WO2021097554A1

WO2021097554A1 - Wood pellets and binder therefor

Info

Publication number: WO2021097554A1
Application number: PCT/CA2020/051498
Authority: WO
Inventors: Vladimir Neburchilov; James Butler; William Skrivan
Original assignee: National Research Council Of Canada
Priority date: 2019-11-18
Filing date: 2020-11-05
Publication date: 2021-05-27

Abstract

A binder for a wood pellet comprises a lignosulfonate and an aqueous acid solution in a weight ratio of 1:1 to 1:4 lignosulfonate : aqueous acid solution. The wood pellet has wood biomass bound into the pellet with 6-24 wt% of a binder, based on total weight of the pellet, and has 2-6 wt%, based on total weight of the pellet, of a hydrophobic agent impregnated into the pellet. The wood pellet exhibits a desirable balance between mechanical durability and water repellency. The wood pellet may be used as a combustible fuel.

Description

WOOD PELLETS AND BINDER THEREFOR

Cross-reference to Related Applications

This application claims the benefit of United States Provisional Patent Application USSN 62/936,674 filed November 18, 2019, the entire contents of which is herein incorporated by reference.

Field

This application relates to a binder for wood pellets and wood pellets produced with the binder.

Background

Pellet fuels, also called energy pellets, are biofuels made from compressed organic matter or biomass. Pellets can be made from any one of five general categories of biomass: industrial waste and co-products, food waste, agricultural residues, energy crops, and virgin lumber. Wood pellets are the most common type of pellet fuel and are generally made from compacted sawdust and related industrial wastes from the milling of lumber, manufacture of wood products and furniture, and construction. Thermally treated wood pellets, developed to be used in existing coal-fired power plants, are made of torrefied wood and refined to resemble hard coal. Pellets can be used as fuels for power generation, commercial or residential heating, and cooking. Pellets are dense and can be produced with a low moisture content (below 10% by weight), which allows the pellets to be burned with a high combustion efficiency.

Wood pellets are produced by compressing wood biomass that has first passed through a mill to provide a wood biomass having wood particles of generally uniform size, often in a uniform dough-like mass. The milled biomass is fed to a press and squeezed through a die having holes of the size required (for example about 6 mm in diameter or larger). The high pressure of the press causes the temperature of the wood to increase, and lignin in the wood plasticizes slightly, forming a natural binder that holds the pellet together as the pellet cools.

Unfortunately, wood pellets have low mechanical durability and low water repellency under high-humidity transportation and storage conditions. For commercial energy pellets, high mechanical durability at high humidity during the transportation and storage is one of the main requirements. To provide high mechanical durability under high humidity conditions, efforts have been made to use an effective binder. The binder also reduces handling hazards such as dust from the wood biomass. For example, various polymers, resins, hydrolyzed proteins with crosslinking, lignin with plasticizer, lignosulphonate, etc. and various hydrophobic agents (e.g. oils) have been tried. Some examples of these efforts are disclosed in U.S. patent publications US 2013/0031830 and US 2005/0272892, and U.S. patents US 7,282,117, US 5,911,045, US 7,282,117 and US 3,454,508. However, the pellets produced by such methods do not have an optimized combination of mechanical durability and water repellency.

There remains a need for binders for energy pellets, in particular wood pellets, which provided a better balance of mechanical durability and water repellency, especially under high- humidity conditions.

Summary

There is provided a binder for a wood pellet, the binder consisting essentially of a lignosulfonate and an aqueous acid solution in a weight ratio of 1:1 to 1 :4 lignosulfonate : aqueous acid solution.

There is further provided a wood pellet comprising wood biomass and 6-24 wt% of the binder as defined above, based on total weight of the pellet.

There is further provided a wood pellet comprising: wood biomass; 6-24 wt% of a binder to bind the wood biomass into the pellet, the binder comprising a lignosulfonate and an aqueous acid solution in a weight ratio of 1 : 1 to 1 :4 lignosulfonate : aqueous acid solution; and, 2-6 wt% of a hydrophobic agent impregnated into the pellet, all weights based on total weight of the pellet.

