WO2017025511A1

WO2017025511A1 - Mixed composition biomass pellets

Info

Publication number: WO2017025511A1
Application number: PCT/EP2016/068891
Authority: WO
Inventors: Christopher John Stanley; Rune SKOVGAARD-PETERSON; Kristian Petersen NØRGAARD; Mads PEDERSEN
Original assignee: Biogasol Aps
Priority date: 2015-08-10
Filing date: 2016-08-08
Publication date: 2017-02-16

Abstract

The present invention relates to a method for the production of a pelleted biomass material, the method comprising taking a first pelletable biomass material that has not been subjected to a process for reducing the hemicellulose content of the biomass material, admixing with the first biomass material a second pelletable biomass material that is the product of subjecting a starting biomass material to a process hydrolysing hemicellulose therein which has reduced the size of fibres in said starting biomass material, and forming pellets of the resulting mixture of the first and second biomass materials.

Description

Mixed composition biomass pellets

This invention describes a mixed composition biomass pellet with superior mechanical, moisture resistance and combustion properties.

Wood pellets are becoming an important fuel source, both in the domestic environment and in large-scale power generation. Wood pellets are most commonly manufactured from softwood, although hardwoods and rarely wheat straw are also used, although in the latter case the content of potassium and chlorine can be unacceptably high. The first step in the standard production method for wood pellets is the drying of the biomass material in order to reduce the moisture content to 5-15%. Terpenes, mainly monoterpenes and sesquiterpenes, evaporate during the drying process. The dried material is then ground, generally using a hammer mill, and then pressed through cylindrical holes in a pellet-forming matrix. The pellet strings are cut with a knife, or broken automatically. The pellets are left to cool before sieving, storing and transporting. Fuel pellets made in this way are in our view lacking in strength, lacking in resistance to disintegration when moist, and lacking in calorific value.

Steam explosion of biomass is a technique that has been widely used and has been found to be an efficient method of pre-treatment for both herbaceous and woody biomass prior to subjecting the material to a process such as acid extraction, super critical extraction or enzymatic hydrolysis to remove C5 and C6 sugars. In general, steam explosion is a process in which biomass is treated with hot steam (160 to 240 °C) under pressure (0.6 to 3.5 MPa) followed by an explosive decompression of the biomass that results in a rupture of the rigid structure of the biomass fibres. The sudden pressure release defibrillates the cellulose bundles and, depending on residence time and temperature, steam explosion can generate anything from small cracks in the wood structure to total defibrillation of the wood fibres. During the process acetic acid is released from the wood and this results in partial hydrolysis of the cell wall components under mild acid conditions, also known as autohydrolysis. It has been shown that the use of diluted mineral acids (i.e. sulphuric, phosphoric, acetic or nitric acid) can accelerate this process i.e. result in substantially higher hydrolysis rates and conversion of the hemicelluloses. Steam explosion treatment has been used as a treatment process in the wood pellets industry as a method to obtain more dimensionally stable and durable pellets. During steam explosion, lignin is softened, released from the cell wall and distributed more evenly onto the surrounding material and this is believed to be the reason for higher stiffness and water resistance in the pellets manufactured from this material. It has been reported that a steam explosion process will lead to the formation of new chemical bonds, which in turn create more durable pellets. Steam explosion treatment increases the calorific value of biomass due to the removal of hygroscopic components and volatiles. The carbon content of the biomass increases since oxygen and hydrogen are removed from the biomass during the treatment. The removal of hydroxyl groups via dehydration reactions results in a more hydrophobic pellet surface. Thus steam explosion brings about significant changes in the starting biomass material as the hemicelluloses become hydrolysed and water soluble, cellulose is partly hydrolysed and depolymerized, and the lignin softens and is depolymerized.

It is known that the major role of hemicelluloses in wood is to impart viscoelastic properties. Thus the hydrolysis of hemicellulose makes wood more brittle and rigid. This brittleness imparts better grindability and imparts higher moisture resistance to the resulting wood pellet. The steam-exploded biomass has more "coal like" properties, which facilitates its use in existing coal fired heat and power plants.

Pellets made from biomass that has undergone a steam explosion process have been reported to be darker brown in colour and stiffer than conventional, untreated wood pellets. They are less abrasive, generate less dust on handling and exhibit reduced moisture uptake compared to conventional wood pellets. It has been reported that the bulk density of pellets made from steam-exploded biomass is relatively high in comparison to conventional, untreated wood pellets.

