FIELD

[0001] Wooden fuel pellets and other biomass fuel pellets containing wax compositions, and corresponding methods of making such fuel pellets, are provided.

BACKGROUND

[0002] Wood based fuels are becoming an increasingly significant source of fuel for both industrial and individual consumer use. Wood pellets can be formed by extruding woody biomass (such as wood particles) under pressure through a die of an appropriate size. The pressure during extrusion can result in heating of the extruded mass, which is believed to assist with maintaining the integrity of the extruded mass after it cools to form a wood pellet.

[0003] U.S. Patent 4,612,017 discloses formation of wood pellets where a wax emulsion and/or iignosulfonate are incorporated during the process of forming the pellet. The amount of wax and/or lignosulfate solids in the pellet corresponds to 0.25 wt% to 2.5 wt% of the pellet. Pellets were formed with various woody biomass types both with and without wax additive. In the examples, formation of some types of pellets showed an improvement in durability with little or no change in power consumption during pellet formation. Other examples of pellet formation showed a reduction in power consumption during pellet formation but also a reduced pellet durability.

[0004] Unfortunately, some types of woody biomass have a lower suitability for formation of wood pellets with desired properties. Desirable properties for wood pellets can include high pellet durability and low moisture uptake during storage. It would be beneficial to identify ways to modify the composition of the woody biomass used for pellet formation and/or modify the method of pellet formation in order to improve the resulting properties of the wood pellet. For example, such modifications could potentially expand the types of woody biomass that are suitable for pellet formation. More generally, it would be beneficial to identify ways to modify the composition of various types of biomass that are potentially suitable for fuel pellet formation in order to improve the resulting properties of the biomass pellet.

SUMMARY

[0005] In various aspects, a pressed biomass pellet composition is provided. The pressed biomass pellet composition includes 0.15 wt% to 5.0 wt% of a wax composition relative to a weight of the biomass pellet composition according to an embodiment of the invention. The wax composition includes n-paraffins and oil-in-wax according to an embodiment of the invention. A weight of the n-paraffins is greater than a weight of the oil-in-wax by at least 0.10 wt% relative to the weight of the biomass pellet composition, such as by at least 0.15 wt%, or at least 0.20 wt%, or at least 0.25 wt% according to various embodiments of the invention. Optionally, a pressed biomass pellet composition is formed from a biomass composition that corresponds to a mixture of biomass, less than 25 wt% water, and 0.15 wt% to 5.0 wt% of a wax composition (or 0.15 wt% to 2.5 wt%) relative to a weight of the biomass composition, according to embodiments of the invention.

[0006] In various aspects, a method of forming a biomass pellet is provided. The method includes pressing a mixture comprising biomass and a wax composition through a die to form a pressed wood pellet comprising 0.15 wt% to 5.0 wt% of the wax composition (or 0.15 wt% to 2.5 wt%). The wax composition can include n-paraffins and oil-in-wax. A weight of the n-paraffins can be greater than a weight of the oil-in-wax by at least 0.10 wt% relative to the weight of the biomass pellet composition, such as by at least 0.15 wt%, or at least 0.20 wt%, or at least 0.25 wt%. Optionally, pressing the mixture to form a pressed biomass pellet can include a power consumption of less than 2500 amps ² /g. Optionally, pressing the mixture to form a pressed biomass pellet can include a power consumption corresponding to less than 60% of a power consumption for pressing a reference mixture comprising less than 0.01 wt% of the wax composition. A biomass composition or mixture for forming a biomass pellet composition can include 20 wt% or less of water.

[0007] In some aspects, the wax composition can have a particle size of at least 4.0 μιτι, or at least 6.0 μιτι, or at least 8.0 μιη. Additionally or alternately, the wax composition can include at least 1.0 wt% oil-in-wax relative to a weight of the wax composition, or at least 5.0 wt%, or at least 10 wt%, or at least 15 wt%.

[0008] In some aspects, a biomass pellet composition can include a surface wax concentration that differs from an interior wax concentration by less than 60 wt%, or less than 50 wt%, or less than 40 wt% relative to the surface wax concentration. Additionally or alternately, a biomass pellet composition can have a pellet durability index of at least 97.0 wt%, or at least 97.5 wt%, or at least 98.0 wt%. Additionally or alternately, a biomass pellet composition comprises a reduced tendency to adsorb water relative to a reference biomass pellet composition that contains less than 0.01 wt% of the wax composition, and optionally wherein the wood pellet composition comprises a reduced tendency to produce CO as an off-gas during storage relative to the reference biomass pellet composition.

[0009] In various aspects, the biomass incorporated into a biomass pellet composition, a biomass composition, and/or a mixture can correspond to woody biomass, grassy biomass, residual agricultural biomass, industrial paper waste biomass, or a combination thereof. In some aspects, the biomass can correspond to woody biomass.

BRIEF DESCRIPTION OF THE DRAWINGS [0010] FIG. 1 shows pellet durability versus wax content for wood pellets made from woody biomass mixtures containing a variety of wax compositions.

[0011] FIG. 2 shows pellet durability versus the difference between n-paraffin and oil-in-wax content for wood pellets made from woody biomass mixtures containing a variety of wax compositions.

[0012] FIG. 3 shows pellet durability versus n-paraffin content for wood pellets made from woody biomass mixtures containing a variety of wax compositions.

[0013] FIG. 4 shows pellet durability versus oil-in-wax content for wood pellets made from woody biomass mixtures containing a variety of wax compositions.

