Sludge drying device and sludge drying method

Abstract

To simplify quality control.SOLUTION: A sludge drying device comprising: a drying furnace that dries sludge granules obtained by stirring sludge with a drying gas to generate pellets; a separation unit that separates the pellets generated in the drying furnace from the drying gas used for drying; a conveyor that has a cylindrical casing forming a space for conveying the pellets and conveys the pellets from the separation unit to a predetermined position; and a temperature sensor that detects a temperature of a predetermined portion of the casing.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present disclosure relates to a sludge drying apparatus and a sludge drying method. [Background technology]

[0002] Patent Document 1 discloses a sludge drying apparatus that includes a drying device that produces solid fuel by drying granulated sludge formed from dried sludge and dewatered sludge using a drying gas. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-52408 Summary of the Invention [Problem to be solved by the invention]

[0004] The present disclosure provides a sludge drying apparatus and a sludge drying method that are useful for simplifying quality control. [Means for solving the problem]

[0005] [1] A sludge drying apparatus comprising: a drying furnace that dries sludge granules obtained by stirring sludge using a dry gas to produce pellets; a separation section that separates the pellets produced in the drying furnace from the dry gas after being used for drying; a conveyor having a cylindrical casing that forms a space for transporting the pellets and that transports the pellets from the separation section to a predetermined position; and a temperature sensor that detects the temperature at a predetermined location of the casing.

[0006] [2] The sludge drying apparatus described in [1] above, further comprising a classifier that classifies the pellets after being transported by the conveyor to obtain product pellets, and an estimation unit that estimates the moisture content of the product pellets based on the detection value of the temperature sensor.

[0007] [3] A sludge drying apparatus as described in [2] above, wherein the estimation unit estimates the moisture content of the product pellets based on a prediction model pre-constructed to show the relationship between the temperature of the casing and the moisture content of the product pellets and the detection value of the temperature sensor.

[0008] [4] A sludge drying apparatus described in any one of [1] to [3] above, wherein the conveyor further has a heat-insulating member covering the periphery of the casing, and the temperature sensor is installed on the outer surface of the casing.

[0009] [5] A sludge drying apparatus described in any one of [1] to [4] above, wherein the temperature sensor is installed on the bottom wall of the casing in a section of the conveyor where the pellets are transported in a direction inclined relative to the vertical direction.

[0010] [6] A sludge drying apparatus described in any one of [1] to [5] above, wherein the temperature sensor is installed on the bottom wall of the casing in a section of the conveyor where the pellets are transported horizontally.

[0011] [7] A sludge drying apparatus according to any one of [1] to [6] above, further comprising a heating unit that heats the drying gas supplied to the drying furnace, and a heating control unit that changes the amount of heat imparted to the drying gas by the heating unit based on the detection value of the temperature sensor.

[0012] [8] A sludge drying apparatus as described in [7] above, further comprising a target temperature setting unit that changes the outlet target temperature based on the detection value of the temperature sensor, and the heating control unit controls the heating unit so that the temperature of the drying gas after being discharged from the drying furnace follows the outlet target temperature.

[0013] [9] The sludge drying apparatus described in [8] above, wherein the target temperature setting unit further calculates an inlet target temperature so that the temperature of the drying gas after being discharged from the drying furnace follows the outlet target temperature, and the heating control unit controls the heating unit so that the temperature of the drying gas before being introduced into the drying furnace follows the inlet target temperature.

[0014]

[10] A sludge drying method comprising: a drying step of drying sludge granules obtained by stirring sludge with a dry gas to produce pellets; a separation step of separating the pellets produced in the drying step from the dry gas after use in drying; a transport step of transporting the pellets after separation from the dry gas to a predetermined position using a conveyor having a cylindrical casing that forms a space for transporting the pellets; and a temperature detection step of detecting the temperature at the predetermined position using a temperature sensor installed at a predetermined position in the casing. [Effects of the Invention]

[0015] According to the present disclosure, a sludge drying apparatus and a sludge drying method are provided that are useful for simplifying quality control. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a schematic diagram showing an example of a system configuration of a sludge drying apparatus. [Figure 2] FIG. 2 is a schematic diagram showing an example of a conveyor. [Figure 3] FIG. 3 is a schematic diagram showing an example of the internal configuration of the conveyor. [Figure 4] FIG. 4 is a schematic diagram showing an example of the internal configuration of the conveyor. [Figure 5] FIG. 5 is a graph showing an example of the measurement results of the moisture content of the product pellets and the detected temperature value. [Figure 6] FIG. 6 is a block diagram illustrating an example of a functional configuration of the control device. [Figure 7] FIG. 7 is a block diagram illustrating an example of a hardware configuration of the control device. [Figure 8] FIG. 8 is a flowchart showing an example of a series of processes executed by the control device. [Figure 9] FIG. 9 is a flowchart showing an example of a series of processes executed by the control device. DETAILED DESCRIPTION OF THE INVENTION

[0017] An embodiment will be described below with reference to the drawings. In the description, the same elements or elements having the same functions are designated by the same reference numerals, and redundant description will be omitted.

[0018] [Sludge drying equipment] FIG. 1 schematically illustrates a sludge drying apparatus according to one embodiment. The sludge drying apparatus 1 illustrated in FIG. 1 is an apparatus for drying sludge (a muddy substance containing organic matter) collected in sewage treatment, wastewater treatment, or the like, to obtain a pellet-shaped product. The product obtained by the sludge drying apparatus 1 can be used as solid fuel. Hereinafter, in this disclosure, the pellet-shaped product obtained from sludge will be simply referred to as "pellets."

[0019] The sludge drying apparatus 1 includes, for example, a mixer 10, a sludge supply unit 20, a drying furnace 30, a gas supply unit 40, a heating unit 50, a recovery unit 60, and a control device 100.

[0020] The mixer 10 agitates the sludge to produce sludge granules (granulated sludge). The mixer 10 is, for example, a twin-shaft mixer, and includes a pellet receiving inlet 12, a sludge receiving inlet 13, screws 14, 14, and a discharge outlet 15. The pellet receiving inlet 12 receives recycled pellets from a recycling hopper 65, which will be described later. The sludge receiving inlet 13 receives sludge. The screws 14, 14 are twin-shaft screws, and each screw 14 is a screw-shaped mixing blade. An electric motor is connected to the screws 14, 14. The electric motor drives the screws 14, 14, respectively, thereby agitating the recycled pellets and sludge, and forming sludge granules with the recycled pellets as a core. The discharge outlet 15 discharges the sludge granules to the drying furnace 30.

