FI75357B - SAETT ATT TORKA IN BIOMASS. - Google Patents

Description

2 75357

Method for drying biomass Technical field

The invention relates to the treatment of biomass for the production of fuel 5. In particular, the invention relates to a combined system for peat extraction, pretreatment (including fractionation), dewatering and further processing (including heat drying). The different parts of the system work together in a synergistic way to obtain an optimal result 10 in terms of the overall economy, the usability of the system, the different types of peat and peat pulps, the quality of the final product and the environmental aspects. However, parts of the system can be truncated to connect to conventional or other systems, and it is also possible to utilize certain parts of the system to treat biomass other than peat. The latter applies above all to the drying section.

BACKGROUND OF THE INVENTION 20 In the case of peat processing, the state of the art is described in detail in Resultat-rapport NE 1982: 5, published by Nämnden för Energiproduktionsforsk-Ning, and in Vattenfalls Torvutredning 1982. 2. Usually the peat is raised as either milled peat or lump peat. Both of these lifting methods are weather dependent, which is a very big drawback. If, on the other hand, the peat is lifted by excavation or the so-called hydraulic peat method, the lifting period can be extended considerably. However, these methods do not allow any complete “year-round lifting” because they do not provide any economically acceptable technique for thawing frozen peat in winter.

Among the pre-treatment methods, e.g. fractionation-35. Thus, it is known that the removal of water from peat by compression can be considerably facilitated if the fine portion from which water is difficult to remove is separated from the 2,75,357 fibrous portion from which water is more easily removed. Such pretreatment of peat, fractionation, can be carried out mechanically according to the prior art in many different ways, e.g. by arc sieving, centrifugation; 5 With the so-called Float-Wash technique, etc. With these known methods, it is possible in itself to raise the dry matter content of the pumped suspension to the order of $ 10. However, these known techniques require that the incoming peat be free of ice, which limits the useful life of the plants. It is also not economically feasible to supplement plants with equipment for heating raw peat to melt ice. Therefore, continuous production regardless of the season is not possible, which is a significant limitation of the system.

15 It has long been known that peat forms a colloidal system with a strong affinity for water and that water colloidally bound to peat can be removed as easily as possible by pretreatment with various electrolyte additions. In particular, the water in the colloidal fine fraction of peat is bound in this way, which is why it is appreciated the importance of obtaining simpler methods or additives, so that water can also be removed mechanically from this part of the fraction. However, the polyelectrolyte additive represents a significant cost factor. This is especially true when the peat contains a large proportion of the fine fraction, which is the case for peat types with a high degree of decomposition and / or if the fine fraction of the fine fraction is recycled in the system so that the fine fraction of incoming raw peat increases.

By squeezing, the dry matter content can be increased to about 30% according to known methods. According to said result report NE 1981: 5, many types of presses have been tested, including a so-called multi-belt press in combination with 35 roll presses. With this known technique, a dry matter content of about 30% can be achieved. Financial calculations based on the I-! 3 75357 indicate that compression does not need to be used for longer than this dry matter content. In order to achieve a higher dry matter content, long compression times are required, whereby the capacity is clearly reduced. As a result, the costs become unreasonably high. Therefore, additional dewatering steps had to be used, namely drying. However, drying of peat with a moisture content of up to 70% is expensive, so that it is desirable to be able to increase the dry matter content before drying, which is not economical with known techniques.

Therefore, it cannot be said that the known compression methods work in synergy with the other devices in the system for the whole system.

Several methods and devices have also been developed for the drying section. One method is described, for example, in Swedish patent application 78 10558-2. According to this method, the material is dried in a steam dryer in which water vapor under overpressure surrounds the material. Steam is generated in a separate steam boiler, which substantially tilts the system and makes it unrealistic, at least in small or medium-sized plants. A more advantageous device in this respect is described in Swedish patent application 78 06720-4, which, however, requires a device which only operates by direct drying at ambient pressure, which does not give high efficiency.