[0014] FIG. 5 shows pellet durability versus total paraffin content for wood pellets made from woody biomass mixtures containing a variety of wax compositions.

[0015] FIG. 6 shows power consumption during pellet formation versus wax content for wood pellets made from woody biomass mixtures containing a variety of wax compositions.

[0016] FIG. 7 shows power consumption during pellet formation versus the difference between n-paraffin and oil-in-wax content for wood pellets made from woody biomass mixtures containing a variety of wax compositions.

[0017] FIG. 8 shows moisture uptake versus wax content for wood pellets made from various woody biomass mixtures.

[0018] FIG. 9 shows moisture uptake versus wax content for wood pellets made from various woody biomass mixtures.

DETAILED DESCRIPTION

[0019] All numerical values within the detailed description and the claims herein are modified by "about" or "approximately" the indicated value, and take into account experimental error and variations that would be expected by a person having ordinary skill in the art.

Overview

[0020] In various aspects, a wax composition can be incorporated into a woody biomass mixture for formation of pellets, such as wooden fuel pellets. It has been unexpectedly discovered that addition of wax compositions with suitable amounts of n-paraffins relative to the amount of oil-in-wax can result in wood pellets with improved properties. In particular, addition of a suitable wax composition to a woody biomass mixture (i.e., a mixture that comprises woody biomass) prior to pellet formation can allow for formation of wood pellets with improved durability and reduced tendency to uptake moisture during storage. Addition of a suitable wax composition can also allow for formation of wood pellets with improved properties while also reducing or minimizing the power requirements during pellet formation. This combination of improved pellet durability and reduced power consumption during pellet formation is unexpected, as conventionally it would be expected that an additive that improves pellet durability would require increased power consumption during pellet formation.

[0021] More generally, addition of a suitable wax composition can also be beneficial for fuel pellets formed from other types of biomass, including other types of biomass alone or in combination, as is contemplated by the present invention in various embodiments. Examples of suitable types of biomass contemplated by the present invention include, but are not limited to, woody biomass (including wood derived from trees, bushes, and/or shrubs); grassy biomass (including biomass derived from grasses such as miscanthus, switchgrass, wheat straw, barley straw, and bamboo); residual agricultural biomass (including biomass derived from corn stover, palm kernel, coconut shells, bagasse, peanut hulls, and cocoa hulls); and biomass derived from industrial paper waste; and any combination of the foregoing.

[0022] Wood pellets are a potential fuel source that can be made from woody biomass, such as wood particles, that would otherwise have a low economic value. In this discussion, wood particles are defined to be a type of woody biomass. Sawdust and other wood residues from commercial or industrial wood processing are examples of suitable sources of woody biomass for pellet formation. Burners for wood pellets can also have relatively high efficiency of conversion of fuel to heat, which can make wood pellets an attractive fuel source in various applications, such as residential heating. More generally, biomass fuel pellets are a potential fuel source that can be made from various suitable types of biomass (as noted above) that would otherwise have a relatively low economic value. In some aspects, biomass pellets (such as wood pellets) can have a biomass content of at least 80 wt%, or at least 85 wt%, or at least 88 wt%, or at least 90 wt%, such as up to 96 wt% biomass or possibly still higher. Additionally or alternately, biomass pellets can have a water content of 10 wt% or less, or 9.0 wt% or less, or 8.0 wt% or less, or 7.0 wt% or less, such as down to 2.0 wt% or less or possibly still lower.

[0023] Unfortunately, the ability to convert a biomass mixture comprising woody biomass to wood fuel pellets can be limited based on various concerns related to storage and transport. One difficulty with wood pellets is the tendency for wood pellets to chip or otherwise partially degrade back into particles not necessarily related in size to the original particles used for pellet formation. The tendency for wood pellets (and/or other biomass pellets) to partially degrade and form smaller particles can be characterized using a Pellet Durability Index, as described in ISO 17831-1. Briefly, an initial or sample amount of pellets screened in a 6.5 mesh screen and then weighed. After the initial screening, the pellets can have an initial weight, such as 100 g. The pellets can then be introduced into a mechanical tumbler for a specified period of time, such as 10 minutes in the tumbler at 50 revolutions per minute. The contents of the mechanical tumbler can then be passed through the screen again, so that any fine particles created during the tumbling are allowed to pass through. The remaining particles are then weighed again to determine the amount of weight retained in the sample of particles after tumbling. The durability can be expressed as the percentage of weight retained in the sample of particles after tumbling. Unless otherwise specified, measurements of durability herein correspond to measurements made in accordance with ISO 17831-1.

[0024] Other difficulties with wood pellets can relate to moisture uptake during storage. Although wood pellets typically have a water content, it is desirable to reduce or minimize the amount of additional water content absorbed by a pellet after formation. The total moisture in a wood pellet sample can be determined according to ISO 18134-1. By using multiple samples of wood pellets, a difference between the water content of a sample before exposure to humidity and the water content of a sample after exposure to humidity can be determined. The difference in water content before and after exposure to humidity corresponds to additional moisture uptake by the wood pellets. Unless otherwise specified, measurements of moisture content of pellets herein correspond to measurements made in accordance with ISO 18134-1.

[0025] More generally, Table 1 shows ISO pellet fuel quality standards for production of fuel pellets for various classes of fuel pellets. In addition to durability and total moisture content, Table 1 also shows a range of desired average sizes, desired maximum fines content, and desired maximum additives contents for fuel pellets in the United States.

