Field of the invention

The invention relates to a method for torrefying biomass. The invention also relates to a fluidized bed boiler, a torrefaction system, and the use of a fluid- ized bed boiler. Background of the invention

The use of biomass in various energy production processes is increasing. In particular, the use of wood material as an energy source which is renewable and neutral in view of carbon dioxide emissions has increased and contrib- utes to replacing the use of non-renewable energy sources which generate carbon dioxide emissions, such as oil, peat and natural gas. In the present application, the term "biomass" refers to any materials, of biological origin which are suitable for use in the production of biocoal. Biomass typically comprises virgin materials or waste materials originating from plants, such as wood or grass. In particular, biomass may comprise wood-based materials, that is, so-called forest fuel. Forest fuels include, for example, wood, bark, wood chips, logging residue, stumps, branches, and brushwood.

The use of biomass as an energy source involves problems, too: the quality and the moisture content of the biomass may be inconsistent, its energy density may be low, the biomass may get wet during the storage, the moist biomass may be decomposed by micro-organisms, particularly fungi, and further, the grinding of the biomass may be difficult. These problems can be reduced by torrefying, that is roasting, the biomass before it is used for energy. For these reasons, particularly the torrefaction of wood has been intensely developed in recent years.

Torrefaction is one method to roast material, such as biomass. Torrefaction is a thermal process ro improve the quality of the final product and to give the raw material added value. Known technology is described by P.C.A. Bergman, A.R. Boersma, R.W.R. Zwart, and J.H.A. Kiel in "Torrefaction for biomass co-firing in existing coal-fired power stations", ECN-C-05-013 (2005). In torrefaction, biomass is heated under inert conditions, that is, low in oxygen, and at atmospheric pressure, at a temperature of about 200 to 320 °C. During the process, water and a small part of both biopolymers and other volatiles are removed from the biomass. Furthermore, the micro-organisms are destroyed. The process is characterized by a long retention time, normally from about 15 minutes to one hour. The long retention time is due to the very low heating rate of the material, which is normally lower than 50°C/min. During the process, the biomass loses about 30% of the mass, among other things with the moisture, but only 10% of the energy content is lost. Thus, the energy density of the fuel increases to an about 1.3 fold value during the torrefaction. For the torrefaction, it is advantageous that the biomass is already relatively dry (with a moisture content of about 15 to 20%) and that the biomass has been crushed or chipped into pieces of a relatively small size (<50 mm). To secure sufficient dryness of the biomass, it is normally dried before the actual torrefaction. Typically, the dryness of the biomass may be, for example, about 40 to 60% before the drying, and the crushing can be implemented either before or after the drying.

In the torrefaction process, the biomass is heated to the torrefaction tem- perature, at which it is kept for a sufficiently long time, that is, for the retention time. The actual torrefaction stage with the pre-heatings may take 60 to 90 minutes. The torrefaction process is implemented in a torrefaction reactor, to which the heat is supplied directly or indirectly from a heat source. According to prior art, the heat source may be a combustion process, and the heat can be recovered from the flue gases of this combustion process by means of heat transfer medium in a heat exchanger, wherein such a heat exchanger is placed in a flue gas duct. In direct heat supply, the heat is carried by hot heat transfer medium which is led to a torrefaction reactor and brought into contact with the biomass to be torrefied. In direct heat supply according to prior art, gas is normally used as the heat transfer medium. In indirect heat supply, the wall, the bottom or the top of the torrefaction reactor is heated with the heat transfer medium, wherein heat is transferred through the wall, the bottom or the top of the torrefaction reactor to the biomass. In this case, oil is often used as the heat transfer medium. The torrefaction reactor may be, for example, cylindrical, and in the reactor, the biomass can be transferred by means of a screw conveyor or the like. It is also known to dry the biomass before the actual torrefaction. In arrangements of prior art, the dry- ing is normally implemented by supplying heat directly to the biomass, for example by means of hot air or flue gas.

According to US 2010/0251616, it is also known to torrefy biomass, particu- larly wood chips, by supplying the heat to the biomass directly by means of a fluid medium, such as oil. Such a process, in which the medium is not in gaseous state, is called torrefaction with a medium. It is possible that the fluid medium accelerates the torrefaction process. According to said publication, the torrefaction with a medium can be implemented with, for example, two preheating steps and one thermal processing step, each step taking about 15 minutes.

