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2023

Identification of Optimal Binders for Torrefied Biomass Pellets (1)

Keywords:

torrefied biomass,binders,pellets

Source: internal company

1. Introduction

There are a number of challenges impeding the use of biomass forestry residues for generation of heat and power. In unprocessed form, forestry residues have a low bulk density (<400 kg/m³), high moisture content (>30%) and low calorific value (<20 MJ/kg), resulting in high transportation and storage costs and safety hazards. Densification of the biomass into pellets or briquettes is essential. Untreated biomass pellets still suffer from issues with weathering and are prone to biological degradation and self-heating as a result of their hydrophilic nature. They require covered and ventilated storage, which significantly increases fuel switching costs for existing fossil fuel facilities. Other impediments to the use of untreated wood fuel are its poor grindability and high oxygen content, which have limited their usage for cofiring in pulverized coal fired plants to a maximum of 20%. Torrefaction followed by densification has the potential to create a fuel that has properties closer to that of coal and overcomes many of the above challenges; however, torrefaction typically reduces the strength and durability of the densified product, requiring very high pelletizing pressures and temperatures or the addition of binders for the creation of a durable pellet. Densification can reduce net energy consumption and GHG emission by 200–1000 kJ MJ⁻¹ and 9–50 CO₂-eq (g MJ⁻¹), respectively. The current work offers a review of the densification process, a review of commonly used binders as well as some identified materials that have potential for use as binders, and a technoeconomic comparison of binder options.

2. Approaches for Torrefied Pellet Production

2.1 Torrefaction

Torrefaction is a process by which biomass is heated to between 200–300 °C in an oxygen-free environment. During torrefaction, biomass undergoes drying and partial thermal decomposition of the components that make up the biomass: hemicellulose, cellulose, and lignin. The result is a dry material that is lower in H/C and O/C ratios, hydrophobic, resistant to biological degradation/off-gassing, and more friable due to the breakdown of the cell wall structure. This process also improves the fixed carbon. Torrefaction of coffee bean grounds (CBG), and rice husks (RH) (75:25) at 275 °C obtains carbon and oxygen contents of 59.84 wt% and 25.75 wt%, respectively, with a synergetic value of 2.89 for high heating value compared to biosolids. Torrefaction occurs in four stages:

• Nonreactive drying (50–150 °C): Evaporation of free water on the surface and in the pores of the wood, softening of lignin, and reduced porosity due to moisture loss;

• Reactive drying (150–200 °C): Breakage of hydrogen and carbon bonds, hemicellulose decomposition, emission of lipophilic compounds (i.e., fatty acids, sterols), and thermo-condensation of chemically bound water (>160 °C);

• Destructive drying (200–250 °C): Disruption of most inter- and intramolecular hydrogen bonds and C-C and C-O bonds, mass loss still at a minimum, devolatilization and carbonization of hemicellulose begins, and depolymerization of lignin occurs (>230 °C) ;

• Hemicellulose decomposition (250–300 °C): Complete decomposition of hemicellulose into volatiles and char, limited devolatilization and depolymerization of lignin and cellulose, and complete destruction of cell structure and loss of fibrous nature.

Of the three primary biomass constituents—cellulose, hemicellulose, and lignin—hemicellulose is the most degraded by torrefaction conditions. Hemicellulose decomposition occurs at temperatures as low as 150 °C via fragmentation, deacetylation, and depolymerization, with the major decomposition reactions occurring in the range of 200–300 °C . At 200 °C, amorphous cellulose degradation starts, while relatively higher temperatures, i.e., >250 °C, are required for crystalline cellulose. Cellulose decomposition begins around 240 °C, beginning with depolymerization and restructuring. The areas of more crystalline cellulose resist degradation better than the amorphous regions, which hold free water that—when converted to steam—assists in breaking apart the cellulose structure.

The numerous hydroxyl groups of the biomass constituents are responsible for the highly hydrophilic nature of biomass. Torrefaction removes many of these OH groups through depolymerization reactions, generating water vapor through chemical condensation. This creates nonpolar compounds that do not bind with water. Hemicellulose is particularly hydrophilic and must be completely degraded to create a hydrophobic pellet/briquette. The biomass is upgraded through breaking down the long-chain hydrogen bonds of cellulose, decomposition of hemicellulose, and depolymerization of lignin.

