Sand drying system and sand drying method

Abstract

To provide a sand drying system and a sand drying method that can be relatively easily adopted and can efficiently dry sand.SOLUTION: The sand drying system comprises a hot air generation furnace 2 generating hot air by self-burning of wood fuel and first and second sand dryers 3 and 4 for drying wet sand, which are juxtaposed, in a crushing plant, with a hot air extraction duct 21 provided on the hot air generation furnace 2, being connected to one end of a first drum 19 of the first sand dryer 3, an exhaust duct 25 being connected to the other end thereof, and a heat exchanger 5 being interposed midway in the first exhaust duct 25 to exchange heat between exhaust coming out of the first drum 19 of the first sand dryer 3 and outdoor air supplied by a blower 32 for supplying outdoor air to heat the outdoor air using latent condensation heat of steam included in the exhaust upon condensation, is characterized by comprising a supply duct 34 that introduces outdoor air heated by the heat exchanger 5 into a second drum 20 of the second sand dryer 4.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a drying system and method for drying sand, particularly sand used as a raw material for asphalt mixtures. [Background technology]

[0002] In asphalt plants at asphalt mixture manufacturing factories that produce asphalt mixtures for road paving, aggregates such as gravel and sand, which are the raw materials for the asphalt mixture, are heated to, for example, about 170°C, and then mixed with stone powder, molten asphalt, etc. in specified amounts to produce the desired asphalt mixture.

[0003] When the aggregate is heated to about 170°C, a large amount of fuel (e.g., about half of the total fuel consumption) is consumed to dry the moisture contained in the aggregate. Sand, which has a large specific surface area, is often supplied to the plant with a relatively high moisture content (e.g., about 15%), so a significant amount of fuel is consumed for the drying process. Therefore, if the moisture content of the aggregates used in asphalt plants, particularly sand, could be reduced in advance, it would be possible to effectively save energy and also reduce CO2 emissions.

[0004] In response to the above-mentioned problems, the present applicant has proposed, as shown in Patent Document 1 (JP 2004-36330 A) and Patent Document 2 (JP 2011-226109 A), a sand dryer, which is a dedicated dryer (rotary kiln) that pre-dries wet sand with a high moisture content, to be installed within the facilities of an asphalt mixture manufacturing factory (near the asphalt plant), and which effectively utilizes, as its heat source, high-temperature hot air (waste heat) exhausted from a deodorizing furnace already installed within the factory facilities. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-36330 [Patent Document 2] Japanese Patent Application Laid-Open No. 2011-226109 Summary of the Invention [Problem to be solved by the invention]

[0006] However, as mentioned above, installing a separate sand dryer within the facilities of an asphalt mixture manufacturing plant requires a certain amount of expense, and depending on the size of the plant, it may be difficult to adopt. Furthermore, even if a sand dryer is installed within the plant facilities, in asphalt plants that are forced to operate intermittently, it is not possible to secure a heat source, at least while the plant (deodorizing furnace) is not operating, and the sand dryer cannot be operated, and it is thought that there is room for improvement in terms of efficient sand drying processing.

[0007] In view of the above, an object of the present invention is to provide a sand drying system and a sand drying method that are relatively easy to adopt and can efficiently dry sand. [Means for solving the problem]

[0008] As a result of extensive research into solving the above problems, the inventors have come to the conclusion that if sand dryers that pre-dry wet sand were installed at a crushing plant that supplies various aggregates, including sand, to multiple asphalt mixture manufacturing plants, rather than at the asphalt mixture manufacturing plant as in the past, it would be possible to eliminate the need for sand dryers at each individual asphalt mixture manufacturing plant, making this a relatively easy-to-adopt solution. Although this would inevitably increase costs for the crushing plant and CO2 emissions, it would be possible to improve efficiency by allowing the crushing plant to dry large quantities of sand to be supplied to multiple asphalt mixture manufacturing plants in a batch (continuous) manner. Furthermore, by being able to supply pre-dried sand to each asphalt mixture manufacturing plant, the added value would be increased, making the investment worthwhile. Furthermore, considering the total CO2 emissions, including those of each asphalt mixture manufacturing plant to which the sand is supplied, this could be considered a reduction, and this led to the completion of the present invention.

[0009] That is, in the sand drying system according to claim 1 of the present invention, a hot air generating furnace that generates hot air by spontaneously combusting wood fuel, and first and second sand dryers that dry moistened sand by contacting the hot air, are installed in parallel in a crushed stone factory, and the hot air generating furnace comprises a kiln body having a supporting burner that uses natural gas as fuel, a wood fuel supply means, and a combustion air supply fan that supplies combustion air necessary for spontaneous combustion of the wood fuel, and a secondary combustion chamber that combusts and decomposes volatile matter generated by spontaneous combustion of the wood fuel in the kiln body, and A hot air outlet duct provided in the secondary combustion chamber of the hot air generating furnace is connected to one end of the first drum of the first sand dryer, while an exhaust duct is provided at the other end. A heat exchanger is provided in the middle of the exhaust duct, which exchanges heat between exhaust gas discharged from the first drum of the first sand dryer and outside air supplied from an outside air supply fan, and heats the outside air by utilizing the latent heat of condensation when water vapor contained in the exhaust gas condenses. A supply duct is also provided for introducing the outside air heated by the heat exchanger into the second drum of the second sand dryer. a first branch duct for introducing a portion of the outside air heated by the heat exchanger into an exhaust duct of a first sand dryer downstream of the heat exchanger; It is characterized by the fact that

[0010] In addition, the sand drying system described in claim 2 is characterized by having a first moisture sensor that detects the moisture content of the dried sand discharged from the first drum of the first sand dryer, and a first controller that adjusts and controls at least one of the combustion amount of the auxiliary burner of the hot air generator furnace, the amount of wood fuel supplied into the kiln body of the hot air generator furnace, the amount of sand supplied into the first drum of the first sand dryer, and the rotation speed of the first drum of the first sand dryer so that the moisture content of the dried sand detected by the first moisture sensor is maintained at a predetermined moisture content.

