Field of the invention The present invention relates to a process for treating bulk material. The invention also relates to a system for treating bulk material.

Background of the invention Drying of bulk material in a pneumatic dryer is previously known. Wet bulk material is fed into a drying enclosure of the 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 dried bulk material is typically used as fuel in different industrial, commercial or domestic applications.

To improve the drying process and the quality of the final product different operations are used for treating the bulk material in connection with the pneumatic dryer. These operations are energy consuming and costly. Summary of the invention

It is an object of the present invention to provide an energy efficient process and system for treating bulk material. This object and other objects are met by the invention as defined in the independent claims.

According to a first aspect of the present invention there is provided a process for treating bulk material. Said process comprises feeding bulk material by an inlet feeding device into a steam filled circulation conduit of a pneumatic steam dryer operating above atmospheric pressure, heating said steam by heating means, drying said bulk material in a drying conduit by heat transfer from said steam thereby forming surplus steam, separating said bulk material from said steam by a first separating means, pneumatically conveying said bulk material suspended in said steam through said drying conduit from said inlet feeding device to said first separating means, releasing surplus steam from said circulation conduit and discharging said bulk material by an outlet feeding device from said pneumatic steam dryer thereby generating flash and leakage steam. The process further comprising utilizing said flash and leakage steam in a bulk material handling system connected to said pneumatic steam dryer.

According to a second aspect of the present invention there is provided a system for treating bulk material. Said system for treating bulk material comprising a pneumatic steam dryer being adapted to operate above atmospheric pressure, said pneumatic steam dryer comprising a circulation conduit which in operation is filled with steam, heating means connected to said circulation conduit for heating said steam, an inlet feeding device connected to said circulation conduit for feeding bulk material into said circulation conduit, a drying conduit connected into said circulation conduit wherein said bulk material is dried by heat transfer from said steam thereby forming surplus steam, first separating means connected to said drying conduit for separating said bulk material from said steam, a fan connected into said circulation conduit for pneumatically conveying said bulk material suspended in said steam through said drying conduit from said inlet feeding device to said first separating means, a surplus steam outlet connected to said circulation conduit for releasing surplus steam from said circulation conduit, an outlet feeding device for discharging said bulk material from said pneumatic steam dryer thereby generating flash and leakage steam. Said system further comprises a bulk material handling system connected to said pneumatic steam dryer for utilizing said flash and leakage steam. Flash and leakage steam is generated when bulk material is depressurized by discharge from the internal pressurized environment of the pneumatic steam dryer to a lower pressure outside the pneumatic steam dryer. Residual moisture which is contained in the bulk material at the end of the drying process vaporizes and forms flash steam. Addtionally leaks some pressurized steam from the outlet feeding device and forms leakage steam. Some minor amounts of steam may also be generated by elements of the bulk material handling system. This is added to the flash and leakage steam. The flash and leakage steam also comprises volatiles like hydrocarbons and terpens evaporated from the bulk material.

The present invention thus provides a process as well as a pneumatic steam dryer which enables treating of bulk material energy efficiently since the flash and leakage steam is a source of energy of low cost which provides several functional benefits. One advantage is that several final products of bulk material having uniform particle size distribution are produced in a safe, substantially oxygen free environment. Another advantage is that the process provides for energy efficient conditioning of wet bulk material. The moisture content of the wet bulk material is consequently decreased in advance of feeding the material into the dryer and less energy is consumed in the pneumatic steam dryer. The heat economy of the pneumatic steam dryer is thereby improved.

A further advantage is that the quality of the treated final product is improved in an environmentally responsible way. Another advantage is that by using flash and leakage steam in the bulk material handling system it is avoided to use other more expensive energy sources to provide the same desirable effects.

Furthermore, the latent heat in the flash and leakage steam is efficiently utilized and recovered in the bulk material handling system. Other devices for recovering the energy in the flash and leakage steam are not necessary. Moreover, it is not necessary to dedust the flash and leakage steam (e.g. by filtering or similar) before recovering the latent heat which is advantageous. 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.

In one embodiment of the invention the process and system for treating bulk material according to the present invention comprises a pneumatic steam dryer and a bulk material handling system connected to the bulk material outlet of said pneumatic steam dryer. In this embodiment of the invention the bulk material handling system comprises a controlling section connected to the outlet feeding device of the pneumatic steam dryer wherein the flash and leakage steam is utilized to provide a safe environment for treating dried bulk material and as transport medium. Furthermore the heat in the flash and leakage steam is used to improve the operations of a milling means and/or pelleting press.

In another embodiment of the invention the process and system for treating bulk material according to the present invention comprises a pneumatic steam dryer and a bulk material handling system connected to the bulk material inlet of said pneumatic steam dryer. In this embodiment of the invention the bulk material handling system comprises a bulk material conditioning device wherein the energy in the flash and leakage steam is utilized and recovered by condensation.