Disclosure of the Invention It is an object of the present invention to provide improvements to one or more of the following steps, namely peat extraction, pretreatment, dewatering and drying, which may be part of a combined system where different compartments or steps can work synergistically with the whole system.

It is also an object of the invention to provide new solutions to the above-mentioned problems and drawbacks of the known methods with regard to the extraction, pretreatment, dewatering and drying of peat and / or other biomasses, in the case of at least the last part.

According to the first aspect of the system, a new procedure for extracting peat is thus proposed. According to the invention, the peat is slurried as a pumpable slurry or suspension which is fed to a dewatering plant. In this case, the invention is characterized in that wastewater is returned from the dewatering plant, which is applied to the bog over the area at a suitable distance from the peat extraction site at least partially to displace and use the bog water for sludge production, while the bog water is preferably discharged to the drained peat extraction site. The wastewater then also contains a fine fraction separated in the dewatering equipment, which is applied to the bog together with warm wastewater, whereby the fine fraction mixes with the coarser material in the upper layer of the bog. Gradually, the major part of the fine fraction applied to the bog returns to the peat intake site after the fine fraction particles have passed through the raw peat layers and / or dewatering equipment of the bog one or more times. To prevent substantial amounts of applied water from flowing directly to the peat intake, without passing through its deepest portions, a raised peat wall can be made between the edge of the peat intake and the swamp to which the water is applied. A particular advantage of the combined peat extraction and subsequent dewatering is also that the warm wastewater in winter can be used to thaw frozen peat so that it can be lifted and slurried into a suspension. The invention thus comprises the possibility of a real year-round peat extraction. Preferably, the warm wastewater also contains peat ash, which on the one hand causes the colloidal bonds of the peat to break and on the other hand raises the pH of the water which is discharged to the peat extraction site together with the fine fraction. This will create improved conditions for the re-use of raised peat in forestry and land-35 cultivation. In addition to or as an alternative to peat ash, other chemicals may be added to break the colloidal bonds of the raised peat by a method known per se.

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75357 way.

Both standard and newly developed equipment can be used to lift the peat itself. In this case, the excavation or hydraulic safety principle can be used. If an excavator is used, it must be remote-controlled, unmanned and not sensitive to stumps.

It is also an object of the invention to provide a pretreatment of suspended peat which can be advantageously combined with both the peat extraction 10 just described and the continuous dewatering by compression to a high concentration. In this case, the purpose of the pretreatment is also to improve the drying capacity of the peat and to increase the concentration. According to this aspect of the invention, water is removed from the peat-water suspension in a plurality of dewatering devices connected in series, at the same time as the fine fraction of peat is removed with the wastewater, whereby the sieve fabrics of the dewatering devices are rinsed clean with warmer rinsing water. The rinsing water is preferably the warmer wastewater from the next device. 20 The last dewatering device connected in series can use hot wastewater from the next final compression system.

Using warm rinsing water improves the drying capacity of the peat. To further improve the drying ability, suitable chemicals are known in a manner known per se, in particular metal compounds acting as electrolytes (positive metal ions), by a combination of chemical, thermal and mechanical treatment to cleave part of the peat colloidal system. Most preferably, said chemicals are yk-30 per se or in part peat ash, which can be obtained from the drying section included in the system. The mechanical treatment comprises, on the one hand, a homogenized peat cake from which water has been removed and obtained from each device and, on the other hand, between the warmer waste water 35 from the next device. A closed dehumidifier is most suitable.

The concentration of the suspension fed to the first dewatering device connected in series is at most 5% »6 75357 preferably a dry matter content of 2-3% and a temperature of 10-30 ° C, while the concentration of the slurry leaving the last dewatering device is 5-12% , preferably the dry matter content is 6-10%, i.e. approximately the same dry matter content as in the untreated bog raw peat and the temperature is 40-80 ° C, preferably 50-70 ° C. Chemicals, preferably peat ash, are added to the buffer storage before the first dewatering device.