[0011] In addition, the sand drying system described in claim 3 is characterized by having a second moisture sensor that detects the moisture content of the dried sand discharged from the second drum of the second sand dryer, and a second controller that adjusts and controls at least one of the amount of outside air blown from the blower, the amount of sand supplied into the second drum of the second sand dryer, and the rotation speed of the second drum of the second sand dryer so that the moisture content of the dried sand detected by the second moisture sensor is maintained at a predetermined moisture content.

[0013] Also, claims 4 The sand drying system described is characterized by having a second branch duct that supplies a portion of the outside air heated by the heat exchanger to the combustion air supply fan of the hot air generating furnace.

[0014] Also, claims 5 In the sand drying system described above, a storage tank for storing liquefied natural gas at low temperature and high pressure and a vaporizer are provided near the hot air generating furnace, and the heat exchanger vessel The apparatus is characterized by being provided with a hot water supply pipe that supplies hot water generated as water vapor contained in the exhaust gas from the first drum of the first sand dryer is condensed to the vaporizer as a heat source.

[0015] Also, claims 6In the sand drying method described, a hot air generating furnace that generates hot air by spontaneously combusting wood fuel and first and second sand dryers that dry moist sand by contact with the hot air are installed in parallel in a crushing plant. In the first sand dryer, hot air discharged from the hot air generating furnace is introduced into the first drum of the first sand dryer to dry the sand. In the second sand dryer, heat is exchanged between exhaust gas discharged from the first drum of the first sand dryer and outside air, and the outside air is heated using the latent heat of condensation when water vapor contained in the exhaust gas condenses. The heated outside air is then introduced into the second drum of the second sand dryer to dry the sand. At the same time, hot water generated as the steam contained in the exhaust gas from the first drum of the first sand dryer condenses during the heat exchange is mixed with the sludge water of the ready-mixed concrete, and the carbon dioxide contained in the hot water reacts with the calcium ions contained in the sludge water to recover calcium carbonate. It is characterized by the following.

[0016] Also, claims 7 The sand drying method described is characterized in that the first and second sand dryers perform the drying process so as to maintain the sand at a predetermined moisture content. [Effects of the Invention]

[0018] According to the sand drying system of claim 1 of the present invention, the sand dryer is installed on the crushed stone plant side, so there is no burden on the asphalt mixture manufacturing plant, etc., which is the sand supply destination.Also, even on the crushed stone plant side, of the pair of sand dryers installed on the premises, one dryer is supplied with hot air generated by the spontaneous combustion of wood fuel, and the other dryer is supplied with outside air heated by utilizing the latent heat of condensation when the water vapor in the exhaust gas condenses.This makes it possible to efficiently dry the large amounts of sand produced at the crushed stone plant while saving energy and reducing CO2 emissions, and makes it possible to construct a sand drying system that is relatively easy to adopt. Furthermore, despite the simple configuration, the temperature of the exhaust gas after heat exchange can be raised above the dew point, and problems such as condensation inside the exhaust duct can be effectively suppressed.

[0019] In addition, according to the sand drying system described in claim 2, by deliberately not drying the dried sand discharged from the first sand dryer completely but adjusting it so that it maintains a predetermined moisture content, some of the residual moisture in the sand after discharge can be evaporated using the sensible heat of the sand, thereby reducing energy loss due to simple heat radiation from the dried sand and enabling the sand to be dried more efficiently.

[0020] In addition, according to the sand drying system described in claim 3, by deliberately not drying the dried sand discharged from the second sand dryer completely but adjusting it so that it maintains a predetermined moisture content, some of the residual moisture in the sand after discharge can be evaporated using the sensible heat of the sand, thereby reducing energy loss due to simple heat radiation from the dried sand and enabling the sand to be dried more efficiently.

[0022] Also, claims 4 According to the described sand drying system, the combustion air supplied to the hot air generating furnace can be preheated, improving the combustion efficiency of wood fuel, which in turn reduces the fuel consumption of the auxiliary burner, thereby enabling further energy savings and CO2 reduction effects.

[0023] Also, claims 5 According to the sand drying system described, the liquefied natural gas that is the fuel for the auxiliary burner can be vaporized without the need for a separate heat source such as a boiler for vaporization, making it even easier to adopt.

[0024] Also, claims 6 According to the described sand drying method, the sand dryer is installed on the side of the crushed stone plant, so there is no burden on the asphalt mixture manufacturing plant, which is the sand supply destination.Also, even on the side of the crushed stone plant, a pair of sand dryers is installed on the premises, and one dryer is supplied with hot air generated by the spontaneous combustion of wood fuel, and the other dryer is supplied with outside air heated by utilizing the latent heat of condensation when the water vapor in the exhaust gas condenses.This makes it possible to efficiently dry the large amounts of sand produced at the crushed stone plant while saving energy and reducing CO2 emissions, and makes it possible to construct a sand drying system that is relatively easy to adopt. Furthermore, the CO2 contained in the exhaust gas can be recovered as calcium carbonate, a useful substance that can be used as a material for asphalt mixtures, and the volume of sludge water, which is an industrial waste product, can be reduced at the same time, making this an ideal system.

[0025] Also, claims 7According to the described sand drying method, the dried sand discharged from the first and second sand dryers is not dried completely, but is adjusted so that it maintains a predetermined moisture content. This allows some of the residual moisture in the sand after discharge to evaporate using the sensible heat of the sand, thereby reducing energy loss due to simple heat radiation from the dried sand and enabling the sand to be dried more efficiently. [Brief explanation of the drawings]

[0027] [Figure 1] 1 is a schematic explanatory diagram showing an embodiment of a sand drying system and a sand drying method according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0028] In the sand drying system and sand drying method of the present invention, for example, a hot air generator that generates hot air by self-combustion of wood fuel, and first and second sand dryers that dry the wet sand with a high moisture content produced at the stone crushing plant by contacting it with the hot air, are installed side by side within the facilities of a stone crushing plant that produces and supplies various aggregates, which are the raw materials for asphalt mixtures, to asphalt mixture manufacturing plants, etc.

[0029] The hot air generating furnace has a supporting burner fueled by natural gas, a wood fuel supply means for supplying wood fuel such as wood chips, and a combustion air supply fan for supplying the combustion air necessary to cause the wood fuel to spontaneously combust.It also consists of a cylindrical kiln body that is supported at an angle so that it can rotate freely, and a secondary combustion chamber that completely combusts and decomposes the volatile matter (wood gas) that is produced as a result of the spontaneous combustion of the wood fuel within the kiln body.

