The present invention relates generally to the field of Selective Catalyst Reactor (SCR) temperature control and in particular to a system and method for maintaining the combustion or flue gas entering the SCR system at or above the optimal catalytic reaction temperature, even when operating the boiler at reduced loads.
In operating a boiler with a Selective Catalyst Reactor (SCR) system, the effectiveness of the SCR is dependant upon the flue gas temperature entering the catalyst reactor. Most can operate within a temperature range of about 450 degrees F. to about 840 degrees F. Optimum performance may typically occur between about 570 degrees F. to about 750 degrees F. Typically, the desired gas temperature entering the SCR is about 580 degrees F. or greater. At a temperature of about 580 degrees F., the reaction of ammonia with NOx is optimized and the amount of the ammonia needed for the catalytic reaction is minimized. Therefore, for economic reasons the desired gas temperature entering the catalyst reactor should be maintained within the optimum temperature range of about 570 degrees F. to about 750 degrees F. at all loads.
However, as boiler load varies, the boiler exit gas temperature will drop below the optimal temperature of about 580 degrees F. To increase the gas temperature to about 580 degrees F., current practice has been to use an economizer gas bypass. The economizer gas bypass is used to mix the hotter gases upstream of the economizer with the cooler gas that leaves the economizer. By controlling the amount gas through the bypass system, a boiler exit flue gas temperature of about 580 degrees F. can be maintained at lower boiler loads.
With this approach, static mixing devices, pressure reducing vanes/plates and thermal mixing devices are required to make the different temperature flue gases mix before the gas mixture reaches the inlet of the catalytic reactor. In most applications, obtaining the strict mixing requirements for flow, temperature and the mixing of the ammonia before the catalyst reactor is often difficult.
In another approach to dealing with decreasing flue gas temperature entering a SCR reactor at reduced boiler loads, an economizer was fitted with a feedwater bypass to partially divert the feedwater away from the economizer in order to maintain the flue gas temperature.
Additional details of SCR systems for NOx removal are provided in Chapter 34 of Steam/its generation and use, 41st Edition, Kitto and Stultz, Eds., Copyright © 2005, The Babcock & Wilcox Company, the text of which is hereby incorporated by reference as though fully set forth herein. Flue gas temperature control using conventional economizers are described in U.S. Pat. Nos. 7,021,248 to McNertney, Jr. et al. and 6,609,483 to Albrecht et al., the texts of which are hereby incorporated by reference as though fully set forth herein.
It is an object of the present invention to provide a system and method for increasing the outlet temperature of flue gas passing through the economizer by reducing the water flow in selected tubes and/or sections of the economizer without the need to divert feedwater away from the economizer. When these selected tubes or sections are reduced in flow, the remaining sections or tubes in the economizer are overflowed so that the total flow is maintained through the economizer. To increase the economizer gas outlet temperature, a certain percentage of the tubes in the economizer will have their heat transfer reduced by decreasing the flow through these tubes. The increase in water flow in the remaining tubes has a minimal effect on the heat transfer of the remaining tubes, resulting in an overall decrease in the total gas side heat transfer of the economizer and as a result increases the gas outlet temperature from the economizer.
It is another object of the present invention to provide a system and a method for maintaining a desired economizer outlet gas temperature across a range of boiler loads by providing two or more sections or compartments of liquid-cooled heat transfer surfaces or tubes in the flow path of the flue gas, wherein the flow rate of each section or compartment is controlled independently of the other sections or compartments, determining the flow rate that is required in each section or compartment in order to produce a combined/overall heat transfer capacity sufficient to maintain the desired economizer outlet gas temperature, and adjusting of the flow rate of each section or compartment of the economizer.
In one aspect, the system is configured to maintain the flue gas entering a catalytic reactor within a desired temperature range that will promote optimal catalytic reaction, irrespective of the boiler load. Preferably, the flue gas temperature is maintained within a range of about 570 degrees F. to about 750 degrees F., preferably about 580 degrees F. In a normal boiler application, the water side of the economizer is used to cool the flue gas that flows over the surface that is installed in the boiler. The system of the present invention separates the heat transfer surfaces of the economizer to increase the outlet temperature of the flue gases to the desired temperature of about 570 degrees F. to about 750 degrees F., preferably, 580 degrees F. at lower boiler loads. This is accomplished by selectively changing the flow rates through different portions of the economizer. By determining the proper amounts and locations of the heating surface, the desired economizer outlet gas temperature can be maintained within the desired temperature range or at a desired temperature across the desired steam generator load range through the control of the flow rates of water through the different sections of the economizer.
