Patent Grant
8910478
The subject of this patent relates to automatic control of power plants, and more particularly to a method and apparatus for intelligently controlling Circulating Fluidized-Bed (CFB) Boilers and Once-Through Supercritical Circulating Fluidized-Bed (OTSC CFB) Boilers.
For the U.S. to reach its future energy objectives, visions to build ultra-clean and highly efficient energy plants of the future have to be realized. In parallel with the development of sensors, more robust and flexible process control technologies must be developed to build an intelligent control system that can yield a fully automated operation and be adaptive to changing process needs and fuel availability. It must be safe, reliable, and easy to install, maintain, and operate. The intelligent control system is aimed to control conventional boilers as well as advanced boilers including Once-Through Supercritical Boilers, Circulating Fluidized-Bed (CFB) Boilers, and Supercritical CFB Boilers in future energy plants that can deliver maximum-energy-efficiency, near-zero-emissions, fuel-flexibility, and multi-products.
First introduced in 1997, the Model-Free Adaptive (MFA) control technology overcomes the shortcomings of traditional Proportional-Integral-Derivative (PID) controllers and is able to control various complex processes that may have one or more of the following behaviors: (1) nonlinear, (2) time-varying, (3) large time delay, (4) multi-input-multi-output, (5) frequent dynamic changes, (6) open-loop oscillating, (7) pH process, and (8) processes with large load changes and disturbances.
Since MFA is “Model-Free”, it also overcomes the shortcomings of model-based advanced control methods. MFA is an adaptive and robust control technology but it does not require (1) precise process models, (2) process identification, (3) controller design, and (4) complicated manual tuning of controller parameters. A series of U.S. patents and related international patents for Model-Free Adaptive (MFA) control and optimization technologies have been issued. Some of them are listed in Table 1.
Commercial hardware and software products with Model-Free Adaptive control have been successfully installed in most industries and deployed on a large scale for process control, building control, and equipment control.
In the U.S. patent application No. 61/473,308, we described a 3×3 MFA control system to control key process variables including Power, Steam Throttle Pressure, and Steam Temperature of Boiler-Turbine-Generator (BTG) units in conventional and advanced power plants. Those advanced power plants may comprise Once-Through Supercritical (OTSC) Boilers, Circulating Fluidized-Bed (CFB) Boilers, and Once-Through Supercritical Circulating Fluidized-Bed (OTSC CFB) Boilers.
In this patent, we expand the invention by introducing a multivariable Model-Free Adaptive control system to control a 5-Input-5-Output (5×5) combustion process of Circulating Fluidized-Bed (CFB) Boilers and Once-Through Supercritical Circulating Fluidized-Bed (OTSC CFB) Boilers. We will also describe a novel MFA control system for controlling combined 3×3 BTG process and 5×5 CFB combustion process.
In the accompanying drawings:
In this patent, the term “mechanism” is used to represent hardware, software, or any combination thereof. The term “process” is used to represent a physical system or process with inputs and outputs that have dynamic relationships.
Without losing generality, all numerical values given in controller parameters in this patent are examples. Other values can be used without departing from the spirit or scope of our invention.
For simplicity, all engineering values in the time-amplitude diagrams used to show control system performance are converted to the scale of 0 to 100.
A. Advanced Power Boilers
Compared with sub-critical fixed bed conventional boilers, there are 3 types of advanced boilers: (1) Once-Through Supercritical (OTSC) Boilers, (2) Circulating Fluidized-Bed (CFB) Boilers, and (3) Once-Through Supercritical Circulating Fluidized-Bed (OTSC CFB) Boilers. Generally speaking, a power plant that is equipped with any number of advanced boilers can be called an advanced power plant.
Boilers used in energy plants are either “drum” or “once-through” types, depending on how the boiler water is circulated. Heat is transferred through the furnace tubes and into the water passing through the tubes to generate steam. In drum-type boilers, the steam-flow rate is typically controlled by the fuel-firing rate. In once-through boilers, the steam-flow rate is established by the boiler feedwater and the superheated steam temperature is controlled by the fuel-firing rate. A boiler is called supercritical when the master steam pressure is over 22.129 Mpa. In general, when water goes over the critical point (Pressure=22.129 Mpa, and Temperature=234 degree C.), it becomes steam. Therefore, a steam drum cannot be used and the Once-Through design is the only choice for supercritical boilers. Once-Through Supercritical boilers run at higher steam temperature and pressure so that better energy efficiency is achieved. But they are difficult to control as summarized in Table 2.
