**3. Solutions to biomass handling challenges**

#### **3.1. Co-optimization of feeding equipment and material properties**

Failure to recognize the extent of material variability during equipment and process design is a common cause of feeding and handling problems. Systems can be designed to accommodate the full range of material variability; however, costs often increase as systems are made more robust. In the end, the selected design becomes a trade-off between increased capital costs for more robust systems (which is a near term, well-defined expense) and increased operating expenses due to additional down time if less expensive equipment fails. Importantly, the impact of increased operating costs is farther in the future and is rarely well defined. Relying on uniform bulk densities for gravity feed, low moisture and consistent particle-size distributions allows equipment designs to be simple and low cost. As long as the material meets the desired specifications, no problems are anticipated, but when material properties deviate outside narrow design specifications, equipment efficiency and reliability suffer, often dramatically.

such as a bin/auger feeder. The range of anticipated material properties and the corresponding design specifications of the hypothetical equipment are illustrated in the regions labeled "anticipated feed" and "system design," respectively. Variation of material properties, due to unavoidable diversity of sources and supply conditions, including seasonal and weather effects, over the course of operation, often breaches equipment design specifications as depicted by the region labeled "actual feed." Ensuring that the reliable operational envelope of the process completely encompasses the actual operating conditions requires consideration and control of both the equipment design and material properties, such as bulk density and moisture content, as well as particle size/shape distributions and roughness. The combination of improved equipment design and better control of material properties is illustrated at the right side of **Figure 8** by the expanded system design envelope and the reduced envelope of actual feed properties that is achieved by actively managing the variation of raw material properties. The objective of this holistic approach is the simultaneous optimization of both

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Achieving an optimal balance between minimizing the cost and complexity of equipment and managing the variation of feedstock properties requires a comprehensive understanding of the material properties and the factors that impact those properties. A common mistake identified by Bell [7] is believing that feeding and handling problems can be readily solved during start up. In truth, retrofitting equipment and processes can be very expensive and drawn-out because problems are often discovered one at a time as successive pieces of equipment come online. Actions that are taken to solve one problem may have unintended consequences that ripple through downstream operations and can add to the confusion between causes and effects. Fully characterizing all potential feedstocks and carefully managing material properties to match handling and conversion equipment is crucial to minimizing the probability of unexpected operating inefficiencies and

Solving biomass feeding and handling challenges will require a combination of techniques and capabilities, including numerical simulation, comprehensive material characterization, and mechanical tests. Numerical simulations to date have not had great impact in evaluating the flowability of biomass in handling equipment because of the extreme complexity of the flow problem. It is recognized that Cauchy equations of force and momentum conservation are insufficient to simulate solids flow because of the interactions of the various forces, including wet and dry friction, capillary, gravity, Coulomb, and elastic windup [28]. However, attempting to empirically solve solids flow problems through a series of tests to classify or rank biomass materials in all possible flow situations is not practical. Tests would have to be conducted for each equipment geometry at all scales using all types of biomass materials and all types of biomass preprocessing options that impact the dominant flow properties of bulk density, particle size and shape distributions, particle surface friction, particle rigidity, and moisture content. The number of tests that would be required is prohibitive, and correctly interpreting the large database of properties and test results would be daunting if not

cost and performance.

failures.

infeasible.

**3.2. Recommended future research directions**

There are two primary approaches to addressing material handling problems. First, the equipment systems may be engineered to anticipated material properties, or second, the feed material may be engineered to perform properly in the equipment systems. The first approach follows traditional engineering design concepts and tends to gain the most attention. In truth, a balanced approach that carefully considers both methods is usually best, especially for processes that are intended to handle different feedstock materials or materials that do not have well-defined and controlled properties.

**Figure 8** depicts how this dual approach of more robust equipment design and better control of feedstock material properties can improve the reliability of a hypothetical operation,

**Figure 8.** The combined approach for solving biomass handling and feeding problems through improved system design and improved preprocessing operations that control feedstock properties to meet to specifications. The scenario on the right in which the equipment systems and feedstocks have been optimized will likely exhibit superior and more reliable performance.

such as a bin/auger feeder. The range of anticipated material properties and the corresponding design specifications of the hypothetical equipment are illustrated in the regions labeled "anticipated feed" and "system design," respectively. Variation of material properties, due to unavoidable diversity of sources and supply conditions, including seasonal and weather effects, over the course of operation, often breaches equipment design specifications as depicted by the region labeled "actual feed." Ensuring that the reliable operational envelope of the process completely encompasses the actual operating conditions requires consideration and control of both the equipment design and material properties, such as bulk density and moisture content, as well as particle size/shape distributions and roughness. The combination of improved equipment design and better control of material properties is illustrated at the right side of **Figure 8** by the expanded system design envelope and the reduced envelope of actual feed properties that is achieved by actively managing the variation of raw material properties. The objective of this holistic approach is the simultaneous optimization of both cost and performance.

Achieving an optimal balance between minimizing the cost and complexity of equipment and managing the variation of feedstock properties requires a comprehensive understanding of the material properties and the factors that impact those properties. A common mistake identified by Bell [7] is believing that feeding and handling problems can be readily solved during start up. In truth, retrofitting equipment and processes can be very expensive and drawn-out because problems are often discovered one at a time as successive pieces of equipment come online. Actions that are taken to solve one problem may have unintended consequences that ripple through downstream operations and can add to the confusion between causes and effects. Fully characterizing all potential feedstocks and carefully managing material properties to match handling and conversion equipment is crucial to minimizing the probability of unexpected operating inefficiencies and failures.

#### **3.2. Recommended future research directions**

**Figure 8.** The combined approach for solving biomass handling and feeding problems through improved system design and improved preprocessing operations that control feedstock properties to meet to specifications. The scenario on the right in which the equipment systems and feedstocks have been optimized will likely exhibit superior and more reliable performance.

