Method for drying renewable raw materials and a drying arrangement

Abstract

The invention relates to a method for drying renewable raw materials (4) in a drying arrangement comprising steps of thermally processing fuel in a combustion boiler (1) generating flue gases comprising carbon dioxide, discharging the flue gases from the combustion boiler (1) and supplying the flue gases into a dryer (2), feeding renewable raw materials (4) into the dryer (2), drying the renewable raw materials (4) in the dryer (2) by the flue gases received from the combustion boiler (1) and flowing through the dryer (2), whereby volatile organic compounds are released from the drying of renewable raw materials (4), discharging the flue gases together with the volatile organic compounds out from the dryer (2) and directing the flue gases to a cleaning unit (3), and cleaning the flue gases in the cleaning unit (3) and capturing carbon dioxide from the flue gases.

Description

[0001]METHOD FOR DRYING RENEWABLE RAW MATERIALS AND A DRYING ARRANGEMENT FIELD OF THE INVENTION The present invention relates to a method for drying renewable raw 5 materials and more particularly to a method as defined in the preamble of the independent claim 1. The present invention also relates to a drying arrangement and more particularly to a drying arrangement as defined in the preamble of the independent claim 10. 10 BACKGROUND OF THE INVENTION It is known from publication US 2023 / 0295516 to treat biomass in a drying device wherein moisture contained in the biomass is reduced. The dried biomass is fed to a pyrolysis reactor thermally decomposing the biomass into pyrolysis coke and pyrolysis gas. The pyrolysis gas is combusted in a combustion 15 device to form flue gas and the fed into a mixing device, where oxygen is added to the flue gas to form drying gas containing sulfur dioxide or sulfur trioxide. The drying gas is discharged from the mixing device and fed into the drying device, guided through the drying device, wherein the sulfur dioxide and / or the sulfur trioxide chemically reacts with the ammonia to form ammonium sulfite and / or 20 ammonium sulfate. The treatment of biomass according to the disclose provides a physical and thermochemical treatment of the biomass. A problem of the prior art is that the flue gases cannot be released to the atmosphere after being used as drying gas in the dryer. Another problem of the prior art is that the arrangement is complex and requires many steps in different 25 devices. BRIEF DESCRIPTION OF THE INVENTION An object of the present invention is to provide a method for drying renewable raw materials and a drying arrangement which at least alleviate the problems of the prior art. 30 The objects of the invention are achieved by a method and an arrangement which are characterized by what is stated in the independent claims. The preferred embodiments of the invention are disclosed in the dependent claims. The invention is based on the idea of using renewable raw materials, especially biomass derived from forestry wastes or other organic materials, as a fuel in a combustion boiler which the renewable raw material is dried with the heat produced by the combustion boiler, and further cleaning the flue gases generated in the combustion boiler and used for the drying of renewable raw materials so that the cleaned flue gases can be released to the atmosphere and the carbon 5 dioxide from the burning and from the organic material is captured. Method according to the invention for drying renewable raw materials in a drying arrangement comprises steps of thermally processing fuel in a combustion boiler generating flue gases comprising carbon dioxide, discharging the flue gases from the combustion boiler and supplying the flue gases into a dryer, 10 feeding renewable raw materials into the dryer, drying the renewable raw materials in the dryer by the flue gases received from the combustion boiler and flowing through the dryer, whereby volatile organic compounds are released from the drying of renewable raw materials, discharging the flue gases together with the volatile organic compounds out from the dryer and directing the flue gases 15 together with the volatile organic compounds to a cleaning unit, and cleaning the flue gases in the cleaning unit and capturing carbon dioxide from the flue gases before releasing cleaned flue gases out from the cleaning unit. In the steps of thermally processing fuel in the combustion boiler generating flue gases and discharging the flue gases from the combustion boiler 20 and supplying the flue gases into the dryer, all the generated flue gases comprising carbon dioxide are directed to the drying arrangement to be used in the dryer. Therefore, the exhaust of flue gases from the combustion boiler elsewhere than the dryer is prevented. Burning in the combustion boiler is usually performed at a temperature between 800^C - 1000^C, which means that the generated flue gases 25 are also hot. The dryer is fed by renewable raw materials which are moist and comprise organic material. The flue gases received to the dryer are arranged to flow through the renewable raw material, which then dries and releases volatile organic compounds in the dryer to mix with the flue gases. In other words, the volatile organic compounds mix with the flue gases flowing through the renewable 30 raw materials so that eventually the flue gas comprises volatile organic compounds released during drying. The flue gases comprising carbon dioxide and the formed volatile organic compounds are