[0030] On the other hand, the first and second sand dryers are of a rotary kiln structure in which a cylindrical drum with multiple scraping blades attached to its inner surface is supported at an angle so as to be freely rotatable, and moist sand with a high moisture content is supplied into the drum together with hot air.The sand is repeatedly scraped up by the scraping blades and brought into direct contact with the hot air, thereby heating and drying the sand.

[0031] One end of the first drum of the first sand dryer is connected to a hot air outlet duct provided in the secondary combustion chamber of the hot air generator, while the other end is equipped with an exhaust duct, and a heat exchanger is installed in the exhaust duct to exchange heat between the high-temperature, high-humidity exhaust gas discharged from the first drum of the first sand dryer and ambient air at room temperature supplied by an ambient air supply fan, and to heat the ambient air using the latent heat of condensation generated when water vapor contained in the exhaust gas condenses during this heat exchange. A supply duct is also installed to supply and introduce the ambient air (warm air) heated by the heat exchanger into the second drum of the second sand dryer.

[0032] Although the exhaust gas discharged from the first drum of the first sand dryer is somewhat cooled due to its use in the sand drying process, it is still at a relatively high temperature and is in a highly humid state, containing a large amount of water vapor that has evaporated from the wet sand with a high moisture content.The water vapor contained in the exhaust gas easily condenses when it exchanges heat with the outside air in the heat exchanger, and the latent heat of condensation generated at that time can be used to efficiently heat the outside air at room temperature.

[0033] Here, although the temperature of the outside air heated through the heat exchanger is not as high as that of the hot air drawn out from the secondary combustion chamber of the hot air generating furnace, the relative humidity drops significantly as the temperature rises, so that the second sand dryer to which this air is supplied can adequately dry the sand.

[0034] In addition, the hot air generator that supplies hot air to the first sand dryer self-burns wood fuel such as wood chips, which can be considered to have zero CO2 emissions from a carbon-neutral perspective, and also uses natural gas, which has a low CO2 emission coefficient, as fuel for the auxiliary burner that assists the self-burning of the wood fuel.This makes it possible to reduce CO2 emissions compared to, for example, when the first sand dryer is directly equipped with a burner that uses heavy oil A or the like as fuel.

[0035] Furthermore, the volatile matter (wood gas) produced by the spontaneous combustion of wood fuel in the kiln body in the upstream stage of the hot air generator is completely combusted and decomposed in the secondary combustion chamber in the downstream stage and then supplied to the first sand dryer as hot air.Since natural gas, which serves as fuel for the auxiliary burner, does not originally contain substances such as sulfur and chlorine that cause the generation of acidic gases, the large amount of condensed water produced when the water vapor in the exhaust gas discharged from the first sand dryer is condensed in the heat exchanger does not become acidic and can be released as is without any special neutralization treatment, and the impact of acid corrosion on the heat exchanger, exhaust duct, etc. can be suppressed.

[0036] It is also preferable to provide a first moisture sensor that detects the moisture content of the dried sand that has been dried and is discharged from the first drum of the first sand dryer, and a first controller that adjusts and controls at least one of the combustion amount of the auxiliary burner of the hot air generator, the amount of wood fuel supplied into the kiln body of the hot air generator, the amount of sand supplied into the first drum of the first sand dryer, and the rotation speed of the first drum of the first sand dryer, so that the moisture content of the dried sand detected by the first moisture sensor is maintained at a predetermined moisture content that does not exceed bone dryness but leaves some moisture.

[0037] If the dried sand discharged from the first drum of the first sand dryer is to be heated and dried until it is bone dry, suitable for use as a raw material for asphalt mixtures, the temperature of the sand must be raised to a high temperature. However, if this hot sand is to be transported from the crushing plant to a remote asphalt mixture manufacturing plant, heat will be dissipated during the transport, causing the temperature to drop, which simply results in energy loss.

[0038] On the other hand, by intentionally leaving some moisture in the sand when it is discharged from the first drum as described above, it is possible to gradually evaporate some of the residual moisture using sensible heat during storage or transportation of the sand (effective use of thermal energy), thereby reducing energy loss due to simple heat dissipation. Furthermore, since it is not necessary to dry the sand until it is bone dry, the amount of fuel consumed in the sand drying process can be reduced, and CO2 emissions can also be expected to be reduced.

[0039] It is also preferable to provide a second moisture sensor that detects the moisture content of the dried sand discharged from the second drum of the second sand dryer, and a second controller that adjusts and controls at least one of the amount of outside air blown from the blower, the amount of sand supplied into the second drum of the second sand dryer, and the rotation speed of the second drum of the second sand dryer so that the moisture content of the dried sand detected by the second moisture sensor is maintained at a predetermined moisture content that does not exceed bone dryness but leaves some moisture.

[0040] As with the first sand dryer, the second sand dryer intentionally maintains a small amount of moisture in the dried sand discharged from the second drum, allowing sensible heat to be used to gradually evaporate some of the residual moisture during storage or transportation of the sand (making it possible to effectively utilize thermal energy), thereby reducing energy loss due to simple heat dissipation.

[0041] Furthermore, it is preferable to provide a first branch duct that introduces a portion of the outside air (hot air) heated by the heat exchanger into the exhaust duct of the first sand dryer downstream of the heat exchanger. Although the exhaust gas temperature after heat exchange may fall below the dew point temperature in some cases, which may cause problems such as condensation in the exhaust duct or white smoke emitting from the chimney at the end of the exhaust duct, the provision of the first branch duct allows a portion of the outside air (hot air) that has been heated and temperature-raised by heat exchange and has a reduced relative humidity to be mixed into the exhaust gas in the exhaust duct, thereby raising the exhaust gas temperature above the dew point, and thus effectively suppressing the above problems despite the simple configuration.

[0042] Furthermore, it is preferable to provide a second branch duct that supplies a portion of the outside air (hot air) heated by the heat exchanger to the combustion air supply fan of the hot air generating furnace. By providing the second branch duct, the combustion air to be supplied into the hot air generating furnace can be preheated (the preheated air can be supplied as combustion air), and the combustion efficiency of the self-combustion wood fuel can be improved, as a result of which the fuel consumption of the auxiliary burner can be reduced and further energy savings and CO2 reduction effects can be expected.