**3. Solutions to biomass handling challenges**

130 Advances in Biofuels and Bioenergy

well-defined and controlled properties.

**3.1. Co-optimization of feeding equipment and material properties**

Failure to recognize the extent of material variability during equipment and process design is a common cause of feeding and handling problems. Systems can be designed to accommodate the full range of material variability; however, costs often increase as systems are made more robust. In the end, the selected design becomes a trade-off between increased capital costs for more robust systems (which is a near term, well-defined expense) and increased operating expenses due to additional down time if less expensive equipment fails. Importantly, the impact of increased operating costs is farther in the future and is rarely well defined. Relying on uniform bulk densities for gravity feed, low moisture and consistent particle-size distributions allows equipment designs to be simple and low cost. As long as the material meets the desired specifications, no problems are anticipated, but when material properties deviate outside narrow design specifications, equipment efficiency and reliability suffer, often dramatically.

There are two primary approaches to addressing material handling problems. First, the equipment systems may be engineered to anticipated material properties, or second, the feed material may be engineered to perform properly in the equipment systems. The first approach follows traditional engineering design concepts and tends to gain the most attention. In truth, a balanced approach that carefully considers both methods is usually best, especially for processes that are intended to handle different feedstock materials or materials that do not have

**Figure 8** depicts how this dual approach of more robust equipment design and better control of feedstock material properties can improve the reliability of a hypothetical operation,

> Solving biomass feeding and handling challenges will require a combination of techniques and capabilities, including numerical simulation, comprehensive material characterization, and mechanical tests. Numerical simulations to date have not had great impact in evaluating the flowability of biomass in handling equipment because of the extreme complexity of the flow problem. It is recognized that Cauchy equations of force and momentum conservation are insufficient to simulate solids flow because of the interactions of the various forces, including wet and dry friction, capillary, gravity, Coulomb, and elastic windup [28]. However, attempting to empirically solve solids flow problems through a series of tests to classify or rank biomass materials in all possible flow situations is not practical. Tests would have to be conducted for each equipment geometry at all scales using all types of biomass materials and all types of biomass preprocessing options that impact the dominant flow properties of bulk density, particle size and shape distributions, particle surface friction, particle rigidity, and moisture content. The number of tests that would be required is prohibitive, and correctly interpreting the large database of properties and test results would be daunting if not infeasible.

In contrast, a close coupling between instrumented lab and pilot scale tests and multiscale modeling may be able to elucidate the appropriate constitutive relations that are needed to augment the Cauchy equations of force and momentum conservation for successful continuum modeling. The powerful outcome of empirically-based numerical simulations is that the results would be scalable within any reasonable equipment size and the impact of specific material properties, such as those described above, could be determined to understand the operational envelope of specific processes. The multiscale models would operate as a direct transfer function to translate microscopic and macroscopic material properties that can be measured in the laboratory to material flow performance in biomass feeding and handling systems. The flow simulations could be used to identify cost effective approaches to modify the biomass materials and/or the transportation and handling equipment to reduce supply chain costs and also to minimize the equipment down-time due to material feeding problems. Continuum models may also be augmented by discrete element method (DEM) modeling that can simulate the motion and even the deformation of each particle in a flow field. **Figure 9** show an example of DEM model of a material that consists of particles with different shapes flowing in a wedge-shaped hopper. Simulating each individual particle in the flow offers the possibility of realistically capturing particle size and shape effects that cannot be directly incorporated into continuum models; however, such models have very high computational costs, so they are typically limited to simulations that involve not more than a few million particles with relatively simple shapes.

A final need that should be addressed is real-time, inline feeding and handling quality assurance (QA) and quality control (QC). Even with near perfect understanding of how material

> attributes impact flowability performance, feeding and handling problems can still arise if variation in harvest, storage, or preprocessing results in localized material that does not meet the specifications. Data recently obtained at Idaho National Laboratory, Idaho in which the author participated indicates a manner in which an in-line test can be rapidly performed [29] using a custom V-shaped hopper with sliding walls as shown in **Figure 10**. The proposed apparatus offers real-time, inline measurement of material flow performance. Installing this or similar QA/QC equipment in biomass feeding and handling systems can prevent out-of-spec material from causing expensive down-time and potential damage to processing equipment.

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**Figure 10.** Wedge-shaped flow hopper with rotating and sliding walls for monitoring flow properties in real-time.

Feeding and handling of biomass has been a primary factor causing pioneer industrial biorefineries to struggle to achieve production targets. The primary biomass properties that impact feeding behavior include bulk density, moisture content, compressibility, elasticity or spring back, particle size and shape distributions, cohesive strength, unconfined yield strength, internal friction angle, and wall friction angle (a property shared with the container surface). The primary issues in the design of hoppers and chutes are: (1) solid flow pattern, (2)

**4. Conclusions**

**Figure 9.** DEM model of flow in a wedge-shaped hopper. The material consists of particles with different shapes as indicated by particle color. Image courtesy of Hai Huang and Yidong Xia at Idaho National Laboratory.

**Figure 10.** Wedge-shaped flow hopper with rotating and sliding walls for monitoring flow properties in real-time.

attributes impact flowability performance, feeding and handling problems can still arise if variation in harvest, storage, or preprocessing results in localized material that does not meet the specifications. Data recently obtained at Idaho National Laboratory, Idaho in which the author participated indicates a manner in which an in-line test can be rapidly performed [29] using a custom V-shaped hopper with sliding walls as shown in **Figure 10**. The proposed apparatus offers real-time, inline measurement of material flow performance. Installing this or similar QA/QC equipment in biomass feeding and handling systems can prevent out-of-spec material from causing expensive down-time and potential damage to processing equipment.