discharged from the dryer to the cleaning unit in which the flue gases are washed with cleaning liquid and carbon dioxide contained in the flue gases is removed. The cleaned flue gases can then be released to the 35 atmosphere. According to the invention in the step of cleaning the flue gases in the cleaning unit, the flue gases are cleaned by directing the flue gases upwards in the cleaning unit, by supplying cleaning liquid from a plurality of nozzles into the flue gases, by colliding the flue gases with the cleaning liquid, whereby the flue gases mix with the cleaning liquid, and by separating the cleaned flue gases from the 5 cleaning liquid and directing the cleaned flue gases to a discharge outlet of the cleaning unit for releasing the cleaned flue gases out from the cleaning unit. The flue gases flow upwards in the cleaning unit and meet droplets of a cleaning liquid which are injected towards the flue gases from the plurality of nozzles. In other words, the cleaning liquid droplets sprayed from the multiple 10 nozzles flow downwards in a counter flow to the flue gases flow. Alternatively, the cleaning liquid may be injected to the flow direction of the flue gases or in a direction transverse to the flow direction of the flue gases. The flue gases collide with the cleaning liquid creating a turbulent zone where the gas / liquid interface is continuously and rapidly renewed. The cleaning of the flue gases is effectively 15 accomplished by thorough mixing of the flue gases and the cleaning liquid. The cleaned, washed flue gases are discharged from the cleaning unit through a discharge outlet at the top part of the cleaning unit. The cleaning liquid is removed from the cleaning unit separately. According to the invention, the method further comprises the steps of 20 removing the dried renewable raw materials from the dryer, and feeding the dried renewable raw materials into the combustion boiler as the fuel for thermally processing. In other words, the renewable raw materials that has been dried in the dryer by flue gases flowing through the dryer is fed to the combustion boiler as fuel 25 to be burned in the combustion boiler, which then generate flue gases to be used in the dryer to dry the following renewable raw materials. The feeding the dried renewable raw materials into the combustion boiler may also comprise storing the dried renewable raw materials in an intermediate storage before feeding into the combustion boiler. 30 According to the invention the method further comprises the steps of removing the dried renewable raw materials from the dryer, and removing part of the dried renewable raw materials out from the drying process for further use and feeding the rest of the dried renewable raw materials into the combustion boiler as the fuel for thermally processing. 35 In other words, part of the renewable raw materials that have been dried in the dryer by flue gases flowing through the dryer are removed from the drying arrangement for further use somewhere else, and the rest of the renewable raw materials are fed to the combustion boiler as fuel to be burned in the combustion boiler, which then generate flue gases to be used in the dryer to dry the following renewable raw materials. 5 According to the invention the method further comprises the steps of removing the dried renewable raw materials from the dryer, and removing the dried renewable raw materials out from the drying process for further use. According to the invention the method further comprises the step of regulating the supply of the flue gases to the dryer with a valve in a first supply line 10 between the combustion boiler and the dryer. According to the invention in the step of discharging the flue gases from the combustion boiler and supplying the flue gases into a dryer, the flue gases are supplied into the dryer at a temperature of 100 ^C – 300 ^C. Alternatively, in the step of discharging the flue gases from the combustion boiler and supplying the 15 flue gases into a dryer, the flue gases are supplied into the dryer at a temperature of 150 ^C – 200 ^C. According to the invention in the step of drying the renewable raw materials in the dryer by the flue gases coming from the combustion boiler and flowing through the dryer, the renewable raw materials are dried by feeding the 20 flue gases through the renewable raw materials in the dryer to dry the renewable raw materials with convection provided by the flue gases. In other words, the flue gases flow through the dryer such that the flue gases flow through the renewable raw material so that the renewable raw material dries because of the flue gases flying through it. The flue gases discharged from the 25 dryer are at a temperature between 40^C – 80 ^C so the flue gases release a lot of heat during the drying process in the dryer. The dryer is a convective dryer which means that hot and dry gases are used both to supply heat necessary for evaporation and to remove water vapor from the material to be dried. The flue gases are hot and dry while the renewable raw material comprises moisture to be 30 removed. While the renewable raw materials which are organic materials dry, they aslo release volatile organic compounds which mix with the flue gases. According to the invention in the step of capturing carbon dioxide from the flue gases, carbon dioxide is captured from the flue gases by introducing an additive to the cleaning liquid to make the cleaning liquid to make the cleaning 35 liquid alkaline, and by allowing the alkaline cleaning liquid to react with the flue