JP7734564B2 - Sand drying system and sand drying method - Google Patents

Description

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

At asphalt plants at asphalt mixture manufacturing plants 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, around 170°C, and then mixed with stone powder, molten asphalt, etc. in predetermined amounts to produce the desired asphalt mixture.

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. In particular, 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 very large amount of fuel is consumed for the drying process. Therefore, if the moisture content of the aggregates used in asphalt plants, especially sand, could be reduced in advance, it would be possible to effectively save energy and also to reduce _CO2 emissions.

In response to the above issues, 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, a dedicated dryer (rotary kiln) that pre-dries wet sand with a high moisture content, to be installed within the asphalt mixture manufacturing factory (near the asphalt plant), and that effectively utilizes, as its heat source, high-temperature hot air (waste heat) exhausted from a deodorizing furnace already installed within the factory facility.

Japanese Patent Application Laid-Open No. 2004-36330 JP 2011-226109 A

However, as mentioned above, installing a separate sand dryer within the facilities of an asphalt mixture manufacturing plant would incur 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 were installed within the plant facilities, in asphalt plants that are forced to operate intermittently, a heat source cannot be secured, at least when the plant (deodorizing furnace) is not in operation, and the sand dryer cannot be operated. Therefore, it is believed that there is room for improvement in terms of efficient sand drying.

In view of the above, the present invention aims to provide a sand drying system and method that are relatively easy to adopt and can efficiently dry sand.

The inventors of the present invention have conducted extensive research to solve the above problems and have concluded that if a sand dryer for pre-drying 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 eliminate the need to install a sand dryer at each individual asphalt mixture manufacturing plant, making it relatively easy to adopt. 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. In addition, the added value of being able to supply pre-dried sand to each asphalt mixture manufacturing plant would be increased, making it potentially worth the investment. Furthermore, it could be considered a reduction in total _CO2 emissions, including emissions from each asphalt mixture manufacturing plant to which the sand is supplied. This led to the completion of the present invention.

In other words, the sand drying system described in claim 1 of the present invention is characterized in that first and second sand dryers that dry wet sand by contacting it with hot air are installed side by side in a crushed stone factory, and one end of the first drum of the first sand dryer is equipped with a gas burner for supplying hot air fueled by natural gas, and the other end is equipped with an exhaust duct, and a heat exchanger is installed 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 outside air supplied from an outside air supply blower, and the outside air is heated by using the latent heat of condensation when the water vapor contained in the exhaust gas condenses, and a supply duct is installed to introduce the outside air heated by the heat exchanger into the second drum of the second sand dryer, and a branch duct is installed 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 .

Furthermore, the sand drying system described in claim 2 is characterized by including 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.

Furthermore, the sand drying system described in claim 3 is characterized by including 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.

In addition, the sand drying system described in claim 4 is characterized in that it is provided with a storage tank for storing liquefied natural gas at low temperature and high pressure and an evaporator near the first sand dryer, and a hot water supply pipe 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 evaporator as a heat source.

In addition, a sand drying method according to claim 5 comprises the steps of: providing first and second sand dryers in parallel at a crushing plant, which dry wet sand by contacting it with hot air; 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; 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 of water vapor contained in the exhaust gas when it condenses, and this heated outside air is introduced into the second drum of the second sand dryer to dry the sand ; and mixing the hot water generated as the water vapor contained in the exhaust gas from the first drum of the first sand dryer condenses with sludge water from ready-mixed concrete, allowing the carbon dioxide contained in the hot water to react with the calcium ions contained in the sludge water, thereby recovering calcium carbonate .

In a sixth aspect of the present invention, there is provided a method for drying sand, characterized in that the first and second sand dryers perform the drying process so as to maintain the sand at a predetermined moisture content.