[0021] The sludge supply unit 20 supplies sludge to the mixer 10. The sludge supplied from the sludge supply unit 20 is, for example, dewatered sludge. The moisture content of the dewatered sludge supplied from the sludge supply unit 20 may be approximately 65% ​​to 85%. The sludge supply unit 20 sends the sludge to the mixer 10 based on an operation instruction from the control device 100. The sludge supply unit 20 may be configured to include a weighing hopper and a feeder. Alternatively, the sludge supply unit 20 may be configured to include a pump and a flow meter.

[0022] The drying furnace 30 dries the sludge granules obtained by stirring the sludge with a dry gas to produce pellets. In this disclosure, for convenience of explanation, the pellets produced by the drying furnace 30 are referred to as "dried pellets." The dry gas used by the drying furnace 30 is a gas for drying the sludge granules. The dry gas is, for example, air. The drying furnace 30 receives the sludge granules and the dry gas. The drying furnace 30 uses the dry gas to dry the sludge granules produced by the mixer 10 to produce dried pellets.

[0023] The drying furnace 30 may be a rotary kiln and may be installed so as to be rotatable about a horizontal rotation axis. The drying furnace 30 has, for example, a granulated material receiving inlet 32, a gas receiving inlet 33, and a delivery outlet 34. The granulated material receiving inlet 32 ​​and the gas receiving inlet 33 are provided at one end of the drying furnace 30 in the axial direction along which the rotation axis extends. The granulated material receiving inlet 32 ​​receives granulated sludge, and the gas receiving inlet 33 receives drying gas. The delivery outlet 34 is provided at the other end of the drying furnace 30 in the axial direction along which the rotation axis extends, and delivers dried pellets and drying gas.

[0024] The moisture content of the granulated sludge received through the granulated material receiving inlet 32 ​​may be approximately 15% to 40%. The temperature of the granulated sludge received through the granulated material receiving inlet 32 ​​may be approximately 40°C to 65°C. The drying furnace 30 agitates the granulated sludge by rotation and sends it to the delivery outlet 34 while exposing it to a drying gas. During this time, contact with the drying gas promotes evaporation of the moisture contained in the granulated sludge, producing dry pellets. The drying furnace 30 is also sometimes called a drying drum.

[0025] The gas supply unit 40 supplies dry gas to the drying furnace 30. The gas supply unit 40 may be configured to circulate the dry gas sent out from the delivery outlet 34 of the drying furnace 30 to the gas receiving inlet 33. The gas supply unit 40 includes, for example, a blower 41. The blower 41 is a fan or a blower, and pressure-feeds the dry gas sent out from the delivery outlet 34 to the gas receiving inlet 33 along a circulation flow path.

[0026] The heating unit 50 heats the dry gas supplied to the drying furnace 30 by the gas supply unit 40. In other words, the gas supply unit 40 supplies the dry gas heated by the heating unit 50 to the drying furnace 30. The temperature of the dry gas supplied to the drying furnace 30 may be approximately 350°C to 500°C. The heating unit 50 includes, for example, a combustion furnace 51, a burner 52, and a heat exchanger 53.

[0027] The combustion furnace 51 generates heating gas by burning fuel such as heavy oil. The heating gas is used to heat the dry gas (dry gas before being supplied to the drying furnace 30) to be supplied to the drying furnace 30. The burner 52 burns the fuel while taking in air so that the interior of the combustion furnace 51 is heated and heating gas is generated. The heating gas contains carbon dioxide generated as the fuel is burned. The heat exchanger 53 receives the dry gas flowing from the blower 41 toward the gas inlet 33 and the heating gas generated in the combustion furnace 51, and heats the dry gas by heat exchange with the heating gas. The heat exchanger 53 is, for example, a plate-type heat exchanger. The fuel may be digester gas (a combustible gas containing methane obtained by fermenting sludge), or it may be LNG, LPG, or city gas.

[0028] The recovery section 60 recovers the dried pellets produced in the drying furnace 30. The recovery section 60 has, for example, a separation section 61, a classifier 62 (classification section), a product hopper 64, and a recycle hopper 65.

[0029] The separation unit 61 separates the dried pellets produced in the drying furnace 30 from the dry gas used for drying. The separation unit 61 is connected to the outlet 34 of the drying furnace 30 and receives the dried pellets and the dry gas used for drying from the drying furnace 30. The temperature of the dry gas introduced into the separation unit 61 may be approximately 100°C to 150°C. The separation unit 61 may be a bag filter with a built-in filter cloth. The separation unit 61 sends the separated dry gas to the heat exchanger 53.

[0030] The recovery section 60 has a conveyor 70, and the dried pellets after being separated from the dry gas in the separation section 61 are transported (carried) to the classifier 62 by the conveyor 70. The temperature of the dried pellets discharged from the separation section 61 to the conveyor 70 may be about 80°C to 120°C. Details of the conveyor 70 will be described later.

[0031] The classifier 62 classifies the dried pellets after they are transported by the conveyor 70. The classifier 62 separates the dried pellets into multiple groups (for example, two groups) according to the particle size of the pellets. The classifier 62 is, for example, a vibrating sieve, and has multiple types of screens with different mesh sizes.

[0032] The product hopper 64 receives dry pellets from the group suitable for use as solid fuel among the plurality of groups and stores them as product pellets. As described above, the product pellets are obtained by classifying the dry pellets using the classifier 62. The recycle hopper 65 receives dry pellets from the group whose particles are smaller than the product pellets and stores them as the recycle pellets.

[0033] The sludge drying apparatus 1 includes a condenser 43 and a blower 44. A portion of the dry gas that has passed through the separation section 61 is pressure-fed by the blower 41 to the gas inlet 33 of the drying furnace 30. A remaining portion of the dry gas that has passed through the separation section 61 is discharged via the condenser 43 and the heating section 50. The condenser 43 condenses the moisture in the dry gas by cooling the dry gas using, for example, cooling water. The dry gas that has passed through the condenser 43 is introduced into the combustion furnace 51 by the blower 44.

[0034] The blower 44 pressure-feeds the dry gas that has passed through the condenser 43 to the combustion furnace 51. The blower 44 is, for example, a fan or a blower, and is provided in the flow path of the dry gas from the condenser 43 to the combustion furnace 51. The dry gas introduced from the condenser 43 into the combustion furnace 51 becomes part of the heating gas and is discharged. At this time, malodorous substances in the dry gas are burned, so the emission of odors accompanying the discharge of the dry gas is suppressed.

