Steam temperature control in a boiler system using reheater variables

A technique of controlling a boiler system such as that used in a power generation plant includes using manipulated variables associated with or control inputs to a reheater section of the boiler system to control the operation of the furnace, and in particular to control the fuel/air mixture provided to the furnace or the fuel to feedwater ratio used in the furnace or boiler. In the case of a once-through boiler type of boiler system, using the burner tilt position, damper position or reheater spray amount to control the fuel/air mixture or the fuel to feedwater flow ratio of the system provides better unit operational efficiency.

TECHNICAL FIELD

This patent relates generally to the control of boiler systems and in one particular instance to the control and optimization of once-through boiler type of steam generating systems having both a superheater section and a reheater section.

BACKGROUND

A variety of industrial as well as non-industrial applications use fuel burning boilers which typically operate to convert chemical energy into thermal energy by burning one of various types of fuels, such as coal, gas, oil, waste material, etc. An exemplary use of fuel burning boilers is in thermal power generators, wherein fuel burning boilers generate steam from water traveling through a number of pipes and tubes within the boiler, and the generated steam is then used to operate one or more steam turbines to generate electricity. The output of a thermal power generator is a function of the amount of heat generated in a boiler, wherein the amount of heat is directly determined by the amount of fuel consumed (e.g., burned) per hour, for example.

In many cases, power generating systems include a boiler which has a furnace that burns or otherwise uses fuel to generate heat which, in turn, is transferred to water flowing through pipes or tubes within various sections of the boiler. A typical steam generating system includes a boiler having a superheater section (having one or more sub-sections) in which steam is produced and is then provided to and used within a first, typically high pressure, steam turbine. To increase the efficiency of the system, the steam exiting this first steam turbine may then be reheated in a reheater section of the boiler, which may include one or more subsections, and the reheated steam is then provided to a second, typically lower pressure steam turbine. While the efficiency of a thermal-based power generator is heavily dependent upon the heat transfer efficiency of the particular furnace/boiler combination used to burn the fuel and transfer the heat to the water flowing within the various sections of the boiler, this efficiency is also dependent on the control technique used to control the temperature of the steam in the various sections of the boiler, such as in the superheater section of the boiler and in the reheater section of the boiler.

However, as will be understood, the steam turbines of a power plant are typically run at different operating levels at different times to produce different amounts of electricity based on energy or load demands. However, for most power plants using steam boilers, the desired steam temperature setpoints at final superheater and reheater outlets of the boilers are kept constant, and it is necessary to maintain steam temperature close to the setpoints (e.g., within a narrow range) at all load levels. In particular, in the operation of utility (e.g., power generation) boilers, control of steam temperature is critical as it is important that the temperature of steam exiting from a boiler and entering a steam turbine is at an optimally desired temperature. If the steam temperature is too high, the steam may cause damage to the blades of the steam turbine for various metallurgical reasons. On the other hand, if the steam temperature is too low, the steam may contain water particles, which in turn may cause damage to components of the steam turbine over prolonged operation of the steam turbine as well as decrease efficiency of the operation of the turbine. Moreover, variations in steam temperature also causes metal material fatigue, which is a leading cause of tube leaks.

Typically, each section (i.e., the superheater section and the reheater section) of the boiler contains cascaded heat exchanger sections wherein the steam exiting from one heat exchanger section enters the following heat exchanger section with the temperature of the steam increasing at each heat exchanger section until, ideally, the steam is output to the turbine at the desired steam temperature. In such an arrangement, steam temperature is controlled primarily by controlling the temperature of the water at the output of the first stage of the boiler which is primarily achieved by changing the fuel/air mixture provided to the furnace or by changing the ratio of firing rate to input feedwater provided to the furnace/boiler combination. In once-through boiler systems, in which no drum is used, the firing rate to feedwater ratio input to the system may be used primarily to regulate the steam temperature at the input of the turbines.