Table 1 - ISO Pellet Fuel Quality Standards

[0026] It has been unexpectedly discovered that addition of a minor portion of a wax composition to a woody biomass mixture prior to pellet formation can provide improved durability and/or reduced moisture uptake for the resulting wood pellets. In particular, the amount of wax composition added to the woody biomass mixture can correspond to 0.15 wt% to 5.0 wt% of the woody biomass mixture and/or of the resulting wood pellet, or 0.15 wt% to 2.5 wt%, or 0.25 wt% to 5.0 wt%, or 0.25 wt% to 2.5 wt% of the woody biomass mixture and/or resulting wood pellet. It has further been discovered that selection of the correct type and amount of the wax composition can depend on multiple factors. In particular, it has been discovered that the benefits of improved durability and/or reduced moisture uptake can be achieved by adding a wax composition that has an n-paraffin content that is sufficiently greater than the oil-in-wax content. Surprisingly, the benefits of improved durability and/or reduced moisture uptake are not correlated with either the n-paraffin content or the oil-in-wax content of a wax composition as separate quantities. Relative to the weight of the pellet (and/or the biomass mixture used to form a pellet), the n-paraffin content can be greater than the oil-in-wax content by at least 0.10 wt%, or at least 0.15 wt%, or at least 0.20 wt%, or at least 0.25 wt%, such as up to 2.5 wt% or 5.0 wt% or possibly still higher.

[0027] Without being bound by any particular theory, it is believed that pellets having reduced moisture uptake can also provide a benefit by having reduced production of off-gases during pellet storage. Wood pellets (and other biomass pellets) can produce off-gases including CO, CO2, and/or CH ₄ during storage of the pellets prior to use. Production of such off-gases is believed to be related to uptake of moisture, which then can facilitate breakdown of wood fibers (or other biomass fibers) in a fuel pellet. By reducing the moisture uptake of a pellet, it is believe that the rate of breakdown of wood fibers can be reduced or minimized, which would result in a correspondingly lower rate of production of off-gases (CO, CO2, and/or CH ₄ ) from pellets during storage. The reduced rate of moisture adsorption and/or the reduced rate of off-gas production (such as reduced rate of CO production) can correspond to a reduced rate relative to a reference wood pellet. Such a reference wood pellet can have substantially the same composition, with the exception that the reference wood pellet can contain 0.01 wt% or less of the wax composition in the wood pellet.

[0028] In addition to improved durability and/or reduced moisture uptake, it has been unexpectedly discovered that addition of a wax composition, with an appropriate amount of n- paraffins versus oil-in-wax, can also produce benefits with regard to reducing the power required during pellet formation. Wood pellets are typically formed by extruding a woody biomass mixture through a die at elevated pressure to press the mixture into a desired pellet shape. Conventionally, additives which are beneficial for increasing durability of a wood pellet are believed to also require increased power when forming the wood pellet. By contrast, addition of a wax composition as described herein to wood particles or pulp prior to pellet formation can substantially reduce the power required for pellet formation while increasing the durability of the pellet. In various aspects, pressing a mixture that includes a wax composition to form a pressed biomass pellet can have a power consumption corresponding to less than 60% of a power consumption, or less than 50%, or less than 40%, for pressing a reference mixture comprising less than 0.01 wt% of the wax composition. The power consumption can be defined, for example, based on the square of the amps used to power the pellet press per gram of biomass pellet that is produced.

[0029] In this discussion, a biomass pellet corresponds to a biomass composition that contains compressed biomass or compacted biomass, such as a biomass composition that is formed by applying pressure to biomass to form a desired shape, such as applying pressure to biomass during and/or after extrusion through a die. Biomass pellets as described herein can be formed using any suitable method such as any type of pellet mill and/or pellet press.

Wax Compositions: N-Paraffin Content and Oil-in-Wax

[0030] In various aspects, a wax composition can be incorporated into a (woody) biomass mixture for formation of biomass pellets, such as wooden fuel pellets. The wax in the wax composition can be a mineral wax such as paraffin wax or microwax (microcrystalline wax); vegetable derived bio-wax such as soy wax; animal derived bio-wax such as tallow; synthetic wax such as Fischer- Tropsch wax or polyethylene wax; semi-crystalline waxes; or combinations of two or more waxes and/or two or more types of waxes. Mineral waxes can correspond to mineral waxes from any convenient crude oil source and/or from any convenient refinery feedstock and/or from any convenient refinery product stream or intermediate stream. Optionally, the amount of synthetic wax and/or Fischer-Tropsch wax in the wax composition can correspond to 25 wt% or less of the wax composition, or 10 wt% or less. In some aspects, n-paraffins can correspond to 10 wt% to 70 wt% of the wax composition, or 10 wt% to 50 wt%, or 20 wt% to 50 wt%, or 10 wt% to 40 wt%. The amount of n-paraffins in a wax sample can be characterized by any suitable nonionizing method, such as gas chromatography. Such characterization of n-paraffin content using non-ionizing methods is conventional for those of skill in the art. In some aspects, oil-in-wax can correspond to 0.5 wt% to 30 wt% of the wax composition (according to ASTM D721), or 0.5 wt% to 25 wt%, or 1.0 wt% to 30 wt%, or 5.0 wt% to 30 wt%, or 1.0 wt% to 20 wt%. Optionally, the amount of oil-in-wax can correspond to at least 1.0 wt% of the wax composition, which can (optionally) correspond to a wax composition that is at least partially composed of non-synthetic wax and/or non-Fischer- Tropsch wax. Optionally, the amount of oil-in-wax can correspond to at least 5.0 wt% of the wax composition, or at least 10 wt%, or at least 15 wt%, which can represent an unrefined or only partially refined wax composition such as a slack wax. The wax composition can have a melting point, according to ASTM D87, of 100°C or less, or 80°C or less, or 70°C or less, such as down to 25°C or possibly still lower. Generally, suitable wax compositions can have an average number of carbon atoms per paraffin for all paraffins in the wax of 20 to 50, or 25 to 50. When considered as a distribution centered on the average carbon number, the width of the distribution required to include 95 wt% of the paraffins in the wax composition can be 15 carbon atoms to 40 carbon atoms, or 15 carbon atoms to 35 carbon atoms, or 20 carbon atoms to 40 carbon atoms. In some aspects, suitable wax compositions can have an average number of carbon atoms for n-paraffin compounds in the wax composition of 20 to 50, or 20 to 25.