[0043] Furthermore, it is preferable to provide a storage tank for storing liquefied natural gas at low temperature and high pressure, and a vaporizer near the hot air generator, and to provide a hot water supply pipe for supplying condensed water (hot water) produced by the condensation of water vapor contained in the exhaust gas from the first drum of the first sand dryer in the heat exchanger to the vaporizer as a heat source. This makes it possible to vaporize the liquefied natural gas that serves as fuel for the auxiliary burner without providing a separate heat source such as a boiler for the vaporization process, making it easy to adopt from the standpoints of cost and maintenance.

[0044] When the sand is dried using the sand drying system, the wet sand with a high moisture content produced at the crushed stone plant is not shipped as is, but is first fed into the first and second drums of the first and second sand dryers installed within the crushed stone plant. In the first sand dryer, hot air generated by spontaneous combustion of wood fuel in a hot air generator located upstream is introduced into the first drum to dry the sand. In the second sand dryer, heat is exchanged between the high-temperature, high-humidity exhaust gas discharged from the first sand dryer and ambient air at room temperature supplied by a blower, and the ambient air is heated using the latent heat of condensation of the water vapor contained in the exhaust gas. This heated ambient air (hot air) is then introduced into the second drum to dry the sand in the same manner as in the first sand dryer.

[0045] The dried sand discharged from the first and second drums of the first and second sand dryers is then transported to its destination, such as an asphalt mixture manufacturing plant, as appropriate. In the asphalt plant dryer at the asphalt mixture manufacturing plant, the sand is heated to a predetermined temperature without consuming much thermal energy (fuel) to dry the sand (while reducing CO2 emissions), and is then mixed with various other materials to produce the desired asphalt mixture.

[0046] Thus, with the above-mentioned sand drying system and sand drying method, by installing a sand dryer at the crushed stone plant, it is possible to reduce costs and management burdens compared to installing one at each asphalt mixture manufacturing plant to which sand is supplied. Also, even at the crushed stone plant, one of a pair of sand dryers installed on the premises is supplied with hot air generated by the spontaneous combustion of wood fuel, and the other is supplied with outside air heated by utilizing the latent heat of condensation when water vapor in the exhaust gas condenses. This makes it possible to build a sand drying system that is relatively easy to adopt and can efficiently dry the large amounts of sand produced for each asphalt mixture manufacturing plant while saving energy and reducing CO2 emissions.

[0047] If the above sand drying system and sand drying method are adopted, it is undeniable that there will be an increase in costs for the crushed stone plant and in CO2 emissions. However, by pre-drying the sand that was previously supplied from the crushed stone plant to each asphalt mixture manufacturing plant in a state with a high moisture content, it is possible to increase added value and also reduce weight, which is expected to improve transportation efficiency. This means that the investment is likely to be well worth it. In addition, when considering the total CO2 emissions, including those emitted by each asphalt mixture manufacturing plant to which the sand is supplied, this can be seen as a reduction, and it is believed that this system can be adopted favorably.

[0048] In addition, the hot water produced when the water vapor contained in the exhaust gas from the first drum of the first sand dryer condenses during the heat exchange can be mixed with sludge water from ready-mixed concrete, which is generated in large quantities during mixer cleaning in ready-mixed concrete manufacturing plants, and the carbon dioxide contained in the hot water can be reacted with the calcium ions contained in the sludge water to recover calcium carbonate.

[0049] This allows the CO2 contained in the exhaust gas to be recovered as calcium carbonate, a useful substance that can be used, for example, as a material for asphalt mixtures, and simultaneously reduces the volume of sludge water that would otherwise need to be treated as industrial waste, making it particularly environmentally friendly. [Example]

[0050] An embodiment of the present invention will now be described with reference to the accompanying drawings.

[0051] In the figure, reference numeral 1 denotes a sand drying system according to the present invention, which is installed, for example, in the facilities of a crushed stone factory that produces and supplies various aggregates, which are the raw materials for asphalt mixtures, to asphalt mixture manufacturing plants, etc. The sand drying system 1 is mainly composed of a hot air generator 2 that generates hot air by self-combustion of wood fuel, a first sand dryer 3 and a second sand dryer 4 that dry the wet sand with a high moisture content produced at the crushed stone factory by contacting it with the hot air, and a heat exchanger 5 that exchanges heat between the exhaust gas discharged from the first sand dryer 3 and outside air to heat the outside air.

[0052] The hot air generating furnace 2 supports a cylindrical kiln body 6, the inner surface of which is lined with heat-storing castable refractories (not shown), at an angle so as to be freely rotatable. One end of the kiln body 6 (the upstream end as viewed from the direction of the hot air and wood fuel flow) is equipped with a natural gas-fueled auxiliary burner 7, a feeding hopper 8 into which wood fuel such as wood chips is fed, a screw feeder 9 (wood fuel supply means) that sequentially feeds the wood fuel discharged from the bottom of the feeding hopper 8 into the kiln body 6, and a combustion air supply fan 10 that supplies the combustion air needed to self-combust the wood fuel. Reference numeral 11 in the figure denotes an outside air intake port for the combustion air supply fan 10, which is equipped with an open / close damper 12.

[0053] The other end of the kiln body 6 (the downstream end as viewed from the direction of the hot air and wood fuel flow) is equipped with a secondary combustion chamber 13 that completely burns and decomposes the volatile matter (wood gas) that is produced as a result of the spontaneous combustion of the wood fuel.The secondary combustion chamber 13 is equipped with a small auxiliary burner 14 that uses natural gas as fuel to assist in the complete combustion of the volatile matter, and the lower part of the secondary combustion chamber 13 is equipped with a discharge hopper 15 that discharges the incineration ash of the wood fuel.

[0054] Also, near the hot air generator 2, there are provided a storage tank 16 for storing liquefied natural gas (LNG) at low temperature and high pressure, a vaporizer 17 for vaporizing the liquefied natural gas in the storage tank 16 to produce natural gas that is fuel for the auxiliary burner 7 and auxiliary burner 14, and a buffer tank 18 for temporarily storing the natural gas produced by vaporization in the vaporizer 17.