After the torrefaction, the final result, that is, the roasted material, is normally dark coke, which is often called biocoal as well. The biocoal obtained by tor- refaction is a homogeneous, hydrophobic and very well preservable product which can be crushed with a significantly lower consumption of electricity than normal pellets. Finally, crushed biocoal can be easily pelletized to an energy-dense product for preservation and transportation. Torrefaction can also be regarded as a mild form of pyrolysis. In pyrolysis, the aim is to remove pyrolysis vapour from biomass, which pyrolysis vapour is condensed to pyrolysis oil. In the pyrolysis, the biomass is treated at a very high temperature, for example at about 400 to 800 °C, wherein primarily pyrolysis vapour is formed in the process, and a smaller quantity of coke is obtained as a side product. Compared with the pyrolysis, the temperature used in the torrefaction process is considerably low, wherein no significant pyrolysis vapours are released from the biomass. The energy content of the biocoal obtained by torrefaction is substantially independent of the gases and vapours released from the biomass during the torrefaction. Furthermore, because only a small quantity of energy-containing gases are released from the biomass during the torrefaction, these gases are not condensed into another product but they can be guided to the boiler. The main products of the pyrolysis process are such gas and oil condensed from it. An essential part of the pyrolysis apparatus is a condenser for condensing pyrolysis vapours into pyrolysis oil. A torrefaction reactor does not comprise such a condenser, but means for removing excessive torrefaction gas from the tor- refaction process. Such an excess can be, for example, burnt, and some tor- refaction systems of prior art comprise a separate burner for burning the excessive torrefaction gas.

The indirectly heated torrefaction process of prior art may be slow, and the fuel produced by it can be expensive. According to the prior art, the heat of the boiler can be recovered from the flue gases. Thus, the heat exchanger needed may be bulky and difficult to install in the flue ducts of the boiler. For energy use of biomass, the use of a liquid medium has been suggested to accelerate the torrefaction process, but no technical solutions have been presented for implementing the process in an economically viable way.

Brief summary of the invention

It is an aim of the present invention to reduce the drawbacks presented above. It is presented that the torrefaction reactor can be integrated in a flu- idized bed boiler, wherein in an embodiment of the torrefaction process it is possible to use a solid heat transfer medium. In this embodiment, it is possible to use a considerably shorter retention time than in solutions of prior art. In another embodiment, the heat for the torrefaction can be recovered from the furnace of the boiler by means of a superheater, wherein it is possible to use a smaller superheater or smaller superheaters than in solutions of prior art, in which a heat exchanger in a flue gas duct is utilized for recovering heat from the flue gases. The fluidized bed boiler according to the present invention is primarily characterized in that the fluidized bed boiler comprises:

- means for heating heat transfer medium,

- means for transferring the heat transfer medium to a torrefaction reactor, and

- means for receiving the heat transfer medium from the torrefaction reactor.

The method according to the present invention is primarily characterized in

- heating heat transfer medium with a fluidized bed boiler,

- transferring the heat transfer medium from said fluidized bed boiler to a torrefaction reactor, and - transferring said heat transfer medium from said torrefaction reactor to said fluidized bed boiler.

The torrefaction system according to the present invention is primarily cha- racterized in that the torrefaction system comprises:

- a fluidized bed boiler for heating heat transfer medium ,

- means for conveying said heat transfer medium from said fluidized bed boiler to said torrefaction reactor, and

- means for conveying said heat transfer medium from said torrefaction reactor to said fluidized bed boiler.

With a fluidized bed boiler according to an embodiment of the invention, it is possible to perform the torrefaction of biomass in a significantly more viable way than with conventional technical solutions. Furthermore, it is possible that the presented torrefaction process with a medium is considerably faster than the techniques for torrefying wood according to the prior art.

Description of the drawings In the following, the invention will be described in more detail with reference to the appended drawings, in which:

Fig. 1 a shows a bubbling fluidized bed boiler according to prior art, Fig. 1 b shows a circulating fluidized bed boiler according to prior art,

Fig. 2 shows a torrefaction reactor integrated with a bubbling fluidized bed boiler.

Fig. 3 shows another torrefaction reactor integrated with a circulating fluidized bed boiler,

Fig. 4 shows a third torrefaction reactor integrated with a bubbling fluidized bed boiler, Fig. 5a shows a fourth torrefaction reactor integrated with a bubbling fluidized bed boiler,

Fig. 5b shows a torrefaction reactor according to Fig. 5a, in which the direction of circulation of the torrefaction gases has been changed,

Fig. 6a shows a separator based on a fluidized bed, and Figs. 6b and 6c

show a side view and a top view, respectively, of a separator based on a sieve.

In Figs. 1 to 6, corresponding numerals or symbols are used for correspond- ing elements.