In a further embodiment of the invention the process and system for treating bulk material comprises a pneumatic steam dryer and a bulk material handling system connected both to said inlet and said outlet of the pneumatic steam dryer. In this embodiment the two previously mentioned embodiments are combined, whereby further functional benefits and advantageous effects are achieved which are described below.

In this specification 'bulk material' has a broad meaning which includes any moist or wet solid organic bulk material with a size distribution and particle density suitable for pneumatic transport. The bulk material particularly includes biomass originating from plants. Examples include agricultural residues and wastes (stalks, straw etc), wood based feedstock (wood chips), forest residues; sawmill wastes, peat, energy crops. These bulk materials usually have an initial moisture content of at least 40% moisture on a weight basis, typically above 50% moisture.

The present invention relates to a process and a system for treating preferably biomass based bulk materials, more preferably wood and peat based bulk materials.

Brief description of figures

Embodiments will now be described, by way of example, with reference to the accompanying flow schemes in which

Figure 1 shows a first embodiment of the system according to the present invention

Figure 2 shows a second embodiment of the system according to the present invention Figure 3 shows a third embodiment of the system according to the present invention

Detailed description of preferred embodiments of the invention Fig. 1 shows a first embodiment of the system according to the invention for treating bulk material which comprises a pneumatic steam dryer 100 and a bulk material handling system 200 connected to a bulk material inlet of said pneumatic steam dryer. In this embodiment is flash and leakage steam utilized in the bulk material handling system 200 connected to a bulk material outlet of a pneumatic steam dryer.

The invention provides a pneumatic steam dryer 100 for drying bulk material 101 comprising a drying section 1 10 and a heating section 120 connected to each other to form a circulation conduit 105. In operation of the pneumatic steam dryer 100, the circulation conduit 105 is pressurized and filled with steam 102 such as superheated or saturated steam, thereby forming a substantially oxygen free, inert, internal environment which prevents hazardous dust explosions. The oxygen content is preferably below 10%, more preferably below 5%. The circulation conduit further comprises ducts for conducting bulk material and steam between the elements of the pneumatic steam dryer. All elements and auxiliary equipments are designed to be operated above atmospheric pressure.

The drying section comprises a bulk material inlet 130, a drying conduit 11 and separating means 150. The bulk material inlet 130 comprises an inlet feeding device 131 connected into the circulation conduit 05 downstream of the heating section 120 in relation to the direction of the flow of steam. Wet bulk material is continuously or intermittently fed into the flow of said steam by the inlet feeding device 31. The wet bulk material comprises particles of organic matter. The pneumatic steam dryer 100 typically receives particles of the size <25 mm at the bulk material inlet of the pneumatic steam dryer. The particles have a size distribution and particle density suitable for pneumatic transport. The size distribution of the particles at the bulk material outlet of the dryer is typically the same as before drying.

The inlet feeding device 131 comprises a pressure tight cellular 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 of the pneumatic steam dryer and the pressure inside the circulation conduit.

A drying conduit 1 1 1 is connected between the bulk material inlet 130 and the separating means 150. The drying conduit 11 1 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 residence time of the bulk material in the drying section is very short,

approximately less than 2 minutes. However also other types of chambers which can be connected into the pneumatic steam dryer and through which pneumatic transport of the bulk material in a fluidized state are likewise possible. The bulk material is dried by the superheated steam in the drying conduit while being transported to the separating means. Moisture and volatiles such as hydrocarbons and terpens in the wet bulk material vaporizes and forms surplus steam. The temperature of the steam decreases when the moisture evaporates from the bulk material. Consequently the moisture content of the bulk material is reduced.

The dried bulk material suspended in steam is conducted to the separating means 50 connected to the drying conduit 1 11. The separating means 150 comprises at least one cyclone which separates the dried bulk material 101 from said steam 102. The dried bulk material 103 is collected at the lower portion of the cyclone where an outlet feeding device 141 is connected and discharges the dried bulk material. The cyclone comprises a gas outlet 152 in the upper portion of the cyclone 150, said gas outlet 152 is connected to the heating section 120 of the circulation conduit 105 whereby the separated steam is returned to the heating section. The outlet feeding device 141 is pressure tight and typically of the same kind as the inlet feeding device 131 described above. A bulk material outlet 140 of the pneumatic steam dryer is connected to the outlet feeding device 141 .

The heating section 120 comprises a fan 160, heating means 121 and a surplus steam outlet 170. The fan 160 generates a flow of steam 102 circulating in the circulation conduit 105. The heating means 121 is connected into the circulation conduit between the fan and the inlet feeding device of the drying section. The fan conveys steam 102 through the heating section. The heating means 121 comprises a heat exchanger which indirectly superheats the steam to above 200 °C. The heat exchanger is supplied by primary heat from flue gas, pressurized steam, hot water, thermal oil or similar from a 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 100.

