WO2008100147A1

WO2008100147A1 - A method for production of peat-based fuel balls

Info

Publication number: WO2008100147A1
Application number: PCT/NO2008/000039
Authority: WO
Inventors: Roald Klausen
Original assignee: BIOENERGI NORD AS
Current assignee: BIOENERGI NORD AS
Priority date: 2007-02-12
Filing date: 2008-02-07
Publication date: 2008-08-21
Anticipated expiration: 2009-08-12
Also published as: NO20070812L; EP2118244A1

Abstract

The invention comprises a method for production of peat- based fuel balls, the method comprising: using wet peat from a bog deposit as a natural raw material in the method, such wet peat typically having a moisture content in the range of 80-95 percent by- weight; converting the peat into ball -shaped or approximately ball -shaped units; and drying the ball-shaped units until having a remaining moisture content of maximum 12 percent by weight. The distinctive characteristic of the method is that it also comprises: mixing the wet peat together with at least one moisture -absorbing, biological fibrous material having a lower moisture content than the moisture content of the wet peat.

Description

A METHOD FOR PRODUCTION OF PEAT-BASED FUEL BALLS

Area of invention

The present invention concerns a method for production of a solid fuel in the form of peat-based fuel balls. Among other things, such fuel balls are suitable as fuel in ordinary wood-burning stoves and pellet stoves, but also in industrial heating installations adapted to this type of solid fuel. As a fuel means, such balls advantageously may substitute and/or supplement ordinary wood pellets.

The invention also concerns use of moisture-absorbing, biological fibrous material as an additive in wet peat for production of said fuel balls from a mixture of said materials .

Background of the invention

Biofuel in the form of pellets have become an increasingly more common fuel means, both for domestic purposes and for industrial purposes. Most of such fuel pellets are based on pure residual products from the wood industry, for example pure sawdust and/or shavings. This results from today's modern society consuming vast amounts of wood for other purposes than firing. As such, there are vast amounts of residual wood easily accessible for production of, among other things, wood pellets, wood briquettes and other shapes of wood-based fuel products. Pure peat, or a mixture of peat and such residual wood, is also used to a certain extent for production of such fuel products.

In accordance with today's production method, such pure residual wood go through relatively comprehensive, equipment- demanding, energy-demanding and thus expensive process in order to arrive at the final fuel product, for example wood pellets. The same process is also used for production of pellets from peat, or from a mixture of peat and residual wood. This production process and its fuel product, especially wood pellets, are encumbered with several technical, practical and economical disadvantages, which will be discussed in further detail below.

There are, however, a number of other combustible raw materials being used to a small or no extent for production of such solid fuels, even though they oftentimes are inexpensive, easily accessible and advantageously could be used for such purposes. Such other fuel raw materials may comprise tree species normally not used for production of wood pellets in accordance with said production process, and/or it may involve types of residual wood considered to be inferior for this purpose. Fuel raw materials of interest may also comprise residual products from other industrial enterprises, for example recycled paper and cardboard. Yet further, such raw materials may comprise residual products from agricultural enterprises, for example plant fibres and similar biological residual material, which may have an exotic origin. Alternate fuel raw materials may also be extracted more or less directly from nature, for example in the form of peat, coal, coal dust, etc.

The reasons for the lacking exploitation of alternate fuel raw materials may be plentiful and varied. It may, for example, involve a situation where the resource potentials have not been imagined or studied. Moreover, start-up and/or operation of such projects may be inhibited by poor economics, little interest and willingness to contribute, environmental considerations, political controls, etc. The economics and viability of such projects depend, among other things, on the nature and availability of the raw material relative to the production site in question. Geographical distances, topography, climate, infrastructure and available means of transport therefore are of great importance to such projects. Yet further, there may be a lack of required technology for extraction, conveyance and production of the alternate biofuel in question. It may, for example, involve a situation where the technology does not exist or is insufficient, and/or a situation where the technology is too expensive to acquire and/or use.

A need therefore exists for providing technology allowing for economically interesting exploitation of alternate fuel raw materials for production of new types and/or shapes of biofuel. Thus, the present invention seeks to provide a relatively simple and cost-efficient method for production of fuel balls based on peat. Furthermore, at least one other combustible and fibre-containing additive is used in the method. Thus, the present invention seeks to exploit the extensive deposits of peat present on Earth. For example, the land areas of the northern hemisphere consist of ca. 3 % bog, from which ca. 1/3 of the bog mass constitutes peat of fuel quality, so-called fuel peat. Besides, peat contains a natural adhesive advantageously exploited in the present invention.

