WO2009040490A1

WO2009040490A1 - Method and apparatus for reducing palm stems to biomass

Info

Publication number: WO2009040490A1
Application number: PCT/GB2007/003618
Authority: WO
Inventors: Graham Colin Bell
Original assignee: Palm-Organics Global Ltd
Priority date: 2007-09-24
Filing date: 2007-09-24
Publication date: 2009-04-02

Abstract

A method of reducing palm stems to biomass by rotating about a rotation axis a plurality of axially-extending blades (14), at least some of which are offset circumferentially about the axis to one another, and which describe cutting paths which meet or overlap in the axial direction, and a cutter element rotatable about a rotation axis and including a plurality of axially-extending blades, at least some of which are offset circumferentially about the axis to one another, and which describe cutting paths which meet or overlap in the axial direction.

Description

METHOD AND APPARATUS FOR REDUCING PALM STEMS TO

BIOMASS

This invention relates to a method of and apparatus for reducing palm stems to biomass, and to the resulting biomass.

Palm trees, of the family Arecaceae, are grown on an industrial scale in many tropical and sub-tropical countries as a source of a wide range of products. Examples include the coconut palm (Cocos nucifera), the date palm (Phoenix dactylifera), the camauba palm (Copernicia prunifera), and the oil palm species (Elaeis guineensis and Elaeis oleifera).

Oil palm trees, for example, are grown for the commercial production of palm oil. Palm oil is extracted from the fruit of the oil palm, and has historically been used for many purposes including cooking, lubrication of machinery, and soap manufacture. More recently, the use of palm oil in the manufacture of biodiesel fuel has become important, and the market for palm oil is continually growing.

Figure 1 of the accompanying drawings is a schematic diagram of an oil palm tree 40. Other types of palm have the same basic structure. The tree 40 comprises a trunk or stem 42, leaves or fronds 44, fruit bunches, one of which is indicated at 46, and roots (not shown). The fronds 44 and fruit bunches 46 grow from the upper regions of the stem 42. The fruit bunches 46 typically contain over 100 fruits, from which palm oil is extracted. One adult tree can produce up to 100 kg of fruit per year, from which around 22 kg of palm oil can be extracted from the pericarp or flesh of the fruit. Oil can also be extracted from the seed kernels of the fruit.

Oil palm trees start producing palm oil in useful quantities about three years after planting. The palm oil yield then rises and reaches a peak level when the palm tree is between six and twelve years old, after which the yield steadily declines. As the yield declines and the height of the tree increases, so the cost of harvesting the oil rises and the profitability of the tree diminishes until, around 20 to 25 years after planting, the oil palm tree reaches the end of its profitable commercial life. Other species of palm tree, grown for other products, also have a limited commercial life.

In any palm-growing operation, it is desirable to fell and remove those trees which are no longer commercially viable. Trees which are left to age tend to harbour diseases and pests which can spread to younger trees. Removal of the unviable trees mitigates this problem and makes the land available for re-planting.

The removal of felled trees can be difficult and costly. The felled palm trees may be burnt, but the considerable environmental damage that results from burning is generally unacceptable. Furthermore, burning is a slow process due to the high water content of the stems. Each tree takes about one day to burn completely, and accelerants may be required to achieve complete incineration. If left to grow, mature palm trees can reach 12 to 15 metres or more in height. The stems oftrees felled after 20 years are, on average, about ten metres long and approximately 0.5 metres in diameter, which makes felled trees unwieldy to transport unless the stems are first cut into smaller pieces. However, palm tree stems tend to cause blunting of conventional cutting tools such as chainsaws, so that more expensive, specialist equipment must be used if the stem is to be reduced in size after felling.

Another way of removing the felled trees is to cut or shred the tree into small pieces which can then be readily removed from the plantation. Various cutting machines are known in the art. In general terms, a cutting machine or pulverizer typically includes a cutter drum having an arrangement of blades, means for feeding the trees towards the cutter drum, and means for removing the cut material from the drum.

The cutter drum of one existing pulverizer consists of a cylinder-like drum rotatable about an axis and having a circumferential wall and a plurality of cutter blades mounted in pockets provided in the circumferential wall of the cylinder drum. The pockets extend axially along the length of the drum. However, dirt, excess chips, palm sap, gum and foreign matter may easily become trapped inside the pockets during the use of the cutter drum, which results in frequent breakdowns of the pulverizing machine. In addition, the cutter blades are prone to wear and tear, due to the blunting effect previously described which is exacerbated by the tendency for matter to collect in the pockets.

It would be desirable, therefore, to provide a cutter element for use in a cutting machine for clearing tree stems, stumps or whole trees, which offers effective reduction of the palm stems into smaller pieces. It would also be desirable to provide a cutter element which is strong and easy to maintain, and in which dirt, excess chips and foreign matter do not tend to collect in use. Furthermore, a need exists to minimise the amount of waste from palm-growing industries, especially by converting waste material that would otherwise be disposed of, such as pulverized tree stems, into commercially valuable products. By doing so, the environmental and economic penalties of disposing of this waste material are avoided or at least mitigated. It would be desirable, therefore, to provide a system for conversion of trees which are too old to be used for producing palm oil, coconuts, dates or other products into other useful and valuable products.

Accordingly, in accordance with one aspect of the invention there is provided a growing medium made from palm tree stems and including chopped vascular bundles and crushed parenchyma.

The growing medium or palm stem biomass utilises palm tree stems that would be left to rot, burnt, or otherwise wasted. Therefore, the present invention offers a solution to the problem of disposing of the stems of unviable palm trees.

The growing medium is a colloidal, loamy, peaty soil-like material. It can be used alone as a substrate for growing plants, for example grass. Alternatively, the growing medium can be added to soil to impart the benefits of the growing medium on the soil.

The combination of the chopped vascular bundles and crushed parenchyma provides a growing medium having excellent water retention qualities compared to other growing media, such as peat, compost, chipped bark, clay pebble or rockwool containing products, for example. Because of the water retention quality, the need for irrigation can be reduced by up to 75% when the growing medium of the present invention is used instead of other media. Therefore, the growing medium of the invention has advantages in dry climates, such as in the Middle East.

The growing medium has high proportions of carbohydrates, starch, short-chain polysaccharides and free sugars. It does not contain heavy metals, chemical derivatives, stimulants or additives. Consequently, the growing medium is totally organic, and is therefore suitable for use in so-called organic farming, where the use of man-made fertilisers is avoided.

The structure of the growing medium aids the establishment and maintenance of populations of soil microbes, thus creating an environment which is naturally receptive to plant growth, and which promotes quick rooting and propagation of plants introduced to the growing medium.

The growing medium promotes the growth of strong, healthy plants by acting as an organic catalyst. In other words, the growing medium produces conditions in which the rates of chemical reactions within the growing medium, or within soil containing the growing medium, are increased. Such chemical reactions include, for example, the nitrification of ammonium and nitrites to nitrates.

