Sand drying system and sand drying method

Abstract

To provide a system and a method of drying sand, relatively easily employable and capable of efficiently drying sand.SOLUTION: The system of drying sand, comprises a first and a second sand dryers 2, 3 drying wet sand juxtaposed in a crushing plant, a gas burner 7 for feeding hot air using natural gas as fuel, provided on one end of a first drum 5 of the first dryer 2, a first exhaust duct 14 provided on the other end thereof, a heat exchanger 4 interposed midway in the first exhaust duct 14 to exchange heat between exhaust coming out of the first drum 5 of the first sand dryer 2 and outdoor air supplied from a fan 21 supplying outdoor air to heat outdoor air using the condensation latent heat of steam contained in the exhaust upon its condensation, and a feed duct 22 that introduces outdoor air heated by the heat exchanger 4 into a second drum 6 of the second sand dryer 3.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, first and second sand dryers for drying wet sand by contacting it with hot air are installed in parallel in a crushing plant, a gas burner for supplying hot air using natural gas as fuel is provided at one end of the first drum of the first sand dryer, and an exhaust duct is provided at the other end, and a heat exchanger is provided in the middle of the exhaust duct for heat exchange between exhaust gas discharged from the first drum of the first sand dryer and outside air supplied from an outside air supply fan, and the outside air is heated by utilizing the latent heat of condensation when water vapor contained in the exhaust gas condenses, and a supply duct is provided for introducing the outside air heated by the heat exchanger into the second drum of the second sand dryer. a 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 comprising 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 gas burner, 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 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 first sand dryer, 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.

[0014] Also, claims 5In the sand drying method described, first and second sand dryers that dry wet sand by contacting it with hot air are installed in parallel in a crushing plant, and in the first sand dryer, hot air supplied from a gas burner fueled by natural gas is introduced into the first drum of the first sand dryer to dry the sand, while 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 generated when water vapor contained in the exhaust gas condenses, and this heated outside air is 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.

[0015] Also, claims 6 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]

[0017] 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, only one dryer is equipped with a burner, and the other dryer is supplied with heated outside air using 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, making 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.

[0018] In addition, according to the sand drying system described in claim 2, the dried sand discharged from the first sand dryer is not dried completely, but is adjusted so that it maintains a predetermined moisture content. This makes it possible to reduce the fuel consumption of the burner while evaporating some of the residual moisture in the discharged sand 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.

[0019] 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.

[0021] Also, claims 4 According to the sand drying system described, the liquefied natural gas used as the burner fuel can be vaporized without the need for a separate heat source such as a boiler for vaporization, making it even easier to adopt.

[0022] Also, claims 5 According to the described sand drying method, 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, only one dryer is equipped with a burner, and the other dryer is supplied with heated outside air using the latent heat of condensation when the water vapor in the exhaust gas condenses.This means that the large amount of sand produced at the crushed stone plant can be dried efficiently, making 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.

[0023] Also, claims 6 According 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]

[0025] [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

[0026] In the sand drying system and sand drying method of the present invention, for example, first and second sand dryers are installed side by side within 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., and dry the wet sand with a high moisture content produced at the crushed stone factory by contacting it with hot air.

[0027] 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.

[0028] One end of the first drum of the first sand dryer is equipped with a gas burner fueled by natural gas for supplying hot air, and 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 air blower, and to heat the ambient air by utilizing 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.

[0029] 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.

[0030] Here, although the temperature of the outside air heated through the heat exchanger is not as high as the hot air supplied from the burner of the first sand dryer, the relative humidity drops significantly as the temperature rises, so that the second sand dryer to which this air is supplied can sufficiently dry the sand.

[0031] Furthermore, by using a gas burner fueled by natural gas, which has a low CO2 emission coefficient, for the burner provided in the first sand dryer, it is possible to reduce CO2 emissions compared to burners fueled by, for example, heavy oil A. Furthermore, because natural gas does not inherently contain substances that cause the generation of acidic gases, such as sulfur or chlorine, the large amount of condensed water produced when the water vapor in the exhaust gas 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.

[0032] 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 gas burner, 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.

[0033] 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.

