Economizer and boiler

Abstract

Provided is an economizer in which a heat transfer pipe (2) for heating feedwater is arranged within a housing (1) through which exhaust gas flows, the economizer comprising a nozzle (3) that supplies a dropwise condensation promotor to a latent heat recovery section in which condensed water is generated on the surface of the heat transfer pipe. The nozzle (3) is disposed so as to supply the dropwise condensation promotor to a region where the surface temperature of the heat transfer pipe is less than or equal to the thermal decomposition temperature of the dropwise condensation promotor. The dropwise condensation promotor comprises at least one among a sarcosine compound and a long-chain aliphatic amine.

Description

Economizers and boilers

[0001] The present invention relates to an economizer that heats boiler feedwater using exhaust gas from a boiler, and more particularly to an economizer that also heats feedwater using the heat of condensation (latent heat) of steam in the exhaust gas. The present invention also relates to a boiler equipped with such an economizer.

[0002] As an economizer that heats feedwater using exhaust gas from a boiler or the like, there is one that heats feedwater using sensible heat of the exhaust gas and further heats the feedwater using latent heat (heat of condensation) (Patent Documents 1 and 2).

[0003] Patent Document 1 describes providing a means for changing the temperature of exhaust gas between a dry economizer for recovering sensible heat and a condensing economizer for recovering latent heat.

[0004] Patent Document 2 describes an economizer that uses fin tubes as heat transfer tubes, recovers sensible heat with the upstream fin tubes, and recovers latent heat with the downstream fin tubes, in which the spacing between the fins of the downstream fin tube is increased to make it easier for condensed water to fall.

[0005] The method of liquefying and condensing steam by bringing it into contact with a solid wall whose temperature is lower than the saturation temperature includes film condensation, in which the condensate forms a continuous liquid film on the wall surface, and dropwise condensation, in which the condensate forms a large number of droplets without wetting the wall surface. Compared to film condensation, dropwise condensation has a heat transfer coefficient several to several tens of times higher, so dropwise condensing agents are sometimes used to condense steam (Patent Document 3).

[0006] JP 2012-57860 A JP 2013-108643 A Japanese Patent No. 6506865 A

[0007] When a drop-like condensation promoter is added to the exhaust gas introduced into the economizer, depending on the temperature conditions, much of the drop-like condensation promoter may be decomposed by the heat of the exhaust gas, making it difficult to improve the heat recovery efficiency in the latent heat recovery section of the economizer.

[0008] An object of the present invention is to improve the heat recovery efficiency in the latent heat recovery section of an economizer.

[0009] The economizer of the present invention is characterized in that a heat transfer tube for heating feedwater is arranged in an exhaust gas flow section through which exhaust gas flows, and the economizer further comprises a dropwise condensation promoter supply means for supplying a dropwise condensation promoter to a latent heat recovery section where condensed water is formed on the surface of the heat transfer tube.

[0010] In one aspect of the present invention, the dropwise condensation promoter supply means is arranged to supply the dropwise condensation promoter to a region where the temperature of the heat transfer tube surface is equal to or lower than the thermal decomposition temperature of the dropwise condensation promoter.

[0011] In one aspect of the present invention, the dropwise condensation promoter supplying means has a supply amount control means for controlling the amount of dropwise condensation promoter supply in accordance with the amount of fuel used by the boiler that generates the exhaust gas.

[0012] In one embodiment of the present invention, the dropwise condensation promoter comprises at least one of a sarcosine compound and a long-chain aliphatic amine.

[0013] The boiler of the present invention is equipped with the economizer of the present invention.

[0014] In the economizer of the present invention, by adding a dropwise condensation promoter to the latent heat recovery section of the economizer, dropwise condensation of water vapor is promoted in the latent heat recovery section, and the heat recovery efficiency in the latent heat recovery section is improved.

[0015] 2 is a cross-sectional view of an economizer according to an embodiment, and FIG. 3 is a cross-sectional view of an economizer according to another embodiment, and FIG. 4 is a perspective view of a heat transfer tube of the economizer of FIG.

[0016] Hereinafter, an embodiment will be described with reference to the drawings.

[0017] 1 is a schematic cross-sectional view of an economizer installed in the exhaust gas duct of a boiler. This economizer has a metal heat transfer tube 2 disposed within a housing 1. The combustion exhaust gas from the boiler flows into the housing 1 through an inlet 1a at one end thereof, comes into contact with the heat transfer tube 2 to lower its temperature, and then flows out of an outlet 1b at the other end thereof.

[0018] Boiler feed water is introduced into the heat transfer tube 2 from the downstream side (outlet 1b side) in the direction of exhaust gas flow, and heated feed water is taken out from the upstream side in the direction of exhaust gas flow.

