JP2012251687A - Heat exchanger system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a heat exchanger system preventing the deposition of a calcium carbonate in a heat exchanger.SOLUTION: The heat exchanger system includes: the heat exchanger 14 having a gas flow path 15 of a high temperature side and a liquid flow path 16 of a low temperature side; a gas supply path 17 for supplying a hot gas to an entrance side of the gas flow path 15; a gas discharge path 18 for discharging a combustion exhaust gas after a heat exchange that flows out of an outlet side of the gas flow path 15; a liquid feed path 19 for supplying a tap water to an entrance side of the liquid flow path 16; a liquid discharge path 20 for discharging liquid after the heat exchange that flows out of the outlet side of the liquid flow path 16 to load equipment; and a combustion exhaust gas injection means 24 for injecting the combustion exhaust gas into the tap water of the liquid feed path 19.

Description

The present invention relates to a heat exchanger system that can efficiently suppress scale (CaCO ₃ ) that is likely to precipitate in the heat exchanger when heat exchange is performed on tap water (city water) with a heat exchanger.

  In a hot water supply apparatus such as a boiler or a hot water heater, fuel such as city gas is burned, and water is heated by the heat generated at that time to generate hot water. In a domestic cogeneration system using an engine or a fuel cell that has been commercialized in recent years (hereinafter, the cogeneration system is abbreviated as a cogeneration system), fuel is supplied to the power generation unit, and heat generated as a by-product at that time is supplied. Water is heated with (combustion exhaust gas) to produce hot water.

  These are equipped with a fuel combustion heat recovery heat exchanger system. Here, as an example, a description will be given of an example of a cogeneration system that performs exhaust heat recovery of combustion exhaust gas of a SOFC fuel cell device. Is performed as follows. FIG. 3 is a process flow diagram showing an example of a cogeneration system that performs exhaust heat recovery of combustion exhaust gas of the SOFC fuel cell device.

  In FIG. 3, 1 is a SOFC power generation unit as a SOFC fuel cell device, 2 is a fuel cell, 3 is a pre-reformer, 4 is an evaporator, 5 is an air preheater, 6 is an air electrode, 7 is a fuel electrode, 10 Is a hot water supply unit, 11 is a hot water tank, 12 is a water supply pump, 13 is a switching valve, 14 is a heat exchanger, 15 is a gas flow path, 16 is a liquid flow path, 17 is a gas supply path, 18 is a gas discharge path, 19 Is a liquid supply path, 20 is a liquid discharge path, 21, 22 and 23 are pipes around the hot water supply tank 11.

  In the SOFC power generation unit 1, high-temperature combustion exhaust gas discharged from the fuel cell 2 sequentially passes through a pre-reformer (or reformer) 3, an evaporator 4 and an air preheater 5, respectively, and a fuel gas such as city gas, Heat is exchanged with water and air to raise the temperature to a predetermined temperature. The air heated by the air preheater 5 is supplied to the air electrode of the fuel cell 2, and the water vapor evaporated by the evaporator is supplied to the pre-reformer 3, where the city gas reacts with the water vapor in the presence of the reforming catalyst. Thus, a reformed gas containing hydrogen is generated, and the generated reformed gas is supplied to the fuel electrode 7 of the fuel cell 2.

  The combustion exhaust gas discharged from the SOFC power generation unit 1 to the outside of the unit is supplied to the inlet side of the gas flow passage 15 of the heat exchanger 14 constituting the hot water supply unit 10 through the gas supply path 17, and from the outlet side of the gas flow passage 15. The exhaust gas after the heat exchange to be discharged is discharged out of the cogeneration system via the discharge path 18. For example, in a household cogeneration system, the water supplied to the hot water supply unit 10 is tap water (city water), and the tap water is supplied from the pipe 21 to the hot water supply tank 11.