[0034] 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 remaining 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 fuel consumption of the burner used in the sand drying process can be reduced, and CO2 emissions can also be expected to be reduced.

[0035] 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.

[0036] 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.

[0037] Furthermore, it is preferable to provide a 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 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.

[0038] 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 first sand dryer, 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 burner fuel without providing a separate heat source such as a boiler for the vaporization process, making it easy to adopt from the standpoints of cost, maintenance, etc.

[0039] 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 is introduced into the first drum from a gas burner fueled by natural gas to dry the sand, while 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, heating the ambient air 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.

[0040] 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.

[0041] Thus, with the above-described sand drying system and method, by installing a sand dryer at the crushed stone plant, costs and management burdens can be reduced compared to installing one at each asphalt mixture manufacturing plant to which sand is supplied. Furthermore, even at the crushed stone plant, only one of a pair of sand dryers installed on the premises is equipped with a burner, 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 efficiently dry the large amounts of sand produced for each asphalt mixture manufacturing plant while saving energy and reducing CO2 emissions, making it possible to build a sand drying system that is relatively easy to adopt.

[0042] 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.

[0043] 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.

[0044] 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]

[0045] An embodiment of the present invention will now be described with reference to the accompanying drawings.

[0046] In the figure, reference numeral 1 denotes a sand drying system according to the present invention, which is installed, for example, within the facilities of a crushed stone plant 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 first sand dryer 2 and a second sand dryer 3 that dry the wet sand with a high moisture content produced at the crushed stone plant by contacting it with hot air, and a heat exchanger 4 that exchanges heat between the exhaust gas discharged from the first sand dryer 2 and outside air to heat the outside air.

[0047] The first and second sand dryers 2, 3 are primarily used to dry wet sand with a high moisture content. They consist of a first cylindrical drum 5 and a second cylindrical drum 6, each of which is rotatably supported at an angle and fitted with a number of scraping blades (not shown) on its inner periphery. When high-moisture sand is fed into the first drum 5 and the second drum 6 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.

[0048] One end of the first drum 5 of the first sand dryer 2 (the upstream end as viewed from the direction of the hot air and sand flowing downstream) is equipped with a gas burner 7 for supplying hot air fueled by natural gas, as well as an input hopper 8 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 9 which sequentially supplies the high moisture content sand discharged from the bottom end of the input hopper 8 into the first drum 5.

[0049] Near the first sand dryer 2, there are a storage tank 10 for storing liquefied natural gas (LNG) at low temperature and high pressure, a vaporizer 11 for vaporizing the liquefied natural gas in the storage tank 10 to produce natural gas that is fuel for the gas burner 7, and a buffer tank 12 for temporarily storing the natural gas produced by vaporization in the vaporizer 11.

[0050] The other end of the first drum 5 of the first sand dryer 2 (the downstream end as viewed from the direction of the hot air and sand flowing down) is provided with an exhaust chamber 13, the upper part of which is connected to a first exhaust duct 14 for discharging exhaust gas, and the lower part is provided with a discharge hopper 15 for discharging dried sand, which is sand that has undergone a drying process. In the figure, 16 is a first moisture sensor that continuously detects the moisture content of the dried sand discharged from the discharge hopper 15.

[0051] Along the first exhaust duct 14, from upstream to downstream as viewed in the direction of exhaust gas flow, are interposed a dust collector 17 such as a bag filter, the heat exchanger 4, a main damper 18 for adjusting the exhaust air volume, and an exhaust fan 19, and a chimney 20 is provided at the end of the first exhaust duct 14. Reference numeral 21 in the figure denotes a blower that supplies outside air to the heat exchanger 4 and is equipped with an inverter so that the blowing volume can be adjusted. The outside air supplied by the blower 21 is heated by the heat exchanger 4 and then introduced into the second drum 6 of the second sand dryer 3 via a supply duct 22 located downstream.

[0052] The heat exchanger 4 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 5 of the first sand dryer 2 and the ambient air at room temperature (e.g., about 15°C) supplied by the ambient air supply fan 21, 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.