Yet another object of the invention is to provide a pressing method which does not require long pressing times, which means that a large capacity is achieved. A short pressing time can be achieved by removing the peat from a substantial part of the fine fraction, by breaking the colloidal bonds by adding said chemicals, i.e. by peat ash, and by carrying out the pressing at an elevated temperature. More specifically, the temperature is higher than in the first dewatering devices, preferably above 90 ° C. A particular feature of the invention is that the water in the peat cake is preferably displaced by warmer water under successive pressure. Usually, the displacement of water in the peat cake takes place via warmer water in the liquid phase, but it may also be possible for the displacement of water in the peat cake to be carried out with steam, but without the evaporation of water in the peat cake.

Many different devices are possible to perform the compression. Most preferably, however, a device is used which, in principle, consists of a closed washing press in which the peat cake is subjected to dewatering, washing or displacement with warmer water and thus heating and rolling. The specific pressure in the roller compression interval must rise to at least 300 bar, suitably at least 400 bar and preferably more than 500 bar. Several such presses can be used, but in the usual case the last press does not displace the water in the peat cake with warmer water. The effluent from the first press is used as rinsing water li 7 75357 in the last initial dewatering device. In order to heat the peat cake and to displace the water in the press or presses in which the washing or displacement takes place, the condensate obtained from the subsequent drying of the peat 5 is preferably used.

Yet another object of the invention is to provide an improved drying method which can also be used for biomass other than peat, for example fine bark, sawdust, forest waste, etc. According to this aspect of the invention, the pulp is dried in at least two steps, in the first step by hot flue gas exchange. phase, using a higher vapor pressure than in the next step and wherein the steam generated after separation from the mass is used in said next step.

Preferably, the pulp is dried in a first stage in a tube-heat exchanger, where the tube or tubes in which the biomass is conveyed are heated by hot flue gases. Preferably, the partially dried biomass from the first stage is then mixed with the flue gases leaving said first heat exchanger so that the water turns to steam while cooling the flue gases, after which the mass is separated from the flue gases. The dried biomass can then be passed through pipes, ducts or the like to a second heat exchanger, preferably a tubular heat exchanger formed as a column through which steam obtained by drying the biomass in the first stage with flue gas heat having a higher temperature than the pulp flows, and steam 30 in said pipes or ducts, so that the pulp is further dried by indirect heating in a manner known per se due to the transfer of steam energy, which steam has a higher temperature than steam at lower temperatures in the pipes or ducts carrying the biomass in said heat exchanger.

Finally, the biomass can be completely dried in a rotating, double-jacketed drum, in which steam at a temperature higher than the inner temperature of the drum flows through the space between the outer jacket and the inner jacket. The mixture of steam and air coming from inside the drum can be used as combustion air in the furnace where said flue gases are formed.

Other objects and features of the system, as well as the various parts belonging to the system, will become apparent from the following description of an embodiment of the combined system and from the claims set out below.

10 Brief Description of the Drawings

In the following description of a preferred embodiment of the combined system, reference is made to the accompanying drawings, in which Figure 1 shows schematically a top view of a peat bog with equipment for raising peat, dewatering and drying peat, Figure 2 shows a flow diagram of pretreatment and pressing equipment, Figure 3 illustrates operation of a dewatering device 20 for use in pretreatment and fractionation of peat initial dewatering, Fig. 4 shows an apparatus for use in peat compression, and Fig. 5 shows a diagram of a drying apparatus.

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Description of a preferred embodiment

The apparatus which will be described below with reference to the figures of the drawings is a designed apparatus. It should be noted that the values of temperatures, concentrations and pressures given in the drawings or mentioned in the text are generally estimated values and that the values obtained in practice will deviate from the estimated values due to factors that are difficult to predict at the design stage. Therefore, the stated numerical values 35 do not limit the principles of the invention, but are to be considered as merely illustrative of the inventive idea. Referring first to Figure 1, reference numeral 1 denotes

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9 75357 peat bogs and 2 peat extraction sites separated by reference number 2. The edge of this peat extraction site 2 is indicated by the reference number 3. The main parts of the plant are a peat excavator near the peat extraction site 5 for dewatering and drying 3 and lines 6 and 7 for conveying peat sludge to the plant 5 and wastewater for conveying. The peat is lifted to full depth by a peat excavator, which is preferably remotely controlled and unmanned, ground, screened at the lifting site and then transported at a concentration of about 10 2.5 ϊ to the buffer storage 8 of plant 5. The peat excavator 4 is preferably tracked. is arranged between the peat excavator i) and the plant 5 to receive the excess hose, while the hose sections so-15 between the arrangement station 9 and the plant 5 are most recently lowered to an unfrozen depth in the bog 1.