[0031] Examples of polymer waxes can include polyethylene waxes, polypropylene waxes, Fischer- Tropsch waxes, polymerized alpha-olefins waxes, polyethylene-block-polyethylene glycol waxes, and polyethylene mono-alcohol waxes. Microcrystalline waxes typically comprise isoparaffinic, naphthenic and n-alkane saturated hydrocarbons. Microcrystalline waxes can have a melt point from about 54°C to about 99°C and a melt viscosity at 99°C of about 8 to about 25 centipoise. Microcrystalline waxes can have an oil content from about 0.5 wt % to about 12 wt %. Paraffin waxes can include from about 30 wt % to about 100 wt % n-alkane straight chain saturated C20- to C6o-hydrocarbons. The paraffin waxes can have a melt point typically from about 35°C to about 85°C, a melt viscosity at 99°C commonly of about 2 centipoise to about 15 centipoise, and typically contain less than about 25 wt % oil. Examples of semi-crystalline waxes include without limitation, polyethylene-block-polyethylene glycol waxes, polyethylene monoalcohol waxes, and mixtures thereof. Petroleum waxes are yet another type of wax. Petroleum waxes comprise a mixture of paraffin and microcrystalline waxes.

Manufacture of Wood Pellets

[0032] For production of wood pellets, a woody biomass mixture can be prepared that include a wax composition. The woody biomass mixture can have a water content of 10 wt% to 25 wt% relative to a weight of the wood particle or pulp mixture. Optionally, the water content can be 22 wt% or less, or 20 wt% or less, or 18 wt% or less, or 16 wt% or less.

[0033] A wax composition can be introduced into the woody biomass mixture by any convenient method that allow for introduction as a wax, as opposed to introducing the wax as a water-based emulsion. The water content of the wax composition can be 10 wt% or less, or 5 wt% or less, such as down to having substantially no water content (a water content of 0 - 0.1 wt%). Such a wax composition can be introduced into a woody biomass mixture, for example, by spraying molten wax into the mixture using a heated spray gun to form wax droplets. The spraying of the molten wax can be performed in a tumbler or other vessel that allows for mixing of the wax droplets into the mixture prior to extrusion for pellet formation. The spraying can be performed to form wax droplets of a convenient size, such as wax droplets having an average diameter of 3.0 μπι to 25 μπι, or 5.0 μπι to 25 μπι, or 10 μπι to 25 μπι. This is in contrast to the typical wax droplet size for waxes in an emulsion, where the average diameter of the wax droplets can be about 0.5 to 2.0 μηι. The amount of wax composition added to the woody biomass mixture can correspond to 0.15 wt% to 5.0 wt% of the mixture in an embodiment of the invention, or 0.15 wt% to 2.5 wt% in another embodiment of the invention, or 0.2 wt% to 5.0 wt% in another embodiment of the invention, or 0.2 wt% to 2.5 wt% in another embodiment of the invention, or 0.2 to 0.8 wt% in another embodiment of the invention or 0.25 wt% to 0.75 wt% in another embodiment of the invention.

[0034] After incorporation of the wax composition into the woody biomass mixture, the wax composition can be fed to a device for pellet formation and pressed to form wood pellets. The wood pellets can be formed in a conventional manner, such as by using conventional pellet press. An example of a pellet press is a ring-type pilot-scale pellet press available from the Dieffenbacher company of Eppingen, Germany. The wood pellets can be formed by extruding the woody biomass mixture under roller pressure through a die of a desired size to from a pressed pellet having a target pellet size. The pressed pellet can then be cooled to room temperature.

[0035] One significant cost of production of wood pellets from a biomass mixture is for the high amount of energy required by the pellet press equipment needed to compress the biomass into the desired wood pellet. A biomass mixture that can be compressed to proper density at reduced energy will permit manufacturing at a lower cost of production while still maintaining constant production rates. Alternately, fuel pellet formation can be increased if production is supported by other plant processes and resources.