Lignin softening occurs at 160–190 °C. While the cleavage of α- and β-aryl-alkyl ether linkages occurs at 150–300 °C. Brosse et al. showed that lignin depolymerization occurs at temperatures as low as 230 °C (7 h thermal treatment), which could negatively affect its ability to act as a binder. Lignin begins to significantly decompose above 280 °C and is completely carbonized at 500 °C. The upper limit for torrefaction is typically 300 °C to limit the amount of lignin decomposition so that it may be used as a binder in pelletizing of the torrefied material. At temperatures above 300 °C, lignin and cellulose decompose into char and volatiles. Lignin is hydrophobic relative to hemicellulose, and cellulose and is less easily dehydrated and therefore converts more easily to char. Dehydration of biomass during torrefaction causes fractional decarboxylation, and de-carbonylation, which increase the calorific value of the solid fuel.

2.2 Densification

Solid fuels are densified primarily to reduce transport cost and complexity by increasing the bulk density and reducing fines. In the case of sawdust, the density can be increased from 40–400 kg/m³ to >1000 kg/m³ for wood pellets, making the densified biomass suitable for thermal conversion processes, i.e., gasification, pyrolysis, combustion, and cofiring with coal or coke. Densification is divided into three stages including the rearrangement of particles, plastic and elastic deformation, and the mechanical interlocking of particles. It improves biomass combustion by increasing the ignition, burnout, and composite combustion indexes. Particulate material (PM) emissions depend on the feedstock and can be increased or decreased by densification. For example, PM emissions of agricultural residues, such as cornstalk, are reduced considerably by densification, while PM emissions of woody biomass, such as camphorwood, are increased. The drawback of densification is the energy intensity and potential emissions of CO₂ and CH₄. There are three main methods of densification of solid fuels: pelletization, briquetting, and granulation.

Granulation is used to agglomerate/densify fine powders (<500 µm) and is not suitable for larger particles. It requires secondary thermal or chemical treatment to strengthen the small granules, which have a large size distribution (2 mm < d_p < 15 mm). Briquetting, developed in the late 1800 s for compaction of coal screenings, involves compressing the particulates between two heated roller dies to produce briquettes. Mechanical or hydraulic piston presses apply load on a die filled with biomass particles. Granulation and briquetting have been primarily applied to the densification of coal and ore fines.

Pelletization is an extrusion process, generating heat and pressure in the die as the material is pushed through, this softens the lignin, binding the particles together. Extrusion using a screw or piston press to produce large diameter densified products often referred to as briquettes is similar to pellet extrusion and can be treated in the same respect. Briquettes are normally produced from waste biomass and have larger diameters than pellets—diameter of 50–100 mm and length of 60–120 mm. Ground biomass passes through 6–8 mm holes and is cut into lengths of 3 to 40 mm. A partially torrefied “bio-coke” can be produced by extruding biomass at ~120–200 °C and 20 MPa. Higher pressure lowers the heating temperature and minimizes the carbonization and volume loss of the raw materials.

Pellets and briquettes must be sufficiently durable to withstand bulk solid handing systems and transportation. The durability depends on the forces that bind the individual particles in the pellet together. The binding mechanisms between particles can be categorized into five different groups:

• Solid bridges;

• Adhesion and cohesion forces;

• Attractive forces (van der Waals, valence, electrostatic and magnetic forces);

• Interfacial forces and surface tension of liquid films;

• Mechanical interlocking.

Rumpf compared the theoretical strength of these different bonds, shown in Figure 1. Regions I and II represent solid bridge bonding where tensile strength is theoretically independent of particle size. Failure of agglomerates occurs between particles, as intraparticle bonding is much stronger than interparticle bonding, and failure would occur in the solid bridges. Finer particles can increase the strength of these bonds but to a lesser extent. Region I represents the strength of briquettes with binders in conjunction with high particle-to-particle surface contact of very fine particles (10 µm diameter and 10 Å separation) where van der Waals forces can be significant, whereas Region II represents particle-size-independent bonding by crystallized salts. The inclined lines in Figure 1 divide the plot into regions of different particle-size-dependent bonding mechanisms.

