Drying system and drying method using superheated steam

Abstract

PURPOSE: To provide a batch-type superheated steam circulation drying system for producing a dried product of high quality.SOLUTION: There is provided a drying system 1 using superheated steam, comprising: a circulation circuit 6 through which superheated steam for drying a drying object present in a drying furnace 2 repeatedly flows into the drying furnace 2; a fan 8 for circulating superheated steam in the circulation circuit 6; an exhaust port 10 provided on a specified position in the circulation circuit 6 to release a portion of steam flowing out of the drying furnace 2 from the circulation circuit 6; a boiler 16 fluidly communicating with the circulation circuit 6 so as to be able to supply steam into the circulation circuit 6; and a heater 18 for heating steam present in the circulation circuit to generate superheated steam. The amount of steam to be supplied into the circulation circuit 6 from the boiler 16 is controlled in such a way that the pressure in the drying furnace 2 changing according to the amount of released steam, does not fall negative relative to the environmental pressure.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a drying system and a drying method that use superheated steam to dry an object to be dried, and more particularly, to a drying system and a drying method that can produce high-quality products while minimizing energy consumption, such as electricity, consumed in the drying process. [Background technology]

[0002] In recent years, mass food waste has become a social issue. Food waste in Japan is approximately 5.22 million tons per year (2020), and in March 2020, a basic policy based on the Food Loss Reduction Promotion Act was approved by the Cabinet, with the goal of reducing commercial food waste by 2.73 million tons by 2030. Furthermore, in October 2019, a new target for the food recycling rate in 2024 was set as the basic policy of the Food Recycling Act, with top priority given to feed production, mainly in the restaurant and food retail industries.

[0003] In order to reduce food waste, foods such as cooked rice, bread, confectionery, noodles, meat, and fish can be dried to produce livestock feed, compost, and other products. Fruits and vegetables can also be dried to make dried fruit, which can be stored for a long time. Furthermore, calcium carbonate-containing materials such as shells can be dried to become raw materials for chalk.

[0004] As shown in Patent Documents 1 to 3, drying systems using superheated steam have been disclosed in the past. The advantage of superheated steam is that the raw material to be dried is less likely to come into contact with air, making it suitable for drying highly flammable raw materials that contain a lot of oil and fat, and preventing discoloration of the material to be dried due to oxidation. Superheated steam sterilizes the material to be dried, has an extremely high heat transfer ability, and can maintain a high product temperature, so it can meet the requirements for making livestock feed for pigs and other animals (for example, requirements such as a heating history of 60 minutes at 90°C). However, if the inside of the treatment tank for the drying treatment is placed in a negative pressure state as described in Patent Document 2, outside air (air) flows into the treatment tank, accelerating the oxidation of the material to be dried. In addition, when a small discharge hole is formed in the jacket structure as described in Patent Document 3, the superheated steam condenses as soon as it is blown out of the small discharge hole into the wide space inside the mixing tank. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 3908251 [Patent Document 2] Patent No. 3502339 [Patent Document 3] Patent No. 6531150 Summary of the Invention [Problem to be solved by the invention]

[0006] In order to implement a drying system and method that utilizes superheated steam, it is desirable to recycle and reuse the used steam flowing out of the drying oven. This is because the fuel cost required to generate superheated steam from water is not cheap. However, as the drying progresses, the moisture contained in the material to be dried is lost, and the amount of steam in the drying oven decreases. In order to replenish the steam, it is necessary to generate steam in a boiler and replenish it in the circulation circuit, but from the viewpoint of cost, it is desirable to minimize the amount of steam replenished. Also, in order to produce high-quality material to be dried, it is desirable to carry out the drying process in such a way that the material to be dried is not exposed to oxygen as much as possible. [Means for solving the problem]

[0007] In one embodiment of the present invention, there is provided a drying system using superheated steam, comprising: a drying furnace configured to accommodate and agitate materials to be dried; a steam circulation circuit configured to allow superheated steam for drying the materials in the drying furnace to repeatedly flow into the drying furnace; a fan for circulating steam in the circulation circuit; an exhaust port provided at a predetermined position of the circulation circuit to release a portion of the steam flowing out of the drying furnace from the circulation circuit, the exhaust port being configured to have an adjustable opening degree of an opening / closing valve of the exhaust port; a boiler fluidly connected to the circulation circuit to allow steam to be replenished into the circulation circuit; and a heating device for heating the steam in the circulation circuit to generate superheated steam, wherein the amount of steam replenished from the boiler into the circulation circuit can be adjusted so that the magnitude of the air pressure inside the drying furnace, which may fluctuate depending on the amount of steam released through the exhaust port, does not become negative relative to the environmental air pressure.