[0036] In some aspects, a woody biomass mixture that also includes a wax composition can be formed into wood pellets while consuming an unexpectedly low amount of power during pellet formation. The unexpected nature of the power consumption was accompanied by an unexpected corresponding increase in durability with certain wax compositions. Conventionally, additives to a biomass mixture that increase pellet durability are believed to also result in increased power consumption during pellet formation. By contrast, for wax compositions that provide an n-paraffin amount that is greater than the oil-in-wax amount by more than a threshold value in the woody biomass mixture (or wood pellets), the durability of the pellets can be increased while also reducing the power required to form the wood pellets. Relative to the weight of the pellet (and/or the woody biomass mixture used to form a wood pellet), the n-paraffin content can be greater than the oil-in- wax content by at least 0.10 wt% in an embodiment of the invention, or at least 0.15 wt%, or at least 0.20 wt% in another embodiment, or at least 0.25 wt% in another embodiment, or at least 0.30 wt% in another embodiment or at least 0.40 wt% in another embodiment or at least 0.50 wt% in another embodiment. [0037] The wax composition can be incorporated roughly evenly throughout the woody biomass mixture and/or the wood pellet. This is in contrast to a wood pellet formed without additional wax that is subsequently coated with a wax composition. For a pellet coated with a wax composition, the wax content of the coated pellet at the pellet surface will be substantially different from the wax content in the interior of the pellet. For a wood pellet formed from a woody biomass mixture containing a wax composition, the wax content or concentration at the surface of the wood pellet can differ from the wax content at the geometric center of the pellet by less than 60 wt% (relative to the surface wax concentration), or less than 50 wt%, or less than 40 wt%.

Examples

[0038] In the following examples, wood pellets were formed from woody biomass mixtures that included either no additional wax or that included one of the wax compositions shown in Table 2. The wax compositions in Table 2 are examples of typical commercially available wax compositions, with a variety of n-paraffin and oil-in-wax contents.

Table 2 - Wax Compositions

[0039] In Table 2, the average number of carbon atoms per paraffin molecule is provided both for the total paraffins in the wax composition and for just the n-paraffins in the wax composition. The "95% spread" refers to the width (in number of carbon atoms) that is required for a symmetric distribution around the average number of carbon atoms to include 95 wt% of the total paraffins in the wax composition. The percentage of n-paraffins refers to the weight percentage of n-paraffins relative to the total weight of the wax composition. It is noted that wax compositions with still higher weight percentages of n-paraffins could also be used as part of a woody biomass mixture (or other biomass mixture). These compositional analysis values were determined by gas chromatograph. The oil-in-wax and melting point values correspond to the expected definitions based on ASTM D721 and ASTM D87, respectively. It is noted that wax compositions A and C represent by-products from refinery processes to form other higher value products. Thus, it would be desirable to find additional end uses for wax compositions similar to wax compositions A and C. However, as shown below, it appears that wax composition A (150 N foots oil) is not suitable for addition to a woody biomass mixture for wood pellet formation, even though wax composition A has a relatively high n-paraffin content. Wax composition B (scale wax) corresponds to a partially refined slack wax, and therefore represents wax composition that would typically be considered a higher value product than a slack wax. Wax composition D corresponds to a fully refined wax. Wax composition E represents a combination of wax compositions B and C.

[0040] Pellet fuel used in this study was manufactured in a Dieffenbacher custom made flat die type pilot-scale pellet press. The type wood fiber used for forming the woody biomass mixture was Southern Yellow Pine. Unless otherwise specified, the woody biomass mixtures used for pellet formation had an initial moisture content of 17%. The pellet press die was sized to produce 6 mm pellets. Wax was incorporated into the biomass by spraying molten wax through a DeVilbiss heated spray gun to spray molten wax into a 5 cubic foot tumbler of woody biomass before feeding the resulting mix into the pellet press. After exiting through the press, the pellets were cut from the die, collected, and allowed to cool to ambient conditions before doing durability and moisture absorption testing.

[0041] The pellet press was powered by a 3.7 kW electric motor. Power consumption readings were taken directly from the Variable Frequency Drive (VFD) connected to the Programmable Logic Controller (PLC) on the pellet press. Real-time readings of all the power being consumed in pellet production were recorded. Power consumption readings were confirmed by a Fluke Clamp Meter model 434 Power Quality Analyzer placed directly around the power line of the pellet press, also providing real-time power readings. Readings from the Fluke meter confirmed results recorded by the PLC. The amount of sample made during each run was weighed and the power consumption was calculated based on the power consumed divided by mass of the pellets produced and reported as amps ² /gram pellet.

[0042] Moisture absorption was conducted using a humidity cabinet. Weighed samples of each individual batch were placed onto an aluminum tray and weighed. Pellet fuel samples were then placed in the cabinet at 40°C and 92% relative humidity. For the data shown in FIG. 8, each tray was removed from the cabinet for a short period and the increase in moisture content was measured and recorded (g H2O/ g pellet). It is noted that the scale used for measurement of the moisture content was inside a moisture and temperature controlled testing room. The samples were then placed back in the humidity cabinet. For the data shown in FIGS. 8 and 9, the total excursion time for the samples outside of the moisture cabinet during a single measurement event was 2 minutes or less. This was done every hour for the first six hours and at 24 and 48 hours. In the comparison of the various wax types in FIG. 9, moisture content readings were made periodically for only the first 8 hours.

Example 1 - Addition of Slack Wax to Woody Biomass Composition

[0043] In this example, wood pellets were formed from a woody biomass compositions containing 1.0 wt% of a slack wax (Wax C from Table 2) at various moisture concentrations for the woody biomass composition. For comparison, wood pellets were also formed from a woody biomass composition having a moisture content of 17 wt% that did not include a wax composition. Table 3 provides an example of the results observed for pellets formed from woody biomass mixtures including 1.0 wt% of Wax C at moisture contents of 17 wt% and 20 wt%.

