Burner Performance Monitoring Using Advanced Analysis of Optical Signals

Transcription

1 Technical Paper BR-1908 Burner Performance Monitoring Using Advanced Analysis of Optical Signals Authors: T.A. Fuller Babcock & Wilcox Power Generation Group, Inc. Barberton, Ohio, U.S.A. C.S. Daw C.E.A. Finney Oak Ridge National Laboratory Knoxville, Tennessee, U.S.A. Presented to: Power-Gen Europe Date: June 3-5, 2014 Location: Cologne, Germany

2 Burner Performance Monitoring Using Advanced Analysis of Optical Signals BR-1908 Presented to Power-Gen Europe Cologne, Germany, June 3-5, 2014 Tim Fuller Babcock & Wilcox Power Generation Group, Inc. 20 S. Van Buren Avenue Barberton, OH C. Stuart Daw Charles E. A. Finney Oak Ridge National Laboratory National Transportation Research Center 2360 Cherahala Boulevard Knoxville, TN Abstract Numerous economic and environmental factors are accelerating development of tighter controls for utility boilers. Such control may not be possible, however, with conventional technology alone. Advanced boiler optimization systems are a key part of these controls, and it is now recognized that accurate burner monitoring can be of great benefit to successful boiler optimization. This is particularly true for advanced low NO x (nitrogen oxides) burners because they are more sensitive to changes in coal quality and boiler operation. Continuous monitoring is especially crucial because significant shifts in flame quality can occur in just a few minutes due to unexpected changes in fuel and furnace operation. Engineers and boiler operators have long recognized that optical flame flicker patterns are correlated with burner emissions performance and flame quality. Several attempts have been made to develop burner monitoring systems based on conventional statistical and time series analysis of the optical flame flicker patterns. These systems have met with limited success, however, because the flicker patterns reflect the highly complex and nonlinear nature of the underlying combustion process. Over the last decade, Babcock & Wilcox Power Generation Group, Inc. (B&W PGG) has worked with researchers from the Oak Ridge National Laboratory (ORNL) to develop a new approach for analyzing the optical flicker patterns from burner flames. This new 1

3 approach relies on advanced analysis techniques from the field of nonlinear dynamics, chaos and nonlinear systems theory. This paper will discuss the application of these new techniques to optical signals from burner flames. The paper will also present field results showing how the application of this new monitoring approach can lead to significant reductions in emissions and an improvement in efficiency. Introduction Worldwide economic pressures and concerns with power plant emissions are leading to the need for even tighter control of fossil-fueled utility boilers. In particular, there is a need for boiler management systems that can continuously optimize efficiency and emissions over all operating conditions. This need has lead to the development and adoption of advanced combustion optimizers that work in conjunction with a boiler s basic control system to continuously improve combustion performance. These optimizers often adjust individual burners to achieve the desired goals; however, the optimizers rarely have direct information related to the performance of the individual burners. Burner performance monitoring systems that accurately reflect burner status are extremely beneficial to the success of advanced boiler optimization systems. Accurate monitoring of burner conditions is even more critical for advanced low NO x burners because they are more sensitive to changes in operating parameters and fuel feed system variations than conventional burners. Conventional combustion monitoring systems only provide information that has been averaged over many burners and long time scales (i.e., measurements of excess air, coal feed, or NO x emissions at time scales of several minutes or hours). It is now recognized that large NO x and carbon burnout fluctuations can occur in individual burners over short time scales (i.e., time scales of 10 seconds down to small fractions of a second). Such fluctuations are of concern because they can produce widely different boiler performance for operating conditions that appear to be otherwise indistinguishable. Combustion diagnostics should thus reflect both long- and short-time-scale transients to be more useful for boiler optimization. B&W PGG, a world leader in advanced combustion technologies, has long recognized the need for improved burner performance monitoring. Additionally, B&W PGG recognized that traditional analysis techniques were inadequate for the complex flame patterns found in utility burners. Seeking to develop an advanced burner performance monitoring system, B&W PGG collaborated with researchers from ORNL to apply cutting-edge analysis techniques to signals from individual burners. The entire initial development effort was performed with sponsorship from the Electric Power Research Institute (EPRI). 2

