Field of invention

The present invention relates to a process for treating bulk material. The invention also relates to a pneumatic dryer 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. It is increasingly more difficult to find high quality feedstock at a reasonable cost, thus lower quality feedstock is more frequently used. The quality of the final product from the pneumatic dryer needs nevertheless to be maintained and preferably improved. Lower quality feedstock renders an increased content of impurities in the dried bulk material. Furthermore, the dried particles in the bulk material have a non-uniform size, and there is a significant span in particle size distribution as well as moisture content. These aspects influence the quality of the final dried bulk material and the applicability of the dried bulk material to further applications such as bio fuel pellets. Moreover, dried bulk material is easily ignited and it is necessary to handle such material carefully to avoid hazardous dust explosions. Drying of bulk material in a pneumatic dryer is very energy consuming and it is desirable to avoid an increase of energy consumption due to the lowered quality and/or different type of feedstock.

Summary of the invention

One object of the invention is to provide a process for treating bulk material originating from a variety of feedstock and a pneumatic dryer carrying out the process for treating such bulk material.

According to a first aspect of the present invention this object is achieved by a process for treating bulk material in a pneumatic dryer comprising a circulation conduit system filled with steam and operating above atmospheric pressure. The process comprises generating a flow of said steam by a first ventilator in the direction from heating means to a first cyclone and indirectly superheating said steam by said heating means. The process also comprises feeding moist bulk material into said circulation conduit system by pressure-sealing inlet feeding means and conveying said bulk material by said flow of superheated steam through a drying conduit and drying said moist bulk material in said drying conduit by heat transfer from said superheated steam whereby moisture vaporizes from said bulk material thereby forming surplus steam.

The process also comprises separating the dried bulk material from said steam by said first cyclone. The process also comprises classifying said bulk material by a first classifier into two fractions, a first accept fraction and a first reject fraction and milling one of said fractions by a first mill to reduce the size of the particles in said fraction and discharging the bulk material by pressure-sealing outlet feeding means, where said first classifier and said first mill are integrated in the circulation conduit system of the pneumatic dryer.

As the classifier and the mill are integrated in the pressurized environment of the circulation conduit system an efficient control of the particle size distribution and the size of the particles in the dried material is achieved. Furthermore, safe size reduction of the particles of the dried material is provided.

As the process comprises classifying said bulk material by a first classifier into two fractions, a first accept fraction and a first reject fraction, the particle size distribution of the dried bulk material is controlled. The first accept fraction comprises the finer particles, and the first reject fraction comprises the coarser particles of the dried bulk material. By separating these different fractions and treating the fractions separately according to a desired result, a homogeneous final product having a well defined particle size distribution is achieved.

Furthermore, as the process comprises milling of one of said fractions, said particle size distribution of the dried bulk material is also controlled and modified by particle size reduction, resulting in a homogenous final product.

Another advantage is that the pneumatic dryer operates above atmospheric pressure and the circulation conduit system is filled with steam, thus the oxygen level is kept low which provides for an inert environment where classifying and milling can be carried out safely and eliminating the risk of dust explosions.

Another advantage of combining classifying and milling is that the performance of the milling means is improved since only the oversized particles of the dried bulk material are introduced in the grinding zone of the mill. Thereby the milling process is more efficient which lowers the specific energy consumption and wear rate.

Another advantage of the present invention is that due to the pressurized environment in the pneumatic dryer is the steam saturation temperature in the range of the softening temperature of lignin, which lowers the necessary force to disintegrate and reduce the particle size by milling. Hence the power consumption is lower in comparison with a mill operating under normal ambient temperature.

Another advantage of the present invention is that the quality of the final product is less influenced by the quality of the feedstock, since the size of the particles and the particle size distribution of the dried material is controlled.