While changing the fuel/air ratio and the firing rate to feedwater ratio provided to the furnace/boiler combination operates well to achieve desired control of the steam temperature over time, it is difficult to control short term fluctuations in steam temperature at the various sections of the boiler using only fuel/air mixture control and firing rate to feedwater ratio control. Instead, to perform short term (and secondary) control of steam temperature, saturated water is sprayed into the steam at a point before the final heat exchanger section located immediately upstream of the turbine. This secondary steam temperature control operation typically occurs before the final superheater section of the boiler and/or before the final reheater section of the boiler. To effect this operation, temperature sensors are provided along the steam flow path and between the heat exchanger sections to measure the steam temperature at critical points along the flow path, and the measured temperatures are used to regulate the amount of saturated water sprayed into the steam for steam temperature control purposes.

Of course, both of these types of control can be generally performed using measurements of the initial output temperature of the boiler (called the water wall temperature), as well as an indication of the desired spray. In traditional boiler operations, a distributed control system (DCS) is used to provide control of both the fuel/air mixture provided to the furnace as well as control of the amount of spraying performed upstream of the turbines. As will be understood, however, the spray control technique can only operate to reduce the temperature of the steam over that developed within the various sections of the boiler, and thus the steam temperature at the outputs of the various sections of the boiler must be assured to be higher than otherwise might be necessary to assure that the steam temperature at the input of the turbines is high enough. Thus, use of the spray technique (which always operates to reduce the steam temperature at the spray nozzle) reduces the efficiency of the overall power generation system and thus should ideally be minimized. Moreover, depending on the power requirements of the electricity generation or other power generation system and the temperature of the spray feed, a lot of water may have to be sprayed into the steam to produce a significant reduction in steam temperature, meaning that it may be difficult to effectively use the spray technique to provide the necessary control in all situations.

None-the-less, in many circumstances, it is necessary to rely heavily on the spray technique to control the steam temperature as precisely as needed to satisfy the turbine temperature constraints described above. For example, once-through boiler systems, which provide a continuous flow of water (steam) through a set of pipes within the boiler and do not use a drum to, in effect, average out the temperature of the steam or water exiting the first boiler section, may experience greater fluctuations in steam temperature and thus typically require heavier use of the spray sections to control the steam temperature at the inputs to the turbines. In these systems, the tiring rate to feedwater ratio control is typically used, along with superheater spray flow, to regulate the furnace/boiler system. However, the desired superheater spray flow setpoint used to regulate superheater spray flow is quite arbitrary because its impact on heat rate (efficiency) is minimal, depending upon where the spray flow is drawn. Thus, while the spray flow technique is very effective in controlling steam temperature, its usage decreases the boiler efficiency and, as a result, it is harder to obtain optimum efficiency in the these types of systems.

SUMMARY

A technique of controlling a steam generating system includes using manipulated variables or control inputs of the reheater section of the boiler system to control the operation of the furnace/boiler portion of the system, such as to control the firing rate to feedwater input ratio used in the furnace/boiler combination. In particular, it is believed that, for example, in the case of a once-through boiler type of steam generating system, using signals indicative of the burner tilt position(s), damper position(s) or reheater spray amount associated with the reheater section of the system to control the fuel to feedwater flow ratio into the furnace/boiler section of the system provides better efficiency over current systems.

DETAILED DESCRIPTION

FIG. 1illustrates a block diagram of a once-through boiler steam cycle for a typical boiler100that may be used, for example, in a thermal power plant. The boiler100may include various sections through which steam or water flows in various forms such as superheated steam, reheated steam, etc. While the boiler100illustrated inFIG. 1has various boiler sections situated horizontally, in an actual implementation, one or more of these sections may be positioned vertically with respect to one another, especially because flue gases heating the steam in various different boiler sections, such as a water wall absorption section, rise vertically (or, spirally vertical).

In any event, as illustrated inFIG. 1, the boiler1001includes a furnace and a primary water wall absorption section102, a primary superheater absorption section104, a superheater absorption section106and a reheater section108. Additionally, the boiler100may include one or more desuperheaters or sprayer sections110and112and an economizer section114. During operation, the main steam generated by the boiler100and output by the superheater section106is used to drive a high pressure (HP) turbine116and the hot reheated steam coming from the reheater section108is used to drive an intermediate pressure (IP) turbine118. Typically, the boiler100may also be used to drive a low pressure (LP) turbine, which is not shown inFIG. 1.