Table 3 - Pellets Formed from Woody Biomass Compositions Including Slack Wax

[0044] To expedite testing, durability testing was performed using a Holmen HP100 tester. As shown in Table 3, addition of slack wax to the woody biomass mixture resulted in wood pellets with increased durability and reduced moisture absorption, even when the amount of moisture content in the woody biomass mixture was increased relative to the control amount of 17 wt%. More generally, it was observed that increasing the moisture content of the woody biomass mixture resulted in lower pellet durability. Based on that observation, a moisture content of 17 wt% was used in the woody biomass mixtures for the remaining tests. Table 3 also shows a reduction in the amount of moisture absorbed by wood pellets that include 1.0 wt% of Wax C. Moisture uptake will be discussed in additional detail in Example 4 below.

Example 2 - Variations in Wax Composition

[0045] To further investigate the impact of adding wax compositions to woody biomass mixtures prior to pellet formation, the wax compositions shown in Table 2 were added in an amount of either 0.25 wt% or 0.50 wt% to a woody biomass mixture. Pellets were then formed, and the durability of the resulting pellets was measured. FIG. 1 shows the durability results for the various woody biomass mixtures containing a wax composition, and a control set of pellets that did not contain additional wax. [0046] As shown in FIG. 1, addition of wax at either 0.25 wt% or 0.5 wt% actually resulted in a decrease in pellet durability for waxes A, B, and E. For Wax C, addition of 0.25 wt% of wax also led to a decrease in pellet durability, while addition of 0.5 wt% resulted in an increase in pellet durability. For Wax D, either wax concentration resulted in an increase in pellet durability.

[0047] The results in FIG. 1 can initially appear to contain conflicting results. First, it appears that many types of waxes are not suitable for improving pellet durability, as introduction of small amounts of wax resulted in lowering of particle durability, and the maximum amount of "additives" that is permitted when attempting to comply with some industry standards is limited to only a few weight percent. However, the data in FIG. 1 also show that increasing the amount of wax to 0.5 wt% resulted in an increase in durability relative to the pellets containing 0.25 wt% wax. This is further illustrated when the durability results from Table 3 are considered, where addition of 1.0 wt% of Wax C resulted in improved durability relative to the control wood pellets (no wax addition).

[0048] In order to explain this behavior, further analysis was performed while considering the nature of the wax compositions. FIG. 3 provides a plot of the durability data shown in FIG. 1 versus the n-paraffin content of the woody biomass mixture due to addition of either 0.25 wt% or 0.5 wt% of the various waxes. In FIG. 3, the data points shown as circles correspond to data points with durability values below the durability of the control sample, while the square data points correspond to higher durability values. As shown in FIG. 3, the data points to not appear to have strong or meaningful correlation with the amount of n-paraffin in the woody biomass mixture due to the wax additive. A potential linear fit to the data is also shown in FIG. 3, but the proposed linear fit is clearly not representative of the data in a meaningful way.

[0049] FIG. 4 shows a plot similar to FIG. 3, but with a comparison of pellet durability with the oil-in-wax in the woody biomass composition due to the added wax. FIG. 4 similarly shows little or no obvious correlation between the durability and the oil-in-wax content. Although two of the data points with increased durability have low oil-in-wax, it is noted that one of the highest oil-in-wax values in the data set also resulted in increased durability.

[0050] FIG. 5 shows another plot similar to FIG. 3, but with a comparison of pellet durability with the total paraffin in the woody biomass composition due to the added wax. Again, the data points appear to largely correspond to a random scatter.

[0051] It was unexpectedly discovered a correlation could be observed in the data by plotting the durability versus a quantity corresponding to the amount of n-paraffin wax minus the amount of oil-in-wax for the woody biomass mixture due to the added wax composition. FIG. 2 shows a plot of the durability versus the compound variable "n-paraffin minus oil-in-wax". As shown in FIG. 2, a logarithmic relationship can be observed between the durability values and the variable "n-paraffin minus oil-in-wax". A functional form was roughly fit to the data points, with fit values corresponding to Equation (1).

(1) Pellet Durability Index = 112.98 + 7.2847*ln[(wt% n-paraffin) - (wt% oil-in-wax)]

[0052] While the n-paraffin content, oil-in-wax content, and total paraffin content of a wax composition all correspond to values that are routinely characterized, it was unexpected to find that a difference between n-paraffin content and oil-in-wax content would correspond to a meaningful variable. This unusual dependence on both n-paraffin content and oil-in-wax content also helps to explain the behavior of the pellets formed using the various waxes. As shown in FIG. 2, having an n-paraffin minus oil-in-wax content of greater than roughly 0.1 wt% in a wood pellet results in an increased pellet durability. For Wax A, the n-paraffin content and oil-in-wax content are relatively similar in the wax composition. Thus, it is difficult to add enough of the wax composition to achieve the needed threshold difference between the n-paraffin content and the oil- in-wax content that provides improved durability. By contrast, Wax B has a large difference between the n-paraffin content and the oil-in-wax content, but the absolute value of the n-paraffin content is relatively low, so it is also difficult to achieve the needed threshold difference. For the highly refined wax composition corresponding to Wax D, the wax composition has both a high n- paraffin content and a low oil-in-wax content. Thus, addition of almost any meaningful amount of Wax D can achieve the threshold difference between n-paraffin content and oil-in-wax content. Wax C presents an intermediate situation. At low amounts of addition of Wax C to a woody biomass mixture, the modest difference between n-paraffin content and oil-in-wax for Wax C means that the threshold value is not achieved for improving pellet durability. However, addition of larger amounts of Wax C does result in an increase in pellet durability. A variation on this behavior is demonstrated by Wax E, which corresponds to a 30 / 70 blend of Wax B and Wax C. Although Wax E has a lower oil-in-wax content, it also has a lower n-paraffin content. As a result, addition of 0.5 wt% of Wax E is still not sufficient to achieve the threshold value. It is believed that addition of larger amounts of Wax E, such as 0.75 wt% or more, would result in a wood pellet with increased durability.