[0008] In addition, when the measurement value of a pressure gauge provided to measure the pressure of the fluid flowing through the exhaust port is not within a set range, the amount of steam replenished from the boiler to the circulation circuit is changed to return the measurement value of the fluid pressure to a value within the set range.

[0009] In addition, when the oxygen concentration value measured by an oxygen concentration meter installed in the drying furnace exceeds a predetermined threshold value, the amount of steam supplied from the boiler to the circulation circuit is increased to return the oxygen concentration value to a value below the threshold value.

[0010] The drying furnace further includes a jacket structure arranged to surround at least the bottom of the drying furnace, the jacket structure having a flow path formed therein that is fluidly connected to the boiler, and a shaft extending generally horizontally within the drying furnace and configured to be rotatable about its axis, with one or more stirring blades extending outward from the outer circumferential surface of the shaft and with a flow path extending therein that is fluidly connected to the boiler.

[0011] The drying oven further includes a vacuum pump or suction blower in fluid communication with the interior of the oven so that gas within the oven can be sucked out to create a vacuum or low pressure.

[0012] A solid-liquid separation means is provided in the drain receiver provided at the bottom of the drying furnace so that oil can be separated and discharged from the oily residue that accumulates in the drain receiver.

[0013] Furthermore, a drying method is presented, which includes the steps of circulating steam in the circulation circuit using a fan provided in the circulation circuit, discharging a portion of the steam flowing out from the drying furnace through an exhaust port provided at a predetermined position in the circulation circuit, replenishing steam into the circulation circuit using a boiler fluidly connected to the circulation circuit, using a heating device provided in the circulation circuit, which is located immediately upstream of the drying furnace in the circulation circuit, to heat the steam in the circulation circuit and generate superheated steam, and adjusting the amount of steam supplied from the boiler to the circulation circuit as necessary so that the magnitude of the air pressure in the drying furnace, which may vary depending on the amount of steam released through the exhaust port, does not become negative relative to the environmental air pressure.

[0014] The step of adjusting the amount of steam replenished from the boiler into the circulation circuit also includes, when a measurement value of a pressure gauge provided to measure the pressure of the fluid flowing through the exhaust port is not within a set range, changing the amount of steam replenished from the boiler into the circulation circuit to restore the pressure value of the fluid to a value within the set range.

[0015] In addition, the step of adjusting the amount of steam supplied from the boiler to the circulation circuit includes, when the oxygen concentration value measured by an oxygen concentration meter installed in the drying furnace exceeds a threshold value, returning the oxygen concentration value to a value below the threshold value by increasing the amount of steam supplied from the boiler to the circulation circuit.

[0016] The drying furnace is also provided with a jacket structure arranged to surround at least the bottom of the drying furnace, the jacket structure having a flow path formed therein that is fluidly connected to the boiler, and a shaft extending generally horizontally within the drying furnace and configured to be rotatable about its axis, with one or more stirring blades extending outward from the outer circumferential surface of the shaft and with a flow path extending therein that is fluidly connected to the boiler. [Brief description of the drawings]

[0017] [Figure 1] 1A and 1B are a perspective view and a side view showing an example of a drying furnace in the drying system of the present invention. [Diagram 2] FIG. 1 is a schematic diagram showing a drying system according to an embodiment of the present invention. [Diagram 3] FIG. 4 is a schematic diagram of a drying system according to another embodiment. [Figure 4] FIG. 13 is a schematic diagram showing a drying system according to yet another embodiment. [Diagram 5] 5A and 5B are diagrams showing how an exhaust port valve opens and closes; [Figure 6] FIG. 4 is an enlarged view of the drain pipe and drain receiver at the bottom of the drying furnace. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] The drying system and drying method of the present invention will be described below with reference to the accompanying Figs. 1 to 6.