[0055] On the other hand, the first and second sand dryers 3, 4 are used exclusively to dry wet sand with a high moisture content. They have a first cylindrical drum 19 and a second cylindrical drum 20, each of which is rotatably supported at an angle and has a plurality of scraping blades (not shown) attached to its inner surface. When high-moisture sand is fed into the first drum 19 and the second drum 20 together with hot air, the high-moisture sand is heated and dried by being repeatedly scraped up by the scraping blades and coming into direct contact with the hot air. In this embodiment, a parallel-flow heating type dryer is used, in which the hot air and sand flow in the same direction.

[0056] One end of the first drum 19 of the first sand dryer 3 (the upstream end as viewed from the direction in which the hot air and sand flow) is connected to a hot air outlet duct 21 provided at the top of the secondary combustion chamber 13 of the hot air generating furnace 2, and is equipped with an input hopper 22 into which moist sand with a high moisture content (for example, about 15%) produced at the crushed stone plant is input, and a screw feeder 23 which sequentially supplies the high moisture content sand discharged from the lower end of the input hopper 22 into the first drum 19.

[0057] The other end of the first drum 19 of the first sand dryer 3 (the downstream end as viewed from the direction of the hot air and sand flowing down) is provided with an exhaust chamber 24, the upper part of which is connected to a first exhaust duct 25 for discharging exhaust gas, and the lower part is provided with a discharge hopper 26 for discharging dried sand, which is sand that has been subjected to a drying process. 27 in the figure is a first moisture sensor that continuously detects the moisture content of the dried sand discharged from the discharge hopper 26.

[0058] Along the first exhaust duct 25, from upstream to downstream as viewed in the direction of exhaust gas flow, are interposed a dust collector 28 such as a bag filter, the heat exchanger 5, a main damper 29 for adjusting the exhaust air volume, and an exhaust fan 30. A chimney 31 is provided at the end of the first exhaust duct 25. Reference numeral 32 in the figure denotes a blower that supplies outside air drawn in through an outside air intake 33 to the heat exchanger 5 and is equipped with an inverter to enable adjustment of the air volume. The outside air supplied by the blower 32 is heated by the heat exchanger 5 and then introduced into the second drum 20 of the second sand dryer 4 via a supply duct 34 located downstream.

[0059] The heat exchanger 5 indirectly exchanges heat between the high-temperature (e.g., about 120°C) and high-humidity (e.g., about 50%) exhaust gas discharged from the first drum 19 of the first sand dryer 3 and the ambient air at room temperature (e.g., about 15°C) supplied by the ambient air supply fan 32, and is configured to efficiently heat the ambient air by utilizing the latent heat of condensation generated when the large amount of water vapor contained in the exhaust gas condenses during this heat exchange. The latent heat of condensation of water vapor has thermal energy five times or more the sensible heat of water, making the heat exchange involving condensation as described above extremely efficient.

[0060] According to the inventor's calculations, if the amount of outside air supplied from the blower 32 is adjusted to about 10 times the amount of exhaust gas discharged from the first drum 19 of the first sand dryer 3, it is expected that the outside air discharged into the supply duct 34 after heat exchange in the heat exchanger 5 can be heated to about 70°C. However, this may seem considerably lower than the hot air temperature of about 850°C supplied from the hot air generator 2. However, even if the temperature rises by about 50-60°C, the relative humidity drops significantly (for example, if the humidity of outside air at room temperature is about 50%, the relative humidity will drop to about 3% as it is heated to about 70°C). Therefore, the second sand dryer 4, to which a large amount of this air (about 10 times) is supplied, can adequately dry the sand.

[0061] The temperature and amount of exhaust gas discharged from the first drum 19 of the first sand dryer 3 vary depending on the moisture content and supply amount of the sand supplied into the first drum 19. Therefore, if the temperature of the outside air supplied to the second drum 20 of the second sand dryer 4 is to be maintained constant (for example, about 70°C), it is necessary to appropriately adjust the amount of outside air supplied while taking into account the temperature of the outside air supplied to the heat exchanger 5. In this embodiment, a temperature sensor 35 is provided in the supply duct 34, and the amount of air blown from the blower 32 is adjusted (feedback controlled) so that the outside air temperature after heating detected by the temperature sensor 35 is maintained at a predetermined temperature (for example, about 70°C).

[0062] Furthermore, as described above, the second drum 20 of the second sand dryer 4 may have a larger capacity than the first drum 19 of the first sand dryer 3 so as to be able to introduce (accept) a large amount of warm air, or the second drum 20 may be made up of multiple drums (such as by arranging them side by side).

[0063] As described above, the heat exchanger 5 intentionally condenses the large amount of water vapor in the exhaust gas discharged after the sand drying process in the first sand dryer 3, and effectively utilizes the latent heat of condensation generated in this process to efficiently heat the outside air. Therefore, if the exhaust gas contains a large amount of substances that cause the generation of acidic gases, such as sulfur and chlorine, not only will it be costly to neutralize the large amount of acidic condensed water that is generated, but it will also be necessary to take measures to prevent acid corrosion of the heat exchanger 5, the first exhaust duct 25, the exhaust fan 30, etc.

[0064] Therefore, in the present invention, in the hot air generating furnace 2 upstream of the first sand dryer 3, wood fuel is allowed to spontaneously combust, and the volatile matter (wood gas) produced by this spontaneous combustion is completely combusted and decomposed in the secondary combustion chamber 13 before being supplied to the first sand dryer 3 as hot air.In addition, gas burners that use natural gas (mainly methane) as fuel are used for the auxiliary burner 7 that assists the spontaneous combustion of the wood fuel and the auxiliary burner 14 that assists the complete combustion of the volatile matter (wood gas).

[0065] Although the wood gas contains some acidic components, it can be completely combusted and decomposed in the secondary combustion chamber 13 in an atmosphere of about 850°C while flowing downward for about 1 to 2 seconds. Furthermore, since natural gas fuel does not originally contain substances that cause acidic gases, such as sulfur and chlorine, the occurrence of the above-mentioned problems can be suppressed even when these exhaust gases are condensed. Furthermore, wood fuels such as wood chips can be considered to have zero CO2 emissions from a carbon-neutral perspective, and the natural gas used in the auxiliary burner 7 and auxiliary burner 14 has the lowest CO2 emission coefficient of any fossil fuel, and can reduce CO2 emissions by about 25% compared to a burner that uses heavy oil A, for example, making it suitable for use in this system.