The process and pneumatic dryer according to the invention is particularly suitable for producing a homogeneous final product with a well defined particle size distribution in order to meet specific demands for the final use of the dried material. Corresponding advantages are achieved by corresponding features of a pneumatic dryer according to a second aspect of the invention, said pneumatic dryer for treating bulk material is provided for carrying out the process, said pneumatic dryer comprises a circulation conduit system being adapted to be filled with steam and to operate above atmospheric pressure. The pneumatic dryer also comprises a first ventilator for generating a flow of steam in the direction from heating means to a first cyclone. The pneumatic dryer further comprises heating means connected to the first ventilator to indirectly superheat said steam. The pneumatic dryer also comprises pressure-sealing inlet feeding means connected downstream of the heating means for feeding moist bulk material into said circulation conduit system and a drying conduit connected to the inlet feeding means wherein the bulk material is dried by heat transfer from said superheated steam, while conveying said bulk material through said drying conduit by said flow of superheated steam. The pneumatic dryer also comprises a first cyclone connected to said drying conduit for separating the dried material from said steam. The pneumatic dryer also comprises a first classifier connected to said first cyclone for classifying the bulk material into two fractions, a first accept fraction and a first reject fraction and a first mill connected to said first classifier for reducing the size of the particles in one of said fractions by milling and pressure-sealing outlet feeding means, where said first classifier and said first mill are integrated in the circulation conduit system of the pneumatic dryer. 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 where several embodiments of the process and pneumatic dryer for treating bulk material according to the invention are provided. One embodiment of the process and pneumatic dryer according to invention is particularly suitable for classifying and size reduction of the particles in the coarsest fraction of the dried material to achieve a particle size distribution corresponding to the specific requirements of a final product, such as a product suitable producing bio fuel pellets. This embodiment of the invention is advantageous when the moist organic feedstock contains coarse, large particles, and allows for flexibility in quality and type of feedstock, still achieving a homogeneous, high quality final product.

Another embodiment of the process and pneumatic dryer according to invention is particularly suitable for classifying and size reduction of the particles in the finest fraction of the dried material. This is advantageous when a final product comprising a high proportion of fine particles is required, such as a product suitable as pulverized fuel.

In yet another embodiment of the process and pneumatic dryer according to invention a combination of the previous embodiments is shown which comprises classifying and particle size reduction of the coarsest fraction to achieve a final product suitable for pellet production and classifying and particle size reduction of the finest fraction to achieve an increased proportion of fines in the final product.

These and other objects and advantages of this invention will become apparent to one skilled in the art upon consideration of the written specification, appended claims and attached figures wherein are set forth, by way of illustration and examples certain embodiments of this invention. Embodiments will now be described, by way of example, with reference to the accompanying figures of flow schemes in which

Figure 1 shows a first embodiment of a pneumatic dryer according to the present invention. Figure 2 shows a second embodiment of a pneumatic dryer according to the present invention.

Figure 3 shows a third embodiment of a pneumatic dryer according to the present invention. Figure 4 shows a fourth embodiment of the pneumatic dryer according to the present invention. Detailed description of preferred embodiments

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 feedstock of biomass derived from plants and animals. Examples include agricultural residues and wastes (stalks, straw etc), wood based feedstock (wood chips), forest residues; sawmill wastes, peat, energy crops and livestock wastes (from dairy, hog and poultry farms). This feedstock of 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 in particular to a process and a pneumatic dryer for treating biomass bulk materials, preferably wood and peat bulk materials.

With reference to figure 1 a first embodiment of the present process and a pneumatic dryer 100 for treating bulk material according to the invention is shown. This embodiment of the invention is particularly advantageous when the wet bulk material contains high proportion of coarse material.

In operation of the pneumatic dryer 100 one type of final product with a particle size distribution corresponding to specific requirements is produced. The final product may for example be used in the production of bio fuel pellets. A flow chart of the pneumatic dryer 100 is shown in figure 1 providing a circulation conduit system. The circulation conduit system comprises a heating section 10, a drying section 20 and a controlling section C1 being connected to each other thereby forming a substantially closed circulation conduit system. In operation of the pneumatic dryer 100, the circulation conduit system is filled with pressurized superheated or saturated steam thereby forming an oxygen free, inert, internal environment which eliminates the risk of dust explosions. The circulation conduit system further comprises ducts and couplings connecting the elements of the pneumatic dryer together for conducting bulk material and steam through the circulation conduit system

