Patent Application
20080050684
Some of the objects of the invention have been set forth above. Other objects and advantages of the invention will appear as the description of the invention proceeds when taken in conjunction with the following drawings, in which:
Referring now specifically to the drawings, a typical cyclone of the type utilized in the practice of the methods according to this invention is shown generally in
The cyclone 10 includes a barrel 12 having a re-entrant throat 14. A burner 16 is mounted so as to project a flame into the re-entrant throat 14. The burner 16 may be of the radial, scroll or vortex type. A secondary air throat 18 mounted in an offset position between the burner 16 and the re-entrant throat 14. The interior of the barrel 12 defines a central vortex zone 20 and a surrounding recirculation zone 22. A main flame scanner 24 and a lighter scanner 26 are positioned to detect conditions and generate signals within the barrel 12 and burner 16 indicative of the operation of the system.
The main flame scanner 24 detects flame instabilities resulting from insufficient primary and/or tertiary air in the burner 16. The lighter scanner 26 detects flame instabilities resulting from insufficient secondary air in the barrel 12. The signals for the main flame and lighter scanners 24, 26 are relatively independent, thereby allowing for independent control of the primary air flow to the burner 16 and secondary air flow to the barrel 12. In general, instabilities in the burner 16 or barrel 12 have only a minor effect on the combustion conditions in the other zone. This finding was unexpected and non-obvious. It was expected that a strong correlation would exist between combustion events in the burner 16 and barrel 12. Contrary to this expectation it was discovered that the combustion in the burner and barrel zones is essentially independent. Cross correlation analysis did not show a strong dependency. Therefore, analyses of the burner 16 and barrel 12 can be decoupled and the corrective action implemented independently.
Variations in statistics based on simple visual qualitative inspection of the statistics do not emerge until the air flows drop below a critical value. The sensitivity is improved by applying quantitative statistical tests. The critical value corresponds to the onset of instability in the air flow distribution and combustion.
Referring now to
An optimal condition is achieved by adjusting the corresponding air flow slightly above the critical value. Below the critical value, the statistics change in proportion to the deviation from optimal. Therefore, the statistics can provide a quantitative indication of the adjustment that is needed to restore optimal operation.
Above the critical value, the statistics do not change with increases in air flow. Once a stable flame is achieved, typical variations in air flow do not substantially affect stability since the cyclone is inherently stable when operated unstaged.
Typically, the optimal primary and tertiary air split is approximately 5% of the total amount of air supplied as tertiary and 10-15% supplied as primary air. Cyclone burners operate best with minimal tertiary air. Tertiary air is adjusted to keep the burner face cool.
A series of tests was performed in which the primary and tertiary air split was varied over a wide range of combinations. The statistics of skewness, kurtosis, temporal asymmetry and symbol sequence clearly indicated a condition in which the sum of primary air and tertiary air flow is too low. However, the analysis did not uniquely discriminate a condition in which the total flow is too low due to insufficient primary air or tertiary air individually.
As is shown in
Two types of information are provided to the operator. First, the technique identifies the current status of the cyclone. It is beneficial to have an indication of the current state of the condition of each cyclone, and a comparison of the performance of the cyclones with each other.
An example of the change in overall assessment of the cyclone condition is shown in
Windbox pressure can have a significant effect on cyclone flame stability. Insufficient windbox pressure adversely affects the flow patterns in the cyclone resulting in poor mixing, and consequently, poor combustion.
The coal blend can have a significant impact on the stability of the cyclone. For example, for a given level of staging, a richer blend of eastern bituminous coal (e.g., 70/30 PRB/Bituminous vs 80/20 PRB/Bituminous) may be more stable. Therefore, conceivably, the optimum cyclone operating conditions could be adjusted as a function of coal blend.
The analysis techniques claimed in U.S. Pat. Nos. 6,775,645 B2 and 6,901,351 B2 were enhanced to track drift in cyclone operation and identify the potential root cause(s) of the drift. Variance bands, as shown in
With these considerations and principles in mind, a methods to adjust the air flow distribution is now described with reference to
The lighter scanner 26 is used to adjust secondary air flow through the secondary air damper 34, and to detect instabilities due to improper distribution in secondary air flow and slag properties.