[0019] Fig. 1(a) is a perspective view showing an example of a drying oven 2 that can be included in the drying system of the present invention. In the drying system and drying method of the present invention, for example, "Pam Apex Mixer WB" manufactured by Pacific Machinery Works, Ltd. is used as the drying oven 2 that also has the ability to mix the material 11 to be dried. This "Pam Apex Mixer WB" can be used for a wide range of applications, from mixing, dispersing, defibrating, and moist coating of the material to be treated to granulation, reaction, and drying. Fig. 1(b) shows a schematic cross section of the drying oven 2 from a side view.

[0020] In order to prevent oxidation of the material 11 to be dried, the air pressure in the drying oven 2 during operation for performing the drying process in the drying system of the present invention is kept higher than the surrounding atmospheric pressure. Also, the oxygen concentration of the gas in the drying oven 2 during operation is usually kept at a few percent (i.e., less than 10%).

[0021] FIG. 2 is a schematic diagram of an embodiment of the drying system 1 of the present invention. The drying system 1 has a capacity of, for example, about 37.5 L (0.0375 m 3 ) ~ approx. 1500L (1.5m 3), a steam circulation circuit 6 configured so that used steam flowing out of the drying furnace 2 can repeatedly flow into the same drying furnace 2, a fan 8 for circulating the steam 4 in the circulation circuit 6, an exhaust port 10 for discharging a part of the steam flowing out of the drying furnace 2 from the circulation circuit 6 as exhaust steam, a boiler 16 configured so that steam can be supplied into the circulation circuit 6, and a heating device 18 disposed downstream of a position where auxiliary steam (steam supplied from the boiler 16) 14 can be supplied and immediately upstream of the drying furnace 2, the heating device 18 heating the steam 4, (14) to generate superheated steam. In such a drying system 1, an object to be dried 11 to be dried is input from an input port 50 on the upper side of the drying furnace 2, and can be taken out from an output port 52 on the lower side after drying.

[0022] The timing and amount of auxiliary steam to be supplied are adjusted according to the moisture content of the material to be dried. However, auxiliary steam may not be necessary when drying vegetables with a high moisture content. This is because if steam is supplied when drying materials with a high moisture content of 90% or more, the materials will be in a state similar to steam-baking, and drying will not be possible. The amount of auxiliary steam supplied can also be adjusted according to the rotation speed of the fan 8 circulating the steam 4 in the circulation circuit 6. In general, when the rotation speed of the fan 8 is low, a relatively small amount of steam is supplied, and when the rotation speed of the fan 8 is high, a relatively large amount of steam is supplied. However, if the balance between the rotation speed of the circulating fan 8 and the supply of steam is lost, the pressure inside the drying oven 2 will become negative relative to the ambient air pressure. When the pressure inside the drying oven 2 becomes negative, outside air (air) will flow into the drying oven 2 from one or more exhaust ports formed in the drying system 1.

[0023] 2, the temperature of the fluid flowing out of the drying oven 2 and flowing through the circulation circuit 6 after the drying process has dropped to about 90° C. (it may be a different temperature). When steam 14 is supplied from the boiler 16 to the circulation circuit 6 and heated, the temperature of the fluid in the circulation circuit 6 rises to, for example, about 130° C. (it may be a different temperature). When the steam in the circulation circuit 6 immediately before flowing into the drying oven 2 is heated by the heating device 18 and becomes superheated steam, it may reach a temperature of about 350° C. (it may be a different temperature).

[0024] As the boiler used in the drying system 1 of the present invention, for example, a once-through boiler having an evaporation capacity of about 100 kg / h may be adopted. As the heating device (heat exchanger) that generates superheated steam, an electric heater or a gas-fueled burner may be adopted as long as it has a heating capacity sufficient to generate superheated steam.

[0025] In one embodiment, a jacket structure 28 (or a double-wall structure constituting the outer periphery of the drying oven 2) is arranged to surround at least the bottom 24 of the drying oven 2, and a flow path fluidly connected to the boiler 16 is formed inside the jacket structure 28. A flow path fluidly connected to the boiler 16 is also formed inside a shaft 30 that extends horizontally inside the drying oven 2 and is configured to be rotatable by a shaft driver 31. The flow path in the jacket structure 28 and the shaft 30 is also configured so that the steam 14a can be supplied from the boiler 16 to the flow path. In one embodiment, the amount of steam 14 distributed from the boiler 16 to the circulation circuit 6 and / or the jacket structure 28 can be controlled using a three-way control valve 20.