  Part of the liquid (water or hot water) stored in the hot water supply tank 11 is supplied to the inlet side of the liquid flow passage 16 in the heat exchanger 14 through the liquid supply path provided with the water supply pump 12, and discharged from the outlet side thereof. The water after the heat exchange is discharged to the hot water supply tank 11 as a load facility through the liquid discharge path 20. As an incidental facility for the hot water tank 11, a three-way selector valve 22a as shown is provided as needed. However, the three-way selector valve 22a supplies tap water directly to an unillustrated facility without passing through the hot water tank 11. Used to operate.

  In general, the temperature of combustion exhaust gas discharged from the SOFC power generation unit 1 is supplied to the heat exchanger 14 in a relatively high temperature state of about 300 ° C., and exchanges heat with tap water there. On the other hand, the temperature of tap water that is heat-exchanged by the heat exchanger 14 and discharged is usually in the range of about 75 ° C to 80 ° C.

  The tap water supplied to the heat exchanger 14 usually contains a scale component (hardness component), which precipitates as a scale (precipitate) as the temperature rises. Precipitates adhere to the inner wall of the pipe inside the heat exchanger 14 and cause clogging of the pipe, a decrease in flow rate, a deterioration in heat transfer coefficient, and the like, leading to a decrease in heat exchange efficiency.

As the scale component, calcium (Ca), magnesium (Mg), silica (Si) and the like are present, but among the precipitates, calcium carbonate (CaCO ₃ ) derived from the calcium component is the most, and the calcium carbonate It is an important factor to operate the heat exchanger 14 stably for a long period of time and with high efficiency.

  The main cause of precipitation of calcium carbonate is the increase in water temperature in the heat exchanger as described above, but as a mechanism, the solubility of carbon dioxide contained in tap water decreases due to the increase in water temperature, This occurs when carbon dioxide dissolved in water is degassed and its equilibrium changes.

That is, the equilibrium is _{"CaCO 3 + CO 2 + H 2} O⇔Ca 2+ + 2HCO 3 - " are shown by the reaction formula, the process proceeds react with carbon dioxide (CO ₂₎ is degassed to the left, calcium carbonate (CaCO ₃ ) Precipitates.

  In order to solve the above calcium precipitation problem, Patent Document 1 and Patent Document 2 describe a method in which carbon dioxide is supplied from a cylinder or the like to water supplied to a heat exchanger. These methods are intended to solve the problem by injecting carbon dioxide to prevent the equilibrium equation from proceeding to the right and generating precipitates.

JP 2010-78239 A JP 2010-223525 A

  However, in the above-mentioned prior art, carbon dioxide is supplied from a specially prepared carbon dioxide cylinder or the like, so that the cylinder is installed due to safety or space restrictions in handling high-pressure equipment, such as a cogeneration system for home use. If this is difficult, there is a problem that its application is difficult and the operating cost is also increased. Therefore, the present invention aims to solve such a problem, and an object thereof is to provide a heat exchanger system that does not require the use of a carbon dioxide source such as a cylinder.

The heat exchanger system of the present invention that solves the above problems includes a heat exchanger having a high temperature side gas flow passage and a low temperature side liquid flow passage, and a gas supply passage that supplies high temperature gas to the inlet side of the gas flow passage. A gas discharge path for discharging the gas after heat exchange flowing out from the outlet side of the gas flow path, a liquid supply path for supplying tap water to the inlet side of the liquid flow path, and heat flowing out from the outlet side of the liquid flow path It is a heat exchange system including a liquid discharge path for discharging the exchanged liquid (tap water) to load equipment, and combustion exhaust gas injection means for injecting combustion exhaust gas into the tap water in the liquid supply path. A combustion exhaust gas containing CO ₂ discharged from a combustion system is supplied to the gas supply path, and a gas injection path for injecting the combustion exhaust gas after heat exchange flowing through the gas discharge path is communicated with the combustion exhaust gas injection means. The liquid discharge path is provided with gas-liquid separation means, and the gas-liquid separation means is configured to separate the gas contained in the liquid after heat exchange (claim 1).