Alternative methods of treating biomass include torrefaction which involves a thermal process consisting of heating to a moderate temperature, generally between 200 and 300 °C under inert or nitrogen atmosphere. This thermal process mainly removes moisture and low molecular weight organic components, eventually leading to depolymerisation of the long hemicellulose and cellulose polysaccharides. In a torrefaction process the hygroscopic raw biomass is converted to a hydrophobic material which has easier grindability. The biomass can also be treated with an aqueous organic solvent at temperatures ranging from 140 to 220°C to break down lignin and hemicellulose to facilitate their removal, leaving a depleted solid material comprising mainly cellulose. This is the well-known Organosolv' process. Solvents used include acetone, methanol, ethanol, butanol, ethylene glycol, formic acid and acetic acid. The concentration of solvent in water ranges from 40 to 80%.

A further biomass treatment process that can remove at least a portion of the hemicellulose present comprises a wet oxidation method that uses a temperature of 170 °C and up to 10 % hydrogen peroxide or similar oxidising agent to hydrolyse the hemicellulose polymers.

WO 2013/166225 describes a process to convert biomass into an energy-dense form, such as pellets, that uses steam or a hot water extraction solution followed by separation of the hydrolysed hemicellulose extract liquor and the cellulose-rich solids; the latter is then further treated in a torrefaction process before pelletisation. The extraction solution may be acidic to improve hydrolysis of the hemicellulose content of the biomass.

WO 2013/142320 describes a steam extraction/hot water process to produce an extract liquor from the biomass containing hydrolysed hemicellulose and

suspended cellulose-rich solids, separating and dewatering the said cellulose-rich solids followed by a further hydrolysis step to generate a further cellulose hydrolysate and intermediate solids; the latter can then be processed to form an energy-rich biomass material.

WO 2013/142317 describes a steam extraction/hot water process that utilises added acetic acid to catalyse the hydrolysis of hemicellulose to oligomers and monomers. The process also generates further acetic acid from acetyl groups present in the biomass released during the hydrolysis step, which is then removed by evaporation and recycled in the continuing process. After evaporation a concentrated extract liquor containing hemicellulosic sugars and impurities is obtained. The dried, dewatered solids obtained at the end of the process are reported to be low in ash content which is an advantage when the pellets are fired subsequently in a boiler.

WO 2014/124399 describes a process that comprises using steam at a

temperature higher than 300°F (149°C) injected into biomass in a reactor, wherein the biomass contains from 0 to 60% by weight moisture, and maintaining a pressure of at least 52 psig (359kPa) for 1 to 30 minutes, followed by separating the water phase and forming pellets from the solid phase. An acid catalyst such as acetic acid or nitric acid with a boiling point less than 200°C may be added prior to the introduction of steam at up to 5% of the biomass by weight to accelerate the rate of hemicellulose hydrolysis. The pellets so produced are said to have higher energy content and higher bulk densities than pellets made from the same feed without such treatment.

Description of the invention The inventors have found that pellets made solely from treated biomass material that had been previously subjected to a highly efficient hemicellulose removal process involving steam explosion and acid hydrolysis exhibited a higher calorific value but significantly lower strength compared to pellets made from the equivalent untreated biomass. The process turns biomass from a tough flexible material into a brittle, rigid material. The change in resulting properties may be principally ascribed to the fact that pellets formed from untreated biomass contain fibres of biomass material, whereas in pellets formed from treated material the fibres are transformed into particles of low aspect ratio.

It is not commercially viable to replace the production of biomass pellets from untreated materials by treating all biomass in the manner described above. The added complexity involved in the production of pellets from such treated biomass would not be justified. To some degree, the production from the starting biomass of a stream of extracted sugars produced by such treatment is worthwhile in its own right, but the amounts of such sugars that are commercially useful are

disproportionately small in comparison to the need for fuel pellets.

The inventors have found that pellets may be made which have properties that are improved over pellets of untreated material, but which are improved also over pellets formed from treated material. Such pellets contain a mixture of longer fibres from the untreated material and shorter, narrower, more particulate fibres from the treated material. Furthermore, such pellets may have properties that are in valuable respects better than those of pellets made entirely from processed biomass in which a substantial proportion of the fibres have been transformed into shorter more particulate fibres.