It is a further object of the present invention to provide a system for maintaining a flue or combustion gas stream being directed into downstream device such as an SCR assembly within a desired temperature range or at a desired (e.g., optimal) temperature comprising: an economizer located upstream of and in fluid communication with the SCR assembly, wherein the economizer comprises at least two tubular configurations having different heat transfer characteristics, disposed in a cross and or counter-current heat exchange relationship with the flow path of the gas stream generated by a boiler and having a flue inlet and a flue outlet, the boiler being located upstream of and in fluid communication with the economizer, each tubular configuration comprises a feedwater inlet and a feedwater outlet, the outlet of both tubular configurations being attached to an outlet header and the inlet of each tubular configuration being attached to a separate inlet header, and a control system configured to independently control the flow of feedwater through each tubular configuration while maintaining a substantially constant total flow of feedwater through the economizer, the flow of feedwater through each tubular configuration is adjusted in a manner that transfers an appropriate amount of heat from the gas stream to maintain the gas stream at the desired optimal temperature.
It is a further object of the present invention to provide a method of maintaining a gas stream being directed into a downstream device such as an SCR assembly within a desired temperature range or at a desired (e.g., optimal) temperature, the SCR assembly being located downstream of and in fluid communication with an economizer, the method comprising disposing, within the economizer, at least two tubular configurations in a cross and or counter-current heat exchange relationship with the flow path of the gas stream, the economizer having a flue inlet and a flue outlet, each tubular configuration comprises a feed water inlet and a feed water outlet, the outlet of both tubular configurations being attached to an outlet header and the inlet of each tubular configuration being attached to a separate inlet header, monitoring the gas temperature at the flue inlet or flue outlet, the feedwater temperature at the feedwater inlet and outlet, and the flow of feedwater through the economizer, controlling the flow of feedwater conveyed through each tubular configuration, based on the measured temperatures and flow, to provide the tubular configurations with a combined heat transfer capacity that is effective to maintain the gas temperature at the desired level, wherein the heat transfer capacity of the tubular configurations is decreased by increasing the flow of feedwater through at least one of the tubular configurations and by reducing the flow of feedwater through the other tubular configurations.
While the present invention is particularly suited to maintaining a desired flue gas temperature entering a downstream SCR device, it will be appreciated that the invention may be used to maintain a desired gas temperature which may required by other types of downstream devices, and for other purposes. One type of downstream device could be an air heater which typically uses the heat in the flue gas leaving the steam generator to heat the incoming air for combustion. In some cases it is desirable to control the flue gas temperature entering the air heater within a desired range or at a desired temperature above the acid dew point temperature, such as during low load operation, to reduce the possibility of condensation occurring which could form acidic compounds which could lead to corrosion of the air heater. Other types of downstream devices include various types of pollution control equipment; e.g., particulate removal devices such as electrostatic precipitators or fabric filters, and flue gas desulfurization devices such as wet or dry flue gas desulfurization equipment.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.
In the drawings:
Referring now to the drawings, in which like reference numerals are used to refer to the same or functionally similar elements,
Each tubular configuration 1, 2 is attached on one end to an inlet header 11, and on the other end, the tubular configurations 1, 2 may each be connected to a separate (not shown) or to a common outlet header 12, which is supported by stringer tubes S. A feedwater line 15 is connected to each inlet header 11, and on each feedwater line 15 there is preferably provided a control valve 5. Each feedwater line 15 may also include a bypass line 7 installed around the control valve 5 for cleaning or flushing the feedwater lines 15 or the tubular configurations 1, 2, or for performing maintenance on the control valve 5. The feedwater lines 15 are connected to the main feedwater line 16 through a distributor 8. While individual sets of control valve 5 and bypass valve 7 may be installed on each feedwater line 15, it will be appreciated that a single control valve 5, bypass line 7 “pair” which is installed in only one feedwater line 15 may be required. The provision of a control valve 5, bypass line 7 pair on all feedwater lines 15 ensures optimum control of the flow through each of the tubular configurations 1, 2, and may be particularly useful at lower boiler loads, but this degree of sophistication and control may not be required in all applications.
