**1. Introduction**

Feeding and handling of materials represent a substantial challenge in biomass feedstock supply systems. Conventional feeding, conveying, and storage systems for dry bulk solids are generally not suitable for lignocellulosic feedstocks because of their low densities and elastic nature. Reports indicate that industrial plants that handle bulk solids operate on an average at 77% of their design capacity, which is considerably lower than that of plants that handle liquids or gases [1]. Importantly, however, many of the surveyed plants handled simple powders for which there are decades of handling experience in multiple industries, including food, pharmaceutical, powder metallurgy, ceramics and plastics. Such powders often have favorable properties, such as low cohesion, small particle sizes and distributions, high densities and low compressibilities that facilitate feeding and handling. In contrast, feedstocks for lignocellulosic biofuels production tend to be cohesive, have large particle size variations, low

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

densities and are highly elastic, leading to greater challenges. It has been estimated that in 2016, the biofuels production achieved only 7% of the total active 58 million gallons per year of nameplate capacity [2]. Delayed startup times and operation below the designed capacity can have serious consequences in terms of the cost of the final product and missed business opportunities. As indicated in **Figure 1**, an increase in plant down time by 10% (decrease in the minimum on-stream factor) can increase the minimum fuel selling price by nearly a USD per gallon. Achieving 80% time on-stream compared to design capacity, even for a short time period of 2 weeks, is still considered a cellulosic biofuel breakthrough as evidenced by a recent press release by POET-DSM [3].

and primary impacts of dominant material properties. The second section presents a brief explanation of biomass flow in silos, bins and hoppers and includes key analyses needed to understand biomass flowability in the context of shear deformation, which is how bulk solids flow. The third section focuses on recent advances that have been made in biomass handling and briefly points to areas in which future effort should focus to have the greatest impact to

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Biomass energy systems are typically considered to consist of several processes, including resource production, collection, transportation, storage, feeding, conversion and transmission of biofuel or energy to end users. Production and collection include farming and forestry activities, as well as collection of waste materials that are suitable for bioenergy production. Transportation of biomass to conversion plants is usually performed by truck, barge or rail car. Short-term and long-term storage at the plant is necessary to ensure that sufficient material is on-hand to keep the plant operating through small disturbances in supply or to mitigate exceedance, in which spikes in biomass supply exceeds conversion capacity, such as could occur in the fall for agricultural residues. The topics associated with research production through storage are wide-ranging and too voluminous to be considered in this chapter. Basu [5] has provided a brief summary of those activities and their implications. The primary

The ability of a real feedstock material to flow through a particular assembly system is a function of the design of the structure and the rheological properties of the biomass material. These properties include bulk density, moisture content, compressibility, elasticity or spring back, particle size and shape distributions, cohesive strength, unconfined yield strength, adhesive strength (shear stress required to initiate motion on a surface), angle of internal friction (theoretical angle describing stress at failure), wall friction angle (shallowest angle at which a material slides on a surface), and permeability (ability of a material to allow gas or liquid to pass through it). These physical properties are important for both biochemical and thermochemical conversion processes [8, 9]. Thermochemical reactions are known to be sensitive to particle geometry, especially for fast reaction rates and short particle residence times. Biochemical conversion processes are generally more tolerant of variations in particle sizes and shapes, such that constraints on physical properties are primarily imposed by require-

A wide range of feedstock particle-size requirements have been reported for supplying feedstocks for biofuels conversion applications [12]. Feedstocks for fast pyrolysis are approximately 2 mm in size [13, 14], while those for biochemical conversion processes are much larger, varying from 6 to 75 mm. Excessive quantities of fine particles contribute to nuisance dust, clogged filters, and reduced permeability of the bulk solid to gases and liquids. Oversized particles create a different set of problems, such as incomplete conversion as well as plugging of air locks and pneumatic transfer lines. Particle grinding and other preprocessing steps have a strong impact on feeding behavior. For example, Idaho National Laboratory reported that replacing a hammer mill with a knife mill with the same nominal screen size solved a blowout problem with a pressure seal [12]. The level of fines can also increase as particles pass through consecutive unit operations and can cause negative impacts on the

alleviate the challenges that currently plague the emerging biofuels industry.

concern of this chapter is biomass handling at the plant prior to conversion.

ments of the feeding and handling systems [10, 11].

Several factors underlie the difficulty of feeding and handling biomass. Particulate solid materials belong to the family of yield stress materials that have flow behavior that is intermediate between those of solids and liquids [4]. These materials can support imposed stresses without significant deformation until the stress attains a threshold value. At that threshold value, permanent deformation occurs that can include complex localized elastic and plastic behavior due to discrete particle effects that are not present in liquids, creams, or gels. The threshold stress value is closely related to inter-particle friction, which depends strongly upon stress and deformation histories. The complexities associated with the flow of bulk solids make them much more challenging to handle than traditional solids, liquids, or gases. Common problems include uncontrollable flow that takes the form of plugging, obstructed or limited discharge, and erratic flow, as well as particle segregation and loss of live storage due to material adhering to container walls. Under extreme cases, flow problems can even cause high transient stresses that result in structural failure.