Thus, the present invention now provides a method for production of a pelleted biomass material, said method comprising: taking a first pelletable biomass material that has not been subjected to a process for reducing the hemiceNulose content of the biomass material, admixing with said first biomass material a second pelletable biomass material that is the product of subjecting a starting biomass material to a process hydrolysing hemiceNulose therein which has reduced the size of fibres in said starting biomass material, and forming pellets of the resulting mixture of first and second biomass materials.

Processes that may be regarded as not being for reducing the hemiceNulose content of a biomass include, but are not limited to, soaking drying, cutting, chopping, grinding, sieving, grading and washing.

Processes that may be regarded as being for reducing the hemiceNulose content of a biomass include steam explosion, hydrolysis, including acid hydrolysis whether with added acid or with internally produced acid, torrefaction, solvent extraction including organic solvent extraction and supercritical solvent extraction, and wet oxidation.

Preferably, the second biomass material is the product of a process including a step of subjecting the starting biomass material to a thermal hydrolysis process hydrolysing hemiceNulose therein. Depending on the biomass, the preferred ranges are: temperature: 150-230°C, time: 5-25 min. Preferably, the second biomass material is the product of subjecting the starting biomass material to process including performing a steam explosion and acid hydrolysis process hydrolysing hemiceNulose therein. Depending on the biomass, the preferred ranges are: temperature: 130-200°C, time: 2-20 min, pH: 1 -4 Optionally, the second biomass material is the product of subjecting the starting biomass material to a torrefaction, organic extraction or wet oxidation process hydrolysing hemiceNulose therein.

Optionally, the starting biomass material used to produce the second pelletable biomass material is the same as a biomass material used to produce the first pelletable biomass material. However, the respective starting biomass materials will often be different to some degree as the hemiceNulose reducing treatment may well be carried out by a different party to that carrying out the selection and preparation of the first biomass material as well as the mixing and pelletisation with the second biomass material.

The first and second biomass materials may be mixed in a weight ratio range of 99:1 to 50:50, or in a weight ratio range of 90:10 to 60:40, or in a weight ratio range of 80:20 to 70:30.

The calorific value of the pellets is preferably at least 1 % greater, more preferably from 1 to 4.5% greater, compared to like pellets formed from the first pelletable biomass material.

Compression energy used to form the pellets is preferably reduced by at least 1 %, more preferably by at least 3%, e.g. by 3 to 10%, compared to the compression energy that would be required to form like pellets from the first pelletable biomass material.

The pellets are preferably formed by a die extrusion process. This may preferably be carried out at a temperature from 50-190°C, a pressure range between 520 - 800 MPa and a die aperture of 5-30 mm.□

The invention further provides a pelleted biomass material comprising a mixture of a first pelletable biomass material that has not been subjected to a process for reducing the hemiceNulose content of the biomass material and a second pelletable biomass material that is the product of subjecting a starting biomass material to a process hydrolysing hemiceNulose therein which has reduced the size of fibres in said starting biomass material. Compared to the first pelletable biomass material, such a pelleted material may preferably have a lesser content of hemiceNulose. Mixing ratios for the first pelletable biomass material and the second pelletable biomass material are preferably as given above.

Such improved pellets may be made by forming pellets from a mixture of the original untreated biomass with the solid material from a treatment process, where at least a proportion of the hemicellulose has been removed, leading to an optimal combination of properties in the pellet including mechanical strength, density, calorific value, hygroscopicity and impurity content.

The production of such improved pellets therefore allows the production of pellets which are an improvement over the pellets formed from untreated material without necessitating the treatment of biomass materials on a non-commercially viable scale.

The treated material is preferably the solid residue remaining after a steam explosion and acid hydrolysis process has been employed to facilitate removal of a large fraction of the hemicellulose content in the form of soluble oligo- and monosaccharides in an extracted liquor. In addition it is believed that a process of this type alters the composition and location of other components such as lignin and cellulose.

The steps of a preferred hemicellulose treatment process are described in

WO2A081478A1 , WO2A081476A1 , WO2A081477A1 and US8563277. These treatment process steps comprise the steps of: a) soaking, dispersing, chopping and dewatering the biomass in a dilute mineral acid solution such as sulphuric acid, b) mixing the biomass with steam with simultaneous cutting to substantially reduce particle size, c) hydrolysing the hemicellulose and separating the xylose and/or mannose sugar-rich liquor and d) drying the remaining solids. Table 1 shows the typical composition of hemicellulose in a range of softwoods.

