5. Inorganic Binders
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.

5.1 Lime
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. 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. 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. 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. 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.

5.2 Calcium Chloride
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.

5.3 Caustic Soda
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. 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.

5.4 Bentonite
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.

5.5 Sulfuric Acid
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.

5.6 Silicate Salts
Silicate salts, sodium silicate (Na2SiO3), and potassium silicate (K2SiO3), forms an oxygen–silicon polymer, with the alkali metal forming ionic bonds with the oxygen, shown in Figure 6. 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.

 

Figure 6. Sodium silicate molecule, polymer chain represented by dashed lines.

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

6. Binder Comparison
The binders identified and discussed in the previous section were compared on a technoeconomic basis. Prices were determined through consultations with suppliers, shown in Table 3. Concentrations were taken from previous studies on binders for biomass or coal or, if none were available, were assumed based on similar binders. The added cost per ton of wood pellets was then determined. The solubility of the binder in water is an important factor in determining both the ability of the binder to disperse well within the material and potentially its ability to help solubilize the thermally altered lignin to act as a binder.

To determine the optimal binders for use in torrefied pellets, 10 parameters were used to quantitatively compare the most promising binders discussed in previous sections. In this comparison, lignin refers to Kraft lignin added to the torrefied pellet, not native lignin. It was decided that not all of the comparison parameters were of equal value. As such, a weighting was given to each. The parameters and weightings are shown in Table 4. For each parameter, the binder received a score of 0–4, with 4 being very good and 0 being very poor for that parameter. For each binder the weighting was multiplied by the score for each parameter, and the sum of these values gave an overall score for the binder, with a maximum overall score of 100. These price values is obtained provided by suppliers.

Table 4. Parameters and weighting used for binder comparison.

Parameter	Weighting
Durability	5
Hydrophobicity	5
Cost	4
Contaminants	3
Heating value	2
Toxicity (human health and safety)	2
Toxicity (environmental)	1
Availability	1
Ash	1
Food source	1

 

The highest weighting was given to durability and hydrophobicity. The durability of the resulting pellet is essential for binders, as this is the binder's primary function. Hydrophobicity is of near equal importance in the case of torrefied pellets, as one of the major benefits of producing torrefied pellets is their resistance to weathering and biological degradation. The durability and hydrophobicity scores were assessed based on a thorough literature review, as highlighted in previous sections. In some cases, the potential binder had not been experimentally tested in pellets or briquettes, and the potential durability improvement is unknown. In these cases, a durability score of 2 was given.

The scoring methodologies for certain quantifiable parameters are listed below in Table 5. The score for environmental toxicity was assigned by reviewing the Material Safety Data Sheet (MSDS) for each binder and environmental toxicology reports. For the "contaminants" parameter, the score was given based on the potential effects of elemental impurities (S, Na, K, Cl, heavy metals, etc.) on combustion systems, described in Table 3. For example, waste glycerol contains NaOH or KOH impurities, and Na and K combined with other elements, causing fouling in combustion systems. The cost was determined by obtaining quotes from industrial suppliers for 1 ton of the binder material with delivery to Vancouver, Canada.

 

Table 5. Scoring methodology for quantitative parameters.

	Score	Heating Value (HHV)	Toxicity/H&S (WHMIS Rating)	Availability (Suppliers)	Pellet Ash Increase (wt%)	Cost (USD/ton)
4	Very Good	>25 MJ/kg	0	Many North America	<0.1%	<$2
3	Good	>15 MJ/kg	1	Many international	<0.5%	<$5
2	Acceptable	>10 MJ/kg	2	Dozens	<1%	<$10
1	Poor	>5 MJ/kg	3	Few	<2%	<$20
0	Very Poor	<5 MJ/kg	4	None yet	>2%	>$20

 

The increase in ash was calculated based on the inorganic fraction of the binder and the range of concentration required based on previous binder studies. Finally, the score for "Food Source" was determined based on the binder's usage in food products or processing. This parameter reflects the importance of minimizing bioenergy's impact on food supply or prices.
The scoring matrix for the binders examined is displayed in Table 6. Overall, the organic binders show a higher potential for use in torrefied energy pellets. This is in part due to the lack of information regarding potential durability improvements from in-organic binders. Additionally, the reduced heating value and generally higher toxicity, ash, and contaminants result in the lower score for inorganic binders.

Table 6. Parametric scoring matrix of selected binders.