According to the sand drying system 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, which is the sand supply destination. Even at the crushed stone plant side, 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 amount of sand produced at the crushed stone plant and to construct a sand drying system that is relatively easy to adopt. Furthermore, despite its simple configuration, the exhaust gas temperature after heat exchange can be raised above the dew point, effectively suppressing problems such as condensation in the exhaust duct.

Furthermore, 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 reduces the burner's fuel consumption while also allowing some of the residual moisture in the discharged sand to evaporate using the sand's sensible heat. This reduces energy loss due to simple heat radiation from the dried sand, allowing for more efficient sand drying.

Furthermore, 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 discharged sand can be evaporated using the sensible heat of the sand, reducing energy loss due to simple heat radiation from the dried sand and enabling the sand to be dried more efficiently.

Furthermore, according to the sand drying system of claim 4 , 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.

According to the sand drying method of claim 5 , the sand dryer is installed at the crushed stone plant, so there is no burden on the asphalt mixture manufacturing plant, which is the sand supply destination. 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 the water vapor in the exhaust gas condenses. This allows for efficient drying of the large amount of sand produced at the crushed stone plant, making it possible to construct a relatively easy-to-adopt sand drying system. Furthermore, the CO2 contained in the exhaust gas can be recovered as calcium carbonate, a useful substance that can be used, for example, as a material for asphalt mixtures, and the volume of sludge water, which is an industrial waste, can be reduced at the same time, making this a desirable system.

In addition, according to the sand drying method described in claim 6 , 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 discharged sand 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.

1 is a schematic explanatory diagram showing an embodiment of a sand drying system and a sand drying method according to the present invention.

In the sand drying system and method of the present invention, first and second sand dryers are installed side by side 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., to dry the wet sand with a high moisture content produced at the crushed stone plant by contacting it with hot air.

The first and second sand dryers use a rotary kiln structure in which a cylindrical drum with multiple scraping blades attached to its inner surface is supported at an angle for free rotation, and moist sand with a high moisture content is supplied into the drum along with hot air. The sand is repeatedly scraped up by the scraping blades while being brought into direct contact with the hot air, thereby heating and drying the sand.

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. A heat exchanger is installed midway through 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 room-temperature outside air supplied by an outside air supply fan, and heats the outside 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 outside air (warm air) heated by the heat exchanger into the second drum of the second sand dryer.

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, but is still relatively hot and highly humid due to the 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 upon heat exchange with the outside air in the heat exchanger, and the latent heat of condensation generated during this process allows the room-temperature outside air to be efficiently heated.

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, making it possible for the second sand dryer to adequately dry the sand.

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 a burner fueled by, for example, heavy oil A. Furthermore, because natural gas does not inherently contain substances that cause acid gas generation, 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 discharged as is without any special neutralization treatment, and the impact of acid corrosion on the heat exchanger, exhaust duct, etc. can be suppressed.

It is also preferable to provide a first moisture sensor that detects the moisture content of the dried sand discharged from the first drum of the first sand dryer after the drying process, 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.

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 an asphalt mixture material, the temperature of the sand must be raised to a high temperature. However, if this hot sand is transported from the crushing plant to a remote asphalt mixture manufacturing plant, heat will dissipate during the process, causing the temperature to drop, which simply results in energy loss.

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 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. 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.

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.

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. 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.

It is also 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. In some cases, the exhaust gas temperature after heat exchange may fall below the dew point temperature, potentially causing problems such as condensation in the exhaust duct or white smoke emitting from the chimney at the end of the exhaust duct. However, by providing the branch duct, a portion of the outside air (hot air) that has been heated and warmed by heat exchange and has a reduced relative humidity can be mixed into the exhaust gas in the exhaust duct. This allows the exhaust gas temperature to be raised above the dew point, effectively suppressing the above problems despite the simple configuration.

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 allows the liquefied natural gas burner fuel to be vaporized without the need for a separate heat source such as a boiler for the vaporization process, making it easy to adopt from the standpoints of cost, maintenance, etc.