  In the heat exchange system, the combustion exhaust gas injecting means may be configured to inject combustion gas into tap water by an ejector method (claim 2).

  In the heat exchange system, a microbubble generator can be arranged in the combustion exhaust gas injection means.

The heat exchange system of the present invention supplies combustion exhaust gas containing CO ₂ discharged from a combustion system to the gas supply path, and circulates the gas exhaust path to the combustion exhaust gas injecting means. A gas injection path for injecting combustion exhaust gas after heat exchange is communicated, gas-liquid separation means is provided in the liquid discharge path, and gas contained in the liquid after heat exchange is separated by the gas-liquid separation means. It is composed. As described above, when the combustion exhaust gas of the combustion system after heat exchange with the heat exchanger is used as the carbon dioxide source, it is not necessary to prepare a special carbon dioxide cylinder or the like as in the conventional case, and the operation cost is also suppressed. Since the gas component mixed in the liquid is separated and removed by the gas-liquid separation means, the hot water supply system and the like are not affected.

  In the above heat exchange system, as described in claim 2, the combustion exhaust gas injecting means may be configured to inject combustion gas into tap water by an ejector method. When such an ejector method is employed, it is not necessary to provide special gas transfer power in the combustion exhaust gas path.

In the heat exchange system, as described in claim 3, the combustion exhaust gas injection means is provided with a micro bubble generator, so that the combustion exhaust gas can be injected as micro bubbles. By making CO ₂ rich combustion exhaust gas into microbubbles, the gas-liquid contact area is expanded, and the surface tension of water makes the pressure inside the bubbles higher than atmospheric pressure, improving the solubility of CO ₂ in water. And the rate of dissolution of carbon dioxide in water can be increased.

In addition, the microbubbles are not easily bonded because the microbubbles are easily repelled, and formation of large bubbles due to the bonding between the microbubbles hardly occurs. Therefore, the dispersibility of CO ₂ in water is improved by making the combustion exhaust gas into microbubbles as described above.

The process flow figure of the heat exchange system of this invention. The process flow figure which expands the heat exchanger 14 shown in FIG. 1 and its peripheral equipment. The process flow figure which shows one example of the conventional cogeneration system. Graph showing the effect of temperature on the CO ₂ solubility in water. The graph which shows the relationship between the bubble diameter and bubble internal pressure in each temperature.

Next, the best mode for carrying out the present invention will be described with reference to the drawings.
FIG. 1 is a process flow diagram showing an example of a cogeneration system using the heat exchanger system of the present invention, and FIG. 2 is an enlarged view of the heat exchanger 14 shown in FIG. 1 and its peripheral devices.
In these drawings, the same parts as those in FIG. 3 are denoted by the same reference numerals, and redundant description is omitted.

  In FIG. 1, the combustion exhaust gas discharged from the SOFC power generation unit 1 is supplied to the heat exchanger 14 through a gas supply path 17 constituted by piping. The combustion exhaust gas after heat exchange discharged from the heat exchanger 14 is discharged from a gas discharge path 18 constituted by piping.

On the other hand, the lower part of the hot water supply tank 11 of the hot water supply unit 10 is replenished with tap water from a pipe 21, and a part of tap water (or hot water) stored in the hot water supply tank 11 is from a liquid supply path 19 constituted by the pipe and the water supply pump 12. The liquid (tap water) that is supplied to the heat exchanger 14 and has undergone heat exchange is discharged from the heat exchanger 14 through a liquid discharge path 20 configured by piping. A combustion exhaust gas injection means 24 is provided in the liquid supply path 19. The combustion exhaust gas injection means 24 uniformly injects combustion exhaust gas containing carbon dioxide into the tap water supplied to the heat exchanger 14 as fine (or fine) bubbles, and the combustion exhaust gas for CO ₂ injection is gas. It is supplied from a gas injection path 25 constituted by a pipe communicating with the discharge path 18.