Prior art and disadvantages thereof

For centuries, and especially before electricity became commonplace, pieces of peat have been taken from the bogs and been dried outside during the summer season, thereby allowing the peat to be utilized as fuel. This type of peat was also an important substitute for fire wood in places having little woodland. The best fuel peat was made from extremely transformed bog, so-called black peat, having large energy content and few ash agents. Although utilizing peat as fuel in today's modern society too, this takes place on a relatively small scale, and then usually as a refined fuel product, such as said peat pellets. On the other hand, peat is utilized extensively for other purposes and, among other things, for production of building materials and for roofing, for purposes of plant cultivation and soil improvement, and as peat strew in connection with animal husbandry. It is particularly the so-called fibrous peat which is utilized on a large scale today.

Peat is a soil type consisting mainly of transformed plant remains, which are found in moist areas, such as in bogs. Virtually all peat is formed in an acidic environment under water. The very transformation of organic material to peat progresses very slowly. To a large extent, the properties of peat will be determined by the plants originally growing at the locality, but also by the degree of transformation (degree of humification) of the peat. Most peat deposits are therefore found in the temperate zones of the Earth.

The specific gravity of the peat increases proportional with its degree of transformation. For this reason, peat is classified according to its degree of transformation, also termed degree of humification. Therefore, and qualitatively speaking, peat is oftentimes referred to as insignificantly transformed, moderately transformed, well transformed and completely transformed. For more quantitative classification of peat, however, it is customary to use the scale of Lennart von Post for degree of peat humification, the scale of which comprises humification degrees Hl, H2 , H3.. HlO. Fibrous peat, which is used extensively for building purposes and peat strew, among other things, thus consists of humification degrees H1-H4, whereas fuel peat consists of humification degrees H5-H10, and especially humification degrees H6-H9.

The method according to this invention involves many features resembling those of the method according to NO 308525, which concerns production of a peat-based building material. The method according to NO 308525 is about forming peat into peat balls, which are dehumidified to a remaining moisture content of maximum 12 %, and which are subsequently impregnated with a wax emulsion and is dried. Finally, the dried and wax- impregnated peat balls are glued together to a building material of desired shape and thickness. Although the method according to NO 308525 has a lot in common with the present method, the latter method comprises essential technical features not mentioned in NO 308525, the features of which are of great technical, practical and economical significance with respect to providing an alternate fuel product capable of competing with known fuel pellets. This aspect will be discussed in further detail below.

In this context, however, the aforementioned process for production of biofuel in the form of pellets based on (a) residual wood, (b) peat, or (c) a mixture of residual wood and peat, is considered to represent the closest prior art. According to this known process, pure wood is ground up into wood chips and/or wood flour, which is dried in one or several rounds, finally allowing the ground up wood pulp to have a moisture content of ca. 5-10 percent by weight. This is an extensive process requiring a lot of energy and being encumbered with large dust problems, thus being associated with the danger of ignition and fire. Then the ground up and dried wood pulp is introduced into a pellet press and is pressed at high pressure through a metal matrix (apertured plate) . From the metal matrix, cylindrical objects are extruded so as to break off, or to be cut up, into short, cylindrical rods. When the ground up wood pulp is forced at high pressure through the metal matrix, the temperature increases within the wood pulp, and the substance lignin is liberated therefrom. The lignin acts as a binder in the extruded rods upon subsequently being cooled so as to form fuel pellets. Due to this extrusion process, the pellets obtain a shiny and smooth cylinder surface, and they become hard and, as such, are to be relatively stable in shape. They are thus to better endure the subsequent handling, packing and transport onwards to the sites of use. A further wood pellet hardness and moisture-resistance may be obtained by boiling the wood pulp before being introduced into the pellet press. Thereby, the wood fibres in the wood pulp are split and liberate more lignin as a binder in the final wood pellets, which are usually referred to as brown wood pellets (in contrast to ordinary, white wood pellets) . For production of wood pellets, also wet wood pulp may be used and extruded in the manner described above. Utilization of wet pulp, however, is not of interest for extrusion and production of peat pellets. According to the Finnish manufacturer Vapo Oy, the peat mass must always be dried before being introduced into and pelletized in said pellet press.

It is thus obvious that the prior art for production of pellets is comprehensive, equipment-demanding, energy- demanding and expensive, which calls for large-scale production in order to be economically interesting.