4 Development Measuring Combustion Performance The initial development effort focused on the type of coal-fired, low NO x utility burner illustrated in Figure 1. This burner is designed to achieve reduced NO x emissions by staging the combustion so that the peak flame temperature is reduced. The flame front for this type of burner develops at the interface between the fuel rich inner zone and fuel lean outer zone. The shape and volume of the flame depends on the relative flow velocities, temperature distribution, and concentration of volatiles and devolatilized char particles. While the staging process typically extends the volume of the combustion region and thereby reduces peak temperature, it also makes the flame less stable. That is, the flame is operating closer to extinction (a type of dynamic tipping point) and is sensitive to small changes in local conditions due to turbulence or shifts in operating parameters. This instability can lead to the emergence of large-scale fluctuations of the flame which can contribute to higher emissions and loss of efficiency. Existing flame scanner Figure 1 Typical Wall-Fired Low NO x Burner With Flame Scanner Boiler engineers and operators have long understood the importance of visual flame observations. Often, the visual appearance of a flame will tell an experienced operator if the combustion is proceeding as desired or if burner adjustments are required. In more recent years, engineers and operators have recognized that fluctuations in optical signals from flames are related to emissions levels. For these reasons, optical signals from commercially available flame scanners were selected as the basis for the advanced burner monitoring system. 3

5 Flame scanners were chosen over other optical sensors because they are already installed on nearly every burner in the utility and industrial industries. Also, the typical placement of flame scanners in a burner (see Figure 1) provides a view of the critical initial flame ignition front. By reusing existing sensors, the new monitoring system can be applied with minimal effort and hardware modifications. The major visible characteristics of a flame such as shape and ignition front location are known as the flame s state. A flame state is characterized by a distinct variation in output light intensity, which is often referred to as flicker. Figure 2 illustrates the scanner signals corresponding to three very distinct burner states. The X-axis in this set of plots is time while the Y-axis of each plot is scanner voltage. In the stable, wellattached case, most of the flicker is caused by combustion fluctuations associated with turbulent mixing of the coal and air. Because the shape and spatial position of the flame do not change much, this flicker is small in amplitude and appears to be largely random. The intermittent partial detachment state involves a rapid back-and-forth movement of the ignition front due to reduced ignition stability, which creates large characteristics spikes in the scanner signal. When the intermittent detachment becomes more severe, flicker becomes even more severe as the flame front jumps between two distinct spatial positions. This state has similar attributes to a noisy twowell potential. This spiking pattern is a type of emergent phenomenon, which is a characteristic feature of many complex nonlinear systems. Stable, well-attached flame Intermittent, partially detached flame Intermittent, fully detached flame Figure 2 Example Flame Scanner Data 4

6 Analyzing Combustion Performance The essential concept behind the new burner monitoring system is that the spiking pattern or flicker fingerprint can be used to identify the flame state of each burner. Knowing the flame state, B&W PGG can then apply its extensive knowledge of combustion to relate the flame state to anticipated burner emissions performance as well as to the root cause of any performance issues. The key challenge in this approach is to determine a set of quantitative metrics that describe the emergent flicker patterns and thus unambiguously characterize the flame state. Attempts by others have been made to characterize scanner signals using standard linear analysis methods such as Fourier decomposition. While some flame states can be distinguished in this way, many important differences become blurred due to the noisy nonlinear instabilities inherent in the combustion process. An example of this problem is illustrated in Figure 3 which depicts the Fourier power spectra for three different scanner signals. The two dashed lines represent very similar burner states where the primary air-to-coal flow ratio (PA/C) was near 1.8. The solid line, on the other hand, represents a very different burner state where the PA/C ratio was near 4. The Fourier spectra, however, would indicate that one of the low ratio flames was the same as the high ratio flame. Since the Fourier spectra comparison incorrectly matches these very different flame states, it is clear that such standard linear analysis techniques alone are not sufficient for achieving the desired level of flame discrimination PA/C=4.0 PA/C=1.80 PA/C=1.81 Power Frequency (Hz) Figure 3 Fourier Power Spectra for Three Different Flame States 5