Optionally, a further heat exchanger may be connected into the drying section 1 10 to further heat the steam and the bulk material suspended therein.

Furthermore, the heating section 120 also comprises a surplus steam outlet 170 which releases pressurized surplus steam evaporated from the bulk material into energy recovery means. The operating conditions such as the pressure and/or the temperature of the pneumatic steam dryer 100 are thereby controlled and maintained.

The energy recovery means comprises for example a steam regenerator. In the energy recovery means, the surplus steam is cooled and depleted of energy, whereby clean steam and waste condensate is generated.

The dried bulk material 103 discharged from the pneumatic steam dryer 100 is typically used as fuel in combustion chambers in industrial , commercial or domestic applications. Another application of the dried bulk material is in wood based polymer composites.

In particular when the bulk material 101 is originating from wood or forest products, the energy content in the dried material 103 is high and can be used for production of fuel pellets or be used as fuel in a combined heat and power plant or steam generation plant supplying the pneumatic with heat. The bulk material is advantageously dried to different moisture contents depending on the final use of the dried product. In case the dried bulk material is to be used in the production of fuel pellets, the moisture content is advantageously below 12 % by weight.

If the dried bulk material is used in the production of wood polymer composites the moisture content is advantageously below 2% by weight, preferably below 1 % by weight, most preferably below 0,5 % by weight.

The pneumatic steam dryer 100 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 100. The system in Fig.1 shows furthermore a bulk material handling system 200 connected to the bulk material outlet 140 of the pneumatic steam dryer 100 described above. Flash and leakage steam 104 is generated when the bulk material is discharged from the pneumatic steam dryer. The flash and leakage steam is utilized in the bulk material handling system 200 in accordance with the present invention.

In this embodiment of the invention the process comprises utilizing of said flash and leakage steam 104 by a bulk material handling system 200 connected to a bulk material outlet 140 of said pneumatic steam dryer 100. The process comprises receiving said flash and leakage steam and dried bulk material by said bulk material handling system 200 from said outlet feeding device 141 and controlling of said bulk material by a controlling section 300.

The bulk material handling system 200 comprises a controlling section 300 connected to said outlet feeding device 141 of said pneumatic steam dryer for receiving the flash and leakage steam and the bulk material from said outlet feeding device. The flash and leakage steam 104 is efficiently utilized to improve the operation of the controlling section 300.

The feedstock fed to the pneumatic steam dryer varies. It is increasingly more difficult to find high quality feed stock to a reasonable cost and a lower quality and/ or different type is more frequently used. Nevertheless the quality of the final products from the pneumatic steam dryer needs to be maintained and preferably improved. Furthermore, the drying process is very energy consuming and it is necessary to avoid any increase of energy consumption due to the lowered quality and/or different feedstock.

In addition, dried bulk material has a moisture content of typically less than 12% after discharge from the pneumatic steam dryer. Handling of heated dried bulk material in an environment with an oxygen content of more than 10% has proven to be very dangerous with regards to the risk of dust explosions. To address this problem this embodiment of the present invention provides a bulk material handling system 200 comprising a controlling section 300 connected the bulk material outlet 40 of the pneumatic steam dryer which utilizes the low cost flash and leakage steam to provide a safe environment for treating the dried bulk material. The controlling section 300 provides for controlling of the particle size distribution of the dried bulk material. Furthermore, indirectly also the moisture content in the final dried products is controlled and the mass ratio between the products is controlled. This means that in operation of the system, one or more final products are produced. Each product comprises a predefined particle size distribution.

According to the present invention the process comprises advantageously pneumatically conveying said flash and leakage steam through said controlling section thereby forming a substantially oxygen free environment inside said controlling section. This has the functional benefit that the risk of hazardous dust explosions is eliminated which improves the safety of the controlling section. The conduits, ducts and operating elements of said controlling section forms a controlling section loop wherein the flash and leakage steam is circulated.

The controlling section is adapted to operate near atmospheric pressure, at a slight overpressure. This means that the controlling section operates in the interval 0.1 -1.0 Bar, preferably 0.2-0.8 Bar, most preferably 0.3-0.7 Bar.

The controlling section 300 comprises a first classifier 310, a second classifier 320, a second separating means 330, a fan 370, milling means 340 and pellet press 350. The controlling section is connected to the bulk material outlet 140 of said pneumatic steam dryer. Said bulk material outlet 140 comprises an outlet feeding device 141 and means for coupling said controlling section to said outlet feeding device. The bulk material is discharged into the controlling section whereby flash and leakage steam is generated. Flash and leakage steam 104 and dried bulk material 103 suspended therein is pneumatically conveyed in the direction towards a first classifier 310. The process preferably comprises classifying of said bulk material into at least a first fraction and a second fraction by a first classifier 310 inside said controlling section 300.