Circulating Fluidized-Bed (CFB) boilers are becoming strategically important in power and energy generation. The unique design of CFB boilers allows fuel such as coal powders to be fluidized in the air so that they have better contact with the surrounding air for better combustion. CFB boilers can burn low-grade materials such as waste coal, wood, and refuse derived fuel. Most importantly, less emissions such as COx and NOx are produced compared to conventional boilers. The critical process variables and their control challenges for a CFB boiler are listed in Table 3. For a CFB boiler, the control challenges are mainly related to the combustion process of its furnace.
B. Supercritical CFB Boilers and BTG Units
Once-Through Supercritical Circulating Fluidized-Bed (OTSC CFB) Boilers or Supercritical CFB Boilers combine the merits of once-through supercritical and circulating fluidized-bed technologies. As a strategically important clean coal technology, Supercritical CFB boilers can significantly improve combustion and energy efficiency, reduce emissions, and have fuel flexibility. It is the most promising boiler for future energy plants because of all its outstanding advantages.
A Supercritical CFB boiler based electric power plant also consists of three key components: (1) Boiler, (2) Turbine, and (3) Generator. Similar to conventional boilers, a Supercritical CFB boiler produces superheated steam to turn the turbine to allow the generator to generate electricity. Operating as a set, the combined Boiler, Turbine, Generator, and all auxiliaries make up a BTG unit.
For a Supercritical CFB boiler, most of the control challenges in Supercritical boilers and in CFB boilers still exist. Since the Supercritical CFB boiler combines the chaotic operating conditions of a CFB boiler and the once-through nature of a supercritical boiler, the control challenges could double. For such a boiler, maintaining a dynamic material and energy balance becomes a big challenge. In general, for a Supercritical CFB boiler, its BTG process and its CFB combustion process are much more dependent on a good automatic control system in order to keep the energy and material balance. If not careful, the entire system can get into vicious cycles causing serious consequences. For instance, when a steam demand increases, it will cause the steam pressure to go down, which will quickly affect the boiler firing condition and then the fluidized-bed condition. The changed combustion condition will result in more changes in steam temperature and pressure and therefore a vicious cycle will build up causing major operation and safety problems. Conventional control methods including coordinated control of steam turbine and boiler control will have major difficulties in controlling Supercritical CFB boilers.
In a power generation network, a BTG unit may be base-loaded to generate at a constant rate, or may cycle up and down as required by an automatic dispatch system. In either case, the boiler control system manipulates the firing rate of the furnace to generate the steam required to satisfy the demand for power. It is also necessary to maintain an adequate supply of feedwater and the correct mixture of fuel and air for safe and economic combustion. These requirements are actually the same for a conventional BTG unit or a BTG unit that employs an advanced power boiler such as a Supercritical boiler, a CFB boiler, or a Supercritical CFB boiler.
C. MFA Control of BTG Units
As introduced in the U.S. patent application No. 61/473,308, the multivariable MFA control system design method has the following key points:
As introduced in the U.S. patent application No. 61/473,308, a 3×3 MFA control system is designed to control the critical process variables of the BTG unit including Power (JT), Steam Throttle Pressure (PT), and Steam Temperature T1. The process has 3 inputs and 3 outputs and is called a Power-Pressure-Temperature (PPT) process. The 3×3 PPT process of a BTG unit includes 9 sub-processes G11, G21, . . . , G33 as listed in Table 5.
The importance of the variable pairing is that we want to make sure the 3 main processes G11, G22, and G33 have a strong direct or reverse acting relationship so that they have good controllability.
The 3×3 PPT process has nine sub-processes G11 through G33 as listed in Table 5. The process variables y1, y2, and y3 are Power (JT), Steam Throttle Pressure (PT), and Steam Temperature T1, respectively. They are the feedback signals for each of the main control loops and compared with the setpoints r1, r2, and r3 at adders 22 to produce error signals e1, e2, and e3. The outputs of the 3×3 MFA controller u1, u2, and u3 manipulate the manipulated variables Firing Rate (RF), Throttle Valve (VT), and Water Feed (FW) to control the Power (JT), Steam Throttle Pressure (PT), and Steam Temperature T1, respectively.
D. Combustion Process of a Supercritical CFB Boiler
Through the coal Feeder, fuel is fed to the lower furnace where it is burned in an upward flow of combustion air. Unburned fuel and ash leaving the furnace are collected by the Cyclone Separator and returned to the lower furnace. Limestone is also fed to the lower furnace for emission reduction.