The water wall absorption section102, which is primarily responsible for generating steam, includes a number of pipes through which water or steam from the economizer section114is heated in the furnace. Of course, feedwater coming into the water wall absorption section102may be pumped through the economizer section114and this water absorbs a large amount of heat when in the water wall absorption section102. The steam or water provided at output of the water wall absorption section102is fed to the primary superheater absorption section104, and then to the superheater absorption section106, which together raise the steam temperature to very high levels. The main steam output from the superheater absorption section106drives the high pressure turbine116to generate electricity.

Once the main steam drives the high pressure turbine116, the steam is routed to the reheater absorption section108, and the hot reheated steam output from the reheater absorption section108is used to drive the intermediate pressure turbine118. The spray sections110and112may be used to control the final steam temperature at the inputs of the turbines116and118to be at desired setpoints. Finally, the steam from the intermediate pressure turbine118may be fed through a low pressure turbine system (not shown here), to a steam condenser (not shown here), where the steam is condensed to a liquid form, and the cycle begins again with various boiler feed pumps pumping the feedwater through a cascade of feedwater heater trains and then an economizer for the next cycle. The economizer section114is located in the flow of hot exhaust gases exiting from the boiler and uses the hot gases to transfer additional heat to the feedwater before the feedwater enters the water wall absorption section102.

As illustrated inFIG. 1, a controller120is communicatively coupled to the furnace within the water wall section102and to valves122and124which control the amount of water provided to sprayers in the spray sections110and112. The controller120is also coupled to various sensors, including temperature sensors126located at the outputs of the water wall section102, the desuperheater section110, the second superheater section106, the desuperheater section112and the reheater section108as well as flow sensors127at the outputs of the valves122and124. The controller120also receives other inputs including the firing rate, a signal (typically referred to as a feedforward signal) which is indicative of and a derivative of the load, as well as signals indicative of settings or features of the boiler including, for example, damper settings, burner tilt positions, etc. The controller120may generate and send other control signals to the various boiler and furnace sections of the system and may receive other measurements, such as valve positions, measured spray flows, other temperature measurements, etc. While not specifically illustrated as such inFIG. 1, the controller120could include separate sections, routines and/or control devices for controlling the superheater and the reheater sections of the boiler system.

FIG. 2is a schematic diagram128showing the various sections of the boiler system100ofFIG. 1and illustrating a typical manner in which control is currently performed in once-through boilers in the prior art. In particular, the diagram128illustrates the economizer114, the primary furnace or water wall section102, the first superheater section104, the second superheater section106and the spray section110ofFIG. 2. In this case, the spray water provided to the superheater spray section110is tapped from the feed line into the economizer114.FIG. 2also illustrates two control loops130and132which may be implemented by the controller120ofFIG. 1or by other DCS controllers to control the fuel and feedwater operation of the furnace102.

In particular, the control loop130includes a first control block140(illustrated in the form of a proportional-derivative-integral (PID) control block) which uses, as a primary input, a setpoint in the form of desired superheater spray. This desired superheater spray setpoint is typically set by a user or an operator. The control block140compares the superheater spray setpoint to a measure of the actual superheater spray amount (e.g., superheater spray flow) currently being used to produce a desired water wall outlet temperature setpoint. The water wall output temperature setpoint is indicative of the desired water wall outlet temperature needed to control the temperature at the output of the second superheater106to be at the desired turbine input temperature, using the amount of spray flow specified by the desired superheater spray setpoint. This water wall outlet temperature setpoint is provided to a second control block142(also illustrated as a PID control block), which compares the water wall outlet temperature setpoint to a signal indicative of the measured water wall steam temperature and operates to produce a feed control signal. The feed control signal is then scaled in a multiplier block144, for example, based on the firing rate (which is indicative of or based on the power demand). The output of the multiplier block144is provided as a control input to a fuel/feedwater circuit146, which operates to control the firing rate to feedwater ratio of the furnace/boiler combination or to control the fuel to air mixture provided to the primary furnace section102.