		Durability	Hydrophobicity	Heating Value	Toxicity/H&S	Toxicity/Environmental	Availability	Contaminates	Ash	Cost	Food Source	Overall Score
Organic Binders	Lignin	4	4	3	3	3	1	2	4	0	4	70
	Biomass Tar	4	3	2	2	1	0	4	4	1 †	4	68
	Tall Oil Pitch	2 *	4	4	3	3	3	3	4	0	4	67
	Starch (gelatinized)	4	2	2	3	4	4	4	4	0	0	64
	Lignosulfonate	3	4	2	2	3	3	0	4	1	4	61
	Glycerol	3	0	2	3	4	3	2	3	4	4	61
	Starch (dry)	2	0	2	3	4	4	4	4	3	0	56
	Protein	2	3	2	3	4	1	4	1	0	0	53
	Calcium Stearate	2 *	3	4	2	3	4	1	4	0	3	54
	Fiber (soluble)	2	0	1	3	4	1	4	4	2	0	47
	Molasses	3	0	1	4	4	1	2	4	0	2	42
	CMC	4	0	2	2	3	1	2	3	0	4	45
	Coal Tar Pitch	4	4	4	0	0	2	0	3	1	4	61
	Asphalt	3	4	3	2	2	1	0	2	0	4	54
Inorganic Binders	Lime (hydrated)	4	3	0	1	2	4	2	1	3	4	66
	Sulfuric Acid	2 *	4	0	1	0	4	0	3	3	4	55
	Bentonite	3	2	0	2	3	4	2	0	1	4	50
	Sodium Silicate	3 *	3	0	1	1	4	0	1	2	4	50
	Calcium Chloride	2 *	2	0	2	0	4	0	1	0	4	33
	Caustic Soda	2 *	1	0	1	2	4	0	2	2	4	37
* Binding ability remains uncertain; further experimental studies are required. † Cost is uncertain as there are no commercial producers.

 

The binder with the highest potential is lignin, with lime, lignosulfonate, tall oil pitch, biomass tar, and starch all close, as can be seen in a list of the top ten binders in Figure 7. It should be noted that no binder is perfect and that each has its drawbacks. In the case of lignin and lignosulfonate, the presence of sulfur may be unacceptable for certain end users depending on local regulations. The current European regulations for wood pellets (CEN/TS 14961) limit sulfur content to 0.05 wt%. This would limit the amount of lignin binder added and prevent the use of some lignosulfonate. New lignin production/separation processes aim to reduce sulfur content. Additionally, there is still uncertainty around the price of lignin and biomass tar as there are few large-volume production facilities.

Figure 7. Parametric comparison study scores from top ten potential binders.

Starch is a proven strong binder, but its hydrophilic nature could lead to potential weathering and biological degradation. If sufficiently gelled before or during pelletization/briquetting, starch can be made less water absorbing; however, it may still be prone to biological degradation. Further investigation is required into the effect of binders on the weathering and biological degradation of torrefied pellets/briquettes. Additionally, pregelatinized starch is much more expensive than dry starch. If the gelatinization process was performed immediately before or after pelletization using dry starch, gelatinized starch would be less expensive and more attractive as a binder. CMC, lignin, starch, and glycerol improved the HHV of torrefied palm pellets 15.07–21.23%; and HHV of pellets reached 20.68–21.24 MJ/kg. Hydrated lime was the best-performing inorganic binder and only one to be in the top ten. This is because of it's proven strong binding ability, non-reactivity with water, availability, and cost. It could have the added benefit of acting as a sulfur scrubber in solid fuel boilers and gasifiers. One potential drawback of lime is the release of carbon dioxide during its production. If the torrefied pellets are to be used to offset coal to obtain carbon credits, this could limit or prevent its use as a binder.

Technoeconomic analyses of torrefied lignocellulosic biomass pellets demonstrate the selection of an appropriate binder and quantity can improve the quality and consistency of the densified product significantly. Binders may negatively influence the profitability of the torrefied pellet production by altering the equipment requirements, capital costs, and the amount of required binder. For example, using distillers for dried grains or corn starch is more cost-effective than soybeans.