The first classifier 3 0 comprises a mechanical separation device, further described below. The first classifier 310 may alternatively comprise an aerodynamic classifier, further described below.

Said first classifier 310 comprises a first classifier housing. The first classifier housing comprises a first classifier inlet 311 , a first fraction outlet 312 and a second fraction outlet 313.

The first fraction outlet 312 corresponds to the reject outlet of said first classifier 310. Hence the first fraction comprises the rejected coarse particles of the bulk material.

The second fraction outlet 313 corresponds to the accept outlet of said first classifier 310. Hence the second fraction comprises the accepted particles of the bulk material.

Said first classifier inlet 31 1 of the first classifier 310 is connected to the outlet feeding device 141 to receive the bulk material and whereby said first classifier 310 classifies said bulk material into at least a first fraction and a second fraction. The flash and leakage steam is pneumatically conveyed to the first classifier together with the bulk material.

The first fraction is discharged from the first classifier via the first fraction outlet 312. The first fraction is supplied to milling means 340 preferably by gravitation. Flash and leakage steam is led to the milling means 340 simultaneously with the first fraction. Said milling means are further described below.

The second fraction is discharged from the first classifier 310 via the second fraction outlet 313.

Advantageously the process comprises classifying of said second fraction by a second classifier 320 into a third fraction and a fourth fraction inside said controlling section 300. The controlling section preferably comprises a second classifier 320 connected to the second fraction outlet 313 of the first classifier 310. The second fraction is pneumatically conveyed from the second fraction outlet to said second classifier 320.

The second classifier 320, preferably an aerodynamic classifier, comprises a second classifier housing, second classifier housing comprises a second classifier inlet 321 , a third fraction outlet 322 and a fourth fraction outlet 323.

The third fraction outlet 322 corresponds to the reject outlet of said second classifier 320. Hence the third fraction comprises the rejected particles of said second fraction fed to the second classifier. The fourth fraction outlet 323 corresponds to the accept outlet of said second classifier 320. Hence the fourth fraction comprises the accepted particles of said second fraction fed to the second classifier.

The second classifier 320 receives said second fraction from said first classifier 310 and classifies said second fraction into a third fraction and a fourth fraction. The second classifier 320 comprises preferably an aerodynamic classifier further described below.

The second fraction outlet 313 of the first classifier 310 may alternatively be connected to a pellet press inlet 351 of a pellet press 350 further described below.

In the embodiment of the system shown in Fig. 1 is the third fraction outlet 322 of the second classifier 320 connected via an outlet feeding device to a pellet press inlet 351 such as a ring-and-roll press or an extruding press. The third fraction is preferably discharged by gravitation into said pellet press 350.

The process advantageously comprises separating said fourth fraction from the conveying flash and leakage steam by a second separating means 330. The system in Fig 1. comprises a second separating means 330 connected to said second classifier 320 to receive and separate said fourth fraction from the flash and leakage steam. The second separating means such as a cyclone 330 is connected to said fourth fraction outlet 323. Said separating means comprises a second separating means inlet 331 , a gas outlet 333 and a material outlet 332. Said fourth fraction suspended in flash and leakage steam is pneumatically conveyed from said second classifier 320 to said second separating means 330. Said fourth fraction is separated and discharged via said material outlet 332.

A fan 370 is connected to the gas outlet 333 of the second separating means. Said fan propels the flash and leakage steam through the controlling section loop, see Fig.l

The controlling section also comprises a flash and leakage steam outlet indicated by 360 to release flash and leakage steam from the controlling section loop to control the pressure in the loop. The flash and leakage steam which has been released from the controlling loop is advantageously further recovered in an external heat recovery system like a district heating net or the like. The process comprises advantageously classifying said bulk material by aerodynamic classification in said first classifier 310. Such classifiers separates the bulk material according to size and/or specific gravity of the particles and is known to achieve a sharp particle size distribution, a sharp cut-point, between the separated fractions. An aerodynamic centrifugal classifier, also known as rotary aerodynamic classifier, is particularly suitable. However also a zigzag aerodynamic classifier may be used to achieve the same effect. The first classifier 310 preferably comprises a rotary aerodynamic classifier. Also the second classifier 320 preferably comprises a rotary aerodynamic classifier. The skilled person appreciates that the cut point of the rotary aerodynamic classifier can be adapted according to desirable particle size distribution of the fractions. Consequently also the mass flow ratio between the separated fractions is controlled.