gases in the step of colliding the flue gases with the cleaning liquid. Adding an additive to the cleaning liquid to make it alkaline can be provided by for example introducing an aqueous solution of sodium carbonate or other alkaline solution to raise the pH value of the washing liquid. According to the invention in the step of capturing carbon dioxide from 5 the flue gases, carbon dioxide is captured from the flue gases by introducing calcium hydroxide to the cleaning unit to react with the carbon dioxide to form calcium carbonate and water. In other words, calcium hydroxide is introduced to the flue gases coming from the dryer comprising volatile organic compounds. Then calcium 10 hydroxide reacts with the carbon dioxide contained in the flue gases and generate calcium carbonate and water, so that the flue gases released from the cleaning unit no longer contain carbon dioxide when introduced to the atmosphere. A drying arrangement for renewable raw materials according to the invention comprises a combustion boiler, a dryer, and a cleaning unit. The 15 combustion boiler is arranged to thermally process fuel from which flue gases are generated. The combustion boiler has a flue gas outlet for discharging the flue gases out from the combustion boiler. The dryer has a first gas inlet in a flow communication with the flue gas outlet of the combustion boiler via a first supply line for receiving flue gases from the combustion boiler to the dryer, a gas outlet 20 for discharging the flue gases flowing from the first gas inlet through the dryer, a supply connection for feeding the renewable raw materials into the dryer, and an outlet connection for removing renewable raw materials out from the dryer. The cleaning unit has a second gas inlet in a flow communication with the gas outlet of the dryer via a second supply line for receiving flue gases into the cleaning unit, a 25 discharge outlet arranged to discharge cleaned flue gases out from the drying arrangement, a cleaning liquid supply connection arranged to supply cleaning liquid from a liquid supply line to the cleaning unit, a cleaning liquid discharge connection arranged to discharge cleaning liquid from the cleaning unit, and a plurality of nozzles arranged to inject cleaning liquid to the flue gases. The cleaning 30 unit further comprising a carbon dioxide capture arrangement arranged to capture carbon dioxide from the flue gases in the cleaning unit. The combustion boiler further comprises a fuel inlet for receiving fuel to be thermally processed in the combustion boiler. The fuel inlet is in a material supply connection with an intermediate storage arranged to receive dried renewable raw material from the 35 dryer. In the drying arrangement, the combustion boiler and the dryer are in a flow communication with each other for feeding flue gases from the combustion boiler to the dryer. The drier is in a material supply connection with the intermediate storage for feeding dried renewable raw material from the dryer to the intermediate storage and the intermediate storage is in a material supply 5 connection with the combustion boiler for feeding the dried renewable raw material as fuel to the combustion boiler. The renewable raw material to be supplied as fuel to the combustion boiler is dried in the dryer by the flue gases coming from the combustion boiler. The dryer comprises a feed line for receiving the renewable raw materials and a first gas inlet for receiving the flue gases to the 10 dryer so that the flue gases flow through the renewable raw materials in the dryer and are discharged from the dryer after passing through the renewable raw materials. The flue gases are discharged through the gas outlet to be guided to the cleaning unit. The cleaning unit is preferably a vertical cleaning unit in which the second gas inlet is in the bottom part of the cleaning unit and the discharge outlet 15 is in the top part of the cleaning unit. The plurality of nozzles are arranged in the cleaning unit such that they are arranged to spray cleaning liquid to the flue gases. The nozzles are preferably arranged in a row in a transverse direction towards a flow path of the flue gases. The nozzles may be arranged to inject cleaning liquid towards the flow of the flue gases or along the flow of the flue gases. Alternatively, 20 the nozzles are arranged along the flow path of the flue gases to spray from both sides of the flow path towards the flue gases. According to the invention the dryer is a convection dryer such as a flash dryer, a fluidized bed dryer, a belt dryer, or an impingement dryer. According to the invention the cleaning unit is arranged such that the 25 second gas inlet is arranged in a bottom part of the cleaning unit, the discharge outlet is arranged in a top part of the cleaning unit and the plurality of nozzles are provided in the cleaning unit between the second gas inlet and the discharge outlet, so that the flue gases are arranged to collide with cleaning liquid sprayed from the multiple nozzles when flowing upwards in the cleaning unit. 30 According to the invention the carbon dioxide capture arrangement comprising an additive supply unit in a fluid communication with the cleaning liquid supply line for providing additives to the cleaning liquid to make the cleaning liquid alkaline. The additives are in a form of an aqueous solution so that the additive 35 supply unit forms the fluid connection with the cleaning liquid supply line. According to the invention the carbon dioxide capture arrangement having a feed for calcium