[0026] When steam 14a is supplied from the boiler 16 into the jacket structure 28, the temperature of the inner surface of the drying furnace 2 that comes into contact with the material to be dried rises to about 130°C (sometimes other temperatures) and the material to be dried can be heated. In other words, the latent heat released when the steam condenses and returns to water is used for heating, while the condensed water is discharged so that the inside of the jacket structure 28 is filled with steam. By combining the highly efficient heat conduction by the jacket structure with superheated steam that enables drying and sterilization under low oxygen conditions, the temperature of the material to be dried 11 can be raised quickly, and even materials with high moisture content or high viscosity can be dried in a short time.

[0027] In one embodiment, a pressure gauge 12 for measuring the pressure of the fluid passing through the exhaust port 10 is provided near the exhaust port 10, and the measured value of the pressure gauge 12 can be used to know whether the air pressure in the drying oven 2 is positive or negative relative to the environmental pressure (atmospheric pressure). As an example, the measured value of the pressure gauge 12 can be set to indicate a negative value when the pressure in the drying oven 2 is negative relative to the environmental pressure. When the air pressure in the drying oven 6 becomes negative relative to the environmental pressure, the outside air (containing about 20% oxygen) sucked from the exhaust port 10 flows into the drying oven 2 through the circulation circuit 6, which may impair the quality of the material 11 to be dried. Therefore, there is a case where the measured value of the pressure gauge 12 is not within the set range, the amount of steam supplied from the boiler to the circulation circuit 6 is changed to return the measured value of the pressure of the fluid to a value within the set range. As an example of the setting range, in the case where the pressure is set to 0.00 MPa to 0.01 MPa, the amount of steam supplied from the boiler 16 can be increased when the air pressure inside the drying furnace 2 is lower than 0.00 MPa, while the supply of steam from the boiler 16 is stopped or the boiler 16 is operated intermittently when the air pressure inside the drying furnace 2 is higher than 0.01 MPa.

[0028] In another embodiment, when the oxygen concentration value measured by the oxygen concentration meter 34 provided at a predetermined position (e.g., near the superheated steam inlet) in the drying oven 2 exceeds a predetermined threshold, the amount of steam supplied from the boiler 16 may be increased to return the oxygen concentration value in the drying oven 2 to a value equal to or lower than the threshold. As an example, if the oxygen concentration measured by the oxygen concentration meter 34 provided in the drying oven 2 is set to 10% as the threshold, when the measured value exceeds 10%, it is determined that the pressure in the drying oven 2 is negative or is about to become negative, and the amount of steam supplied from the boiler 16 to the circulation circuit 6 is increased until the measured oxygen concentration value returns to a value equal to or lower than 10%.

[0029] In one embodiment, the amount of auxiliary steam 14 supplied from the boiler 16 to the circulation circuit 6 can be adjusted by adjusting the opening of a steam supply valve of the boiler 16. In another embodiment, a three-way control valve 20 provided at the branch point of the auxiliary steam 14 and the steam 14a may be configured to receive the measurement values ​​of the pressure gauge 12 and / or the oxygen concentration meter 34, and calculate and control the distribution amount of steam to be supplied from the boiler 16 to the circulation circuit 6.

[0030] In one embodiment, in order to precisely adjust the amount or pressure of auxiliary steam 14 to be supplied from the boiler 16 to the circulation circuit 6, an injector using a nozzle needle-like tube having a smaller diameter than the outlet of a conventional steam supply valve may be used at the steam outlet. In this case, it may be possible to change the pressure of the steam supplied from the boiler 16 to the circulation circuit 6 on the order of 0.01 MPa.

[0031] In the drying system 1a of the embodiment shown in FIG. 3, a plurality of shovel-shaped agitating blades 32 extend from the shaft 30 of the drying furnace 2, and an independently driven crushing blade (chopper) 54 is provided at a predetermined position on the lower side of the drying furnace 2 (for example, a position between the agitating blades 32 spaced apart from each other in the axial direction of the shaft 30). Such a drying system 1a is suitable for drying a highly viscous material 11 that is difficult to dry, and can crush and stir the highly viscous material 11 that tends to become lumpy using the shovel-shaped agitating blades 32 and the crushing blades 54. In other embodiments, a sawtooth-shaped agitating blade 32 or a flat-shaped agitating blade 32 may be used depending on the viscosity of the material 11 to be dried.