[0053] According to the inventor's calculations, if the amount of outside air supplied by the blower 21 is adjusted to about 10 times the amount of exhaust gas discharged from the first drum 5 of the first sand dryer 2, it is expected that the outside air discharged into the supply duct 22 after heat exchange in the heat exchanger 4 can be heated to about 70°C. However, this may seem considerably lower than the temperature of the hot air supplied from the gas burner 7, which is about 1,800°C. 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 3, to which a large amount of this air (about 10 times) is supplied, can adequately dry the sand.

[0054] The temperature and amount of exhaust gas discharged from the first drum 5 of the first dryer 2 vary depending on the moisture content and supply amount of the sand supplied into the first drum 5, and so if it is desired to maintain a constant temperature of the outside air supplied to the second drum 6 of the second dryer 3 (for example, about 70°C), it is necessary to appropriately adjust the amount of outside air supplied while taking into consideration the temperature of the outside air supplied to the heat exchanger 4. In this embodiment, a temperature sensor 23 is provided in the supply duct 22, and the amount of air blown from the blower 21 is adjusted (feedback controlled) so that the outside air temperature after heating detected by the temperature sensor 23 is maintained at a predetermined temperature (for example, about 70°C).

[0055] Furthermore, as described above, the second drum 6 of the second sand dryer 3 may have a larger capacity than the first drum 5 of the first sand dryer 2 so as to be able to introduce (accept) a large amount of warm air, or the second drum 6 may be made up of multiple drums (such as by arranging them side by side).

[0056] As described above, the heat exchanger 4 intentionally condenses the large amount of water vapor in the exhaust gas discharged after the sand drying process in the first sand dryer 2, 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 4, exhaust duct 14, exhaust fan 19, etc.

[0057] Therefore, in the present invention, a gas burner fueled by natural gas (mainly methane) is used for the burner installed for drying the sand in the first sand dryer 2. Natural gas fuel does not inherently contain substances that cause acid gas generation, such as sulfur and chlorine, so the occurrence of the above-mentioned problems can be suppressed even when the exhaust gas is condensed. In addition, natural gas has the smallest CO2 emission coefficient of all fossil fuels, and can reduce CO2 emissions by approximately 25% compared to a burner fueled by, for example, heavy oil A, making it suitable for use in this system.

[0058] Also, 24 in the figure is a branch duct that connects the supply duct 22 and the first exhaust duct 14, and allows a portion of the outside air (warm air) heated by the heat exchanger 4 to be introduced into the exhaust duct 14 downstream of the heat exchanger 4 depending on the opening and closing operation of an opening / closing damper 25 interposed in the middle of the branch duct 24.

[0059] 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 14 downstream of the heat exchanger 4 or in the exhaust fan 19, or white smoke (water vapor in the exhaust gas condensed into an aerosol) emitting from the chimney 20 at the end of the first exhaust duct 14. However, by providing the branch duct 24, 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 14, 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.

[0060] The opening and closing operation of the opening / closing damper 25 can be performed, for example, by providing a separate temperature sensor in the first exhaust duct 14 downstream of the heat exchanger 4, and when the temperature sensor detects a predetermined temperature (for example, dew point temperature) or lower, the opening / closing damper 25 is opened to mix in some of the heated outside air (warm air). In some cases, however, a constant amount of air may be mixed in at all times while the system is operating.

[0061] In addition, one end of the second drum 6 of the second sand dryer 3 (the upstream end as viewed from the direction of the flow of the outside air (warm air) and sand) to which the supply duct 22 is connected is equipped, similar to the first drum 5 of the first sand dryer 2, with an input hopper 26 for inputting moist sand with a high moisture content (for example, about 15%) produced at the crushing plant, and a screw feeder 27 for sequentially supplying the high moisture content sand discharged from the lower end of the input hopper 26 into the second drum 6.

[0062] The second drum 6 of the second sand dryer 3 is provided with an exhaust chamber 28 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 29 for exhaust gas discharge is connected to the upper part of the exhaust chamber 28, while a discharge hopper 30 for discharging dried sand is provided at the lower part. 31 in the figure is a second moisture sensor that continuously detects the moisture content of the dried sand discharged from the discharge hopper 30.