The heating section 10 of the circulation conduit system is connected downstream of a first cyclone 30 in the drying section 20. The heating section comprises a first ventilator 50 and heating means 60. The first ventilator 50 such as a fan, blower or ejector generates a flow of steam which circulates in the circulation conduit system in the direction from said heating means 60 to a first cyclone 30. The heating means 60, such as a heat exchanger comprises a heat exchanger inlet 61 and a heat exchanger outlet 62. Said heat exchanger inlet 61 is connected by a conduit to the ventilator 50. The heat exchanger 60 operates as a super heater and superheats the steam. The heat exchanger 60 is supplied by primary heat from any type of steam source such as a steam generation plant or preferably a steam turbine. The heat exchanger outlet 62 is connected to the drying section 20 and an inlet feeding means 22. The heating section is bypassed by the bulk material conveyed through the drying section 20 and the controlling section C1 , thereby avoiding deposits of bulk material inside the heating section and the heat exchanger 60.

The drying section 20 comprises a drying conduit 21 , said inlet feeding means 22, and a first cyclone 30. The moist bulk material is fed into said flow of steam in the drying conduit 21 of the drying section via the inlet feeding means 22 connected downstream of said heating means 60. The inlet feeding means 22 is pressure-sealing and comprises a cellular feeder or a plug screw feeder or a similar device providing a marginal steam leakage at the actual differential pressure thereby being adapted to avoid pressure loss from the circulation conduit system of the pneumatic dryer.

The pneumatic dryer 100 typically receives wet biomass particles of the size <25 mm at the bulk material inlet of the pneumatic dryer. The bulk material is dried by the superheated steam in the drying conduit 21 while being transported to said first cyclone 30. Drying is achieved at steady state conditions, meaning fully developed pneumatic transport of said material by the steam. Moisture and volatiles such as hydrocarbons and terpenes in the wet bulk material vaporizes and forms surplus steam. Drying i.e. evaporation of moisture from the bulk material is causing the temperature of the superheated steam to drop in the direction of flow in the drying conduit.

The first cyclone 30 is connected to the drying conduit 21 downstream of the inlet feeder 10. The first cyclone 30 separates the bulk material from the steam. The steam is extracted via a first cyclone steam outlet 302 and recirculated into said heating section connected to said first cyclone 30. The dried bulk material is discharged from said first cyclone 30 via a first cyclone material outlet 303 to a first classifier 81 via a first classifier inlet 811 in a controlling section C1 of the pneumatic dryer.

Furthermore, the circulation conduit system of the pneumatic dryer 100 also comprises said controlling section C1 , where the size of the particles and the particle size distribution of the dried material is controlled and reduced. The controlling section is connected to said first cyclone material outlet 303. The controlling section C1 comprises a first classifier 81 , a coarse mill 91 , a second cyclone 31 , a first outlet feeding means 70 and a second ventilator 51.

The controlling section C1 is pressurized and steam filled and the elements of the controlling section C1 are adapted to operate above atmospheric pressure. The first classifier 81 comprises an aerodynamic classifier, such as a rotary classifier. The first classifier 81 is connected to the first cyclone material outlet 303 of the first cyclone 30, and receives the bulk material from the first cyclone 30. Additionally, the first classifier 81 withdraws steam from said bulk material outlet 303 to provide the necessary flow rate for the aerodynamic classification. Aerodynamic classifiers are known to achieve a sharp particle size distribution, a well defined cut-point, between the separated fractions. 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.

Said first classifier 81 comprises a first classifier inlet 811 , a first classifier accept outlet 812 and a first classifier reject outlet 813. The object of the first classifier 81 is to aerodynamically separate the particles which have an acceptable size from those particles in the bulk material having the largest equivalent diameter and/or particles which have the highest density due to high moisture content. The probability for a given particle to become rejected by a rotary classifier increases with the product of the size and particle density i.e. moisture content in accordance with the Stokes law applied in a centrifugal force field created by respectively within the rotor of the classifier.

