RU2543096C1 - METHOD AND DEVICE FOR SELECTIVE CATALYTIC REDUCTION OF NOx IN POWER BOILER - Google Patents

Abstract

FIELD: power industry.

SUBSTANCE: flue gas flow containing NO_x leaves a furnace via a flue gas duct to a vent pipe and is cooled down in a heat utilising area containing an economiser section located in the flue gas duct. At least some part of NO_x is reduced to N₂ on a NO_x reducing catalyst contained in the flue gas duct downstream relative to the economiser section. A power boiler includes an additional air heater installed in the flue gas duct downstream relative to the NO_x reducing catalyst. The gas air heater and the additional air heater are connected in parallel.

EFFECT: method improvement.

12 cl, 2 dwg

Description

BACKGROUND OF THE INVENTION

FIELD OF THE INVENTION

The present invention relates to a method and system for selective catalytic reduction (SCR) of NO _x using a solid or liquid hydrocarbon fuel power boiler. More specifically, the present invention relates to controlling the temperature of the flue gas entering the boiler NO _x reduction catalyst.

Description of the Related Art

Nitrogen oxides, also known as NO _x , contribute to acid rain and smog. In accordance with the Environmental Protection Act, require the reduction of NO _x emissions to acceptable levels, NO _x recovery during and after the combustion process is an important objective in the design and operation of modern plants.

Nitrogen oxides are by-products of the combustion of solid and liquid hydrocarbon fuels, such as powdered coal or fuel oil, and are present in two main forms. If nitrogen is formed by combustion in air, NO _x is called “thermal NO _x ”. Thermal NO _{x is} formed when molecular nitrogen (N ₂ ) is exposed to temperatures exceeding approximately 1500 ° C, which cause it to decompose to form atomic nitrogen (N), which can then combine with atomic or molecular oxygen to form NO or NO ₂ . If nitrogen comes from organically bound nitrogen in a fuel, NO _x is called the term “fuel NO _x ”.

A variety of methods are used to reduce nitrogen oxide emissions. One method is selective catalytic reduction (SCR), which uses a catalyst and a reducing agent, typically ammonia gas, to reduce NO _x to nitrogen gas and water according to the following reaction equations:

4NO + 4NH ₃ + O ₂ => 4N ₂ + 6H ₂ O

2NO ₂ + 4NH ₃ + O ₂ => 3N ₂ + 6H ₂ O

Since NO _{x is} approximately 95% composed of NO, the first reaction is the predominant process. The ideal temperature range for the implementation of SLE is usually from about 300 to about 400 ° C. When the operating temperature drops well below 300 ° C, the possibility of depositing ammonium bisulfate and sulfur trioxide on the surface of the catalyst increases. This may cause a gradual loss of catalyst activity. Above 400 ° C, ammonia can decompose, reducing the efficiency of the process. If the temperature exceeds approximately 450 ° C, the activity of the catalyst may gradually decrease due to sintering.

A typical energy boiler using SCR as a method for reducing NO _x includes a furnace in hydraulic connection with a flue gas channel. The combustion of hydrocarbon fuel occurs in the furnace with the formation of hot flue gases, which rise inside the furnace, giving up part of their energy to generate steam on the evaporative surfaces of the furnace walls. The flue gases then pass through the heat recovery region (TUO) of the flue gas channel, where they give off additional energy to superheat the steam and heat the feed water on the economizer surfaces. The flue gases leaving the economizer section pass through a NO _x reduction catalyst, an air heater and possible flue gas treatment systems, finally leaving the exhaust pipe through the exhaust pipe.

In a typical SCR system, at some point in the flue gas channel upstream of the catalyst section, a reagent, such as ammonia gas or a urea solution in water, enters and mixes with the flue gas stream. The mixture of reagent and flue gas then enters the catalyst section in which the catalytic reduction of NO _x occurs in the reaction of the reagent and the excess oxygen in the flue gas.

The catalyst, as a rule, includes many layers of solid catalytic material located in the path of the flow of flue gas. The most common types of catalytic material used and the approximate temperature ranges of the flue gas at which they are effective as catalysts are the following: titanium dioxide (270-400 ° C), zeolite (300-430 ° C), iron oxide (380- 430 ° C) and activated carbon / coke (100-150 ° C).

US Pat. No. 5,555,849 describes a fossil fuel-fired power plant with an economizer system upstream of the NO _x reduction catalyst, where the economizer system includes a bypass on the water side to maintain the desired temperature of the flue gas in the NO _x recovery catalyst even at low load.

European Patent Publication EP 0753701 A1 describes a boiler with a NO _x reduction catalyst located in a flue gas channel between two economizers, having a bypass flue gas channel for the economizer upstream of the NO _x reduction catalyst.