When added to soil, the growing medium also increases the soil's carbon exchange capacity, i.e. the amount of cationic species that can be held by a given mass of soil. This means that a greater amount of nutrient species are available in the soil, so the nutrient take-up of the plants increases, promoting healthy and strong growth and significantly reducing the amount of fertiliser that must be applied for successful growth.

The chopped vascular bundles provide a fibrous component which maintains an open structure through which plant roots can easily grow. Furthermore, the ground parenchyma may facilitate root growth within the growing medium.

Unlike many other growing media, the palm stem biomass growing medium of the present invention is clean to handle. It does not contain hazardous mineral fibres, and does not produce excessive dust.

The growing medium can be manufactured in situ where the palm trees are felled, so that the stems need not be transported in whole pieces, nor cut for transportation in smaller pieces. Instead, the growing medium can be conveniently bagged on site, or can be loaded into a suitable vehicle for packaging elsewhere. Alternatively, the growing medium can be manufactured in a factory, in which case the stems are transported to the factory whole or in large pieces.

In a further aspect, the invention extends to the use of palm stem biomass as a growing medium, and in another aspect the invention resides in the use of palm stems in the manufacture of a growing medium.

The growing medium can, for example, be used as a landscaping substrate, either on its own or mixed with other growing media such as soil. Grass will grow successfully on a layer of palm stem biomass with a thickness of 25 mm, irrespective of the underlying surface. This offers a convenient and inexpensive way to produce grassed areas on any surface, for example on concrete, barren land such as reclaimed land or land that has become unproductive through desertification, soil degradation or mining operations. Use of the growing medium on concrete is particularly suited for development and urban beautification projects and for the establishment of sporting fields and golf courses.

The palm stem biomass is suitable for a range of applications in addition to its use as a growing medium, as will now be described.

The palm stem biomass can also be used as a horticultural growing medium to grow crops of vegetable and flowers, especially for commercial use. The growing medium can be used as a replacement for peat-based compost, which is advantageous because peat mining causes significant environmental damage and is being phased out, particularly in Europe. Again, the palm biomass may be used alone or in conjunction with other growing media, depending on the application.

Because of the high nutrient content and catalysis function of the biomass, it is suitable for use as an organic fertilizer, especially when combined with animal waste such as manure and composted using a thermophylic process to kill pathogens. The biomass improves the rate of release of nutrients from the animal waste as well as adding its own nutrient content to the soil or other growing medium.

Another use of the palm stem biomass is as animal feed. The biomass is edible and non- toxic, and provides a nutritionally well-balanced animal feed when combined with other vegetable materials such as fruit and vegetables. The use of palm stem biomass as an animal feed is advantageous in that it contains no animal-derived products.

The palm biomass has a high cellulose content (typically around 30% by volume). Therefore, the palm biomass can be used as a source of cellulose for the production of cellulosic ethanol (bioethanol), which is being increasingly used as a replacement for petroleum fuels in motor vehicles. Cellulose can be extracted from the biomass using an esterification process, or by using a two-phase enzyme/microbial process.

If cellulose and lignin is extracted from the palm stem biomass, for example to manufacture cellulosic ethanol as described above, the remaining fibrous material can be used in the production of fibre-based products such as medium-density fibreboard, hardboard, paper, and disposable cups and plates.

Furthermore, various chemical components of the palm stem biomass can be extracted and used in, for example, biochemical products for pharmaceutical or cosmetic applications.

The palm stem biomass can itself be used as a fuel, for example for electricity generation. The biomass is clean burning and, because the biomass burns slowly, the thermal energy of combustion is released slowly and is therefore easy to utilise, for example in a heat exchanger.

In another aspect, the invention resides in a method of reducing palm stems to biomass by rotating about a rotation axis a plurality of axially-extending blades, at least some of which are offset circumferentially about the axis to one another, and which describe cutting paths which meet or overlap in the axial direction.

The biomass produced in this way may, for example, be used as a growing medium as previously described.

In another aspect of the invention, there is provided a cutter element rotatable about a rotation axis and including a plurality of axially-extending blades, at least some of which are offset circumferentially about the axis to one another, and which describe cutting paths which meet or overlap in the axial direction.

In one embodiment of this aspect of the invention, the cutter element includes at least three radial support members spaced along the axis to define spaces therebetween, wherein the said plurality of blades are disposed within the spaces, each said blade describing a cylindrical cutting path on rotation of the element, wherein the blades within a space are circumferentially offset from the blades within each adjacent space, and wherein the cutting path described by the blades in a space meets or overlaps with the cutting path described by the blades in each adjacent space.

The cutter blades are significantly shorter than the axial length of the cutter element. The use of a long blade is therefore avoided, so the risk of fracture or cracking of the blades is reduced. The provision of smaller individual cutter blades also allows easier handling of the blades, in comparison to a single long blade, because the shorter blades have smaller mass. Therefore, the blades are easier to remove and replace, for example for sharpening, than the cutter blades of known cutter elements.

The offset arrangement of the cutter blades results in a well-balanced cutter element, because the mass of the cutter blades is evenly distributed around the circumference of the drum. Also, the applicant has shown that the offset arrangement of the blades results in an optimised cutting action for producing biomass.

The arrangement of the cutter element of the present invention allows successful cutting of palm stems at feed rates substantially higher rate than previously known cutting machines. For example, feed rates of 7 linear metres of stem per minute or higher can be achieved without damage to the cutter element.

Because the cutter blades are mounted on the support plates, rather than directly on an axial shaft, for example, the cutter element has a shape which is relatively aerodynamically efficient. In this way, the 'braking' effect on the cutter element, which occurs due to the cutter blades meeting air resistance and slowing the rotation of the drum, is minimised. In other words, the frictional forces acting on the cutter element as it rotates are minimised. When the cutter element is used in a cutting machine and driven by an engine, via a pulley and belt transmission for example, the frictional forces acting on the engine and on the associated pulleys and belts are likewise minimised. In this way, the wear and tear on the engine and transmission of the cutting machine is kept to a minimum.

In accordance with another aspect of the invention, there is provided cutting machine for processing palm stems, including a cutter element as previously described, and drive means to effect rotation of the cutter element.

In one embodiment of the cutter machine according the invention, a transmission means is provided to transmit power from the drive means to the cutter element. The transmission means may, for example, comprise a resiliently extensible drive belt. Because the drive belt is resiliently extensible or flexible, sudden changes in load on the belt can be accommodated by extension or flexion of the drive belt, so that the changes are not transmitted to the drive means. Thus the machine is tolerant to the changes in load that occur when the machine starts and when the blades of the cutter element impact the stem. Alternatively, power may be transmitted from the drive means to the cutter element by a driveshaft or a chain drive. However, in those cases damage to the transmission is more likely because sudden changes in load cannot be as readily accommodated as when a drive belt is used. In these circumstances, therefore, an alternative means for accommodating changes in load may be used, such as a fluid coupling.

