NÄSLUND, Bengt.Olof (Umedalsallén 31, Umeå, S-903 64, SE)
ATTERHEM, Lars (Långnäset 18, Burträsk, S-937 93, SE)
NÄSLUND, Bengt.Olof (Umedalsallén 31, Umeå, S-903 64, SE)
| Claims 1. Pneumatic steam dryer (1.1 ; 1.2; 1.3; 1.4) for producing fuel pellets, said pneumatic steam dryer being adapted to operate above atmospheric pressure, comprising -a circulation conduit system which in operation of the pneumatic steam dryer (1.1 ; 1 .2; 1.3; 1.4) is pressurized and filled with steam, -a heat exchanger (6,6') connected into said circulation conduit system for heating said steam, -an inlet feeding device (8) connected to said circulation conduit system for feeding bulk material into said circulation conduit system, -a drying conduit (1 1) connected into said circulation conduit system wherein said bulk material is dried by heat transfer from said steam thereby forming surplus steam, - separating means (12; 12') connected to said drying conduit (11) for separating said bulk material from said steam, said separating means (1 1) comprising a steam outlet (14) and a bulk material outlet ( 5; 15'), -a fan (5) connected into said circulation conduit system for pneumatically conveying said bulk material suspended in said steam from said inlet feeding device (8) to said separating means (12;12'), characterized in that a pelleting device (16.1 ; 16.2) is integrated in the circulation conduit system, where said pelleting device (16.1 ;16.2) is connected to said bulk material outlet (15;15') of the separating means (12; 12'), and the total mass flow of bulk material is conveyed from the separating means (12; 12') to the pelleting device (16.1 ;16.2). 2. A pneumatic steam dryer according to claim 1 wherein the pelleting device comprises a pellet press (16.1 ; 16.2). 3. A pneumatic steam dryer according to claim 1 or 2 wherein the pelleting device comprises an extrusion pellet press (16.2). 4. A pneumatic steam dryer according to any of claims 1-3 wherein the pellet press comprises a ring-and-roll pellet press (16.1). 5. A pneumatic steam dryer according to any of claims 1-3 wherein the pellet press comprises a screw type extruder (16.2). 6. A pneumatic steam dryer according to any of the claims 1-5 comprising a mill (21) connected into the circulation conduit system. 7. A pneumatic steam dryer according to any of the claims 1-6 wherein the mill (21) is connected between the bulk material outlet (15; 15') and the pelleting device (16). 8. A pneumatic steam dryer according to any of the claims 1-7 wherein said separating means comprises at least two cyclones (12') connected in parallel into said circulation conduit system. 9. Process for producing fuel pellets in a pneumatic steam dryer (1.1 ; 1.2; 1.3; 1.4) according to one or more of the claims 1 -8 comprising -feeding bulk material by an inlet feeding (8) device into a steam filled circulation conduit system of a pneumatic steam dryer operating above atmospheric pressure, -heating said steam by heating means (6;6'), -drying said bulk material in a drying conduit (1 1) by heat transfer from said steam thereby forming surplus steam, -pneumatically conveying said bulk material suspended in said steam through said drying conduit (1 1) from said inlet feeding device (8) into said separating means (12;12'), -separating said bulk material from said steam by a separating means, said process characterized i n -conveying the total mass flow of bulk material from the separating means (12; 12') to a pelleting device (16) and - pelletizing said bulk material into pellets by the pelleting device (16). 10. A process according to claim 9 wherein said pelletizing is carried out by extrusion. 1 1. A process according to claim 9 or 10 comprising comminuting said bulk material by a mill (21 ) in advance of said pelletizing. |
Field of the invention The present invention relates to a pneumatic steam dryer for producing bio fuel pellets. The invention also relates to a process for producing bio fuel pellets in a pneumatic steam dryer.
Background of the invention Drying of bulk material of wooden or peat origin in a pneumatic dryer is previously known. Wet bulk material is fed into a drying conduit of a pneumatic dryer and dried by heated gas. The dried bulk material is separated from the gas by separating means and discharged from the pneumatic dryer. The drying process is very energy consuming and is therefore preferably carried in a pneumatic dryer comprising a closed system. The bulk material is typically used as fuel in different industrial, commercial or domestic applications. The dried bulk material is cooled down after the drying process and transported to a separate factory where the bulk material is further treated, for example pelletized. To prepare the dried bulk material for pelleting the material is first conditioned, heated, to about 80 °C in advance of the pelleting operation. This is very energy demanding. Moreover is the dried bulk material easily ignited and it is therefore necessary to handle such material carefully.