Table 1 . Constituent Scots Pine Spruce Eucalyptus Silver Birch

(Pinus (Picea (Eucalyptus (Betula

sy Ives Iris) glauca) camalduleMsis) verrucosa)

Cellulose (%) 40 39.5 45.0 41.0

Hemicellulose

-Glucomaiman (%) 16.0 17.2 3..1 2.3

-Glucuroiioxylan (%} 8.9 10.4 14.1 27.5

-Other polysaccharides (%) 3.6 3.0 2.0 2.6

Liguin (%) 27.7 27.5 31.3 22.0

Total extractives (%) 3.5 2.1 2.8 3.0

An analysis of the treated material derived from softwoods generated in such a process showed that it had a residual hemicellulose content of <10% of the original content, the original fibrous content of the softwood content had been reduced to <5% (where a fibre is defined as <1 mm in length and <20 μιτι in diameter) and the material had a significant content of spherical or semi-spherical particulates falling mainly in the 10 m to 1 mm diameter range.

When pellets were formed using standard methods from this treated material the pellets had, as expected from the prior art, advantageous properties including a higher calorific value, lower hygroscopicity and a higher density. However, pellets composed entirely of this treated material were found to have poorer mechanical strength. This reduced mechanical strength has been observed by the inventors in pellets made from a range of biomass feedstocks including softwood, hardwood and wheat straw.

The inventors believe that whilst the conversion of fibres to smaller fibres or particles in the treatment process is necessary for optimal hemicellulose hydrolysis in acidic conditions and subsequently for improved combustion properties of the pellets they compromise the mechanical strength of the pellets. The inventors also believe that the removal of the hemicellulose backbone weakens the remaining cellulose fibres. In addition the pre-treatment process as described may partially remove and reposition lignin within the biomass structure, which also reduces the pellet strength.

Thus a treated biomass material that is optimal for hemicellulose removal, where the fibre content is substantially altered and partially replaced with smaller fibres or particulates and where the hemicellulosic content has been substantially reduced is sub-optimal for achieving the desired pellet mechanical strength. Hence the economic advantages of extracting hemicellulosic sugars from biomass prior to pelletisation (in that a second commercially valuable product is obtained) are offset by the poorer mechanical properties of the resulting pellets; which make them less desirable in a commercial or domestic power generation application.

The composition of pellets formed from mixes of untreated to treated biomass material can be as suggested above or may also be in the range from 99:1 %(w/w) to 1 :99 %(w/w) respectively, or can be 50:50 %(w/w), or more preferably from 80:20 %(w/w) to 20:80 %(w/w) respectively.

A further advantage of a treatment process is that undesirable impurities can be substantially reduced in the solid product. WO 2013/142317 describes a steam extraction/mild acid hot water process that reduces the ash content in the solids. The inventors have also observed that a steam explosion/acid hydrolysis process leads to a substantial reduction in the ash content of the product, particularly where the biomass is wheat straw. Thus the low ash solids from a treatment process can be mixed with the untreated biomass in various proportions to reduce the overall ash content of the resulting pellets whilst maintaining the desired physical and thermal properties. Other biomass materials that could be treated and used for manufacture of wood pellets include alfalfa, bagasse, corn stover, corn cobs, corn kernels, rice hulls and sugarcane.

The invention will be further described and illustrated by the following Examples which make reference to the accompanying drawings which show the following: Figure 1 shows a flow diagram for a process according to the invention of hemicellulosic sugar removal and mixing of wood biomass to produce solid fuel pellets;

Figure 2 shows graphs illustrating the pellet strength and the energy content of mixed fuel pellets of the invention; Figure 3 shows the loss of fines on dipping pellets into water as measured in Example 6;

Figure 4 shows the loss of fines on dipping pellets into water for a second time as measured in Example 6; Figure 5 shows the distribution of fibre length before and after 'Carbofrac' treatment as measured in Example 7; and

Figure 6 shows the distribution of fibre width before and after 'Carbofrac' treatment as measured in Example 7

Examples Example 1 .