There is further provided a torrefied wood pellet consisting essentially of: 70-92 wt% torrefied wood biomass; 6-24 wt% of a binder binding the torrefied wood biomass into the pellet, the binder consisting essentially of sodium lignosulfonate and acetic acid in a weight ratio of 1 :1 sodium lignosulfonate : acetic acid; and, 2-6 wt% of a hydrophobic agent impregnated into the pellet, all weights based on total weight of the pellet

The binder provides excellent mechanical durability for wood pellets. Further, with the inclusion of the hydrophobic agent, the wood pellet has increased water repellency without significant loss in mechanical durability. Thus, with the binder and the hydrophobic agent, the wood pellet surprisingly exhibits a desirable balance between mechanical durability and water repellency. The wood pellet may be used as a combustible fuel.

Further features will be described or will become apparent in the course of the following detailed description. It should be understood that each feature described herein may be utilized in any combination with any one or more of the other described features, and that each feature does not necessarily rely on the presence of another feature except where evident to one of skill in the art.

The present invention provides a mixed binder for binding wood biomass into an energy pellet. Lignosulfonates in the binder are water-soluble anionic polyelectrolyte polymers that are byproducts from the production of wood pulp using sulfite pulping. Being anionic, lignosulfonates have a counter cation, for example, sodium, potassium, calcium, magnesium or another metal of Group 1 or Group 2 of the Periodic Table. The counter cation is preferably sodium. Sodium lignosulfonate is a preferred lignosulfonate. The aqueous acid solution in the binder may be any suitable aqueous solution of a mineral acid, an organic acid or a mixture thereof. The acid may be, for example, sulfuric acid, phosphoric acid, hydrochloric acid, acetic acid or mixtures thereof. Sulfuric acid, hydrochloric acid and acetic acid are preferred. Acetic acid is particularly preferred. The aqueous acid solution preferably contains a concentration of acid that is at least 50% of the acid’s maximum concentration in water at a given temperature, where the maximum concentration of the acid in water is 100% at the given temperature.

The lignosulfonate and aqueous acid solution are present in the binder in a relative weight ratio of 1:1 to 1:4 lignosulfonate : aqueous acid solution. Preferably, the weight ratio is 1 :1 to 1:2, more preferably 1 :1. While not wishing to be bound by any theory or mechanism of action, it is thought that the acid binds wood pellets by disrupting inter-molecular lignin bonds in the wood biomass, softening lignin prior to pelletizing and allowing the re-strengthening of the lignin during cooling to ‘glue’ particles of the wood biomass together to form the pellets. The lignosulfonate is thought to bind by adhering to the surface of biomass particles and forming solid bridges between the particles. However, while lignosulfonate alone is hydroscopic, the presence of acid leads to its quick solidification thereby mitigating the hydroscopic nature of the lignosulfonate. The wood pellet may comprise wood biomass bound into the pellet with 6-24 wt% of a binder, and 2-6 wt% of a hydrophobic agent impregnated into the pellet, all weights based on total weight of the pellet. The wood biomass may be present in the pellet in an amount of 70- 92 wt%, based on total weight of the pellet. Preferably, the wood biomass comprises the balance of the weight of the pellet after considering weights of the binder and hydrophobic agent. In some embodiments, the amount of wood biomass may be 79-92 wt%. In some embodiments, the amount of wood biomass may be 83-92 wt%. In some embodiments, the amount of wood biomass may be 88-92 wt%. In some embodiments, the amount of binder may be 6-15 wt%. In some embodiments, the amount of binder may be 6-11 wt%. In some embodiments, the amount of binder may be 6-8 wt%. In some embodiments, the amount of hydrophobic agent may be 4-6 wt%.

In one embodiment, the wood pellet consists essentially of 70-92 wt% of a wood biomass, 6-24 wt% of a binder binding the wood biomass into the pellet and 2-6 wt% of a hydrophobic agent impregnated into the pellet. In another embodiment, the wood pellet consists of 70-92 wt% of a wood biomass, 6-24 wt% of a binder binding the wood biomass into the pellet and 2-6 wt% of a hydrophobic agent impregnated into the pellet.