[0032] 2 and 3, a vacuum cooling device 48 is further provided, which is composed of a vacuum pump 42, a condenser 44, and a collector 46, which are fluidly connected to the inside of the drying furnace 2. After the drying process is completed, the inside of the drying furnace 2 is made into an airtight space, and the fluid in the drying furnace 2 is sucked by the vacuum cooling device 48 until it is in a vacuum state or a state close to a vacuum, so that the material 11 to be dried in the drying furnace 2 is cooled before being removed from the drying furnace 2. In this way, the exhaust steam from the drying furnace 2 is passed through the vacuum cooling device to be drained, thereby suppressing the generation of odors.

[0033] In the drying system 1b shown in Fig. 4, a low-pressure device 58 consisting of a condenser 44 and a suction blower 56 with weak suction power is provided instead of the vacuum cooling device 48. In other embodiments, a low-pressure device 58 consisting of only the suction blower 56 may be provided. This makes it possible to lower the temperature of the material 11 to be dried in the drying oven 2 using the suction blower 56 of the low-pressure device 58 after the drying process in the drying oven 2 is completed. However, in still other embodiments, the drying system may not include either the vacuum cooling device 48 or the low-pressure device 58.

[0034] FIG. 5 shows the opening and closing of the on-off valve 23 of the exhaust port 10 provided at a predetermined position of the circulation circuit 6 (for example, a position immediately downstream of the drying oven 2). The upper part of FIG. 5(a) to (c) shows a plan view of the on-off valve 23 provided at the exhaust port 10, and the lower part shows a front view of the exhaust port 10. The on-off valve 23 is preferably an on-off valve 23 whose opening degree can be adjusted steplessly from fully closed to fully open, either automatically or manually. A large amount of moisture is entrained in the steam flowing out of the drying oven 2 immediately after the start of the drying process (i.e., high humidity). In order to release this, the user of the drying system 1 opens the on-off valve 23 to an appropriate opening degree, for example, 30 degrees to 45 degrees (FIG. 5(b)), and releases part of the steam through the exhaust port 10. As the drying process progresses and the amount of steam flowing out of the drying furnace 2 decreases, the user narrows the opening of the on-off valve 23, for example to about 15 degrees (Figure 5(c)), which reduces the amount of steam released from the circulation circuit 6 and increases the amount of steam to be reused as superheated steam.

[0035] FIG. 6(a) shows an enlarged view of the drain pan 36 provided at the bottom 24 of the drying oven 2 and its vicinity. In one embodiment, the drain pan 36 has a solid-liquid separation means for separating oil from the oily residue accumulated at the bottom 24 of the drying oven 2, and a drain pipe 38 connected to the drain pan 36. FIG. 6(b) is a plan view of a punched metal plate 62 which is the upper cover of the drain pan 36. The punched metal plate 62 can separate only the oil from the oily residue accumulated in the drain pan 36 provided at the bottom of the drying oven 2 by screening. The separated oil is discharged through the drain pipe 38. In this way, the solid-liquid separation means consisting of the punched metal plate 62 can be provided in the drain pan 36. In other embodiments, other well-known solid-liquid separation means may be provided.

[0036] In the present invention, since the drying system and drying method are of a batch type (not a continuous type), the moisture content of the product obtained after drying can be appropriately adjusted by adjusting the drying process time and the temperature and amount of superheated steam according to the moisture content of the material to be dried, etc. Also, since the drying process is performed using the minimum necessary amount of superheated steam so that the material to be dried is hardly exposed to oxygen, it is possible to produce a high-quality product while reducing energy consumption. [Explanation of symbols]

[0037] 1. Drying system 1a…Drying system 1b…Drying system 2…Drying oven 4...(Exhaust) steam 6...Circulation circuit 8. Fan 10…Exhaust port 11...Materials to be dried 12...Pressure gauge 14...(Auxiliary) Steam 14a…Steam 16…Boiler 18...Heating equipment 20...Three-way (control) valve 22...Rotation axis 23...Shut-off valve 24...Bottom 26...Flow path 28…Jacket structure 29...Bearing 30...Shaft (rotating axis) 31...Shaft driver 32...Mixing blade 34...Oxygen concentration meter 36…Drain receiver 38…Drain pipe 40...Filter body 42…Vacuum pump 44…Condenser 46…Collector 48...Vacuum cooling device 50...Inlet 52...Outlet 53...Crushing blade driver 54...Chopper 55...Communication pipe 56…Suction blower 58...Low voltage equipment 62...Punched metal plate 64...Exhaust port

