Densification Impact on Raw, Chemically and Thermally Pretreated Biomass

The following sections are included:

• Dedication
• Preface
• About the Editor
• Acknowledgments
• Disclaimer
• Contents
• List of Abbreviations

The biomasses such as agricultural residues, energy crops, forest residues, and other wastes usually have irregular shapes and a low-bulk density, resulting in loose harvest formats and a corresponding energy density lower than coal or other fossil fuels. They also have high moisture content, speeding up degradation during storage. The low-bulk density of the biomass creates problems, such as logistics and storage, handling and feeding, transportation costs, a large storage footprint, inefficiencies in processing in the biorefineries that can increase the downtime, and the overall size of the reactor and the conversion equipment. Besides the physical properties challenges, other chemical composition and energy content challenges also exist. These challenges limit the biorefineries from operating at their designed capacities. Various chemicals and thermal and mechanical pretreatment and preprocessing methods are used to improve the biomass quality and make it suitable for the biorefineries to operate at their designed capacities. This chapter discusses enhancing biomass quality in terms of physical properties and chemical composition using mechanical, thermal, and chemical preprocessing and pretreatment technologies.

Various fundamental models were developed to understand the compaction characteristics of powders. Many authors have used these models to understand the compaction behavior of biomass particles. Biomass’s compression, relaxation, and frictional (adhesion) properties depend on compressive force, moisture content, and particle size and shape. The various models commonly used for biomass are the Spencer and Heckel model, the Walker model, the Jones model, the Cooper–Eaton model, the Kawakita–Lüdde model, the Sonnergaard model, the Panelli–Filho model, and the asymptotic modulus model. Various researchers tested all these models for different biomass grinds. For example, the Cooper– Eaton model parameters for biomass grinds can help understand prominent compaction mechanisms, such as particle rearrangement and elastic and plastic deformation. However, the mechanism of mechanical interlocking and the ingredient melting phenomenon during biomass compression are also important to understanding the compaction mechanism. The Kawakita–Lüdde model helps relate the biomass grinds’ initial porosity and the compacts’ yield strength. Numerical software, such as Pro/Engineer, PFC3D, and others, are gaining importance in understanding biomass grinds’ compaction behavior during the densification process. The common systems used for biomass densification are pellet mill, briquette press, and screw extruder. Among the densified products, the pellets produced using a pellet mill are widely used worldwide for biopower production.

Different biomaterials have different chemical compositions and since the densification properties are strongly related to feedstock, it is important to increase knowledge about biomass relationship to densification properties. The purpose of this chapter is to give a brief introduction to the development of the plant kingdom, the chemical composition of biomasses, and how different components can affect the densification characteristics. A study where 11 different pure substances (cellulose, hemicelluloses, lignin, etc.), added to pine and beech, are pelletized in a single pellet press and results are presented showing that polysaccharides can play an important role when biomasses are densified as single sources or blends solutions in a densification process.

Future pellet plants will need to be more flexible regarding the need to vary the feedstock species and increase the areas for biomass. By blending biomass species, pellets can be a solution that meets new desirable specifications for different conversion pathways, such as biochemical, thermochemical, and biopower. In this chapter, the authors address opportunities for blending and densification to homogenize the chemical composition of biomaterials and improve the potential for other processing and transformation methods. The advantages of blending biomasses are many: increasing the potential supply of biomass, creating new markets, reducing raw material costs, and improving biomass flow and pelletizing properties. Important densification parameters to keep in mind are highlighted, and five different studies are summarized; these include investigations from a single pellet press study to briquette applications. Some conclusions are that blending woody and herbaceous biomasses yields products with uniform product quality in terms of physical properties and chemical composition. Furthermore, blending pine and switchgrass yields a compressed product with higher density and durability while reducing energy consumption.

Applications involving processing of biomass feedstocks have increased significantly in recent years. Reliable storage and handling of biomass are critical for the success of these applications. Given the potential flow issues associated with inadequately designed biomass handling and processing equipment, severe consequences could occur to the process reliability and efficiency. Thus, a thorough understanding of the flow behavior of the given biomass feedstock and the use of this information in the design of storage, handling, and processing systems are essential. Flow behavior of bulk solids is a complex phenomenon. Multiple parameters associated with bulk solids flow need to be characterized and studied to obtain a complete understanding. In addition, effects of environmental conditions, process, and operation-related parameters on the flow behavior also need to be studied. This chapter introduces the basic principles and methods used for characterizing the flow behavior of bulk solids, with an emphasis on herbaceous biomass feedstocks. Then, illustration is provided about how to use this information obtained from characterizing flow behavior in design for handling equipment for pelletization of biomass feedstocks. Finally, principles associated with good project management are also discussed.

Lignocellulosic biomass has low energy content and is high in oxygen content. The proximate and ultimate composition of lignocellulosic biomass is inferior compared to coal. The grinding properties of biomass are completely different compared to coal. Biomass is fibrous, whereas coal is brittle. One way to make biomass look like coal is through torrefaction. The biomass is roasted in an oxygen-free environment at temperatures of 200–300°C for different residence times. During torrefaction, the biomass loses the low energy content of volatiles and produces a solid product that is high in energy content. The solid fraction rich in carbon is torrefied biomass or ‘biocoal’. Biocoal represents a renewable energy commodity that can substitute coal. The torrefied biomass has superior biomass in terms of proximate and ultimate composition and physical properties such as grinding and particle size, but the challenge is low in bulk density. The low bulk density is primarily due to the loss of low energy content volatiles during the torrefaction process. One way to increase the density of the torrefied biomass is through densification. The densification systems commonly used today are pellet mills and briquette presses. Densification helps improve the transportation and handling of low-density torrefied biomass. The challenge is in making a durable pellet using torrefied biomass as the biomass loses its binding ability. Typically, binders are used for making torrefied and densified biomass. The challenge of adding binders is introducing foreign material to the biomass and changing the composition. In addition, adding binders can change the torrefied material properties, such as hydrophobicity. The other option to overcome this limitation is to torrefy the densified biomass. This chapter looks at the production of torrefied and densified biomass and its physical properties.