When drying sand 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 supplied to 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. In the second sand dryer, heat is exchanged between the high-temperature, high-humidity exhaust gas discharged from the first sand dryer and ambient-temperature outside air supplied by a blower, heating the outside air using the latent heat of condensation generated when the water vapor contained in the exhaust gas condenses. This heated outside air (hot air) is then introduced into the second drum, where the sand is dried in the same manner as in the first sand dryer.

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.

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.

Here, if the above-mentioned 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 etc. 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. Therefore, it is expected that the investment will be well worth it. Furthermore, when considering the total _CO2 emissions, including those emitted by each asphalt mixture manufacturing plant to which the sand is supplied, it can be seen as a reduction, and it is thought that this system can be adopted preferably.

In addition, the hot water produced when 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 at 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.

This allows the _CO2 contained in the exhaust gas to be recovered as calcium carbonate, a useful substance that can be used as a material for asphalt mixtures, and simultaneously reduces the volume of sludge water that must be treated as industrial waste, making it particularly environmentally friendly.

One embodiment of the present invention will be described below with reference to the drawings.

In the diagram, 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 and the like. The sand drying system 1 is mainly composed of a first sand dryer 2 and a second sand dryer 3 that dry the high-moisture wet sand 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.

The first and second sand dryers 2, 3 are used exclusively to dry wet sand with a high moisture content. They consist of a cylindrical first drum 5 and a cylindrical second drum 6, each with multiple scraper blades (not shown) attached to their inner surfaces, which are supported at an angle and rotatable. When high-moisture sand is supplied into the first drum 5 and the second drum 6 along with hot air, the high-moisture sand is heated and dried by being repeatedly scraped up by the scraper blades and coming into direct contact with the hot air. This embodiment uses a parallel-flow heating type dryer, in which the hot air and sand flow in the same direction.

One end of the first drum 5 of the first sand dryer 2 (the upstream end when viewed from the direction of the hot air and sand flowing downstream) is equipped with a gas burner 7 that supplies hot air fueled by natural gas, as well as a feeding hopper 8 into which moist sand with a high moisture content (e.g., approximately 15%) produced at the crushed stone plant is fed, and a screw feeder 9 that sequentially feeds the high moisture content sand discharged from the bottom of the feeding hopper 8 into the first drum 5.

Near the first sand dryer 2 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 serves as fuel for the gas burner 7, and a buffer tank 12 for temporarily storing the natural gas produced by vaporization in the vaporizer 11.

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 flow) is equipped with an exhaust chamber 13, the upper part of which is connected to a first exhaust duct 14 for exhaust gas discharge, and the lower part is equipped with a discharge hopper 15 for discharging dried sand, which is sand that has undergone a drying process. 16 in the figure is a first moisture sensor that continuously detects the moisture content of the dried sand discharged from the discharge hopper 15.

The first exhaust duct 14 is equipped with, from upstream to downstream in the direction of exhaust gas flow, 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. A chimney 20 is also 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 to allow adjustment of the air volume. 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.

The heat exchanger 4 indirectly exchanges heat between the high-temperature (e.g., approximately 120°C) and high-humidity (e.g., approximately 50%) exhaust gas discharged from the first drum 5 of the first sand dryer 2 and room-temperature (e.g., approximately 15°C) outside air supplied by the outside air supply blower 21. This heat exchange efficiently heats the outside air by utilizing the latent heat of condensation generated when the large amount of water vapor contained in the exhaust gas condenses. Because the latent heat of condensation of water vapor has more than five times the thermal energy of the sensible heat of water, this type of heat exchange involving condensation is extremely efficient.

According to the inventor's calculations, if the amount of outside air supplied from the blower 21 is adjusted to approximately 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 approximately 70°C. However, this may at first glance seem considerably lower than the approximately 1,800°C temperature of the hot air supplied from the gas burner 7. However, even if the temperature rises by approximately 50-60°C, the relative humidity drops significantly (for example, if the humidity of outside air at room temperature is approximately 50%, the relative humidity will drop to approximately 3% as it is heated to approximately 70°C). Therefore, the second sand dryer 3, which receives a large amount of this air (approximately 10 times the humidity), will be able to adequately dry the sand.

