BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
FIG. 1 is a schematic of a gas temperature control system according to a first embodiment of the present invention;
FIG. 2 is a schematic of an embodiment of the present invention showing two tubular configurations positioned adjacent one another in a non-overlapping relationship;
FIG. 3 is a schematic of an embodiment of the present invention showing three tubular configurations positioned adjacent one another in a non-overlapping relationship;
FIG. 4 is a schematic of an embodiment of the present invention showing application of the invention to a parallel gas path convection pass design;
FIG. 5 is a schematic of an embodiment of the present invention showing application of the invention to a longitudinal flow economizer, where the concept is applied to control the flow to individual panels of tubes forming the economizer;
FIG. 6 is a schematic rear view looking into the convection pass of FIG. 5;
FIG. 7 is a schematic view illustrating a partial rear view of the tubes in the serpentine arrangement of FIG. 1 to show the variations in fluid flow and flue gas temperature resulting therefrom;
FIGS. 8 and 9 are schematic views illustrating a partial rear view of the tubes in a serpentine arrangement similar to that shown in FIG. 1 to show how variations in economizer outlet fluid temperature due to the variations in fluid flow and flue gas temperature can be accommodated in the outlet headers and supporting stringer tubes; and
FIGS. 10 and 11 are schematic views of a control system as applied to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to the drawings, in which like reference numerals are used to refer to the same or functionally similar elements, FIG. 1 shows an economizer 3 for receiving flue gas generated by a boiler (not shown), located upstream of and in fluid communication with the economizer 3. As used in the present application and as is known to those skilled in the art, the term boiler is used herein to broadly refer to apparatus used for generating steam and may include both drum-type boilers and those of the once-through type. For a general description of such types of boilers or steam generators, the reader is referred to the aforementioned STEAM 41st reference, particularly the Introduction and Selected color plates, and Chapters 19, 20, and 26, the text of which is hereby incorporated by reference as though fully set forth herein. The economizer 3 includes a flue inlet and a flue outlet, and is located in a convection pass 13 upstream of and in fluid communication with a Selective Catalytic Reactor (SCR) assembly (not shown). Within the economizer 3, there is arranged two or more tubular configurations 1, 2 for providing modular heat transfer surfaces for recovery or extraction of heat from the flue gas. The tubular configurations 1, 2 are preferably disposed in a cross and or counter-current heat exchange relationship with respect to the flow path 14 of the flue gas. It is also contemplated that the tubular configurations may be disposed in a cross and or co-current heat exchange relationship with the flow path 14 of the flue gas.
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 FIG. 1, the stringer tubes S that are used to support convective superheat heat transfer surface (not shown; located above the economizer 3) are shown. In most cases, these stringer tubes S will require the full flow from the economizer 3 because the gas temperatures increase in the upper regions of the convection pass and the need for cooling would be greater for these stringer tubes S to meet the stress requirement for supporting these additional heat transfer surfaces.
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.
FIG. 2 shows the tubular configurations in the economizer positioned adjacent one another in a non-overlapping relationship. The heat transfer of the overall economizer system can be reduced and the desired outlet gas temperature can be obtained by changing the flow rates in the adjacent tubular configurations 1′, 2′. In both embodiments, the two different water pathways through the economizer have two different heat transfer characteristics. For example the tube or pathway 1′ in FIG. 2 is shorter than the tube 2′. In the embodiment of FIG. 1, the tubes may have different heat transfer characteristic due to different surface treatments of the tubes, different diameters of the tubes, different placement in the gas flow path or different lengths.
FIG. 3 is a schematic of an embodiment of the present invention showing three tubular configurations positioned adjacent one another in a non-overlapping relationship, and is otherwise similar in concept and operation to FIG. 2. This concept may be particularly useful for controlling gas temperature to prevent it from falling below the acid dew point temperature at which condensation may begin to occur, reducing the possibility of condensation occurring which could form acidic compounds that can corrode downstream devices such as air heaters. Again, while each feedwater line 15 may also include a bypass line 7 installed around its associated 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, it will be appreciated that a control valve 5, bypass line 7 “pair” does not need to be installed in each feedwater line 15; in a three tubular configuration arrangement, a control valve 5, bypass line 7 pair need only be supplied on two of the three tubular configurations. This arrangement may again be particularly useful at lower boiler loads, depending upon the degree of control desired.