[0035] The sludge drying apparatus 1 includes a blower 47 and an exhaust pipe 48. The blower 47 pressure-feeds the heating gas that has passed through the heat exchanger 53 (the heating gas after being used to heat the drying gas) to the exhaust pipe 48. The blower 47 is, for example, a fan or a blower, and is provided between the heat exchanger 53 and the exhaust pipe 48. The exhaust pipe 48 exhausts the heating gas sent from the heat exchanger 53.

[0036] The sludge drying apparatus 1 includes a temperature sensor 82 and a temperature sensor 83. The temperature sensor 82 detects the temperature of the drying gas used for drying in the drying oven 30 and sent to the separation section 61 (hereinafter referred to as the "outlet temperature of the drying oven 30"). The temperature sensor 82 is provided, for example, in the flow path of the drying gas from the outlet 34 to the separation section 61. The temperature sensor 82 only needs to be able to detect the temperature of the drying gas that has essentially passed through the drying oven 30, and therefore may be provided near the outlet 34 or the outlet 34 inside the drying oven 30 instead of in the flow path. The temperature sensor 82 outputs an electric signal indicating the outlet temperature of the drying oven 30 (the temperature of the drying gas after being used for drying in the drying oven 30) to the control device 100. The temperature sensor 82 may be of any type, and is, for example, constituted by a thermocouple.

[0037] The temperature sensor 83 detects the temperature of the drying gas passing through the heating section 50 (heat exchanger 53) and sent to the drying oven 30 (hereinafter referred to as the "inlet temperature of the drying oven 30"). The temperature sensor 83 is provided, for example, in the flow path of the drying gas from the heat exchanger 53 to the gas inlet 33. The temperature sensor 83 only needs to be able to detect the temperature of the drying gas before it is actually introduced into the drying oven 30, so it may be provided near the gas inlet 33 or the gas inlet 33 inside the drying oven 30 instead of in the flow path. The temperature sensor 83 outputs an electric signal indicating the inlet temperature of the drying oven 30 (the temperature of the drying gas before it is introduced into the drying oven 30) to the control device 100. The temperature sensor 83 may be any type of sensor, and is, for example, constituted by a thermocouple.

[0038] 2, 3, and 4 schematically show an example of the conveyor 70. The conveyor 70 transports the dry pellets from the separation unit 61 to a predetermined position. As described above, the conveyor 70 is configured to transport the dry pellets from the separation unit 61 to the classifier 62. In this case, the conveyor 70 receives the dry pellets from the separation unit 61 after they have been separated from the dry gas, and transports the dry pellets to the classifier 62. By transporting the dry pellets to the classifier 62, the dry pellets are supplied to the classifier 62.

[0039] As shown in FIG. 2, the conveyor 70 includes three sections: a first conveying section 70a, a second conveying section 70b, and a third conveying section 70c. The first conveying section 70a receives the dry pellets from the separating section 61 and conveys the dry pellets in a direction inclined relative to the vertical direction. Part or all of the first conveying section 70a may convey the dry pellets horizontally. At least part of the first conveying section 70a may convey the dry pellets diagonally upward or diagonally downward.

[0040] The second conveying section 70b has its upstream end connected to the downstream end of the first conveying section 70a and conveys the dry pellets vertically from bottom to top. The third conveying section 70c has its upstream end connected to the downstream end of the second conveying section 70b and conveys the dry pellets in a direction inclined relative to the vertical. Part or all of the third conveying section 70c may convey the dry pellets horizontally. At least part of the third conveying section 70c may convey the dry pellets diagonally upward or diagonally downward.

[0041] In this disclosure, the terms "upstream" and "downstream" are used with reference to the direction in which pellets, such as dry pellets, are transported, and with reference to the direction in which gas, such as dry gas, flows. That is, pellets are transported from upstream to downstream, and gas flows from upstream to downstream. The conveyor 70, which is composed of the first conveying section 70a, the second conveying section 70b, and the third conveying section 70c, is an example, and the configuration of the conveyor 70 can be changed as needed depending on the installation positions of the separator 61 and the classifier 62.

[0042] 3 and 4, the conveyor 70 has a casing 71, a heat-insulating member 72, a plurality of scrapers 75, and an operating mechanism 76. In FIGS. 3 and 4, dried pellets (lumps of dried pellets) are indicated by "P." "D1" represents the direction (orientation) from upstream to downstream on the conveying path of the conveyor 70, and "D2" represents the direction (orientation) from downstream to upstream on the conveying path of the conveyor 70.

[0043] The casing 71 is a cylindrically formed member that forms a space (hereinafter referred to as "space S") for transporting the dried pellets that have passed through the separation section 61. The casing 71 is formed to extend along the transport path of the conveyor 70, and thus the space S is formed along the transport path of the conveyor 70. The casing 71 may be formed continuously over most of the transport path of the conveyor 70. In other words, the space S surrounded by the casing 71 is formed over most of the transport path of the conveyor 70.

[0044] The casing 71 may be made of metal, for example, stainless steel. In a cross section perpendicular to the direction D1 (or the direction D2), the casing 71 may have a rectangular frame shape, as shown in Fig. 4. When focusing on the section of the conveyor 70 where the dried pellets are transported in a direction intersecting the vertical direction, the casing 71 may be configured with a bottom wall 71a, an upper wall 71b, a side wall 71c, and a side wall 71d.

[0045] The thickness t1 of each of the bottom wall 71a, the top wall 71b, the side wall 71c, and the side wall 71d may be approximately 2 mm to 8 mm. The bottom wall 71a and the top wall 71b face each other in the vertical direction, and the side wall 71c and the side wall 71d face each other in the horizontal direction. When focusing on the section of the conveyor 70 where the dried pellets are transported in the vertical direction, the casing 71 may be composed of four side walls. The casing 71 is also called a shell (conveyor shell).

[0046] The heat insulation member 72 suppresses a decrease in temperature in the space S within the casing 71. The heat insulation member 72 is provided so as to cover the outer surface of the casing 71. The heat insulation member 72 may be made of any material as long as it can suppress a decrease in temperature in the space S. In a cross section perpendicular to the direction D1 (or the direction D2), the shape of the heat insulation member 72 may be a rectangular frame. The thickness t2 of the heat insulation member 72 may be larger than the thickness t1 of the casing 71 (each wall). The thickness t2 of the heat insulation member 72 is, for example, about 10 to 20 times the thickness t1 of the casing 71.

