Organic binders have the benefit of low ash and impurities, important factors when producing a torrefied pellet/briquette for use in an existing coal-fired plant. They generally suffer from poor resistance to moisture and biological degradation, with some exceptions. The following section describes the most promising organic binders for use in torrefied wood pellets.

Starch is most commonly used as a binder in food products. Starch addition can increase the pellet hardness and reduce abrasion during transportation. It is also used in pharmaceutical applications and animal feed pelletization. It can be used to bind biomass pellets in much the same way. Starch binders are common in Australia, where biomass pellet utilization is widespread. Typically, concentrations of <2 wt% of starch are utilized. Starch is also widely used in Asia for the production of coal briquettes for residential heating and cooking.

Starch can be derived from a number of different plants—including wheat, potato, corn, rice, and pea—and food waste [35,36]. It consists of a number of glucose rings joined together by oxygen side-chain bonds. Depending on the source plant, starch is typically composed of 20–25% amylose and 75–80% amylopectin [35]. To save on processing costs, flour is typically used as the binder, which contains 85–95 wt% starch. Tabil et al. found that an addition of only 0.5 wt% pea starch significantly increased the durability of alfalfa feed pellets [37]. Kuokkan et al. found that the addition of 1% potato flour increases pellet durability from 96.5% to 98% compared to binder-free wood pellets [35]. They also found that the addition of potato flour did not significantly increase the biological degradation of the pellet. Margl and Kiefer produced a raw wood pellet, 11–14 wt% moisture, using a corn flour binder, 1–5 wt%, and found improved abrasion resistance [38]. Tapioca starch and corn starch have a low cost. Tapioca starch increases the strength of briquettes more than corn starch due to its higher lignin content. The addition of tapioca starch solution to biomass at a ratio of 100:20 (TS1) provides the highest fixed carbon content (56.94), lower volatile matter (26.42), and lower ash content. It is able to increase the HHV of biomass by 30% [39].

The main drawback of using starch is its hydrophilic nature. If dry starch is used as a binder, it will absorb moisture and potentially deteriorate. Gelatinized starch is created through the addition of water and heating to between 55 and 85 °C and can be promoted through the use of corrosives, i.e., sodium hydroxide [40]. This breaks apart the starch granule through swelling and irreversible absorption of water. The intermolecular bonds are broken, and starch molecules leave the granule, allowing the intermeshing of starch molecules in solution. The mechanical shearing and friction heating during pelletization can also cause some degree of starch gelatinization, but this is limited [18].

Trubiano and Kasica produced a “compressible” starch with a partially broken-down granular structure through treatment with an acid or enzyme [41]. Heimann et al. produced a raw wood pellet using a partially gelatinized starch (10 wt%) and an alkali metal hydroxide (0.02 wt%) with a durability index of 99.99 [42]. The greater the degree of starch gelatinization, the higher the pellet durability and the lower the moisture absorption potential; however, it would still be susceptible to biological deterioration. Wood found that pregelatinized starch resulted in higher pellet hardness and durability [43]. Franke et al. created hydrophobic coal briquettes using a gelatinized starch wetted and re-dried at 100–150 °C [40]. To create a hydrophobic pellet, a drying/hardening step at ~270 °C for 1 h was required. The resulting pellets did not deteriorate in water, absorbing only 7 wt% water when submerged for 24 h. Dry and gelatinized starches are treated separately in the binder comparison.

Fiber can be either water-soluble or insoluble. The former increases the viscosity of the feed allowing for better pelletizing and increase pellet strength [18]. The long fiber molecules entangle and wrap around the biomass particles. Insoluble fibers can entangle or fold between particles to increase strength [14]. Increasing the fiber content from 18–27 wt% has been shown to increase the durability of alfalfa pellets by about 5% [44]. Native fiber present in the feedstock can decrease the durability of pellets. Fiber is stiff and highly elastic, causing it to re-expand after pelletization [45]. This could require chemical pretreatment of the high-fiber material to break down the long chains. Long fibers in the pellet can also make weak points for fragmentation. Soluble fiber can be solubilized during the heat and pressure of pelletization and subsequently recrystallized upon cooling to form solid bridges [46]. This is in addition to fiber entanglement, which occurs in soluble and insoluble fibers. Due to this added binding mechanism, only soluble fibers will be used in the comparative analysis.