Detailed description of the invention

Figure 1 a shows a fluidized bed boiler 100 of prior art. A furnace 106 is limited on the sides by walls 104 of the fluidized bed boiler. From below, the furnace is limited by a grate 108. The furnace of the fluidized bed boiler comprises incombustible solid bed material 12, such as sand. Figure 1a shows a bubbling fluidized bed boiler (BFB boiler). Air is supplied through the grate to the furnace, as shown by an arrow 110. By means of the supply of air 110, the sand is fluidized, and inflammable material mixed in the sand is burnt. The quantity of air to be supplied to the bubbling fluidized bed boiler is relatively low, wherein the bed material is fluidized primarily on the bottom of the furnace, on the grate, as shown in Fig. 1a. Heat can be recovered from flue gases by heat exchangers 114 and 116. The path of the flue gases is illustrated with arrows 120 and 122.

Figure 1 b shows another fluidized bed boiler 150 according to prior art. The furnace 156 is limited on the sides by the walls 154 of the fluidized bed boiler. From below, the furnace is limited by a grate 158. The furnace of the fluidized bed boiler comprises incombustible solid heat transfer medium, such as sand. The fluidized bed boiler of Fig. 1 b is a circulating fluidized bed boiler (CFB boiler). Air is supplied through the grate to the furnace, as shown by an arrow 160. By means of the supply of air 160, the sand is fluidized and circulated, and the inflammable material mixed in the sand is burnt. Consequently, the quantity of air to be supplied to the circulating fluidized bed boiler is so high that the bed material and the material to be burnt rise upwards in the furnace 156. The circulation of the bed material is denoted with arrows 170, 172 and 174, whereas the circulation of the combustion air is denoted with arrows 160 and 170. In a cyclone 184, bed material is separated from flue gases and returned into the furnace. In the lower parts of the cyclone, heat contained in the bed material can be recovered by a heat exchanger 182. The cyclone extends down to the bottom 192, from where the bed material is circulated further to the furnace 156. The heat exchanger 182 may also be placed in the upper parts of the cyclone. The path of flue gases separated in the cyclone is illustrated with an arrow 190. Heat exchangers 180 can be used to recover heat from the flue gases.

One idea of the invention is to utilize energy of the hot bed material or hot flue gases of the fluidized bed boiler or radiant energy of the combustion process as a source of heat for torrefaction. The energy required for the torrefaction can be produced in the fluidized bed boiler, for example by heat- ing the torrefaction gases in the furnace or the flue gas ducts of the fluidized bed boiler, or heat can be introduced with hot bed material into the torrefaction reactor. When direct heat transfer is used, it is necessary to heat the heat transfer medium, to bring the heated heat transfer medium into contact with the biomass, and to circulate the heat transfer medium that was in con- tact with the biomass, to be reheated. In the embodiments shown in Figs. 2 to 5, the bed material of the fluidized bed boiler is sand, but it would also be possible to use other corresponding inflammable bed material.

Figure 2 shows the use of a fluidized bed boiler according to Fig. 1 a as a source of heat for a torrefaction reactor 200. Heat is recovered by a heat exchanger 210 placed in the furnace of the boiler, where the torrefaction gases 212 to be led to torrefaction process are heated. The torrefaction gases heated in the heat exchanger are denoted with the reference numeral 412. The torrefaction gases 412 heated in the heat exchanger 210 are transferred to the torrefaction reactor 200 by suitable means, such as a pipe 213. In a corresponding manner, cooled torrefaction gases 212 are transferred from the torrefaction reactor 200 to the heat exchanger 210 by suitable means, such as a pipe 215. The circulation of torrefaction gases can be boosted by a fan 214. A heat exchanger for heating torrefaction gases can also be provided in the bed area, that is, inside the bed material 112. Such a heat exchanger is denoted with the reference numeral 118. Advantageously, the heat exchanger is arranged in the furnace 106, to achieve sufficiently efficient heat transfer from the furnace to the torrefaction gases.

The biomass 202 to be torrefied is supplied to the torrefaction reactor 200. The heat transfer in the torrefaction may be advantageously direct; that is, the torrefaction gases can also be guided to the torrefaction reactor, wherein the biomass is brought into contact with the torrefaction gases. It is also possible that the heating of the biomass is arranged indirectly, wherein the torrefaction reactor is heated from the outside with hot gases. In the torrefaction reaction, gases are released from the biomass, wherein the excess 216 of the torrefaction gases can be guided to the fluidized bed boiler, for burning. The excess of the torrefaction gases can be guided to the furnace by suitable means, such as a tube 219.