[0047] The multiple scrapers 75 are plate members (scrapers) that move the dry pellets. The multiple scrapers 75 are arranged at intervals on the conveying path of the conveyor 70. When focusing on a section of the conveyor 70 where the dry pellets are transported in a direction intersecting the vertical direction, some of the multiple scrapers 75 move in direction D1 in the lower half of the casing 71. Some of the multiple scrapers 75 move (transport) the dry pellets in direction D1 in the lower half of the casing 71. At least some of the dry pellets being transported by the scrapers 75 contact the bottom wall 71a of the casing 71.

[0048] In the upper half of the casing 71, some of the remaining scrapers 75 are moving in direction D2. The scrapers 75 moving in direction D2 do not move (convey) dried pellets. The operating mechanism 76 is a mechanism that moves some of the multiple scrapers 75 in direction D1 and moves some of the remaining scrapers 75 in direction D2.

[0049] Focusing on one scraper 75, for example, the scraper 75 is moved in direction D1 by the operating mechanism 76 from the upstream end of all or part of a section on the conveying path of the conveyor 70 to reach the downstream end of the section. During this time, the scraper 75 transports the dried pellets. After reaching the downstream end of the section, the scraper 75 is moved in direction D2 by the operating mechanism 76 to reach the upstream end of the section. As described above, the operating mechanism 76 moves the multiple scrapers 75 so as to alternate between movement in direction D1 and movement in direction D2. The conveyor 70 may be a scraper-type conveyor.

[0050] The sludge drying apparatus 1 is equipped with a temperature sensor 79. The temperature sensor 79 detects the temperature at a predetermined location of the casing 71. The temperature sensor 79 is installed at any location of the casing 71. The temperature sensor 79 may be installed on the outer surface of the casing 71. In this case, the temperature sensor 79, together with the casing 71, is covered by the heat insulation member 72. The temperature sensor 79 may be installed inside a wall portion that constitutes the casing 71, instead of on the outer surface of the casing 71.

[0051] The temperature detection location by the temperature sensor 79 may be located in any section on the conveying path of the conveyor 70. For example, the temperature sensor 79 is installed on the bottom wall 71a of the casing 71 in a section of the conveyor 70 where the dried pellets are conveyed in a direction inclined with respect to the vertical direction. The temperature sensor 79 may also be installed on the bottom wall 71a of the casing 71 in a section of the conveyor 70 where the dried pellets are conveyed horizontally. In one example, the temperature sensor 79 is installed on the bottom wall 71a of the casing 71 in the first conveying section 70a or the third conveying section 70c.

[0052] The temperature sensor 79 outputs an electrical signal indicating the temperature at a predetermined location of the casing 71 to the control device 100. The temperature sensor 79 may be any type of sensor, and is configured, for example, by a thermocouple. Heat is transferred to the casing 71 from the dried pellets that are discharged from the separation section 61 onto the conveyor 70 and are being transported. In other words, when the temperature of the dried pellets changes, the temperature of the casing 71 may also change. Therefore, the temperature of the dried pellets can be substantially detected from the detection result by the temperature sensor 79.

[0053] The inventors have found that there is a correlation between the temperature of the casing 71 (temperature of the dried pellets) and the moisture content of the product pellets. FIG. 5 shows the measurement results of the temperature (°C) detected by the temperature sensor 79 and the moisture content (%) of the product pellets. In the conveying section corresponding to the third conveying section 70c, the temperature sensor 79 was installed on the bottom wall 71a of the casing 71 to measure the temperature of the casing 71. The moisture content of the product pellets was measured using a moisture meter MOC63u manufactured by Shimadzu Corporation.

[0054] In the graph shown in Fig. 5, multiple data sets, each consisting of a data set of detected temperature values ​​and moisture content of product pellets, are plotted using crosses. It can be seen from the graph shown in Fig. 5 that as the temperature of the casing 71 increases, the moisture content of the product pellets tends to decrease, and as the temperature of the casing 71 decreases, the moisture content of the product pellets tends to increase.

[0055] A high temperature of the casing 71 (temperature of the dried pellets) means that the calorific value of the dry gas supplied to the drying furnace 30 is large. If the calorific value of the dry gas supplied to the drying furnace 30 increases (if more heat is applied to the sludge granulated material), the drying of the generated dry pellets is accelerated, and as a result, the moisture content of the dried pellets decreases. On the other hand, a low temperature of the casing 71 (temperature of the dried pellets) means that the calorific value of the dry gas supplied to the drying furnace 30 is small. If the calorific value of the dry gas supplied to the drying furnace 30 decreases (if less heat is applied to the sludge granulated material), the drying of the generated dry pellets is suppressed, and as a result, the moisture content of the dried pellets increases. From the above, it is possible to estimate the moisture content of the dried pellets using the detection value of the temperature sensor 79.

[0056] The control device 100 is a computer that controls at least the heating unit 50. As shown in Fig. 6, the control device 100 has, as functional components (hereinafter referred to as "functional modules"), for example, a temperature information acquisition unit 112, an estimation model holding unit 114, a moisture content estimation unit 116, a target temperature setting unit 118, a heating control unit 120, and a result output unit 122. The processing executed by each of these functional modules corresponds to the processing executed by the control device 100.

[0057] The temperature information acquisition unit 112 acquires information indicating the temperature of the casing 71 from the temperature sensor 79. The temperature information acquisition unit 112 repeatedly acquires the information indicating the temperature of the casing 71 from the temperature sensor 79 at predetermined measurement intervals (at each measurement interval), for example.

[0058] The estimation model holding unit 114 holds (stores) a prediction model M that is constructed in advance to indicate the relationship between the temperature of the casing 71 and the moisture content of the product pellets. The prediction model M is a model that outputs the moisture content of the product pellets in response to an input of the temperature of the casing 71. For example, as shown in FIG. 5, the prediction model M is a linear approximation expression that indicates the relationship between the temperature of the casing 71 and the moisture content of the product pellets. Instead of a linear approximation expression, the prediction model M may be a quadratic or higher approximation expression, or may be a model constructed by machine learning. The prediction model M may also be discrete table data. The prediction model M may be constructed by an operator in the early stages of operation of the sludge drying apparatus 1 and then input to the control device 100.

[0059] The moisture content estimation unit 116 (estimation unit) estimates the moisture content of the product pellets based on the detection value of the temperature sensor 79. The moisture content estimation unit 116 may estimate the moisture content of the product pellets based on the prediction model M stored in the estimation model storage unit 114 and the detection value by the temperature sensor 79. The moisture content estimation unit 116 may repeat estimating the moisture content of the product pellets for each measurement period in accordance with the measurement period of the temperature sensor 79.