In operation of the pneumatic dryer 100 the first classifier 81 classifies the dried bulk material into two fractions demonstrating respectively different particle size distributions, hereafter called a first accept fraction comprising fine particles and a first reject fraction comprising coarse particles. A rotary classifier is particularly suitable for classifying bulk material in a pressurized environment.

A first mill 91 is connected to the first classifier reject outlet 813 of the first classifier 81 , and receives one of said fractions, the first reject fraction which comprises the most coarse particles of the bulk material. The first mill 91 has the objective to reduce the top size of the dried bulk material by reducing the size of the particles in said fraction by milling. Thus the first mill 91 is adapted to operate as a coarse mill. The first mill 91 has a mill outlet conduit 911 connected to the drying conduit 21 of the drying section 20. The bulk material obtained by milling is recirculated into the drying conduit 21 for further drying.

An advantageous control of the particle size distribution and moisture content of the dried material is thus achieved. The quality aspects of the final product corresponding to the first accept fraction is improved since the quality is proportional to the particle size distribution and deviation of the particles end moisture content. Since the first mill 91 is integrated in the pneumatic dryer it must be designed for operation in a pressurized environment. There are several suitable milling means such as a refiner mill, a pin mill or a vertical rotor mill. All these milling means except the refiner mill require a separate mill steam aspiration circulation. This is a necessity for safe transporting of the bulk material through the milling zone and to avoid local over heating in these milling means.

The first reject fraction comprises particles which are rejected by the first classifier 81 due to size and high particle density i.e. high moisture content. Size reduction by milling and recirculation of these particles into the drying conduit 21 makes it possible to achieve the desired final moisture content of the product with a shorter drying conduit.

The controlling section C1 further comprises a second cyclone 31 connected to the first classifier 81 via the first classifier accept outlet 812. The second cyclone 31 receives and separates the first accept fraction from steam. The second cyclone 31 has a second cyclone material outlet 313 and a second cyclone steam outlet 312. The second cyclone material outlet 313 is connected to an outlet feeding means 70 for discharging the first accept fraction which has a particle size distribution corresponding to the requirements of a specified final product. The outlet feeding means 70 is pressure-sealing and comprises a cellular feeder of a similar type as the inlet feeder 22 described above.

Surplus steam corresponding to the amount of steam that has been vaporized from the bulk material is released from the circulation conduit system via a surplus steam outlet 40, connected to the second cyclone steam outlet 312. The pressure level in the circulation conduit system is thereby controlled and maintained. Said surplus steam is conveyed into steam processing means (not shown in the figure) connected to the pneumatic dryer.

One advantage of releasing surplus steam from the second cyclone steam outlet 312 conveyed from the first cyclone material outlet 303 of said first cyclone 30 is that the steam flow rate for aerodynamic classification by the classifier is provided. If the flow rate of surplus steam is insufficient for optimal operation of the rotary classifier 81 , additional steam is withdrawn, extracted, by a second ventilator 51 connected to the second cyclone steam outlet 312. The second ventilator 51 is further connected to the heating section with a steam return conduit 511 to return the steam to the heat exchanger inlet 61 to convey the extracted steam to the heat exchanger 60 for superheating. Another advantage of withdrawing steam from the bulk material outlet 303 is that the cyclone vortex is moved downwards towards the material outlet. The dust concentration is then reduced in the steam discharged via the first cyclone gas outlet 302, thereby increasing the efficiency of the cyclone 30.

The conduit system of the pneumatic dryer 100 is configured to be substantially closed to maintain the pressure inside the pneumatic dryer in operation at a desired level. The pneumatic dryer operates at a pressure level above the atmospheric pressure, typically at 1-10 bar, more preferably 2-5 bar overpressure. The pressure level and temperature are selected and controlled depending on the type of bulk material, the moisture content of the feedstock and the requirements on the final product.

In one example biomass of wood is fed to the pneumatic dryer and dried. It is then advantageous to select a steam pressure level inside the pneumatic dryer which corresponds to a particular steam saturation temperature. This steam saturation temperature is preferably within the range of the lignin softening temperature given the specific material and its specific moisture content. When the lignin is softened, the milling step to reduce the particle size in the bulk material is more efficient which significantly reduces specific power consumption in the mill or mills.

