Abstract:
A method for refining pellets, in which, in a first step, pellets that have been pressed from a biomass are provided. A heat treatment is carried out, involving heating the pellets to a temperature between 210° C. and 390° C. The heat treatment takes a period of time of between 1 minute and 30 minutes. Also described is a device that is suitable for carrying out the method. The pellets treated in this way are water-repellent and so can be stored outdoors.

Description:
BACKGROUND 
       [0001]    The invention relates to a method for refining pellets. The method is used in the case of pellets which have been pressed from biomass. The invention also relates to a device suitable for carrying out the method. 
         [0002]    It is widespread practice to press biomass to form pellets. The pellets can be used, for example, as fuel—in particular in the form of cofiring in power stations—in order to generate heat or electrical energy. However, the pellets produced from biomass have the property of losing their shape under the influence of moisture and of rapidly rotting. It is therefore not possible to store the pellets outdoors. This is unfavorable for use in power stations since it is generally customary there to store the fuels outdoors. 
       SUMMARY 
       [0003]    A method and a device with which the storability of pellets can be improved is presented. 
         [0004]    In the method, the pellets are subjected to a heat treatment. In the process, the pellets are heated to a temperature of between 210° C. and 390° C. The heat treatment extends over a period of time of between 1 min and 30 min. 
         [0005]    A few terms will be explained first. Pellets pressed from biomass are spoken about if the starting material from which the pellets are pressed is predominantly biomass. This does not exclude other materials, such as, for example, refuse, being added to the starting material. The proportion of said other materials should be smaller than 30%. After the pressing operation, the pellets have a greater density than the starting material. The term biomass comprises, for example, all herbaceous and ligneous renewable raw materials and mixtures thereof. The pellets can consist entirely of biomass. 
         [0006]    The biomass from which the pellets are pressed can be, for example, wood, the moisture content of which is typically between 30% and 60% in the starting state. Another example of biomass is straw with a moisture content of between 15% and 20% in the starting state. Overall, the original moisture content of the biomass can therefore lie, for example, within the range of between 15% and 60%. 
         [0007]    Said starting material has to be dried according to the type of biomass before pelletization—customarily to 6% to 12% in the case of wood and 10% to 20% in the case of straw. A further portion of the moisture is lost from the material by the pelletization, and therefore the moisture content is lower in the pellets pressed from biomass. In the case of pellets made from straw, the moisture content can be, for example, at 8%, and at 9% in the case of pellets made from bamboo wood and at 11% in the case of pellets made from prairie grass. 
         [0008]    Stated more generally, the moisture content in the pellets can be, for example, between 6% and 12% before the treatment. The moisture content is indicated in each case in percent by weight. These details make it clear that the biomass pellets which are the subject matter of the invention differ significantly from carbon pellets as are described, for example, in U.S. Pat. No. 4,412,840, FR1187244 and GB1010452. Carbon pellets have nothing to do with the invention. 
         [0009]    The heat treatment takes place within a temperature range which is higher than the temperature at which pellets are dried and which is lower than the temperature at which pellets are burned. In the temperature range, processes which are not known in every detail are set into motion in the pellets. Said processes probably include a pyrolytic breakdown of inorganic compounds and escape of various volatile components. Tests have shown that, as a result of the treatment, pellets which have highly water-repelling properties are obtained. The pellets are therefore refined in such a manner that they can be stored outdoors even over a relatively long period of time. Tests have shown that, in addition, the energy content of the pellets (MJ/kg) is increased. 
         [0010]    If heat acts on the pellets, a temperature gradient can be set within the pellets because a certain amount of time may elapse before the heat has penetrated into the center of the pellets. The temperature specification relates to the surface of the pellets. 
         [0011]    The fact that changes may occur in the biomass within the temperature range lying between drying and burning does not constitute a new discovery as such. For example, it is known that biomass chopped into small pieces (chips) and subjected to a heat treatment within the corresponding temperature range will undergo a considerable change to the internal structure thereof. The corresponding process is also referred to as torrefaction. 