Alternatively the process comprises classifying said bulk material by mechanical separation in said first classifier 310. Mechanical separation devices has a broad meaning and includes sifters, sieves and screens which in operation separates the bulk material into at least two fractions by vibration, oscillation, tilting, wobbling or the like. The skilled person appreciates that minor adaptations of the feeding system are necessary to provide for efficient transport of the dried bulk material into the mechanical separation device.

By classifying and separating the dried bulk material received by the controlling section 300 into at least a first fraction and a second fraction it is possible to control the dried bulk material and to achieve final products of high quality. The first fraction contains the particles of said bulk material which are coarse and/or insufficiently dry. These particles are rejected by the first classifier.

The second fraction contains the particles of said bulk material which are smaller than a particular size specification which corresponds to the cut-point of the first classifier. These particles are accepted by the first classifier. However, the second fraction also contains all the fines of the dried bulk material.

The second classifier 320 separates the second fraction into a third and a fourth fraction. The third fraction comprises the coarser particles of the second fraction rejected by the second classifiers. The fourth fraction comprises the finer particles of the second fraction which are accepted by the second classifier.

The third fraction is particularly suitable to be used in the production of bio fuel pellets for industrial/ commercial/domestic purposes. As the fines have been separated from the third fraction, the bio fuel pellets release less dust which improves the quality of the fuel pellets. Alternatively, the third fraction may also be used in the production of wood polymer composites.

The fourth fraction contains the fines. The fines contains an elevated level of ash in comparison with the third fraction which makes the fourth fraction less suitable for combusting in small typically domestic combusting chambers. On the contrary, the fourth fraction is particularly suitable as pulverized fuel in large scale pulverized fuel fired combustion chambers.

The process comprises advantageously disintegrating of said first fraction by milling means 340 inside said controlling section. The controlling section comprises a milling means connected to the first fraction outlet 312 of said first classifier 310 to receive the coarse first fraction and to disintegrate said coarse first fraction.

The milling means 340 comprises a milling means inlet 341 suitable to receive bulk material and flash and leakage steam, a milling chamber where said disintegrating of said first fraction is carried out and a milling means outlet 342 suitable for discharging disintegrated bulk material and flash and leakage steam out of said milling means 340. The milling means 340 are preferably adapted to disintegrate coarse particles. The milling means 340 comprises preferably an aspirated type of mill for example a hammer mill or a vertical rotor mill.

The flash and leakage steam 104 conveys the dried bulk material 103 through the controlling section. The temperature of the flash and leakage steam and the bulk material is

approximately 100 °C after discharging by the outlet feeding device of the pneumatic steam dryer. The temperature of the bulk material is therefore still elevated when the bulk material is fed into the milling means. Depending on the selected mill this is particularly advantageous because the milling operation improves and the specific power consumption of the milling means decreases. A vertical rotor mill is particularly suitable for this. The bulk material is also further dried in the milling operation.

Furthermore also flash and leakage steam is fed with the dried bulk material into the milling means 340. Low oxygen content inside the milling means is thus maintained which eliminates hazardous dust explosions.

The mill generates disintegrated bulk material. The embodiment shown in Fig 1. indicates several alternative variants to further treat the disintegrated bulk material obtained by milling. Valves indicated by 515 allows for the different alternative variants to be implemented.

In one variant of the process is the disintegrated bulk material obtained from said milling means 340 supplied to the inlet feeding device 131 of the pneumatic steam dryer 100 and feeding said disintegrated bulk material into said circulation conduit of the pneumatic steam dryer, thereby further drying the disintegrated bulk material. To accommodate for this is the inlet feeding device 131 connected to said milling means outlet 342 via a third separating means 510 to receive disintegrated bulk material obtained by milling and to feed the disintegrated bulk material into said circulation conduit of said pneumatic steam dryer.

In an alternative variant of the process is the disintegrated bulk material from said milling means supplied to a pellet press 350' further described below. To accommodate for this in the embodiment of the system shown in Fig. 1 is said milling means outlet 342 connected to a pellet press inlet 351 ' to supply disintegrated bulk material obtained by milling of said first fraction to said pellet press 350' to further pelletize said disintegrated bulk material.

In a further alternative variant the process is the disintegrated bulk material from said milling means 340 supplied into said first classifier 310 and the disintegrated bulk material obtained from said milling means 340 is classified by said first classifier. To accommodate for this is said milling means outlet 342 connected to the first classifier inlet to supply disintegrated bulk material obtained by milling of said first fraction to said first classifier to further classify said disintegrated bulk material (not shown in Fig. 1). The process comprises advantageously pelleting of said third fraction by a pellet press 350 connected into said controlling section 300. Hence, the system comprises preferably a pellet press 350 connected to the third fraction outlet 322 of said second classifier 320 to receive and pelletise said third fraction. The process may alternatively comprise pelleting of the second fraction by a pellet press 350 connected to the controlling section. To accommodate for this alternative, the controlling section comprises said first classifier 310, said second separating means 330, a cyclone, connected to the first fraction outlet of the first classifier 310 and a pellet press 350 connected to the material outlet of said second separating means to receive and pelletise said second fraction (not shown in figures). The pellet press 350 comprises a roller-and-ring press or alternatively an extruder. A roller- and-ring press is typically used for pelleting the dried bulk material into fuel pellets.

