(87a) Mechanistic Investigation of Preprocessing Specifications Impact on Flow Related Issues in Biomass Handling Systems Using Experiment and Discrete Particle Simulations

Biomass is an abundant, versatile source of renewable energy that can be used to generate power on demand and produce bio-based fuels, chemicals, and materials. Harnessing the full potential of bioenergy as a strategic player in the transition to carbon-neutral energy sources is contingent upon the deployment of this technology at commercial scale becoming economically viable. To achieve this goal, efficient processes need to be developed for different unit operations along the bioenergy value chain, from harvesting the raw biomass materials all the way up to their lifetime in the biorefinery. The unique technical challenges associated with the life cycle of biomass feedstocks is mainly attributed to the undesirable characteristics of their raw form, e.g., high variability, high moisture and oxygen content, low bulk density, low calorific value, and hydrophilic nature. Accordingly, biomass preprocessing is required to produce high-quality feedstocks using mechanical, chemical, and thermal processes that change the physical properties and chemical composition of the raw materials. The complex nature of biomass feedstocks, including their irregular shapes, variable moisture content, and heterogeneous composition, contributes to their poor flowability characteristics. As a result, biomass particles often exhibit non-uniform flow patterns, arching phenomena, and cohesive behavior, leading to flow obstructions, materials jamming, clogging, erratic flow, particle segregation, and loss of live storage volumes in storage and handling systems. These flow-related issues not only disrupt material flow but also increase energy consumption, maintenance costs, and operational downtime in biomass processing facilities. Understanding the underlying mechanisms of biomass poor granular flowability is crucial for optimizing the design and operation of biomass handling facilities and mitigating flow-related issues.

The flow behavior of particulate biomass materials is typically intermediate between solids and liquids. Individual particles and bulk biomass have complex viscoelastic deformation under stress. Under short-term low loading, the yield stress can withstand imposed stresses without any significant deformation up to a threshold value. Stresses above this threshold value causes permanent deformation in a complex fashion that involves localized elastic and plastic strains, arising from the discrete nature of the particles, and exhibits nonlinear flow response to the applied external forces. The threshold stress required for the onset of permanent deformation depends strongly upon the inter-particle friction and is very sensitive to the stress and deformation histories. Difficulties associated with the handling and feeding of biomass feedstocks are not originating only from the traditional rheological properties such as bulk density, moisture content, compressibility, cohesive and adhesive strengths, as it is also affected by the chemical composition of individual particles, internal microstructure, entrapped gases, and temperature. For example, while some anatomical fractions of loblolly pine (bark and needle) exhibited direct relation between the moisture content and the rheological properties, no influence was observed for other fractions (stem and whole). In addition, the impact of the moisture content on the rheological properties was only present for the smaller particle size of bark and needle. Moreover, same-size particles from different fractions showed different flowability. This demonstrates the inherent variability of the biomass feedstocks attributes and their impact on the final product quality. So, it is extremely important to thoroughly quantify the impact of different material attributes on the performance of different unit operations along the supply chain, to understand the interplay between them and assess the critical parameters.

Under dynamic loading conditions, the granular flow behavior can be classified into three different regimes, namely, 1) quasi-static shearing, 2) inertial, and 3) intermediate transitional flows. While the quasi-static shearing flow regime represents one extreme that occurs in dense and confined granular assemblies, the inertial flow is the opposite extreme that is typically encountered in the dilute limit where the granular particles are loose and interacting mainly through uncorrelated binary collisions. Theoretical formulation of elastoplastic solids and the kinetic theory of dense gases can be adequately applied to the two different regimes, respectively. On the other hand, the intermediate transitional flow regime is so far still poorly understood. Thus, numerical experiments are needed to complement theoretical prediction and aid in bridging the gap between the microscopic properties and the macroscopic behavior of granular materials. The present research explores the challenges associated with the poor granular flowability of biomass feedstocks in different materials handling and feeding unit operations along with validation efforts using experiment. This includes screw feeder, roll screen, hopper, and compressive screw pump systems. Applications of several Discrete Element Method (DEM) models are sought to quantify the flowability of granular biomass feedstocks (using mass throughput and specific energy consumption as benchmarking metrics) and study the impact of different comminution techniques and preprocessing specifications on the performance of handling systems under consideration. Specifically, following quality-by-design approach, different critical material attributes (particle size, particle size distribution, particle shape, and moisture content) and critical process parameters were investigated. Effects of factors such as particle properties, inter-particle interactions and interlocking, wall friction, and surface roughness on jamming phenomena and transitions between flow regimes are simulated. Simulation results shed light on the fundamental mechanisms leading to jamming events and help advance the modeling of flow-related issues in industrial applications. This will enable the development of mitigating strategies and innovative solutions for optimizing biomass handling systems, improving process efficiency, and promoting sustainable bioenergy production.