         [0012]    However, chips are not directly comparable in terms of the properties thereof with pellets, since, inter alia, the density of the chips is significantly lower than the density of the pellets. In the case of pellets, it was not to be expected that the structural change would extend into the center of the pellets. It would be unfavorable for the storability of the pellets if only the surface of the pellets obtained a water-repelling structure while the components in the center of the pellets remain substantially unchanged. 
         [0013]    The heat can be applied to the pellets by a suitably temperature-controlled air stream. This has the advantage that the volatile components emerging from the pellets are directly entrained by the air stream. 
         [0014]    The power of the air stream is preferably such that the pellets are set into motion by the air stream. This promotes the admission of heat into the pellets. In addition, by means of the constant impacts, volatile components can more easily escape from the pellets. 
         [0015]    For a particularly intensive transmission of heat and thorough mixing, the air stream can be set in such a manner that the pellets are carried upward by the air stream and are then deflected together with the air stream to the side. The pellets then drop downward out of the deflected air stream. An inclined surface via which the pellets slide back again to the starting point can be arranged there. Tests have shown that the pellets can withstand even this powerful movement and the intensive impacts without breaking. This stability of the pellets results from the fact that components having considerable binding forces are contained in the biomass from which the pellets have been pressed. These components include those which are dissolved by the heat treatment. In this respect, it is advantageous for the method that the pelletization has already taken place before the heat treatment. 
         [0016]    It is not ruled out that the treatment of the pellets takes place continuously, for example by the pellets being continuously transported further during the heat treatment. This can take place, for example, by means of a device in which a drum rotates about a substantially horizontal axis. The pellets are introduced into the drum at one end side and move through the drum to the opposite end side where they emerge again from the drum. The circumferential surface of the drum can be provided with inwardly pointing blades with which the pellets are conveyed forward by rotation of the drum. In addition or alternatively to the blades, a worm can also be provided in order to transport the pellets. 
         [0017]    For the heat treatment, a suitably temperature-controlled air stream can be conducted through the drum, said air stream preferably likewise moving from the one end surface to the opposite end surface. The air stream can move through the drum in the same direction as, or in the opposite direction to, the pellets. The air stream can also be used in order to heat the drum itself. For example, the air stream can first of all be conducted past the drum on the outside and then conducted through the drum. For this purpose, the drum can be surrounded by a casing. Such a device can be used in order to carry out the method. 
         [0018]    If the treatment takes place batchwise, it is simpler to separate the chamber in which the treatment takes place from the surroundings. This is desirable because the materials emerging from the pellets are, under some circumstances, combustible and may constitute a safety risk if they pass into the surroundings. 
         [0019]    In the case of batchwise treatment, the quantity of air with which the treatment of the pellets takes place can easily be controlled. This variable can be used in order to adjust the process. The volume of the air stream is preferably at 6 m 3  to 20 m 3  per kilogram of pellets. This specification relates to the entire quantity of air which is supplied to the pellets during the heat treatment. 
         [0020]    There are other possibilities of transmitting heat to the pellets. For example, the pellets can be brought into contact with a warm object or can be placed onto a warm surface. In comparison thereto, a very intensive transmission of heat to the pellets takes place by means of an air stream. The transmission of heat is preferably configured in such a manner that a treatment duration of between 3 min and 16 min is sufficient. 
         [0021]    In general, the structure of the pellets changes more rapidly, the higher the temperature is. However, at very high temperatures, the risk is also greater that the change in the structure will not take place uniformly but rather that undesirable burning will already take place at the surface whereas the structure has not yet changed at all in the center. Good results both in respect of the duration of the heat treatment and in respect of the uniform structure of the pellets are obtained within the temperature range of between 250° C. and 300° C. In addition, it can be of advantage for the reactions in the pellets if the heat treatment is carried out in an atmosphere with a reduced content of oxygen. The content of oxygen can be, for example, smaller than 10%, preferably smaller than 6%, with reference to the volume. 
