WASTE HEAT RECOVERY SYSTEMS AND METHODS

Abstract

A waste heat recovery system and method are disclosed. The waste heat recovery system disclosed herein comprises: a waste heat recovery boiler; a waste heat supply section configured to supply waste heat to the waste heat recovery boiler; and a water tank configured to communicate fluidically with the waste heat supply section.

Description

Description WASTE HEAT RECOVERY SYSTEMS AND METHODS Invention Engineering Field Disclosed is a waste heat recovery system and method. More specifically, disclosed is a waste heat recovery system and method that allows continuous operation without downtime. Background of the Invention A waste heat recovery boiler is a device that produces steam and hot water by recovering the heat energy from flue gases discharged from incinerators, furnaces, smelting furnaces, chemical processes, and the like. Therefore, a waste heat recovery boiler can save energy and protect the environment by recycling the discarded waste heat, and the steam or hot water produced can thus be used in industrial processes, to generate electricity using steam turbines, or to heat nearby facilities using hot water. Waste heat recovery boilers allow boiler water to flow inside tubes located inside the path where the flue gas flows, and vaporize the boiler water by using the heat from the flue gas to produce steam, and then discharge the steam through pipes to recover the waste heat. The discharged cooling water (i.e. boiler water) is then evaporated due to the high temperature of the flue gas, thus producing steam, and this steam is channeled through pipes to recover waste heat. Thus, the waste heat recovered from the waste heat recovery boiler is then used to preheat air, generate steam, heat oil, or the like. Comparative documents regarding waste heat recovery boilers include Korean Application Publication No. 10-2019-0071087 “APPARALYZER FOR WASTE HEAT RECOVERY FROM EXHAUST GAS”, Korean Application Publication No. 10-0751036 “NOZZLE FOR HEAT RECOVERY STEAM GENERATOR”, and Korean Application Publication No. 10-0436940 “NOZZLE STRUCTURE FOR BOILER COOLING HEADER”, etc. In addition, since in the case of high-temperature flue gas (500°C or higher) containing a large amount of highly viscous dust, handling and removal of such dust is not possible, excessive differential pressure occurs in the inlet duct of the waste heat recovery boiler due to the unavoidably accumulated highly viscous dust mass, which then requires a series of measures such as facility shutdown and internal maintenance / cleaning / removal of dust, which makes normal operation impossible, and is a major factor in the decline in productivity. Brief Description of the Invention Technical Issues One aspect of the present disclosure provides a waste heat recovery system that can operate continuously without operational downtime. Another aspect of the present disclosure provides a method of waste heat recovery that allows continuous operation without operational downtime. Technical Solutions According to one aspect of the present disclosure, a waste heat recovery system comprising: waste heat recovery boiler; a waste heat supply section configured to supply waste heat to a waste heat recovery boiler; and a water tank configured to communicate fluidically with the waste heat supply section. A waste heat recovery boiler may include a flue gas flow path, and a plurality of coolant pipes spaced apart from each other in the flue gas flow path. The waste heat supply section comprises a main channel, an inlet channel, and an outlet channel, wherein the inlet channel is configured to fluidly communicate with a side portion of the main channel, the outlet channel is configured to fluidly communicate with an upper portion of the main channel, and a water tank is configured to fluidly communicate with a lower portion of the main channel. The waste heat recovery system may further include, if the flue gas dust is collected in the waste heat supply section, an air blower configured to forcibly move the collected dust to a water tank. The waste heat recovery system may further include a flexible connection placed between the main line and the water tank and configured to connect the main line and the water tank to each other. The waste heat recovery system shall not include a dust barrier section between the main duct and the water tank. The waste heat recovery system may further include a water supply pipe and a water discharge pipe, each configured to communicate fluidly with the water tank, and circulation pipes branching from the water discharge pipe. The waste heat recovery system may further include a water level gauge configured to measure the level of a liquid filled in a water tank, a pump configured to remove the liquid