Summary of the invention
It is an object of the present invention to provide an energy efficient and safe pneumatic steam dryer for producing fuel pellets.
This object and other objects are met by the invention as defined in the independent claims. Example embodiments of the invention are defined in the dependent claims. In addition the present invention has other advantages and features apparent from the description below.
According to a first aspect of the present invention a pneumatic steam dryer for producing bio fuel pellets is provided, said pneumatic steam dryer being adapted to operate above atmospheric pressure. The pneumatic steam dryer comprises a circulation conduit system which in operation of the pneumatic steam dryer is filled with steam, a heat exchanger connected into said circulation conduit system for heating said steam, an inlet feeding device connected to said circulation conduit for feeding bulk material into said circulation conduit system and a drying conduit connected into said circulation conduit system wherein said bulk material is dried by heat transfer from said steam thereby forming surplus steam and a fan connected into said circulation conduit system for pneumatically conveying said bulk material suspended in said steam from said inlet feeding device to the separating means.
The pneumatic steam dryer comprises further a separating means connected to said drying conduit for separating said bulk material from said steam where said separating means comprises a steam outlet and a bulk material outlet.
The pneumatic steam dryer comprises further a pelleting device integrated in the circulation conduit system, wherein said pelleting device is connected to said bulk material outlet of the separating means, and the total mass flow of bulk material is conveyed from the separating means to the pelleting device.
According to a second aspect of the present invention a process is provided for producing bio fuel pellets in a pneumatic steam dryer according to the invention. The process comprises the steps of feeding bulk material by an inlet feeding device into a steam filled circulation conduit system of a pneumatic steam dryer operating above atmospheric pressure and heating said steam by a heat exchanger. Furthermore the process comprises drying said bulk material in a drying conduit by heat transfer from said steam thereby forming surplus steam and pneumatically conveying said bulk material suspended in said steam through said drying conduit from said inlet feeding device into said separating means and separating said bulk material from said steam by a separating means. The process further comprises conveying the total mass flow of bulk material from the separating means to a pelleting device and pelletizing said bulk material into bio fuel pellets by the pelleting device. The present invention thus provides a pneumatic steam dryer as well as a process that enables pelletizing of dried bulk material into fuel pellets in a substantially oxygen free, inert safe environment.
A further advantage of integrating the pelleting device into the steam filled pressurized circulation conduit system is that the temperature of the dried bulk material is elevated after the drying process and is approximately above 140 °C at the bulk material outlet of the cyclone. This is beneficial since the lignin in bulk material originating from wooden material is softened. The softened lignin has a glueing effect on the particles and binds the particles together in the forming and pressing operation in the pelleting device. The pelleting operation is therefore easier and less energy consuming. Thus no other means for heat conditioning of the dried bulk material in advance of the pelleting device is necessary which improves the energy efficiency of the process for producing bio fuel pellets These and other advantages are further described below and defined in the appended dependent claims. The present invention relates in particular to a pneumatic steam dryer for producing bio fuel pellets of preferably biomass based bulk materials, more preferably wood and peat based bulk materials. These bulk materials usually have an initial moisture content of at least 40% moisture on a weight basis, typically above 50% moisture. Biomass based bulk material has high energy content and is particularly suitable for production of bio fuel pellets. The bulk material is preferably dried to moisture content below 12 % by weight.
Brief description of figures
Embodiments will now be described, by way of example, with reference to the accompanying figures in which
Figure 1 shows a flow scheme of a first embodiment of the pneumatic steam dryer 1.1 according to the present invention.
Figure 2 shows a flow scheme of a second embodiment of the pneumatic steam dryer 1.2 according to the present invention.
Figure 3 shows a flow scheme of a third embodiment of the pneumatic steam dryer .3 according to the present invention.
Figure 4 shows a flow scheme of a fourth embodiment of the pneumatic steam dryer 1.4 according to the present invention.