[0053] As a final note, the results shown in FIGS. 1 - 5 are based on incorporation of wax into the woody biomass mixture prior to pellet formation. Additional tests were performed where wood pellets were formed from a woody biomass mixture without addition of wax, followed by attempts to coat the resulting wood pellets with a wax composition. It was observed that attempting to coat the pellets with wax after pellet formation did not produce a benefit with regard to either pellet durability or moisture uptake. Of course, attempts to add wax to pellets after pellet formation (such as by coating) also did not impact the power required to form pellets.

[0054] The data shown in FIGS. 1 - 5 is also summarized in Table 4. In Table 4, the scale wax (Wax B) corresponds to a high melt scale wax with an oil-in-wax content of 1.5 wt% - 5.0 wt% relative to weight of the wax. The slack wax (Wax C) corresponds to a high melt slack wax with an oil-in-wax content of 8 wt% to 15 wt% relative to a weight of the wax. The refined wax (Wax D) corresponds to a high melt fully refined wax with less than 0.5 wt% oil-in-wax. The blend (Wax E) corresponds to a blend of 30 wt% Wax B and 70 wt% Wax C. Either 0.25 wt% or 0.50 wt% of a wax was added to the woody biomass prior to pellet formation. The "fraction" column corresponds to the weight fraction of each wax that corresponds to n-paraffin and oil-in-wax. The "pellet content" column corresponds to the resulting weight percent of n-paraffin, oil-in-wax, and (n-paraffin - oil-in-wax) in a wood pellet produced from the corresponding woody biomass mixture. The power consumption corresponds to the power required to form a gram of wood pellets using the corresponding woody biomass mixture.

Table 4 - Pellet Compositions

Example 3 - Power Consumption During Pellet Manufacture

[0055] In addition to showing durability, the data in Table 4 include the power consumption during manufacture of the various pellets. The power consumption is represented as the square of the amount of current per gram of pellet produced. FIG. 6 provides a plot of the power consumption values in Table 4. As shown in FIG. 6, Waxes B - E show reduced power consumption during pellet manufacture, while Wax A resulted in increased power consumption.

[0056] It was unexpectedly discovered that the power consumption was also correlated with the difference between n-paraffin content and oil-in-wax content of the wood pellets. FIG. 7 shows the power consumption data plotted against the difference in n-paraffin content and oil-in-wax content. Similar to FIG. 2, the power consumption data appears to asymptotically approach a limiting value in power consumption during pellet manufacture, with a minimum threshold difference being needed before a reduced power consumption is observed. Based on FIG. 7, it appears that a threshold value of at least 0.05 wt% difference between the n-paraffin content and the oil-in-wax content is needed to provide reduced power consumption during pellet manufacture.

[0057] Based on FIG. 2 and FIG. 7, it is noted that addition of a sufficient amount of a wax composition with a sufficient difference between n-paraffin content and oil-in-wax content can allow for both an increase in pellet durability and a decrease in the power required to produce the pellets. This is an unexpected combination of benefits. Conventionally, it would be expected that an additive suitable for increasing durability would result in increased power consumption.

Example 4 - Moisture Uptake of Wood Pellets

[0058] Addition of a wax composition with a sufficient amount of n-paraffin content relative to oil-in-wax content can also provide benefit with regard to reducing the moisture uptake of wood pellets. FIG. 8 shows results for moisture uptake for wood pellets containing from 0.25 wt% to 2.0 wt% of Wax C, along with the control wood pellets that contain no additional wax. FIG. 8 shows that the change in moisture uptake with respect to additional wax varies depending on the amount of wax added. For addition of 0.25 wt% of Wax C, addition of the wax composition results in an increase in moisture uptake during exposure of wood pellets to moisture in a humidity cabinet. This increase is observed at all of the times shown in FIG. 8. By contrast, addition of a sufficient amount of Wax C to achieve an increase in durability (0.5 wt% or more) also results in a decrease in moisture uptake at all times shown in FIG. 8. Based on the data it appears that moisture protection is optimized at around 0.50 wt % wax. Additional wax beyond 0.5 wt % offers little, if any, improvement in the wax concentrations studied.

[0059] Based on FIG. 8, it might appear that the change in moisture uptake due to wax addition follow a pattern similar to the pattern for durability. However, testing of moisture uptake with other waxes shows that a different pattern is being followed. FIG. 9 shows a plot similar to FIG. 8 for addition of 0.25 wt% or 0.5 wt% of Wax D to the wood pellets. As shown in FIG. 9, addition of 0.25 wt% of Wax D results in an increase in moisture uptake by the resulting wood pellets. This is in contrast to the durability results, where addition of 0.25 wt% of Wax D was sufficient to achieve an increase in pellet durability. Addition of 0.5 wt% of Wax D does result in a reduced moisture uptake, at least for the longer time periods shown in FIG. 9. Based on the data in FIG. 9, it appears that the impact of wax addition on moisture uptake may be related to a more conventional variable, such as a direct relation to the amount of wax and/or amount of paraffin added to the wood pellet.