[0066] Also, 36 in the figure is a first branch duct that connects the supply duct 34 and the first exhaust duct 25, and allows a portion of the outside air (warm air) heated by the heat exchanger 5 to be introduced into the first exhaust duct 25 downstream of the heat exchanger 5 depending on the opening and closing operation of an opening / closing damper 37 interposed midway through the first branch duct 36.

[0067] In some cases, the exhaust gas temperature after heat exchange may fall below the dew point temperature, which may cause problems such as condensation in the first exhaust duct 25 downstream of the heat exchanger 5 or in the exhaust fan 30, or white smoke (water vapor in the exhaust gas condensed into an aerosol) emitting from the chimney 31 at the end of the first exhaust duct 25. However, by providing the first branch duct 36, it is possible to mix some of the outside air (warm air) that has been heated and temperature-raised by heat exchange and has a lowered relative humidity into the exhaust gas in the first exhaust duct 25, and as a result, the exhaust gas temperature can be raised above the dew point, and this simple configuration makes it possible to suppress the above problems.

[0068] The opening and closing operation of the opening / closing damper 37 may be performed, for example, by providing a separate temperature sensor in the first exhaust duct 25 downstream of the heat exchanger 5, and when the temperature sensor detects a predetermined temperature (for example, dew point temperature) or lower, the opening / closing damper 37 is opened to mix in some of the heated outside air (warm air). In some cases, however, a constant amount may be mixed in at all times while the system is operating.

[0069] Also, 38 in the figure is a second branch duct that connects the supply duct 34 and the combustion air supply fan 10 of the hot air generator 2, and makes it possible to supply a portion of the outside air (warm air) heated in the heat exchanger 5 to the combustion air supply fan 10 depending on the opening and closing operation of an opening and closing damper 39 interposed in the middle of the second branch duct 38.

[0070] By providing the second branch duct 38, it is possible to preheat the combustion air to be supplied into the hot air generator 2 (preheated outside air can be supplied as combustion air), and as a result, the combustion efficiency of the self-combustion wood fuel can be improved, which reduces the fuel consumption of the auxiliary burner 7 and is expected to result in further energy savings and CO2 reduction effects. Note that when the hot air generator 2 is started up (when operation begins), etc., while it is not possible to supply a sufficient amount of preheated outside air from the heat exchanger 5, it is advisable to open the open / close damper 12 provided on the outside air intake 11 of the combustion air supply fan 10 and supply outside air at room temperature drawn in through the outside air intake 11 into the hot air generator 2.

[0071] In addition, one end of the second drum 20 of the second sand dryer 4 (the upstream end as viewed from the direction of the flow of the outside air (warm air) and sand) to which the supply duct 34 is connected is equipped, similar to the first drum 19 of the first sand dryer 3, with an input hopper 40 for inputting moist sand with a high moisture content (for example, about 15%) produced at the crushing plant, and a screw feeder 41 for sequentially supplying the high moisture content sand discharged from the bottom end of the input hopper 40 into the second drum 20.

[0072] The second drum 20 of the second sand dryer 4 is provided with an exhaust chamber 42 at the other end (the downstream end as viewed from the direction of the flow of the outside air (hot air) and sand), and a second exhaust duct 43 for exhaust gas discharge is connected to the upper part of the exhaust chamber 42, while a discharge hopper 44 for discharging dried sand is provided at the lower part. 45 in the figure is a second moisture sensor that continuously detects the moisture content of the dried sand discharged from the discharge hopper 44.

[0073] As in the case of the first sand dryer 3, the second exhaust duct 43 is also provided with, in order from the upstream side when viewed from the downstream direction of the exhaust gas, a dust collector 46 such as a bag filter, a main damper 47 for adjusting the exhaust air volume, and an exhaust fan 48, and a chimney 49 is provided at the end of the second exhaust duct 43.

[0074] In the figure, reference numeral 50 denotes a hot water supply pipe that supplies condensed water (hot water) produced by the condensation of water vapor contained in the exhaust gas from the first drum 19 of the first sand dryer 3 in the heat exchanger 5 to the vaporizer 17 for liquefied natural gas vaporization as a heat source, with a supply pump 51 interposed along the hot water supply pipe. According to the inventor's calculations, the condensed water can be recovered as hot water of approximately 40°C. This allows it to be fully utilized as a heat source for the vaporizer 17 without the need for a separate heat source such as a boiler for the vaporization process, which is advantageous in terms of cost and maintenance. As mentioned above, the condensed water that has cooled after being used as a heat source in the vaporizer 17 is no longer acidic, and therefore does not require any special neutralization treatment and can be directly discharged to the outside through a drainage pipe 52.

[0075] In the figure, 53 is a first controller that adjusts and controls at least one of the combustion amount of the auxiliary burner 7 of the hot air generator 2, the amount of wood fuel supplied into the kiln body 6 of the hot air generator 2 (e.g., the supply speed of the screw feeder 9), the amount of sand supplied into the first drum 19 of the first sand dryer 3 (e.g., the supply speed of the screw feeder 23), and the rotation speed of the first drum 19 of the first sand dryer 3, so that the moisture content of the dried sand discharged from the first drum 19 of the first sand dryer 3, as detected by the first moisture sensor 27, is maintained at a predetermined moisture content excluding bone dryness.

[0076] For example, if the moisture content of the dried sand is higher than a predetermined moisture content (e.g., about 5%), the system adjusts and controls the following: increase the combustion rate of the auxiliary burner 7; increase the amount of wood fuel supplied to the kiln body 6 (increase the supply speed of the screw feeder 9); decrease the amount of sand supplied to the first drum 19 (decrease the supply speed of the screw feeder 23); increase the rotational speed of the first drum 19; or perform a combination of these. Increasing the rotational speed of the first drum 19 increases the amount of sand scraped up per unit time in the first drum 19, thereby increasing the chance of contact with the hot air from the hot air generator 2 and reducing the moisture content of the sand.