Figure 1. Theoretical tensile strength of agglomerates adapted from [14].

The elevated pressure and temperature from the biomass-wall friction during the pelletization process can lead to the formation of solid bridges due to the softening and diffusion of molecules between particles, a chemical reaction on the particle surface, and/or a solidification of melted components or binders between particles. In the case of wood pellets, lignin forms these solid bridges. Viscous binders, such as resin or tar, adhere to adjacent particle surfaces, creating a strong bond similar to a solid bridge. Some of these harden at ambient temperature, creating solid bridges. If large, irregular particles are pelletized—as in the case of alfalfa pellets—overlapping and folding will bind particles together through mechanical interlocking.

Solid attractive forces are typically weak and depend highly on the contact surface area and distance between particles, with van der Waals forces and valence forces effective at 0.1 μm and 10 A, respectively. These forces are too weak to create a pellet or briquette durable enough to withstand handling during transport.

Mechanical interlocking is a factor in binding more fibrous agricultural materials, such as hay. This interlocking is dependent on complete crushing of the plant stems, adhesion of the stems via the other aforementioned binding mechanisms, and interlacing of the stem and leaf materials. Agricultural biomass typically has less lignin content than woody biomass, and mechanical interlocking is more important.

2.3 Order of Torrefied Pellet Production

The order of torrefaction and densification affects the quality and cost of pellets as well; the two modes are torrefaction prior to densification and densification prior to torrefaction. Torrefaction removes the moisture and volatiles and decomposes the lignin, making the densification more difficult than for raw biomass. Torrefied biomass with 10 wt% moisture content provides a high-quality pellet. Pelletizing the mixture of kitchen and garden waste in both modes showed that increased torrefaction temperature decreased the wettability index and increased the higher heating value. The pellets produced with torrefaction prior processes showed a better calorific value but worse mechanical properties, particularly durability, compared to prior torrefaction processes. Sarker et al. studied two process modes in the torrefaction and pelletization of canola seed hulls and oat hulls with mustard meal as a waste binder. Torrefaction prior to densification provides a more energy-dense pellet, although the durability, mechanical strength, and porosity of pelletizing following torrefaction is higher. The co-pelletization of two or more treated and untreated biomass feeds through extrusion has attracted attention recently as it requires lower die temperature and pressure in a single pelletizer. Ghiasi et al. performed extensive work on both pathways and found that pelletization prior to torrefaction was preferable and torrefied particles required a binder for pelletization. Torrefaction after pelletization can be challenging and require more energy input for the size reduction of raw wood vs. torrefied wood.

2.4 Binders

Torrefied pellets have lower strength and durability than raw wood pellets. Binders are additives used primarily to improve the durability of briquettes and pellets as well as to reduce the production of fines during transportation and handling. They can also be used to improve the combustion characteristics of the fuel or to help lubricate the pellet press to reduce the energy of production. Binders can be divided into three broad categories based on the method in which they bind the pellet particles together:

• Matrix;

• Film;

• Chemical reaction.

Binding forces include solid bridges, attraction forces, mechanical interlocking bonds, adhesive and cohesion forces, and interfacial forces and capillary pressure. The different binding force mechanisms are summarized in Table 1.

Table 1. Binding forces and their mechanisms.

Binding Forces	Mechanism
Solid bridges	Formed between particles due to crystallization of some molecular components, chemical reaction, hardening of binders, and solidification of melted components and are mainly formed after the cooling of pellets.
Attraction forces	Formed within solid particles of pellets. They are short-range attracting forces; such forces are molecular, hydrogen, electrostatic, and magnetic forces. Attraction forces must take place via chemical bonding between primary biopolymers. Temperature plays a significant role in this process.
Mechanical interlocking bonds	Bonding together of fibers and particles during compression. They play a limited or no role in the total strength of biomass due to the absence of atomic forces. The strength of the mechanical interlocking effect depends on the binder material, its concentration, and the amount of compression applied during pellet production.
Adhesion and cohesion forces	High-viscosity binders which adhere to the solid particles surfaces to produce strong bonds very similar to solid bridges in the bonding mechanism.
Interfacial forces and capillary pressure	Free moisture between particles in a wet agglomeration process causes bond-cohesive forces generated from interfacial tension at the liquid–gas interface. Interfacial forces and capillary pressure bonds generated during agglomeration disappear immediately; the free moisture evaporates, and possibly, some other binding mechanisms may take over.