The cutting machine of the invention is preferably used in a static location, such as a factory. This is because the components of the cutting machine, including components of a cutter element according to the invention, can be designed to maximise the robustness and rigidity of the cutting machine. The cutting machine may instead be used in a temporary location, and may be adapted to allow ease of transportation on a lorry. In that case, the components of the cutting machine may be designed to minimise the mass and size of the cutting machine. In another variant, the cutting machine is self-mobile, for example by the provision of a tracked or wheeled chassis.

Although the invention will be described primarily with reference to oil palm trees, it will be appreciated that other members of the palm family are suitable for carrying out the invention.

Reference has already been made to Figure 1 of the accompanying drawings, which is a schematic diagram of an oil palm tree. The invention will now be described, by way of example only, with reference to the remaining accompanying drawings, in which like reference numerals indicate like features, and in which:

Figure 2 is a schematic diagram of a cross-section through the stem of an oil palm tree;

Figure 3 is a schematic diagram of a growing medium according to one aspect of the present invention;

Figure 4 is a schematic diagram illustrating the use of the growing medium of Figure 3 as a landscaping substrate;

Figure 5 is a perspective view of a first embodiment of a cutter element according to the invention; Figure 6 is a front view of the cutter element of Figure 5;

Figure 7 is a cross-sectional view of the cutter element of Figure 5;

Figures 8a and 8b are front and cross-sectional views, respectively, of a second embodiment of a cutter element according to the invention;

Figures 9a and 9b are front and side views, respectively, of a cutter blade and cutter holder of the cutter element of Figures 8a and 8b;

Figures 10a and 10b are front and side exploded views, respectively, of the cutter blade and cutter holder of Figures 9a and 9b;

Figure 11 is a side view of an alternative cutter blade;

Figure 12 is a side view of an end support plate of the cutter element of Figures 8a and 8b;

Figure 13 is a side view of an inner support plate of the cutter element of Figures

8a and 8b;

Figure 14 is a side view of a central support plate of the cutter element of Figures 8a and 8b;

Figures 15a and 15b are front and side views, respectively, of the cutter element of Figures 8a and 8b, with the cutter blades and cutter holders removed and showing first bracing plates;

Figures 16a and 16b are front and side views, respectively, of the cutter element of Figures 8a and 8b, with the cutter blades and cutter holders removed and showing second bracing plates;

Figures 17a and 17b are front and side views, respectively, of the cutter element of Figures 8a and 8b, with the cutter blades and cutter holders removed and showing third bracing plates; Figures 18a and 18b are front and side views, respectively, of the cutter element of Figures 8a and 8b, with the cutter blades and cutter holders removed and showing fourth bracing plates;

Figure 19 is a side view of a second embodiment of a cutter element according to the invention, mounted in a cutting machine; and

Figure 20 is a front view of the cutter element and cutting machine of Figure 19, with the cutter holders and blades of the cutter element removed.

Referring first to Figure 2 of the accompanying drawings, the stem 42 of the oil palm tree is made up of vascular bundles 48 and parenchyma tissue 50. The vascular bundles 48 are, in turn, made up of xylem and phloem tissue. The xylem tissue contains cells with thickened cell walls, arranged to form passages for the transport of liquid along the stem, primarily nutrient-bearing water from the roots to the leaves. The phloem tissue contains cells known as sieve cells, which are arranged to convey carbohydrates along the stem, primarily the products of photosynthesis from the leaves to the rest of the tree. The vascular bundles also contain fibres. The cell walls of the fibre cells which make up the fibres contain a large amount of lignin, which gives the fibres high strength and toughness. These fibres, which are known as schlyrenchyma fibres, give support to the stem of the tree.

The vascular bundles 48 are distributed throughout the stem. The parenchyma tissue 50 occupies the space between the vascular bundles 48, and consists of cells with relatively soft, flexible cell walls. The parenchyma cells bind the vascular bundles 48 together.

Figure 3 is a schematic diagram showing the structure of a growing medium produced by a cutter element to be described. The growing medium, or palm stem biomass 52, is made from palm tree stems 42, and includes fibrous material in the form of chopped vascular bundles 54, and starchy material in the form of ground parenchyma tissue 56. Both components are produced by cutting palm tree stems 42 into small pieces, with typical dimensions of less than approximately 15 mm. By way of the cutting process, the vascular bundles 48 of the stem 42 are chopped to form the fibrous material 54, and the parenchyma tissue 50 is largely separated from the vascular bundles 48. The cutting action acts to grind the soft parenchyma tissue 50 into small particles or grains 56, so that the parenchyma 56 is a fine powder.

The following table shows the distribution of particle sizes, exemplified by particle diameters, in a typical sample of palm stem biomass 52 made from oil palm stems.

Particle diameter analysis of palm biomass growing medium

The majority of particles are less than 1 mm in diameter, giving the growing medium a colloidal consistency.

The fibres 54 are typically less than 2 mm in diameter, and have a relatively uniform length. In a given sample, the fibre length may be within a few millimetres of an average value in a range between 1 mm and 16 mm, although fibre lengths outside this range may also be provided. The length of fibres 54 provided may be optimised for a given application. For example, when the biomass 52 is to be used with clay-rich soils, a short fibre length of around 1 mm may be appropriate, while for sand-rich soils a longer average fibre length of around 15 mm may be chosen.

Typically, the palm stem biomass 52 of the invention has a pH of between 5.0 and 7.5 and an low electrical conductivity, due to the low salt content of the biomass 52. It will be appreciated that these properties, and the particle size analysis results for different samples of biomass 52, will vary according to a number of factors, including the type of palm used, the growing conditions of the palms, the storage conditions of the biomass 52, the cutting parameters such as rotation speed and feed speed, and so on. Because some of these parameters can be controlled, the particle size of the biomass can, if necessary, be adjusted according to the requirements of particular applications. The biomass 52 typically includes approximately 95% or more by weight of organic matter, of which approximately 30% is cellulose, 25% is starch, and 10% is simple carbohydrate in the form of glucose, xylose, galacose, arabinose and rhamnose. The carbon-to-nitrogen (C:N) ratio is typically around 74 by weight.

The palm biomass contains many important nutrients. The The following table shows the nutrient concentration in percent by dry weightfor a typical sample of the growing medium made from oil palm stems.

Nutrient analysis of palm biomass growing medium

It will be appreciated that the nutrient content may vary significantly between samples of the biomass, depending on the type of palm used, the growing conditions of the palms, the age of the stems, and many other factors. Indeed, a particular nutrient content may be desired for particular applications, in which case suitable stems are chosen for reduction into biomass.

Most of the nutrient content of the biomass originates from the parenchyma cells of the palm stem. The crushing of the parenchyma cells helps to make these nutrients available for uptake by roots or for leaching into surrounding material such as soil. The fibrous chopped vascular bundles give a free-draining structure to the biomass.