[0077] If the dried sand discharged from the first drum 19 of the first sand dryer 3 is to be heated and dried until it is bone dry, suitable for use as a raw material for asphalt mixtures, the temperature of the sand must inevitably be high. However, if this hot sand is to be transported from the crushing plant to a remote asphalt mixture manufacturing plant or the like, heat will dissipate during the transport, causing the temperature to drop, which simply results in energy loss.

[0078] On the other hand, by intentionally leaving a small amount of moisture in the sand before it is discharged from the first drum 19, it is possible to gradually evaporate some of the residual moisture by utilizing sensible heat during storage or transportation of the sand (effective use of thermal energy), thereby reducing energy loss due to simple heat dissipation. Also, since it is not necessary to dry the sand until it is bone dry, fuel consumption in the hot air generator 2 is reduced, and CO2 emissions can also be expected to be reduced.

[0079] Furthermore, with the above method, it is expected that sand at room temperature with a small amount of moisture remaining (even after drying using sensible heat) will ultimately be recovered (for example, according to the inventor's calculations, sand discharged from the first drum 19 at a temperature of approximately 70°C and a moisture content of approximately 5% will still have moisture of approximately 3% even when it returns to room temperature of approximately 15°C, although some evaporation occurs due to sensible heat).

[0080] However, at asphalt plants in asphalt mix manufacturing plants, when asphalt mix is ​​produced, sand supplied from crushed stone factories is heated to around 170°C in a dryer, and is dried to bone dry at the same time, so there is no particular problem. Furthermore, because the moisture content of the sand is significantly reduced compared to normal (before drying) conditions (for example, moisture content drops from about 15% to about 3%), energy savings and CO2 reduction effects can be expected. Furthermore, having some moisture remaining in the sand can be expected to have a dust-proofing effect, and because it is at room temperature, no special measures are required to keep it warm during storage or transportation, making it easier to handle.

[0081] Also, reference numeral 54 in the figure denotes a second controller that adjusts and controls at least one of the amount of outside air blown from the blower 32, the amount of sand supplied into the second drum 20 of the second sand dryer 4 (for example, the supply speed of the screw feeder 41), and the rotation speed of the second drum 20 of the second sand dryer 4 so that the moisture content of the dried sand discharged from the second drum 20 of the second sand dryer 4, as detected by the second moisture sensor 45, is maintained at a predetermined moisture content excluding bone dryness.

[0082] For example, if the moisture content of the dried sand is higher than a predetermined moisture content (e.g., about 5%), the system adjusts and controls the amount of outside air blown from the blower 32 to increase, the amount of sand fed into the second drum 20 to decrease (the feed speed of the screw feeder 41 to decrease), the rotation speed of the second drum 20 to increase, or a combination of these. Note that increasing the amount of outside air blown from the blower 32 increases the amount of low-humidity (low-relative-humidity) warm air introduced into the second drum 20 of the second sand dryer 4, improving drying efficiency and enabling the moisture content of the sand to be reduced.

[0083] As with the first sand dryer 3, the second sand dryer 4 intentionally leaves some moisture in the sand before discharging it from the second drum 20. This allows sensible heat to be used to gradually evaporate some of the residual moisture during storage or transportation of the sand (making it possible to make effective use of thermal energy), thereby reducing energy loss due to simple heat dissipation.

[0084] When the sand drying system 1 is used to dry sand, wet sand with a high moisture content produced at a crushed stone plant is first supplied to the first drum 19 of the first sand dryer 3 and the second drum 20 of the second sand dryer 4, both of which are installed within the crushed stone plant. In the first sand dryer 3, hot air generated by spontaneous combustion of wood fuel in the hot air generator 2 located upstream is introduced into the first drum 19 to dry the sand. In the second sand dryer 4, high-temperature, high-humidity exhaust gas discharged from the first sand dryer 3 is heat-exchanged with ambient temperature outside air supplied by the blower 32 in the heat exchanger 5, and the outside air is heated using the latent heat of condensation generated when the water vapor contained in the exhaust gas condenses. The heated outside air (hot air) is then introduced into the second drum 20 to dry the sand.

[0085] At this time, in the first sand dryer 3, the first controller 53 adjusts and controls at least one of the combustion amount of the auxiliary burner 7 of the hot air generator 2, the amount of wood fuel supplied into the kiln body 6 of the hot air generator 2, the amount of sand supplied into the first drum 19 of the first sand dryer 3, and the rotation speed of the first drum 19 of the first sand dryer 3, so that the moisture content of the dried sand detected by the first moisture sensor 27 is maintained at a predetermined moisture content excluding bone dryness.

[0086] Meanwhile, in the second sand dryer 4, the second controller 54 adjusts and controls at least one of the amount of outside air blown from the blower 32, the amount of sand supplied into the second drum 20 of the second sand dryer 4, and the rotation speed of the second drum 20 of the second sand dryer 4 so that the moisture content of the dried sand detected by the second moisture sensor 45 is maintained at a predetermined moisture content other than bone dry.

[0087] The dried sand discharged from the first drum 19 of the first sand dryer 3 and the second drum 20 of the second sand dryer 4, with the moisture content maintained at a predetermined level, is then transported to its destination, such as an asphalt mixture manufacturing plant, as appropriate. During transportation or storage, the sensible heat of the dried sand gradually evaporates much of the moisture remaining in the sand.

[0088] When the dried sand is actually used as a raw material for asphalt mixtures in the asphalt plant of an asphalt mixture manufacturing factory, it will still have a small amount of residual moisture (it will be wet). In the asphalt plant's dryer, the sand can be heated to a specified temperature (approximately 170°C) using a relatively small amount of fuel (while reducing CO2 emissions) without consuming much fuel (thermal energy) to dry the sand, and then mixed with various other materials to produce the desired asphalt mixture.

[0089] In this way, with the sand drying system 1, the amount of fuel (thermal energy) required to dry sand with one sand dryer can be used to dry the amount of sand used by two (or more) sand dryers.According to simulation calculations conducted by the inventors, it is possible to dry 50 tons of sand per hour with the same amount of fuel required to dry 30 tons of sand with a moisture content of 15% to a moisture content of 5%, which is expected to result in an energy saving effect of approximately 65%.