         [0022]    In tests, it has proven advantageous if the pellets have a diameter of between 4 mm and 20 mm. Before the method is carried out, the pellets preferably have a moisture content of 5% to 15%, with reference to the weight. 
         [0023]    When the heat treatment of the pellets is finished, a quantity of heat is stored in the pellets. It is possible that, fed by said quantity of heat, the reactions will continue in the interior of the pellets, even though no more heat is fed in from the outside. This is undesirable because the heating power of the pellets may be lost as a result. It is therefore advantageous if the pellets are quenched directly after the heat treatment. For the quenching, use can be made, for example, of water which is preferably not warmer than room temperature. 
         [0024]    The pellets obtained by the method can be used for what is referred to as cofiring in power stations. The pellets are therefore burned as an accompaniment to the actual fuel, for example coal, in the power station. For this purpose, the pellets have to be pulverized such that, for example, 95% by weight of the particles have a fineness of less than 2 mm. It has been shown that the method results in the particles having a significantly increased brittleness. The pellets can therefore be ground with a substantially smaller consumption of energy than classic pellets. In addition, the mills customarily present in power stations can be used for the grinding, which has not been possible in the case of the classic pellets made from untreated biomass. Also for this reason, the pellets produced are better suited for a cofiring use than classic pellets. In addition, the energy content of the pellets is increased by the treatment. 
         [0025]    The invention also relates to a device for refining pellets. The device comprises a treatment chamber with a supporting floor, a first side wall and a second side wall. The supporting floor is provided with a plurality of apertures. The first side wall and the second side wall are adjacent to the supporting floor on opposite sides. The second side wall has a chute which leads onto the supporting floor. A channel for an air stream is provided, said channel leading below the supporting floor. The air stream can be heated and conducted from below through the apertures by means of a heating device and a driving device. A deflecting surface is arranged above the supporting floor such that the air stream and the pellets entrained by the air stream are deflected in a direction of movement which points from the first side wall in the direction of the second side wall. A baffle plate is arranged between the first side wall and the second side wall such that the pellets move upward between the first side wall and the baffle plate. 
         [0026]    In this device, the batch to be treated is placed onto the supporting floor and an air stream is conducted through the apertures in the supporting floor, the power of the air stream being such that the air stream carries the pellets upward. In the process, the pellets move substantially parallel to the first side wall of the treatment chamber. The air stream is deflected by the deflecting surface in such a manner that said air stream moves together with the pellets in the direction of the second side wall. The pellets drop out of the air stream, strike against the chute of the second side wall and move back on the chute in the direction of the supporting floor. 
         [0027]    It is of advantage for the movement of the air stream and the pellets if the region in which the movement upward takes place is separate from the region in which the pellets move downward. A baffle plate between the first side wall and the second side wall is therefore proposed, said baffle plate being arranged in such a manner that the pellets primarily move upward between the baffle plate and the first side wall. The pellets are only slightly damaged by collisions with the baffle plate. The pellets are therefore sufficiently robust that the use of a baffle plate is possible. 
         [0028]    The pellets can cross the plane of the baffle plate above and below the baffle plate. For this purpose, the baffle plate is preferably designed in such a manner that there is a distance in each case between the supporting floor and the lower end of the baffle plate and between the upper end of the baffle plate and the upper end of the deflecting surface. By contrast, the baffle plate can extend in width substantially beyond the entire treatment chamber. In an advantageous embodiment, the baffle plate is arranged substantially parallel to the first side wall. 
         [0029]    In order to be able to adjust the flow conditions in the treatment chamber, the distance between the baffle plate and the first side wall can be adjustable. If the baffle plate is arranged above the supporting floor, the air stream is preferably conducted in such a manner that the pellets primarily move upward between the baffle plate and the first side wall. The supporting floor can therefore be designed in such a manner that the apertures in the vicinity of the first side wall are of larger dimensions than in the vicinity of the second side wall. A powerful air stream then arises precisely in the region in which the pellets are intended to be moved upward. 