hydroxide, a reaction space for the reaction between the calcium hydroxide and the carbon dioxide, and an outlet for the calcium carbonate and water. According to the invention the carbon dioxide capture arrangement is 5 part of the cleaning unit. In this application, renewable raw material means material that originates from nature. The renewable raw material may be for example recycled fibre or biomass or forestry waste. An advantage of the invention is that the carbon dioxide contained in 10 the flue gases used in the drying of the renewable raw materials is captured in the cleaning unit together with cleaning of the flue gases and the volatile organic compounds. Another advantage of the invention is that the renewable raw material that is dried in the drier can be used as fuel of the combustion boiler for generating flue gases that are used in the dryer, and the flue gases together with the volatile 15 organic compounds from the renewable raw material are cleaned so that clean gases can be released from the drying arrangement to the atmosphere. BRIEF DESCRIPTION OF THE DRAWINGS The invention is described in detail by means of specific embodiments with reference to the enclosed drawing, in which 20 Figure 1 shows a drying arrangement according to the invention. DETAILED DESCRIPTION OF THE INVENTION Figure 1 shows a drying arrangement according to the invention, in which fuel is burned in a combustion boiler 1. The combustion boiler 1 generates flue gases during burning the fuel which the fuel gases are led out from a flue gas 25 outlet 10 of the combustion boiler 1. The flue gases are prevented from exiting the combustion boiler 1 elsewhere than through the flue gas outlet 10, such that the combustion boiler 1 comprises for example an exhaust valve 12 which is in a closed position. The combustion boiler 1 further comprises a fuel inlet 11 for receiving fuel to be burned in the combustion boiler 1. 30 The drying arrangement further comprises a dryer 2 for drying renewable raw material 4. The moist renewable raw material 4 is supplied to the dryer 2 through a supply connection 22 in the dryer 2 which is in a material supply connection through a feed line 500 to a material container or other material source. The renewable raw material 4 is dried inside the dryer 2 with hot flue gases 35 supplied from the combustion boiler 1 through a first supply line 100 extending between the combustion boiler 1 and the dryer 2 and the dryer 2 receives the hot flue gases through a first gas inlet 20 in a flow communication with the flue gas outlet 10 of the combustion boiler 1 through the first supply line 100. The first supply line 100 comprises a valve 13 which is arranged to regulate the supply of 5 the flue gases from the combustion boiler 1 to the dryer 2. The dryer 2 receives the flue gases from the combustion boiler 1 so that the flue gases flow through the renewable raw material 4 provided in the dryer 2 and then the flue gases are removed from the dryer through a gas outlet 21. Together with the flue gases which comprise carbon dioxide also volatile organic 10 compounds generated from the renewable raw materials 4 during drying are discharged through the gas outlet 21 to a second supply line 200 extending between the dryer and a cleaning unit 3. The dried renewable raw materials 4 are removed from the dryer 2 through an outlet connection 23 of the dryer 2 to an intermediate storage 5 15 provided between the drier 2 and the combustion boiler 1. A material feed line 300 is arranged to supply dried renewable raw material 4 from the intermediate storage 5to the combustion boiler 1 as fuel for the combustion boiler 1. In an alternative embodiment of the invention, part of the dried renewable raw material 4 can be guided away through an discharge line 400 from the drying arrangement 20 and reused somewhere else. In a still alternative embodiment of the invention, all the dried renewable raw material 4 can be guided away through the discharge line 400 from the drying arrangement and reused somewhere else. The flue gases comprising carbon dioxide and volatile organic compounds discharged from the dryer 2 are led to the cleaning unit 3 through a 25 second gas inlet 31. In the cleaning unit 3 the flue gases are cleaned with cleaning liquid injected from plurality of nozzles 33 provided inside the cleaning unit 3. Together or separately with the cleaning liquid, the flue gases are exposed to a carbon dioxide capture arrangement 34 where carbon dioxide is captured from the flue gases. This can be performed for example so that calcium hydroxide is fed to 30 react with carbon dioxide in the flue gases, so that calcium carbonate and water are generated. The cleaned flue gases from which also carbon dioxide is removed is discharged out from the cleaning unit through a discharge outlet 32. Alternatively, an aqueous solution of additive is supplied from an additive supply unit 37 in a fluid communication with a cleaning liquid supply line 35a for providing additives 35 to the cleaning liquid to make the cleaning liquid alkaline. The cleaning liquid supply line 35a is arranged to provide cleaning liquid to the cleaning unit 3 through a cleaning liquid connection 35. The cleaning unit 3 further comprises a cleaning liquid discharge connection 36 arranged to discharge cleaning liquid from the cleaning unit 3. The invention has been described above with reference to the example 5 shown in the figure. However, the invention is in no way restricted to the above example but may vary within the scope of the claims.