In the torrefaction reactor 200, the biomass 202 is roasted to biocoal 204. The biomass may advantageously comprise at least one of the following: wood chips, straw, and hay. Furthermore, the raw material of the process may comprise, for example, waste paper, municipal solid waste, or both of these. The conveying of the biomass in the torrefaction reactor may be arranged with a conveyor, for example a screw or belt conveyor, or the torrefaction reactor may be arranged at an angle to the horizontal plane, in which case the biomass can be moved in the reactor by gravity. The torrefaction reactor 200 may be a retort furnace, referring, for example, to a cylindrical reactor whose axis may be substantially parallel to the horizontal plane or arranged at said angle to the horizontal plane. Biomass is transferred to the reactor, and the reactor may be arranged to rotate around its axis, wherein the rotary movement mixes the biomass and the biomass is heated relatively evenly. The torrefaction reactor may also be a fluidized bed reactor, in which the biomass is fluidized by torrefaction gases. In the torrefaction reactor, the biomass is heated to a torrefaction temperature which is advan- tageously 200 to 320 "C, and the biomass is kept at the torrefaction temperature for a retention time. In torrefaction implemented with direct heat transfer and with torrefaction gas as the heat transfer medium, the retention time is advantageously about 10 to 30 min. In torrefaction implemented with indirect heat transfer, the retention time may be 15 to 60 min.

Direct heat transfer by means of torrefaction gases will require circulation of the torrefaction gases, because the torrefaction gases have to be low in oxygen (inert) to avoid burning of the biomass during the torrefaction. The oxygen content of the torrefaction gas may be lower than 3%, advantageously lower than 0.5% and preferably lower than 0.1 %. The oxygen content should be low, because oxygen reacts with the biomass to be torrefied and thereby reduces the quantity of biocoal to be obtained from the process. In an embodiment of the invention, flue gases can be admixed to the torrefaction gases. The oxygen content of the flue gases may be for example 2 to 6%, and the torrefaction gas may comprise for example deoxygenized flue gases, or low quantities of unprocessed flue gases can be admixed to the torrefaction gas. If the torrefaction gas contains oxygen, the oxygen content of the torrefaction gas will decrease as a result of burning in the torrefaction process. Furthermore, if the torrefaction gas is recirculated in the process, the oxygen content of the torrefaction gases will reduce to a low level. Possibly, the oxygen content of the torrefaction gases may be higher at the initial stage of the process than in the later stages of the process.

It is obvious that a torrefaction reactor of the type shown in Fig. 2 can also be integrated in a circulating fluidized bed reactor according to Fig. 1 b. Thus, the heat exchanger 210 for the torrefaction gases can be arranged not only in connection with the furnace but also in connection with the circulation of the bed material; for example, the heat exchanger 182 can be used as s such a heat exchanger, or a heat exchanger may be provided in the upper part 184 of the cyclone (Fig. 1 b). Furthermore, the torrefaction gases can be heated by heat exchangers 114, 116 or 180 in the flue gas ducts. After the torrefac- tion process, heat can be recovered from the hot biocoal by a heat exchanger 220. If necessary, the hot biocoal can also be cooled in another low-oxygen environment.

Figure 3 shows another fluidized bed boiler, in which a torrefaction reactor 300 is integrated. The fluidized bed boiler shown in Fig. 3 is of the circulating fluidized bed boiler type, but it is obvious that also a bubbling fluidized bed boiler could be used as the source of heat. In the embodiment of Fig. 3, the heat transfer medium is the bed material of the fluidized bed boiler, for example sand. Hot bed material can taken out of the fluidized bed boiler via a bed channel 302. The channel 302 may be arranged, for example, in connection with the circulation of bed material in a cyclone 184. In particular, the channel may be arranged at the bottom 192 of the cyclone 184. The hot bed material can be led via a heat exchanger 303 and another channel 305 to the torrefaction reactor 300. In the cyclone, the temperature of the bed material may be, for example, as high as 800 to 900 °C, wherein the bed material is advantageously cooled before it is supplied to the torrefaction reactor. At the torrefaction reactor, the heat can be transferred directly or indirectly to the biomass. In direct heat transfer, which is illustrated in Fig. 3, hot bed material is supplied to the torrefaction reactor and brought into contact with the biomass, wherein the biomass is heated and torrefied. In indirect heat transfer, hot bed material is used to heat the torrefaction reactor containing biomass. In indirect heat transfer, hot bed material is used to heat the torrefaction reactor from the outside.