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.
FIG. 4 illustrates application of the principles of the present invention to a parallel gas path convection pass design. The parallel gas paths in the convection pass 20 are established by a baffle 22 as is known to those skilled in the art. As shown therein, the economizer 3 may have a lower portion which extends across both of the parallel gas paths, while an upper portion may reside only in a single one of the parallel gas paths. Opposite the upper portion of the economizer 3, in the other gas path, may be provided steam cooled surface, such as superheater or reheater surface 24. The baffle 22 may or may not extend into the lower portion of the economizer 3, and may be steam or water cooled surface depending upon the flue gas temperatures.
FIGS. 5 and 6 are drawn to an embodiment of the present invention as applied to a longitudinal flow economizer, where the concept is applied to control the flow to individual panels 26 of tubes forming the economizer 3. The individual panels 26 of tubes are provided with panel inlet headers 28 and panel outlet headers 30. Feedwater from the economizer inlet headers 11 are fed to the panel inlet headers 28 by means of supply tubes 32. Feedwater flow through the panels 26 and is collected at the panel outlet headers 30. Feedwater is then conveyed from the panel outlet headers 30 via riser tubes 34 to the economizer outlet header 12.
FIG. 6 is a schematic rear view looking into the convection pass of FIG. 5, viewed in the direction of arrows 6-6 of FIG. 5. It is understood that while two tubular panel configurations 1, 2 are shown, an additional third tubular panel configuration flow path could be employed as well.
FIG. 7 is a schematic view illustrating a partial rear view of the tubes in the serpentine arrangement of FIG. 1 to show the variations in fluid flow and flue gas temperature resulting therefrom. The tubes comprising flow path 1 (higher velocity economizer fluid) are denoted by solid dark circles, while the tubes comprising flow path 2 (lower velocity economizer fluid) are denoted by open circles. The higher velocity economizer fluid tubes extract more heat from the flue gas passing across these tubes, and as a result the flue gas temperature leaving these banks of tubes is lower than the flue gas temperature leaving those banks of tubes which have a lower economizer fluid flow therethrough.
FIGS. 8 and 9 are schematic views illustrating a partial rear view of the tubes in a serpentine arrangement similar to that shown in FIG. 1 to show how the variations in economizer 3 outlet fluid temperatures due to the variations in fluid flow and flue gas temperature can be accommodated in outlet headers 12, 12′ and supporting stringer tubes S. As before, the tubes comprising flow path 1 (higher velocity economizer fluid) are denoted by solid dark circles, while the tubes comprising flow path 2 (lower velocity economizer fluid) are denoted by open circles. In some economizer arrangements, the economizer outlet header may be a continuous header 12, with a single common interior portion, where feedwater heated by the various tubular configurations in the economizer 3 is collected and then dispersed via the stringer tubes S. While theoretically the economizer feedwater may travel anywhere along the length of this outlet header 12, in practice the feedwater travels the shortest route from the tubular configurations feeding the outlet header 12 into the nearest adjacent stringer tubes S. This type of economizer outlet header is schematically illustrated in FIG. 8. In other types of economizer arrangements, the economizer outlet header may be formed of a plurality of separate, shorter headers which are then field girth welded together at their ends E to make the entire economizer outlet header. In this type of economizer outlet header, designated 12′ and schematically illustrated in FIG. 9, the feedwater can only be conveyed into and out of the interior portions of each separate header, the ends E of each header preventing fluid flow into adjacent separate headers. It will thus be appreciated that fewer tubular configurations supply feedwater to these separate headers and fewer stringer support tubes S convey feedwater from these separate headers. Significant temperature differences between the temperature of the fluid within the stringer tubes S are to be avoided since such temperature differences can lead to differential thermal expansion of the stringer tubes S. In order to encourage mixing of the hotter and cooler feedwater fluids entering either type of economizer header 12 or 12′, a baffle means B may be employed to encourage mixing of the hotter and cooler feedwater streams within the headers 12, 12′ prior to the feedwater exiting into the stringer support tubes S, thereby equalizing the temperatures within the stringer tubes S. The baffle means B may be a simple plate located to cause the feedwater flow to divert as desired, or it may be a more complex structure such as a perforated plate with holes sized and/or spaced in a particular configuration.