Claims

CLAIMS 1. Method for drying renewable raw materials (4) in a drying arrangement, c h a r a c t e r i z e d in that the method comprises steps of:- thermally processing fuel in a combustion boiler (1) generating flue 5 gases comprising carbon dioxide, - discharging the flue gases from the combustion boiler (1) and supplying the flue gases into a dryer (2), - feeding renewable raw materials (4) into the dryer (2), - drying the renewable raw materials (4) in the dryer (2) by the flue 10 gases received from the combustion boiler (1) and flowing through the dryer (2), whereby volatile organic compounds are released from the drying of renewable raw materials (4), - discharging the flue gases together with the volatile organic compounds out from the dryer (2) and directing the flue gases together with the 15 volatile organic compounds to a cleaning unit (3), and - cleaning the flue gases in the cleaning unit (3) and capturing carbon dioxide from the flue gases before releasing cleaned flue gases out from the cleaning unit (3).20 2. Method according to claim 1, c h a r a c t e r i z e d in that in the stepof cleaning the flue gases in the cleaning unit (3), the flue gases are cleaned by directing the flue gases upwards in the cleaning unit (3), by supplying cleaning liquid from a plurality of nozzles (33) into the flue gases, 25 by colliding the flue gases with the cleaning liquid, whereby the flue gases mix with the cleaning liquid, and by separating the cleaned flue gases from the cleaning liquid and directing the cleaned flue gases to a discharge outlet (32) of the cleaning unit (3) for releasing the cleaned flue gases out from the cleaning unit (3). 30 3. Method according to claim 1 or 2, c h a r a c t e r i z e d in that in thestep of capturing carbon dioxide from the flue gases, carbon dioxide is captured from the flue gases by introducing an additive to the cleaning liquid to make the cleaning 35 liquid alkaline, and by allowing the alkaline cleaning liquid to react with the flue gases inthe step of colliding the flue gases with the cleaning liquid.

4. Method according to claim 1 or 2, c h a r a c t e r i z e d in that in thestep of capturing carbon dioxide from the flue gases, carbon dioxide is captured 5 from the flue gases by introducing calcium hydroxide to the cleaning unit (3) to react with the carbon dioxide to form calcium carbonate and water.

5. Method according to any previous claim, c h a r a c t e r i z e d in10 that the method further comprises the steps of: - removing the dried renewable raw materials (4) from the dryer (2), and - feeding the dried renewable raw materials (4) into the combustion boiler (1) as the fuel for thermally processing. 15 6. Method according to any of claims 1-4, c h a r a c t e r i z e d in thatthe method further comprises the steps of: - removing the dried renewable raw materials (4) from the dryer (2), and 20 - removing part of the dried renewable raw materials (4) out from the drying process for further use and feeding the rest of the dried renewable raw materials (4) into the combustion boiler (1) as the fuel for thermally processing.