from the water tank, and a third flow control valve configured to be installed at the rear end of a circulation pipe as the rear end of the pump and control the amount of liquid removed according to the water level gauge signal. The waste heat recovery system may further include an equalization water tank configured to communicate fluidly with the water tank, and may be configured to supply water to the water tank from the equalization water tank according to a pressure difference between the water pressure of the equalization water tank and the fluid pressure of the water tank. According to another aspect of the present disclosure, a waste heat recovery method comprising: step (S10) supplying high temperature exhaust gas of 500 °C or more to the waste heat supply section in the form of a duct; step (S20) waste heat recovery by supplying, to the waste heat recovery boiler, a portion of the flue gas that has passed through the waste heat supply section; and step (S30) supplying, to the water tank, the remaining portion of the flue gas that has passed through the waste heat supply section. Step (S30) can be configured to allow some of the dust contained in the flue gas to fall into the water tank due to gravity without being obstructed by the falling dust barrier section. The waste heat recovery method may further include step (S40) of maintaining the concentration of sodium carbonate in the liquid filled in the water tank at a reference value or less. Step (S40) may include step (S40-1) supplying water to the water tank at a first flow rate, step (S40-2) discharging the liquid filled into the water tank at a second flow rate, step (S40-3) recirculating a portion of the liquid discharged in step (S40-2) to the water tank at a third flow rate, and step (S40-4) discharging a portion of the remaining liquid discharged in step (S40-2) at a fourth flow rate out of the water tank. The waste heat recovery method may further include step (S50) measuring the height of the liquid filled in the water tank, and the fourth flow rate in step (S40-4) may be determined according to the height of the liquid in the water tank measured in step (S50). Beneficial Effects Waste heat recovery systems and methods according to embodiments of the present disclosure have the following beneficial effects: (1) Continuous operation without stopping operation can be made possible by continuously removing dust that enters the dead zone during operation. (2) Blockage of the space between the cooling tubes installed inside the waste heat recovery boiler by dust can be repaired. (3) By reducing the problem of reduced heat transfer efficiency due to dust covering the surface of the cooling pipes installed in waste heat recovery boilers, an improvement in the waste heat utilization performance of waste heat recovery boilers, as well as an increase in the amount of steam production and waste heat recovery capacity, can be expected. Short Description of Image Figure 1 is a partial perspective view of a waste heat recovery system according to an embodiment of the present disclosure. Figure 2 is a front view of the waste heat recovery system in Figure 1. Figure 3 is a side view of the waste heat recovery system in Figure 1. Figure 4 is a perspective view of the waste heat recovery system in Figure 1. Figure 5 is a diagram that schematically shows the water tank and the ancillary facilities connected to it of the waste heat recovery system of Figure 1. Figure 6 is a diagram showing the equalization water tank added to the waste heat recovery system from Figure 1. Figure 7 is a diagram showing a simulation model of the waste heat recovery system in Figure 1. Figure 8 is a diagram showing the velocity contours as a result of the simulation of the waste heat recovery system in Figure 1. Figure 9 is a diagram showing the temperature contour lines as a result of the simulation of the waste heat recovery system in Figure 1. Complete Description of the Invention Mode for Invention Below, a waste heat recovery system according to an embodiment of the present disclosure is described in more detail with reference to the drawings. In this specification, “fluid communication” means that two or more parts are connected in such a way that fluid can flow within the parts. Figure 1 is a partial perspective view of the waste heat recovery system (100) according to an embodiment, Figure 2 is a front view of the waste heat recovery system (100) of Figure 1, Figure 3 is a side view of the waste heat recovery system (100) of Figure 1, and Figure 4 is a perspective view of the waste heat recovery system (100) of Figure 1. Referring to Figures 1 to 4, the waste heat recovery system (100) according to the embodiments of the present disclosure includes a waste heat recovery boiler (not shown), a