[0060] The target temperature setting unit 118 changes the target outlet temperature, which represents a target value for the outlet temperature of the drying oven 30, based on the detection value of the temperature sensor 79. The target temperature setting unit 118 changes the target outlet temperature, for example, so that the detection value of the temperature sensor 79 falls within a predetermined target range. In addition to changing the target outlet temperature, the target temperature setting unit 118 calculates a target inlet temperature, which represents a target value for the inlet temperature of the drying oven 30, so that the outlet temperature of the drying oven 30 follows the target outlet temperature. The target temperature setting unit 118, for example, calculates the deviation between the detection value of the temperature sensor 82 (detection value of the outlet temperature) and the target outlet temperature, and calculates the target inlet temperature so as to reduce the deviation.

[0061] The heating control unit 120 changes the amount of heat imparted to the drying gas by the heating unit 50 based on the detection value of the temperature sensor 79. The heating control unit 120 changes the amount of heat imparted to the drying gas by the heating unit 50 so that the detection value of the temperature sensor 79 falls within a predetermined target range. For example, the heating control unit 120 controls the detection value of the temperature sensor 79 so that it falls within a predetermined target range, thereby producing product pellets (dried pellets) such that the moisture content of the product pellets falls within the target range.

[0062] The heating control unit 120 controls the heating unit 50, for example, so that the outlet temperature of the drying oven 30 (the value detected by the temperature sensor 82) follows the outlet target temperature. In one example, the heating control unit 120 controls the drying oven 30 so that the inlet temperature of the drying oven 30 (the value detected by the temperature sensor 83) follows the inlet target temperature calculated by the target temperature setting unit 118. The heating control unit 120 may calculate the deviation between the value detected by the temperature sensor 83 (the detected value of the inlet temperature) and the inlet target temperature, and change the amount of heat imparted to the drying gas by the heating unit 50 so as to reduce the deviation.

[0063] Changing the amount of heat imparted to the drying gas by the heating unit 50 based on the detection value of the temperature sensor 79 also includes changing the amount of heat imparted to the drying gas by the heating unit 50 based on the moisture content of the product pellets obtained based on the detection value of the temperature sensor 79. Changing the target outlet temperature based on the detection value of the temperature sensor 79 also includes changing the target outlet temperature based on the moisture content of the product pellets obtained based on the detection value of the temperature sensor 79.

[0064] The result output unit 122 outputs the moisture content of the product pellets obtained based on the detection value of the temperature sensor 79. To the control device 100, for example, an output device 102 is connected. The output device 102 is a device for notifying an operator of information from the control device 100. The output device 102 is, for example, a monitor. The result output unit 122 may output the estimated result of the moisture content of the product pellets to the output device 102.

[0065] FIG. 7 is a block diagram illustrating an example of the hardware configuration of the control device 100. The control device 100 is configured with one or more control computers. As shown in FIG. 7, the control device 100 has a circuit 150. The circuit 150 includes one or more processors 151, a memory 152, a storage 153, an input / output port 154, and a timer 155. The storage 153 has a computer-readable storage medium such as a non-volatile semiconductor memory. The storage 153 stores a program that causes the control device 100 to control the heating unit 50 and the like according to a preset control procedure. For example, the storage 153 stores a program for configuring each of the above-mentioned functional modules.

[0066] The memory 152 temporarily stores programs loaded from a storage medium in the storage 153 and calculation results by the processor 151. The processor 151 executes the programs in cooperation with the memory 152 to configure each functional module of the control device 100. The input / output port 154 inputs and outputs electrical signals to and from the heating unit 50, temperature sensor 79, temperature sensor 82, temperature sensor 83, output device 102, etc. in accordance with commands from the processor 151. The timer 155 measures elapsed time, for example, by counting reference pulses at a fixed interval.

[0067] The circuit 150 is not necessarily limited to one that configures each function by a program. For example, the circuit 150 may configure at least some of its functions by a dedicated logic circuit or an ASIC (Application Specific Integrated Circuit) that integrates such a dedicated logic circuit.

[0068] [Sludge drying method] Next, as an example of a sludge drying method, a description will be given of multiple processes executed in the sludge drying apparatus 1. The sludge drying method includes, for example, a granulation process, a drying process, a separation process, a conveying process, a classification process, a temperature detection process, and a drying control process. These multiple processes are executed in parallel while the sludge drying apparatus 1 is operating.

[0069] The granulation process is a process in which sludge is agitated in the mixer 10 to obtain sludge granules. The granulation process includes supplying sludge from the sludge supply unit 20 to the mixer 10 and supplying recycled pellets from the recycle hopper 65 to the mixer 10. The granulation process includes sending the sludge granules obtained in the mixer 10 to the drying furnace 30.

[0070] The drying process is a process in which the sludge granules obtained by stirring the sludge are dried with a dry gas to produce dry pellets. The drying process includes supplying the dry gas heated in the heating section 50 to the drying furnace 30. In the drying process, for example, the sludge granules are dried with the dry gas while the drying furnace 30 rotates around its rotation axis, thereby producing dry pellets. The drying process includes sending the dry pellets obtained in the drying furnace 30 and the dry gas used to dry the sludge granules to the separation section 61.

[0071] The separation process is a process of separating the dried pellets produced in the drying process from the drying gas used for drying. In the separation process, the dried pellets and the drying gas are separated, for example, by a vacuum filter functioning as the separation unit 61. The separation process includes supplying the dried pellets separated from the drying gas to the conveyor 70.

[0072] The conveying process is a process of conveying the dried pellets after separation from the drying gas to a predetermined position using the conveyor 70. In the conveying process, the conveyor 70 is used to convey the dried pellets from the separation unit 61 to the classifier 62. In the conveying process, heat is transferred from the dried pellets being conveyed by the conveyor 70 to the casing 71 of the conveyor 70.

[0073] The classification process is a process of classifying the dried pellets after they have been transported by the conveyor 70. In the classification process, the dried pellets are separated into product pellets and recycled pellets. In the classification process, some of the dried pellets after they have been transported by the conveyor 70 are introduced into the product hopper 64 as product pellets. After being temporarily stored in the product hopper 64, the product pellets may be shipped as solid fuel.

[0074] The temperature detection process is a process in which a temperature sensor 79 installed at a predetermined location of the casing 71 detects the temperature of the predetermined location. In the temperature detection process, for example, in a section of the conveyor 70 where dried pellets are transported in a direction inclined with respect to the vertical direction, the temperature sensor 79 detects the temperature of a predetermined location on the bottom wall 71a of the casing 71.

[0075] The drying control process is a process of controlling the heating section 50 based on the detection value of the temperature sensor 79 so that the moisture content of the product pellets is maintained within a certain target range. The drying control process is executed by the control device 100. An example of a series of processes executed by the control device 100 will be described below. In the following, a case will be described in which the detection value of the temperature sensor 79 is used directly to perform control without estimating the moisture content from the detection value of the temperature sensor 79.