The suspension or sludge is pumped along line 6 to the plant and the wastewater is returned to a return line 7 with a wastewater temperature of about 30 ° C. The warm wastewater contains the fine fraction of peat separated in plant 5 and 20 peat ashes added in plant 5 to break the colloidal bonds of the peat-water suspension. This wastewater is applied to the peat bog behind the peat excavator at a suitable distance from the edge 3 · No fresh water is added to the system. The effluent, thus applied to the bog 25, penetrates the raw peat layer of the bog and displaces a substantial amount of existing colder bog water, which is filtered into the peat intake site 2. The effluent application point is indicated by reference numeral 10, the distance between point 10 and edge 3 being selected does not flow in a substantial amount over the edge 3, but instead penetrates the bog 1. The area thus irrigated is indicated by reference numeral 11 in Fig. 1. Further to ensure that this water does not flow in a substantial amount over the edge 3> a suitable wall 12 is placed on the raised peat between area 11. Due to the fact that the colder bog water is thus separated, the wastewater will heat the peat with good power. The main part of the wastewater, which is applied to the peat water, is reused to prepare a suspension, which means that a substantial part of the fine fraction separated in the factory 5 and returned via line 7 will also be recycled with the sludge in line 6.

5 Usually, the deeper layers of the peat bog are more degraded than the upper layers, which contain coarser material. By applying the fine fraction-containing wastewater to the bog into a coarser material, a certain homogenization is obtained between the coarser and the finer fraction. The system also implies that when the suspension in line 6 has reached equilibrium, it will contain a higher proportion of fine fraction than the untreated crude peat in the bog 1, but by passing the peat ash with the wastewater to the peat gut, the chemical treatment of peat begins at the lifting site.

This means that the cleavage of the colloidal bonds already starts in the bog and in the sludge line 6, which essentially compensates for the higher fine fraction. Peat ash, which contains a high proportion of CaO, also raises the pH of the peat water, which is advantageous when the raised peat is used to cultivate 20 forests or land.

In addition to a high proportion of CaO, for example 40% CaO, peat ash also contains other oxide metal compounds such as sodium, potassium, iron, magnesium and silicon oxides. Of these, divalent or trivalent positive ions are important for the decoagulation of colloids, i.e. for breaking colloidal bonds. One example of the presence of metal in peat ash, leading to efficient electrolytes, is calcium (Ca 2+), iron (Pe 3+) and Aluminum (Al 3+), etc. Peat ash is added to buffer stock 8. Preferably it is added in excess so that it can effectively assist in the cleavage of colloidal bonds in all parts of the system, including treatment in the bog 1. It is also conceivable to add peat ash to the return line 7. In addition to the buffer storage 8, the plant 5 comprises 35 pre-treatment and compression compartments 13, drying equipment 14 and peat silo 15. reference number 16 and the exit from peat extraction point 2

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11 75357 quantities with reference number 17.

Fig. 2 schematically shows the structure of the pretreatment and pressing section 13 shown in Fig. 1. It in turn consists of a pretreatment compartment 18 and a compression compartment 19.