waste heat supply section (110), and a water tank (120). A waste heat recovery boiler may include a flue gas flow path (not shown) therein and a plurality of cooling tubes (not shown) spaced apart from each other within the flue gas flow path. Cooling water can flow through each cooling tube. Therefore, the flue gas passing through the flue gas flow path can be cooled through contact with each cooling tube, and at the same time, the cooling water flowing inside each cooling tube can be heated. The heated cooling water can be recovered as hot water or steam and used for heating nearby facilities, etc. A waste heat recovery boiler may be installed at the top of the outlet duct (113) of the waste heat recovery section (110) described below, and may be configured to be in fluid communication with the top of the outlet duct (113). The waste heat supply section (110) may include a main line (111), an inlet line (112), and an outlet line (113), which are connected in fluid communication with each other. The inlet flow channel (112) may be configured to communicate fluidly with a side portion of the main channel (111). The outlet flow channel (113) may be configured to communicate fluidly with the top of the main channel (111). For example, the main channel (111), the inlet channel (112), and the outlet channel (113) can be formed as one body. After the flue gas discharged from the incinerator, blast furnace, smelting furnace, chemical process, etc., is supplied to the waste heat supply section (110), most of the flue gas can flow into the waste heat recovery boiler and be cooled by heat loss through heat exchange with the cooling tubes, and in this cool state can leave the waste heat recovery boiler. Exhaust gas contains dust, and this dust is characterized by low density, high viscosity, and high solubility in water. In addition, if such dust is collected at the lower end of the waste heat recovery boiler facility (e.g., the lower end of the waste heat supply section (110)) in a concentrated manner, it may obstruct the flue gas flow path to the waste heat recovery boiler and create excessive differential pressure that makes normal operation of the facility impossible, and in particular, handling of such dust may become very difficult at high temperatures of 500 °C or more. In addition, some of the flue gas may enter the water tank (120), which will be explained below, and in this case, the dust contained in the flue gas may fall into the water tank (120) due to the force of gravity. As explained, because some of the dust contained in the flue gas falls into the water tank (120) and is then discharged directly from the water tank (120) when the waste heat recovery boiler is in operation, it is possible to prevent the accumulation of dust in the waste heat supply section (110), thereby reducing the differential pressure occurring in the waste heat supply section (110) and reducing the amount of dust supplied to the waste heat recovery boiler.Furthermore, it is possible to improve (for example, by 5% to 10%) the problem of the space between the cooling tubes installed inside the waste heat recovery boiler being clogged by dust and at the same time, improve the problem of decreased heat transfer efficiency due to the cooling tube surface being covered by dust. As a result, the benefits of improvement in the waste heat recovery performance of the waste heat recovery boiler, increased hot water or steam generation amount, and increased waste heat recovery capacity can be expected. The water tank (120) functions to mix the dust introduced into it with water to form an aqueous solution. In addition, the water tank (120) may be configured to communicate fluidly with the waste heat supply section (110). For example, the water tank (120) may be configured to communicate fluidly with the lower section of the main duct (111). Therefore, the water tank (120) may be installed in a dead zone, e.g., a region separated from the main flow of high-temperature flue gases that successively pass through the inlet flow duct (112), the main duct (111), and the outlet flow duct (113), and as a result may be less affected by high temperatures. For example, if the water tank (120) is not installed on the waste heat recovery system (100), the direct removal of the dust accumulated in the waste heat supply section (110) becomes impossible, in which case, only after internal obstruction occurs, the entire facility must be shut down and cooled, and the accumulated dust must be manually cleaned from the inside, thereby incurring associated costs and operational downtime costs. In addition, the waste heat recovery system (100), in cases where dust originating from the flue gas collects in the waste heat supply section (110), may further include an air blower (not