  As described above, the liquid discharged from the heat exchanger 14 flows out to the liquid discharge path 20, and the gas-liquid separation means 30 is provided in the middle of the liquid discharge path 20, and the gas (gas) mixed in the liquid there. The components are separated. As the gas-liquid separation means 30, a centrifugal separation type, a filter type or the like generally used in this field can be used. The liquid after the gas separation is returned to the upper part of the hot water supply tank 11 as a load facility of the heat exchanger 14 through the liquid discharge path 20 on the outlet side of the gas-liquid separation means 30.

  On the other hand, the gas separated by the gas-liquid separation means 30 flows out from the gas discharge path 31 constituted by piping, but the tip of the gas discharge path 31 communicates with the gas discharge path 18, and the gas discharge It is discharged out of the system together with the flue gas after heat exchange via the path 18.

  In FIG. 2, the heat exchanger 14 is provided with the gas flow path 15 and the liquid flow path 16 as described above. The gas supply path 17 communicates with the inlet side 15a of the gas flow path 15 and the gas is supplied to the outlet side 15b. The discharge path 18 communicates, the liquid supply path 19 downstream of the combustion exhaust gas injection means 24 communicates with the inlet side 16a of the liquid flow path 16, and the gas discharge path 20 communicates with the outlet side 16b.

  The heat exchanger 14 includes a high-temperature gas flow passage 15 and a low-temperature liquid flow passage 16 in the casing. At least one of the gas flow passage 15 and the liquid flow passage 16 is formed of a metal tube having a circular or elliptical cross section. For example, when the liquid flow passage 16 is formed of a tube, the outer space of the tube forms a gas flow passage 15, both flow passages are separated from each other by the metal wall of the tube, and the flue gas and tap water are passed through the metal wall. The mutual heat exchange is performed by indirect contact. Conversely, the gas flow passage 15 may be formed of a tube, and the outer space thereof may be the liquid flow passage 16.

  The combustion exhaust gas injection means 24 in the present embodiment includes an ejector 26 and a microbubble generator 27 as shown in FIG. The combustion exhaust gas supplied from the gas injection path 25 is sucked by the ejector 26 and introduced into the microbubble generator 27. The micro bubble generator 27 is preferably a swirl type or venturi type static mixer type using the discharge pressure of the feed water pump 12 arranged on the upstream side of the combustion exhaust gas injection means 24 because of its small size and simple structure.

  Next, in the present embodiment configured as described above, an embodiment of a method for preventing scale deposition on the water side of the heat exchanger according to the present invention will be described.

(Scale precipitation mechanism)
Scale components include calcium (Ca), magnesium (Mg), silica (Si), etc., among which calcium is the most contained in tap water. The precipitation of calcium carbonate as a scale is closely related to CO ₂ in water.

FIG. 4 is a diagram (graph) illustrating the temperature dependence of the CO ₂ concentration in water. FIG. 4 shows the solubility of CO _{2 in} tap water (mole fraction × 10 6), but here it is calculated from the CO ₂ concentration of 0.04% in general air.

In the CO ₂ solubility in the tap water in FIG. 4, the CO ₂ concentration in water (dissolved CO ₂ concentration) decreases as the water temperature increases. From this, it can be understood that the degassing of dissolved CO ₂ dissolved in water occurs due to the water temperature rising due to the heating of water. For example, the arrow shown in FIG. 4 indicates the amount of CO _{2 that} is not completely dissolved and degassed when the water temperature rises from 20 ° C. to 80 ° C.

Thus, when CO ₂ degassing occurs,
Since the equilibrium state shown by the reaction formula of “CaCO ₃ + CO ₂ + H ₂ O⇔Ca (HCO ₃ ) ₂ ” is shifted and the reverse reaction is promoted, the precipitation of calcium carbonate is accelerated. Therefore, in the heat recovery type heat exchanger 14 accompanied by heat transfer to the water side, CO ₂ is easily degassed from the liquid flow passage inlet side 16a side to the outlet side 16b, and the shift of the equilibrium state occurs. In particular, it is apparent that precipitation of calcium carbonate occurs in the vicinity of the outlet side 16b heated to a water temperature close to 80 ° C.