The pretreatment compartment 18 comprises four dewatering devices 20, 21, 22 and 23 connected in series. From the buffer storage 8 to which the peat ash is added and distributed to the slurry from the peat extraction site 4, the slurry is fed to the first dewatering device 20 at 2.5% and about 30 ° C. In the first dewatering device 20 the dry matter content is increased to 3> 8% and the temperature to 35 ° C and in the subsequent devices 21, 22 and 23 to 4.9% and 42 ° C, 6.2% and 50 ° C and 8% and 60 ° C. Thus, a dry matter content 15 is obtained from the last device 23, which is approximately equal to the dry matter content of the raw peat in the bog 1. The fine fraction separated in the first dewatering device 20 corresponds to a dry matter content of about 0.6 o / o, and then in the subsequent dewatering devices 21-23 the fine fraction corresponds to a dry matter content of 0.4 o / oo in the wastewater. The wastewater returned to line 7 thus contains a dry matter content of about 0.5 o / oo in the form of a fine fraction.

Figure 3 schematically illustrates the operation of four dewatering devices 20-23 connected in series. The peat suspension-25 is led through the inlet pipe 24 to the space 25 between the outer jacket 26 and the rotating strainer drum 27, which strainer drum is covered by a strainer fabric in a manner known per se. The pressure P0 inside the strainer drum 27 is lower than the inflow pressure P1 outside the drum. Thus, the water and the fine fraction 30 are sucked into a sieve cloth having a density of 130 mesh and through a sieve drum 27 and are preferably discharged from the axial. Gradually, the mass becomes thicker in state 25. At the end of the accounts 25 there is, in a known manner, a removal plate 28 and a scraper 29 in the area of the removal line 30. From the inside of the rotating strainer drum 27, the rinsing liquid is directed against the inside of the strainer drum in the space between the discharge plate 28 and the scraper. The rinsing water inlet is denoted by reference numeral 31. The rinsing water thus pressed through the strainer drum 27 is obtained from the following strainers, as schematically shown in Figure 2. The peat cake obtained is removed with a scraper 29 under the influence of warmer rinsing water 5 31. the warmer rinsing water and the peat cake are homogenized using a rotor. This contributes mechanically to the cleavage of the colloids, so that continuous dewatering and fractionation is further facilitated in the following devices, which is one of the effects achieved by the series connection of several devices compared to a single large device. Also, the gradually higher temperature enhances the decomposition of the colloids as well as the effect of the peat ash added to the buffer storage 8.

15 The significance of the higher temperature for the drying rate and thus the dewatering capacity is determined by Darcy's equation: d V ^ A · Δ p - ~ -E—, where dty · L · cv 20 V = filtrate volume A = compression range Δρ = pressure difference y = liquid viscosity 25 L = peat cake thickness cv = drying resistance factor

As a result of the higher temperature, the y-value decreases, i.e. the viscosity, as a result of which the drying rate increases.

30 By repeating the thickening in all four dewatering devices connected in series, the fractionation degree 20-23 increases, i.e. the difference of the finest peat fraction. The ratio between the incoming and the outgoing concentration is controlled by the speed of the drum 27 and by the separation pressure between the inlet of the suspension (P ,.) and the outlet of the filtrate (P_). The backwash stream, i.e. the warmer wastewater stream introduced through line 31, is passed in a sufficient amount to catch the desired concentration 13 7535 7 in line 30, i.e. in the inlet line to the next dewatering stage. By introducing warm return water as the rinsing fluid, the process gradually replaces the colder incoming suspension fluid with a warmer fluid, allowing direction 5 to optimize the continued dewatering process.

As a device for pressing operations, two series-connected presses are used in compartment 19, which may be modified washing presses, indicated by reference numerals 33 and 34 in Fig. 2. More specifically, presses of the type which can remove water from biomass at a temperature of 90-130 ° C and also at excess-atmospheric pressure. The first press 33 of these two presses simultaneously performs three operations: dewatering, washing or displacement, and roller pressing. To this end, a suitable clamp is shown in a schematic-ic spread in Figure 4. The clamp, which may be made, as well as the cellulose pulp in a wash press comprises a casing 35 »siivilärummusta 36, which rotates in the direction of arrow 47 direction, a press tusvalssista 38 and peat cake 40 irroitinkaapimesta 39-20 Water removal in the press on to the same basic principle as the closed dewatering devices 20-23 described above · Peat or similar biomass is fed via line 41 into the converging space between the jacket 35 and the strainer drum 37, the strainer drum of which, as in the previous dewatering devices 25, is covered by a strainer plate in a known manner. The pulp slurry follows the speed of the drum 37 in a converging space 42, whereby water is squeezed out through the perforated surface of the drum 37. The filtrate is removed from inside the drum. At the end of the converging space 42 there are a plurality of flaps 43, 44 and 45. In the space 46 between these flaps 30 and the screen drum 37, high temperature water, 90-130 ° C, preferably above 100 ° C, is pressed and pressed through the pulp layer against the drum 37. In this case, the suspension water remaining in the pulp is displaced, whereby said liquid is pressed into the drum 37 through the perforations and replaced with a cleaner and warmer liquid, which causes the water to be removed to a higher dry matter content in subsequent rolling operations.