Multiple fans and dampers are used to form the Primary Air, Secondary Air, and Exhaust Air as manipulated variables to achieve the following control objectives: (1) hold the proper CFB circulating condition, (2) keep the combustion fuel-air-ratio, and (3) control the furnace negative pressure. Since each manipulated variable can affect all three control objectives, this is a strongly coupled multivariable process. The air system of a CFB furnace is much more complex than a fixed-bed furnace because the CFB circulating condition has to be held as an additional control objective.
In a CFB furnace, there are 4 regions based on the vertical distribution of solids, which can be coal or fuel powder. They are the Bottom Region, Dense Region, Dilute Region, and Exit Region. The Bed Thickness can be roughly described as a process variable representing the thickness or the height of the dense region. It can be estimated using the pressure differential in the Dense Region of the CFB furnace. CFB boilers are typically operating in 50:1 ash to coal ratio. That means, during normal operation, only 2% of fresh coal or fuel powder is mixed with 98% coal ash that still has a lot of energy. Since the Dense Region has the highest heat transfer efficiency through direct contact to the furnace wall, it is important to run the CFB furnace at an optimal Bed Thickness.
If the Bed is too thin, the heat transfer efficiency will be low. If the Bed is too thick, it will not hold-up since it is the fluidized bed, which requires a sufficient amount of air and pressure to establish the bed. So, it is desirable to run the CFB furnace at the maximum Bed Thickness possible, while not causing other operating condition problems such as a fuel and air ratio mismatch. This indeed is a very complex problem, where the industry still does not have good answers to many of the questions. Typically, a trial-and-error based operation is the practice in real power plants, and the Bed Thickness is fixed at a relatively conservative and safe position. This results in low efficiency and potential CFB furnace shutdowns if the fuel type and size suddenly change. Automatic control of Bed Thickness is very important for the new generation of CFB boilers, especially Supercritical CFB boilers.
Slag Disposal is the ash leaving the CFB furnace. Because it affects the Bed Thickness directly, we use Slag Disposal as the manipulated variable for controlling the Bed Thickness. The Solids Recycle Feed is another process variable that can affect the Bed Thickness. Since manipulating this variable can only cause a temporary change to the Bed Thickness, it is best to leave it running at a constant rate.
Based on the multivariable MFA control system design method, we selected 5 pairs of variables with 25 sub-processes to form a 5-Input-5-Output CFB combustion process. The process inputs as manipulated variables and the process outputs as the process variables to be controlled are listed in Table 6.
In
The importance of the variable pairing is that we want to make sure the 5 main processes H11, H22, H33, H44, and H55 have a strong direct or reverse acting relationship so that they have good controllability. As part of Model-Free Adaptive (MFA) control system design strategy, we use S (Strong), M (Medium), and W (Weak) to represent the degree of connections between the input and output of each sub-process. We also use the plus or minus sign to represent whether the process is direct or reverse acting. The detailed qualitative input and output relationship among all 25 sub-processes is analyzed and presented in Table 7. They provide valuable information when we design and configure the MFA control system for controlling this complex process.
E. Optimal CFB Combustion Control
Combustion is a complex sequence of exothermic chemical reactions with fuel and oxygen producing heat. For industrial furnaces that use fossil fuel (gas, oil, or coal), good combustion control is desirable. Good combustion requires the correct amount of oxygen. Too little air results in CO formation, soot, and even explosion. Too much air will result in excessive NOx emissions and low efficiency due to the heat loss. In practice, an optimal combustion control condition can be considered at the point where Excess O2=2%, and CO2, H2, and CO are all under 100 ppm (portion per million).
Optimal combustion control is about finding the optimal fuel-air-ratio dynamically in the sense of most efficient combustion and meeting the emission requirements of COx, NOx and SOx. There are many ambient and atmospheric conditions that can affect the optimal fuel-air-ratio. For example, cold air is denser and contains more oxygen than warm air; wind speed affects the stack; and barometric pressure affects the draft, etc. Using oxygen sensors to measure the excess O2 in flue gas, O2 trim control can be implemented with an O2 control loop.
vf(t)=L[uf(t)], (1)
u1(t)=a1vf(t)+Δua1(t), (2a)
u2(t)=a2vf(t)+Δua2(t), (2b)
u3(t)=a3vf(t)+Δua3(t). (2c)
In these equations, uf(t) is the fuel flow signal, L(.) is a scaling function to scale the fuel flow signal uf(t) to a control signal vf(t) in the range of 0 to 100, Δua1(t), Δua2(t), Δua3(t) are controller output incremental signals from the 3×3 MFA controller or the 3×3 controller, a1, a2, a3 are fuel-air ratio parameters, and u1(t), u2(t), u3(t) are controller outputs of the 3×3 Fuel-Air Ratio Controller. The fuel-air ratio parameters are related to the fuel type and grade, and can be determined by certain formulas and experimentation.