In addition to numerous research articles, several books and book chapters have been written on the topic of biomass handling and feeding [5, 6]. As explained by Bell [7], there have been significant, though relatively few, contributions from various researchers in powder mechanics, solids flow, and related topics over the last 100 years. This chapter is divided into three sections. The first section briefly summarizes the primary topics of biomass handling

**Figure 1.** Gallon of gasoline equivalent (GGE) selling price as a function of minimum on-stream factor. Adapted from [2].

and primary impacts of dominant material properties. The second section presents a brief explanation of biomass flow in silos, bins and hoppers and includes key analyses needed to understand biomass flowability in the context of shear deformation, which is how bulk solids flow. The third section focuses on recent advances that have been made in biomass handling and briefly points to areas in which future effort should focus to have the greatest impact to alleviate the challenges that currently plague the emerging biofuels industry.

densities and are highly elastic, leading to greater challenges. It has been estimated that in 2016, the biofuels production achieved only 7% of the total active 58 million gallons per year of nameplate capacity [2]. Delayed startup times and operation below the designed capacity can have serious consequences in terms of the cost of the final product and missed business opportunities. As indicated in **Figure 1**, an increase in plant down time by 10% (decrease in the minimum on-stream factor) can increase the minimum fuel selling price by nearly a USD per gallon. Achieving 80% time on-stream compared to design capacity, even for a short time period of 2 weeks, is still considered a cellulosic biofuel breakthrough as evidenced by a

Several factors underlie the difficulty of feeding and handling biomass. Particulate solid materials belong to the family of yield stress materials that have flow behavior that is intermediate between those of solids and liquids [4]. These materials can support imposed stresses without significant deformation until the stress attains a threshold value. At that threshold value, permanent deformation occurs that can include complex localized elastic and plastic behavior due to discrete particle effects that are not present in liquids, creams, or gels. The threshold stress value is closely related to inter-particle friction, which depends strongly upon stress and deformation histories. The complexities associated with the flow of bulk solids make them much more challenging to handle than traditional solids, liquids, or gases. Common problems include uncontrollable flow that takes the form of plugging, obstructed or limited discharge, and erratic flow, as well as particle segregation and loss of live storage due to material adhering to container walls. Under extreme cases, flow problems can even cause high

In addition to numerous research articles, several books and book chapters have been written on the topic of biomass handling and feeding [5, 6]. As explained by Bell [7], there have been significant, though relatively few, contributions from various researchers in powder mechanics, solids flow, and related topics over the last 100 years. This chapter is divided into three sections. The first section briefly summarizes the primary topics of biomass handling

**Figure 1.** Gallon of gasoline equivalent (GGE) selling price as a function of minimum on-stream factor. Adapted from [2].

recent press release by POET-DSM [3].

118 Advances in Biofuels and Bioenergy

transient stresses that result in structural failure.

Biomass energy systems are typically considered to consist of several processes, including resource production, collection, transportation, storage, feeding, conversion and transmission of biofuel or energy to end users. Production and collection include farming and forestry activities, as well as collection of waste materials that are suitable for bioenergy production. Transportation of biomass to conversion plants is usually performed by truck, barge or rail car. Short-term and long-term storage at the plant is necessary to ensure that sufficient material is on-hand to keep the plant operating through small disturbances in supply or to mitigate exceedance, in which spikes in biomass supply exceeds conversion capacity, such as could occur in the fall for agricultural residues. The topics associated with research production through storage are wide-ranging and too voluminous to be considered in this chapter. Basu [5] has provided a brief summary of those activities and their implications. The primary concern of this chapter is biomass handling at the plant prior to conversion.

The ability of a real feedstock material to flow through a particular assembly system is a function of the design of the structure and the rheological properties of the biomass material. These properties include bulk density, moisture content, compressibility, elasticity or spring back, particle size and shape distributions, cohesive strength, unconfined yield strength, adhesive strength (shear stress required to initiate motion on a surface), angle of internal friction (theoretical angle describing stress at failure), wall friction angle (shallowest angle at which a material slides on a surface), and permeability (ability of a material to allow gas or liquid to pass through it). These physical properties are important for both biochemical and thermochemical conversion processes [8, 9]. Thermochemical reactions are known to be sensitive to particle geometry, especially for fast reaction rates and short particle residence times. Biochemical conversion processes are generally more tolerant of variations in particle sizes and shapes, such that constraints on physical properties are primarily imposed by requirements of the feeding and handling systems [10, 11].