Figure 4 shows one possible application of the biomass 52 as a growing medium, in particular a landscaping substrate. A layer of biomass 52 is applied to a base 58. The base 58 may, for example, be a concrete slab, a roadway, a building roof or a ground surface, such as rock, compacted earth, soil, rubble and so on. Grass seeds or, more preferably, grass stolons are distributed on the surface of and within the layer of biomass 52, so that grass 60 grows from the biomass 52. The roots of the grass 60 extend within the biomass 52, and draw nutrients and water from the biomass 52.

The layer of biomass 52 may be any suitable thickness. Grass will grow successfully on a layer of biomass 52 with a thickness of approximately 25 mm, for example. Grass seeds or stolons can be distributed on the biomass 52 after the biomass 52 has been applied to the base 58, for example by spreading and raking, or alternatively grass seeds could be mixed with the biomass 52 before the biomass 52 is applied to the base 58. The biomass may be mixed with soil to assist germination when seeds are used.

A cutter head constituting a cutter element according to the present invention will now be described. The cutter element, when used in a suitable cutting apparatus or machine to cut palm tree stems, produces palm stem biomass which is particularly suited for use as a growing medium as previously described.

A first embodiment of the cutter element is shown in Figures 5, 6 and 7 of the accompanying drawings. The cutter element 10 comprises a hollow shaft 11 , first and second end flanges or support plates 12a, 12b mounted to the hollow shaft 11, first and second inner flanges or support plates 13a, 13b spaced apart from and mounted parallel to the end flanges 12a, 12b, and a plurality of cutter blades 14. The end flanges 12a, 12b form the outer support for the cutter element 10. The hollow shaft 11 is arranged to accommodate within its bore a rotary shaft (not shown) of a drive apparatus (not shown). The drive apparatus imparts rotary motion to the cutter element 10, in use.

The blades 14 are mounted at the outer periphery of the support plates 12a, 12b, 13a, 13b, so that the blades 14 extend across the space between the support plates 12a, 12b, 13a, 13b and lie generally parallel to the shaft 11. The blades 14 are arranged around the circumference of the cutter element 10.

Additional support plates, which can be described as bracing plates 15a, 15b, are mounted on the outer surface of the hollow shaft 11 and extend radially outwards from the shaft 11 to the outer circumferential surface of the support plates 12a, 12b, 13a, 13b, and axially along the rotation axis of the cutter element 10 in between an end support plate 12a, 12b and an inner support plate 13a, 13b. The bracing plates 15a, 15b act to strengthen the cutter element 10, and prevent twisting or shear deformation of the cutter element 10. A first space is defined between the first end support plate 12a and the first inner support plate 13a, a second space is defined between the first inner support plate 13a and the second inner support plate 13b, and a third space is defined between the second end support plate 12b and the second inner support plate 13b, as shown in Figure 5. Each of the support plates 12a, 12b, 13a, 13b is formed with an aperture 16 for receiving the hollow shaft 11 therethrough. The support plates 12a, 12b, 13a, 13b consist of disc-like plates spaced apart along the hollow shaft 11 so as to form flanges on the shaft 11.

As shown most clearly in Figure 6, the bracing plates 15 are mounted on opposing sides of the outer surface of the hollow shaft 11 in each of the first, second and third spaces between the inner support plates 13a, 13b and the end support plates 12a, 12b. The bracing plates 15a, 15b in adjacent spaces are offset by 90 degrees so that, for example, a first bracing plate 15a in the first space is perpendicular to a second bracing plate 15b in the second space.

The cutter element 10 also includes cutter holders or bars 17, which extend across the spaces between the support plates 12a, 12b, 13a, 13b and parallel to the hollow shaft 11. Each bar 17 is positioned approximately in line with a bracing plate 15a, 15b in an adjacent one of the spaces between the support plates 12a, 12b, 13a, 13b. The bars are mounted or attached to the support plates 12a, 12b, 13a, 13b by way of welded joints or other suitable connections.

The support plates 12a, 12b, 13a, 13b are formed with recesses 18 adjacent the position where the bars 17 are mounted on the support plates 12a, 12b, 13a, 13b. A cutter blade 14 is attached to each one of the bars 17 at an angle with an attachment means 14a, as shown most clearly in Figure 6. The attachment means 14a are releasable so that the blade 14 may be easily removed and replaced. For example, the attachment means 14a may be bolts which engage with threaded holes provided in the bars 17. Each blade has a cutting face 19 which is positioned outside the envelope of the element defined by the circular edges of the support plates 12a, 12b, 13a, 13b, so that, when the element 10 is rotated in use, the cutting faces 19 impinge upon the material fed towards the cutter element 10 to shred the material.

As shown in Figure 7, the recesses 18 are located beneath the cutting faces 19 of the blades 14. The recesses 18 extend approximately to the mid-plane of each support plate 12a, 12b, 13a, 13b so that the blades 14 can also extend to the mid-plane of each support plate 12a, 12b, 13a, 13b. In this way, when the cutter element 10 is rotated in use, the cutting faces 19 of the blades 14 sweep a continuous cutting path along the length of the cutter element 10. In other words, when the cutter element 10 rotates, the cylindrical cutting paths swept by adjacent blades 14 meet or overlap.

The recesses 18 extend beneath the cutting faces 19 of the blades 14, so that material cut by the blades 14 does not get trapped beneath the blades 14 close to the support plates 12a, 12b, 13a, 13b. Instead, the material is directed, by way of the recesses 18, into the spaces between the support plates 12a, 12b, 13a, 13b. For this reason, the recesses 18 have a curved, smooth shape to eject the cut material into the spaces and to prevent trapping of material.

The arrangement of the blades 14 and the recesses 18 allows a complete and continuous cutting action, with no 'dead zones' where stems fed towards the cutter element 10 are not cut by the rotating blades, as would be the case if the paths swept by the cutter blades 14 in adjacent spaces did not meet or overlap. In this way, the cutting action of the present invention is similar to that of a cutter element with long blades extending the full length of the element, but, because of the arrangement of the bracing plates 15a, 15b and the support plates 12a, 12b, 13a, 13b, the cutter element 10 is considerably stronger than an element with long blades.

The blades 14 within the first and third spaces are circumferentially offset by 90 degrees from the blades 14 within the second space. In this way, mass of the cutter element 10 is balanced and evenly distributed around the circumference of the cutter element 10, so that the cutter element 10 can rotate without excessive vibration. Balancing masses (not shown) may also be affixed to the cutter element 10 to compensate for minor imbalance of the cutter element 10, for example due to manufacturing tolerances.