Claims

1. A drying furnace configured to accommodate and agitate the material to be dried; a steam circulation circuit configured to allow superheated steam for drying the material to be dried in the drying oven to repeatedly flow into the drying oven; A fan that circulates steam in the circulation circuit; an exhaust port provided at a predetermined position of the circulation circuit so as to release a part of the steam flowing out from the drying furnace from the circulation circuit, the exhaust port having an opening degree of an opening / closing valve configured to be adjustable; a boiler in fluid communication with the circulation circuit to provide make-up steam into the circulation circuit; a heating device for heating the steam in the circulation circuit to generate superheated steam; Equipped with A drying system using superheated steam, which is configured so that the amount of steam replenished from the boiler to the circulation circuit can be adjusted so that the air pressure inside the drying furnace, which can fluctuate depending on the amount of steam released through the exhaust port, does not become negative relative to the ambient air pressure.

2. 2. The drying system of claim 1, wherein when a measurement value of a pressure gauge provided to measure the pressure of the fluid flowing through the exhaust port is not within a set range, the amount of steam replenished from the boiler to the circulation circuit is changed to return the measurement value of the pressure of the fluid to a value within the set range.

3. 2. The drying system according to claim 1, wherein, when an oxygen concentration value measured by an oxygen concentration meter provided in the drying furnace exceeds a predetermined threshold, the amount of steam replenished from the boiler to the circulation circuit is increased to return the oxygen concentration value to a value equal to or lower than the threshold.

4. a jacket structure disposed to surround at least a bottom portion of the drying oven, the jacket structure having a flow path formed therein that is in fluid communication with the boiler; a shaft extending generally horizontally within the drying oven and configured to be axially rotatable, the shaft having one or more stirring blades extending outward from an outer circumferential surface of the shaft and a flow passage extending within the shaft and fluidly communicating with the boiler; The drying system of any one of claims 1 to 3, further comprising:

5. 4. The drying system of claim 1, further comprising a vacuum pump or suction blower in fluid communication with the drying oven so as to suction gas within the drying oven to a vacuum or low pressure.

6. 4. The drying system according to claim 1, further comprising a solid-liquid separation means provided in a drain receiver provided at the bottom of the drying oven so that oil can be separated and discharged from a residue rich in oil and fat that accumulates in the drain receiver.

7. A circulation circuit configured so that superheated steam repeatedly flows into a drying furnace capable of accommodating and agitating a material to be dried, circulating steam in the circulation circuit using a fan provided in the circulation circuit; discharging a portion of the steam flowing out from the drying furnace through an exhaust port provided at a predetermined position of the circulation circuit; providing make-up steam into the circulation circuit using a boiler in fluid communication with the circulation circuit; using a heating device disposed in the circulation circuit to heat steam in the circulation circuit to generate superheated steam; adjusting the amount of steam supplied from the boiler to the circulation circuit as necessary so that the pressure in the drying oven, which may vary depending on the amount of steam discharged through the exhaust port, does not become negative relative to the environmental pressure; A drying method comprising the steps of:

8. The step of adjusting the amount of steam supplied from the boiler to the circulation circuit includes:

8. The drying method according to claim 7, further comprising, when a measurement value of a pressure gauge provided for measuring the pressure of the fluid flowing through the exhaust port is not within a set range, changing an amount of steam replenished from the boiler to the circulation circuit to return the pressure value of the fluid to a value within a set range.

9. The step of adjusting the amount of steam supplied from the boiler to the circulation circuit includes:

8. The drying method according to claim 7, further comprising, when an oxygen concentration value measured by an oxygen concentration meter provided in the drying furnace exceeds a predetermined threshold, increasing an amount of steam replenished from the boiler to the circulation circuit to return the oxygen concentration value to a value equal to or lower than the threshold.

10. a jacket structure disposed to surround at least a bottom portion of the drying oven, the jacket structure having a flow path formed therein that is in fluid communication with the boiler; a shaft extending generally horizontally within the drying oven and configured to be axially rotatable, the shaft having one or more stirring blades extending outward from an outer circumferential surface of the shaft and a flow passage extending within the shaft and fluidly communicating with the boiler; The drying method according to any one of claims 7 to 9, wherein the drying oven is provided with a

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