         [0030]    The supporting floor can be arranged in an inclined manner. This has the advantage that the pellets move out of the device under the influence of gravitational force when an opening is opened up next to the lower end of the supporting floor. The device can thereby be easily emptied. The chute of the second side wall is preferably adjacent to the upper end of the inclined supporting floor. The chute may have a greater slope than the supporting floor. 
         [0031]    Under some circumstances, a device can also be used without such a baffle plate for carrying out the method. In an advantageous embodiment, the supporting floor is arranged in an inclined manner. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]    The invention is described by way of example below using advantageous embodiments with reference to the appended drawings, in which: 
           [0033]      FIG. 1  shows a first embodiment of a device for carrying out the method; 
           [0034]      FIG. 2  shows a detail from  FIG. 1  in an enlarged illustration; 
           [0035]      FIG. 3  shows a second embodiment of a device for carrying out the method; 
           [0036]      FIG. 4  shows a device; and 
           [0037]      FIG. 5  shows a depiction of comparison tests. 
       
    
    
     DETAILED DESCRIPTION 
       [0038]    A device shown in  FIG. 1  comprises a treatment chamber  14  which is designed for heat treatment of the pellets. As the enlarged illustration in  FIG. 2  shows, the lower end of the treatment chamber  14  comprises an inclined supporting floor  15  which is provided with a multiplicity of apertures  16 . A distribution space  17 , to which an air stream can be supplied via a feed line  18 , is formed below the supporting floor  15 . The air stream can be distributed in the distribution space  17  and can pass from below through the apertures  16  in the supporting floor  15 . 
         [0039]    A fan  19  and a burner  20  are furthermore arranged in the feed line  18 . Fresh air supplied via an inlet  21  is heated by the burner  20  to a temperature of approximately 300° C. The air stream is accelerated by the fan  19  in the direction of the distribution space  17 . The air stream passes through the apertures  16  in the supporting floor  15  into the treatment chamber  14  and first of all moves substantially vertically upward there. At the upper end of the treatment chamber  14 , the air stream strikes against a deflecting surface  22 , by means of which the air stream is deflected to the left in a lateral direction. The air stream is deflected again on the opposite wall, and therefore the air stream moves upward and is output to the surroundings through an outlet  23 . The wall at which the air stream is deflected upward is designed as a chute  24  which, coming from the left, leads onto the supporting floor  15 . 
         [0040]    In addition, the device comprises a storage container  25  which is connected to the upper end of the treatment chamber  14  via a sluice  26 . The lower end of the treatment chamber  14  is provided with a sluice  27 , via which material lying on the supporting floor  15  can be removed from the treatment chamber  14 . 
         [0041]    The storage container  25  is filled with pellets which have been pressed from biomass as the starting material. The starting material can be, for example, herbaceous or ligneous renewable raw materials or mixtures of said materials. In addition, the pellets can contain a proportion of up to 30% of non-renewable materials, such as, for example, refuse. The pellets can have, for example, a diameter of 6 mm and a length of few centimeters. 
         [0042]    A batch of pellets is admitted to the treatment chamber  14  via the sluice  26 . In the treatment chamber  14 , the pellets drop downward and accumulate on the supporting floor  15 . The burner  20  and the fan  19  heat the air stream to a temperature of approximately 300° C. and conduct said air stream into the distribution chamber  17  and subsequently through the supporting floor  15 . 
         [0043]    The power of the air stream is such that the air stream raises the pellets from the supporting floor  15  and carries said pellets upward. The air stream together with the pellets is deflected in the lateral direction on the deflecting surface  22 . The pellets drop out of the air stream because of gravity and land on the chute  24 , via which the pellets pass back to the supporting floor  15 . The pellets therefore follow a circuit in which they are repeatedly lifted upward by the air stream and then pass back via the chute  24  onto the supporting floor. 