Advantageously, the heat transfer is arranged as direct heat transfer. Thus, the solid medium is in contact with the biomass, and the torrefaction can be significantly accelerated. In the torrefaction, moisture and other volatiles are released, which can be led into the furnace 156. This is denoted with an arrow 304 which illustrates, on one hand, the excess of the torrefaction gases and, on the other hand, the means by which said excess is led into the furnace. The gases can be led into the furnace by suitable means, such as a pipe, and the gas flow can be boosted by a fan 214.

When direct heat transfer is used, the bed material, such as sand, has to be separated from the biocoal in a separator 306 after the torrefaction. The separator is advantageously arranged in connection with the torrefaction reactor 300. From the separator 306, biocoal 204 can be led into the heat exchanger 220, in which it is cooled. The heat exchanger is not necessarily always needed. The sand separated in the separator is recirculated to the process. The separated bed material from the separator is denoted with the reference numeral 308. From this bed material, part 310 can be recirculated to the torrefaction reactor along a recirculation line 31 1 , and part 312 can be returned to the fluidized bed boiler. The return of the bed material 312 to be reheated can be arranged, for example, via a return channel 313. It is also possible that all the separated bed material 308 is returned to reheating, in which case the separated bed material is not divided into parts 310 and 312 but the bed material 308 is returned to the fluidized bed boiler along the return channel 313.

Consequently, hot sand via the channel 305 as well as biomass 202 are supplied to the torrefaction reactor, and sand 310 separated by the separator can also be supplied. By adjusting the relative quantities of these it is possible to change the temperature prevailing in the torrefaction reactor. Advanta- geously, the torrefaction temperature is 200 to 320 °C, as mentioned above. When the process is running, advantageously the same quantity of sand 308 is removed from the torrefaction reactor by the separator after the torrefaction, as is supplied to the torrefaction reactor via channels 3 and 305. In a corresponding manner, substantially the same quantity of sand 312 is recirculated to be heated in the fluidized bed boiler as is taken as a source of heat along the channel 305 from the fluidized bed boiler. If the heat exchanger 303 does not act as a storage or source of sand, the quantity of sand 302 taken from the fluidized bed is substantially equal to the quantity of hot sand supplied to the torrefaction reactor along the channel 305. In an embodiment, the heat exchanger 303 is used as a storage of sand to level out the quantity of sand taken from the fluidized bed boiler. The temperature of the torrefaction reactor can be adjusted further by adjusting the temperature of the hot sand 305 by means of the heat exchanger 303. In this way, it may be possible to simultaneously adjust the quantity of the sand to be recir- culated and the temperature of the torrefaction reactor. The mass ratio between the bed material and the bio mass in the torrefaction reactor can be, for example, between 1 :2 and 50:1 , advantageously between 5:1 and 30:1 , and preferably about 10:1. In some embodiments, it is also possible to use lower or higher mass ratios. The temperature of the bed material to be sup- plied to the torrefaction reactor in the channel 305 is not higher than in the fluidized bed boiler and is at least the process temperature required for torrefaction.

In an embodiment, the temperature of the bed material to be supplied to the torrefaction reactor can be adjusted with the heat exchanger 303. The suitable temperature for the bed material 305 to be supplied to the torrefaction reactor can be determined, for example, by calculating the quantity of thermal energy to be input in the process by means of the energy balance of the process, when the torrefaction temperature required for the biomass and the quantity of thermal energy required for this heating are known. This temperature that is suitable for the bed material to be supplied to the torrefaction reactor is influenced, among other things, by the ratio between the quantity of the bed material 310 circulated from the torrefaction reactor and the hot bed material introduced into the reactor via the channel 305, as well as the mass ratio between the bed material and the biomass in the torrefaction reactor 300. The moisture content of the biomass also has slight effect. In some cases, for example if a lot of bed material 310 is recirculated from the torrefaction reactor compared with the quantity of bed material supplied from the fluidized bed boiler along the channel 305, or if there is a lot of biomass compared with the quantity of the bed material, the determined suitable temperature may be equal to the temperature in the fluidized bed boiler or in the cyclone. In this case, adjustment of the temperature of the bed material may not be necessarily needed, nor is a heat exchanger 303 necessary.

The torrefaction reactor 300 may be a retort furnace, which refers, for example, to a cylindrical reactor where the axis of the cylinder may be substantially parallel to the horizontal plane. Biomass is transferred to the reactor, and the reactor may be arranged to rotate around its axis, wherein the rotary movement mixes the biomass and the biomass is heated relatively evenly. In the reactor, the biomass can also be transferred by a conveyor, for example by a screw or belt conveyor. It is obvious that the torrefaction reactor in this embodiment may be different from the embodiment of Fig. 2, because the mass in the reactor may be even considerably greater in the embodiment of Fig. 3.