[0076] 8 and 9 are flowcharts showing a series of processes executed by the control device 100 in the drying control step. The series of processes shown in Fig. 8 is a process flow for setting a target outlet temperature that represents a target value for the outlet temperature of the drying furnace 30. The series of processes shown in Fig. 9 is a process flow for controlling the heating unit 50 based on the target outlet temperature. The series of processes shown in Fig. 8 and the series of processes shown in Fig. 9 are executed in parallel.

[0077] With the prediction model M already constructed and the outlet target temperature set to an initial value, the control device 100 executes step S11 as shown in Fig. 8. In step S11, for example, the temperature information acquisition unit 112 waits until a predetermined measurement timing arrives. The predetermined measurement timing is set so that temperature detection of the casing 71 is repeated, for example, every one minute to several minutes. The initial value and measurement timing may be set in advance by an operator.

[0078] Next, the control device 100 executes steps S12 and S13. In step S12, for example, the temperature information acquisition unit 112 acquires information indicating the temperature of the casing 71 from the temperature sensor 79. In step S13, for example, the target temperature setting unit 118 determines whether the temperature acquired in step S12 is equal to or higher than a predetermined threshold value Th1. The threshold value Th1 may be set in advance by an operator. The threshold value Th1 is set, for example, based on the upper limit of the target range of the temperature of the casing 71 that corresponds to the target range of the moisture content of the product pellets (for example, it is set to a value that is 1°C to several°C lower than that upper limit).

[0079] If it is determined in step S13 that the temperature obtained in step S12 is equal to or higher than threshold value Th1 (step S13: YES), the process executed by control device 100 proceeds to step S14. In step S14, for example, target temperature setting unit 118 sets a value obtained by subtracting a predetermined value from the current outlet target temperature as a new outlet target temperature. This changes the outlet target temperature to a smaller value. The predetermined value representing the amount by which the temperature is decreased may be set in advance by an operator, or may be set to a value of about 1°C to several°C.

[0080] On the other hand, if it is determined in step S13 that the temperature obtained in step S12 is lower than threshold value Th1 (step S13: NO), the process executed by the control device 100 proceeds to step S15. In step S15, for example, the target temperature setting unit 118 determines whether the temperature obtained in step S12 is equal to or lower than a predetermined threshold value Th2. The threshold value Th2 may be set in advance by an operator. For example, the threshold value Th2 is set based on the lower limit of the target range of the temperature of the casing 71 corresponding to the target range of the moisture content of the product pellets (for example, it is set to a value that is 1°C to several degrees Celsius higher than that lower limit). The threshold value Th2 is a value lower than threshold value Th1.

[0081] If it is determined in step S15 that the temperature obtained in step S12 is equal to or lower than threshold value Th2 (step S15: YES), the process executed by control device 100 proceeds to step S16. In step S16, for example, target temperature setting unit 118 sets a value obtained by adding a predetermined value to the current outlet target temperature as the new outlet target temperature. This changes the outlet target temperature to a larger value. The predetermined value representing the increase may be set in advance by an operator, or may be set to a value of about 1°C to several°C.

[0082] On the other hand, if it is determined in step S15 that the temperature obtained in step S12 is greater than threshold value Th2 (step S15: NO), the process executed by the control device 100 returns to step S11. In this case, the outlet target temperature is maintained at its current value. As described above, if the temperature of the casing 71 is equal to or greater than threshold value Th1, the outlet target temperature is changed to decrease, and if the temperature of the casing 71 is equal to or less than threshold value Th2, the outlet target temperature is changed to increase.

[0083] After executing step S14 or step S16, the control device 100 executes step S17. In step S17, for example, the target temperature setting unit 118 waits until a predetermined time has elapsed since the execution of the previous step. The predetermined time may be set in advance by an operator, and is set, for example, to a value (e.g., about 5 to 20 minutes) that allows a change in the outlet target temperature to actually be reflected in the drying status in the drying oven 30.

[0084] After step S17 is executed, the process executed by the control device 100 returns to step S11. As described above, if the target outlet temperature is not changed, at the next measurement timing, it is determined again whether or not the target outlet temperature needs to be changed based on the temperature of the casing 71. On the other hand, if the target outlet temperature is changed, after waiting for a predetermined time, it is determined again whether or not the target outlet temperature needs to be changed based on the temperature of the casing 71.

[0085] During the execution of the series of processes shown in Fig. 8, the control device 100 executes step S21 as shown in Fig. 9. In step S21, for example, the heating control unit 120 acquires information indicating the outlet temperature of the drying oven 30 (the temperature of the drying gas after being used for drying in the drying oven 30) from the temperature sensor 82.

[0086] Next, the control device 100 executes step S22. In step S22, for example, the target temperature setting unit 118 calculates an inlet target temperature representing a target value for the inlet temperature of the drying furnace 30 based on the outlet temperature obtained in step S21 and the current outlet target temperature. The target temperature setting unit 118 may calculate a deviation between the current outlet target temperature and the outlet temperature obtained in step S21, and perform a proportional operation, a proportional-integral operation, or a proportional-integral-differential operation on the deviation to calculate the inlet target temperature.

[0087] Next, the control device 100 executes step S23. In step S23, for example, the heating control unit 120 acquires, from the temperature sensor 83, information indicating the inlet temperature of the drying oven 30 (the temperature of the drying gas after being heated by the heating unit 50 and before being introduced into the drying oven 30).

[0088] Next, the control device 100 executes step S24. In step S24, for example, the heating control unit 120 changes the amount of heat imparted to the drying gas by the heating unit 50 based on the inlet target temperature calculated in step S22 and the inlet temperature obtained in step S23. In one example, the heating control unit 120 may calculate the deviation between the inlet target temperature calculated in step S22 and the inlet temperature obtained in step S23, and perform a proportional operation, a proportional-integral operation, or a proportional-integral-differential operation on the deviation to calculate the manipulated variable of the combustion furnace 51. The heating control unit 120 then outputs the calculated manipulated variable to the combustion furnace 51. Specific examples of the manipulated variable of the combustion furnace 51 include manipulated variables for the opening of a fuel valve and an air valve. By adjusting the manipulated variable of the combustion furnace 51, the amount of heat imparted to the drying gas by the heating unit 50 is adjusted.