In one embodiment, each tubular configuration 1, 2 comprises a plurality of serpentine or stringer tubes arranged horizontally or vertically back and forth within the economizer 3. The tubes in one tubular configuration may be positioned in an offset relationship with respect to the tubes of the other tubular configuration. The tubes may be offset vertically, horizontally, diagonally, or longitudinally or offset in a combination of two or more such orientations. Preferably, the tubular configurations 1, 2 are positioned adjacent to one another in the convection pass 13 in an overlapping or non-overlapping relationship, and extend or expand substantially along a flow path 14 of the flue gas passing across the economizer 3. In an alternate embodiment, the heat transfer capacity of each tubular configuration is not identical. It will also be appreciated that the tubes forming the tubular configurations 1, 2 may or may not employ extended surface such as fins to achieve a desired amount of heat transfer to the feedwater flowing through the economizer 3.
An existing economizer 3 can be modified or retrofitted according to the present invention, such that a selected tubular configuration is fed with sufficient feedwater to effectively reduce the overall heat transfer capacity of the economizer 3.
The remaining feedwater is circulated into the remaining tubes in the other tubular configuration. The tubes in the selected tubular configuration would receive more than the normal flow which will slightly increase the heat transfer of this tubular configuration. Also, by determining the appropriate quantity of tubes for each tubular configuration or economizer bank, the effective heat transfer of the economizer 3 can be reduced so that the desired economizer outlet gas temperature is obtained. In
The temperature monitoring required to adjust the proportioning values of the system can be monitored by knowing the outlet gas temperature or by knowing the inlet gas temperature along with the water side temperatures, both inlet and outlet, and the water side fluid flow through the system. Preferably, temperature and flow rate monitoring, and adjustment of the flow rate in each tubular configuration or economizer bank are carried out by a controller 9.
In operation, temperature sensors are provided at the flue inlet and/or at the flue outlet 4, at the feedwater inlet and at the feedwater outlet. A flow meter (not shown) is also provided for the main feedwater line to measure the economizer 3 fluid flow through the system. The temperature sensors and flow meter are in signal communication 10 with controller 9 and are calibrated to transmit measurements to the controller 9 for the feedback control of the flow of feedwater through each tubular configuration 1, 2.
For example, when the controller 9 detects a drop in the boiler load or in the gas temperature at the economizer flue inlet or outlet, the flow of feedwater through each tubular configuration is adjusted to reduce the combined heat transfer capacity of the economizer. This can be achieved by increasing the flow of feedwater through one tubular configuration to decrease the flow and heat transfer of the other tubular configuration.
In addition, under certain low flow conditions, it may be necessary to provide orifice means at one or both of the inlets and outlets of individual tubes in a given tubular configuration to provide additional pressure drop for flow stability in these tubes. Orificing these tubes, particularly the lower velocity flow paths, provides additional pressure drop which will tend to equalize the flow distribution between each of the tubes in that tubular configuration.
While two types of economizer outlet headers 12, 12′ are shown in
As discussed earlier, the present invention is particularly suited to maintaining a desired flue gas temperature entering a downstream SCR device. However, it will be appreciated that the invention may be used to maintain a desired gas temperature which may required by other types of downstream devices, and for other purposes. One type of downstream device could be an air heater which typically uses the heat in the flue gas leaving the steam generator to heat the incoming air for combustion. In some cases it is desirable to control the flue gas temperature entering the air heater within a desired range or at a desired temperature above the acid dew point temperature, such as during low load operation, to reduce the possibility of condensation occurring which could form acidic compounds which could lead to corrosion of the air heater. Other types of downstream devices include various types of pollution control equipment; e.g., particulate removal devices such as electrostatic precipitators or fabric filters, and flue gas desulfurization devices such as wet or dry flue gas desulfurization equipment.
While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles. For example, the present invention may be applied to new boiler or steam generator construction involving selective catalytic reactors or other types of downstream devices, or to the replacement, repair or modification of existing boilers or steam generators where selective catalytic reactors or other types of downstream devices and related equipment are or have been installed as a retrofit. In some embodiments of the invention, certain features of the invention may sometimes be used to advantage without a corresponding use of the other features. Accordingly, all such changes and embodiments properly fall within the scope of the following claims.