Softwood biomass was divided into two portions. A biomass pre-treatment process was carried out on a portion of the softwood biomass comprising the steps of soaking and cutting in a 1 % sulphuric acid solution, dewatering, further cutting in a reactor with simultaneous steam injection followed by hydrolysis and separation of the liquor containing the pentose sugars. This is referred to as the "Carbofrac" process and is summarised in Figure 1 . The solids remaining after liquor removal were then dried in a Nabertherm P330 heating cabinet to 10 % water content. An untreated portion of the softwood was size reduced in a Pulverisette 19 cutting mill using a 6 mm screen, dried to achieve 10 % moisture content and then was optionally mixed with the dried pre-treated material prior to the Pellet Mill step depicted in Figure 1 . Pellets were then formed from the pre-treated material, the untreated material and from the mixtures of pre-treated and untreated material using an 8 mm press channel die at 125°C. 0,750 g sample was compressed at 100 mm/min at a maximum pressure at 300 MPa. The pressure was held for 10 seconds.

Example 2. Physical properties of the mixed pellets

Mechanical properties were assessed on the wood pellets manufactured according to Example 1 using a die temperature of 125°C and a moisture content of 10%. Pellets were composed of 100% pretreated softwood (WIS), a 50% WIS/50% mix with untreated softwood (50/50), or a 80% WIS/20% mix with untreated softwood (80/20), or a 20% WIS/80% mix with untreated softwood (20/80). The tests used were:

• Compression energy required to compress the biomass sample into a pellet (Wcomp)

• Friction of the pellet in the press channel; static friction (Max F) and dynamic friction (WFric).

• Strength and density

For Wcomp measurements an 8 mm press channel die was used at 125°C. A 0.75g sample was compressed at 100 mm/min at a maximum pressure at 300 MPa. The pressure was held for 10 seconds.

For MaxF measurements, following compression, the stop piston was removed from the die and the pellet was pressed out of the channel at 100 mm/min. The force required to initiate the pellet motion was defined as the static friction. The energy to push the pellet a total distance of 20 mm in the channel (the dynamic friction (Wfric) was also calculated.

For strength and density measurements cylindrical pellets of 8 mm in diameter and 10 - 12 mm in length were left to cool overnight and were then placed on a flat surface and subjected to increasing orthogonal force until the pellets were crushed. The applied energy to achieve complete crushing was recorded. The length, diameter and pellet mass were measured for the density calculation. For all measurements of pellet properties, 8 replicates were made for each sample. Table 2 summarises the physical properties of the wood pellets.

Table 2. Physical properties of the mixed pellets

The data in Table 2 shows that Wcomp was significantly lower for the WIS material (100% pretreated softwood) compared to untreated material (SW). The mixes of 50/50, 20/80 and 80/20 showed intermediate results with a clear trend towards reduced Wcomp with increasing WIS. Thus, a worthwhile reduction in Wcomp involved in making pellets can be achieved if untreated biomass is supplemented with as little as 20% treated biomass.

The Wfric data suggests that increasing the content of WIS led to a higher force requirement to move the pellet in the channel but this was only evident at 80%w/w WIS or higher. A similar effect at 80%w/w WIS or higher was observed for Max F.

Thus, the benefit in Wcomp discussed above can be obtained without significant penalty in Wfric provided that not more than 80%w/w treated biomass is used.

Similarly, the benefit in Wcomp discussed above can be obtained without significant penalty in pellet strength provided that not more than 80%w/w treated biomass is used.

A significant reduction in the pellet strength was not observed until the WIS content was increased above 50 %w/w in the mixes and this was still moderate at 80%w/w.

There was a slight increase in density observed as the WIS content in the pellet was increased.

Conclusion

Mixing treated biomass with untreated biomass improves the properties of the pellets as regards compressive force required without excessive increase in the friction in the die or reduced strength. Density is also improved.

Example 3. Combustion properties of the mixed pellets

Table 3 shows the data on combustion of the mixed pellets. There is an increase in the calorific value in the pellets as the proportion of WIS increases. Figure 2 provides a graphical representation of the key data on pellet strength and calorific value taken from Tables 2 and 3 and shows clearly that compared to standard wood pellets, mixed pellets containing up to 50% WIS show minimal reduction in strength whilst exhibiting significantly increased energy content. The 80WIS:20SW pellets are still an acceptable compromise as well with a minor reduction in strength but a significantly enhanced calorific value (7% increase). Table 3. Combustion data for mixed pellets

The values for SW and WIS are determined by analysis, whereas the values for 20:80, 50:50 and 80:20 are calculated from weighted averages.