In this embodiment, therefore, the combustion exhaust gas discharged from the SOFC power generation unit 1 shown in FIG. 1 is injected into the liquid flow passage inlet side 16a via the gas flow passage 17 and the combustion gas injection means 24. The CO ₂ content in the combustion exhaust gas of a general SOFC power generation unit is about 5%, which is extremely rich in CO ₂ compared to the average CO ₂ concentration of 0.04% in the atmosphere. .

  Further, since the combustion exhaust gas passes through the microbubble generator 27 arranged in the combustion gas injection means 24, it is injected into the tap water in the liquid flow passage inlet side 16a in the form of microbubbles. At this time, since the pressure inside the bubble rises in inverse proportion to the radius of the bubble due to the surface tension of the bubble, the pressure inside the bubble becomes higher than the water pressure without using a pressure vessel or a compressor. It becomes possible to improve the solubility of.

FIG. 5 shows a graph of the relationship between the bubble diameter and the bubble internal pressure at each temperature. As shown in FIG. 5, when the bubble diameter is smaller than 10 μm, it is clear that the bubble internal pressure becomes extremely large. In the vicinity of the gas-liquid boundary liquid side of the combustion exhaust gas microbubbles injected into the tap water in the liquid flow passage 16 of the heat exchanger 14 due to the high CO ₂ concentration in the combustion exhaust gas and the effect of increasing the bubble internal pressure due to the microbubbles. , High CO ₂ solubility can be achieved.

In FIG. 4, solubility data from the bubbles (near the gas-liquid boundary liquid side) at each bubble diameter of the SOFC combustion exhaust gas is additionally shown. As can be seen from this data, it is clear that even at a temperature of 80 ° C., extremely high CO ₂ solubility occurs in the vicinity of the bubbles of the combustion exhaust gas.

Furthermore, by making the combustion exhaust gas into microbubbles, the resilience between the microbubbles can be used, and the dispersibility of CO _{2 in} the combustion exhaust gas into water can be improved. Thereby, since the microbubbles ride on the water flow in the liquid flow passage 16 and are efficiently transported to the heat exchange surface of the liquid flow passage 16 as well, the presence distribution of CO ₂ in water can be made uniform. it can. Furthermore micro bubbles can increase the surface area for a fine cell, the CO ₂ bubbles. Therefore, it is possible to further improve the solubility of CO ₂ in water.

Furthermore, the heat exchange amount of injected microbubbles of CO ₂ by the flue gas injection means 24, as by an amount exceeding the saturation solubility of CO ₂ in water, water and 2-phase flow microbubbles undissolved remained It passes through the vessel 14. As a result, even if a difference in the concentration distribution of CO ₂ occurs in the water due to such degassing near the heat transfer surface where CO ₂ is easily degassed by heating, the microbubbles dissolve quickly, so ₂ can be reduced, and the generation of scale can be suppressed.

In summary, the high CO ₂ solubility in the vicinity of the microbubbles can be realized by injecting the combustion exhaust gas microbubbles in the tap water in the liquid flow passage 16. In addition, although CO ₂ deaeration occurs according to the solubility of the bulk conditions as the distance from the micro bubbles is increased, the high dispersibility of the micro bubbles realizes a uniform distribution of CO ₂ in water and a solubility higher than the bulk conditions. be able to.

(Experimental example)
An experiment for confirming the precipitation of calcium carbonate (CaCO ₃ ) inside the heat exchanger 14 was performed using the system shown in FIG. The heat exchanger 14 was constituted by ten circular pipes having a length of 0.15 m and a cross section of 0.05 cm ² as the liquid flow path 16 in the casing, and the gas flow path 15 was used as the outer space.