The wood biomass may be prepared by comminuting (e.g. milling, grinding or the like) a wood biomass into a wood biomass having particles of generally uniform size. The mass- averaged diameter of the particles in the wood biomass is preferably 1 mm or less, for example in a range of 0.5 mm to 1 mm. The wood biomass may be from any suitable wood source, for example hardwoods, softwoods or mixtures thereof. Some examples of wood sources include pine, spruce, fir, hemlock, cedar, ash, aspen, balsam, beech, birch, walnut, cherry, chestnut, elm, oak, hickory, maple, bamboo, palm and willow. Willow wood is particularly preferred. The wood biomass is preferably subjected to torrefaction, which involves subjecting the wood biomass to intense heat for a period of time. Torrefaction comprises a thermochemical treatment of the biomass at 200°C to 320°C under atmospheric pressure for 1-3 hours and in the absence of oxygen. During the torrefaction process, water contained in the biomass as well as superfluous volatiles are released, and biopolymers (e.g. cellulose, hemicellulose and lignin) partly decompose giving off various types of volatiles. The final product is the remaining solid, dry, blackened material that is referred to as torrefied biomass.

The hydrophobic agent preferably comprises a liquid, preferably a high-viscosity liquid, for example an oil. The hydrophobic agent is preferably a bio-based oil, more preferably a vegetable oil, for example canola oil, castor oil, soybean oil, rapeseed oil, olive oil, palm oil, rice bran oil, coconut oil, sunflower oil and the like and mixtures thereof. Canola oil and castor oil are preferred, especially canola oil. Impregnation of the pellet with the hydrophobic agent may be accomplished by any suitable method, for example spraying or immersion. Immersion may be performed submerging the wood pellet into liquid hydrophobic agent at a temperature in a range of 20-80°C for a time of 3 seconds or more under a vacuum of -20 mmHg or less, and then blotting away the excess hydrophobic agent. Spraying is preferably used to provide better control over the impregnation of the hydrophobic agent into the pellet. Spraying techniques are generally known in the art of pelletizing.

The wood pellets have excellent mechanical durability, having a normalized crush strength as high as 100 MPa or higher, even 120 MPa or higher. With the addition of hydrophobic agent, the wood pellets are very water resistant having a stability in an submersion test of at least 10 minutes.

EXAMPLES

Willow wood biomass was subjected to torrefaction at 280°C for 20 min to produce a torrefied willow biomass.

To form pellets from the torrefied willow biomass, the torrefied willow biomass was first ground in an IKA grinder at 6000 rpm for 2 min. Binder as listed in Table 1 was then added to the ground biomass and the biomass was mixed in a portable IKA ball mill for 5 min. Torrefied wood pellets (TWP) were prepared by pressing the ground biomass in a hydraulic press (Carver Corp.) through a die having an internal diameter of 6.35 mm (MTI Corp.) at 130°C for 2 min at a load of 3300 lb to produce cylindrical pellets having a diameter of 6.35 mm.

To impregnate the TWP with an oil, two methods were conducted. In a first method, the TWP were submerged in 5 ml of oil at a temperature of 20°C for 3 seconds in a portable vacuum chamber at a pressure of -30 mmHg. The pellets were dried for 24 hours on porous Kimtech™ wipes to remove excess of oil. The TWP were weighed before and after oil application to control the oil content. In a second method, the TWP were sprayed with oil, providing better control over the amount of the oil impregnated into the TWP. The compositions in Table 1 where canola oil was used are for TWP that were impregnated with canola oil by the spray method. Mechanical durability of the TWP was tested by applying an axial load of 5 N to the cylindrical pellets (length (L) = 6-8 mm, diameter (D) = 6.35 mm) between press plates in a computer-controlled press (TestResources Corp.). Peak load represents the maximum load borne by the pellet before breaking. The results are tabulated in Table 1 as normalized crush strength in MPa.