shown) configured to forcibly move the collected dust to a water tank (120). In particular, the air blower may be configured to emit pressurized air onto the collected dust in the waste heat supply section (110) (e.g., the inlet duct 112), thereby forcibly sweeping the collected dust into the water tank (120). In addition, the waste heat recovery system (100) may further include flexible connections (not shown). A flexible connection can be placed between the main line (111) and the water tank (120) and is configured to connect the main line (111) and the water tank (120) to each other. When the main line (111) and the water tank (120) are subjected to thermal expansion and thermal contraction, the flexible connection can accommodate the thermal expansion and thermal contraction, thereby serving to ensure a secure connection between the main line (111) and the water tank (120) at all times. In addition, the waste heat recovery system (100) shall not include a falling dust barrier portion (not shown) between the main duct (111) and the water tank (120). A falling dust barrier member refers to any member that prevents dust included in the exhaust gas from falling from the main duct (111) to the water tank (120) due to gravity. For example, a falling dust barrier member may include an electrostatic precipitator, a filter, or a combination thereof. For example, if a falling dust barrier member is installed between the main duct (111) and the water tank (120), highly viscous dust may adhere to the falling dust barrier member, resulting in dust buildup over time, the dust buildup becoming so severe that dust buildup may occur even within the waste heat supply member (110), defeating the original purpose of installing the water tank (120). Although the waste heat supply portion 110 is not limited to any particular shape or size, and similarly, the water tank 120 is not limited to any particular shape or size, examples are illustrated in Figures 1 to 4. In addition, the waste heat recovery system (100) may further include a water supply pipe L1, water discharge pipes L2, L4, and a circulation pipe L3, each configured to communicate fluidly with the water tank (120). Water supply pipe L1 serves to supply water to the water tank (120). The L2 water drain pipe functions to drain liquid (for example a mixture of water and dust) from the water tank (120). The L3 circulation pipe, which is a branch of the L2 water drainage pipe, functions to return some of the liquid that flows through the L2 water drainage pipe to the water tank (120). The L4 water drain pipe functions to discharge to the outside, thereby removing some of the liquid that is discharged from the water tank (120) through the L2 water drain pipe. In addition, the waste heat recovery system (100) may further include a first flow control valve V1 and a second flow control valve V2. The first flow control valve V1 is installed on the water supply pipe L1 and serves to control the flow rate of water supplied to the water tank (120) through the water supply pipe L1. The second flow control valve V2 is installed on the circulation pipe L3 and functions to control the flow rate of the liquid that is returned to the water tank (120) via the circulation pipe L3, between the liquid that is discharged from the water tank (120) via the water discharge pipe L2. The first flow control valve V1 and the second flow control valve V2 may each be a globe valve, but the present disclosure is not limited thereto. In addition, the waste heat recovery system (100) may further include a water level gauge LT, a pump P, and a third flow control valve V3. The LT water level gauge can be configured to measure the height of a liquid filled in a water tank (120). Pump P may be configured to forcibly remove liquid from the water tank (120). The third flow control valve V3 can be installed at the rear end of the circulation pipe L3 as the rear end of the pump P and configured to control the liquid discharge amount according to the signal from the water level gauge LT. The liquid that has passed through the third flow control valve V3 can be discharged to the outside through the water discharge pipe L4. In addition, FT flow meters can be installed on each of the L2 water discharge pipe and L3 circulation pipe. Another embodiment of the present disclosure provides a method of waste heat recovery that can be carried out using the waste heat recovery system (100) mentioned above. Below, waste heat recovery methods according to embodiments of the present disclosure are described in more detail with reference to Figures 1 to 5. The waste heat recovery method includes the step (S10) of supplying flue gas having a high temperature of 500 °C or more to a duct-shaped waste heat supply section (110); a step (S20) of recovering