[0090] In addition to the above-mentioned device configuration, wood fuel such as wood chips, which can be considered to have zero CO2 emissions from a carbon-neutral perspective, is self-combusted as fuel for the hot air generator 2 that supplies hot air to the first sand dryer 3, and natural gas, which has a low CO2 emission coefficient, is also used as fuel for the auxiliary burner 7 that assists the self-combustion of the wood fuel.As a result, according to the simulation calculations, a CO2 reduction effect of approximately 75% can be expected.

[0091] The above simulation is based on the premise that a large amount of wet sand is continuously dried, but it is believed that the above preconditions are fully met in a crushing plant that produces and ships large amounts of sand (aggregate) to multiple asphalt mixture manufacturing plants, etc.

[0092] Although not shown in the figure, in a ready-mixed concrete manufacturing plant or the like, a large amount of sludge water from ready-mixed concrete is generated when washing concrete mixers and agitator vehicles, and the like. This sludge water is appropriately transported into the facilities of a crushed stone plant where the sand drying system 1 is installed, and mixed with the hot water (condensed water) generated in the heat exchanger 5, and the carbon dioxide contained in the hot water (a large amount of CO2 generated by the combustion of natural gas is dissolved as carbon dioxide (H2CO3)) and calcium ions (Ca 2+ The sludge water may be directly showered (sprayed) onto the exhaust gas flowing down through the first exhaust duct 25 downstream of the heat exchanger 5, and the CO2 in the exhaust gas may be recovered as calcium carbonate.

[0093] In either case, the CO2 contained in the exhaust gas can be recovered as calcium carbonate, which can be used as a material for asphalt mixtures (stone powder, filler), and the volume of sludge water can be reduced at the same time, making it even more environmentally friendly. [Industrial Applicability]

[0094] The present invention can be widely used in the drying process of wet sand with a high moisture content produced in a crushing plant. [Explanation of symbols]

[0095] 1. Sand drying system 2. Hot air generator 3...First sand dryer 4...Second sand dryer 5...Heat exchanger 6...Kiln body 7...Auxiliary combustion burner 9...Screw feeder (wood fuel supply means) 10... Combustion air supply fan 13... Secondary combustion chamber 16...Storage tank (liquefied natural gas) 17...Vaporizer 19...First drum 20...Second drum 21...Hot air outlet duct 25...First exhaust duct 27...First moisture sensor 32...Blower 34...supply duct 36...first branch duct 38...Second branch duct 43...Second exhaust duct 45...Second moisture sensor 50...Hot water supply pipe 53...First controller 54...Second controller

Claims

1. A hot air generating furnace that generates hot air by spontaneously combusting wood fuel, and first and second sand dryers that dry moist sand by contacting the hot air, are installed in parallel in a crushed stone plant. The hot air generating furnace comprises a kiln body having a natural gas-fueled auxiliary burner, wood fuel supply means, and a combustion air supply fan that supplies the combustion air necessary to spontaneously combust the wood fuel, and a secondary combustion chamber that combusts and decomposes volatile matter generated by the spontaneous combustion of the wood fuel in the kiln body. One end of the first drum of the first sand dryer is connected to a hot air outlet duct provided in the secondary combustion chamber of the hot air generating furnace. On the other hand, an exhaust duct is provided at the other end, and a heat exchanger is provided in the middle of the exhaust duct to exchange heat between the exhaust gas discharged from the first drum of the first sand dryer and the outside air supplied from a blower for supplying outside air, and to heat the outside air by utilizing the latent heat of condensation when the water vapor contained in the exhaust gas condenses.A sand drying system characterized by having a supply duct to introduce the outside air heated by the heat exchanger into the second drum of the second sand dryer, and a first branch duct to introduce a portion of the outside air heated by the heat exchanger into the exhaust duct of the first sand dryer downstream of the heat exchanger.

2. The sand drying system described in claim 1, characterized in that it is equipped with a first moisture sensor that detects the moisture content of the dried sand discharged from the first drum of the first sand dryer, and a first controller that adjusts and controls at least one of the combustion amount of the auxiliary combustion burner of the hot air generator furnace, the amount of wood fuel supplied into the kiln body of the hot air generator furnace, the amount of sand supplied into the first drum of the first sand dryer, and the rotation speed of the first drum of the first sand dryer so that the moisture content of the dried sand detected by the first moisture sensor is maintained at a predetermined moisture content.

3. 3. The sand drying system of claim 1, further comprising a second moisture sensor for detecting the moisture content of the dried sand discharged from the second drum of the second sand dryer, and a second controller for adjusting and controlling at least one of the amount of outside air blown from the blower, the amount of sand supplied into the second drum of the second sand dryer, and the rotation speed of the second drum of the second sand dryer so that the moisture content of the dried sand detected by the second moisture sensor is maintained at a predetermined moisture content.

4. A sand drying system as described in any one of claims 1 to 3, characterized in that it is provided with a second branch duct that supplies a portion of the outside air heated by the heat exchanger to the combustion air supply fan of the hot air generating furnace.

5. A sand drying system as described in any one of claims 1 to 4, characterized in that a storage tank for storing liquefied natural gas at low temperature and high pressure and a vaporizer are provided near the hot air generating furnace, and a hot water supply pipe is provided to supply hot water produced as water vapor contained in the exhaust gas from the first drum of the first sand dryer condenses in the heat exchanger to the vaporizer as a heat source.

6. a hot air generator that generates hot air by spontaneously combusting wood fuel, and first and second sand dryers that dry moist sand by contacting the hot air, which are installed in parallel at a crushing plant; the first sand dryer dries the sand by introducing hot air from the hot air generator into a first drum of the first sand dryer; the second sand dryer exchanges heat between exhaust gas discharged from the first drum of the first sand dryer and outside air, heating the outside air using the latent heat of condensation when water vapor contained in the exhaust gas condenses, and introducing this heated outside air into the second drum of the second sand dryer to dry the sand; and mixing warm water generated as a result of the condensation of water vapor contained in the exhaust gas from the first drum of the first sand dryer during the heat exchange with sludge water from ready-mixed concrete, allowing carbon dioxide contained in the warm water to react with calcium ions contained in the sludge water, resulting in recovery of calcium carbonate.

7. 7. The method for drying sand according to claim 6, wherein the first and second sand dryers dry the sand so as to maintain a predetermined moisture content.

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