An extruder is typically used for producing wood polymer composite pellets which are suitable to be used in a variety of applications such as in the production of construction materials.

To produce the wood polymer composite pellets is the third fraction mixed with granulate formed thermoplastic material and the mixture is extruded by an extruder connected into said controlling section. To accommodate for this alternative process comprises the controlling section an extruder which is connected to said second classifier 320 to receive said third fraction. Granulate formed thermoplastic material is additionally fed to the extruder from an external container 380 and mixed with the third fraction.

One advantage of feeding the pellet press 350 from the first classifier 310 or the second classifier 320 in the controlling section is that pelleting (or extruding) is carried out while the temperature of the dried bulk material is still elevated. The presence of flash and leakage steam in the conduits of the controlling section improves forming of pellets. Pellets are more easily formed/extruded when the bulk material is warm. No other means for heat conditioning the dried bulk material is necessary which improves the energy efficiency. The flash and leakage steam is also beneficial for safety reasons as previously described.

The process and system for treating bulk material according to this embodiment of the invention is suitable for producing homogenous final dried products with well defined particle size distribution. The following are non limiting examples of final products which can be produced by the process and system for treating bulk material according to this embodiment of the invention. Coarse final product 4mm < d ₉₇ < 10 mm particularly for pellets and briquettes or similar. Fine final product 0.7mm < d ₉₇ < 2.5 mm, particularly for pulverized fuel firing or similar. Very fine final product 200 pm < d ₉₇ < 400 pm, particularly for wood polymer composites or similar. In operation of the system for treating bulk material shown in Fig. 1 two final products with respectively different particle size distributions are produced represented by the third fraction and the fourth fraction.

Since the quality of the final products is controlled in the controlling section, a variety of quality/type of bulk material can be treated by system for treating bulk material according to the invention.

Second embodiment of the present invention Fig. 2 shows a second embodiment of system according to the invention which comprises a pneumatic steam dryer 00 (previously described) and a bulk material handling system 200' connected to a bulk material inlet of said pneumatic steam dryer. The bulk material handling system 200' is connected to a bulk material outlet 140 of said pneumatic steam dryer 100 to receive and utilize the flash and leakage steam.

This embodiment comprises the previously described pneumatic steam dryer 100 and the drying process carried out by said pneumatic steam dryer which is therefore not further described here.

Furthermore, like references are used for like features. In this embodiment of the invention the bulk material handling system comprises a bulk material conditioning device wherein the energy in the flash and leakage steam is utilized and recovered by condensation. Preferably comprises the process utilizing and recovering of said flash and leakage steam by conditioning wet bulk material in a conditioning section 400 of said bulk material handling system 200'. Advantageously comprises the process supplying flash and leakage steam to said conditioning section 400 from said outlet feeding device 141 of said pneumatic steam dryer.

Advantageously comprises the process conditioning said bulk material in a bulk material conditioning device 402 directly preceding an inlet feeding device 401. Preferably comprises the process conditioning said bulk material by condensing said flash and leakage steam in said bulk material in a steam condensing screw feeder 402.

Advantageously comprises the process supplying said flash and leakage steam counter currently to the direction of movement of said bulk material inside said steam condensing screw feeder 402.

Advantageously comprises the process compressing and dewatering of said bulk material by said inlet feeding device 401 in advance of feeding the bulk material into the pneumatic steam dryer 100.

The process comprises utilizing and recovering of flash and leakage steam 104 by a bulk material handling system 200' connected to a bulk material inlet 130 of said pneumatic steam dryer.

The process according to this embodiment of the invention is carried out by a system for treating bulk material shown in Fig. 2. The system for treating bulk material comprises a bulk material handling system 200' which comprises a conditioning section 400 connected to said bulk material inlet 130 of said pneumatic steam dryer for receiving wet bulk material 01 , conditioning said wet bulk material, dewatering the conditioned bulk material and feeding the dewatered bulk material into said pneumatic steam dryer. Said conditioning section 400 is connected to the bulk material outlet 140 via a flash and leakage steam conduit 403 to receive flash and leakage steam which is to be utilized. The conditioning section 400 comprises a wet bulk material inlet 420, a bulk material conditioning device 402 and an inlet feeding device 401. A container 421 is provided for holding wet bulk material 101 and a screw feeder 422 for feeding said material via the wet bulk material inlet 420 into the bulk material conditioning device 402.