the waste heat by supplying a portion of the flue gas that has passed through the waste heat supply section (110) to a waste heat recovery boiler (not shown); and the step (S30) of supplying a remaining portion of the flue gas that has passed through the waste heat supply section (110) to a water tank (120). Step (S30) may be configured to allow a portion of the dust included in the flue gas to fall into the water tank (120) due to gravity without resistance from the falling dust barrier portion (not shown). In addition, the waste heat recovery method may further include a step (S40) of maintaining a concentration of sodium carbonate in the liquid filled in the water tank (120) at a reference value (e.g., 10% by weight) or less. Step (S40) may include step (S40-1) supplying water to the water tank (120) at a first flow rate (e.g., 10 m3 / h), step (S40-2) discharging liquid contained in the water tank (120) at a second flow rate (e.g., 60 m3 / h), step (S40-3) returning a portion of the liquid discharged in step (S40-2) to the water tank (120) at a third flow rate (e.g., 50 m3 / h), and step (S40-4) discharging a portion of the remaining liquid discharged in step (S40-2) out of the water tank (120) at a fourth flow rate (e.g., 10 m3 / h). In addition, the waste heat recovery method may further include a step (S50) of measuring the level of liquid filled in the water tank (120). In this case, a fourth flow rate in step (S40-4) may be determined according to the level of liquid in the water tank (120) measured in step (S50). For example, if the amount or temperature of the flue gas fed into the water tank (120) increases, the amount of water evaporated from the water tank (120) increases, and to address such situation, the fourth flow rate may be reduced. In addition, in an emergency situation where a large amount of water is directly evaporated, or a large amount of water is drained due to leakage from the water tank (120), the waste heat recovery method may further include the step (S60) of supplying water to the water tank (120) from an equalization water tank (130) in fluid communication with the water tank (120), according to the pressure difference between the water pressure of the equalization water tank (130) and the fluid pressure of the water tank (120). Figure 6 is a diagram showing an equalization water tank (130) added to the waste heat recovery system (100) of Figure 1. Referring to Figure 6, the waste heat recovery system (100) may further include an equalization water tank (130). The equalization water tank (130) is for appropriately handling emergency situations in which a large amount of water is instantly evaporated or a large amount of water is drained due to leakage from the water tank (120), and the equalization water tank (130) serves to prevent the water tank (120) from running out of water in such emergency situations. The equalization water tank (130) may be configured to communicate fluidly with the water tank (120). The equalization water tank (130) may be filled with water. In this case, the waste heat recovery system (100) may be configured such that water is supplied from the equalization water tank (130) to the water tank (120) according to the pressure difference between the water pressure of the equalization water tank (130) and the liquid pressure of the water tank (120).For example, the water tank (120) and the equalization water tank (130) are connected in fluid communication through their respective bottoms via an equalization pipe EQL, and if the above-mentioned pressure difference occurs, water is supplied from the equalization water tank (130) to the water tank (120) via the equalization pipe EQL and then the flow control valve V4 operates according to the water level drop signal measured by the level gauge LT installed in the equalization water tank (130), so that water is supplied to the equalization water tank (130) from the outside via the water supply pipe L5, so that the water level in the equalization water tank (130) remains constant. For example, the temperature of the liquid filled in the water tank (120) may be more than 80 °C, and the temperature of the water filled in the equalization water tank (130) may be less than 50 °C, but the present disclosure is not limited to thereof. Figure 7 is a diagram showing the simulation model of the waste heat recovery system (100) in Figure 1, Figure 8 is a diagram showing the velocity contours as a result of the simulation of the waste heat recovery system (100) in Figure 1, and Figure 9 is a diagram showing the temperature contours as a result of the simulation of the waste heat recovery system (100) in Figure 1. Below, the simulation results of a waste heat recovery method that discharges collected dust or falling dust directly using a