As mentioned, such known fuel pellets also have several technical and practical disadvantages significantly avoidable by using peat-based fuel balls produced in accordance with the present method. As mentioned above, such known fuel pellets oftentimes occur in the form of short, cylindrical rods typically having a diameter of ca. 6-10 mm and a length of ca. 5-30 mm. Although being a natural consequence of the production process, the cylinder-shape of the rods leads to a series of practical disadvantages. Although having a shiny and smooth cylinder surface, cylindrical pellet rods usually have uneven, rough and splintered end surfaces. Irregular end surfaces constitute structural weaknesses which, when subjected to mechanical loads, easily cause splintering and fragmentation of the pellets and thus weaken the shape stability thereof. This form of pellet disintegration produces undesired dust and micro-splinters. Moreover, the lignin binding in such pellets may be poor in part, which further assists the pellet disintegration and the associated formation of dust and micro-splinters. Besides weakening the structural integrity of the rod pellets, the formation of dust and micro-splinters may also produce partially dangerous and/or unfavourable conditions in connection with the very firing process. Among other things, disintegration of pellets in connection with firing may restrain the combustion process by virtue of dust and micro- splinters obstructing, completely or partially, the supply of air to the combustion process. Dust and micro-splinters also increase the possibility of a dust explosion in the combustion furnace/stove and/or associated equipment. For this reason, the pellet industry, among others, warns against allowing fuel pellets engage with plastics and plastics objects, whereby static electricity may arise and cause a dust explosion.

A cylindrical pellet shape also restrains the pellet feeding into the combustion furnace/stove. For this reason, feeding chambers for such pellet combustion furnaces/stoves are generally provided with a feed screw or similar feeding device for force-feeding cylindrical rod pellets into the combustion furnace/stove. The disadvantage of this, however, is that the complexity and price of the combustion furnace/stove increases, which is a direct consequence of said pellet shape.

The aforementioned, known process for production of fuel pellets, but also the cylindrical rod shape of such pellets, is therefore associated with a series of substantial problems . The objects of the invention

The primary object of the invention is to provide technology for allowing a more varied and cost-efficient exploitation of alternate raw materials for production of solid fuel based on biological material, especially peat.

A further object of the invention is to avoid or substantially reduce the aforementioned disadvantages related to prior art for production of fuel pellets, but also disadvantages associated with the shape of such pellets.

How to achieve the objects

Said objects are achieved by means of features disclosed in the following description, the features of which the subsequent claims are based upon.

According to a first aspect of the invention, a method for production of peat-based fuel balls is provided, the method comprising:

- using wet peat from a bog deposit as a natural raw material in the method, such wet peat typically having a moisture content in the range of 80-95 percent by weight;

- converting the peat into ball-shaped or approximately ball- shaped units; and

- drying the ball-shaped units until having a remaining moisture content of maximum 12 percent by weight. The distinctive characteristic of the method is that it also comprises :

- mixing the wet peat together with at least one moisture- absorbing, biological fibrous material having a lower moisture content than the moisture content of the wet peat; and - adding such moisture-absorbing fibrous material until the mixture of peat and fibrous material assumes a production moisture in the range of 50-75 percent by weight.

The present fuel balls do not need to have a completely spherical shape. They may just as well have a somewhat irregular ball-shape, for example an oval ball-shape.

By using the present method, the aforementioned problems and disadvantages of the prior art may be avoided to a large extent .

Firstly, one avoids or substantially reduces the large dust problems and danger of ignition and fire that the aforementioned, known process for production of fuel pellets is encumbered with, which follows from grinding up and drying the wood in large drying ovens prior to pelletizing the ground up wood. In the present method, however, wet peat is mixed together with at least one moisture-absorbing, biological fibrous material being drier than the peat. Thereby, the peat is dehumidified in a cold state through moisture transmission onto the drier fibrous material. This is a considerably cheaper and safer manner of dehumidifying the primary raw material in the production process.

Secondly, one avoids said handling- and user-related disintegration that known, rod-shaped fuel pellets are encumbered with. This relates to the fact that fuel balls, which are produced in accordance with the present method, have a continuous, even and strong surface exhibiting a far greater structural integrity than that of cylindrical rod pellets. Moreover, the fuel ball is reinforced further by virtue of the fibrous material of the raw material mixture forming a reinforcing lattice within the ball. This comes in addition to the natural adhesive being exploited advantageously for binding the raw material mixture together in a ball. Said reinforcing lattice is of considerable importance both in the formation and drying of such fuel balls, but it also is of great significance in connection with the subsequent handling, transport and firing with the balls. The reinforcing effect is of particularly great importance in connection with the final drying of the balls, insofar as the fibrous reinforcement prevents cracking of the balls upon drying and shrinking somewhat. The natural reinforcing lattice of the additive fibres thus prevents disintegration of the fuel balls throughout their overall life.