[0019] The upstream side of the heat transfer tube 2 in the gas flow direction is the sensible heat recovery section, and the downstream side of the gas flow direction is the latent heat recovery section. This latent heat recovery section is a region where the surface temperature of the heat transfer tube 2 (hereinafter sometimes referred to as the heat transfer tube metal temperature) is 100°C or less, and a chemical injection nozzle 3 is provided to supply a dropwise condensation promoter as a chemical to this region. Supplying the dropwise condensation promoter from this chemical injection nozzle 3 improves the heat recovery efficiency in the latent heat recovery section.

[0020] FIG. 2 is a cross-sectional view of an economizer according to another embodiment, and FIG. 3 is a perspective view of a heat transfer tube thereof.

[0021] A heat transfer tube 6 is installed in a housing 5 installed in the exhaust gas flue of the boiler. As shown in Figure 3, the heat transfer tube 6 is a fin tube having a tubular heat transfer tube body 6a and disk-shaped fins 6b provided so as to spread out in the radial direction from the outer circumferential surface of the heat transfer tube body 6a.

[0022] The exhaust gas flows into the housing 5 through an exhaust gas inlet 5a provided at one end of the housing 5, comes into contact with the heat transfer tube 6 to lower its temperature, and then flows out through an exhaust gas outlet 5b provided at the other end of the housing 5.

[0023] The heat transfer tubes 6 are arranged in multiple rows and columns so that the tube axis direction of the heat transfer tube bodies 6a is perpendicular to the direction connecting the inlet 5a and the outlet 5b.

[0024] In this embodiment as well, a chemical injection nozzle 7 is provided so as to supply droplets of condensation accelerator to the latent heat recovery section downstream in the gas flow direction.

[0025] By supplying the droplet-like condensation promoter from this chemical injection nozzle 7 to the latent heat recovery section, the heat recovery efficiency in the latent heat recovery section is improved.

[0026] When the thermal decomposition temperature of the dropwise condensation promoter is lower than 100° C., it is preferable that the chemical injection nozzles 3, 7 are provided so as to supply the dropwise condensation promoter to a region of the latent heat recovery section where the metal temperatures of the heat transfer tubes 2, 6 are equal to or lower than the thermal decomposition temperature. Conversely, when the thermal decomposition temperature of the dropwise condensation promoter is higher than 100° C., the chemical injection nozzles 3, 7 may be installed so as to supply the dropwise condensation promoter to the sensible heat recovery section upstream of the latent heat recovery section.

[0027] Furthermore, a plurality of chemical injection nozzles may be installed at different installation positions. In this case, the chemical injection nozzle to be used for chemical injection may be switched depending on the operating conditions of the boiler and the thermal decomposition temperature of the dropwise condensation promoter.

[0028] In the present invention, a dropwise condensation promoter may be added to the exhaust gas after examining in advance the distribution ratio for each temperature (concentration of the dropwise condensation promoter in the gas phase / concentration in the aqueous phase), the adsorption characteristics for each metal, and the decomposition characteristics for each temperature, so that the dropwise condensation promoter will adsorb to the heat transfer tube surface (metal surface) even when the pH of the condensed water is on the acidic side. For example, an optimal dropwise condensation promoter is selected as shown in Table 1 below, and one that maximizes the latent heat recovery promotion effect is selected. In addition to the fuels listed in Table 1, ammonia co-firing, in which ammonia is partially used, may also be used.

[0029] In a typical small boiler, for example, the exhaust gas temperature at the economizer inlet is 200 to 300°C, and after passing through the temperature range of the latent heat recovery section, the exhaust gas temperature at the economizer outlet drops to approximately the temperature of the boiler feedwater. In this case, it is preferable to select a dropwise condensation promoter that does not decompose significantly even at 200 to 300°C depending on the material of the heat transfer tube surface. For example, the dropwise condensation promoter is preferably at least one of a sarcosine compound and a long-chain aliphatic amine.

[0030] In gas-fired boilers, the sulfur content of fuel is generally low, resulting in low condensate acidity. Therefore, not only stainless steel heat transfer tubes but also carbon steel heat transfer tubes may be used outside the condensate generation area (latent heat recovery section). The condensate generation area may also change due to fluctuations in feedwater temperature. Therefore, in the case of economizers equipped with carbon steel heat transfer tubes, it is preferable to use oleyl propylene diamine, which is easily adsorbed to carbon steel under weakly acidic conditions, as a dropwise condensation promoter. Furthermore, to address the generation of condensate on stainless steel heat transfer tubes, it is more preferable to use a sarcosine compound in addition to oleyl propylene diamine. In this case, it can be economically applied without the use of an alkaline agent, but an alkaline agent may be added depending on the quality of the condensate. The alkaline agent may be a volatile amine, NH ３ , NaOH, phosphates, etc. The volatile amine may be, for example, ethanolamine, which is a low volatility amine that is soluble in condensed water.

[0031] In heavy oil-fired boilers, sulfuric acid is generated due to the sulfur content in the fuel, which tends to increase the acidity of the condensed water. Therefore, corrosion-resistant stainless steel (SUS316L) and other materials are used for the heat transfer tubes. In the case of economizers equipped with stainless steel heat transfer tubes, sarcosine compounds are preferred as the dropwise condensation promoter. When the acidity increases due to the fuel properties or when a dropwise condensation promoter other than a sarcosine compound is used, the pH may be adjusted to the optimum range by using the alkaline agent described above in combination.

[0032] In hydrogen boilers with high flame temperatures, thermal nox is generated from atmospheric nitrogen, which is then oxidized to form high concentrations of nitric acid, resulting in high condensate acidity. Therefore, corrosion-resistant stainless steel and other materials are used for heat transfer tubes. In this case, a sarcosine compound is preferred as the dropwise condensation promoter. Because an extremely low pH in the condensate can change its adsorption characteristics, it is preferable to use the alkaline agent in combination with the condensate to maintain the pH of the condensate within the optimal range for the dropwise condensation promoter used.

[0033] In any of the boilers described above, the alkaline agent may be added together with the dropwise condensation promoter, or may be added separately, or may be added by spraying boiler blow water containing the alkaline agent.

[0034]

[0035] The surface of the heat transfer tube may be subjected to a water-repellent treatment, such as coating the surface of the heat transfer tube with a highly water-repellent material such as polytetrafluoroethylene (PTFE) which is a fluororesin or silicone to form a resin layer, or physical treatment to give the surface of the heat transfer tube an uneven structure resembling lotus leaves.

[0036] The dropwise condensation promoter used in the present invention preferably contains at least one of a sarcosine compound and a long-chain aliphatic amine.

[0037] Preferred sarcosine compounds include N-oleoyl sarcosine, N-lauroyl sarcosine, N-stearoyl sarcosine, and N-cocoyl sarcosine.

[0038] As the long-chain aliphatic amine, octadecylamine, oleylamine, oleylpropylenediamine, etc. are preferred.

[0039] The amount of the dropwise condensation promoter added may be sufficient to achieve the effect of promoting dropwise condensation on the surface of the heat transfer tube in the latent heat recovery section, and although this amount varies depending on the type of dropwise condensation promoter, it is usually preferably about 0.01 to 10 mg / kg, and particularly about 0.1 to 1 mg / kg, of the amount of condensed water produced in the exhaust gas. The amount of the dropwise condensation promoter added may also be adjusted according to (for example, proportional to) the amount of fuel used by the boiler.

[0040] The dropwise condensation promoter may be added intermittently or continuously, but is preferably added continuously.

[0041] The dropwise condensation promoter may be dissolved or dispersed in a solvent such as water, methanol, ethanol, or isopropanol and then supplied to the chemical injection nozzle 3 or 7. For example, a solution of the dropwise condensation promoter is stored in a tank, and the dropwise condensation promoter solution is supplied to the chemical injection nozzle 3 or 7 by a pump. A spray tip may be provided at the tip of the nozzle to ensure uniform supply by the chemical injection nozzle. When an orifice-type spray tip is provided, the dropwise condensation promoter solution may be diluted separately and supplied to the economizer in order to adjust the supply pressure and flow rate of the dropwise condensation promoter solution to ranges suitable for the spray tip. The dropwise condensation promoter solution may be diluted with filtered boiler blowdown water or economizer condensate and pressurized to a state suitable for spraying. As described above, the amount of liquid supplied by this pump may be controlled according to the amount of fuel used by the boiler.

[0042] Although the present invention has been described in detail using specific embodiments, it will be apparent to those skilled in the art that various modifications can be made without departing from the spirit and scope of the present invention. This application is based on Japanese Patent Application No. 2023-127187, filed on August 3, 2023, and is incorporated by reference in its entirety.

[0043] 1, 5 Housing 2, 6 Heat transfer tube 3, 7 Chemical injection nozzle

Claims

1. An economizer in which a heat transfer tube for heating feedwater is arranged in an exhaust gas flow section through which exhaust gas flows, the economizer further comprising a dropwise condensation promoter supply means for supplying a dropwise condensation promoter to a latent heat recovery section where condensation water is formed on the surface of the heat transfer tube.

2. An economizer according to claim 1, wherein said dropwise condensation promoter supplying means is arranged to supply the dropwise condensation promoter to an area where the temperature of the heat transfer tube surface is equal to or lower than the thermal decomposition temperature of said dropwise condensation promoter.

3. An economizer according to claim 1, wherein said dropwise condensation promoter supplying means has a supply amount control means for controlling the amount of dropwise condensation promoter supplied in accordance with the amount of fuel used in the boiler which generates said exhaust gas.

4. The economizer according to any one of claims 1 to 3, wherein the dropwise condensation promoter comprises at least one of a sarcosine compound and a long-chain aliphatic amine.

5. A boiler equipped with the economizer according to any one of claims 1 to 4.

Images