Economic production of transportation fuels from biomass sources is an ongoing challenge and opportunity. Hybrid energy systems combine various forms of energy into resilient supplies. This chapter describes biofuel production using biomass gasification. A key step in optimizing the biomass gasification step is densification of various forms of biomass including lignocellulosic biomass. An emphasis of this chapter is providing examples of the impact of feedstock preparation on biomass gasification. These examples show the beneficial impact that mechanical, chemical, and thermal preprocessing technologies can have on feedstock quality which also impacts the quality and quantity of biofuel production. This chapter also includes a discussion of the economics associated with biofuels production from multiple biomass types. Finally, this chapter discusses the key analytical methods used to quantify biofuel quality which also helps demonstrate the feasibility of substituting biofuels generated by biomass gasification for current fossil fuels. This chapter demonstrates that the technology to sustainably generate biofuels currently exists and that the key issue is whether biofuels can be produced in a cost-competitive process as a drop-in compatible fuel using the existing refining and distribution infrastructure currently used by petroleum-based refineries. It’s also important to note that the current petroleum industry evolved over the past one hundred years and has seen several generations of technology evolution to get to where it is today. Biomass densification is an example of how the biofuels industry is evolving today into a cost-competitive environmentally friendly substitute for current fossil fuels.

Crop harvest leaves behind agricultural waste with a significant amount of moisture content. Transporting high moisture agriculture waste biomass to a biorefinery is highly uneconomical. Achieving dry biomass (<10%) for subsequent thermochemical processing such as pyrolysis or gasification is cost-intensive. Hydrothermal liquefaction (HTL) technology is more suited for agricultural waste conversion because it can accept high moisture feedstock to produce biofuels and bioproducts. HTL technology has demonstrated many more advantages over other thermochemical technologies at the laboratory scale. Yet, there is no commercial HTL plant to process agricultural residue or lignocellulosic biomass. Transporting wet biomass and making it HTL-ready are the two significant obstacles to commercializing the HTL technology. Biomass densification may reduce the transportation cost and allow processability at higher biomass loadings. Although densification via pelletization has been reported in the literature, currently, there is no understanding of the influence of densified biomass on HTL product distribution or yield. This chapter covers essential information on HTL technology, reported yields of bioproducts (bio-oil and hydrochar), and the parameters to consider in configuring an HTL-ready feedstock with emphasis on corn stover feedstock densification.

Quality properties of microwave-torrefied oat hull pellets were determined as a result of pelleting trials at different pellet die temperatures, sample particle sizes, and binder formulations. Microwave-torrefied ground biomass samples were compressed at two levels of pellet die temperatures (95°C and 145°C), two particle sizes (fine and coarse), and two levels of binders at different wt% (sodium lignosulfonate and sawdust). Quality of pellets was examined based on relaxed pellet density, pellet tensile strength, moisture absorption, moisture content, ash content, and higher heating value. Results showed that pellets made from fine particle size at high die temperature (145°C) have improved density and tensile strength. The best binder for density and tensile strength improvement was 10 wt% sodium lignosulfonate. However, since raw sawdust is abundantly available and cheaper than lignosulfonate, it is best recommended as a low-cost effective binder for densifying torrefied oat hull into pellets. Overall, all the densified pellets had a moisture content of 4–5% wet basis. Moisture absorption decreased when increasing torrefaction temperature, and pellets formulated with lignosulfonate presented higher moisture absorption and lower calorific value. Moreover, the addition of lignosulfonate significantly increased the pellet ash content, while sawdust significantly decreased the ash content of microwave torrefaction oat hull pellets.

Lignocellulosic biomass is the most abundant cheap resource produced from agriculture and forest residues. Among several thermochemical pretreatments reported in the literature, Ammonia Fiber Expansion (AFEX) pretreatment uses ammonia that can be recovered and re-used benefiting the environment. Ammonia helps open up the complex cell wall by cleaving the ester linkages and increasing the digestibility during enzyme hydrolysis and rumen in the animal gut. The lignin solubilized by ammonia during AFEX pretreatment is partially relocated to the surface of the biomass, while it is released to act as a natural binding when densifying the biomass. The AFEX pellets have a bulk density comparable to corn, can be stored in grain elevators, and can be used for diverse applications, such as feedstock for biofuels, animal feed, anaerobic digestion, and producing biocomposite materials. A local biomass pretreatment depot (LBPD) will help produce the AFEX pellets near the location where the feedstocks are produced and transported using trucks or rail carts. This approach will help overcome the logistic issues of transporting and storing biomass and sold as a commodity product that will enable the biochemical platform and biobased economy.

Lignocellulosic biomass (LCB) represents abundant biomass that is economically feasible and environmentally sustainable to produce biofuels and bioproducts. Despite these lucrative advantages, LCB faces logistical and technological challenges in its utilization. Low bulk density and high moisture content hinder the biomass’s efficient transportation and storage. Densification has become an essential step for efficient transport, storage, and utilization of biomass, pelletization being one of the most common densification methods. Pellets (and other densification techniques) have been studied in great detail for the thermochemical conversion of biomass. However, the extent of knowledge of its effect on sugar yields during biochemical conversions is limited. This chapter summarizes the densification methods used for biomass for biochemical conversion and their possible merits and limitations.

The following section is included:

• Index