Thirdly, the fuel balls provide the favourable effect of being easily fed from a feeding chamber and into a combustion furnace/stove by virtue of being self-rolling due to the shape thereof. Contrary to known pellet furnaces/stoves, the combustion furnace/stove does not need to be provided with a feeding device, for example a feed screw, in order to feed the solid fuel into the furnace/stove. A furnace/stove for the present fuel balls may therefore be of simpler and cheaper construction than that of a pellet furnace/stove.

Due to their ball-shape, the present fuel balls are also easier to dry than rod-shaped pellets. The reason for this is that the drying medium, normally consisting of air, flows more easily through a mass of balls than a mass of rod-shaped pellets.

Advantageously, the peat used in this connection is taken directly from a peat deposit. At that stage, the peat may have a moisture content of e.g. 93 percent by weight. Yet further, such wet peat may be stored in vicinity of the production site. Natural runoff from the peat may thus lower its moisture content to ca. 80-85 percent by weight.

Advantageously, the peat being used may be fuel peat having a degree of humification in the range of H6-H9 according to von Post's scale of humification for classification of peat, which is described above. Fuel balls based on such fuel peat have a heat value measured to ca. 20.2 MJ/kg. In comparison, known wood pellets typically have a heat value in the range of 14.4-18.8 MJ/kg.

According to a preferred embodiment of the method, said fibrous material is added until said peat mixture assumes a production moisture in the range of 60-65 percent by weight.

Yet further, ground-up wood having a moisture content of maximum 40 percent by weight may be used as said fibrous material. For example, easily accessible tree species growing in vicinity of the production site may thus be used, or tree species normally not used for production of wood pellets. Types of residual wood considered to be inferior for this purpose, for example bark and flitch/slab wood, may also be used for this purpose.

As an addition or alternative, ground-up cellulose having a moisture content of maximum 15 percent by weight may also be used as said fibrous material. For example, ground-up cellulose from paper and/or cardboard may be used, such as pure, recycled materials. As a further addition or alternative, other types of moisture-absorbing, biological fibrous materials suitable for this purpose may be used, including dried grass of the type Phalaris, also termed canary grass.

As said fibrous material, one may use, for example, ground-up wood having a moisture content of maximum 40 percent by weight, for instance in the range of 30-40 percent by weight, and also ground-up cellulose having a moisture content of maximum 15 percent by weight, for instance in the range of 2- 10 percent by weight. These additives are used in a peat mixture which may comprise 50-70 percent by weight of wet peat; 10-30 percent by weight of ground-up wood; and 10-30 percent by weight of ground-up cellulose. Preferably, it is used 55-65 percent by weight of wet peat; fS-JS' percent by weight of ground-up wood; and IS-JS percent by weight of ground-up cellulose.

For example, one may thus use ca. 60 percent by weight of wet peat having a degree of humification in the range of H6-H9; ca. 20 percent by weight of ground-up wood chips of birch; and ca. 20 percent by weight of ground-up paper and/or cardboard.

In a preferred embodiment, the method may also comprise the following steps:

- extruding the peat mixture through an apertured plate so as to form a peat mixture string;

- cutting said peat mixture string into units; and

- conveying the units through a peripherally rotating tube so as to form ball-shaped units of the peat mixture. Advantageously, the peripherally rotating tube may also be pivoted back and forth about a pivot axis being transverse to the tube, so-called double rotation. This type of double rotation implies that the ends of the tube are moved alternately up and down simultaneous with the tube rotating about its longitudinal axis. Thereby, said peat mixture units are kneaded until becoming ball-shaped or approximately ball- shaped.

Although any suitable size may be used, said peat mixture string advantageously may be cut into units having a transverse dimension and length in the range of 5-15 mm, and preferably in the range of 8-10 mm. Yet further, it is of advantage for the transverse dimension and the length of the unit to be approximately equal, which facilitates the kneading of the peat mixture units in the rotating tube.

Furthermore, the peat mixture may be extruded through an apertured plate having circular apertures . The peat mixture string, and thereby units thereof, thus assume a circular cross-section, which further facilitates the conversion of the peat mixture units into ball-shaped units.