Detailed description of preferred embodiments. With reference to Fig. 1 a first embodiment of a pneumatic steam dryer 1.1 for producing bio fuel pellets according to the present invention is shown.
The pneumatic steam dryer 1 comprises a drying section 2, a heating section 3 connected to each other to form a circulation conduit system. The circulation conduit system further comprises a pelleting section 4 connected to the drying section 2 via separating means 12.
The pneumatic steam dryer 1.1 and the circulation conduit system are adapted to operate above atmospheric pressure. In operation of the pneumatic steam dryer is the circulation conduit system pressurized and filled with steam such as superheated or saturated steam, thereby forming a substantially oxygen free, inert, internal environment which prevents hazardous dust explosions. The oxygen content in the steam is preferably below 12%, more preferably 5-10%. The steam is used as drying medium for drying the bulk material as well as transport medium of the bulk material for transporting the material through the circulation conduit system. The circulation conduit system further comprises ducts for conducting bulk material and steam between the elements of the pneumatic steam dryer. The heating section 3 comprises a fan 5, a heat exchanger 6 and a surplus steam outlet 7. The fan 5 is connected into the circulation conduit system for pneumatically conveying said bulk material suspended in said steam through the circulation conduit system from an inlet feeding device 8 into the separating means 9. The fan 5 comprises a blower or an ejector. A heat exchanger 6 is connected into the circulation conduit system for heating said steam. The heat exchanger 6 is preferably connected between the fan 5 and the inlet feeding device 8 of the drying section 2, thus only steam is heated by the heat exchanger. This has the functional benefit that deposits of particles of bulk material on the heat conducting surfaces of the heat exchanger are avoided. The heat exchanger 6 superheats the steam in the drying section 3 to a temperature above 200 °C. The heat exchanger 6 is supplied with a primary heating medium 9 such as flue gas, pressurized steam, hot water, thermal oil or similar from an external heat source. In particular industrial processes using steam, for example a combined heat and power plant, a paper mill, a steam generation plant or similar are used for supplying heat to the pneumatic steam dryer 1.1.
Optionally, an additional heat exchanger 6' may be connected into the drying section 2 to heat the steam after the bulk material has been fed into the circulation conduit system.
Furthermore, the heating section 3 also comprises a surplus steam outlet 7 connected to the circulation conduit system for releasing surplus steam from the circulation conduit system. Operating conditions such as the pressure and/or temperature of the pneumatic steam dryer are thereby controlled and maintained. The surplus steam is fed to energy recovery means 10 such as a steam regenerator as part of an energy recovery process. In the energy recovery means 10 is the surplus steam cooled and depleted of energy, whereby clean steam and waste condensate is generated.
The drying section 2 comprises an inlet feeding device 8, a drying conduit 1 1 and separating means 12. The inlet feeding device 8 is connected to the circulation conduit for feeding wet bulk material 13 continuously or intermittently into a flow of steam inside said circulation conduit system.
The wet bulk material 13 fed to the pneumatic steam dryer 1.1 by the inlet feeding device comprises wood based particles. The particles are typically sized < 25 mm having a size distribution and particle density suitable for pneumatic transport.
The size distribution of the particles after drying is typically the same as before drying. The inlet feeding device 8 comprises typically a cellular feeder or a plug screw feeder or a similar feeding device which is adapted to avoid pressure loss from the circulation conduit and only provides for a marginal steam leakage at the actual differential pressure between the outside pressure (typically atmospheric) of the pneumatic steam dryer and the pressure inside the circulation conduit.
A drying conduit 1 1 is connected into said circulation conduit system between the inlet feeding device and the separating means. The drying conduit 11 typically comprises an elongated conduit consisting of a series of vertical pipes which are connected together in the upper and lower ends by bent pipes. The bulk material is dried in the drying conduit by heat transfer from the superheated steam while being pneumatically transported in a fluidized state to the separating means. The residence time of the bulk material in the drying section is very short, approximately less than 2 minutes. Moisture and volatiles such as hydrocarbons and terpens in the wet bulk material evaporates and forms surplus steam. When moisture evaporates from the bulk material the steam temperature decreases. Consequently the moisture content of the bulk material is reduced.