Water repellency of the TWP was tested by submerging the TWP in 100 ml of deionized water to evaluate the duration of TWP stability before swelling or disintegration. The results are tabulated in Table 1 as stability in water.

Table 1

¹ Normalized Crush Strength = (Breaking Force)/(n/4*D²) * (L/D)

As can be seen in Table 1 , water resistant torrefied wood pellets (10 minutes or greater submersion in water prior to disintegration or swelling) of the present invention can be produced with a normalized crush strength of at least 101 MPa (about 14,700 psi) at a load of 67,000 psi. In contrast, thermally treated biomass pellets produced in accordance with US

2013/0031830 using 40 wt% of a protein/alkaline binder exhibit a normalized crush strength of 1095 psi at a load of 300 psi with insufficient water repellency. The amount of binder (6-24 wt%) required to produce the present torrefied wood pellets is also significantly less than the amount of binder (40 wt%) used in the pellets of US 2013/0031830, thereby significantly reducing the cost of producing the pellets. It is of special note that the inclusion of 2-6 wt% of an oil increases water repellency without unduly affecting the normalized crush strength of the torrefied wood pellets.

The novel features will become apparent to those of skill in the art upon examination of the description. It should be understood, however, that the scope of the claims should not be limited by the embodiments, but should be given the broadest interpretation consistent with the wording of the claims and the specification as a whole.

Claims

Claims:

1. A binder for a wood pellet, the binder consisting essentially of a lignosulfonate and an aqueous acid solution in a weight ratio of 1:1 to 1:4 lignosulfonate : aqueous acid solution.

2. The binder of claim 1, wherein the aqueous acid solution is an aqueous solution of sulfuric acid, hydrochloric acid or acetic acid.

3. The binder of claim 1 or claim 2, wherein the lignosulfonate comprises sodium lignosulfonate.

4. A wood pellet comprising wood biomass and 6-24 wt% of the binder as defined in claim 1 , based on total weight of the pellet.

5. The wood pellet of claim 4, further comprising 2-6 wt% of a hydrophobic agent impregnated into the pellet, based on total weight of the pellet.

6. A wood pellet comprising: wood biomass;

6-24 wt% of a binder to bind the wood biomass into the pellet, the binder comprising a lignosulfonate and an aqueous acid solution in a weight ratio of 1:1 to 1:4 lignosulfonate : aqueous acid solution; and,

2-6 wt% of a hydrophobic agent impregnated into the pellet, all weights based on total weight of the pellet.

7. The pellet of any one of claims 4 to 6, wherein the hydrophobic agent comprises a vegetable oil.

8. The pellet of claim 7, wherein the oil is canola oil or castor oil.

9. The pellet of any one of claims 4 to 8, wherein the aqueous acid solution comprises acetic acid.

10. The pellet of any one of claims 4 to 8, wherein the aqueous acid solution comprises sulfuric acid.

11. The pellet of any one of claims 4 to 10, wherein the lignosulfonate comprises sodium lignosulfonate.

12. The pellet of any one of claims 4 to 11, wherein the wood of the wood biomass is a torrefied wood biomass.

13. The pellet of any one of claims 4 to 12, wherein the wood biomass is present in the pellet in an amount of 70-92 wt%, based on total weight of the pellet.

14. The pellet of any one of claims 4 to 13, wherein the wood biomass comprises willow wood biomass.

15. A torrefied wood pellet consisting essentially of: 70-92 wt% torrefied wood biomass;

6-24 wt% of a binder binding the torrefied wood biomass into the pellet, the binder consisting essentially of sodium lignosulfonate and acetic acid in a weight ratio of 1:1 sodium lignosulfonate : acetic acid; and,

16. The pellet of claim 15, wherein the hydrophobic agent is a vegetable oil.

17. The pellet of claim 16, wherein the oil is canola oil.

18. The pellet of any one of claims 15 to 17, wherein the wood biomass is willow wood biomass.

19. Use of the pellet of any one of claims 4 to 18 as a combustible fuel.