7 Recognizing the limitations of linear analysis, B&W PGG and ORNL turned to the field of nonlinear dynamics, chaos, and complex systems theory (referred to collectively as nonlinear systems theory) for more advanced analysis techniques. Analytical techniques of these fields supplement the more traditional techniques of linear analysis and stochastic analysis. In linear analysis, complex problems are made tractable by approximating the real dynamics with linear models. This approach ignores any nonlinear effects which are typically important in any real-world problem of interest. In stochastic analysis, statistics are used to describe the long-term, global behavior of a system. While this approach includes the nonlinear effects, it cannot resolve temporal and spatial features of the system which may be of interest. Unlike the traditional approaches, nonlinear systems theory provides a set of analytical tools that are specifically designed to quantitatively describe the dynamic evolution of complex systems from one state to another. The usefulness of nonlinear systems analysis techniques for flame characterization is illustrated in Figure 4. This is a plot of the relative frequency of a range of possible flicker patterns extracted from the original scanner signals used in Figure 3. This type of analysis is known as symbol sequence analysis. The details of its implementation are beyond the scope of this paper. The basic concept, however, is to scan through a time series signal looking for the occurrence of pre-determined flicker patterns (known as symbol sequences). The frequency of each pattern is then plotted against the range of pattern types (known as sequence numbers) as shown in Figure 4. Figure 4 illustrates that the symbol sequence analysis correctly matches the two similar flame states while clearly separating the third different state. In this case, the intermittent flame detachment occurring at the high PA/C condition produces high frequencies of particular flicker patterns corresponding to unstable periodicities in the flame. Symbol sequence analysis like many other nonlinear systems methods, is more appropriate than traditional methods for detecting the unstable periodicities that can lead to poor burner performance. 6

8 Frequency PA/C=4.0 PA/C=1.80 PA/C= Sequence Number Figure 4 Symbol Sequence Analysis of Scanner Data Testing and Concept Refinement B&W PGG and ORNL performed a significant amount of testing to determine the mix of analysis techniques that would provide the desired flame state discrimination. The testing was performed at B&W PGG s research center in a single burner 5-million Btu/hr facility and a near-commercial-scale, single burner 100-million Btu/hr facility. Each facility was fully instrumented including gas species analyzers and also provided complete visual access to the flame. From the testing, B&W PGG and ORNL determined that a specific mix of linear and nonlinear systems analysis techniques provided the best flame state discrimination. The testing also allowed the research team to settle on the final approach for a meaningful burner performance monitoring system. In general, the diagnostic approach begins by calculating the prescribed set of specialized flame statistics for a given burner that characterize the global dynamic flicker patterns. The set of flame statistics are then compared to a library of reference flame states and the burner is assigned the flame state with the closest match in aggregate statistics. In addition to the flame statistics, the reference flame states contain information about the operating parameters that caused the state, as well as an overall flame rating number. The flame state library is open-ended so the number of possible states can grow with experience. The open library approach allows the performance monitoring system to learn over time by adding new library entries and/or discarding old entries. 7

9 The Flame Doctor System B&W PGG and ORNL have created a hardware and software package that can be used for burner performance monitoring on utility and industrial units. This package is known as the Flame Doctor system. The system hardware contains high speed data acquisition equipment and specialized signal conditioning boards that allow for simultaneous acquisition of up to 200 flame scanner signals. The Flame Doctor data acquisition and conditioning hardware is shown in Figure 5. The software runs all of the calculations required to assign each burner a flame state and overall rating. The overall rating is expressed on a scale of 0 to 100, where 0 is no flame and 100 is the optimal flame. In addition to the flame rating, the software also provides a root cause for any non-optimal burner and guidance on adjusting operating parameters to improve performance. The software also provides other information on the individual burners, including aggregate performance data for burners being fed by a given pulverizer. A partial screen shot of the Flame Doctor software is presented in Figure 6. Figure 5 Portable Flame Doctor Data Acquisition Hardware 8