Under the heat and pressure of the pelletizing process and in the presence of water, protein will plasticize and can be used as a binder [47]. Heat, moisture, and shear also cause proteins to denature, allowing the long protein chain molecules to intertwine and bind together [43,45]. Alternatively, the protein could be hydrolyzed prior to pelletization via heating (120 °C for 45 min) in an acidic or alkaline solution [48]. When the protein collagen is hydrolyzed it forms gelatin, discussed later. Oriented strand board (OSB) can be made with a hydrolyzed soy protein binder cross-linked with formaldehyde [49]. Protein is typically sourced from crops similar to starch, including soybean, wheat (gluten), corn, and alfalfa. It can also be sourced from by-products from animal processing, i.e., collagen.

Steele and Penmetsa produced torrefied pellets with hydrolyzed soy protein and bio-oil. The strongest pellet produced was in a ratio of 1:1 hydrolyzed protein to bio-oil, with bonds formed through cross-linkages between particles [48]. The pellets produced could be immersed in water for extended periods without disintegration, and increasing the binder concentration from 10 to 30% reduced the moisture uptake substantially [48]. The type of protein is important to the pellet durability and the addition of raw protein produces a pellet with greater durability than denatured protein [43]. Cavalcanti found that different proteins produced pellets of varying durability [50]. He found that protein from soybeans had a positive effect on pellet durability, whereas protein derived from corn meal had a negative effect [51].

Gelatin is produced through the hydrolysis of collagen; a process which breaks the bonds between protein polypeptide fibrils into smaller single peptides. This modified protein is less rigidly structured, rearranging more easily and dissolving easily in hot water. Upon cooling, the protein strands bond together in a partial return to helical polypeptides [52]. The collagen strands bind together randomly, forming a three-dimensional network of gelatin molecules known as a semisolid colloidal gel. The formation of the cross-bonds is slow, and the gel strength increases with time.

Molasses is a by-product of the refining of sugar. Molasses binds through a film-type adhesion of the particles. It may also gain some binding effect from the formation of solid bridges resulting from the recrystallization of sugars or solidification upon cooling and drying of the pellet. It is a popular binder in animal feed pellets because it has the added benefit of increasing the calorific value and nutrient content of the feed. It creates very durable pellets but is soluble in water and prone to weathering.

The addition of fats and oils to animal feed pellets typically leads to a decrease in pellet durability [18,53]. Fats and oils act as a lubricant between particles, reducing bonding strength as well as inhibiting the solubility of water-soluble components to form solid bridges. The lubricating effect reduces friction in the pelletizer, leading to a reduction in pelletizing pressure and a further reduction in durability [18]; however, this does reduce the energy requirements of the pelletizer [53]. It is possible that in wood, fats and oils could act as a plasticizer for the polymeric compounds’ lignin and hemi-cellulose, reducing their softening temperature and increasing their binding ability. This could be of benefit to torrefied pellets, in which the glass transition of the lignin has been altered; however, further study is required, and fats and oils will not be included in the binder analysis.

Carboxymethyl cellulose (CMC), or “cellulose gum”, is produced through the reaction of cellulose with chloroacetic acid, producing soluble cellulose. CMC is a widely used food additive. The addition of CMC to biomass causes electric dipole forces between particles, which may increase the cohesion strength of pellets and cause polyelectrolyte formation. A hydrogen bond forms between the electric dipole of water molecules in water and the OH group on the CMC. At the interface of biomass particles and CMC strong bonds—similar to solid bridges—are formed. These interparticle interaction enhancements in pellets increase the quality of biomass as a source of energy. Relaxed density; compressive strength; and durability of cotton stalks, wheat straw, and rape straw pellets increased considerably in CMC content cotton stalks and wheat straw, while the quality content of CMC content rape straw was decreased due to the presence of extractives [54]. CMC is an effective binder for minimal extractive content biomass [54]. Addition of Sodium carboxymethyl cellulose to biomass fuels, besides improving the pellets quality, increases particulate matter (PM) to varying degrees due to the formation of Na-containing species, e.g., NaCl, Na₂SO₄, NaOH, and Na₂CO₃. Adding Si-containing rice husk or SiO₂-rich minerals to biomass fuels can reduce PM emissions by facilitating coarse ash particle formation [55]. Carboxymethylcellulose provided the highest mechanical durability (4449 N), lower ash content (4.2 wt%), and improved HHV (20.68 MJ/kg) compared to other binders for torrefied Palm kernel pellets [56].