Example 4. Chemical properties of the mixed pellets

Table 4 shows the potassium and chlorine content in the treated biomass material prior to use in a pelleting process. Clearly mixed pellets partially composed of this treated material will exhibit a substantially lower content of ash after combustion.

Table 4. Ash content in untreated and treated biomass material

Example 5. Hygroscopic properties of mixed pellets

Standard pellets produced in a die process from mixes of treated and untreated material were dried at 45°C and their initial moisture content was determined. They were then placed in a closed box at a relative humidity of 75% at 25°C. The pellets were weighed regularly and after 1 1 days the equilibrium moisture content had been reached. Table 5 shows the equilibrium moisture content of pellets mixed in different proportions and confirms that pellets containing a higher % of treated material exhibited reduced water absorption.

Table 5. Equilibrium moisture content of pellets after incubation under high humidity conditions (starting moisture content 5%)

Example 6. Effect of immersion in water on the structure of mixed pellets

Standard pellets of the same mass produced in a die process from mixes of treated (WIS) and untreated material were placed in demineralized water for five seconds. After removal from the water the pellets were gently wiped with tissue. Fines that became detached from the pellet during immersion were collected by filtration, dried and weighed. Analyses were conducted in duplicate.

The average mass of fines lost from the pellets after a single immersion in water, as a function of WIS content in the pellet, is shown in Figure 3. The dried pellets were then immersed in water for a second time and the fines collected and weighed as above. Figure 4 shows the average mass of fines lost during this second immersion.

The data shows that increasing the content of WIS material in a mixed pellet significantly reduces the loss of material during immersion in water. From 50 % to 100 % WIS content a similar quantity of fines were produced in the first immersion. A far larger quantity of fines were lost from the pellets after the second immersion and again those pellets with a content of greater than 50% WIS exhibited superior resistance to water-induced loss of material. It was observed that one of the 0% WIS pellets disintegrated completely during the second immersion.

Example 7. Analysis of composition of mixed pellets Hardwood pellets and hardwood-derived WIS material from the Carbofrac process were first processed by a standard Kraft method to remove lignin and generate a fibrous pulp suitable for further analysis. An L&W Fibre Tester which uses image analysis was used to measure the fibre dimensions in the pulp.

Results are seen in Figures 5 and 6. As seen in Figure 5, the length weighted proportion of fibres of different lengths for pulp derived from untreated wood chips shows a maximum at about 0.92mm. On the other hand, following treatment to remove hemicellulose by the 'Carbofrac' process as described above, the corresponding peak length falls to about 0.2mm. The length weighted proportion of fibres having the length of 0.92mm falls from about 140 to about 80. As seen in Figure 6, the width of the fibres is also decreased by the 'Carbofrac' process, but by a lesser proportion.

The acid hydrolysis treatment process was found to have reduced the length weighted average fibre length by 30%, from 0.761 mm to 0.532 mm. Length weighted average fibre width was reduced by 8%, from 21 .2 μιτι to 19.7 μιτι. Thus, the average aspect ratio (length/diameter) of the fibres was reduced from 36 to 27. Thus a mixed composition pellet will contain a bimodal length distribution derived from two clearly identifiable populations of fibres derived respectively from the untreated biomass and the treated material. Dispersing pellets in water to create a slurry and then measuring fibre dimensions will confirm whether the pellets are derived from a single biomass source, whether untreated or partially treated, or whether they are made from a mixture of untreated and treated material.

An analysis can also be made of the hemicellulose content of each population of fibres. After dispersal in water the two fibre populations can be separated using a mesh filter and the hemicellulose content of each population determined using standard analysis methods. The smaller population of fibres separated from the dispersed pellet, which are derived from WIS material will contain <50% w/w, or <20% w/w or preferably <10% w/w of the hemicellulose content of the separated untreated fibres, thus indicating that the pellet contains a proportion of treated material that had been subjected to an acid hydrolysis and washing step to extract the pentoses.