In the final drying process, the ball-shaped units of the peat mixture advantageously are dried in a drying silo, for example a drying silo of the same type used for drying grains. In this connection, fuel balls having a diameter of 8-10 mm have proven most favourable for achieving fast and cost-efficient drying thereof. Under favourable weather conditions, natural air may be used in the drying process, but supply of warm air may also be required under less favourable conditions. Preferably, such ball-shaped units of the peat mixture are dried until having a remaining moisture content of maximum 10 percent by weight.

According to a second aspect, the invention also comprises use of moisture-absorbing, biological fibrous material as an additive in wet peat for production of fuel balls from a mixture of said materials. As mentioned above, the fibrous material acts as an integrity-enhancing reinforcement material in such fuel balls.

Claims

C l a i m s

1. A method for production of peat-based fuel balls, the method comprising:

- converting the peat into ball-shaped or approximately ball-shaped units; and

- drying the ball-shaped units until having a remaining moisture content of maximum 12 percent by weight, c ha r a c t e r i z e d i n that the method also comprises :

- mixing the wet peat together with at least one moisture-absorbing, biological fibrous material having a lower moisture content than the moisture content of the wet peat; and

- adding such moisture-absorbing fibrous material until the mixture of peat and fibrous material assumes a production ,moisture in the range of 50-75 percent by weight .

2. The method according to claim 1, c ha r a c t e r i z e d i n using peat having a degree of humification of H6-H9 according to von Post's scale of humification.

3. The method according to claim 1 or 2 , c h a r a c t e r i z e d i n adding said fibrous material until said peat mixture assumes a production moisture in the range of 60-65 percent by weight.

4. The method according to claim 1, 2 or 3, c ha r a c t e r i z e d i n using, as said fibrous material, ground-up wood having a moisture content of maximum 40 percent by weight.

5. The method according to any one of claims 1-4, c ha r a c t e r i z e d i n using, as said fibrous material, ground-up cellulose having a moisture content of maximum 15 percent by weight.

6. The method according to claim 5, c ha r a c t e r i z e d i n using ground-up cellulose from at least one of the materials paper and cardboard.

7. The method according to any one of claims 1-6, c ha r a c t e r i z e d i n using, as said fibrous material, dried grass of the type Phalaris, also termed canary grass .

8. The method according to claim 1, 2 or 3 , c ha r a c t e r i z e d i n using, as said fibrous material, ground-up wood having a moisture content of maximum 40 percent by weight, and also ground-up cellulose having a moisture content of maximum 15 percent by weight; and

- using, in said peat mixture, 50-70 percent by weight of wet peat; 10-30 percent by weight of ground-up wood; and 10-30 percent by weight of ground-up cellulose.

9. The method according to claim 8, c h a r a c t e r i z e d i n using, in the peat mixture, 55-65 percent by weight of wet peat; 15-25 percent by weight of ground-up wood; and 15-25 percent by- weight of ground-up cellulose.

10. The method according to any one of claims 1-9, c h a r a c t e r i z e d i n that the method also comprises:

- cutting said peat mixture string into units; and

- conveying the units through a peripherally rotating tube so as to form ball-shaped units of the peat mixture.

11. The method according to claim 10, c h a r a c t e r i z e d i n also pivoting the peripherally rotating tube back and forth about a pivot axis being transverse to the tube, so-called double rotation.

12. The method according to claim 10 or 11, ch a r a c t e r i z e d i n cutting said peat mixture string into units having a transverse dimension and length in the range of 5-15 mm.

13. The method according to claim 12, c h a r a c t e r i z e d i n cutting the peat mixture string into units having a transverse dimension and length in the range of 8-10 mm.

14. The method according to any one of claims 10-13, c h a r a c t e r i z e d i n extruding the peat mixture through an apertured plate having circular apertures, whereby said peat mixture string, and thereby units thereof, assume a circular cross-section.

15. The method according to any one of claims 1-14, cha ra ct e ri z e d in drying the ball-shaped units of the peat mixture in a drying silo.

16. The method according to any one of claims 1-15, cha rac t e r i z e d in drying the ball-shaped units of the peat mixture until having a remaining moisture content of maximum 10.percent by weight.

17. Use of moisture-absorbing, biological fibrous material as an additive in wet peat for production of fuel balls from a mixture of said materials, the fibrous material acting as an integrity-enhancing reinforcement material in such fuel balls.