The empty space or receiving area between the support plates 12a, 12b, 13a, 13b and below the blades 14 receives the material which is cut from the stems by the blades 14. The receiving area thus moves in conjunction with the cutter element 10. The cut material can be removed from the receiving area by, for example, arranging a receiving chute beneath the cutter element 10 into which the cut material falls or is ejected by the rotational motion of the cutter element 10, or by providing a conveyor belt to convey the cut material to a store or bagging station. The cutter element 10 is particularly suited to producing the growing medium of the invention, because the continuous and complete cutting action of the cutter element 10 chops the vascular bundles of palm stems cleanly and without snagging, ripping, or tearing of the fibres. In this way, fibrous material with a relatively small particle size can be produced.

A second embodiment of the cutter element of the invention will now be described with reference to Figures 8 to 20 of the accompanying drawings.

Figure 8a shows a front view of a cutter head constituting a cutter element 110 comprising a hollow shaft 111 , first and second end support plates 112a, 112b, and first and second inner support plates 113a, 113b. Each support plate has a central circular opening 116 to accommodate the shaft 111. The end support plates 1 12a, 112b are mounted at or close to the opposing ends of the shaft 111. The inner support plates 113a, 113b are mounted to the shaft 111 parallel to, and spaced from, the end support plates, so as to define a first space between the first end support plate 112a the first inner support plate 113a, a second space between the first inner support plate 113a the second inner support plate 113b, and a third space between the second inner support plate 113b and the second end support plate 112b.

As shown in Figure 8b, which is a cross-section through the cutter element 110 of Figure 8a, a plurality of blades 114 are mounted to cutter holders 120 which are, in turn, mounted to the cutter element 110. The positions of the cutter holders 120 are shown superimposed on Figure 8b, without hatching. Each blade 114 is attached to a cutter holder 120, which is, in turn, welded between an end support plate 112a, 112b and an inner support plate 113a, 113b, or between the two inner support plates 113a, 113b.

Two blades 114 are provided within each of the first, second and third spaces. In each space, the blades 114 are arranged diametrically opposite one another. The blades 1 14 within the first and third spaces are circumferentially offset by 90 degrees from the blades 114 within the second space.

Figures 9a and 9b show front and side views, respectively, of an assembly comprising a cutter blade 114 and cutter holder 120, when removed from the cutter element 1 10.

Figures 10a and 10b show the cutter blade 114 and cutter holder 120 in exploded view. The cutter holder 120 has a generally triangular cross section, so as to define a horizontal top face, a vertical side face and an inclined face, when considered in the orientation shown in Figures 9a and 9b. The cutter holder 120 includes tapped bores 122 which run parallel to the side face, and which intersect the top face and the inclined face.

The cutter blade 114 comprises a 12mm thick plate having an inner surface 114a, an outer surface 114b parallel to the inner surface 114a, and oppositely-facing chamfered edges to define cutting edges or faces 119. Thus each cutter blade 114 is double-edged, allowing reversal of the blade 114 when required to expose a fresh cutting face 119. The angle of the chamfer, indicated at P on Figure 10b, is approximately any angle from 20 to 25 degrees. In other words, the cutting face 119 forms any angle from 20 to 25 degrees with the inner surface 114a of the cutter blade 114 when oriented as shown in Figure 10b.

The cutter blade 114 is provided with frusto-conical bores 124 which extend through the thickness of the blade 114. The cutter blade 114 is held on to the cutter holder 120 by threaded bolts 126 which screw into the tapped bores 112 of the cutter holder 120. The bolts 126 have countersunk or flared heads which mate with the frusto-conical bores 124 of the cutter blade 114 so that the bolt 126 do not protrude above the outer surface 114b of the cutter blade 114.

As shown most clearly in Figure 9a, the cutter blade 1 14 is longer than the cutter holder 120 so that, when the blade 114 is mounted on the holder 120, the ends of the cutter blade 114 overhang the edges of the holder 120.

Referring again to Figures 8a and 8b, the cutter holders 120 are mounted to the cutter element 110 between the support plates 112a, 112b, 113a, 113b. The end faces 128 of the cutter holders 120 abut the support plates 112a, 112b, 113a, 1 13b and are held in position by means of welds or other suitable fixings.

Figure 1 1 shows a side view of an alternative cutter blade 214, which may be substituted in place of the cutter blade 114 shown in Figures 9a and 9b. The alternative cutter blade 214 is similar to the cutter blade shown in Figures 9a and 9b, except in that the cutting faces are double-chamfered. A first cutting face 219a meets the inner surface 214a of the cutter blade 204 at an angle (labelled R on Figure 1 1 ) of 40 degrees. A second cutting face 219b meets the first cutting face and forms and angle (labelled S on Figure 11 ) of 15 degrees with a plane parallel to the inner and outer surfaces 214a, 214b of the cutter blade 214, so that the second cutting face 219b meets the outer surface 214b at an angle of 165 degrees. Frusto-conical bores 224 are provided to allow the blade 214 to be mounted to a cutter holder of the type shown in Figures 9a to 10b. The cutter blade 214 offers good resistance to blunting.

Figure 12 is a side view of the first end support plate 112a. The second end support plate 112b is identical to the first end support plate 112a. The end support plate 1 12a is fabricated from 18 mm thick steel plate. Two cut-outs or recesses 130 are provided in the outer circumference of the support plates 112a, 112b. The cut-outs 130 define a shallow, non-symmetrical ¹V shape, and are situated diametrically opposite one another. Each cut-out 130 has a relatively long edge 132 forming the shallowest arm of the 'V shape, and a relatively short edge 134 forming the steepest arm of the 'V shape. The angle Q formed between the long edge 132 and a tangent to the circumference of the support plate 112a, 112b, taken at the edge of the cut-out 130 is approximately equal to 26 degrees.

Figure 13 shows a side view of the first inner support plate 113a, which is identical to the second inner support plate 113b. The inner support plates 113a, 113b are similar to the end support plates 112a, 112b, except in that four cut-outs 130 are provided. The cutouts are arranged at 90 degree intervals around the circumference of the inner support plate 113a. The shape of the cut-outs in the inner support plates 113a, 113b are identical to the cut-outs provided in the end support plates 112a, 112b.

As shown in Figure 8a, the end support plates 112a, 112b and inner support plates 113a, 113b are mounted on the shaft 111 of the cutter element 110 so that the cut-outs 130 of the first end support plate 112a are aligned with two of the cut-outs 130 of the first inner support plate 113a; the cut-outs 130 of the first inner support plate 113a are aligned with the cut-outs 130 of the second inner support plate 113b; and the cut-outs 130 of the second end support plate 112b are aligned with two of the cut-outs 130 of the second inner support plate 113b. Furthermore, the cut-outs 130 of the first and second end support plates 112a, 112b are aligned with one another.

The cutter holders 120 are attached to the support plates 112a, 112b, 113a, 1 13b as previously described and extend across the first, second and third spaces adjacent to the aligned cut-outs 130 of the support plates. Two cutter holders 120 are provided in each of the first, second and third spaces. The cutter holders 120 provided in the second space are circumferentially offset by 90 degrees from the cutter holders provided in the first and third spaces.