10 Figure 6 Partial Flame Doctor Screen Shot Case Studies During the last several years, B&W PGG has utilized the Flame Doctor system on a large number of utility boilers worldwide to improve combustion performance. The following two case studies highlight the improvements that can be realized through application of advanced burner monitoring. Case Study 1 The first case study is on a 475 MW, B&W PGG Universal Pressure (UP ) boiler that fires an eastern United States (U.S.) bituminous coal. The vertical tower boiler is designed with the outlet being split into three parallel gas paths that encompass the primary superheater, first and second stage reheater, and economizer sections, respectively. Thirty (30) low NO x burners are arranged in two rows on each of the four boiler walls. The burners are fed by ten (10) B&W PGG EL-76 pulverizers. A side view of the unit is shown in Figure 7. 9

11 Figure 7 Case Study 1: Unit Side View B&W PGG was contracted to assist with tuning this unit because of high carbon monoxide (CO) levels. The customer increased O 2 levels to reduce the CO but experienced fan limitations and was not able to reduce the CO to acceptable levels. The increased O 2 levels caused a corresponding increase in NO x levels which put the plant in danger of violating its emissions permit. B&W PGG installed a portable version of the Flame Doctor system on the unit to simultaneously monitor all 30 burners. Using the combustion performance information provided by the Flame Doctor software, B&W PGG made systematic adjustments to the secondary air registers on the burners to add air to the burners with poor performance and remove air from the burners with good performance. B&W PGG started with the poorest performing burners first and continued making adjustments until all of the burners had achieved an acceptable level of performance. O 2 levels and burner adjustments continued until the plant s overall performance goals were reached. Using the Flame Doctor system to redistribute the secondary air to the burners had a positive impact on the O 2 distribution leaving the furnace. Figure 8 shows the starting and ending O 2 distribution from the plant s three O 2 sensors, as well as the unit load. 10

12 For this plant, the O 2 readings are expressed as excess air percentages. Excess air is represented on the left Y-axis while the load is represented on the right Y-axis. Time is represented on the X-axis. The excess air distribution at the start of tuning is given by the points on the left side of Figure 8 while the distribution at the end of tuning is given by the points on the right side. The initial distribution shows a significant spread in excess air of approximately 7% with the middle of the furnace having the lowest readings. After tuning the burners with the Flame Doctor system, the spread in excess air was reduced to approximately 2.4%. This result is typical of using the Flame Doctor system to optimally redistribute the secondary air to the burners Average Excess Air Average Excess Air % % Excess Air % 10.8% 13.6% 13.0% MWG /24/04 12:00 PM 5/25/04 12:00 AM 5/25/04 12:00 PM 5/26/04 12:00 AM 5/26/04 12:00 PM 5/27/04 12:00 AM 5/27/04 12:00 PM 5/28/04 12:00 AM O2 East Baseline O2 Middle Baseline O2 West Baseline O2 East FD Tuned O2 Middle FD Tuned O2 West FD Tuned Gross MW Gross MW Figure 8 Case Study 1: Excess Air Distribution The overall improvement in performance for this unit is shown in Figure 9. This figure shows the before and after tuning values for O 2 (excess air), CO, NO x, and average Flame Doctor burner assessment (diagnosis). As seen in the figure, B&W PGG improved the average performance of all 30 burners by approximately 7 points on the Flame Doctor system s diagnosis scale. The improved burner performance resulted in a 50% reduction in CO emissions. The reduced CO emissions, coupled with the tighter excess air distribution shown in Figure 8, allowed B&W PGG to reduce the overall excess air level by approximately 2%. This reduction in excess air allowed the plant to recover extra capacity on the fans and also had a positive impact on unit efficiency. 11

13 The reduced excess air level coupled with improved burner performance resulted in a 10% reduction in NO x emissions NOx CO O2 FD O NOx 500 CO 80 Diagnosis Baseline Tuned Figure 9 Case Study 1: Unit Performance Improvements Case Study 2 The second case study is on a 150 MW, B&W PGG radiant boiler (RB) that fires a combination of eastern U.S. bituminous coal and synthetic fuel (synfuel). The unit is equipped with 16 low NO x burners arranged in four rows and four columns on a single wall. Coal is fed to the burners from four (4) B&W Roll Wheel size 89N pulverizers. The unit is also equipped with overfire air (OFA) ports on the same wall as the burners. A side view of the unit is shown in Figure