The inlet feeding device 401 is connected between the bulk material conditioning device 402 and the bulk material inlet 130 of the pneumatic steam dryer.

Said bulk material conditioning device 402 is connected directly preceding said inlet feeding device 401. The bulk material conditioning device 402 comprises preferably a steam condensing screw feeder or a rotating tumbler wherein the flash and leakage steam is utilized and recovered by condensation.

The steam condensing screw feeder 402 comprises an elongated body which comprises a first end indicated by 405 connected to said screw feeder 422. The steam condensing screw feeder 402 further comprises a steam inlet 406 in a second end indicated by 407, a feeding screw 408 for feeding said bulk material from said first end 405 to said second end 407, an inner housing 409 enclosing said feeding screw 408, said inner housing is preferably cylindrically formed and having in inner wall. Said inner housing provides a significant gap between the feeding screw 408 and the inside of said inner wall to allow for the wet bulk material to be conditioned. Flash and leakage steam 104 is supplied counter currently to the direction of movement of the bulk material 101. Flash and leakage steam is supplied to the steam inlet 406 via the flash and leakage steam conduit 403 from the outlet feeding device 141 of the pneumatic steam dryer.

By feeding the flash and leakage steam counter currently to the direction of the movement of the bulk material the advantageous effect of deaeration of the bulk material is achieved. The efficiency in following step of dewatering in the plug screw feeder is thereby improved.

The inlet feeding device 401 comprises a plug screw feeder. The object of the plug screw feeder is to feed the bulk material from an atmospheric environment outside the pneumatic steam dryer via the bulk material inlet 130 into the pressurized environment of said dryer with marginal leakage of pressurized steam. The plug screw feeder 401 receives conditioned bulk material from the steam condensing screw feeder 402, Said plug screw feeder 401 comprises preferably dewatering means for dewatering the bulk material. Plug screw feeders with dewatering means are previously known and typically comprises a conically shaped rotating plug feeding screw 412 for feeding said bulk material from a first end indicated by 410 to a second end indicated by 411 and an inner housing enclosing said plug feeding screw 412. The inner housing comprises a conically tapered inner wall comprising passages such as channels and/or orifices in said inner wall for draining of fluids. The plug feeding screw 412 cooperates with said inner wall to mechanically compress said bulk material located between the feeding screw and the inside of the conically tapered wall to form a plug of bulk material 101 and to extract fluids from the material. The wet bulk material 101 is fed from the container 421 into said first end 405 of the bulk material conditioning device 402. Flash and leakage steam 104 is fed into the said second end 407 of the steam condensing screw feeder 402. Flash and leakage steam is supplied counter currently to the direction of movement of said bulk material. Flash and leakage steam is condensed into the bulk material whereby the bulk material is conditioned. The bulk material is there after fed into the plug screw feeder 401. The bulk material is slightly dewatered in the plug screw feeder and thereafter fed into the pneumatic steam dryer.

In this embodiment of the invention said flash and leakage steam is utilized for conditioning the wet bulk material and recovered by condensing the flash and leakage steam into the bulk material. The bulk material conditioning device is followed by an inlet feeding device wherein the bulk material is further compressed and dewatered.

The flash and leakage steam conditions the wet bulk material in the steam condensing screw feeder which improves the efficiency of the dewatering operation in the following plug screw feeder. This has the functional benefit that an amount of liquid is pressed out of the bulk material already before the drying process in the pneumatic steam dryer.

This provides the effect that a significant amount of waste substances such as chlorine and alcalic substances originating from the wet bulk material is removed already before the bulk material is fed into the pneumatic steam dryer. Costly devices and processes for taking care of these substances at a later process step are thereby avoided and furthermore the bulk material is significantly drier already before the pneumatic steam dryer. This also provides for drying of low quality feedstock which typically contains a larger proportion of waste substances by the pneumatic steam dryer.

Since flash and leakage steam is a low cost steam source it is particularly suitable to use for conditioning purposes. Moreover, the dewatering operation is proven to be very effective when the temperature of the conditioned bulk material is approximately 80 °C. The flash and leakage steam is therefore particularly suitable to be used in the conditioning operation. The conditioning operation is of great importance especially when the wet bulk material is of a low temperature or even frozen. Another advantage with utilizing flash and leakage steam as conditioning medium is that the flash and leakage steam already contains some waste substances from the drying process, which makes the flash and leakage steam particularly suitable instead of using other cleaner resources which are more expensive. Third embodiment

Fig. 3 shows a third embodiment of the system according to the invention which comprises a pneumatic steam dryer 100 (previously described) and a bulk material handling system 200" connected to a bulk material inlet of said pneumatic steam dryer 100 and to the bulk material outlet 140 of said pneumatic steam dryer.