The top face of each cutter holder 120 is aligned with the relatively long edge 132 of each cut-out 130. In this way, when cutter blades 114 are mounted on the cutter holders 120, the cutter blades 114 extend into the cut-outs 130, across the thickness of the support plates 112a, 112b, 113a, 113b. In this way, the axial extent of the cutter blades 114 within each space overlaps the axial extent of the cutter blades 114 in the or each adjacent space. The extent of the overlap is approximately equal to the thickness of one of the support plates 112a, 112b, 113a, 113b.

In this way, when the cutter element 110 rotates in use, the cutter blades 114 sweep an area which is uninterrupted along an axis parallel to the shaft 111. The cutter blades 114 therefore provide a complete and continuous cutting action along the axis of the cutter element 110.

A central support plate 140 is fixed to the shaft 111 mid-way between and parallel to the first and second inner support plates 113a, 113b. The central support plate 140, a side view of which is shown in Figure 14, is formed from a circular steel plate and has an outer diameter equal to that of the end and inner support plates 112a, 112b, 113a, 113b. The central support plate 140 is provided with a central circular opening 116 to receive the shaft 111.

The central support plate 140 is provided with two cut-outs 142 which extend from the outer circumference of the central support plate 140 to the central opening 116. Each cut-out is formed by a first edge 144 which lies parallel to and slightly offset from a diameter of the central support plate 140, and a second edge 146 which lies at an angle to the radial direction of the support plate 140. The first and second edges 144, 146 of the cut-outs 142 do not meet at the edge of the circular opening 116, but are instead spaced from one another. In this way, the cut-outs 142 separate the central support plate 140 into two portions. These two portions are held in position with respect to one another by virtue of the welded joint between the central support plate 140 and the shaft 111.

The cut-outs 142 are arranged so that the cutter holders 120 located between the first and second inner support plates 113a, 1 13b are received within the cut-outs 142. Away from the cutter blades 114 and the cutter holders 120, the outermost edge of the central support plate 140 lies at the same radial distance from the shaft 111 as the outermost edges of the end and inner support plates 112a, 112b, 113a, 113b.

The cutter element 110 is provided with a number of bracing plates (not shown in Figures 8a and 8b) to increase the strength and rigidity of the element 110. Figures 15a to 18b show the position of the bracing plates within the cutter element 110, with the cutter blades and cutter holders removed for clarity.

Figures 15a and 15b are front and side views, respectively, of the cutter element 110 showing the position of four bracing plates 150a arranged radially around the shaft 111 within the second space. These bracing plates 150a extend from the shaft 111 to the outermost edges of the inner support plates 113a, 113b, and are each cut into two coplanar portions by the central support plate 140.

Figures 16a and 16b are front and side views, respectively, of the cutter element 110 showing the position of two inclined bracing plates 150b provided within the second space and which extend from a point on the shaft 111 mid-way between the innermost ends of the radial bracing plates 150a shown in Figures 15a and 15b to the outermost ends of two oppositely facing ones of the radial bracing plates 150a. Each inclined bracing plate 150b is cut into two coplanar portions by the central support plate 140.

Figures 17a and 17b are front and side views, respectively, of the cutter element 110 showing the position of four inclined bracing plates 150c provided within the first and third spaces. The inclined bracing plates 150c extend from the shaft 111 to the outermost edges of the end support plates 112a, 112b and inner support plates 113a, 113b, at an angle to the radial direction of the cutter element 110. The outermost edges of the inclined bracing plates 150c lie close to the position of the cutter holders (120, not shown in Figures 17a and 17b) provided within the first and third spaces.

Figures 18a and 18b are front and side views, respectively, of the cutter element 110 showing the position of four radial bracing plates 15Od provided within the first and third spaces. The radial bracing plates 15Od extend from the shaft 111 to the outermost edges of the end support plates 112a, 112b and inner support plates 113a, 113b, parallel to the radial direction of the cutter element 110. The outermost edges of the inclined bracing plates 150c lie close to the position of the cutter holders (120, not shown in Figures 18a and 18b) provided within the first and third spaces.

It will be appreciated that the bracing plates 150a, 150b, 150c, 15Od described with reference to Figures 15a to 18b can be used in appropriate combinations within the cutter element of the present invention.

As in the first embodiment of the cutter element, because the blades 114 within the first and third spaces are circumferentially offset by 90 degrees from the blades 114 within the second space, the mass of the cutter element 110 is balanced and evenly distributed around the circumference of the cutter element 110, so that the cutter element 110 can rotate without excessive vibration. Also, as in the first embodiment, balancing masses or weights (not shown) may be affixed to the cutter element 110, for example to the support plates 112a, 112b, 113a, 113b, bracing plates 150a-d, or cutter holders 120.

The material cut from the stems by the blades 114 tends to be ejected from the blades 114 towards the shaft 111. In this second embodiment of the cutter element, the bracing plates 150a, 150b, 150c, 15Od and in particular the inclined bracing plates 150b, 150c, act to throw the cut material outwards, away from the cutter element 110 and into the space between the cutter element 110 and the housing 116.

In another aspect, the invention resides in a cutting machine including a cutter element according to the invention. Figures 19 and 20 show, respectively, side and front views of a cutting machine 160 constructed according to the invention, which includes a cutter element 110, drive means 162, and a chassis or support frame 164. The cutter element 110 is mounted within a housing 166, which supports the cutter element 110 and provides protection for persons in the vicinity of the cutter element 110. The blades 114 and bracing plates 150a, 150b, 150c, 15Od of the cutter element 110 are not shown in Figure 20, for clarity.

The drive means 162 comprises an engine, for example a diesel engine, which provides a rotary output in the form of a drive wheel 168. The drive wheel 168 is driven by a single-speed direct-drive gearbox. . The cutter element 110 is connected to a pulley 170 by way of a drive shaft 172, which may for example be an extended portion of the hollow shaft 111 of the cutter element 110. The pulley 170 is connected to the drive wheel 168 by a rubber drive belt 174. The drive wheel 168, pulley 170, drive shaft 172 and drive belt 174 may be housed within a shield or housing (not shown) to protect users against injury.

The drive wheel 168, pulley 170 and drive belt 174 are arranged to give a gearing reduction. The elasticity of the rubber drive belt 174 results in a deviation from the mechanical gearing reduction that would be achieved if the drive belt were not extensible.

As shown in Figure 20, the housing 166 is arranged so that the cutter element 110 is exposed to a front aspect of the machine 160. In this way, palm stems can be fed into the machine 160 through the housing 166 and towards the cutter element 110.

Conveyor rollers 176 and a feeder roller (not shown) are provided to feed the stems towards the cutter element 110. In use, a stem is placed on top of the conveyor rollers 176, which rotate to feed the stems towards the cutter element 110 at an appropriate rate. The feeder roller is arranged to apply a downward force to the stem in the vicinity of the conveyor rollers 176, to improve the purchase of the conveyor rollers 176 on the stem. The feeder roller may be driven to feed the stems towards the cutter element, in addition to or instead of the conveyor rollers 176.