Several problems are addressed by pressurizing the steam inside the circulation conduit system of the pneumatic dryer 100. The efficiency of the drying process depends on the recovery rate of the surplus steam formed by the vaporized moisture from the bulk material and the recovery rate of the auxiliary power necessary for operation of the pneumatic dryer, such as the energy supplied to the first ventilator 50. The energy is recovered by indirect heat transfer of latent heat in the surplus steam to a steam processing means such as steam regenerators for condensing steam, heat exchangers producing low pressure steam, a steam turbine, or a district heating net. The possibilities to utilize this vast heat source increases with the temperature i.e. with the pressure in the pneumatic dryer. Additionally, the heat capacity of the steam increases with the dryer pressure making it possible to reduce the flow and the dimensions of the dryer. .

With reference to figure 2 a second embodiment of the process and a pneumatic dryer 200 according to the present invention is shown. In operation of this embodiment of the process and pneumatic dryer 200 two different products are produced with respectively different particle size distributions. The first product comprises coarse bulk material which is particularly suitable in the production of bio fuel pellets and the second product comprises dried bulk material with a high proportion of fines suitable as fuel in pulverized fuel firing. For several reasons the quality expectations on bio fuel pellets require a specifically low proportion of fines. One reason is that the impurities originating from wood based biomass are usually concentrated in the fines of the dried material. These impurities are detrimental for the quality of bio fuel pellets. Another reason is that the fines as such give rise to dust when the bio fuel pellets are used which is undesirable.

It is therefore particularly advantageous to use a pneumatic dryer to produce several final dry products, each having respectively different particle size distribution corresponding to more precise expectations of each product.

In this embodiment of the invention the features of the pneumatic dryer 200 disclosing the same purpose and configuration as in the pneumatic dryer 100 are referred to with corresponding reference numbers. In particular the drying section 20 and the heating section 10 correspond to those previously described.

Figure 2 shows a pneumatic dryer 200 comprising a controlling section C2. The controlling section C2 comprises a first classifier 81 , a first mill 92, a second cyclone 31 , a second ventilator 51 , a first outlet feeding means 71 , a second outlet feeding means 72 and a surplus steam outlet 40.

The controlling section C2 is pressurized and steam filled and the elements of the controlling section C2 are adapted to operate above atmospheric pressure.

The first classifier 81 is connected to the first cyclone 30 as described above. In operation of the pneumatic dryer 200 the first classifier 81 classifies the dried material into two fractions hereafter called first accept fraction and first reject fraction. The dried bulk material rejected by the first classifier 81 comprises a particle size distribution and moisture content corresponding to the requirements of a specified first final product thus the first reject fraction is discharged via a first outlet feeding means 71 connected to the first classifier reject outlet 813.

The first outlet feeding means 71 is pressure-sealing and comprises a cellular feeder of a similar type as the inlet feeder 22 described above.

A second cyclone 31 is connected to the first classifier accept outlet 812. The second cyclone 31 has a second cyclone steam outlet 312 and a second cyclone material outlet 313. The second cyclone 31 receives the first accept fraction, the fines, from the first classifier accept outlet 812, and separates the first accept fraction from the steam.

However, to prepare the first accept fraction to be used for example as pulverized fuel, a further reduction of the particle size is carried out by milling with a first mill 92. Thus in this embodiment is the first mill 92 adapted to operate as fines mill. The first mill 92 comprises a mill of the same type as the first mill 91 as described above. The first mill 92 is connected to the second cyclone material outlet 313 of the second cyclone 31 , thereby receiving the separated bulk material.

Furthermore, the controlling C2 section also comprises a surplus steam outlet 40, which is further described in the first embodiment of the invention.

As previously mentioned, it is advantageous to carry out size reduction by milling within the pressurized environment, specifically since the elevated temperature improves the milling performance A second outlet feeding means 72 is connected to the mill 92 to discharge to the second final product comprising the fine dried material. The second outlet feeding means 72 is pressure-sealing and comprises a cellular feeder of a similar type as the inlet feeder 22 described above.