1 «75357

In the last part of the dewatering process in the press 33, the pulp layer is subjected to high pressure and dewatered in an extended compression gap between the press roll 38 and the screen drum 37. The transition to the pu-5 cross-roll takes place without reducing the pressure, which prevents the pulp cake from breaking in the roller compression gap and back to flow. In the rolling press interval, the characteristic pressure can be raised to 600 bar. Thus, in press 33, the pulp concentration can be rapidly increased from about $ 8 dry matter in the inlet pipe 4l 10 to about 20% in the exit press cake 40 with a combination of high compression pressure and good drying capacity, based on efficient exploitation of the potential of Darcy's equation (see above). . In this connection, it should also be noted that the elastic resistance reduces e.g. high temperature so that the cake can be compressed more efficiently, which can also have a beneficial effect on the drying efficiency.

The filtrate is passed from press 33 to tank 48, for subsequent use in pretreatment compartment 18, as described above. The pulp cake 40, which may have a temperature of 110 ° C and a dry matter content of about 20%, is passed to a retention vessel 49, where the temperature of the pulp is further raised to about 130 ° C by steam. From the retention vessel 49, the pulp is fed to an additional press 34 at said high temperature of about 130 ° C, dewatering to a dry matter content of up to 40%. Press 34 may be similar to press 33, but no washing or displacement is performed at this high temperature. The filtrate is fed to a tank 50 for use on the one hand to dilute the mass 30 from the retention vessel 49 and on the other hand as a displacement liquid in the press 33. The displacement liquid in the press 33 is further used as a condensate from the next drying apparatus 14.

The drying apparatus 14 will now be explained in more detail with reference to Figure 35. Heat for the drying process is generated by burning a certain amount of finally dried peat or biomass in the furnace 51 · This requires about 10% drying

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15 75357 of peat. For drying, the incoming mass is charged to the hopper 52, bypasses the barrier 53 and mixed with steam in line 5 * 1. The fine pulp (hereafter referred to as peat, although other biomasses 5 are also possible) is blown upwards through a plurality of pipes (symbolically indicated by reference numeral 55) to the first pipe heat exchanger column 56. The flue gases are also passed up from the furnace 51 through a column 56 so that the moisture in the tur-10, on the other hand, evaporates under the effect of indirect heating with flue gases in the space 57 surrounding said pipes. In this case, the flue gases cool from a temperature of about 1300 ° C to a temperature of about 300-500 ° C. From this first drying step, the peat is passed to a cyclone 58, 15 where it is separated from the steam which is discharged along line 59. A portion of the steam is discharged through line 54, to mix with the incoming peat as mentioned above, while the peat is led down from the cyclone 58 through the barrier 60 and line 61, to mix in the downcomer 20 62 with flue gases removed from column 56 via line 63. When the flue gases thus meet in the peat line 62, their temperature is 300-500 ° C. Thus, some of the moisture remaining in the peat is further evaporated, while the peat cools the flue gases to a temperature of about 100 ° C before the peat-flue gas mixture reaches the second cyclone 64. In this cyclone 64, the flue gases are separated from the peat. The flue gases are led to a scrubber tower 65 where they are washed before being released into the open air. Together with the flue gases, a certain amount of water is removed in the form of steam, which is the only water loss in the combined system.