[0063] As in the case of the first sand dryer 2, the second exhaust duct 29 is also provided with, in order from the upstream side when viewed from the downstream direction of the exhaust gas, a dust collector 32 such as a bag filter, a main damper 33 for adjusting the exhaust air volume, and an exhaust fan 34, and a chimney 35 is provided at the end of the second exhaust duct 29.

[0064] In the figure, reference numeral 36 denotes a hot water supply pipe that supplies condensed water, generated by the condensation of water vapor contained in the exhaust gas from the first drum 5 of the first sand dryer 2 in the heat exchanger 4, to the vaporizer 11 for liquefied natural gas vaporization as a heat source, with a supply pump 37 interposed along the 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 11 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 11 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 38.

[0065] In the figure, 39 is a first controller that adjusts and controls at least one of the combustion amount of the gas burner 7, the amount of sand supplied into the first drum 5 of the first sand dryer 2 (for example, the supply speed of the screw feeder 9), and the rotation speed of the first drum 5 of the first sand dryer 2 so that the moisture content of the dried sand discharged from the first drum 5 of the first sand dryer 2, as detected by the first moisture sensor 16, is maintained at a predetermined moisture content other than bone dry.

[0066] 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 by increasing the combustion rate of the gas burner 7, decreasing the amount of sand fed into the first drum 5 (by slowing down the feeding speed of the screw feeder 9), increasing the rotational speed of the first drum 5, or a combination of these. Note that increasing the rotational speed of the first drum 5 increases the amount of sand scraped up per unit time in the first drum 5, thereby increasing the chance of contact with the hot air from the gas burner 7 and making it possible to reduce the moisture content of the sand.

[0067] If the dried sand discharged from the first drum 5 of the first sand dryer 2 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.

[0068] On the other hand, by intentionally leaving a small amount of moisture in the sand before it is discharged from the first drum 5, 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, the fuel consumption of the gas burner 7 used in the sand drying process can be reduced, and CO2 emissions can also be reduced.

[0069] 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 5 at a temperature of approximately 70°C and with 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).

[0070] 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.

[0071] Also, reference numeral 40 in the figure denotes a second controller that adjusts and controls at least one of the amount of outside air blown from the blower 21, the amount of sand supplied into the second drum 6 of the second sand dryer 3 (for example, the supply speed of the screw feeder 27), and the rotation speed of the second drum 6 of the second sand dryer 3 so that the moisture content of the dried sand discharged from the second drum 6 of the second sand dryer 3, as detected by the second moisture sensor 31, is maintained at a predetermined moisture content excluding bone dryness.

[0072] 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 21 to increase, the amount of sand fed into the second drum 6 to decrease (the feed speed of the screw feeder 27 to decrease), the rotation speed of the second drum 6 to increase, or a combination of these. Note that increasing the amount of outside air blown from the blower 21 increases the amount of low-humidity (low-relative-humidity) warm air introduced into the second drum 6 of the second sand dryer 3, improving drying efficiency and enabling the moisture content of the sand to be reduced.

[0073] As with the first sand dryer 2, the second sand dryer 3 intentionally leaves some moisture in the sand before discharging it from the second drum 6. 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.

[0074] 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 5 of the first sand dryer 2 and the second drum 6 of the second sand dryer 3, both of which are installed within the crushed stone plant. In the first sand dryer 2, hot air is introduced into the first drum 5 from a gas burner 7 fueled by natural gas to dry the sand. In the second sand dryer 3, heat is exchanged in the heat exchanger 4 between the high-temperature, high-humidity exhaust gas discharged from the first sand dryer 2 and the ambient air at room temperature supplied by a blower 21. The ambient air is heated using the latent heat of condensation generated when the water vapor contained in the exhaust gas condenses, and this heated ambient air (hot air) is then introduced into the second drum 6 to dry the sand.

[0075] At this time, in the first sand dryer 2, the sand drying process is carried out while the first controller 39 adjusts and controls at least one of the combustion amount of the gas burner 7, the amount of sand supplied into the first drum 5 of the first sand dryer 2, and the rotation speed of the first drum 5 of the first sand dryer 2 so that the moisture content of the dried sand detected by the first moisture sensor 16 is maintained at a predetermined moisture content excluding bone dryness.