water tank (120), while recovering waste heat from the flue gas using the waste heat recovery system (100) in Figure 1, will be explained with reference to Figures 7 to 9. The simulation was performed using commercial CFD (Computational Fluid Dynamics) software (Fluent 2020 R1) and the geometry and boundary conditions applied to it are as follows. <geometri> As illustrated in Figure 2, d11 is set to 3,883 mm, d12 is set to 3,200 mm, d13 is set to 2,882 mm, d21 is set to 1,752 mm, d22 is set to 3,441.5 mm, d23 is set to 850 mm, and d24 is set to 950 mm. Additionally, as illustrated in Figure 3, d14 is set to 793 mm, and d15 is set to 1,885 mm. In addition, the material of the waste heat supply part (110) is set to a composite consisting of 125 mm thick heat-resistant material and 6 mm thick CS steel (carbon steel), and the material of the water tank is set to 6 mm thick SS steel (stainless steel). In addition, the density, thermal conductivity, and specific heat capacity of heat-resistant materials, CS steel, and SS steel are set as shown in Table 1. [Table 1] Material Density Thermal conductivity Specific heat capacity kg / m3 (W / m -K) J / kg -K Heat-resistant materials 1, 150 0.3 700 CS steel 7,801 43 473 SS steel 7,850 14 490 Also, the wind speed is set to 2.5 m / s, and the ambient temperature is set to 45°C. <Kondisi batas> The flue gas supplied to the waste heat supply section (110) is set to have a molecular weight of 26.61, a flow rate of 115.676 kg / h, and a temperature of 600°C. In addition, the exhaust gas discharged from the waste heat supply section (110) is set to have a pressure of 70 mbar (gauge pressure). In addition, the water supply pipe L1 connected to the water tank (120) is set to have an inner diameter of 1.5 inches, a flow rate of 2 tons / hour, and a temperature of 40°C. In addition, the water discharge pipe L2 connected to the water tank (120) is set to have an inner diameter of 3 inches and a flow rate of 12 tons / hour. In addition, the circulation pipe L3 connected to the water tank (120) is set to have an inner diameter of 3 inches, a flow rate of 10 tons / hour, and a temperature of 60°C. In addition, the liquid filled in the water tank (120) is set to have a density of 1,150 kg / m3 and a water height of 1,650 mm. <Kondisi Pengaturan> As illustrated in Figure 7, the flue gas flow path, which is the same as the space inside the waste heat supply section (110), is divided by a number of meshes, and simulations are performed. The setting conditions are summarized and shown in Table 2. [Table 2] List of simulation settings Input parameters Simulation Type Transient Realizable K-epsilon turbulence model Wall function Scalable wall function Mixed two-phase model Pressure-velocity coupling SIMPLE Number of mesh elements About 3 million CFD S / W Current 2020 RI Number of iterations About 30,000 <Hasil Simulasi> Referring to Figure 8, the figure on the left is a diagram showing the velocity contour of the flue gas flowing through the flue gas flow path corresponding to the space inside the waste heat supply section (110), and the figure on the right is an expanded view of the dotted rectangular portion of the left figure. As shown in the figure on the right of Figure 8, it can be confirmed that part of the flue gas flows downward into the water tank (120). In this case, the dust included in the flue gas falls into the water tank (120) due to the force of gravity. Referring to Figure 9, it can be confirmed that in the case where 600°C flue gas is supplied to the waste heat supply section (110), the temperature of the flue gas discharged upward by passing through the waste heat supply section (110) is 594°C, and the surface temperature of the liquid filled in the water tank (120) is 108°C. Thus, since the surface temperature of the liquid filled in the water tank (120) is only 108°C, no excess amount of water is evaporated from the water tank (120), and thermal damage to the water tank (120) and surrounding equipment can be eliminated. From this result, it can be confirmed that even if flue gas with a temperature as high as 500°C or more is supplied to the waste heat recovery boiler to recover waste heat, dust can be discharged directly while continuously recovering waste heat without shutting down the waste heat recovery system (100) to discharge dust that may cause an increase in differential pressure. The present disclosure has been explained with reference to the drawings, but these are illustrative examples only, and it will be apparent to those skilled in the comparative documents that various modifications and equivalent implementations may be made thereof. Therefore, the scope of this disclosure shall be determined by the appended claims.< / geometri>

Claims