In the torrefaction reactor, the biomass is heated to the torrefaction tem- perature and kept at this temperature for a retention time. In torrefaction with a medium, the retention time may be shorter or considerably shorter than in torrefaction with a gaseous medium. Because the biomass is heated in the process, the size of pieces of the biomass will affect the retention time. In view of heat transfer principles, particularly the smallest dimension of chips will affect the torrefaction rate. Typically, the size of wood chips to be torrefied is smaller than 50 mm, and typically, the size of wood chips refers to the size of a hole, through which the chips can pass. Thus, the size of a chip in the direction of at least one dimension, the middle one in the order of magnitude, is advantageously smaller than 50 mm. For biomass of this size, the retention time in the torrefaction reactor may be, for example, less than 10 minutes, advantageously less than 5 minutes, and preferably less than 2 minutes. In some embodiments and in the case of a smaller size of pieces of biomass, it may be possible to achieve a retention time less than a minute, even significantly less than a minute. These mentioned times relate to torrefaction with a medium, where the heating is implemented directly. It is also possible to combine the two preceding embodiments. Figure 4 shows an embodiment, in which a torrefaction reactor 400 is integrated with the fluidized bed boiler. The fluidized bed boiler of Fig. 4 is a bubbling fluidized bed boiler, but it could also be a circulating fluidized bed boiler as shown in Fig. 3. In the embodiment of Fig. 4, the torrefaction reactor 400 is supplied with not only biomass 202 but also heated torrefaction gases 412 and hot bed material introduced via the channel 305. In the embodiment of Fig. 4, the torrefaction reactor 400 may be similar to the other embodiments, but it may also be a fluidized bed reactor that contains bed material in addition to the biomass. In this case, therefore, a fluidized bed can be formed of the mixture of the biomass to be torrefied and the bed material in the torrefaction reactor. The fluidized bed can be formed in the torrefaction reactor by providing fluidizing gas channels at the bottom of the torrefaction reactor and by supplying fluidizing gas to the torrefaction reactor along the fluidizing gas channels, wherein the material in the torrefaction reactor is fluidized. The fluidized bed can be fluidized, for example, with torrefaction gases 412 heated in the heat exchanger 210. In this embodiment, the torrefaction gases do not necessarily need to be heated, because a sufficient supply of heat can also be provided by mere recirculation of hot sand in the torrefaction reactor. This is denoted with a broken line 414 which illustrates bypassing of the heat exchanger. For clarity, the figure does not show pipes or the like in which the torrefaction gas 212 or the heated torrefaction gas 412 is conveyed.

In the torrefaction reactor 400, some water vapour and other gases are released, which can be led, for example, into the furnace 106 for burning, for example via a pipe. This is illustrated by an arrow 216. In an embodiment, the material in the torrefaction reactor 400 is fluidized by means of flue gases in the fluidized bed boiler, wherein flue gases 122 are led as fluidizing gases 412. This embodiment is not shown in the figure. If necessary, oxygen can be removed from the flue gases before they are used as fluidizing gases. In the embodiment of Fig. 4, the heating of torrefaction gases with the heat exchanger 210 also supplies more heat energy to the torrefaction reactor. If the temperature of the hot sand suitable to be supplied to the process is determined by calculating the quantity of heat energy supplied to the process by means of the energy balance of the process, the heat energy supplied with the torrefaction gases should be taken into account. The optimum of energies supplied in different ways has to be found case by case, depending on the quality of the biomass, the moisture content of the biomass, the size of the biomass chips, as well as the properties of the bed material. The function of the bed material in the torrefaction process is not only to act as a heat transfer medium but also to enable a faster torrefaction process. The torrefaction process becomes faster and may be significantly accelerated, because the heat transfer between the solid medium and the biomass is more efficient than heat transfer from a gaseous medium to biomass. Yet another embodiment is shown in Figs. 5a and 5b. In Fig. 5a, a torrefaction reactor 500 is arranged in connection with a bubbling fluidized bed boiler. A drier 505 is provided upstream of the actual torrefaction reactor. Biomass 202 and heated torrefaction gases are supplied to the drier. The torrefaction gases 212 have been heated to hot torrefaction gases 412 by a heat exchanger 210. Part 510 of the heated torrefaction gases is led to the drier 505, and part 512 of the gases is led to the torrefaction reactor 500. By adjusting the ratio between the parts 510 and 512, it is possible to adjust the temperature in the drier 505 and in the torrefaction reactor. Moisture is released from the biomass in the drier 505; that is, the biomass dries to at least some extent, whereby vapour is released. The vapour and the torrefaction gases, whose mixture is denoted with the reference numeral 515, are led from the drier to the torrefaction reactor and are further recirculated to the process by a fan 214. The heat stored in the vapour produced in drying can be recovered for example in a condenser 550. In the condenser, the torrefac- tion gases can be cooled so that the moisture contained in the torrefaction gas is condensed into water. The water can be collected from the process. The heat collected by the condenser 550 can be transferred, for example, to district heating. The vapour formed in drying can also be led to the boiler, which is denoted by the reference numeral 216. In this case, the heat contained in the vapour can be recovered by heat exchangers 114 and 1 16 for flue gases. The heat can be used, for example, in a district heating network.