Tap water having an average calcium content of 20 ppm (simulating high hardness tap water) and an average temperature of 20 ° C. is continuously circulated through the liquid flow passage 16 of the heat exchanger 14 at a flow rate of 2 L / min. The combustion exhaust gas simulation gas (5% CO ₂ ) having a temperature of 300 ° C. is supplied to the side 15a, and the combustion exhaust gas simulation gas flow rate is adjusted so that the liquid temperature at the liquid flow passage outlet side 16b is 80 ° C. did.

Furthermore, the combustion exhaust gas was continuously supplied to the combustion exhaust gas injection means 24 at a flow rate of 20 cc / min (void ratio 1%). At this time, the combustion exhaust gas was injected into the liquid flow passage inlet side 16a as microbubbles having a diameter of several tens of micrometers by the microbubble generator 27 arranged in the combustion exhaust gas injection means 24. After three months of continuous experiments, the heat exchanger 14 was disassembled and the inside was inspected. As a result, almost no precipitation of calcium carbonate (CaCO ₃ ) was found inside the pipe of the liquid flow passage 16 of the heat exchanger, and it was confirmed that it could be operated for a considerably long period of time.

  As mentioned above, although the suitable Example (mode) of this invention was described referring an accompanying drawing, it cannot be overemphasized that this invention is not limited to this example. It will be apparent to those skilled in the art that various changes and modifications can be made within the scope of the claims, and these are naturally within the technical scope of the present invention. It is understood.

  The heat exchange system of the present invention can be applied to a heat recovery gas / liquid heat exchanger system disposed in a hot water supply system that recovers heat from high-temperature combustion exhaust gas discharged from a fuel combustion system and generates hot water.

DESCRIPTION OF SYMBOLS 1 SOFC power generation unit 2 Fuel cell 3 Pre-reformer 4 Evaporator 5 Air preheater 6 Air electrode 7 Fuel electrode

DESCRIPTION OF SYMBOLS 10 Hot water supply unit 11 Hot water supply tank 12 Water supply pump 13 Switching valve 14 Heat exchanger 15 Gas flow path 15a inlet side 15b outlet side 16 Liquid flow path 16a inlet side 16b outlet side 17 Gas supply path 18 Gas discharge path 19 Liquid supply path

20 Liquid discharge passages 21, 22 and 23 Pipes around the hot water tank 11 24 CO ₂ injection means 25 Gas injection passages 26 Ejectors 27 Microbubble generators 30 Gas-liquid separation means 31 Gas discharge passages

Claims (3)

A heat exchanger 14 having a high-temperature gas flow passage 15 and a low-temperature liquid flow passage 16, a gas supply passage 17 for supplying high-temperature gas to an inlet side 15 a of the gas flow passage 15, and an outlet side of the gas flow passage 15 The gas discharge path 18 for discharging the gas after heat exchange flowing out from 15b, the liquid supply path 19 for supplying tap water to the inlet side 16a of the liquid flow path 16, and the heat flowing out from the outlet side 16b of the liquid flow path 16 A heat exchanger system comprising a liquid discharge path 20 for discharging the liquid after replacement to a load facility, and a combustion exhaust gas injection means 24 for injecting combustion exhaust gas into the tap water of the liquid supply path 19,
A gas injection passage 25 for supplying combustion exhaust gas containing CO ₂ discharged from a combustion system to the gas supply passage 17 and injecting the combustion exhaust gas after heat exchange flowing through the gas discharge passage 18 into the combustion exhaust gas injection means 24. A gas-liquid separation unit 30 is provided in the liquid discharge path 20 and is configured to separate gas contained in the liquid after heat exchange by the gas-liquid separation unit 30. Heat exchanger system.   2. The heat exchanger system according to claim 1, wherein the combustion exhaust gas injection means 24 is configured to inject combustion gas into tap water by an ejector method.   The heat exchanger system according to claim 2, wherein a microbubble generator (27) is disposed in the combustion exhaust gas injection means (24).

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