Additional Embodiments

[0060] Embodiment 1. A biomass pellet composition comprising 0.15 wt% to 5.0 wt% of a wax composition (or 0.15 wt% to 2.5 wt%) relative to a weight of the biomass pellet composition, the wax composition comprising n-paraffins and oil-in-wax, a weight of the n-paraffins being greater than a weight of the oil-in-wax by at least 0.10 wt% relative to the weight of the biomass pellet composition, the biomass pellet composition optionally comprising a pressed biomass pellet composition.

[0061] Embodiment 2. The biomass pellet composition of Embodiment 1, wherein the weight of the n-paraffins is greater than the weight of the oil-in-wax by at least 0.15 wt% relative to the weight of the biomass pellet composition, or at least 0.20 wt%, or at least 0.25 wt%.

[0062] Embodiment s. A biomass composition comprising woody biomass, less than 25 wt% water, and 0.15 wt% to 5.0 wt% of a wax composition (or 0.15 wt% to 2.5 wt%) relative to a weight of the biomass composition, the wax composition comprising n-paraffins and oil-in-wax, a weight of the n-paraffins being greater than a weight of the oil-in-wax by at least 0.10 wt% relative to the weight of the woody biomass composition.

[0063] Embodiment 4. A method of forming a biomass pellet, comprising: pressing a mixture comprising biomass and a wax composition through a die to form a biomass pellet comprising 0.15 wt% to 5.0 wt% of the wax composition (or 0.15 wt% to 2.5 wt%), the wax composition comprising n-paraffins and oil-in-wax, a weight of the n-paraffins being greater than a weight of the oil-in- wax by at least 0.10 wt% relative to the weight of the biomass pellet.

[0064] Embodiment 5. The method of Embodiment 4, wherein pressing the mixture to form a biomass pellet comprises a power consumption of less than 2500 amps ² /g, or less than 2000 amps ² /g, or less than 1500 amps ² /g; or wherein pressing the mixture to form a biomass pellet comprises a power consumption corresponding to less than 60% of a power consumption (or less than 50%), or less than 40%>) for pressing a reference mixture comprising less than 0.01 wt%> of the wax composition; or a combination thereof.

[0065] Embodiment 6. The biomass composition or method of any of Embodiments 3 - 5, wherein the biomass composition or the mixture comprises 20 wt%> or less of water, or 18 wt%> or less. [0066] Embodiment 7. The biomass composition or method of any of Embodiments 3 - 6, wherein the weight of the n-paraffins is greater than the weight of the oil-in-wax by at least 0.15 wt% relative to the weight of the biomass composition or the mixture, or at least 0.20 wt%, or at least 0.25 wt%.

[0067] Embodiment 8. The biomass pellet composition, biomass composition, or method of any of the above embodiments, wherein the wax composition comprises a particle size of at least 4.0 μιη, or at least 6.0 μιτι, or at least 8.0 μιη.

[0068] Embodiment 9. The biomass pellet composition, biomass composition, or method of any of the above embodiments, wherein the wax composition comprises at least 1.0 wt% oil-in- wax relative to a weight of the wax composition, or at least 5.0 wt%, or at least 10 wt%, or at least 15 wt%, the wax composition optionally comprising 25 wt% or less of Fischer-Tropsch wax, or 10 wt% or less..

[0069] Embodiment 10. A biomass pellet composition made from the biomass composition of any of Embodiments 3 or 6 to 9 or made according to the method of any of Embodiments 4 to 9.

[0070] Embodiment 11. The biomass pellet composition of any of Embodiments 1, 2, or 10, wherein the biomass pellet composition comprises a surface wax concentration that differs from an interior wax concentration by less than 60 wt%, or less than 50 wt%, or less than 40 wt% relative to the surface wax concentration.

[0071] Embodiment 12. The biomass pellet composition of any of Embodiments 1, 2, 10, or

11, wherein the biomass pellet composition comprises a pellet durability index of at least 97.0 wt%, or at least 97.5 wt%, or at least 98.0 wt%.

[0072] Embodiment 13. The biomass pellet composition of any of Embodiments 1, 2, 10, 11, or 12, wherein the biomass pellet composition comprises a reduced tendency to adsorb water relative to a reference biomass pellet composition that contains less than 0.01 wt% of the wax composition, and optionally wherein the wood pellet composition comprises a reduced tendency to produce CO as an off-gas during storage relative to the reference biomass pellet composition.

[0073] Embodiment 14. The biomass pellet composition, biomass composition, or method of any of the above embodiments, wherein the biomass comprises woody biomass, grassy biomass, residual agricultural biomass, industrial paper waste biomass, or a combination thereof; or wherein the biomass comprises woody biomass.

[0074] Embodiment 15. The biomass pellet composition of any of embodiments 1, 2, 10, 11,

12, 13, or 14, wherein the biomass pellet composition comprises at least 80 wt% biomass, or at least 85 wt%, or at least 88 wt%, or at least 90 wt%; or wherein the biomass pellet composition comprises 10 wt% or less of water, or 9.0 wt% or less, or 8.0 wt% or less, or 7.0 wt% or less; or a combination thereof.

[0075] When numerical lower limits and numerical upper limits are listed herein, ranges from any lower limit to any upper limit are contemplated. While the illustrative embodiments of the invention have been described with particularity, it will be understood that various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the spirit and scope of the invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the examples and descriptions set forth herein but rather that the claims be construed as encompassing all the features of patentable novelty which reside in the present invention, including all features which would be treated as equivalents thereof by those skilled in the art to which the invention pertains.

[0076] The present invention has been described above with reference to numerous embodiments and specific examples. Many variations will suggest themselves to those skilled in this art in light of the above detailed description. All such obvious variations are within the full intended scope of the appended claims.