While two types of economizer outlet headers 12, 12′ are shown in FIGS. 8 and 9 it will be appreciated that only one type of economizer outlet header, 12 or 12′, would typically be employed for an economizer in a given steam generator. Similarly, while the earlier Figs. have employed the reference numeral 12 for the outlet header, it will be appreciated that either type of header 12 or 12′ may be employed in all of these embodiments.
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.
In the case of the present invention, and as particularly applied to apparatus and methods for controlling the temperature of flue gas temperature exiting from an economizer, it will be appreciated that economizer gas outlet and/or gas inlet temperatures may be used to control the water flow rate through portions of the heat exchanger for the purpose of affecting the temperature of the flue gas after it has passed across the heat exchanger or economizer. However, due to the low flow rates which may be required for obtaining the desired outlet flue gas temperature, such a flue gas temperature control system may not provide a very quick response with respect to the boiler's capabilities for load control and operation, particularly when there is a large difference between the feedwater rates for the different passes through the economizer or heat exchanger.
Accordingly, another aspect of the present invention involves a control system and operation method which can accommodate such operating conditions. The system and method is not only applicable to the present invention but also to other heat exchanger systems that control the flow through different portions of a heat exchanger to achieve a desired flue gas temperature exiting from the heat exchanger.
FIGS. 10 and 11 illustrate the application of this concept to two different water proportioning or biasing systems. As illustrated in FIGS. 10 and 11, the present invention uses the feedwater flow rate entering the economizer to engage/disengage the economizer water internal proportion or bias system. Once engaged, the measured feedwater flow to the economizer is used to generate a proportioned or biased flow rate demand signal. The demand signal is then compared to the measured underflow proportioned or biased flow rate. If there is a difference between the demand signal flow rate and the measured flow rate, the control valve(s) modulating the proportioned or biased flow is/are adjusted.
More particularly, referring to FIG. 10, there is shown the application of the principles of the present invention to an embodiment of the present invention. In this case, feedwater is provided via main feedwater line 16 to the individual tubular configuration flow paths 1, 2 making up the economizer 3 via feedwater lines 15. A flow element FE 50 provided in line 16 produces a measured feedwater flow signal which is conveyed to a high/low limiter unit 52 and to a demand signal generator unit 54. The demand signal generator unit 54 produces a proportioned or biased flow rate demand signal, here an underflow rate demand signal, which is based upon the measured feedwater flow entering the economizer 3. This underflow rate demand signal is then conveyed to flow controller with bias unit 56. Flow controller with bias unit 56 also receives an actual measured underflow rate from flow element FE 58, indicative of the flow rate through the tubular configuration 1. Flow controller with bias unit 56 compares the underflow rate demand signal from unit 54 with the actual measured underflow rate from flow element FE 58, and produces a control valve signal which is conveyed to control valve 5 to modulate the feedwater flow through tubular configuration 1. The control valve signal from unit 56 is also conveyed to a signal inverter unit 60, and an inverted control valve signal is conveyed to the other control valve 5 to modulate the feedwater flow through tubular configuration 2. Since the unit 56 is a flow controller with bias capability, changes can easily be made to adjust the underflow rate demand signal in order to accommodate actual field performance; i.e., to adjust the flow splits between tubular configurations 1 and 2, in order to achieve a desired flue gas temperature exiting the economizer and entering the downstream device, such as an SCR.
The demand signal generator unit 54 may generate the underflow rate demand signal in any manner known to those skilled in the art; lookup tables, calculations according to a predetermined equation(s) of underflow rate demand as a function of boiler load, etc. The demand signal generator unit 54 may also include a plurality of such tables or equations corresponding to different fuel types to be burned in the boiler.
The high/low limiter unit 52 receives the measured feedwater flow signal from flow element FE 50 and produces a high/low limit signal which is provided to block valve 62, located in the feedwater line 15 flow path supplying feedwater to tubular configuration 1. The high/low limiter is designed to position the block valve 62 at a specific open position, depending upon the boiler load as indicated by the boiler measured feedwater flow signal. For example, assume a boiler with a nominal megawatt (MW) capacity of 600 MW. The boiler operation may be considered to fall into one of three operating ranges: 0-200 MW, 200-400 MW, and 400-600 MW, and these ranges will determine the manner of operation of the control system of the multiple pass economizer according to the present invention.