With reference to figure 3 a third embodiment of the process and a pneumatic dryer 300 according to the present invention is shown.

This embodiment shows a combination of the previous embodiments. In operation of a pneumatic dryer 300 the size of the particles in the first reject fraction is reduced and the proportion of fine particles in the finest fraction is increased by particle size reduction.

The pneumatic dryer 300 produces two final products with respectively different particle size distributions. A first final product comprising a bulk material with intermediate sized particles particularly suitable for producing bio fuel pellets and a second final product comprising bulk material with fine sized particles particularly suitable as pulverized fuel is produced.

In this embodiment of the invention the features of the pneumatic dryer 300 disclosing the same purpose and configuration as in the pneumatic dryers 100 and/or 200 are referred to with corresponding reference numbers. In particular the drying section 20 and the heating section 10 correspond to those previously described.

This embodiment shows a pneumatic dryer 300 comprising a heating section 10, a drying section 20 and a controlling section C3. The controlling section comprises a first classifier 81 , a first mill 91 , a second mill 93, a second classifier 82, a second cyclone 31 , a second ventilator 51 a first outlet feeding means 71 , a second outlet feeding means 72 and a surplus steam outlet 40.

The controlling section C3 is pressurized and steam filled and the elements of the controlling section C3 are adapted to operate above atmospheric pressure. The first classifier 81 is connected to the first cyclone 30 as described above. In operation of the pneumatic dryer 300 the first classifier 81 classifies the dried material into two fractions hereafter called first accept fraction and first reject fraction. The first classifier 81 comprises a first classifier inlet 811 , a first classifier accept outlet 812 and a first classifier reject outlet 813. A first mill 91 is connected to the first classifier 81 via the first classifier reject outlet 813 and receives the first reject fraction, the most coarse fraction. The first mill 91 has the objective to reduce top size of the bulk material by size reduction of particles in the first reject fraction by milling. The first mill 91 is therefore adapted to operate as a coarse mill. The first mill 91 comprises a mill outlet conduit 911 connected to the inlet of the drying conduit 21 in the drying section 20. After reducing the size of the particles, the bulk material obtained by milling is recirculated into the drying conduit 21 for further drying, thus an advantageous control of the particle size distribution and control of the moisture content of the dried material is achieved.

The second classifier 82 is connected to said first classifier 81 via a second classifier inlet 821 and the first classifier accept outlet 812 and receives the first accept fraction from the first classifier 81. The second classifier 82 comprises a second classifier accept outlet 822 and a second classifier reject outlet 823. The second classifier 82 comprises an aerodynamic classifier, such as a rotary classifier. The second classifier 82 aerodynamically classifies the first accept fraction into two further fractions which have different particle size distribution, hereafter called second accept fraction and second reject fraction. The object of the second classifier 82 is to ensure the particle size distribution and the mass flow of the second accept fraction.

The second reject fraction comprises bulk material having an intermediate particle size distribution and is particularly suitable for producing bio fuel pellet of high quality. The advantage with the second reject fraction is primarily that the finest particles, the fines, have been removed and concentrated in the second accept fraction by the second classifier 82. The second classifier reject outlet 823 is connected to a first outlet feeding means 71 for discharging the first final product, the second reject fraction. The first final product is further treated in a non pressurized environment outside the pneumatic dryer. The first outlet feeding means 71 is pressure-sealing and comprises a cellular feeder of a similar type as the inlet feeder 22 described above.

A second cyclone 31 is connected to the second classifier 82 via the second classifier accept outlet 822 and receives the second accept fraction. The second cyclone 31 comprises a second cyclone gas outlet 312 and a second cyclone material outlet 313. A second mill 93 is connected to the second cyclone 31 via the second cyclone material outlet 313 and receives the second accept fraction. The second mill 93 reduces the particle size of the second accept fraction to achieve the second final product comprising a fine dried bulk material which is suitable to be used as pulverized fuel in a pulverized fuel fired boiler without further size reduction. The second mill 93 is therefore adapted to operate as a fines mill. A second outlet feeding means 72 is connected to the second mill 93 for discharging the second final product. The second outlet feeding means 72 is pressure-sealing and comprises a cellular feeder of a similar type as the inlet feeder 22 described above.

