, Apoorva
- Graduate Program:
- Agricultural and Biological Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- December 14, 2017
- Committee Members:
- Virendra M Puri, Dissertation Advisor/Co-Advisor
Virendra M Puri, Committee Chair/Co-Chair
Jude Liu, Committee Member
Daniel Edward Ciolkosz, Committee Member
Md Amanul Haque, Outside Member
Hojae Yi, Special Member - Keywords:
- characterization
switchgrass
coordination number
contact area
pellet
diametral tensile strength - Abstract:
- The quantitative contribution of inter-particle binding mechanisms at the microscale is essential knowledge to engineering the biomass densification. The goal of the presented research was to characterize the mechanical behavior of densified biomass assemblies at the microscale and assess their macroscale quality based on the microscale properties. The objectives aimed to quantify the mechanical properties of particle-particle binding, to characterize the arrangement of particles in a densified assembly, and to develop a relationship between microscale mechanical characteristics and macroscale mechanical biomass assembly properties. Switchgrass was size reduced using one screen size of 3.175 mm and conditioned to have the moisture content of 17.5% (w.b.) for microscale and macroscale mechanical characterizations. First step towards microcharacterization of particle-particle bond was to determine the stress-strain response of single particle. Single switchgrass particle in dry and wet states were tested using an adapted microelectromechanical systems (MEMS) device in conjunction with the imaging techniques. Dry particles exhibited monotonic linear elastic response whereas wet particles exhibited three-phase sigmoidal shaped stress-strain response. The three phases include the first linear elastic zone up to approximately 1.6% strain, the second linear elastic zone up to strain ranging from 1.6 to 2.1% with significantly increased elastic modulus (168.9-223.7%), and the third linear elastic zone beyond 2.1% strain, where elastic modulus declined sharply (down to 90.9% of the second phase). The modulus of elasticity up to 1.5% strain for dry and wet switchgrass particles were 1.60 ± 0.33 GPa and 6.99 ± 1.66 GPa, which were significantly different each other (p < 0.001). Increase in the stiffness of switchgrass particles contributed to the bundling of fibers promoted by the activation of binders due to the increased moisture content. This study further investigated the influence of pressure and temperature on the stiffness of the particle-particle binding formed by the activation of natural binders present in the switchgrass. Bound two-particles were extracted from densified assemblies that were formed using a single action die system at predetermined pressures and temperatures including Treatments A (60 MPa and 75°C), B (100 MPa at 75°C), C (60 MPa and 90°C), and D (100 MPa at 90°C). For these four treatments, the particle-particle binding stiffness values were 0.955 ± 0.368 kN m-1, 0.986 ± 0.266 kN m-1, 0.550 ± 0.248 kN m-1, and 0.703 ± 0.249 kN m-1, respectively. The diametral tensile strength of formed assemblies at treatments A, B, C, and D were 38.1 ± 2.3 kPa, 60.9 ± 7.1 kPa, 7.2 ± 1.4 kPa, and 16.4 ± 6.4 kPa, respectively. The density of formed assemblies at treatments A, B, C, and D were, 619.5 ± 25.4 kg m-3, 655.7 ± 55.7 kg m-3, 421.7 ± 52.5 kg m-3, and 472.5 ± 19.5 kg m-3, respectively. Diametral tensile strength and density of assemblies were linearly correlated to the stiffness of particle-particle bonds (R2=0.838 and R2=0.981). High pressures were documented to form stronger compacts, however, presence of sufficient moisture at low temperature significantly improved the densified assembly properties by assumed lowered the glass transition temperature of lignin to form stronger bonds. Further investigation was performed on the coordination number and associated contact area to investigate a minimum requirement for a stable assembly. Particle arrangements of densified switchgrass assemblies formed at treatments A, B, C, and D were investigated by MicroCT scan images. The range of coordination number ranged from 9 to 12 for Treatment A, 12 to 14 for Treatment B, 6 to 7 for Treatment C, and 8 to 10 for Treatment D, respectively. Since biomass particles are highly irregular in shape, percentage contact area provides additional information on green compacts’ evolving integrity. The percentage of contact area for switchgrass compacts formed at treatments A, B, C, and D were 58.3±8.8, 81.7±4.5, 36.2±4.2, and 40.3±2.1, respectively. Coordination number and percentage contact area of all specimen were found to be positively correlated each other (R² = 0.872). In addition, strong correlations between microscale properties, i.e., coordination number and associated percentage area, and macroscale properties, i.e. densities and strength of assemblies, were also found (R² >0.85). In summary, the combination of moisture, temperature, and pressure can form a stable biomass assembly through both chemical and mechanical changes. Moisture loss at higher temperature can adversely affect the binding strength of particles, resulting in weaker interactions in the assembly. Additionally, this study found the importance of pressure that can induce particles to break into smaller sizes with changed shape, to undergo rearrangement, and therefore, to attain a stable structure with better mechanical interlocking between particles. This study contributes to science and engineering of biomass densification by providing an insight into the formation of a stable biomass assembly with preferred attributes of the very building block, i.e. particles.