This embodiment comprises the previously described pneumatic steam dryer 100 and the drying process carried out by said pneumatic steam dryer which is therefore not further described here. Furthermore, in the following like references are used for like features.

The process according to this embodiment of the invention comprises utilizing of said flash and leakage steam in a bulk material handling system 200" connected to a bulk material outlet 140 and connected to a bulk material inlet 130 of said pneumatic steam dryer 100. In this embodiment of the invention flash and leakage steam is utilized in a bulk material handling system 200" comprising a controlling section 300 connected to said bulk material outlet 140 of said pneumatic steam dryer and a conditioning section 400 connected to said bulk material inlet 130 of said pneumatic steam dryer. All features and alternative variations previously described in the separate first and second embodiments of the invention are combinable with corresponding effects. This means that the controlling section 300 and the conditioning section 400 are separately and independently implemented to the same pneumatic steam dryer 100, providing the same effects and beneficial functions as previously described.

More preferably the process according to this embodiment of the invention comprises utilizing said flash and leakage steam sequentially by first the controlling section 300 and thereafter by the conditioning section 400. Advantageously the process comprises supplying flash and leakage steam from said controlling section to said conditioning section.

Most preferably comprises the process supplying of the flash and leakage steam from said milling means 340 in said controlling section to said steam condensing screw feeder 402 in said conditioning section via a milling loop 500.

In Fig. 3 the third embodiment of the system according to the present invention is shown which comprises a bulk material handling system 200" for utilizing flash and leakage steam which comprises a controlling section 300 connected to said bulk material outlet 140 and a conditioning section 400 connected to said bulk material inlet 130 of said pneumatic steam dryer 100.

The conditioning section 400 is connected to said controlling section 300 to receive said flash and leakage steam 104 from said controlling section. Said controlling section 300 is previously described in the first embodiment of the present invention and said conditioning section 400 is previously described in the second embodiment of the present invention.

Said controlling section comprises a first classifier 310 for classifying said bulk material into at least a first fraction and a second fraction and milling means 340 for disintegrating said first fraction. A second classifier 320 classifies said second fraction into a third and a fourth fraction as previously described. Said controlling section forms a loop wherein flash and leakage steam is circulated by a fan 370. A pellet press 350 is connected to said second classifier to pelletize said third fraction. Said conditioning section 400 comprises a bulk material conditioning device 402 and an inlet feeding device 401. As previously described comprises the bulk material conditioning device preferably a steam condensing screw or a rotating tumbler.

Fig. 3 shows further the bulk material conditioning device, the steam condensing screw feeder 402 in the conditioning section 400 connected to the controlling section 300. Flash and leakage steam 104 is supplied via milling means 340 and a third separating means 510 to the steam condensing screw feeder. Furthermore, the disintegrated bulk material obtained by milling is also supplied to the conditioning section 400.

Fig. 3 shows further the steam condensing screw feeder 402 connected to the controlling section 300, preferably via the milling means 340 to receive said flash and leakage steam 104.

In addition to the controlling section and the conditioning section provides this embodiment a milling loop 500, which connects the controlling section and the conditioning section together.

Said milling loop comprises said milling means 340, a third separating means 510 and a fan. The disintegrated bulk material obtained by milling is pneumatically conveyed with flash and leakage steam to said third separating means 510 and thereafter fed to the conditioning section 400. Thus said bulk material conditioning device, the steam condensing screw feeder 402 is connected to said milling means 340 to receive disintegrated bulk material obtained by milling.

A first partial flow of the flash and leakage steam separated in the separating means 510 is fed to the bulk material conditioning device 402, and a second partial flow of flash and leakage steam is returned to the milling means 340.

This has the further advantage that the pressure in the controlling section can be maintained by releasing said flash and leakage steam into the milling loop and thereafter into the conditioning section.

Said third separating means 510 is preferably a cyclone which separates the disintegrated bulk material from the flash and leakage steam and supplies the material into the bulk material conditioning device 402. The flash and leakage steam is supplied to the bulk material conditioning device via flash and leakage steam conduit 404 for conditioning of the bulk material in the same manner as previously described.

The utilizing of flash and leakage steam in said bulk material handling system 200" has several functional benefits. The flash and leakage steam is sequentially used in the controlling section and the conditioning section. A safe environment inside the controlling section is achieved. Energy efficiency is achieved by using the heat of the flash and leakage steam to keep the bulk material warm thereby improving the milling operation and the pelleting operation and to heat the wet bulk material in the bulk material conditioning device. Furthermore, environmental benefits are achieved by the dewatering operation.

The connection between the milling means and the bulk material conditioning device has the further effect that bulk material suspended in flash and leakage steam can be conveyed from the milling means to the bulk material conditioning device by pneumatic transport.

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 bu the appended patent claims.