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. Therefore, if the temperature of the outside air supplied to the second drum 6 of the second dryer 3 is to be maintained constant (e.g., approximately 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 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 (e.g., approximately 70°C).

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 that a large amount of warm air can be introduced (received), or the second drum 6 may be made up of multiple drums (such as arranged side by side).

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, effectively utilizing the latent heat of condensation generated during this process to efficiently heat the outside air. Therefore, if the exhaust gas contains large amounts of substances that cause acid gas formation, such as sulfur or chlorine, not only will it be costly to neutralize the large amount of acidic condensed water that is generated, but measures to prevent acid corrosion of the heat exchanger 4, exhaust duct 14, exhaust fan 19, etc. will be necessary.

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 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.

Also, reference numeral 24 in the figure denotes 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 in response to the opening and closing operation of an opening/closing damper 25 interposed midway through the branch duct 24.

In some cases, the exhaust gas temperature after heat exchange may fall below the dew point temperature, which can 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 (aerosolized water vapor in the exhaust gas) 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 warmed by heat exchange and has a lowered relative humidity into the exhaust gas in the first exhaust duct 14. This allows the exhaust gas temperature to be raised above the dew point, and this simple configuration makes it possible to mitigate the above problems.

The opening and closing operation of the on-off damper 25 can be controlled, for example, by providing a separate temperature sensor in the first exhaust duct 14 downstream of the heat exchanger 4. When the temperature sensor detects a temperature below a predetermined temperature (e.g., dew point temperature), the on-off damper 25 is opened to mix in some of the heated outside air (warm air). However, in some cases, a fixed amount may be mixed in at all times while the system is operating.

Furthermore, like the first drum 5 of the first sand dryer 2, 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 with a feeding hopper 26 for feeding moist sand with a high moisture content (e.g., approximately 15%) produced at the crushing plant, and a screw feeder 27 for sequentially feeding the high moisture content sand discharged from the bottom of the feeding hopper 26 into the second drum 6.

The second drum 6 of the second sand dryer 3 is also 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 (warm air) and sand). 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. Reference numeral 31 in the figure denotes a second moisture sensor that continuously detects the moisture content of the dried sand discharged from the discharge hopper 30.

As with the first sand dryer 2, the second exhaust duct 29 is equipped with, from upstream to downstream in the direction of exhaust gas flow, 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.

In the figure, reference numeral 36 denotes a hot water supply pipe that supplies condensed water, produced when water vapor contained in the exhaust gas from the first drum 5 of the first sand dryer 2 condenses in the heat exchanger 4, to the vaporizer 11 used for liquefied natural gas vaporization as a heat source. A supply pump 37 is installed along the pipe. According to the inventor's calculations, the condensed water can be recovered as hot water at 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, resulting in advantages 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; it can be directly discharged to the outside via the drainage pipe 38.

In the figure, 39 denotes 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 (e.g., 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 excluding bone dryness.

For example, if the moisture content of the dried sand is higher than a predetermined moisture content (e.g., approximately 5%), the system adjusts and controls the combustion rate of the gas burner 7 by increasing the amount of sand supplied to the first drum 5 (by slowing down the supply speed of the screw feeder 9), by increasing the rotational speed of the first drum 5, or by performing a combination of these. Increasing the rotational speed of the first drum 5 increases the amount of sand stirred up per unit time in the first drum 5, thereby increasing the opportunity for contact with the hot air from the gas burner 7 and making it possible to reduce the moisture content of the sand.

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 an asphalt mixture material, 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 dissipate during the process, causing the temperature to drop, which simply results in energy loss.