A wide range of feedstock particle-size requirements have been reported for supplying feedstocks for biofuels conversion applications [12]. Feedstocks for fast pyrolysis are approximately 2 mm in size [13, 14], while those for biochemical conversion processes are much larger, varying from 6 to 75 mm. Excessive quantities of fine particles contribute to nuisance dust, clogged filters, and reduced permeability of the bulk solid to gases and liquids. Oversized particles create a different set of problems, such as incomplete conversion as well as plugging of air locks and pneumatic transfer lines. Particle grinding and other preprocessing steps have a strong impact on feeding behavior. For example, Idaho National Laboratory reported that replacing a hammer mill with a knife mill with the same nominal screen size solved a blowout problem with a pressure seal [12]. The level of fines can also increase as particles pass through consecutive unit operations and can cause negative impacts on the


significantly smaller than the screen size. Many parameters actually affect the particle size distribution and its mean. For example, the mill type strongly affects particle size and shape. Typically, hammer mills produce more fine particle sizes than knife mills using the same screen size and also result in wider particle size distributions. This is particularly important because knife mills are typically used to prepare samples for laboratory tests, while hammer mills are often used in high-throughput industrial-scale applications. The impacts of moisture

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Proper storage and retrieval of biomass is critical to maintain quality in terms of both chemical properties for conversion and physical properties for feeding and handling. Retrieval or reclamation of biomass from storage is one of the most trouble-prone processes of biomass plant operation [5]. Silos are common in the agricultural and grain industries to store large quantities of material in a protective environment and can be large in diameter (4–15 m) and quite tall. Material is usually augured into the top of the silo and removed at the bottom. Very cohesive materials require specialized and expensive sweeping reclaimers to extract material from the entire bottom cross-section of the silo, where compressive pressures and material strengths can be very high. These systems require extensive engineering and are not discussed further here; however, additional information can be found at http://www.laidig.com/

Less cohesive materials and shorter storage systems in which compressive forces are lower often use less expensive hoppers or chutes to funnel biomass to a small feed discharge mechanism. In the 1960s, Andrew Jenike developed the first complete methodology for the flow of bulk solids within the framework of hoppers, bins, and feeders. His work included test equipment and procedures for measuring the necessary material properties, a theory of bulk solids flow within hoppers and bins, and a procedure to determine the hopper slope and outlet dimensions required for unobstructed gravity flow [6, 16]. The development presented here

As described by Jenike [17], the primary issues in the design of hoppers and chutes are: (1) solid flow pattern, (2) slope angle of discharge, and (3) size of the discharge opening. Although there are a number of flow obstructions that may develop in a bin, two primary types are analyzed here: arching or doming as illustrated in **Figure 2(a)** and ratholing or piping as illustrated in **Figure 2(b)**. Most particulate solids are easily flowable when they are well-aerated but become cohesive and strong when compacted. For example, fluidized bulk solids have very low shear strengths and typically flow with carrier gases; however, the same bulk solids can be made into rigid briquettes or pellets by subjecting them to high compressive stresses, especially in the presence of moisture or binders. The increasing strength of bulk solids with increasing compressive stress allows them to form arches and bridge over openings. In the case of large bins and hoppers, the weight of material in upper layers compresses the lower

content, incoming particle size, and tool speed also vary for different mill types [12].

**2. Silos, bins and hoppers for storing and discharging**

**2.1. Flow obstructions and patterns**

closely follows the formalism that Jenike advanced.

reclaimers.

**Table 1.** Noninclusive summary of feedstock performance related to particle physical and mechanical properties (adapted from [15]).

feeding and conversion performance [7]. Thus, even if ideal specifications are achieved, care must be taken so that the subsequent unit operations do not unintentionally modify material properties.

The behavior of biomass feedstocks in handling and feeding equipment is affected by many factors beyond traditional rheological properties. These factors include chemical composition of particles, temperature, presence of trapped gases and the unique stress and deformation histories of the bulk solid. The impacts of specific parameters are summarized in **Table 1**. Particle size and moisture content often receive the most attention, and it is important to recognize that in some cases the particle size "specification" is based on the screen size of a laboratory mill, rather than a thorough classification of particle-size distribution. Such a screen size specification is often misleading because in most cases the mean product particle size is significantly smaller than the screen size. Many parameters actually affect the particle size distribution and its mean. For example, the mill type strongly affects particle size and shape. Typically, hammer mills produce more fine particle sizes than knife mills using the same screen size and also result in wider particle size distributions. This is particularly important because knife mills are typically used to prepare samples for laboratory tests, while hammer mills are often used in high-throughput industrial-scale applications. The impacts of moisture content, incoming particle size, and tool speed also vary for different mill types [12].