7. Method according to any previous claim, c h a r a c t e r i z e d in25 that the method further comprises the step of: - regulating the supply of the flue gases to the dryer (2) with a valve (13) in a first supply line (100) between the combustion boiler (1) and the dryer (2).

8. Method according to any previous claim, c h a r a c t e r i z e d in30 that in the step of discharging the flue gases from the combustion boiler (1) and supplying the flue gases into a dryer (2), the flue gases are supplied into the dryer at a temperature of 100 ^C – 300 ^C, or at a temperature of 150 ^C – 200 ^C.35 9. Method according to any previous claim, c h a r a c t e r i z e d inthat in the step of drying the renewable raw materials (4) in the dryer (2) by theflue gases coming from the combustion boiler (1) and flowing through the dryer (2), the renewable raw materials are dried by feeding the flue gases through the renewable raw materials (4) in the dryer (2) to dry the renewable raw materials (4) with convection provided by the 5 flue gases.

10. A drying arrangement for renewable raw materials (4), ch a r a c t e r i z e d in that the drying arrangement comprises:a combustion boiler (1) arranged to thermally process fuel from which 10 flue gases are generated, the combustion boiler (1) having a flue gas outlet (10) for discharging the flue gases out from the combustion boiler (1), a dryer (2) having a first gas inlet (20) in a flow communication with the flue gas outlet (10) of the combustion boiler (1) via a first supply line (100) for receiving flue gases from the combustion boiler (1) to the dryer (2), a gas outlet 15 (21) for discharging the flue gases flowing from the first gas inlet (20) through the dryer (2), a supply connection (22) for feeding the renewable raw materials (4) into the dryer (2), and an outlet connection (23) for removing renewable raw materials (4) out from the dryer (2), and a cleaning unit (3) having a second gas inlet (31) in a flow 20 communication with the gas outlet (21) of the dryer (2) via a second supply line (200) for receiving flue gases into the cleaning unit (3), a discharge outlet (32) arranged to discharge cleaned flue gases out from the drying arrangement, a cleaning liquid supply connection (35) arranged to supply cleaning liquid from a liquid supply line (35a) to the cleaning unit (3), a cleaning liquid discharge 25 connection (36) arranged to discharge cleaning liquid from the cleaning unit (3), and a plurality of nozzles (33) arranged to inject cleaning liquid to the flue gases, the cleaning unit (3) further comprising a carbon dioxide capture arrangement (34) arranged to capture carbon dioxide from the flue gases in the cleaning unit (3), 30 the combustion boiler (1) further comprises a fuel inlet (11) for receiving fuel to be thermally processed in the combustion boiler (1), the fuel inlet (11) is in a material supply connection with an intermediate storage (5) arranged to receive dried renewable raw material from the dryer (3).35 11. A drying arrangement according to claim 10, c h a r a c t e r i z e din that the dryer (2) is a convection dryer such as a flash dryer, a fluidized beddryer, a belt dryer, or an impingement dryer.

12. A drying arrangement according to claim 10 or 11, ch a r a c t e r i z e d in that the cleaning unit (3) is arranged such that the second5 gas inlet (31) is arranged in a bottom part of the cleaning unit (3), the discharge outlet (32) is arranged in a top part of the cleaning unit (3) and the plurality of nozzles (33) are provided in the cleaning unit between the second gas inlet (31) and the discharge outlet (32), so that the flue gases are arranged to collide with cleaning liquid sprayed from the multiple nozzles (33) when flowing upwards in 10 the cleaning unit (3).

13. A drying arrangement according to any of claims 10-12, ch a r a c t e r i z e d in that the carbon dioxide capture arrangement (34)comprising an additive supply unit (37) in a fluid communication with the cleaning 15 liquid supply line (35a) for providing additives to the cleaning liquid to make the cleaning liquid alkaline.

14. A drying arrangement according to any of claims 10-12, ch a r a c t e r i z e d in that the carbon dioxide capture arrangement (34) having20 a feed for calcium hydroxide, a reaction space for the reaction between the calcium hydroxide and the carbon dioxide, and an outlet for the calcium carbonate and water.

15. A drying arrangement according to claim 14, c h a r a c t e r i z e d25 in that the carbon dioxide capture arrangement (34) is part of the cleaning unit (3). 30 35

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