14 Figure 10 Case Study 2: Unit Side View B&W PGG installed a portable version of the Flame Doctor system on this unit to help address concerns with flame impingement on the rear wall. The plant was also interested in reducing NO x emissions if possible. Similar to Case Study 1, B&W PGG used the Flame Doctor system s burner performance information to iteratively adjust the secondary air registers on the burners. B&W PGG also adjusted the burner spin vanes to control the length of the flames and prevent flame impingement. The spin vane adjustments were driven by visual observations, but the Flame Doctor system showed the effects of these adjustments on burner performance. The performance of all burners at the start of tuning is shown in Figure 11. This figure shows the Flame Doctor system s main burner performance results screen. Each burner is represented by a colored icon, with red icons representing underperforming burners. The gray icon indicates that the Flame Doctor system detected a signal fault for that burner during the current analysis cycle. Readings from the plant s two (2) O 2 probes are overlaid on the screen shot. 13

15 The pre-tuning assessment of combustion performance given in Figure 11 shows that the majority of the burners were underperforming. Visual observations showed that a large number of flames were impinging on the rear wall. The plant s O 2 readings indicated a large imbalance in the O 2 profile from left to right on the unit. Interestingly, the majority of underperforming burners were located on the side with the lower O 2 reading. 3.6% O 2 6.4% O 2 Figure 11 Case Study 2: Baseline Burner Performance Figure 12 shows the Flame Doctor system s main burner performance screen after the completion of tuning. The plant s O 2 readings are superimposed on the screen shot for reference. As shown in the figure, B&W PGG achieved an acceptable level of performance on almost all burners. In addition, by redistributing the secondary air based on burner performance, the O 2 imbalance on the unit was effectively eliminated. Finally, at the end of tuning, none of the burner flames were impinging on the rear wall of the unit. 14

16 5.6% O 2 5.3% O 2 Figure 12 Case Study 2: Tuned Burner Performance The Case Study 2 unit uses a selective noncatalytic reduction (SNCR) system for final NO x control. The SNCR system injects urea through an injection grid to control the NO x emissions to a targeted level. The plant only measures the NO x at the SNCR outlet and not at the SNCR inlet. Therefore, B&W PGG did not have a direct means of knowing how much the combustion tuning affected the NO x emissions from the boiler. However, the urea injection rate was used as an indirect indicator of boiler NO x emissions because higher emissions would require more urea to achieve the same targeted NO x level. Figure 13 shows a comparison of the urea usage before and after combustion tuning. The urea flow rate is expressed as gallons/day on the Y-axis. Before tuning, the unit required an average flow rate of approximately 260 gallons/day to achieve the targeted NO x level. After tuning, the urea flow rate dropped by 50% to around 120 gallons/day, indicating a significant drop in boiler NO x emissions. The reduction in urea consumption resulted in a significant savings in operating costs for this unit. 15

17 Gallons/day As Found Tuned Figure 13 Case Study 2: SNCR Urea Usage Summary B&W PGG and ORNL have collaborated to develop a burner performance monitoring system using advanced nonlinear systems analysis of optical flame scanner signals. The use of nonlinear analysis techniques allows the system to accurately and reliably assess the combustion performance at each individual burner. The new performance monitoring approach is incorporated in a hardware and software package known as the Flame Doctor system. In addition to the advanced nonlinear analysis techniques, the Flame Doctor system also encompasses B&W PGG s knowledge of combustion through an open-ended flame reference library. The Flame Doctor system has been utilized on a large number of boilers worldwide to reduce emissions and improve efficiency. B&W PGG has also initiated projects to couple the burner performance results provided by the Flame Doctor system with advanced combustion optimization systems. Results from these projects are very encouraging and are expected to have a wider application as power plant owners and operators seek to further optimize overall burner and boiler performance. 16

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