Additional lignin can be added to the feedstock to improve binding. Concentrations of lignin up to 30 wt% can be beneficial for pellet strength. Above about 30 wt%, the pellets can become brittle, and durability is thus decreased. Lignin is a by-product of the pulp and paper industry and is the main constituent of Kraft liquor waste product, about 40% of black liquor. Delignification of the wood fiber in the Kraft pulping process is conducted using a sulfate, which replaces the lignin’s ether bonds with sulfur functional groups, causing depolymerization. The majority of sulfur is recovered as elemental sulfur and polysulfide; however, Kraft lignin still contains 2–3 wt% sulfur. Acid washing can further reduce the sulfur content of the lignin [57]. Lignin is also a by-product of bio-ethanol production. This type of lignin is referred to as hydrolysis lignin and typically has less sulfur than Kraft lignin (>1 wt%). As a binder in pellet production, Kraft lignin must be heated and softened as it has a higher glass transition temperature (T_g ≈ 100 °C) than lignin present in saturated wood [33].

Kong et al. used alkaline lignin (similar to Kraft lignin) as a binder in bio-char pellets in the form of saw dust pyrolyzed at 500 °C for 1 h [34]. They found that bio-char pellets made with 15 wt% lignin swelled considerably and lost all durability when stored for 2 weeks in a 60% relative humidity chamber, absorbing 25 wt% moisture. This was attributed to moisture absorption on the polar functional groups on the alkaline lignin.

Lignosulfonate is a by-product of the less-used sulfite pulping process. In this process, a sulfite (i.e., NaSO₃) is used to break the ester bonds between lignin molecules that bind the cellulose fibers, a process known as delignification. Like Kraft lignin, lignosulfonates come in a broad range of complex compositions and molecular makeups but are less fragmented. They all include the SO₃H side chain of the lignin aromatic alcohols. They are produced from black/red liquor, a waste by-product of the acid pulping of wood fiber in the pulp and paper industry. Lignosulphonates can be precipitated from the liquor through the addition of a metal hydroxide. When used as a binder, bonding between lignosulphonate molecules typically occurs at the free phenolic hydroxyls.

Lignosulphonate is one of the most widely used and effective binders in the production of animal feed pellets, effective at low concentrations of 1–3 wt% [58]. It binds by adhering to the surface of and forming solid bridges between particles. Its major drawback is the potential absorption of water. The sulfonate groups are hydrophilic, and in addition to hydrophilic residual sugars, weathering will likely be a problem, preventing uncovered storage and transportation of pellets. Further studies are needed to test moisture absorption.

The metal salts (Na, Ca) formed during the precipitation of lignosulphonate can cause a decrease in ash melting temperature, leading to fouling of boiler tubes and the formation of large ash agglomerates. Calcium typically produces compounds with higher melting points, so lignosulphonate precipitated using calcium hydroxide is preferable. The sulfur content in lignosulphonates (up to 10 wt%) can lead to the formation of sulfates with high melting points, reducing ash fouling and corrosion tendencies; however, it also increases the production of SO₂, a harmful pollutant. A pellet made with only 1 wt% lignosulphonate increases the sulfur content to 0.57 g/kg, above the CEN/TS 14961 limit for chemically treated biomass of 0.5 g/kg [35].

Pfost added 1–2 wt% lignosulphonate to feed pellets, increasing the durability from 90 to 97% [59]. Kuokkan et al. found that the addition of 1 wt% lignosulfonate increased pellet durability from 96.5% to 97.7% compared to binder-free pellets [35]. They also found that lignosulfonate decreased the pelletizing energy usage and increased production rate. Dobie tested a mixed binder composed of lignosulphonate, 2.4 wt% ammonia, and 50 wt% water [60]. He found that adding 5–10 wt% of this mixture to grass pellets increased the durability from 15–44% to 93–97%, allowing the production of pellets from a difficult-to-densify feedstock.