Furthermore, the controlling C3 section also comprises a surplus steam outlet 40, which is further described in the first embodiment of the invention.

With reference to figure 4 a fourth embodiment of the process and pneumatic dryer 400 according to the present invention is shown.

In this embodiment of the invention the features of the pneumatic dryer 400 disclosing the same purpose and configuration as in the pneumatic dryer 200 are referred to with corresponding reference numbers. In particular the drying section 20 and the heating section 10 correspond to those previously described.

This embodiment is particularly suitable when a high precision cut of oversized particles in a final product is requested. Such a product is particularly suitable in the production of wood polymer composites. Another advantage is that the specific power consumption in the first mill 92 is reduced given a requested limit in the amount of oversized particles.

The pneumatic dryer 400 of this embodiment produces two final products with respectively different particle size distributions. A first final product comprising coarse dried bulk material suitable for production of bio fuel pellets and a second final product comprising fine dried bulk material suitable as pulverized fuel or for production of wood polymer composites.

In figure 4 the pneumatic dryer 400 is shown which comprises a heating section 10, a drying section 20 and a controlling section C4.

The controlling section C4 comprises a first classifier 81 , a second classifier 82, a first mill 91 , a second cyclone 31 , a second ventilator 51 , a first outlet feeding means 71 , a second outlet feeding means 72 and a surplus steam outlet 40. The controlling section C4 is pressurized and steam filled and the elements of the controlling section are adapted to operate above atmospheric pressure.

The first classifier 81 is connected to the first cyclone 30 as described above. In operation of the pneumatic dryer 400 the first classifier 81 classifies the dried material into two fractions hereafter called first accept fraction and first reject fraction. The dried bulk material rejected by the first classifier 81 comprises a particle size distribution and moisture content corresponding to the requirements of a specified final product thus the first reject fraction is discharged as the first final product via a first outlet feeding means 71 connected to the first classifier reject outlet 813. A second classifier 82 is connected via a second classifier inlet 821 to the first classifier accept outlet 812 of the first classifier 81 whereby the second classifier 82 receives the first accept fraction from the first classifier 81. The second classifier 82 comprises preferably a rotary classifier operating as previously described. The second classifier 82 classifies the bulk material of the first accept fraction into a second accept fraction and a second reject fraction. The second classifier 82 comprises a second classifier reject outlet 823, and a second classifier accept outlet 822.

A first mill 92 is connected to the second classifier via the second classifier reject outlet 823. The first mill 92 receives the second reject fraction and reduces the particle size of the second reject fraction by milling, thus the first mill 92 is adapted to operate as a fines mill. The first mill 92 comprises a first mill outlet 921 which is connected to the second classifier inlet 821. The bulk material obtained by milling is recirculated by a flow of steam conveyed from the second ventilator 51 to the second classifier 82 via the second classifier inlet 821. The particle size distribution in the second accept fraction, the finest fraction, is controlled by recirculating the bulk material obtained by milling to the classifier 82 for further classifying, A second cyclone 31 is connected to said second classifier 82 via a second cyclone inlet 312 and a second classifier accept outlet 822. The second cyclone 31 receives the second accept fraction and separates the bulk material from steam. A second outlet feeding means 72 is connected to the second cyclone material outlet 313 whereby the second final product, the fine dried bulk material is discharged.

The first and second outlet feeding means 71 ,72 are pressure-sealing and comprises cellular feeders of a similar type as the inlet feeder 22 described above.

Furthermore, the controlling C4 section also comprises a surplus steam outlet 40, which further described in the first embodiment of the invention. The process and pneumatic dryer for treating bulk material according to the invention are 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 achieved by the process and pneumatic dryer according to the invention.

Coarse final product 4mm < d ₉₇ < 10 mm suitable for pellets and briquettes or similar.

Fine final product 0,7mm < d _g7 < 2,5 mm, suitable for pulverized fuel firing or similar.

Very fine final product 200 pm < d ₉₇ < 400 pm, suitable for wood polymer composites or similar.

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.