Separating means 12 are connected to the drying conduit 1 1 to receive all bulk material and to separate the bulk material from the steam, the separating means 12 comprising a steam outlet 14 and a bulk material outlet 15.
The separating means 12 comprises at least one cyclone which separates the dried bulk material from said steam in one single step. The steam outlet 14 is connected to the heating section and the steam is returned to the heating section 3 for heating. The pneumatic steam dryer 1 .1 further comprises a pelleting section 4. The pelleting section is connected to the bulk material outlet 15 of the separating means 12. The bulk material is typically fed to the pelleting section 4 by gravity. It may alternatively include transport by conveyors or similar, short term storage in a hopper or vessel and final feeding to a pelleting device 16. The pelleting section comprises a pelleting device 16.
The rolling body or screw cooperates with the perforated matrix surface. The dried bulk material is fed into a compression zone where the material is formed into pellets, by pressing it or extruding by the rolling body or screw through the perforations of the matrix.
The pelleting device comprises for example a pellet press 16.1 , such as a ring-and-roll pellet press or a flat bed die pellet press. Ring-and-roll pellet presses and flat bed die pellet presses are previously known. These pellet presses typically comprises a perforated matrix surface and a compressing body such as a rolling body in a compressing zone of the pellet press. The compressing body cooperates with the perforated matrix surface. The dried bulk material is fed into the compression zone where the bulk material is formed into pellets. The rolling body presses or extrudes the material through the perforations of the matrix surface.
The pelleting device may alternatively comprise an extrusion pellet press 16.2 such as a screw type extruder which is further described below.
The pellet press 16.1 , in Fig. 1 illustrated by a ring-and-roll pellet press comprises a pelleting device inlet 18 and a pelleting device outlet 19. The pelleting device inlet 18 is connected to the bulk material outlet 15 of the separating means. The ring-and-roll pellet press 16.1 is integrated in the circulation conduit system. This means that at least the pelleting device inlet side 18 and the compression zone of the pellet press is subjected to overpressure. The total mass flow of dried bulk material is conveyed from the separating means 12 to the ring-and- roll pellet press 16.1 via the bulk material outlet 15. The bulk material is pelletized by the pelleting device into bio fuel pellets.
Bio fuel pellets are typically cylindrically shaped with a diameter of less than 15 mm and a length of less than 70 mm. Other dimensions are equally possible, however the bio fuel pellets are suitably dimensioned for being combusted in commercially available domestic pellet furnaces.
The ring-and-roll pellet press 16.1 is fully integrated into the steam filled pressurized circulation conduit system. Both the inlet side 18 and the outlet side 19 are subjected to overpressure. The ring-and-roll pellet press has to be designed for operation in a
pressurized environment. The ring-and-roll pellet press 16.1 is followed by a pellet discharging device 17 which discharges the fuel pellets to the normal atmosphere. The pellet discharging 17 device comprises a cellular feeder or similar which is pressure-tight and adapted to avoid pressure loss from the circulation conduit and only provides for a marginal steam leakage at the actual differential pressure between the outside pressure (typically atmospheric) of the pneumatic steam dryer and the pressure inside the circulation conduit system. The discharging operation by the cellular feeder is intermittent and it is necessary to assist the feeding flow from the pellet press 16.1 to the cellular feeder 17. Thus a storage container 24 is preferably connected between the pellet press 16.1 and the cellular feeder 17.
Optionally may the pelleting section also comprise a mill 21 connected between the bulk material outlet 15 and the ring-and-roll pellet press 16.1. The mill 21 and the advantages thereof are further described below.
In Figure 2 is a flow scheme of a second embodiment of the pneumatic steam dryer 1.2 according to the present invention shown. Like references are used for like features.
The pneumatic steam dryer 1.2 differs from the first embodiment in that the separating means comprises at least two cyclones 12' connected in parallel into the circulation conduit to provide efficient separation in one single step. The two cyclones are fed in parallel with bulk material suspended in drying steam. Each cyclone comprises a separate cyclone inlet 12'i which is connected to the drying conduit. The cyclone inlets 12'i are equally
dimensioned, thus approximately half of the total mass flow of dried bulk material suspended in steam is conducted through each cyclone inlet 12'i. By improving the separation efficiency the operation of the dryer is improved and the maintenance costs are decreased. The total mass flow of dried bulk material is conveyed from the separating means 12' to the ring-and- roll pellet press 16.1 via the bulk material outlet 15'. The bulk material is pelletized by the pelleting press into bio fuel pellets.