ISO 17225 limits the amount of binder in graded biofuels to less than 4 wt% graded torrefied briquettes. If more than 20 wt% binder is used, the biofuel is classified as a blend. Additionally, the International Maritime Organization codes for the transportation of dangerous goods stipulate a binder concentration of ≤3 wt% to be classified as torrefied wood pellets and briquettes.

Binders have been widely used in the production of agricultural pellets and coal briquettes to increase durability. They can be divided into two types: inorganic and organic binders. Inorganic binders increase the ash content of the densified fuel while typically decreasing the energy content; therefore, they must be used in low concentrations. Inorganic binders have some potential advantages over organic binders, including decreased biological degradation and beneficial effects on combustion systems. Organic binders typically do not increase the pellet ash content and have little effect on pellet energy density; however, they can be more costly and prone to biological degradation. Using oxygen-containing components as binders can increase the oxygen content of the pellets, which is against the purposes of torrefaction—deoxidation and homogenization. The common criteria for binder consideration are:

• Cost—must not add significant cost to pellet production;

• Durability—must increase the durability of the pellet significantly;

• Weathering—must not deteriorate when exposed to moisture;

• Ash—must not significantly increase the amount of ash or unwanted elements;

• Heating value—must not significantly reduce the heating value of the fuel.

In addition to these criteria, other minor criteria are examined in this study: toxicity, availability, the potential as a food source, and contaminants detrimental to combustion systems. The following sections identify a number of binders with potential for use in torrefied pellets based on research into binders for animal feed and coal fines. The binders are evaluated based on the above criteria to identify the most promising binders for future experimental examination. Additionally, the status of commercially available binders is examined.

2.5 Water

Although not a binder, water aids in the binding of particles in a number of different ways. Water is added to different binders to obtain homogeneity. When the material contains soluble components, the water will dissolve the soluble material on the surface and—upon subsequent evaporation—will cause recrystallization of the soluble material between particles. It can also bind particles together through surface tension and by increasing the contact surface area between particles, causing an increase in van der Waals forces, although these forces are minimal. There are different hypotheses to describe the effect of water in briquetting.

The capillary hypothesis: during cold-press briquetting of coal with a binder, pressure is applied to the mixture of coal and water. As pressure increases, water is forced out from the capillaries and covers the surface of the material, forming a thin water film. The water film fills the gaps between particles and creates interaction forces between the molecules, which helps to bind the material together. When the pressure is released, some of the water re-enters the capillaries, while the rest remains on the surface in a crescent shape due to surface tension.

Adhesion molecules hypothesis: water fills the gaps between particles and forms secondary capillary adsorption and surface tension forces.

In the case of wood pellets, water acts as a plasticizer of the lignin and hemicellulose, allowing them to soften and flow between particles under the heat and pressure of pelletization, hardening and forming solid bridges when cooled. Water is of particular importance in torrefied wood pellets since the torrefaction process decomposes the hemicellulose and increases the glass transition temperature of the lignin. Table 2 demonstrates the glass transition of raw and modified lignin. Re-addition of water is typically required to plasticize the lignin, binding the torrefied particles together, and an increase in moisture content from 1.1 to 11.1 wt% increases relative pellet hardness by ~63%.

Table 2. Glass transition temperatures of raw and modified lignin and hemicellulose.

	Tg (°C)	Reference
Lignin (dry)	200–220	[30,31]
Lignin (wet, hardboard)	115	[31]
Lignin (saturated, pine)	58–75	[32]
Hemicellulose (dry)	200	[30]
Hemicellulose (10% moisture)	12	[30]
Kraft lignin	97	[33]
Alkaline lignin	60–90	[34]
Hydrolysis lignin	70–83	[33]
Lignosulphonate	99	[33]

© 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).