US patent No. 6405791 describes a tubular air heater with an inlet chamber, which allows the installation of an upgraded system of selective catalytic reduction (SCR) upstream relative to the air heater in an existing boiler.

In addition to the problem discussed in US Pat. No. 5,555,849, it turned out that, especially when installing an upgraded NO _x reduction catalyst system in an existing power boiler, the temperature of the flue gas on the NO _x reduction catalyst can become excessively high, in particular at high loads. Due to, for example, changes in the fuel composition or the operating mode of the boiler, or even the unsatisfactory design of the boiler, the temperature at the outlet of the economizer may exceed 430 ° C, i.e. be above the optimal temperature range of existing catalysts for the reduction of NO _x .

Thus, when installing SCR downstream of the economizer, the use of a special catalyst may be required to reduce NO _x . Another solution to this problem is the installation of additional economizer surfaces in the heat recovery area (TUO) of the boiler. In this method, however, the temperature of the feed water increases, and if the temperature rises and approaches the saturation temperature of the drum of the steam boiler, this produces an adverse effect on the circulation of water in the boiler and, ultimately, reduces the productivity of the boiler.

Patent documents, including Japanese patent publication JP 550700328 A, European patent publications EP 1956293 A1, EP 1959192 A1 and US patent 5078973 A, describe boilers with an air heater in the flue gas channel upstream relative to the denitration device. Japanese Patent Publication JP 7208701 A describes a tubular air heater upstream of a NO _x reduction catalyst and an air heater downstream of a NO _x reduction catalyst.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method for the selective catalytic reduction of NO _x in a power boiler, by which the problems of the prior art described above can be minimized.

Another objective of the present invention is to provide a device for the selective catalytic reduction of NO _x in a power boiler, by which it is possible to minimize the above-described problems of the prior art.

According to one aspect, the present invention provides a method for the selective catalytic reduction of NO _x in an energy boiler, the method comprising the following steps: (a) burning fuel in a furnace of a boiler and generating a flue gas stream that contains NO _x , (b) passing a flue gas stream from the furnace through the flue gas channel into the exhaust pipe, (c) cooling the flue gas stream in a heat recovery area including an economizer section installed in the flue gas channel, (d) recovering at least frequently NO _x to N ₂ in the NO _x reduction catalyst is located in the flue gas channel downstream of the economizer section, and (e) further cooling the flue gas and the formation of the heated air in the gas air heater installed in the channel of the flue gas downstream of the economizer section and upstream of the NO _x reduction catalyst.

According to yet another aspect, the present invention provides an energy boiler with selective catalytic reduction of NO _x , the boiler including: (a) a combustion chamber for burning fuel in a furnace of a boiler and thereby producing a flue gas stream containing NO _x , (b) a channel flue gas for passing the flow of flue gas from the furnace to the exhaust pipe, (c) a heat recovery area including an economizer section installed in the flue gas channel to cool the flue gas stream, (d) the catalyst is recovered NO _x located in the flue gas channel downstream of the economizer section to recover at least a portion of NO _x to N ₂ ; and (e) a gas air heater installed in the flue gas channel downstream of the economizer section and upstream of the NO _x reduction catalyst, to further cool the flue gas and generate heated air.

The present invention, i.e. the installation of a gas air heater upstream of the NO _x reduction catalyst for cooling the flue gas provides the advantage of the possible installation of a traditional NO _x reduction catalyst using standard catalytic material. The gas air heater is preferably a tubular air heater, but in some cases it may be other types of heat exchangers that transfer heat from the flue gas to the air supplied for combustion in the boiler. According to another preferred embodiment of the present invention, the gas air heater is a recirculating heat exchanger heat exchanger. In some cases, the gas air heater may, alternatively, be another suitable type, for example a heat pipe.

Advantageously, the energy boiler includes a burner or, in practice, a set of burners for burning fuel sent to the burners by a stream of primary air. According to a first embodiment of the present invention, combustion air heated in a gas air heater enters the burners as secondary air. Due to the use of secondary air as a cooling medium in this method there is no risk of overheating of the cooling medium, which may occur when using feed water as a cooling medium. In addition, since the heat transferred to the secondary air can be fully used in the boiler, this method does not affect the operation or efficiency of the existing boiler. Alternatively, the combustion air heated in the gas air heater may also be other types of combustion air, for example the primary air that enters the furnace.