Not discussed in the above analysis is the use of waste biomass as a binder or the use of multiple binders. Using particulate biomass waste as a binder reduces the overall cost. García et al. used grape pomace, almond shell, olive stone, and pine sawdust as solid additives with glycerol in pelletizing pine. Adding 20 wt% grape pomace and 10 wt% glycerol reduced transportation costs by 20%. Lignin and starch improve the strength and the bulk density of the torrefied biomass slightly better than sawdust. The three binders reduced the bulk density of the biomass, which reduces the volumetric energy density of treated pellets compared to control wood pellets.

HHV of pellets can increase or decrease compared to the original biomass depending on the HHV, moisture content, particle size and weight fraction of binder. Xanthan and guar reduced the HHV of spent coffee grounds from 25.4 KJ/g to 24.44 and 24.39. The HHV of noncarbonized coffee husk briquette developed under high pressure was 15.2 MJ/kg. The HHV of coffee briquette with cassava and clay reached 21.9–23.5 and 13–17.2 MJ/kg, respectively. For rice husk, it was 15.9–20.9 and 9.5–12.0 MJ/kg using cassava and clay, respectively. Ahn et al., 2014 investigated the effect of chemical composition of binder on HHV and durability of binders. Rapeseed flour, coffee meal, bark, pinecones, and lignin powder were used as binders in the fabrication of larch and tulip tree pellets. The average higher heating values (HHV) of rapeseed flour, coffee meal, bark, pinecones, and lignin powder were 17.4 MJ/kg, 26.3 MJ/kg, 23.4 MJ/kg, 19.5 MJ/kg, and 20.7 MJ/kg, respectively. High contents of oil/fat and lignin increase the HHV of binders, while high moisture reduces HHV. For example, HHV of pinecones have generally high values due to a certain extractive; however, in this study, the high moisture content of pinecones caused lower HHV in the produced pellets. Lignin powder has a lower HHV than lignin. This is due to the intrinsic cementing property of lignin powder, which causes incomplete combustion at high temperatures.

7. Conclusions
Torrefied biomass pellets/briquettes are a promising near-term fuel alternative in conventional coal fired boilers, furnaces, and kilns. They have superior energy density, grindability, and resistance to weathering and biological degradation compared to conventional white wood pellets. This allows for reduced storage and transportation costs and capital investment required for conversion of existing coal facilities to biomass. The use of binders in agricultural feed pellets and coal briquettes is commonplace, but there is little experience or research on binder use in torrefied biomass materials.

Torrefied material is more difficult to densify due to the partial thermal decomposition of the lignin, a reduction in the lignin plasticity, and a reduction in its binding ability. As such, binders may be needed for the densification of torrefied fuel products to increase the durability of the densified fuel and reduce the generation of fines, which constitute a material loss and a health and safety hazard. Binders for torrefied wood must be able to resist weathering in the same way as the torrefied material itself. Additionally, binders must be cost-effective, as the prices for solid fuel products are already low, making competition from biomass-based sources difficult without policy incentives. An ideal binder would be one that is hydrophobic, low-cost, and capable of generating strong, densified products using a minimal amount. This paper examined the applicability of a number of binders to torrefied pellets/briquettes.

Overall, the organic binders show a higher potential for use in torrefied energy pellets. This is due to the lack of information regarding potential durability improvements from inorganic binders, the reduced heating value, generally higher toxicity, and increased ash and contaminants. The most promising binders are lignin, biomass tar, tall oil pitch, and lime. Due to the highly competitive nature of energy products, it is essential that the added cost of the binder is low.
Hydrated lime was the best-performing inorganic binder and the only one to be in the top ten. This is because of hydrated lime's proven strong binding ability, non-reactivity with water, availability, and cost. It could have the added benefit of acting as a sulfur scrubber in solid fuel boilers and gasifiers. It has the drawback of hazards associated with transportation and handling.

There is a wide variety of torrefaction processes, each producing a somewhat different product due to the treatment conditions (time, temperature, heat and mass transfer, etc.). Additionally, the location of the torrefaction facility will alter the availability/accessibility of different binders. There is unlikely to be a one-binder solution that fits all types of torrefied material and locations. This document should serve as a guide for producers of torrefied pellets/briquettes. Additionally, further experimental investigations into binders are required for torrefied material pellets/briquettes to reduce the uncertainty of the durability of pellets/briquettes produced with different binders.

 

Butler, J. W., Skrivan, W., & Lotfi, S. Identification of Optimal Binders for Torrefied Biomass Pellets. Energies, 16(8), 3390. https://doi.org/10.3390/en16083390

© 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/).