The operation of the superheater spray section110is controlled by the control loop132. The control loop132includes a control block150(illustrated in the form of a PID control block) which compares a temperature setpoint for the temperature of the steam at the input to the turbine116(typically fixed or tightly set based on operational characteristics of the turbine116) to a measurement of the actual temperature of the steam at the input of the turbine116to produce an output control signal based on the difference between the two. The output of the control block150is provided to a summer block152which adds the control signal from the control block150to a feedforward signal which is developed by a block154as, for example, a derivative of the load signal. The output of the summer block152is then provided as a setpoint to a further control block156(again illustrated as a PID control block), which setpoint indicates the desired temperature at the input to the second superheater section106. The control block156compares the setpoint from the block152to a measurement of the steam temperature at the output of the superheater spray section110and, based on the difference between the two, produces a control signal to control the valve122which controls the amount of the spray provided in the superheater spray section110.

Thus, as will be seen from the control loops130and132ofFIG. 2, the operation of the furnace102is directly controlled as a function of the desired superheater spray. In particular, the control loop132operates to keep the temperature of the steam at the input of the turbine116at a setpoint by controlling the operation of the superheater spray section100, and the control loop130controls the operation of the fuel provided to and burned within the furnace102to keep the superheater spray at a predetermined setpoint (to thereby attempt to keep the superheater spray operation or spray amount at an “optimum” level).

FIG. 3illustrates a the typical (prior art) control loop160used in a reheater section108of a steam turbine power generation system, which may be implemented by, for example, the controller120ofFIG. 1. Here, a control block162produces a temperature setpoint for the temperature of the steam being input to the turbine118as a function of the steam flow (which is typically determined by load demands). A control block164(illustrated as a PID control block) compares this temperature setpoint to a measurement of the actual steam temperature at the output of the reheater section108to produce a control signal as a result of the difference between these two temperatures. A block166then sums this control signal with a measure of the steam flow and the output of the block166is provided to a spray setpoint unit or block168as well as to a balancer unit170.

The balancer unit170includes a balancer172which provides control signals to a superheater damper control unit174as well as to a reheater damper control unit176which operate to control the flue gas dampers in the various superheater and the reheater sections of the boiler. As will be understood, the flue gas damper control units174and176alter or change the damper settings to control the amount of flue gas from the furnace which is diverted to each of the superheater and reheater sections of the boilers. Thus, the control units174and176thereby control or balance the amount of energy provided to each of the superheater and reheater sections of the boiler. As a result, the balancer unit170is the primary control provided on the reheater section108to control the amount of energy or heat generated within the furnace102that is used in the operation of the reheater section108of the boiler system ofFIG. 1. Of course, the operation of the dampers provided by the balancer unit170controls the ratio or relative amounts of energy or heat provided to the reheater section108and the superheater sections104and106, as diverting more flue gas to one section typically reduces the amount of flue gas provided to the other section. Still further, while the balancer unit170is illustrated inFIG. 3as performing damper control, the balancer170can also provide control using furnace burner tilt position or in some cases, both.

Because of temporary or short term fluctuations in the steam temperature, and the tact that the operation of the balancer unit170is tied in with operation of the superheater sections104and106as well as the reheater section108, the balancer unit170may not be able to provide complete control of the steam temperature at the output of the reheater section108, to assure that the desired steam temperature at this location is attained. As a result, secondary control of the steam temperature at the input of the turbine118is provided by the operation of the reheater spray section112.

In particular, control of the reheater spray section112is provided by the operation of the spray setpoint unit168and a control block180. Here, the spray setpoint unit168determines a reheater spray setpoint based on a number of factors, taking into account the operation of the balancer unit170, in well known manners. Typically, however, the spray setpoint unit168is configured to operate the reheater spray section112only when the operation of the balancer unit170cannot provide enough or adequate control of the steam temperature at the input of the turbine118. In any event, the reheater spray setpoint is provided as a setpoint to the control block180(again illustrated as a PID control block) which compares this setpoint with a measurement of the actual steam temperature at the output of the reheater section108and produces a control signal based on the difference between these two signals, and the control signal is used to control the reheater spray valve124. As is known, the reheater spray valve124then operates to provide a controlled amount of reheater spray to perform further or additional control of the steam temperature at output of the reheater108.