When using the method according to the present invention, in various load conditions of the boiler, the air flow through the gas air heater can be controlled or stopped to provide the desired temperature of the flue gas entering the catalyst. Thus, the air flow can advantageously be controlled directly depending on the load condition of the boiler or depending on the measured temperature of the flue gas entering the NO _x reduction catalyst. Thus, the present invention provides a simple method for providing optimized catalyst operation under various load conditions, for example, without having to provide an economizer with a by-pass flue gas stream or a by-stream on the water side for low-load operation. The present invention thus provides temperature control over a wide range without the need for any change in the flue gas or steam / water circuit in the boiler. The present invention, therefore, is particularly useful for modernizing the installation, but it can also be used in new power units, for example, to control the temperature of the flue gas supplied to the NO _x reduction catalyst.

The above brief description, as well as additional objectives, features and advantages of the present invention, will become clearer after reading the following detailed description of the currently preferred, but nonetheless illustrative embodiments of the present invention, considered in conjunction with the accompanying drawings.

Brief Description of the Drawings

Figure 1 is a schematic illustration of an exemplary energy boiler according to the present invention.

Figure 2 represents a portion of the flue gas channel of an energy boiler according to another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 is a schematic illustration of a coal-fired powder coal-fired power boiler 10 according to the present invention. The boiler includes a furnace 12 with a burner for introducing into the furnace a mixture of powdered coal 16 and primary air 18 from a coal mill 20. Typically, an energy boiler includes many burners, but for simplicity, only one burner is shown in FIG. The fuel is burned in a furnace using primary air and secondary air 22 entering the furnace through a blow box 24 near the burner, and hot flue gas is generated. Combustion can be practiced in practice completely using tertiary air and / or two-stage combustion of fuel with the introduction of air into the furnace downstream of the burners, but for simplicity, the introduction of tertiary air and / or air for two-stage combustion of fuel is not shown in Fig. 1. The resulting hot flue gases rise inside the furnace, giving part of their energy to the evaporating surfaces 30 on the walls of the furnace to evaporate the feed water 26 and produce steam 28. The flue gas leaves the furnace through a flue gas channel 32 connected to the top of the furnace.

The flue gases then pass through the heat recovery region (TUO) 34 of the flue gas channel, where they give off additional energy to the heating surfaces of the superheater 36 to overheat the generated steam and to the economizer 38 surfaces to heat the feed water to the evaporator surfaces. As a rule, TUO includes many overheating and heating surfaces, but since this is not critical for the present invention, only one superheater 36 is shown in FIG.

The flue gases leaving the economizer 38 pass through a NO _x reduction catalyst 40, an air heater 42, a flue gas cleaning system 44, and an exhaust pipe 46 into the atmosphere. The flue gas channel 32 also includes an injector 48 for introducing an NO _x reducing agent, such as ammonia, upstream of the catalyst 40. The catalyst 40 preferably includes a conventional catalytic material such as titanium dioxide or iron oxide. Typically, the flue gas cleaning system includes several flue gas cleaning units, such as a dust collector and desulfurizer, but since this is not critical to the present invention, only one cleaning gas system 44 is shown schematically.

According to the present invention, the flue gas channel includes a gas air heater, in this case a tubular air heater 50, installed upstream of the NO _x reduction catalyst 40. Using a tubular air heater, it is possible to cool the flue gas, as much as desired, to the optimum temperature range for the catalyst, for example, to a temperature of less than about 400 ° C.

The tubular air heater 50 is advantageously connected so as to provide possible additional heating of the secondary air 22. In some embodiments, the tubular air heater can also be used to heat the primary air 18 or tertiary air and / or air for two-stage combustion of fuel, which is not shown in FIG. 1 . According to a preferred embodiment of the present invention, the tubular air heater 50 is connected in parallel with the air heater 42, which is also referred to herein as the “second air heater” and which is installed in the flue gas channel 32 downstream of the NO _x reduction catalyst 40. Thus, the secondary air flow from the secondary blower 52 can be separated using control devices, such as control valves 54, 54 ', between the tubular air heater 50 and the air heater 42 downstream of the catalyst 40.

The ratio of the air flows through the tubular air heater 50 and the air heater 42 downstream of the catalyst 40 can advantageously be set depending on the load of the boiler or depending on the temperature of the flue gas upstream of the catalyst, which is measured by a temperature-determining device, such as a thermometer 56. Thus , the system preferably includes a controller 58 for controlling the control valves 54, 54 'depending on the measured temperature.

As a rule, at high loads, when the temperature of the flue gas upstream relative to the catalyst tends to increase and exceed the optimum operating temperature of the catalyst, the main part of the secondary air is passed through a tubular air heater 50 by at least a partially closed valve 54 'installed on the branch of the line secondary air passing through the air heater 42, installed downstream relative to the catalyst 40. Accordingly, at low loads, less frequent l secondary air is passed through the tubular air heater by at least partially closing the valve 54 mounted on a branching of the secondary air supply line passing through the tubular air heater 50. Thus, by controlling the separation of the air flow between the tubular air heater 50 and the air heater 42 downstream 40 with respect to NO _x catalyst recovery is possible to optimize the flue gas temperature at the catalyst inlet NO _x regeneration when pa personal load conditions.