The 3×3 MFA controller that can be used in this embodiment has been described in the U.S. patent application No. 61/473,308. The 3×3 controller that can be used in this embodiment are any of a number of well known automatic controllers that are developed based on the control methods described in the “Instrument Engineers' Handbook—Process Control and Optimization,” edited by Bela Liptak, published by CRC Press in 2005, including PID Control, Model-Based Control, Model-Free Adaptive (MFA) Control, Model Predictive Control, and Nonlinear and Adaptive Control.
As shown in
A SISO MFA controller 56 is used to control the Coal Feed flow. Adder 60 is used to form the Coal Feed feedback loop. The SISO MFA controllers that can be used in this embodiment have been described in U.S. Pat. Nos. 6,055,524 and 6,556,980. The Fuel Flow signal uf(t) connected with the Coal Feed setpoint is a critical input signal for the 3×3 Fuel-Air Ratio MFA Controller 38 since it is the leading signal for fuel-air ratio control.
F. Control of Supercritical Circulating Fluidized-Bed Boilers
The control system comprises a 3×3 Fuel-Air Ratio MFA Controller for Air Systems 61, a 5×5 CFB Combustion Process 62, a SISO MFA Controller 65, a 3×3 MFA Controller for BTG Unit 66, and a 3×3 PPT Process of BTG Unit 67. For this 7×7 MFA control system, the setpoints (SP) are r1, r2, . . . , r7; the controller outputs (OP) are u1, u2, . . . , u7; and controlled process variables (PV) are y1, y2, . . . . , y7, respectively. The feedback loops and signal adders are not drawn due to the limited space of the figure. The 3×3 Fuel-Air Ratio MFA Controller for Air Systems 61 has been described in
Within the 3×3 MFA control system for controlling the 3×3 Power-Pressure-Temperature (PPT) process of a Boiler-Turbine-Generator (BTG) unit, there are 3 sub-systems including the Firing Rate and Combustion Sub-System 62, Throttle Valve and Steam Flow Sub-System 69, and Water Flow Sub-System 70. Each of the sub-systems may include various control loops. For instance, the Water Flow Sub-System typically includes a water flow control loop. In this case, control signal u7 from the 3×3 MFA controller 66 is used as the setpoint for the water flow control loop, which is the inner loop of the cascade control system. MFA controllers or conventional controllers could be used to control these sub-systems.
Within the Firing Rate and Combustion Sub-System 62, there are three second layer sub-systems including the Primary Air Sub-System 63, Secondary Air Sub-System 64, and Coal Feed Sub-System 68. SISO MFA controllers can be used in these sub-systems as illustrated and described in
Without losing generality, the control system comprises a 3×3 Fuel-Air Ratio Controller for Air systems 71, a 5×5 CFB Combustion Process 72, a SISO Controller 75, a 3×3 Controller for BTG Unit 76, and a 3×3 PPT Process of BTG Unit 77. For this 7×7 control system, the setpoints (SP) are r1, r2, r7; the controller outputs (OP) are u1, u2, . . . , u7; and controlled process variables (PV) are y1, y2, . . . . , y7, respectively. The feedback loops and signal adders are not drawn due to the limited space of the figure.
Within the 3×3 control system for controlling the 3×3 Power-Pressure-Temperature (PPT) process of a Boiler-Turbine-Generator (BTG) unit, there are 3 sub-systems including the Firing Rate and Combustion Sub-System 72, Throttle Valve and Steam Flow Sub-System 79, and Water Flow Sub-System 80. Each of the sub-systems may include various control loops. For instance, the Water Flow Sub-System typically includes a water flow control loop. In this case, control signal u7 from the 3×3 controller 76 is used as the setpoint for the water flow control loop, which is the inner loop of the cascade control system. Within the Firing Rate and Combustion Sub-System 72, there are three second layer sub-systems including the Primary Air Sub-System 73, Secondary Air Sub-System 74, and Coal Feed Sub-System 78.
The SISO controller and 3×3 controllers that can be used in this embodiment are any of a number of well known automatic controllers that are developed based on the control methods described in the “Instrument Engineers' Handbook—Process Control and Optimization,” edited by Bela Liptak, published by CRC Press in 2005, including PID Control, Model-Based Control, Model-Free Adaptive (MFA) Control, Model Predictive Control, and Nonlinear and Adaptive Control.