[0076] Meanwhile, in the second sand dryer 3, the second controller 40 adjusts and controls at least one of the amount of outside air blown from the blower 21, the amount of sand supplied into the second drum 6 of the second sand dryer 3, and the rotation speed of the second drum 6 of the second sand dryer 3, so that the moisture content of the dried sand detected by the second moisture sensor 31 is maintained at a predetermined moisture content other than bone dry.

[0077] The dried sand discharged from the first drum 5 of the first sand dryer 2 and the second drum 6 of the second sand dryer 3, 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 causes much of the moisture remaining in the sand to gradually evaporate.

[0078] 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.

[0079] Thus, with the sand drying system 1, the burner fuel (thermal energy) required for one sand dryer can be used to dry the amount of sand required for two (or more) sand dryers. According to simulation calculations conducted by the inventors, the fuel required to dry 30 tons of sand with a moisture content of 15% per hour to a moisture content of 5% can be used to dry 50 tons of sand, which is expected to result in energy savings and CO2 reductions of approximately 65%. Note that 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 prerequisites will be fully met in a crushing plant that produces and ships large amounts of sand (aggregate) to multiple asphalt mixture manufacturing plants, etc.

[0080] 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 a concrete mixer or an agitator vehicle is washed, and the sludge water is appropriately transported into the facility 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 4, 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 the calcium ions (Ca 2+ The sludge water may be directly showered (sprayed) onto the exhaust gas flowing down through the first exhaust duct 14 downstream of the heat exchanger 4, and the CO2 in the exhaust gas may be recovered as calcium carbonate.

[0081] 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]

[0082] 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]

[0083] 1...Sand drying system 2...First sand dryer 3...Second sand dryer 4...Heat exchanger 5...First drum 6...Second drum 7...Gas burner 10...Storage tank (liquefied natural gas) 11... carburetor 14... first exhaust duct 16...First moisture sensor 21...Blower 22...Supply duct 24...Branch duct 29... Second exhaust duct 31... Second moisture sensor 36...Hot water supply pipe 39...First controller 40...Second controller

Claims

1. A sand drying system comprising: a first sand dryer and a second sand dryer, which dry wet sand by contacting it with hot air, installed side by side in a crushed stone plant; a gas burner for supplying hot air fueled by natural gas at one end of the first drum of the first sand dryer; an exhaust duct at the other end; a heat exchanger located midway through the exhaust duct, which exchanges heat between exhaust gas discharged from the first drum of the first sand dryer and outside air supplied by a blower for supplying outside air, and which heats the outside air by utilizing the latent heat of condensation when water vapor contained in the exhaust gas condenses; a supply duct for introducing the outside air heated by the heat exchanger into the second drum of the second sand dryer; and a branch duct for introducing 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. 2. The sand drying system of claim 1, further comprising a first moisture sensor for detecting the moisture content of the dried sand discharged from the first drum of the first sand dryer, and a first controller for adjusting and controlling at least one of the combustion amount of the gas burner, 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 a storage tank for storing liquefied natural gas at low temperature and high pressure and a vaporizer are provided near the first sand dryer, and a hot water supply pipe is provided for supplying 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.

5. a first drum of the first sand dryer that dries the sand by contacting it with hot air; a gas burner using natural gas as fuel, the hot air being supplied to the first drum of the first sand dryer; a second drum of the second sand dryer that exchanges heat between exhaust gas discharged from the first drum of the first sand dryer and outside air, the outside air being heated by the latent heat of condensation of water vapor contained in the exhaust gas, and the heated outside air being introduced into the second drum of the second sand dryer to dry the sand; and a sludge water from ready-mixed concrete is mixed with hot water produced by the condensation of water vapor contained in the exhaust gas from the first drum of the first sand dryer during the heat exchange, and the carbon dioxide contained in the hot water reacts with calcium ions contained in the sludge water to recover calcium carbonate.

6. 6. The method for drying sand according to claim 5, wherein the first and second sand dryers dry the sand so as to maintain a predetermined moisture content.

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