Figure 2 represents a part of the flue gas channel 32 of an energy boiler according to another embodiment of the present invention. A gas air heater 50 ′ is installed in the flue gas channel upstream with respect to the catalyst section 40, and a conventional air heater 42 is installed downstream with respect to the catalyst section 40. According to this embodiment, the gas air heater 50 ′ includes a flue gas cooler 60 in the flue gas channel 32 and a separate air heater 62 at the junction 64 of the air supply line 66. The flue gas cooler 60 and the air heater 62 are connected via a pipe 68 for heat transfer ator means of a pump 70.

Although the present invention is described herein by way of examples in connection with what is currently considered the most preferred embodiments, it should be understood that the present invention is not limited to the described embodiments, but is intended to extend to various combinations or modifications of its distinctive features and some other applications included in the scope of the present invention, which are defined in the attached claims.

Claims (12)

1. A method for the selective catalytic reduction of NO _x in an energy boiler (10), the method comprising the following steps:
(a) burning fuel in the furnace (12) of the boiler and generating a flue gas stream that contains NO _x ;
(b) the passage of the flow of flue gas from the furnace through the channel (32) of the flue gas into the exhaust pipe (46);
(c) cooling the flue gas stream in a heat recovery region (34) including an economizer section (38) that is located in the flue gas channel;
(d) recovering at least a portion of NO _x to N ₂ on a NO _x reduction catalyst (40) located in the flue gas channel downstream of the economizer section; and
(e) additional cooling of the flue gas and the formation of heated air in a gas air heater (50) installed in the flue gas channel downstream of the economizer section and upstream of the NO _x reduction catalyst,
moreover, the energy boiler includes an additional air heater (42) installed in the flue gas channel downstream relative to the NO _x reduction catalyst, and this method is characterized in that the gas air heater and the additional air heater are connected in parallel
relative to air flow. 2. The method according to claim 1, in which the gas air heater (50) is connected to the air stream to produce heated air entering as secondary air into the burner (14) located in the furnace. 3. The method according to claim 1, further comprising the step of controlling the flow of air entering the gas air heater (50), depending on the load mode of the energy boiler. 4. The method according to claim 1, further comprising the steps of measuring the temperature of the flue gas entering the NO _x reduction catalyst (40) and controlling the flow of air entering the gas air heater (50), depending on the measured temperature. 5. The method according to claim 1, in which the gas air heater (40) is a tubular air heater. 6. The method according to claim 1, in which the gas air heater is a heat exchanger (50 ') with a recirculating coolant. 7. Energy boiler (10) with selective catalytic reduction of NO _x , and this boiler includes:
(a) a combustion chamber for burning fuel in the furnace (12) of the boiler and thereby producing a flue gas stream containing NO _x ;
(b) a flue gas channel (32) for passing a flue gas stream from the furnace to a chimney (46);
(c) a heat recovery region (34) including an economizer section (38) installed in the flue gas channel to cool the flue gas stream;
(d) a NO _x reduction catalyst (40) located in the flue gas channel downstream of the economizer section to recover at least a portion of the NO _x to N ₂ ; and
(e) a gas air heater (50) installed in the flue gas channel downstream of the economizer section and upstream of the NO _x reduction catalyst, to further cool the flue gas and produce heated air,
moreover, the energy boiler includes an additional air heater (42) installed in the flue gas channel downstream relative to the NO _x reduction catalyst, and characterized in that the gas air heater and the additional air heater are connected in parallel with respect to the air flow. 8. The energy boiler according to claim 7, in which the combustion chamber for burning fuel includes a burner (14), and a gas air heater (50) is connected to the air channel for the passage of heated air into the furnace as secondary air near the burner. 9. The energy boiler according to claim 7, further comprising a regulator (58) for regulating the flow of air entering the gas air heater (50), depending on the load mode of the energy boiler. 10. The energy boiler according to claim 7, further comprising
a thermometer (56) for measuring the temperature of the flue gas entering the NO _x reduction catalyst (40), and a regulator (58) for regulating the flow of air entering the gas air heater (50), depending on the measured temperature. 11. The energy boiler according to claim 7, in which the gas air heater (50) is a tubular air heater. 12. The energy boiler according to claim 7, in which the gas air heater (50 ') is a heat exchanger with a recirculating coolant.

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