G. Control Experiments and Simulation Results
Under the projects of SBIR grant DE-FG02-06ER84599 awarded by the U.S. Department of Energy, extensive research and development have been performed including the development of real-time simulation models for the 3×3 Power-Pressure-Temperature (PPT) process of BTG units, 4×4 CFB combustion processes, 5×5 CFB combustion processes, combined BTG and CFB processes, and Supercritical CFB boilers. In addition, automatic controllers including the 3×3 MFA controller described in the U.S. patent application No. 61/473,308 as well as the 3×3 Fuel-Air Ratio MFA controller described in this patent application have been developed in real-time control software platforms. In
In
In this case, the Bed Temperature SP (Signals 135 and 138) is firstly stepped up from 45 to 60, the Excess O2 SP (Signals 141 and 144) is then stepped down from 60 to 40, and the Furnace Pressure SP (Signals 147 and 150) is lastly stepped up from 3 to 6. It can be seen that each setpoint change causes disturbances to all control loops. The 3×3 MFA air control system is able to deal with the disturbances and keeps the Bed Temp, Excess O2, and Furnace Negative Pressure under control. In contrast, the PID control system cannot effectively control the 3×3 process resulting in oscillations in all 3 loops.
To summarize, the control trends demonstrate outstanding control performance of the 3×3 Fuel-Air Ratio MFA Controller for Air Systems for both tracking and regulating capabilities. The compensators inside the 3×3 MFA controller can effectively decouple and reduce the interactions from the other loops of the multivariable combustion process. The control trends also demonstrate unsatisfactory control performance of the PID control system. Since PID controllers are single-loop controllers and can only treat the 3-Input-3-Output (3×3) multivariable process as three single-input-single-output (SISO) processes, it is very difficult for the PID control system to be effective and achieve good control performance. When there is a setpoint change or disturbance in the process, it will take a long time for the loops to settle down due to interactions among the loops. For instance, when the setpoint of Loop 1 is changed, the PID control action in Loop 1 will disturb Loop 2 and 3 causing their PID controllers to produce control actions, which will come back to disturb Loop 1. The multiple and bi-directional interactions can cause conflicting control actions and trigger a vicious cycle resulting in loop oscillations. Therefore, when applying PID for multivariable control, most PID controllers are significantly de-tuned to avoid potential oscillations or even unstable control. In the real world, a large percentage of multi-input-multi-output (MIMO) processes are treated as single-input-single-output (SISO) processes resulting in poor control performance, inconsistent quality, wasted materials and energy, and plant safety problems.
In
In
To conclude, the 7×7 Model-Free Adaptive (MFA) control system described in this patent shows excellent control performance and robustness in dealing with setpoint changes in different variables, interactions among the process variables, disturbances caused by varying operating conditions, and other uncertainties.
In the foreseeable future, the energy needed to support our economic growth will continue to come mainly from coal, the most abundant and lowest cost resource on this planet. The performance of coal-fired power plants is highly dependent on coordinated and integrated sensing, control, and actuation technologies and products. The control systems and methods described in this patent application as well as in U.S. patent application No. 61/473,308 can be very useful for controlling advanced boilers including Once-Through Supercritical (OTSC) Boilers, Circulating Fluidized-Bed (CFB) Boilers, and Once-Through Supercritical Circulating Fluidized-Bed (OTSC CFB) Boilers in future energy plants that can deliver maximum-energy-efficiency, near-zero-emissions, fuel-flexibility, and multi-products.
This application claims priority to U.S. Provisional Application No. 61/586,411 filed on Jan. 13, 2012, which is herein incorporated by reference.
This invention was made with government support under SBIR grant DE-FG02-06ER84599 awarded by the U.S. Department of Energy. The government has certain rights to the invention.
| Number | Name | Date | Kind |
|---|---|---|---|
| 2315527 | Junkins et al. | Apr 1943 | A |
| 3138000 | Vollmer, Jr. | Jun 1964 | A |
| 3625186 | Herbst | Dec 1971 | A |
| 5027751 | Archer et al. | Jul 1991 | A |
| 5038568 | Gounder et al. | Aug 1991 | A |
| 20120259437 | Cheng et al. | Oct 2012 | A1 |
| Number | Date | Country | |
|---|---|---|---|
| 20130180244 A1 | Jul 2013 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 61586411 | Jan 2012 | US |
| Child | 13739939 | US |