As will be understood from the descriptions of the control loops ofFIGS. 2 and 3, the steam temperature is controlled in the reheater section108primarily by manipulation of the damper or burner tilt positions and secondarily by operation of the reheater spray section112. However, control of the damper or burner tilt positions effects the amount of energy or heat provided to the superheater sections104and106. Moreover, the control of the superheater sections104and106is primarily based on the amount of fuel provided to the furnace (e.g., the fuel to feedwater ratio) which is, in turn, controlled or based on a desired superheater spray setpoint. However, determination of the desired superheater spray setpoint is quite arbitrary, as the impact of this setpoint on the heat rate (efficiency) is minimal and typically is unknown.

A better manner of controlling the boiler system100ofFIG. 1is illustrated inFIG. 4in which similar blocks as those shown inFIG. 2are illustrated with the same reference numbers. As will be noted, the control scheme illustrated inFIG. 4used to control the operation of the furnace102, shown as control loop200, is very similar to the control loop130ofFIG. 2, but instead uses, as the primary input to the control block140, a factor or signal used to control or associated with the reheater section108of the boiler system100instead of a desired superheater spray setpoint. Thus, as illustrated in the control loop200ofFIG. 4, a desired or optimal burner tilt position is input to the control block104. Of course, while the burner tilt position is illustrated inFIG. 4as the input to the control block140, other signals or factors used in the control of or associated with the reheater section108could be used instead or in combination, including for example, signals related to damper positions of the dampers within the boiler system100, signals related to the reheater steam spray, etc. Thus, for example, in implementing this new type of control, the controller120ofFIG. 1may receive signals or use signals related to burner tilt position(s) of one or more burners in the boiler (especially the burners that effect the operation of or the heat provided to the reheater section108) or related to the damper position(s) of one or more dampers used in the boiler to direct heat flow through the reheater section108of the boiler or signals related to the control of the reheater spray section112including, for example, the output of the spray setpoint unit168, the output of the PID control block180, a measure of the position of the valve124, a measure of the actual amount of spray (e.g., flow or temperature reduction) being provided by the reheater spray section112, to produce the water wall outlet setpoint signal for the control block142.

Of course, while certain reheater control related signals are described herein as being input to the control loop200, other reheater control related signals or factors could be used as well or in other circumstances. Likewise, while the diagram ofFIG. 4illustrates a particular cascaded control loop or routine200to implement control of the furnace102, other desired types, kinds or configurations of control loops may be used instead of or in addition to that shown inFIG. 4, as long as these control loops use one or more reheater control or manipulated variable signals to control the operation of the furnace or boiler. Thus, for example, the control loop200could be configured in other manners, could use other types of control blocks or routines (such as other than PID control blocks), and could use other signals in any desired manner to combine with the reheater control related signal or the reheater manipulated variable signals to control the operation of the furnace102. For example, the control loop200could include a multi-input/single-output or a multiple-input/multiple-output control routine (such as a neural network routine, a model predictive control routine, an expert system based control routine, etc.) which accepts a number of inputs including one or more inputs related to or indicative of reheater section control or manipulated variables as well as potentially other inputs, to develop one or more output control signals to control the operation of the boiler/furnace to thereby provide steam temperature control. Additionally, while the control loop200ofFIG. 4is illustrated as producing a control signal for controlling the fuel/air mixture of the fuel provided to the furnace102, the control loop200could produce other types or kinds of control signals to control the operation of the furnace such as the fuel to feedwater ratio used to provide fuel and feedwater to the furnace/boiler combination, the amount or quantity or type of fuel used in or provided to the furnace, etc.