Other preferred aspects of the invention may be summarized as: a. A material composed of a mixture of untreated and treated biomass wherein the treated material has a reduced content of hemicellulose. b. The material in (a) wherein the mixing ratio is from 99:1 to 1 :99 %w/w untreated to pre-treated material respectively. c. The material in (a) wherein the mixing ratio is from 80:20 to 20:80 %w/w untreated to pre-treated material respectively. d. The material in (a) wherein the mixing ratio is 50:50 %w/w untreated to pre-treated material respectively. e. The material in (a) composed of a mixture of untreated and treated

biomass wherein the treated material is produced in a steam explosion and acid hydrolysis process. f. The material in (a) composed of a mixture of untreated and treated

biomass wherein the treated material is produced in a torrefaction, organic extraction or wet oxidation process. g. The material in (a) composed of a mixture of untreated and treated

biomass wherein the compression energy required to form a pellet is reduced by at least 1 % compared to the untreated material. h. The material in (a) composed of a mixture of untreated and treated

biomass wherein the calorific value is at least 1 % greater compared to the untreated material. i. The material in (a) wherein the untreated and treated biomass come from the same source material. j. The material in (a) wherein the untreated and treated biomass come from different source materials. k. A compressed pellet formed in a die extrusion process comprising the material in any preceding lettered paragraph.

I. A compressed pellet according to (k) wherein the mixing ratio of untreated to treated material is selected for optimal properties including pellet strength and calorific value. m. The material in (a) wherein the mixing ratio of untreated to treated

material is selected to reduce the potassium content below a selected level. n. The material in (a) wherein the mixing ratio of untreated to treated material is selected to reduce the chlorine content below a selected level.

In this specification, unless expressly otherwise indicated, the word Or' is used in the sense of an operator that returns a true value when either or both of the stated conditions is met, as opposed to the operator 'exclusive or' which requires that only one of the conditions is met. The word 'comprising' is used in the sense of

'including' rather than in to mean 'consisting of. All prior teachings acknowledged above are hereby incorporated by reference. No acknowledgement of any prior published document herein should be taken to be an admission or representation that the teaching thereof was common general knowledge in Australia or elsewhere at the date hereof.

Claims

Claims:

1 . A method for production of a pelleted biomass material, said method

comprising:

taking a first pelletable biomass material that has not been subjected to a process for reducing the hemicellulose content of the biomass material, admixing with said first biomass material a second pelletable biomass material that is the product of subjecting a starting biomass material to a process hydrolysing hemicellulose therein which has reduced the size of fibres in said starting biomass material,

and forming pellets of the resulting mixture of first and second biomass materials.

2. A method as claimed in claim 1 , wherein the second biomass material is the product of subjecting the starting biomass material to a thermal hydrolysis process hydrolysing hemicellulose therein.

3. A method as claimed in claim 1 , wherein the second biomass material is the product of subjecting the starting biomass material to a steam explosion and acid hydrolysis process hydrolysing hemicellulose therein.

4. A method as claimed in claim 1 , wherein the second biomass material is the product of subjecting the starting biomass material to a torrefaction, organic extraction or wet oxidation process hydrolysing hemicellulose therein.

5. A method as claimed in any preceding claim, wherein the starting biomass material used to produce the second pelletable biomass material is the same as a biomass material used to produce the first pelletable biomass material.

6. A method as claimed in any preceding claim, wherein the first and second biomass materials are mixed in a weight ratio range of 99:1 to 50:50.

7. A method as claimed in claim 6, wherein the first and second biomass

materials are mixed in a weight ratio range of 90:10 to 60:40.

8. A method as claimed in claim 7, wherein the first and second biomass

materials are mixed in a weight ratio range of 80:20 to 70:30.

9. A method as claimed in any preceding claim, wherein the calorific value of the pellets is at least 1 % greater compared to like pellets formed from the first pelletable biomass material.

10. A method as claimed in any preceding claim, wherein compression energy used to form the pellets is reduced by at least 1 % compared to the

compression energy that would be required to form like pellets from the first pelletable biomass material.

1 1 . A method as claimed in any preceding claim, wherein the pellets are formed by a die extrusion process.

12. A pelleted biomass material comprising a mixture of a first pelletable biomass material that has not been subjected to a process for reducing the

hemicellulose content of the biomass material and a second pelletable biomass material that is the product of subjecting a starting biomass material to a process hydrolysing hemicellulose therein which has reduced the size of fibres in said starting biomass material.