By provision of the conveyor rollers 176 and the feeder roller, the rate at which stems are fed towards the cutter element 110 can be controlled. Typically, the conveyor rollers 176 and feeder roller are used to restrain the stem, so that the rate at which the stem moves towards the cutter element 110 is slower than the rate at which the stem would otherwise be drawn into the cutter element 110 by the action of the blades 114 alone.

To provide further control of the rate at which the stem is drawn into the cutter element 1 10, the central support plate 140 acts in combination with the inner support plates 113a, 1 13b to support the stem and preventing it from entering the cutter element 110 at a non- perpendicular angle, which would result in undesirable variation in the particle size of the cut or shredded material. The central support plate 140 also prevents the stem from moving too far into the interior of the cutter element 110, which could result in coarse cut particles, and jamming of the cutter element 110.

As shown most clearly in Figure 20, the chassis or support frame is provided with an opening 178, through which the cut material falls under the influence of gravity. A conveyor belt (not shown) is provided beneath the opening 178. The cut material falls onto the conveyor belt, and is conveyed to a station for storage or packaging.

The cutter element 110 is particularly suited to producing a growing medium, because the continuous and complete cutting action of the cutter element 10 chops the vascular bundles of palm stems cleanly and without snagging, ripping, or tearing of the fibres. In this way, fibrous material with a relatively small particle size can be produced.

Many variations and modifications to the cutter element and cutting machine could be made without departing from the scope of the invention as defined in the appended claims. For example, a longer cutter element could be produced by the provision of additional inner flanges or support plates to form further spaces between the support plates, in which cutter holders, cutter blades and bracing plates can be arranged as previously described. Furthermore, additional bracing plates, cutter holders and cutter blades could be provided to improve the strength and/or cutting frequency of the cutter element.

The cutter element is rotatable about a central axis. In this way, the volume of revolution defined by the cutter element as it rotates in use is cylindrical or drum-shaped, so that the cutter element may be used in place of the cutter drum of a conventional stem cutting apparatus. The cutter element may in this context be referred to as a cutter drum or a cutter head.

The hollow shaft of the cutter elements described above acts as a support member extending axially along the rotation axis of the element. It will be appreciated that, instead of a hollow shaft, any suitable axial support member could be used. For example, a solid shaft could be used. The axial support member need not be circular in cross- section, but could have a different cross-sectional shape, for example a square or hexagon.

An axial support member need not be provided. For example, a cutter element consisting of end support plates, inner support plates, cutter holders, cutter blades, and bracing plates could be made, in which the outer surface of one or both of the end support plates could be provided with connection means, for example a spindle fitted with a pulley or sprocket, for connection to a driving means to rotate the element. The end and inner support plates act as radial support members which support the blades. Instead of or in addition to support plates, any other suitable radial support members could be used. For instance, radial support members consisting of a box- section framework could be used. The framework could, for example, be cartwheel- shaped. As another alternative, a perforated circular support plate could be used.

The number of cutter blades, support plates and bracing plates provided may differ from those in the embodiments of the cutter element described above. In particular, more cutter blades may be provided around the circumference of the cutter element. For example, each space between the support plates may be provided with three or four cutter blades, each with a corresponding cutter holder. An appropriate number and arrangement of cut-outs would be provided in the end, inner and central support plates.

As a further example, more than two inner support plates may be provided, so that four or more spaces are provided between the support plates. In this way, the length of the cutter element can be increased from approximately three blades long, as in the previously described embodiments, to approximately four or more blades long. It is conceivable that the cutter element may be provided in a modular form, so that the length of the cutter element can be increased or decreased by adding or subtracting support plates, cutter blades and other components as appropriate for a particular task.

The cutter blades may be made from any suitable material. For example, the blades may be made from steel, which may be hardened at the cutting edges. Tungsten carbide blades may also be used. The blades may be formed from more than one material. For example, the blades may comprise a steel plate with tungsten carbide cutting edges, so that a composite blade is formed.

The angle formed between the cutter blades and a tangent to the circumference of the support plate (labelled Q in Figure 8b, for example) and the shape of the chamfered edges of the cutter blades (exemplified by the angle P in Figure 10b, and the angles R and S in Figure 1 1 ) may differ from those described above. The cutter blades may, for example, include curved cutting faces or cutting faces with more than two portions. The angle Q and the shape of the cutter blades are chosen to provide the optimum particle size for the cut material, while ensuring that wear and damage to the blades during use of the cutter element is minimised. Furthermore, the shape and orientation of the blades can be chosen to minimise the build-up of gum, sap, resin and moist detritus which tend to accumulate as the stems are cut.

The dimensions of the cutter element of the invention preferably allow the majority of palm stems to be cut with the cutter element. However, the cutter element may be lengthened, as described above, so that large stems can be cut or so that several stems can be cut side-by-side. The diameter of the cutter element can also be increased or decreased according to the application.

The particle size characteristics of the palm biomass produced by the cutter element or cutting machine of the invention depends on a number of factors, including the stem feeding rate (i.e. the rate at which the stems are fed towards and reduced by the cutter element), the number of blades provided, the rotation speed of the cutter element, the shape of the blade cutting surface, and the angle between the blade and a tangent to the support plates.

In particular, for a given cutter element or cutter machine, the particle size characteristics, exemplified by the length of the chopped fibres, can be controlled by changing the stem feeding rate. By selecting a low feed rate, the fibre length of the resulting biomass is short, while at a higher feeding rate the fibre length produced is longer. Typically, fibre lengths in a range between 1 mm and 16 mm can be produced by selecting an appropriate feeding rate. At a given feeding rate, the fibre lengths are uniform to within 2-3 mm. Furthermore, the fibres are well separated, without lumps or clumps of fibres.

Claims

1. A method of reducing palm stems to biomass by rotating about a rotation axis a plurality of axially-extending blades, at least some of which are offset circumferentially about the axis to one another, and which describe cutting paths which meet or overlap in the axial direction.

2. A method according to claim 1 , wherein the biomass comprises chopped vascular bundles and parenchyma.

3. A method according to claim 1 or claim 2, wherein the majority of the biomass has a particle diameter of less than 1 mm.

4. A method according to any preceding claim, wherein the palm stems are fed towards the blades along a feed axis perpendicular to the rotation axis.

5. A method according to claim 4, wherein planar surfaces of the blades meet the feed axis at an angle of between 57 and 62 degrees.

6. A method according to claim 5, wherein planar surfaces of the blades meet the feed axis at an angle of between 58 and 60 degrees.

7. A method of manufacturing a growing medium from palm stems, the palm stems including vascular bundles and parenchyma, the method including applying a simultaneous chopping and crushing action to a palm stem to chop the vascular bundles into fibres and crush the parenchyma.