From the cyclone 64, peat is passed in a steam circulating stream through line 66 to a second tubular heat exchanger column 67. Peat is blown through parallel connected tubes to column 67, wherein said tubes are surrounded by downstream steam in ambient space 69. The steam 68 carries up 75357 pressure (100 ° C, 1 bar) than with downstream steam in the ambient space 69 (150 ° C, 4-5 bar), so that the ambient steam, with immediate heating, still evaporates some of the moisture in the peat. The condensate is led from the ym-5 flowing space 69 via line 70 to the bottom of column 67 for use as a displacement liquid in press 33, Fig.

In column 67, the further dried peat is passed through line 71 at a pressure of 1 bar and a temperature of about 100 ° C via line 71 to a third cyclone 72. In this command 10, cyclone 72 separates the peat from steam and passes through a shut-off valve 73 to a double jacketed rotary dryer 75 on the other hand, via line 76 to go to the circulating steam system as shown, while the part is led to the outlet end of the dryer 75 15, here to lead to the space 77 between the outer and inner sheath of the rotating drum ·

In the drum 75, heat is transferred from the outer space 77 into the drum 78 so that moisture is further removed from the peat which is gradually fed to the drum 75. At the end of the outlet 20, the drum 75 is surrounded by a cover cover 79. The inner and outer sheaths of the drum 75 in this area are provided with openings to remove the dried peat cover cover 79, from which the peat is finally led to a pelletizing device 81.

The pelletizer 8l is surrounded by a cabinet 82 in which fresh air is blown in via a line 83. This fresh air, as well as the dust obtained during pelletization, is passed to an air mixer 84, which is heated by steam and condensate via line 85. From the air mixer 84, warm air is supplied to the feeder 74 for mixing with the peat. The mixture of air and steam is led from hood 79 via line 85 to a heat exchanger 86, where the mixture of air and steam is heated by flue gases from furnace space 51, after which the heated gas is led to furnace space 51 for use as combustion air.

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Claims (8)

17 75 35 7 A method for drying biomass, characterized in that the pulp is dried in at least two steps, in the first step the pulp is dried indirectly in a tube heat exchanger (56) in the vapor phase at a higher vapor pressure than in the next step in the tube heat exchanger (56). the ducts or the like in which the biomass is transported are heated by hot flue gases, that the steam generated in this case is separated from the pulp and that the separated steam is used for indirect heating of the biomass in the next step. A method according to claim 1, characterized in that the partially dried biomass of the first stage is mixed (62) with the flue gases leaving said first heat exchanger, so that the flue gases further dry the pulp by direct drying so that the water turns to steam while cooling the flue gases, then the mass is separated from the flue gases. Method according to one of Claims 1 to 2, characterized in that the pre-dried biomass is passed through pipes (68), ducts or the like to a second heat exchanger (67) through which steam generated in the first stage, which has a higher temperature, flows. 25 space than the pulp and steam in said pipes or ducts in the second heat exchanger, so that the pulp is further dried by indirect heating in a manner known per se from the transfer of energy from the higher temperature steam to the lower temperature steam in the pipes or ducts (68) transporting biomass in said second heat exchanger. Method according to one of Claims 1 to 3, characterized in that the biomass is dried in a rotating double-jacketed drum (75), the steam flowing through the space (78) between the outer jacket and the inner jacket of the drum having a higher temperature than inside the drum (78). . A method according to claim 4, characterized in that the steam-air mixture coming from inside the drum is used as combustion air in a furnace (51) where said flue gases are generated. Method according to one of Claims 1 to 5, characterized in that the combustion line of the furnace contains dust-containing ventilation air from the climatic office space (82). Method according to one of Claims 4 to 6, characterized in that the combustion air of the furnace (51) is preheated in one or more series-connected air heaters supplied with condensate and double-walled dryer 10 steam and used as a carrier for steam and dust drying in the furnace dryer. Method according to one of Claims 1 to 7, characterized in that a part of the drying article is used as fuel in the furnace. 15