[0089] The control device 100 repeatedly executes a series of processes from steps S21 to S24 in a predetermined control cycle (for each control cycle), thereby executing cascade control that includes calculating the target inlet temperature so that the outlet temperature of the drying furnace 30 follows the target outlet temperature, and controlling the heating unit 50 (the manipulated variable of the combustion furnace 51) so that the inlet temperature of the drying furnace 30 follows the target inlet temperature.

[0090] [Variations] The above-described series of processes shown in each of Figures 8 and 9 are examples and can be modified as appropriate. In the above-described series of processes, the control device 100 may execute one step and the next step in parallel, or may execute each step in an order different from that of the above-described example. The control device 100 may execute a process in any step that is different from that of the above-described example.

[0091] 8, the control device 100 may estimate the moisture content of the product pellets based on the temperature of the casing 71 obtained in step S12, and then change the target outlet temperature based on the moisture content of the product pellets. The target temperature setting unit 118 may increase the target outlet temperature when the moisture content of the product pellets is equal to or greater than a first threshold, and may decrease the target outlet temperature when the moisture content of the product pellets is equal to or less than a second threshold that is smaller than the first threshold. The target temperature setting unit 118 may maintain the target outlet temperature at its current value when the moisture content of the product pellets is less than the first threshold and greater than the second threshold.

[0092] In the above example, the control device 100 performs two functions: controlling the heating unit 50 based on the value detected by the temperature sensor 79, and outputting the estimated moisture content of the product pellets, but it may perform only one of these functions. In one example, the result output unit 122 may output the estimated moisture content, and then the control device 100 may control the heating unit 50 based on instructions from an operator who has confirmed the estimated moisture content.

[0093] Instead of the above-described cascade control, the control device 100 (heating control unit 120) may control the heating unit 50 without considering the inlet temperature of the drying furnace 30. In one example, the heating control unit 120 may calculate the deviation between the detected value of the temperature sensor 82 (detected value of the outlet temperature) and the target outlet temperature, and change the amount of heat imparted to the drying gas by the heating unit 50 so as to reduce the deviation.

[0094] Instead of the above-described cascade control, the control device 100 (heating control unit 120) may control the heating unit 50 without taking into consideration the outlet temperature of the drying furnace 30. In one example, the target temperature setting unit 118 sets (calculates) the inlet target temperature based on the detected value of the temperature sensor 79 or the estimated moisture content of the product pellets so that the moisture content of the product pellets is maintained within a target range. The heating control unit 120 may then calculate the deviation between the detected value of the temperature sensor 83 (detected value of the inlet temperature) and the inlet target temperature, and change the amount of heat imparted to the drying gas by the heating unit 50 so as to reduce the deviation.

[0095] The heating unit 50 may have any configuration as long as it can adjust the temperature of the drying gas introduced into the drying furnace 30. The material forming the casing 71 is not limited to metal. The material forming the casing 71 may be any material that transmits the heat of the dried pellets and allows the temperature sensor 79 to detect a temperature correlated to the temperature (moisture content) of the dried pellets at the installation location.

[0096] The temperature sensor 79 may be installed on the top wall 71b, side wall 71c, or side wall 71d instead of the bottom wall 71a in a section of the conveyor 70 where the dried pellets are transported in a direction inclined with respect to the vertical direction. The temperature sensor 79 may be installed on a side wall of the casing 71 in a section of the conveyor 70 where the dried pellets are transported in the vertical direction.

[0097] The sludge drying apparatus 1 may include two or more temperature sensors 79. That is, the sludge drying apparatus 1 may detect temperatures at two or more locations on the casing 71 using two or more temperature sensors 79. The moisture content estimation unit 116 of the control device 100 may calculate a statistical value, such as an average value, of the detection results of the two or more temperature sensors 79, and then estimate the moisture content of the product pellets. The heating control unit 120 of the control device 100 may calculate a statistical value, such as an average value, of the detection results of the two or more temperature sensors 79, and then control the heating unit 50 based on the calculation result.

[0098] Summary of this disclosure The sludge drying apparatus 1 described above comprises a drying furnace 30 that dries the sludge granules obtained by stirring the sludge using dry gas to produce dried pellets, a separation section 61 that separates the dried pellets produced in the drying furnace 30 from the dry gas after being used for drying, a conveyor 70 that has a cylindrical casing 71 that forms a space S for transporting the dried pellets and transports the pellets from the separation section 61 to a predetermined position, and a temperature sensor 79 that detects the temperature at a predetermined location of the casing 71.

[0099] In systems that convert sludge, such as sewage sludge, into solid fuel, maintaining the moisture content of the product pellets (dried pellets) within a certain range is an important quality control issue. In such systems, operators typically periodically manually measure the moisture content (dryness) of the product pellets using a moisture content meter. Therefore, the operator must manually change the operating state to adjust the dryness of the product pellets. This requires manual measurement of the moisture content before operating the system, which increases the workload for quality control. In response to this issue, the sludge drying system 1 is equipped with a temperature sensor 79 that detects the temperature at a predetermined location on the casing 71 of the conveyor 70 that transports the dried pellets that will later become the product pellets. As described above, a correlation has been found between the temperature of the casing 71 and the moisture content of the product pellets. Therefore, by controlling the temperature of the casing 71, the moisture content of the product pellets can be effectively controlled. This reduces the number of manual moisture content measurements. Therefore, the sludge drying system 1 is useful for simplifying quality control. Furthermore, the simplification of the process increases the opportunities for managing (understanding) the moisture content, which may contribute to improving the quality of the product pellets.

[0100] The sludge drying apparatus 1 described above may further include a classifier 62 that classifies the dried pellets transported by the conveyor 70 to obtain product pellets, and a moisture content estimation unit 116 that estimates the moisture content of the product pellets based on the detected value of the temperature sensor 79. In this case, manual operation can be performed by knowing the moisture content of the product pellets, or automatic control can be performed using the moisture content of the product pellets, which is useful for achieving both simplification of quality control and improvement of the quality of the product pellets.

[0101] In the sludge drying apparatus 1 described above, the moisture content estimation unit 116 may estimate the moisture content of the product pellets based on a prediction model M that is constructed in advance to indicate the relationship between the temperature of the casing 71 and the moisture content of the product pellets, and the detection value of the temperature sensor 79. In this case, by using the prediction model M, the moisture content can be estimated with high accuracy.

[0102] In the sludge drying apparatus 1 described above, the conveyor 70 may further include a heat insulating member 72 that covers the periphery of the casing 71. The temperature sensor 79 may be installed on the outer surface of the casing 71. Even if the temperature sensor 79 is installed on the outer surface of the casing 71, the heat insulating member 72 makes the detected value of the temperature sensor 79 less susceptible to the influence of outside air, and the temperature of the dried pellets in the space S can be obtained with high accuracy. Therefore, this is useful for achieving both simplification of the installation work of the temperature sensor 79 and highly accurate estimation of the moisture content.