8. A method according to claim 7, wherein a majority by mass of the chopped vascular bundles and crushed parenchyma have a particle size of less than 1 mm.

9. A method according to claim 7 or claim 8, wherein the simultaneous chopping and crushing action is achieved by rotating about a rotation axis a plurality of axially- extending blades, at least some of which are offset circumferentially about the axis to one another, and which describe cutting paths which meet or overlap in the axial direction.

10 A cutter element rotatable about a rotation axis and including: a plurality of axially-extending blades, at least some of which are offset circumferentially about the axis to one another, and which describe cutting paths which meet or overlap in the axial direction.

11 A cutter element according to claim 10 and including: at least three radial support members spaced along the axis to define spaces therebetween; wherein the said plurality of blades are disposed within the spaces, each said blade describing a cylindrical cutting path on rotation of the element; wherein the blades within a space are circumferentially offset from the blades within each adjacent space; and wherein the cutting path described by the blades in a space meets or overlaps with the cutting path described by the blades in each adjacent space.

12. A cutter element according to claim 11 , wherein each space contains two blades.

13. A cutter element according to claim 12, wherein the blades within each space are diametrically opposed.

14. A cutter element according to claim 12 or claim 13, wherein the circumferential offset is approximately 90 degrees.

15. A cutter element according to any of claims 11 to 14, wherein the radial support members are support plates arranged parallel to one another to define the spaces.

16. A cutter element according to claim 15, wherein the support plates are provided with cut-outs to accommodate portions of the blades.

17. A cutter element according to claim 16, wherein the cut-outs are recesses formed in the support plates.

18. A cutter element according to claim 17, wherein the recesses extend beneath the blades to direct material cut by the blades into the spaces.

19. A cutter element according to claim 17 or claim 18, wherein the recesses extend at least half-way through the thickness of the support plates.

20. A cutter element according to any one of claims 17 to 20, wherein the cutting path of the blades in adjacent spaces overlap by a distance approximately equal to the depth of the recesses less half of the thickness of the support plates.

21. A cutter element according to claim 15 or claim 16, wherein the cutting path of the blades in adjacent spaces overlap by a distance approximately equal to the thickness of the support plates.

22. A cutter element according to any one of claims 11 to 21 , including an axial support member.

23. A cutter element according to claim 22, wherein the axial support member is a shaft upon which the radial support members are mounted.

24. A cutter element according to any one of claims 11 to 23 and provided with cutter holders mounted to the radial support members, wherein the blades are attachable to the cutter holders.

25. A cutter element according to claim 24, wherein the blades are releasably attachable to the cutter holders.

26. A cutter element according to claim 24 or claim 25, wherein the blades are plates with at least one cutting edge.

27. A cutter element according to claim 26, wherein the blades include two opposed cutting edges.

28. A cutter element according to any one of claims 11 to 27, wherein each one of the blades forms an acute angle with a tangent to the rotational envelope of the radial support members, the tangent meeting the radial support member adjacent to the blade.

29. A cutter element according to any one of claims 11 to 28, wherein each blade includes parallel outer and inner surfaces, and a cutting surface inclined to the outer and inner surfaces.

30. A cutter element according to claim 29, wherein the cutting surface is inclined at any angle of from 20 to 25 degrees to the inner surface of the blade.

31. A cutter element according to claim 29, wherein the cutting surface includes two non-coplanar portions.

32. A cutter element according to claim 31 , wherein a first portion of the cutting surface meets the inner surface of the blade at a first angle, and a second portion of the cutting surface meets the outer surface of the blade at a second angle, and wherein the second angle is greater than 180 degrees minus the first angle.

33. A cutter element according to claim 32, wherein the first angle is approximately 40 degrees and the second angle is approximately 165 degrees.

34. A cutter element according to any one of claims 11 to 33, including bracing means to strengthen the cutter element.

35. A cutter element according to claim 34, wherein the bracing means comprise , bracing plates provided within the spaces and extending axially between the radial support members.

36. A cutter element according to claim 35, wherein one or more bracing plates extends in a radial direction.

37. A cutter element according to claim 35 or 36, wherein one or more bracing plates are inclined to the radial direction of the cutter element so as to eject cut material from the spaces on rotation of the cutter element.

38. A cutting machine for processing palm stems, including a cutter element according to any of claims 10 to 37; and drive means to effect rotation of the cutter element.

39. A cutting machine according to claim 38, including transmission means to transmit rotational movement from the drive means to the cutter element.

40. A cutting machine according to claim 39, wherein the transmission means includes a drive belt.

41. A cutting machine according to claim 40, wherein the drive belt is resiliency extensible to absorb changes in load in the transmission means.

42. A cutting machine according to claim 41 , wherein the drive belt is a rubber material.

43. A growing medium made from palm stems and including chopped vascular bundles and ground parenchyma.

44. A growing medium according to claim 43, wherein the maximum particle size is approximately 10 mm.

45. A growing medium according to claim 43, wherein the maximum particle size is between approximately 3 mm and approximately 5 mm.

46. A growing medium according to claim 43, wherein the average length of the chopped vascular bundles is within the range 1 mm to 16 mm.

47. A growing medium according to any of claims 43 to 46, wherein the ground parenchyma facilitates root growth within the growing medium.

48. A growing medium according to any of claims 43 to 47, produced by a cutter element according to any of claims 10 to 37, or by a cutting machine according to any of claims 38 to 42.

49. A growing medium according to any of claims 43 to 47, produced by a method according to any of claims 1 to 9.

50. A cutter drum comprising: a hollow shaft; a pair of end flanges mounted at the opposing ends of said hollow shaft; at least one inner flange mounted to said hollow shaft which is parallel and spaced apart in between said pair of end flanges; and a plurality of blades mounted at the outer periphery in between said outer and inner flanges of the circumferential of said cutter drum, whereby each of said blades having its end portions protruded to said flanges which are provided with recesses for a complete and continuous cutting action.

51. A cutter drum according to claim 50, wherein said blades are mounted to the flanges onto a plurality of bars secured to the flanges.

52. A cutter drum according to claim 51 , wherein said bars are secured at the spaced area in between said end flange and inner flange.

53. A cutter drum according to claim 50, wherein said cutter drum further comprising of a plurality of support plates vertically mounted at the outer surface of said hollow shaft and extended in between said end flange and inner flange to the outer circumferential surface of said flanges to strengthen said cutter drum.

54. A cutter drum according to claim 53, wherein said cutter drum may be added with additional inner flanges to form a longer cutter drum.

55. A cutter drum according to claim 53, wherein said support plates are mounted at predetermined position in each space area between the flanges whereby preferably said support plate in a first space area is mounted perpendicular to the support plate of the next space area.

56. A cutter drum according to claim 50, wherein said blades are attached to said bars at an angle with attachment means with the cutting faces of the blades facing upwards and on top of said recesses.