The dried biomass 522 is transferred from the drier to the torrefaction reactor. Instead of the torrefaction gases, the drying could also be implemented with flue gases 122. A difference to the preceding examples is the fact that a separate drier 505 for biomass is provided in this embodiment. If the system comprises a drier, the system may comprise a condenser 550 for cooling the vapour released from the drier, wherein the moisture is condensed. A drier could also be used in connection with torrefaction with a medium as shown in Figs. 3 and 4. Also in embodiments in which no separate drier is used, torrefaction gases can be cooled by a condenser 550 to condense moisture contained in the torrefaction gases. For clarity, Fig. 5a does not show the pipes in which the torrefaction gases 212, 216, 412, 512, 510, 515 pass. Said pipes are not shown in Fig. 5b either.

Figure 5b shows yet another way of combining a drier and a torrefaction reactor. The embodiment of the figure is, for its essential parts, similar to the embodiment of Fig. 5a. Here, however, the heated torrefaction gases 412 are led directly to the torrefaction reactor 500. In the reactor, the torrefaction gases are cooled to some extent, and furthermore, vapours and gases released in torrefaction are included in the gases. These gases 515 are led into the drier 505. Because the torrefaction gases are cooled in the reactor 500 and the temperature in the drier 505 is, by nature, lower than in the reactor 500, the recirculation of the torrefaction gases in this direction may be advantageous. The dried biomass 522 can be transferred by a conveyor, such as a screw or belt conveyor, from the drier 505 to the torrefaction reactor 500, and the torrefaction gases are automatically passed in the direction of the lower pressure. The circulation of torrefaction gases can be boosted by a fan 214. It is obvious that also this embodiment can be combined with a torrefaction process with a medium. In addition, it is obvious that the heat transfer in both the torrefaction reactor 500 and in the drier 505 can be direct or indirect. Figures 6a to 6c show some alternatives for a separator 306 for separating sand from biocoal. Figure 6a shows a separator 600 based on a fluidized bed. A mixture 652 of sand and biocoal is supplied to the separator along a channel 602. The sand and biocoal make up a bed 604 which is placed sub- stantially in the lower part of the separator 600. The height of the bed in the separator can be adjusted by adjusting the quantities of input and output material. The bed is fluidized by means of an air flow or torrefaction gases 606. The grate 608 is permeable to fluidizing gas but not bed sand. The grate 608 may be, for example, a porous structure or a perforated plate. When the fluidized bed is in a fluidized state, the biocoal 204, which is substantially lighter in weight than the bed sand 112, tends to float on top of the fluidized bed 604. In this way, most of the biocoal 204 rises onto the surface of the fluidized bed, whereas the sand 112, the heavier material, sinks to the bottom. Biocoal can be collected from an upper outlet channel 610, and bed sand can be collected from a lower outlet channel 612. The passage of the biocoal on the surface of the fluidized bed can be boosted, for example, by a conveyor 620. The conveyor may comprises a belt 622, which is arranged to move by means of belt pulleys 624. Furthermore, the belt may be equipped with scrapers 626 or other structures for scraping biocoal 204 from the sur- face of the bed. Scrapers of other types can also be used to intensify the recovery of biocoal. Upstream, downstream, or in place of the fluidized bed separator, it is also possible to use separators based on sieving.