In any event, in the example illustrated inFIG. 4, the control block140compares the actual burner tilt positions with an optimal burner tilt position, which may come from off-line unit characterization (especially for boiler systems manufactured by Combustion Engineering) or a separate on-line optimization program or other source. Of course, in a different boiler design configuration, if flue gas by-pass damper(s) are used for primary reheater steam temperature control, then the signals indicative of the desired (or optimal) and actual burner tilt positions in the control loop200may be replaced or supplemented with signals indicative of or related to the desired (or optimal) and actual damper positions. Still further, instead of or in addition to the burner tilt positions and damper positions, the control block140may use a desired or optimal reheater spray flow setpoint as well as measurements of reheater spray flow to perform control. In this case, the optimal setpoint is generally the flow rate of reheater spray that is kept at a minimum while still being able to regulate steam temperature. Still further the control block140may use some reheater variable (manipulated variable) even if that variable itself is not used to directly control the reheater steam temperature.

It is believed that the use of a reheater manipulated and control variable, such as burner tilt positions, damper positions or reheater spray, to control the operation of the boiler or furnace102provides more direct impact on boiler efficiency and heat rate than, for example, superheater spray. In particular, it is believed that this approach has more direct and immediate control on boiler efficiency and heat rate than superheater spray variables, in addition to controlling the superheat and reheat steam temperatures as usual. For example, burner tilt positions directly affect the fire-ball position and flame temperature in the furnace, which directly affects combustion efficiency. Of course, the optimal setpoint for burner tilt position or damper position, can be determined by a separate procedure. If reheat steam temperature is controlled by reheater spray, the amount of spray flow also has a huge impact on heat rate. In fact, compared with superheater spray flow, the impact of reheater spray flow on heat rate is believed to be approximately 10 times higher, thus making reheater spray flow a better control variable for boiler or furnace control. More particularly, the primary difference between the cost of reheater and superheater sprays relates to the difference in additional energy that needs to be added in the boiler for these sprays. For example, if superheater sprays are used, and they come from the boiler feed pump, the enthalpy entering the boiler is about 320 Btu/lb. If no sprays were used, the same flow would come from final feedwater and enter the boiler at 480 Btu/lb and so an additional 160 btu/lb needs to be added from fuel in the boiler for superheater sprays. For reheater sprays, assuming that they also come from the boiler feed pump at 320 Btu/lb, cold reheat enthalpy is typically 1300 Btu/lb, and hot reheat enthalpy is typically 1520 Btu/lb. So here it is necessary to add about 1200 Btu/lb additional energy, making the use of reheater sprays (or other reheater variables) as a primary boiler control variable more effective in increasing boiler efficiency.

In any event, as will be seen fromFIG. 4, the rest of the control loop200is the same as or is similar to the control loop130ofFIG. 2and operates in essentially the same manner, except that the primary setpoint and control input into the loop200is derived from a reheater control or manipulated variable, instead of the superheater spray. However, as noted above, the details and implementation of the control loop200may be changed or be varied to control the operation of the furnace/boiler and the specific details of the control loop200shown inFIG. 4are not limiting of the invention, which is to control the operation of the furnace/boiler based on a reheater section manipulated or control variable, such as burner tilt position, damper position, reheater spray, etc. Likewise, the control of the superheater spray section110may be performed as illustrated inFIG. 2or4or may be changed in any desired manner inFIG. 4. In a similar manner, and the control of the reheater spray section112may be performed in the system ofFIG. 4using the same control scheme shown inFIG. 3or in any other desired manner. Also, the use of a reheater section manipulated or control variable in the control loop200ofFIG. 4is not limited to a control variable or a manipulated variable used to actually control the reheater section in a particular instance. Thus, it may be possible to use a reheater manipulated variable that is not actually used to control the reheater section108as an input to the control loop200that controls the furnace/boiler operation of the turbine system.

Still further, the control scheme described herein is applicable to steam generating systems that use other types of configurations for superheater and reheater sections than illustrated or described herein. Thus, whileFIGS. 1-4illustrate two superheater sections and one reheater section, the control scheme described herein may be used with boiler systems having more or less superheater sections and reheater sections, and which use any other type of configuration within each of the superheater and reheater sections.

Thus, many modifications and variations may be made in the techniques and structures described and illustrated herein without departing from the spirit and scope of the present invention. Accordingly, it should be understood that the methods and apparatus described herein are illustrative only and are not limiting upon the scope of the invention.