1. A waste heat recovery system comprising: a waste heat recovery boiler; a waste heat supply section configured to supply waste heat to the waste heat recovery boiler; a water tank configured to communicate fluidly with the waste heat supply section; an equalization water tank configured to communicate fluidly with the water tank; a water supply pipe, a water discharge pipe, and a circulation pipe branching from the water discharge pipe, each configured to communicate fluidly with the water tank; a first flow control valve installed on the water supply pipe, and a second flow control valve installed on the circulation pipe;and a water level gauge configured to measure the level of a liquid filled in a water tank, a pump configured to discharge the liquid from the water tank, and a third flow control valve installed at the rear end of a circulation pipe as the rear end of the pump and configured to control the amount of the liquid discharged according to a signal of the water level gauge, wherein the water tank is configured to mix dust introduced therein with water to form an aqueous solution, wherein a waste heat recovery system is configured to maintain a sodium carbonate concentration in the liquid filled in the water tank at a reference value or less, wherein the waste heat recovery system is configured to supply water to the water tank from an equalization water tank according to a pressure difference between the water pressure of the equalization water tank and the liquid pressure of the water tank.; 2. The waste heat recovery system of claim 1, wherein the waste heat recovery boiler comprises a flue gas flow path and a plurality of cooling tubes disposed at a distance apart from each other within the flue gas flow path.

3. The waste heat recovery system of claim 1, wherein the waste heat supply portion comprises a main channel, an inlet channel, and an outlet channel, wherein the inlet channel is configured to fluidly communicate with a side portion of the main channel, the outlet channel is configured to fluidly communicate with an upper portion of the main channel, and a water tank is configured to fluidly communicate with a lower portion of the main channel.

4. The waste heat recovery system of claim 1, further comprising an air blower configured to, if dust originating from the flue gas collects in the waste heat supply section, forcibly transfer the collected dust to a water tank.

5. The waste heat recovery system of claim 1, further comprising a flexible connection disposed between the main line and the water tank and configured to connect the main line and the water tank to each other.

6. The waste heat recovery system of claim 5, not comprising a falling dust barrier portion between the main duct and the water tank.

7. A waste heat recovery method comprising: the step (S10) of supplying high-temperature flue gas of 500°C or more to a duct-shaped waste heat supply section; the step (S20) of recovering the waste heat by supplying, to the waste heat recovery boiler, a portion of the flue gas that has passed through the waste heat supply section; the step (S30) of supplying, to a water tank, the remaining portion of the flue gas that has passed through the waste heat supply section; the step (S40) of maintaining the concentration of sodium carbonate in a liquid filled in the water tank at a reference value or less; the step (S50) of measuring the level of the liquid filled in the water tank; and the step (S60) of supplying water to the water tank from an equalization water tank in fluid communication with the water tank, according to the pressure difference between the water pressure of the equalization water tank and the liquid pressure of the water tank, in an emergency situation where a large amount of water is directly evaporated,or a large amount of water drained due to leakage from a water tank, wherein the water tank is configured to mix dust introduced into it with water to form an aqueous solution, wherein step (S40) comprises step (S40-1) supplying water to the water tank at a first flow rate, step (S40-2) discharging liquid contained in the water tank at a second flow rate, step (S40-3) recirculating a portion of the liquid discharged in step (S40-2) to the water tank at a third flow rate, and step (S40-4) discharging the remaining portion of the liquid dissolved in step (S40-2) out of the water tank at a fourth flow rate, wherein the fourth flow rate in step (S40-4) is determined according to the liquid level in the water tank measured in step (S50)., 8. The waste heat recovery method of claim 7, wherein step (S30) is configured to allow a portion of the dust included in the flue gas to fall into the water tank by gravity without being obstructed by the falling dust barrier portion.

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