Tall oil pitch is the third major by-product from the Kraft fiber production process. It is composed of a number of different compounds including esterified acids (23–38%), free fatty acids (35–52%), and neutral compounds (25–34%) [61]. It is skimmed off after the initial thermal alkaline treatment and is the bottom fraction of subsequent distillation, which is then neutralized with acid. It is used as an ingredient in adhesives and binder/sealant in road construction [62] and is used as the binder for extruded “fire logs” [63]. In wood pellets, it has shown promise as a plasticizer for lignin [63]. It is water-insoluble and not prone to biological degradation, but its effect on improving the durability of pellets has yet to be demonstrated.

Also referred to as “biomass oil” or “pyrolysis oil”, biomass tar is a liquid produced when biomass is heated above 300 °C in an oxygen-free environment. In the temperature range of 200–300 °C, torrefaction occurs, driving off moisture and lighter volatiles, which can be condensed into an aqueous, low-viscosity “oil”. In the range of 300–500 °C, pyrolysis begins, and heavier hydrocarbons are driven off, producing biomass tars. Biomass tar is a complex mixture of hundreds of hydrocarbons produced through the thermal decomposition of lignin, hemicellulose, and cellulose as well as oils, waxes, and other minor components of biomass. It binds the wood particles through formation of a film on the particles and “gluing” the particles together. Concentrations of up to 50 wt% can be used depending on the feedstock; however, typically, concentrations are in the range of 2 to 20 wt%. White stated that a minimum of 3 wt% biomass tar binder was required to ensure sufficient coverage of the biomass particles [64]. The heavier tars are immiscible with water and not prone to weathering. Tar creates very durable pellets, but there could be potential issues associated with leaching and off-gassing of the lighter polycyclic aromatic hydrocarbons (PAHs), some of which could be toxic. Addition of tar to wheat straw increases the mechanical strength and LHV of pellets considerably [65]. By addition of 35 wt% tar, the LHV increased between 20–26% depending on the feedstock [65].

Tar binder can be produced in conjunction with the production of torrefied biomass. It is common practice to use the condensate (mostly water with light tars) from torrefaction to rehydrate the torrefied material prior to pelletization. The binding effect of these lighter volatiles is most likely limited.

Glycerol is a by-product of the biodiesel production process. For every 10 kg of biodiesel produced, 1 kg of glycerol is produced through the transesterification of fats and oils (triglycerides). Glycerol primarily acts as a plasticizer and has the potential to soften other polar molecules, such as lignin. The hydroxyl groups on the glycerol molecule, could disrupt the intermolecular hydrogen bonds of the lignin in much the same way as water.

Lu et al. found that wheat straw pellets made with 5 wt% glycerol increased the fracture and tensile strength of pellets by 67 and 54%, respectively [66]. In torrefied pellets, glycerol could plasticize the lignin, allowing for effective pelletization at lower temperatures and moisture contents. The main drawback of glycerol is its hydrophilic nature, resulting from the hydroxyl groups. If the glycerol is saturated with water during the pelletizing process, then further moisture absorption/weather is not a problem.

Stearic acid is one of the most common saturated fatty acids found in nature, typically in animal fats. It is produced through the saponification of triglycerides (fats and oils). It is used as a lubricant and release agent in pressing processes. The polar end group easily forms ionic bonds with metal cations (Na, Ca, Mg, Zn, etc.). Renirie et al. found that the addition of stearic acid, 2.5 wt%, and rye meal, 1 wt%, reduced the fines of the produced pellets (2.7%) compared to pellets without a binder and 1 wt% flour binder that produced 4.3% and 3.5% fines, respectively [67]. Additionally, the stearic acid had a lubricating effect, reducing the power consumption of the pelletizing process and increased the throughput on an industrial mill.

Metallic stearates are used in the plastics industry as lubricants and release agents. It has been found that the use of calcium stearate, [CH₃(CH₂)₁₆COO^-]₂(Ca²⁺), has been shown to increase the hardness of polymers by acting as a plasticizer [68]. Calcium stearate and other plasticizers have the potential to reduce the energy requirements of the pelletization of difficult to extrude torrefied material. The use of plasticizers in this respect warrants further experimental investigation. Calcium stearate is a good candidate as a plasticizer due to its low cost, insolubility in water, and resistance to biological degradation, but further experimental investigation is required.

Although standards regarding bioenergy products will likely preclude the use of petrochemical binders, they are widely used in the production of coal briquettes and deserve mention.