In the 0-200 MW operating range, both the block valve 62 and the control valve 5 in the flow path supplying the tubular configuration 1 would normally be open. In the 200-400 MW operating range, the flue gas temperatures leaving the economizer 3 have increased over those experienced in the 0-200 MW range, but are typically not yet high enough to permit proper SCR operation. Thus, the principles of the present invention are employed; the block valve 62 would be closed to a specific setting in this case to provide additional flow resistance to tubular configuration 1, thereby allowing the control valves 5 to operate with more flexibility to modulate the feedwater flow through both tubular configurations 1, 2, and the flue gas temperature leaving the economizer 3 increases to a desired value. As boiler megawatt load continues to increase, into the 400-600 MW operating range, the feedwater modulating principles of the present invention can be gradually “phased out” and the control valves 5 can be opened wide and the block valve 62 can be opened in such a manner to obtain balance flow conditions in both tubular configurations 1, 2, since the flue gas temperature exiting from the economizer 3 is above the desired minimum value. While three, equally sized operating “ranges” have been described, unequal operating ranges may be employed. The general idea is that there are typically lower boiler operating ranges where the principles of the invention are generally not applied, an intermediate operating range where the invention is primarily used, and an upper boiler operating range where the principles of the present invention are not required and thus modulation of the control valves is gradually phased out because the flue gas temperatures are above the desired minimum levels.
Turning now to FIG. 11, there is shown the application of the principles of the present invention to another embodiment which is quite similar to the one disclosed in FIG. 10. In this case, one fundamental difference is that instead of two control valves 5 in each of the feedwater flow paths 15 supplying feedwater to tubular configurations 1, 2, there is only a single control valve 5 present and which is modulated to control feedwater flow through a single tubular configuration; in this case, tubular configuration 2 (but which alternatively could have been tubular configuration 1, if desired). Another difference involves that fact that the single control valve 5 and the block valve 62 are located in parallel flow paths, rather than being in series with one another as in FIG. 10. Also, the block valve 62 in FIG. 11 could be operated in various conditions, fully open, partially open or fully closed, since the control valve 5 provides a flow path around block valve 62. Otherwise, the principles of operation are the same as in FIG. 10. Measured feedwater flow to the economizer 3 is used to generate a proportioned or biased flow rate demand signal at unit 54. The demand signal is then conveyed to the flow controller with bias unit 56 where it is compared to the measured underflow proportioned or biased flow rate from flow element FE 58. If there is a difference between the demand signal flow rate and the measured flow rate, the control valve 5 modulating the proportioned or biased flow is adjusted. Bias is applied as required to tune the performance of the system.
While the control system described above and with particular reference to FIGS. 10 and 11 can be used without reference to any measured flue gas temperatures, it is envisioned that a combination of the two approaches can be employed. That is, the measured feedwater flow to the economizer 3 can be used to generate a proportioned or biased flow rate demand signal at unit 54, and establish an initial flow rate. Then, a temperature measurement, such as the flue gas leaving the economizer, can then be used as a check or as a trim value to “fine tune” the positions of the control valve(s) 5.
It will thus be appreciated that the control system and operation method is particularly suited to operating conditions, such as low boiler load conditions, where the major portion of the water flow to the economizer must be proportioned or biased to meet the desired or target flue gas outlet temperature. The normal method of using outlet gas temperature to control water flow is not effective when a major amount of the water is proportioned or biased because the residence time of the water flowing through the (underflow section of the) economizer heating surface can no longer be measured in seconds or several minutes but stretches to almost an hour. This lengthy residence time in turn lengthens the time it takes to effectively make a change in gas temperature to the point that gas temperature can no longer be used to control the proportioned or biased water flow rate.
The advantage of the invention is that it allows systems which proportion or biased water flow rate in the economizer sections for the purpose of raising exiting gas temperature to be effective at lower boiler loads than can be achieved by a system which uses outlet gas temperature to control the feedwater proportioning or biased water flow rate. This allows an SCR located downstream of the boiler, if the boiler is so equipped, to continue to operate at lower boiler loads than previously possible using this type of system.
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.
Patent Application
20070261647