         [0044]    This heat treatment is carried out for a period of time of approximately 8 min and results in volatile materials emerging from the pellets and in various long-chain compounds being broken up. The pellets thereby become hydrophobic and obtain a brittle internal structure. During the heat treatment, a quantity of air of, in total, approximately 8 m 3  per kilogram of pellets is conducted through the treatment chamber  14 . 
         [0045]    After the end of the heat treatment, the sluice  27  is opened and the pellets emerge from the treatment chamber  14  through the sluice  27 . Immediately after the exit therefrom, the pellets are treated with water which is approximately at room temperature, and therefore the pellets are quenched and the reactions taking place in the pellets are abruptly interrupted. 
         [0046]    According to  FIG. 4 , the device can comprise a baffle plate  31  arranged in the treatment chamber  14 . The baffle plate  31  extends parallel to a first side wall  32  of the treatment chamber  14 , the side wall adjoining the lower end of the inclined supporting floor  15 . A clearance remains above and below the baffle plate  31  such that the pellets can cross the plane of the baffle plate  31 . In the other dimension, the baffle plate  31  extends over the entire width of the treatment chamber  14 . The baffle plate  31  is suspended movably in the treatment chamber  14 , and therefore the distance between the first side wall  32  and the baffle plate  31  can be adjusted in order to achieve optimum flow conditions in the treatment chamber  14 . The apertures  16  in the supporting floor  15  are designed in such a manner that a very powerful air stream upward is produced in the region between the baffle plate  31  and the first wall  32 , the air stream carrying the pellets therewith. In the region between the baffle plate  31  and the opposite side wall  32 , on which the chute  24  is formed, the upwardly directed air stream is weaker. The air stream serves here primarily in order to convey the movement of the pellets in the direction of the first side wall  32  such that the pellets can be picked up there by the powerful air stream. 
         [0047]    In the embodiment according to  FIG. 3 , a return line  28  which leads back via a separator  29  to the burner  20  is connected to the outlet  23  of the treatment chamber  14 . In the separator  29 , solid components are separated out from the air stream and accumulated on the floor of the separator  29 . The solid components can be removed at regular intervals via a star wheel  30 . 
         [0048]    An air stream which is freed from the solid components, but in which gaseous components which have been released from the pellets are still contained is therefore guided back to the burner  20 . Said gaseous components can be partially burned and can serve as fuel for the burner  20 . A closed circuit managing without the supply of fresh air and without the supply of new fuel is ideally produced. The content of oxygen in the air stream is optionally reduced, and this may be of advantage for the reactions in the pellets. 
         [0049]    The pellets refined in this manner are hydrophobic and can therefore be stored outdoors even for a relatively long period of time. This and also the high degree of brittleness of the material, enabling the pellets to be easily ground, result in the pellets being highly suitable for processing in the form of cofiring in power stations. In addition, the energy content of the pellets is increased by the treatment. 
         [0050]    The success of the method has been confirmed in series of tests. For example, customary wood pellets with a diameter of 6 mm of certified DinPlus quality with 9% moisture were introduced into the device described above. Said pellets were treated in the device at between 240° C. and 320° C. for between 3 and 21 minutes. The pellets treated in this manner showed a characteristic increase in burning power of, for example, (roughly) 18.5 MJ/kg to 21 MJ/kg. Even after storage in water for 3 days, the pellets were stable. The pellets had a high uniformity of browning even within the pellets, which makes it possible to conclude that the intensity of the treatment is very uniform. 
         [0051]    Further tests were carried out with other wood pellets (diameter 8 mm, quality EN14961-2-B), pellets made from bamboo wood (9% moisture), pellets made from straw (8% moisture) and pellets made from prairie grass (11% moisture). In all of the tests, the characteristic increase in the burning power was demonstrated after the treatment.  FIG. 5  depicts a comparison in each case of the burning power in the unit MJ/kg, wherein the light bar represents the burning power in the starting state of the pellets and the dark bar represents the burning power after carrying out the method according to the invention. The left bars relate here to straw, the second bars to prairie grass, the third bars to wood and the bars completely on the right to bamboo.