Figure 6b shows a side view of an embodiment for separating biocoal 204 from sand 1 12 by means of a sieve 650. A mixture 652 of sand and biocoal is conveyed by a conveyor 654 to a sieve 650. The mesh size of the sieve is selected so that sand 112 falls through the sieve 650 while biocoal 204 does not. The sieve can be arranged at an angle a to the horizontal plane 660. Along the sieve, the biocoal can be transferred, for example, to another con- veyor 656. The sand can be collected, for example, onto a conveyor 658 and transferred, for example, in a direction 662 to be either heated in the fluidized bed boiler or re-used in the torrefaction reactor. Figure 6c shows the sieving arrangement of Fig. 6b in a top view. The sieve 650 preferably comprises bars 670 whose spacing is arranged in such a way that sand falls between the bars whereas biocoal 204 does not. Preferably, the spacing of the bars 670 can be, for example, about 0 mm. In Fig. 6c, the mixture of sand and biocoal is denoted with the reference numeral 652, as in Fig. 6b. The sieving can be intensified by shaking the sieve.

The biomass is typically relatively coarse material, wherein gas, such as air, is easily entrained in the biomass. In the torrefaction reactor, atmospheric oxygen would cause burning of the biomass, which is not desirable. In the different embodiments of the invention, the feeding of the biomass can be performed by means of a silo. Inert gas, such as nitrogen or flue gases, can be supplied to the silo to displace oxygen from the silo. Thus, oxygen is not significantly carried with the biomass into the torrefaction reactor, and the biomass will not burn in the torrefaction. In connection with the feeding of the biomass, it is possible to apply so-called lock hopper technique, in which the feeding can be implemented by means of two feed valves. In the feeding process, the first feed valves is upstream of the second feed valve which is thus closer to the torrefaction reactor in the feeding process. In such an arrangement, both valves can be initially closed. It is then possible, for example, to open the first valve and to fill the silo with biomass. After this, the first valve can be closed and all or part of the oxygen can be displaced from that part of the silo which is limited by the closed feed valves. Finally, the second valve can be opened and biomass can be supplied to the torrefaction reactor. With sand, hardly any oxygen is carried into the torrefaction reactor, because sand is solid material.

With the invention, it is possible to perform the torrefaction of biomass in a significantly less expensive way than with present technical solutions. Furthermore, it is possible that the presented torrefaction process with a medium is considerably faster than the prior art techniques for torrefaction of wood. The heat needed for the torrefaction process according to the present invention can be supplied by a direct contact of a heat transfer medium, such as sand of a fluidized bed boiler, or torrefaction gases, with the biomass, or indirectly. In direct contact, the heat transfer medium is brought into contact with the biomass to be torrefied. In indirect heating, a first heat transfer medium, such as sand or gas, heats the wall, bottom or top of the reactor, and the heat is transferred through the wall, bottom or top to both the biomass and the gas in the reactor. The direct contact of the solid medium may enable even a very fast roasting process. One aim of torrefaction is to maximize the energy content of biocoal. Thus, elements with a low energy content are advantageously removed from the biomass. In particular, the aim is to remove moisture by the torrefaction. However, the aim of the torrefaction is not to remove inflammable compo- nents or gases from the biomass, to keep the energy content of the biocoal high. For this reason, said 320°C can be considered the maximum temperature for the torrefaction process. After the torrefaction, the biocoal can be crushed or pelletized, or pellets can be formed of crushed biocoal. The integration of the torrefaction process with a fluidized bed boiler, as presented above, provides several alternatives and advantages. The advantages include, among other things, lower investment costs, energy efficiency and economy, as well as possibly higher speed of the process. The energy efficiency of the process and the apparatus is good, because when the tor- refaction reactor is integrated with a fluidized bed boiler, the distance for transportation of the heat transfer medium is reduced and lost heat is reduced. Furthermore, the process becomes faster when a torrefaction process with a medium is taken into use. It should also be noted that the presented integration enables a torrefaction process with a long retention time, when torrefaction gases are used as the heat transfer medium, or if the heat transfer in the torrefaction is implemented indirectly. Also in this case, the investment costs of the torrefaction system are reduced when it is integrated with an existing fluidized bed boiler. The investment costs are reduced, because the presented torrefaction system does not require a separate burner for the excess of torrefaction gases, but the excess can be burnt in the fluidized bed boiler. Furthermore, it may be possible to utilize the same means for processing biomass in connection with both the fluidized bed boiler and the torrefaction reactor. Consequently, fewer devices may be included in the integrated system than in separate systems, which reduces the investment costs. Furthermore, retort furnaces are expensive, whereby the presented fluidized bed reactor as the torrefaction reactor may be less expensive in view of the investment costs.

The invention is not limited to the above-presented embodiments, but it can be applied within the scope of the appended claims.