[0103] In the sludge drying apparatus 1 described above, the temperature sensor 79 may be installed on the bottom wall 71a of the casing 71 in a section of the conveyor 70 where the dried pellets are transported in a direction inclined with respect to the vertical direction. The dried pellets transported by the conveyor 70 come into contact with the bottom wall 71a. Therefore, with the above configuration, the temperature of the dried pellets in the space S can be obtained with high accuracy. Therefore, this is useful for estimating the moisture content with high accuracy using the value detected by the temperature sensor 79.

[0104] In the sludge drying apparatus 1 described above, the temperature sensor 79 may be installed on the bottom wall 71a of the casing 71 in a section of the conveyor 70 where the dried pellets are transported horizontally. Compared to sections where the dried pellets are transported diagonally upward or downward, the amount of pellets that come into contact with the bottom wall 71a among the dried pellets transported by the conveyor 70 increases. Therefore, with the above configuration, the temperature of the dried pellets in the space S can be obtained with higher accuracy. Therefore, this is even more useful for estimating the moisture content with high accuracy using the value detected by the temperature sensor 79.

[0105] The sludge drying apparatus 1 described above may further include a heating unit 50 that heats the drying gas supplied to the drying furnace 30, and a heating control unit 120 that changes the amount of heat imparted to the drying gas by the heating unit 50 based on the detected value of the temperature sensor 79. In this case, manual operation by the operator to adjust the moisture content of the product pellets can be omitted, which is useful for reducing the workload of the operator.

[0106] The sludge drying apparatus 1 described above may further include a target temperature setting unit that changes the target outlet temperature based on the detection value of the temperature sensor 79. The heating control unit 120 may control the heating unit 50 so that the temperature of the drying gas discharged from the drying furnace 30 follows the target outlet temperature. In this case, the heating unit 50 is controlled so that the temperature follows the target outlet temperature based on the detection value of the temperature sensor 79. This allows automatic control to be performed to adjust the moisture content of the product pellets to within the target range. This therefore allows for improvement in the quality of the product pellets.

[0107] In the sludge drying apparatus 1 described above, the target temperature setting unit 118 may further calculate an inlet target temperature so that the temperature of the drying gas after being discharged from the drying furnace 30 follows the outlet target temperature. The heating control unit 120 may control the heating unit 50 so that the temperature of the drying gas before being introduced into the drying furnace 30 follows the inlet target temperature. In this case, the influence of disturbances that may occur on the temperature of the drying gas before being introduced into the drying furnace 30 can be adjusted at an earlier timing than when only feedback control is performed based on the temperature of the drying gas after being discharged from the drying furnace 30. Therefore, the influence of disturbances can be suppressed, thereby further improving the quality of the product pellets.

[0108] The sludge drying method described above includes a drying step of drying granulated sludge obtained by stirring sludge with a dry gas to produce pellets, a separation step of separating the dried pellets produced in the drying step from the dry gas used for drying, a transport step of transporting the dried pellets after separation from the dry gas to a predetermined position using a conveyor 70 having a cylindrical casing 71 that forms a space S for transporting the dried pellets, and a temperature detection step of detecting the temperature at the predetermined position using a temperature sensor 79 installed at a predetermined position in the casing 71. Like the sludge drying apparatus 1 described above, this sludge drying method is useful for simplifying quality control. [Explanation of symbols]

[0109] 1...sludge drying device, 30...drying furnace, 61...separation section, 62...classifier, 70...conveyor, 71...casing, S...space, 71a...bottom wall, 72...heat insulation member, 79...temperature sensor, 100...control device, 116...moisture content estimation section, 118...target temperature setting section, 120...heating control section.

Claims

1. a drying furnace for drying the sludge granules obtained by stirring the sludge with a dry gas to produce pellets; a separation unit that separates the pellets produced in the drying furnace from the drying gas after being used for drying; a conveyor having a cylindrical casing that forms a space for conveying the pellets, and that conveys the pellets from the separation section to a predetermined position; A sludge drying apparatus comprising: a temperature sensor that detects the temperature at a predetermined location of the casing.

2. a classifying unit that classifies the pellets after being conveyed by the conveyor to obtain product pellets; The sludge drying apparatus according to claim 1 , further comprising: an estimation unit that estimates a moisture content of the product pellets based on a detection value of the temperature sensor.

3. The sludge drying apparatus of claim 2, wherein the estimation unit estimates the moisture content of the product pellets based on a prediction model pre-constructed to show the relationship between the temperature of the casing and the moisture content of the product pellets and the detection value of the temperature sensor.

4. The conveyor further includes a heat insulating member that covers the periphery of the casing, The sludge drying apparatus according to any one of claims 1 to 3, wherein the temperature sensor is installed on an outer surface of the casing.

5. The sludge drying apparatus according to any one of claims 1 to 3, wherein the temperature sensor is installed on the bottom wall of the casing in a section of the conveyor where the pellets are transported in a direction inclined relative to the vertical direction.

6. 6. The sludge drying apparatus according to claim 5, wherein the temperature sensor is installed on a bottom wall of the casing in a section of the conveyor where the pellets are transported horizontally.

7. a heating unit that heats the drying gas supplied to the drying furnace; The sludge drying apparatus according to any one of claims 1 to 3, further comprising a heating control unit that changes the amount of heat imparted to the drying gas by the heating unit based on the detection value of the temperature sensor.

8. a target temperature setting unit that changes the outlet target temperature based on the detected value of the temperature sensor; The sludge drying apparatus according to claim 7 , wherein the heating control unit controls the heating unit so that the temperature of the drying gas after being discharged from the drying furnace follows the outlet target temperature.

9. the target temperature setting unit further calculates an inlet target temperature so that the temperature of the drying gas after being discharged from the drying furnace follows the outlet target temperature; The sludge drying apparatus according to claim 8 , wherein the heating control unit controls the heating unit so that the temperature of the drying gas before being introduced into the drying furnace follows the inlet target temperature.

10. a drying step of drying the sludge granules obtained by stirring the sludge with a dry gas to produce pellets; a separation step of separating the pellets produced in the drying step from the drying gas used for drying; a conveying step of conveying the pellets after separation from the dry gas to a predetermined position using a conveyor having a cylindrical casing that forms a space for conveying the pellets; a temperature detection step of detecting the temperature at a predetermined location of the casing using a temperature sensor installed at the predetermined location.

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