The pneumatic steam dryer 1.2 also comprises a mill 21. The mill is integrated in said circulation conduit system, and connected between the bulk material outlet and the pelleting device. The mill comprises a mill inlet 22 and a mill outlet 23. The mill inlet is connected to the bulk material outlet 15 and the mill outlet 23 is connected to the ring-and-roll pellet press 16.1. The object of the mill 21 is to comminute the bulk material to achieve an advantageous uniform particle size distribution in advance of pelletizing. The mill 21 comprises for example a vertical rotor mill, a refiner mill or a pin mill. The advantage of integrating the mill into the circulation conduit system is that the temperature of the bulk material is elevated when the bulk material is fed into the mill. Depending on the selected type of mill this is particularly advantageous because the milling operation improves and the specific power consumption of the mill decreases. The safe, inert environment is also beneficial for the milling operation.
Like in previous embodiment may an additional heat exchanger 6' optionally be connected into the drying section 2 to heat the steam after the bulk material has been fed into the circulation conduit system.
Figure 3 shows a flow scheme of a third embodiment of the pneumatic steam dryer 1.3 according to the present invention. Like references are used for like features. The pneumatic steam dryer 1.3 differs from the previous embodiments in that the pelleting device comprises an extrusion pellet press 16.2 such as screw type extruder. Screw type extruders are previously known and typically comprises a housing and a compressing body such as a compressing screw and a perforated matrix surface. The housing, the
compressing screw and the perforated matrix surface forms a compressing zone. The compressing screw cooperates with the housing to compress the dried material located in between the screw and the housing. The compressed bulk material is extruded through the perforated matrix surface. In operation of the screw type extruder the dried bulk material forms a plug in the compression zone. The bulk material plug functions as a pressure-seal. The screw type extruder therefore also operates to pressure-tighten the pneumatic steam dryer.
The extrusion pellet press 16.2 is integrated in the circulation conduit system. The extrusion press comprises an inlet side 18' and an outlet side 19'. The inlet side 18' is connected to the pressurized internal environment of the pneumatic steam dryer, and the outlet side 19' is subjected to atmospheric pressure. Thus a separate pellet discharging device is not necessary. The screw type extruder 16.2 both pelletizes the bulk material and discharges the fuel pellets to the normal atmosphere. The screw type extruder is connected to the bulk material outlet 15 and receives the dried bulk material. The extrusion pellet press has to be designed for operation in a pressurized environment. Optionally may the pelleting section also comprise a mill 21 connected between the bulk material outlet 15 and the ring-and-roll pellet press 16.1. The mill 21 and the advantages thereof are further described above. Like in previous embodiments may an additional heat exchanger 6' optionally be connected into the drying section 2 to heat the steam after the bulk material has been fed into the circulation conduit system.
Figure 4 shows a flow scheme of a fourth embodiment of the pneumatic steam dryer 1.4 according to the present invention. Like references are used for like features.
The pneumatic steam dryer 1 .4 differs from the third embodiment in that the separating means comprises at least two cyclones 12' connected in parallel, as previously described. Furthermore comprises the pelleting section 4 also a mill 21 connected in between the bulk material outlet 15' and the extrusion press 16.2. The effects and advantages of the mill are described above.
Like in previous embodiments may an additional heat exchanger 6' optionally be connected into the drying section 2 to heat the steam after the bulk material has been fed into the circulation conduit system.
The pneumatic steam dryer is advantageously operated above atmospheric pressure. This means that the overpressure range is typically from 1 to 40 Bar, preferably from 2 to15 Bar, more preferably from 2 to 10 Bar, most preferably from 2 to 5 Bar. The overpressure range from 2.5-4.5 Bar has proven to be particularly suitable for the drying process and the evaporation capacity of the drying steam. The possibilities to utilize the vast heat source by recovering the latent heat in the surplus steam increases with the temperature i.e. with the steam pressure inside the pneumatic steam dryer.
The invention has mainly been described above with reference to a few embodiments.
However, as readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims.