Boiler System

Abstract

A boiler system 100 includes: a boiler 10 that combusts fuel gas GF to heat supply water W1; an exhaust line 16 that exhausts exhaust gas GE from the boiler; a circulation line 20 that branches from the exhaust line, has connected thereto an oxygen supply line 30 and an air supply line 26, and supplies air or oxygen gas together with the exhaust gas GE to the boiler as combustion-supporting gas GS. The oxygen supply line 30 includes a main line 30A provided with an ON / OFF valve 32, and a sub line 30B provided with an adjustable opening value 34, and is configured to be capable of adjusting an oxygen concentration in a mixed gas of the oxygen gas and the exhaust gas and supplying the mixed gas to the boiler.

Description

[Technical Field]

[0001] The present invention relates to a boiler system, and more particularly to a boiler system that can increase the CO2 concentration in flue gas discharged from a small boiler. [Background technology]

[0002] In recent years, there has been a demand for a significant reduction in CO2 (carbon dioxide) gas emissions, which are considered to be a factor in global warming. To this end, various systems have been developed to separate and capture CO2 from exhaust gases emitted from combustion equipment such as boilers using methods such as chemical adsorption and solid absorption. However, the CO2 concentration in exhaust gases is typically low, at around 10%, which poses a problem of low capture efficiency for CO2 separation and capture systems.

[0003] In response to this, a method has been proposed for increasing the CO2 concentration in exhaust gas by using oxygen combustion instead of conventional air combustion in the boilers installed in coal-fired power plants, and demonstration tests of CCS (Carbon dioxide Capture and Storage) using oxygen combustion are also being conducted (Non-Patent Document 1 and Patent Document 1).

[0004] Oxyfuel combustion is a method of burning fuel by supplying oxygen gas instead of air as an oxidizing agent or combustion-supporting gas. With oxyfuel combustion, the exhaust gas contains almost no nitrogen components, so the main component of the exhaust gas (for example, more than 90% in the case of coal combustion) can be CO2 gas. Therefore, if oxyfuel combustion is used, it becomes easier to separate and capture CO2 by cooling the exhaust gas, etc.

[0005] However, when oxygen combustion is performed using high-concentration oxygen gas, the flame temperature becomes too high, which may damage the burner or boiler. For this reason, the oxyfuel combustion boiler equipment described in Non-Patent Document 1 and Patent Document 1 circulates part of the exhaust gas, mixes the produced oxygen with recirculated gas (mainly CO2 gas), and supplies the combustion-supporting gas with a reduced oxygen concentration to the boiler.

[0006] In addition, in the above-mentioned coal-fired power plant, in order to perform oxyfuel combustion, an oxygen production unit is installed near the boiler to produce high-purity oxygen from the air taken in by cryogenic separation or the like. However, if electricity is required to produce oxygen, it is possible to continue taking in air and performing air combustion as in the past until power generation begins.

[0007] Patent Document 2 discloses a boiler plant for coal-fired power generation that can switch between oxyfuel combustion and air-fuel combustion. In this boiler plant, air combustion can be performed by supplying air instead of oxygen gas and circulating exhaust gas when the boiler is started up before switching to oxyfuel combustion. Once oxygen gas can be produced using the electricity obtained by air-fuel combustion, the air supply system can be closed and oxygen gas and circulating exhaust gas can be supplied to the boiler, allowing for easy switching to oxyfuel combustion of coal.

[0008] Furthermore, Patent Document 2 describes a technology for controlling the oxygen concentration in a combustion-supporting gas by adjusting the flow rate of the circulating exhaust gas, thereby controlling the ratio of the oxygen amount to the fuel amount to a desired setting. The oxygen concentration can be calculated from each gas flow rate command value as the ratio of the oxygen flow rate to the total gas flow rate. Furthermore, by setting the feedwater temperature based on the calculated oxygen concentration, it is possible to control the temperature of the water and steam flowing through the heat exchanger within an allowable range and ensure safe operation.

[0009] In this way, by performing oxyfuel combustion to increase the CO2 concentration in the flue gas and then capturing the CO2, it is possible to capture and store CO2 more efficiently. Patent Documents 3 and 4 describe specific flue gas CO2 separation and capture devices. Patent Document 5 describes a technology for adjusting the CO2 concentration in flue gas from a relatively small boiler. However, this flue gas is used to neutralize boiler effluent, and the CO2 concentration is simply adjusted by adjusting the amount of air mixed in before being introduced into the boiler effluent. [Prior art documents] [Non-patent literature]

[0010] [Non-Patent Document 1] Terutoshi Uchida, "Towards the realization of a CO2-free coal-fired power plant using oxyfuel combustion - Operational status of the Callide oxyfuel combustion project -", Proceedings of the Thermal and Nuclear Power Generation Conference 2013, received on September 18, 2013, pp. 101-106 [Patent documents]

[0011] [Patent Document 1] Patent No. 6471485 [Patent Document 2] Patent No. 5130145 [Patent Document 3] Patent No. 5351816 [Patent Document 4] Patent No. 3025566 [Patent Document 5] Japanese Patent Application Publication No. 11-244654 Summary of the Invention [Problem to be solved by the invention]

[0012] As mentioned above, in large facilities such as coal-fired power plants, attempts are being made to significantly increase the CO2 concentration in the exhaust gas by performing oxyfuel combustion using oxygen gas produced from the atmosphere and recycled exhaust gas, thereby achieving more efficient CO2 capture and storage. In such a system, nitrogen is removed from the combustion-supporting gas in advance, so the amount of nitrogen oxides (NO x ) can be suppressed.

[0013] However, unlike large facilities such as power plants, small boilers such as small once-through boilers and hot water boilers are required to have larger output fluctuations during use, and the combustion control speed must be orders of magnitude faster than large facilities.For this reason, simply adopting the same oxyfuel combustion boiler configuration as conventional large facilities could result in operational problems such as misfires in small boiler facilities, even if the exhaust gas CO2 concentration could be increased.

[0014] The present invention has been made to solve the above-mentioned problems, and its object is to provide a boiler system that enables oxyfuel combustion while achieving stable combustion and can improve the CO2 concentration in the exhaust gas, even when using a small boiler that requires relatively high responsiveness in combustion control. [Means for solving the problem]

[0015] A boiler system according to an embodiment of the present invention includes a boiler that heats feedwater by burning fuel gas, an exhaust line that exhausts exhaust gas from the boiler, and a circulation gas line that branches off from the exhaust line, to which an oxygen supply line and an atmospheric air supply line are connected, and which can supply the exhaust gas to the boiler together with at least one of atmospheric air and oxygen gas as a combustion-supporting gas, wherein the atmospheric air supply line is provided with a damper for controlling atmospheric air suction, and the oxygen supply line includes a main line provided with an on / off valve and a sub-line provided with a variable-opening valve, and is configured such that by closing the damper of the atmospheric air supply line and controlling the opening and closing of the on / off valve of the oxygen supply line and the aperture of the variable-opening valve, the oxygen concentration in the mixed gas of oxygen gas from the oxygen supply line and the exhaust gas can be adjusted and supplied to the boiler as a combustion-supporting gas.

[0016] In one embodiment, the boiler system further includes a combustion-supporting gas fan located near the boiler for sending the combustion-supporting gas from the circulation gas line to the boiler, and an oxygen sensor for measuring the oxygen concentration in the combustion-supporting gas sent from the combustion-supporting gas fan to the boiler, and is configured so that the adjustable valve is feedback-controlled based on the output of the oxygen sensor.

[0017] In one embodiment, the outlet of the combustion-supporting gas fan is connected to a burner wind box provided in the boiler, the oxygen sensor is configured to measure the oxygen concentration of the gas inside the burner wind box, and the adjustable valve is controlled based on the output of the oxygen sensor so that the oxygen concentration of the gas inside the burner wind box is maintained at 20 to 22%.

[0018] In one embodiment, the main line of the oxygen supply line is composed of multiple lines connected in parallel, each line having an on / off valve, and is configured so that oxygen gas can be supplied from the main line at multiple flow rates by controlling the on / off valves of the multiple lines.

[0019] In one embodiment, a manual valve is provided on at least one of the primary side or secondary side of the adjustable valve provided in the sub-line of the oxygen supply line, so that 80 to 90% of the theoretical amount of oxygen required for combustion can be supplied via the main line, and 0 to 50% of the theoretical amount of oxygen required for combustion can be supplied via the sub-line.

[0020] In one embodiment, the boiler system further includes an economizer that preheats water supplied to the boiler using the heat of exhaust gas from the boiler, and a gas cooler that cools the exhaust gas downstream of the economizer, the exhaust line being configured to exhaust the exhaust gas cooled by the gas cooler, and the gas cooler being configured to cool the outlet temperature of the exhaust gas to 30°C to 50°C.

[0021] In one embodiment, the on / off valve provided in the main line of the oxygen supply line is a solenoid valve, and the adjustable valve provided in the sub-line of the oxygen supply line is a motor-operated valve.

[0022] In one embodiment, the fuel gas is either city gas or LP gas, and the boiler is either a once-through boiler or a hot water boiler. [Effects of the Invention]

[0023] According to the boiler system according to the embodiment of the present invention, even when a small boiler such as a once-through boiler or a hot water boiler is used, it is possible to increase the CO2 concentration in the exhaust gas while realizing stable combustion. [Brief explanation of the drawings]

[0024] [Figure 1] 1 is a schematic diagram illustrating a boiler system according to an embodiment of the present invention. [Figure 2] FIG. 1 illustrates one aspect of an oxygen supply line according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0025] Hereinafter, embodiments of the present invention will be described with reference to the drawings, but the present invention is not limited to the following embodiments.

[0026] 1 shows a boiler system 100 according to an embodiment of the present invention. The boiler system 100 burns a fuel gas GF. to make Burner 11 Rubo The boiler 10 includes an economizer 12 through which exhaust gas GE from the boiler 10 passes, a gas cooler 14 for cooling the exhaust gas GE that has passed through the economizer 12, and an exhaust line 16 for exhausting the exhaust gas GE that has been cooled by the gas cooler 14.

[0027] The boiler 10 used in this embodiment is a relatively small-sized boiler, such as a once-through boiler or hot water boiler. The fuel gas GF used is, for example, city gas (or gas mainly composed of methane) or LP gas (or gas mainly composed of propane or butane). The boiler system 100 is configured to generate and provide steam or hot water using the relatively small-sized boiler 10, and is installed in various factories, residences, accommodation facilities, hot spring facilities, etc., and can provide hot water or steam at temperatures appropriately set according to the application.

[0028] The boiler 10 can generate hot water and steam by heating the feedwater W1 to the boiler 10 by burning the fuel gas GF using the burner 11. The economizer 12 can use the heat of the exhaust gas GE from the boiler 10 to preheat the feedwater W1 before supplying it to the boiler 10, thereby reducing the consumption of the fuel gas GF.

[0029] In this embodiment, the gas cooler 14 is configured to cool the exhaust gas GE by exchanging heat with the supplied cooling water W2 using various heat exchangers (for example, a plate heat exchanger or a tubular heat exchanger). The gas cooler 14 is configured to be able to cool the exhaust gas until the outlet temperature T1 of the exhaust gas reaches, for example, about 30 to 50°C, particularly about 40°C. This allows the steam contained in the exhaust gas to be drained in the gas cooler 14, thereby obtaining dry exhaust gas.

[0030] The blowdown water from the boiler 10, the drain from the economizer 12, and the drain from the gas cooler 14 are sent to the neutralization device 18. The neutralization device 18 neutralizes the wastewater using various methods, adjusts the pH value to comply with industrial wastewater standards, and then discharges it outside.

[0031] The exhaust line 16 is provided to release the cooled exhaust gas GE into the atmosphere from a flue 16A or to transfer it to a CO2 capture facility via a CO2 capture line 16B. As will be described later, the boiler system 100 can increase the CO2 concentration in the exhaust gas by performing oxyfuel combustion, making it possible to efficiently capture CO2 from the exhaust gas sent via the CO2 capture line 16B. Although not shown, the exhaust line 16 may be provided with an optional exhaust gas treatment device such as a bag filter, if necessary.

[0032] In the boiler system 100 of this embodiment, a circulation gas line 20 branching off from the exhaust line 16 is connected to the exhaust line 16. The circulation gas line 20 is configured to circulate the exhaust gas GE cooled by the gas cooler 14 to the boiler 10 using a forced draft fan (combustion-supporting gas fan) 22 provided near the boiler. The circulation gas line 20 is provided with a circulation gas intake damper 24 driven by a motor M, and by adjusting the opening of the circulation gas intake damper 24, the exhaust gas GE can be supplied to the boiler 10 at any flow rate.

[0033] In addition, an air supply line 26 and an oxygen supply line 30 are connected to the circulation gas line 20. This allows the boiler 10 to be supplied with air AIR, oxygen gas O2, and / or exhaust gas GE drawn in from the exhaust line 16 as a combustion-supporting gas GS. In this configuration, the boiler 10 can perform both air combustion and oxygen combustion.

[0034] In the illustrated embodiment, the atmospheric air supply line 26 is connected upstream of the oxygen supply line 30, but the oxygen supply line 30 may be connected upstream instead. The atmospheric air supply line 26 and the oxygen supply line 30 do not necessarily need to be connected at different locations, and may be connected using a common line at the same connection point of the circulation gas line 20, branched off upstream from the common line, and arranged in parallel.

[0035] The atmospheric air supply line 26 and the oxygen supply line 30 may be connected in any manner as long as they communicate with the circulation gas line 20 between the circulation gas intake damper 24 and the boiler 10. In this embodiment, a forced draft fan 22 is used as a combustion-supporting gas fan provided near the boiler to form a gas flow in the circulation gas line 20 toward the boiler, but instead of or in addition to this, another blower (such as a forced draft fan or an induced draft fan) may be provided in the circulation gas line 20. Also, any blower may be provided in the exhaust line 16 as needed.

[0036] The configuration of the circulation gas line 20 will be described in more detail below.

[0037] An air intake damper 28 driven by a motor M is provided in the air supply line 26 connected to the circulation gas line 20. By adjusting the opening of this air intake damper 28, the air (air) can be supplied to the boiler 10 at any flow rate.

[0038] In this way, air whose flow rate can be controlled can be supplied as the combustion supporting gas GS to the boiler 10 via the circulation gas line 20, so that the boiler can be operated by air combustion when it is difficult to stably perform oxyfuel combustion, such as during boiler startup. During the period when air combustion is being performed, air is supplied to the boiler 10 at a controlled flow rate via the air intake damper 28, the opening of which is adjusted, while the circulation gas intake damper 24 and the oxygen supply line 30 are normally kept closed.

[0039] When using city gas 13A as fuel gas, air combustion using air from the atmospheric supply line 26 results in a combustion exhaust gas containing approximately 10 vol% CO2, with most of the remainder consisting of nitrogen gas and water vapor. Therefore, when attempting to separate and capture CO2 from the exhaust gas in air combustion, the efficiency is low and the capture equipment becomes excessively large, resulting in a cost-effective approach. Therefore, after the boiler is started, combustion stabilizes, and a stable circulating supply of exhaust gas GE becomes possible, it is preferable to switch from air combustion to oxygen combustion to improve the CO2 concentration in the exhaust gas.

[0040] The oxygen supply line 30 used for oxyfuel combustion is composed of a main line 30A and a sub-line 30B connected in parallel. In the illustrated embodiment, the main line 30A and the sub-line 30B are connected to the same oxygen supply source, but they may be connected to separate oxygen supply sources. As the oxygen supply source, for example, oxygen gas generated from air by pressure swing adsorption (PSA) can be used.

[0041] An on / off valve 32 is provided in the main line 30A of the oxygen supply line 30, while a variable aperture valve (typically a proportional valve) 34 is provided in the sub-line 30B. In this embodiment, the on / off valve 32 is configured as an electromagnetic valve (solenoid valve S), and the variable aperture valve 34 is configured as an electrically operated valve driven by a motor M. The on / off valve 32 may be configured as, for example, an air-operated valve (AOV).

[0042] In this embodiment, the flow rate of oxygen gas in each line is controlled so that the main line 30A can supply 80 to 90% of the theoretical combustion oxygen amount required for combustion that is compatible with the components and flow rate of the fuel gas GF, and the sub-line 30B can supply 0 to 50% of the theoretical combustion oxygen amount. The oxygen gas supplied from the oxygen supply line 30 is mixed with the flue gas GE introduced by opening the circulating gas intake damper 24, and is supplied to the boiler 10 as combustion-supporting gas GS with an adjusted oxygen concentration.

[0043] In addition, in this embodiment, the sub-line 30B is composed of a single line equipped with a single adjustable valve 34, whereas the main line 30A is composed of multiple parallel lines, each equipped with an on / off valve 32.

[0044] Fig. 2 shows an example in which main line 30A is composed of three lines. As shown in Fig. 2, main line 30A is composed of lines 301, 302, and 303 connected in parallel, and on / off valves (here, solenoid valves) 321, 322, and 323 are provided on lines 301, 302, and 303, respectively. The rated open flow rates (or CV values: the flow coefficient of the fluid passing through the open valve when the primary pressure and secondary pressure values ​​are at specified values) of on / off valves 321, 322, and 323 may be the same or different.

[0045] In this configuration, multi-stage oxygen flow rate control that matches the boiler's combustion pattern can be instantly achieved by controlling the opening and closing of on-off valves 321, 322, and 323. For example, in the case of four-position control that controls the combustion pattern from 0% to 20% to 50% to 100%, 0% control can be achieved with all on-off valves closed, 20% control with only on-off valve 321 open, 50% control with on-off valves 321 and 322 open, and 100% control with all on-off valves 321, 322, and 323 open. In each combustion pattern, the total amount of oxygen supplied through each line is adjusted to 80 to 90% of the theoretical combustion oxygen amount, as described above, in accordance with fluctuations in the fuel gas flow rate.

[0046] Although the above describes an embodiment in which three lines with on / off valves are used to perform four-position control, it goes without saying that two or four or more lines with on / off valves may be used depending on the combustion pattern. Furthermore, if two-stage control (0%-100%) of the flow rate in the main line 30A is sufficient, the main line 30A may also be configured with a single line, similar to the sub-line 30B, as shown in FIG. 1.

[0047] In the main line 30A, each solenoid valve constituting the on / off valve 32 has a sufficiently high response rate compared to a motor-driven electrically operated valve (free-opening valve 34). Therefore, the opening of each solenoid valve can be controlled quickly to match the combustion pattern, thereby making it possible to supply oxygen gas at a flow rate that prevents poor combustion. In addition, since the flow rate is controlled by providing on / off valves 32 on multiple lines and controlling their opening and closing, instantaneous switching between high and low flow rates can be realized relatively easily.

[0048] In particular, small boilers often adjust their output in a very short time (for example, 2 to 3 seconds, often within 5 seconds) to meet heat demands. Furthermore, because combustion is performed using a high-power burner in a small combustion chamber, the amount of oxygen required for fuel combustion can change rapidly with each output change. Therefore, even a slight delay in the supply of the required amount of oxygen can instantly lead to incomplete combustion, oscillating combustion, or even misfires. In contrast, oxygen is supplied from the main line 30A, which, while providing stepwise flow control, is highly responsive and can adjust the flow rate from low to high, ensuring the supply of a minimum amount of oxygen that will prevent poor combustion. Therefore, the boiler system 100 can prevent misfires and poor combustion, even in applications where small boilers experience significant output fluctuations.

[0049] Furthermore, a manual valve 38 may be provided on at least one of the upstream and downstream sides of the adjustable valve 34 in the sub-line 30B of the oxygen supply line 30. FIG. 1 shows an example in which the manual valve 38 is provided on the upstream side of the adjustable valve 34, and FIG. 2 shows an example in which the manual valve 38 is provided on the downstream side of the adjustable valve 34. The manual valve 38 thus arranged can be used to adjust the maximum control flow rate in the sub-line 30B. By using the manual valve 38 to preset the amount of oxygen to be introduced relative to the set aperture of the circulating gas intake damper 24, the proportion of the circulating gas amount can be adjusted.

[0050] To finely adjust the amount of oxygen, a sub-line 30B having a variable valve 34 is used. This allows fine adjustment of the total flow rate of oxygen gas so that the oxygen concentration in the combustion-supporting gas GS formed by the mixture of oxygen gas O2 from the oxygen supply line 30, the exhaust gas from the exhaust line 16, and GE is stably maintained at a ratio of, for example, about 21% (e.g., 20 to 22%), which is equivalent to the oxygen concentration in the atmosphere.

[0051] In addition, in order to reduce the amount of exhaust gas itself, taking into consideration oxygen-enriched combustion in which the oxygen concentration is set to 21% or more, the adjustable valve 34 may be set to a CV value (or rated flow value) that can supply up to approximately half of the required amount of oxygen.

[0052] 1, the boiler system 100 of this embodiment is equipped with an oxygen sensor 36a for measuring the oxygen concentration of the combustion-supporting gas GS supplied to the burner 11. More specifically, the oxygen sensor 36a is attached to the burner wind box 13 to which the outlet of the forced draft fan 22 is connected, and is installed so as to measure the oxygen concentration of the gas inside the burner wind box 13. For example, a galvanic cell type sensor is preferably used as the oxygen sensor 36a, but various types of sensors can be used as long as they are capable of measuring the oxygen concentration.

[0053] An oxygen concentration control circuit 36b is connected to the oxygen sensor 36a. The oxygen concentration control circuit 36b is configured to perform feedback control of the adjustable valve 34 provided in the sub-line 30B of the oxygen supply line 30 based on the output of the oxygen sensor 36a.

[0054] For example, if the set oxygen concentration is 20 to 22%, and the oxygen concentration output by the oxygen sensor 36a is lower than 20%, the aperture of the adjustable valve 34 is opened by an amount of operation corresponding to the difference, thereby increasing the oxygen concentration. On the other hand, if the oxygen concentration output by the oxygen sensor 36a is higher than 22%, the aperture of the adjustable valve 34 is closed by an amount of operation corresponding to the difference, thereby decreasing the oxygen concentration. In this way, by using the oxygen concentration control circuit 36b to feedback-control the adjustable valve 34 so that the difference between the set concentration and the measured concentration approaches zero, it is possible to continuously supply the combustion-supporting gas GS having the desired set oxygen concentration to the burner 11. It goes without saying that the set oxygen concentration may be a constant value (for example, 21%).

[0055] In this way, the boiler system 100 can supply the combustion-stimulating gas GS, which is a mixture of oxygen gas and exhaust gas (mainly CO2), to the boiler 10 via the circulation gas line 20, thereby enabling stable oxy-combustion. During the period when oxy-combustion is being performed, the exhaust gas mixed with oxygen gas is supplied to the boiler 10 as the combustion-stimulating gas GS, while the atmospheric air suction damper 28 is normally closed and the atmospheric air supply line 26 is maintained in a closed state.

[0056] When switching from air combustion to oxyfuel combustion, the circulating gas intake damper 24 and the on / off valve 32 in the main line 30A are opened in synchronization with the closing of the atmospheric air intake damper 28. At the same time, the oxygen concentration is adjusted by adjusting the aperture of the variable aperture valve 34 in the sub-line 30B. The boiler system 100 is configured to close the damper 28 in the atmospheric air supply line 26 and control the opening and closing of the on / off valve 32 in the oxygen supply line 30 and the aperture of the variable aperture valve 34, thereby adjusting the oxygen concentration in the mixed gas of oxygen gas O2 and flue gas GE from the oxygen supply line, and then supplying this to the boiler 10 as combustion-supporting gas GS to perform oxyfuel combustion.

[0057] Therefore, the boiler system 100 can smoothly transition from air combustion to oxyfuel combustion when the boiler is started up, and after switching to oxyfuel combustion, the oxygen supply amount can be quickly changed to match the output change characteristics of the small boiler, thereby preventing poor combustion such as incomplete combustion and misfires, and emitting exhaust gas with a high CO2 concentration.

[0058] Next, the configuration of the gas cooler 14 will be described. As described above, the gas cooler 14 in this embodiment is configured to reduce the exhaust gas outlet temperature T1 to, for example, 30°C to 50°C to obtain dry exhaust gas. In particular, in the case of a small boiler 10 that uses city gas or LP gas as fuel gas, although water vapor is generated by combustion, the exhaust gas GE does not contain corrosive gases such as sulfur, so cooling it to near room temperature does not pose a significant problem. Therefore, the contained water vapor can be converted into drainage by cooling, making it easy to obtain dry exhaust gas with an increased CO2 concentration. Furthermore, in the boiler system 100, dry exhaust gas is mixed with a combustion-supporting gas in oxyfuel combustion and combustion is performed, making it possible to avoid the adverse effects of moisture in the recirculated gas on burner combustion.

[0059] The cooling water W2 used in the gas cooler 14 is typically provided using a cooling water circulation line including a cooling system such as a cooling tower that is provided as a separate system from the supply system for the boiler feed water W1. However, it is also possible to guide part of the cooling water W2 from the outlet of the gas cooler 14 to a softener and use this as the boiler feed water W1.

[0060] Furthermore, a large amount of drainage is generated in the gas cooler 14 when the exhaust gas is cooled. In order to efficiently move and drip this drainage to the bottom of the gas cooler 14, it is preferable that the exhaust gas GE flow downward from the top to the bottom of the gas cooler 14. On the other hand, it is preferable that the cooling water W2 flow upward to prevent air pockets from forming. Furthermore, because the drainage has absorbed CO2 and other substances in the exhaust gas and become acidic, it is preferable that the gas cooler 14 be constructed using an acid-resistant material. It is also preferable that the gas cooler 14 be constructed using a material with a suitable heat-resistant temperature, taking into consideration the inlet exhaust gas temperature when the economizer 12 is operated in bypass mode. [Industrial Applicability]

[0061] A boiler system according to an embodiment of the present invention is suitably used in, for example, a small steam boiler or a water heater to increase the CO2 concentration in the exhaust gas while maintaining stable combustion. [Explanation of symbols]

[0062] 10. Boiler 12 Economizer 14 Gas Cooler 16 Exhaust line 18 Neutralization device 20 Circulation gas line 22 Forced draft fan (combustion-supporting gas fan) 24 Circulating gas intake damper 26 Atmospheric supply line 28 Atmospheric suction damper 30 Oxygen supply line 30A main line 30B Subline 32 On / off valve (solenoid valve) 34 Adjustable valve (motorized valve) 36a Oxygen sensor 36b Oxygen concentration control circuit 38 Manual valve 100 Boiler System GE Exhaust Gas GF Fuel Gas GS Combustion-supporting gas

Claims

1. a boiler that heats feedwater by burning fuel gas; an exhaust line for exhausting exhaust gas from the boiler; a circulation gas line branching from the exhaust line, to which an oxygen supply line and an air supply line are connected, and which can supply the exhaust gas together with at least one of air and oxygen gas to the boiler as a combustion-supporting gas; A boiler system comprising: The air supply line is provided with a damper for controlling air intake, the oxygen supply line includes a main line provided with an on / off valve and a sub-line provided with a variable valve; by closing the damper of the atmospheric air supply line and controlling the opening and closing of the on / off valve of the oxygen supply line and the aperture of the adjustable valve, the oxygen concentration in the mixed gas of the oxygen gas from the oxygen supply line and the exhaust gas can be adjusted and the mixed gas can be supplied to the boiler as a combustion-supporting gas, a boiler system further comprising an economizer that preheats water supplied to the boiler using heat from exhaust gas from the boiler, and a gas cooler that cools the exhaust gas downstream of the economizer, wherein the exhaust line is provided to discharge the exhaust gas cooled by the gas cooler, and the gas cooler is configured to cool the outlet temperature of the exhaust gas to 30°C to 50°C and discharge dry exhaust gas.

2. The combustion supporting gas system further comprises a combustion supporting gas fan provided near the boiler for sending the combustion supporting gas from the circulation gas line to the boiler, and an oxygen sensor for measuring the oxygen concentration in the combustion supporting gas sent from the combustion supporting gas fan to the boiler, 2. The boiler system according to claim 1, wherein the open / close valve is feedback-controlled based on the output of the oxygen sensor.

3. 3. The boiler system according to claim 2, wherein an outlet of the combustion-supporting gas fan is connected to a burner wind box provided in the boiler, the oxygen sensor is configured to measure the oxygen concentration of the gas inside the burner wind box, and the adjustable valve is controlled based on an output of the oxygen sensor so that the oxygen concentration of the gas inside the burner wind box is maintained at 20 to 22%.

4. 4. A boiler system according to claim 1, wherein the main line of the oxygen supply line is composed of a plurality of lines connected in parallel, each line being provided with the on / off valve, and wherein oxygen gas can be supplied from the main line at a plurality of flow rates by controlling the on / off valves of the plurality of lines.

5. 4. The boiler system according to claim 1, wherein a manual valve is provided on at least one of the primary side or secondary side of the adjustable valve provided on the sub-line of the oxygen supply line, and wherein 80 to 90% of the theoretical amount of oxygen required for combustion can be supplied via the main line, and 0 to 50% of the theoretical amount of oxygen required for combustion can be supplied via the sub-line.

6. 4. The boiler system according to claim 1, wherein the on / off valve provided in the main line of the oxygen supply line is a solenoid valve, and the adjustable valve provided in the sub-line of the oxygen supply line is an electric valve.

7. 4. The boiler system according to claim 1, wherein the fuel gas is either city gas or LP gas, and the boiler is either a once-through boiler or a hot water boiler.

8. A boiler system as described in any of claims 1 to 3, wherein the gas cooler is configured to cool the exhaust gas by heat exchange with external cooling water supplied separately from the water supply, and wherein the exhaust gas flows downward from the top to the bottom while the cooling water is supplied to flow upward, and wherein condensate generated from the cooled exhaust gas is discharged from the bottom.

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