On the other hand, by intentionally leaving some 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. Furthermore, 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.

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 a moisture content of approximately 5% will still have a moisture content of approximately 3% even when it returns to room temperature of approximately 15°C, despite evaporation due to sensible heat).

However, in asphalt plants at asphalt mixture manufacturing plants, when producing asphalt mixture, sand supplied from a crushed stone factory is heated to about 170°C in a dryer, and is then dried to bone dryness at that time, so there is no particular problem. Furthermore, since the moisture content of the sand is significantly reduced compared to normal (before drying) conditions (for example, moisture content is reduced 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 provide a dust-proof effect, and since the sand is at room temperature, no special measures such as heat retention are required during storage or transportation, making it easier to handle.

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 (e.g., 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.

For example, if the moisture content of the dried sand is higher than a predetermined moisture content (e.g., approximately 5%), the system adjusts and controls the amount of outside air blown from the blower 21 to increase, the amount of sand supplied to the second drum 6 to decrease (the supply speed of the screw feeder 27 to decrease), the rotation speed of the second drum 6 to increase, or a combination of these. 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, thereby improving drying efficiency and enabling the moisture content of the sand to be reduced.

As with the first sand dryer 2, the second sand dryer 3 intentionally leaves a small amount of 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.

When drying sand using the sand drying system 1, 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 installed within the crushed stone plant. In the first sand dryer 2, hot air is introduced into the first drum 5 from a natural gas-fueled gas burner 7 to dry the sand. Meanwhile, in the second sand dryer 3, heat exchanger 4 exchanges high-temperature, high-humidity exhaust gas discharged from the first sand dryer 2 with ambient-temperature outside air supplied by a blower 21. The outside air is heated using the latent heat of condensation generated when the water vapor contained in the exhaust gas condenses, and this heated outside air (hot air) is then introduced into the second drum 6 to dry the sand.

At this time, the first sand dryer 2 performs the sand drying process by adjusting and controlling at least one of the combustion amount of the gas burner 7, the amount of sand supplied to 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.

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 excluding bone dryness, while drying the sand.

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 specified moisture content maintained, 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.

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 is in a state where only a small amount of moisture remains (is wet).In the dryer of the asphalt plant, the sand is heated to a predetermined 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 the sand can then be mixed with various other materials to produce the desired asphalt mixture.

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%. While the above simulation is based on the premise that a large amount of wet sand is continuously dried, 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.

Furthermore, although not shown, in a ready-mixed concrete manufacturing plant or the like, large amounts of sludge water from ready-mixed concrete generated during the cleaning of concrete mixers and agitator trucks may be transported appropriately to a crushing plant facility where the sand drying system 1 is installed, and mixed with warm water (condensed water) generated in the heat exchanger 4, causing the carbon dioxide contained in the warm water (a large amount of _CO2 generated with the combustion _of natural gas dissolved as carbon dioxide ( _H2CO3 )) to react with the calcium ions (Ca2 ⁺ ) contained in the sludge water, thereby recovering calcium carbonate ( _CaCO3 ). Furthermore, 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.

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), for example, and the volume of sludge water can be reduced at the same time, making it even more environmentally friendly.

The present invention can be widely used in the drying process of wet sand with a high moisture content produced in crushing plants.

DESCRIPTION OF SYMBOLS 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)
REFERENCE SIGNS LIST 11: Vaporizer 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 (6)

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 . The sand drying system described in claim 1 further comprises 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. A sand drying system as described in claim 1 or 2, characterized in that it is equipped with 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. 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. a first drum of the first sand dryer that dries the sand by contacting it with hot air; a gas burner that uses 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 second drum of the first sand dryer that exchanges hot water with sludge water from ready-mixed concrete, the hot water being 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. The hot water is then mixed with sludge water from ready-mixed concrete, and the carbon dioxide contained in the hot water reacts with calcium ions contained in the sludge water to recover calcium carbonate . 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.