Coal tar pitch is produced in a similar fashion to biomass tar, through the heating of coal in a low oxygen environment to produce condensable, heavy hydrocarbon tars. Coal tar pitch has been used in the production of coal briquettes since the beginning of the 20th century [69]. It acts as film type binder, “gluing” particles together. It is insoluble in water and resistant to biological degradation; however, it is highly toxic and a known carcinogen due to the high concentration of PAHs. It can also foul the feed lines to furnaces when warmed through grinding. Zhong et al. used coal tar pitch to produce formed coal briquettes from high-volatility coal for COREX ironmaking process [70].

Asphalt is a by-product of the crude oil refining process, obtained from the distillation bottoms. Material with a boiling point greater than 500 °C is referred to as asphalt. It is a highly viscous amorphous material, and when heated above 100 °C, it softens and can be used as a film-type binder [71,72]. It is highly resistant to weathering and biological degradation but is also toxic. Addition of asphalt as a binder improves the hydrophobicity of pulverized coal [73]. It must be transported from the refinery in heated or insulated tankers or railcars until its final end use, typically for paving or roofing operations. This would make its use as a binder in small quantities difficult.

Using plastic waste as a binder can minimize their effect on the environment as a pollutant. The addition of high-density polyethylene (HDPE) to torrefied wheat and barley straw pellets increased density, tensile strength, and high heating value and reduced ash content and moisture adsorption [74]. Plastic binders are not included in the comparative analysis as their addition would make the pellet a solid recovered fuel rather than virgin wood pellet.

The main disadvantage of using inorganic binders is that they increase the amount of ash in the fuel. This reduces the energy density of the densified product and can make it unsuitable for use in certain systems depending on the impurities present (i.e., S, Na, K, and other fouling agents). They have the benefit of low cost and resistance to biological degradation. Organic binders typically strengthen the pellet through a chemical reaction.

Lime (CaO) is the primary ingredient in cement and, as such, is a very strong binder. It binds through the chemical reactions that form crystallized calcium hydroxide (Ca(OH)₂) and calcium carbonate (CaCO₃) shown in Equations (1) and (2). CaO is highly caustic, and its hydrated form, calcium hydroxide, is often used as a binder to minimize handling hazards despite potential loss in durability. Calcium hydroxide binds through the chemical reaction with carbon dioxide to recrystallize as calcium carbonate (CaCO₃), forming strong ionic bonds between particles. At ambient temperatures (20 °C), the kinetics of the carbonation reaction (2) would be quite slow, and the pellet could increase in strength over time.

It has also been shown that alkali earth metals increase the cross-linking of phenolic resins, which have similar structures and functional groups to lignin [75]. In torrefied pellets the enhanced cross-linking of lignin could be a binding mechanism as important as the hardening effect from re-crystallization.

Ca(OH)₂ binder has been shown to produce very strong bio-char pellets [34]. Water in excess of 10 wt% was required to form durable pellets with Ca(OH)₂ with highly torrefied material (500 °C for 1 h). This and the lack of residual inherent binder suggest a re-crystallization mechanism as the primary binding mechanism. The pellets made with 15 wt% Ca(OH)₂ and 15 wt% moisture showed very high durability, >99.5%, and increased in durability after storage in a humidity chamber at 60% relative humidity. This slow re-crystallization further enhanced the strength and durability of the pellets [34]. Ca(OH)₂ pellets also showed low moisture absorption during humidified storage, only increasing in moisture content by 10% over 2 weeks.

Kong et al. also studied bio-char pellets produced utilizing a lignin binder and chemical hardener, including NaOH, CaCL₂, CaO, and Ca(OH)₂ in a mass ratio of 1:4, hardener to lignin [34]. They found that CaO and Ca(OH)₂ hardeners produced the most durable pellets of all the hardeners tested, with abrasive resistances of 99.63 and 99.77%, respectively. They conducted a moisture uptake test involving storing the pellets for two weeks at 60% relative humidity. The pellets made with NaOH and CaCl₂ hardeners swelled and lost all durability during these moisture uptake tests, whereas the pellets made with CaO and Ca(OH)₂ had increased abrasive resistances of 99.71 and 99.82%, respectively. However, the CaO and lignin pellets had reduced impact resistance strength and compressive strength, likely a result of the reaction between CaO and water to form the more friable Ca(OH)₂. The Ca(OH)₂ and lignin pellet maintained the same impact resistance and had slightly reduced compressive strength. Increasing the ratio of Ca(OH)₂ to lignin had the beneficial effect of reducing the moisture uptake but produced a more brittle and slightly less durable pellet.

Lime is often added to biomass fuel pellets to reduce clinker formation and slagging in high temperature boilers or kilns. The lime (CaO) reacts with other ash components in the fuel, such as silicates, to form compounds with high melting points. In the co-combustion of biomass pellets and coal, the lime has the added benefit of reducing sulfur emissions through the formation of CaSO₄.

Calcium chloride is used to accelerate the curing of cement. If used in conjunction with lime, it could improve the durability of the pellet and reduce drying time. If reacted with water, it will dissolve and form calcium hydroxide and hydrochloric acid:

This would have the double benefit of forming calcium hydroxide, which acts as a binder, and hydrochloric acid, which could free the lignin for better particle binding. CaCl₂ would cause issues with corrosion of pelletizing equipment and during combustion/gasification through the release of HCl.

Sodium hydroxide (NaOH), also known as caustic soda or lye, is a common industrial chemical used as a strong base in many processes. It is produced via the chlor-alkali process through the electrolysis of salt (NaCl) water. NaOH does not directly act as a binder. When used in solution as a pretreatment, NaOH disrupts the ester bonds between lignin and carbohydrates, which solubilizes the lignin, freeing it from the lingo-cellulosic matrix prior to pelletization in much the same way as delignification in the Kraft process. During pelletization, the lignin rebinds the material into the pellet shape.

Kong et al. utilized NaOH as a hardener for a lignin binder in a bio-char pellet at a mass ratio of 1:4 NaOH to lignin [34]. They found that the NaOH increased the pellet durability compared to lignin-only binder. However, when stored for two weeks at 60% relative humidity, the pellets disintegrated and lost all durability.

Bentonite is a hydrated magnesium aluminum silicate clay primarily composed of montmorillonite. It is composed of plate-like particles that are negatively charged on the surface and positively charged on the edges. It is this polarity that gives bentonite its binding ability. The different types of bentonite clay are defined by their dominant element—sodium bentonite, calcium bentonite, or potassium bentonite. Sodium bentonite has a high swelling capacity in water, up to 12 times its volume, whereas calcium bentonite has little swelling capacity. Bentonite is a mined mineral and the United State is the primary producer.

Bentonite is typically used as a binding agent for the production of iron ore pellets at a concentration of 1 wt% and in iron and steel castings. Pfost and Young found that the addition of 2.4 wt% bentonite to feed pellets increased pellet durability by 6% and reduced fines in poultry feed pellets [76].

Sulfuric acid (H₂SO₄) is a common industrial chemical. It binds in much the same way as caustic soda as a plasticizer, by disrupting the inter-molecular lignin bonds, softening the lignin prior to pelletization. This allows the lignin to flow into the interparticle spaces during compression, plasticize, and reharden to form bridges. There are no published results examining the effectiveness of a sulfuric acid binder, but it was expected that it would bind in the same manner as caustic binders as a plasticizer. The use of acid hydrolysis to soften the thermally altered lignin is a novel concept and deserves further research to examine its effectiveness.

Silicate salts, sodium silicate (Na₂SiO₃), and potassium silicate (K₂SiO₃), forms an oxygen–silicon polymer, with the alkali metal forming ionic bonds with the oxygen. Sodium silicate is an inorganic adhesive used in the bonding of cardboard, insulation, and wood. Due to its polarity, it is soluble in water. It binds by means of a highly viscous film created as the adhesive dries. It is also possible due to its polar nature that it could act as a plasticizer or form weak ionic bonds with lignin or cellulose. It has not been applied as a binder in pellet production, but it shows potential. It does not release volatiles during curing or storage, it has a relatively low toxicity, and it is not susceptible to biological degradation; however, moisture absorption and weather could be issues.

McGoldrick patented a binder mixture composed of potassium silicate and a surfactant for the production of biomass agglomerates [77]. The surfactant pushes the binder in solution to the surface of the agglomerate and creates a hard “shell” on the agglomerate surface [78]. This allows the agglomerate to keep its shape during transportation and combustion.