The analog model for the control method is a single acting flow control valve with spring return. The air flows are held to minimum unless otherwise directed by the results of the analysis of the flame scanner signals.
The primary objective is to minimize NOx while maintaining acceptable carbon monoxide emissions, unburned carbon loss, and reliable cyclone operation, i.e., uniformly flowing slag. In a preferred embodiment of the invention, a fixed coal blend is used, with a single secondary air damper 34 and separate primary and tertiary air dampers 30, 32 in a radial burner cyclone design.
The method is preferably implemented in an open-loop, advisory mode.
If the boiler is staged, the secondary air flow is set to a typical damper setting with the secondary air damper 34, providing equal secondary air flow to each cyclone as best determined by plant instrumentation e.g., static pressure in duct, differential pressure across flow measurement device, and the like. Over-fire air flow dampers 36 are set to the same damper position to cause equal air flow through each. The tertiary air flow damper 32 is then closed.
The primary air flow damper 30 is manually adjusted to 90-95% of the target sum of the primary air flow and tertiary air flow as indicated by flow measurement devices or the position of the primary air damper 30. The tertiary air flow damper 32 is then used to provide balance of primary and tertiary air flow as indicated by flow measurement devices. The cyclone performance is assessed by analysis of the flame scanner signals from the main flame scanner 24 and lighter scanner 26. For cyclones 10 indicating instability in burner 16 or barrel 12 as indicated by statistics outside variance limits, the primary air flow or secondary air flow is increased as appropriate to eliminate instability. Instability is indicated by some or all of statistics being outside of ± variance limits.
With all cyclones 10 indicating stable operation in both main flame and scanner lighter signals, the primary air flow is decreased on each cyclone until the main flame scanner signal just begins to exhibit instability, as indicated by statistics drifting toward variance limits. Tertiary air flow is maintained at minimum flow.
After adjusting the primary air flow to the minimum value needed to just achieve stable operation, the secondary air flow on each cyclone is decreased by closing the secondary air damper 34 stepwise, and simultaneously until the lighter scanner 26 just begins to detect instability.
Simultaneously, air flow to all overfire air ports 38 is increased equally to maintain overall excess air while decreasing secondary air flow. This corresponds to minimum secondary air flow and maximum degree of staging for each cyclone 10. If the secondary air flow is reduced further, there would be insufficient heat released in the cyclone 10 to keep the slag free flowing. Note that the air flow and/or damper positions on all cyclones 10 will likely not be the same.
Finally, the overfire air dampers 36 are adjusted as needed to compensate for any O2 or CO imbalances resulting from the adjustments made to the cyclones 10.
This method for tuning the cyclones 10 should result in the maximum staging and minimum emissions of NOx. It provides a direct, non-iterative approach for tuning cyclones. Once tuned, the method can then be used to detect upsets in operation due to other factors such as coal properties or slag properties using the tracking feature.
A temporary upset in operating conditions is compensated for by adjusting cyclone operation. If the instability is apparent in the main flame scanner signal, compensation is achieved by providing a positive bias to the primary air flow until the instability is eliminated.
If the instability is apparent in the lighter scanner signal, compensation is achieved by providing a positive bias to the secondary air flow until the instability is eliminated.
The air biases (primary air bias 40 and secondary air bias 42) are reduced on a timed basis to verify that the temporary upset has passed and conditions can be restored to optimal.
When combined with a neural network boiler optimizing software, the method can be used to identify optimum settings on individual cyclones and over-fire air ports for a given coal blend. The optimal settings for each coal blend can be stored in the control system. As the coal blend changes, the primary air bias 40 and secondary air bias 42 can be adjusted automatically to adjust the cyclone settings and the degree of staging to achieve optimal conditions for that blend.
The method described above reduces the need for the operator to visually inspect the cyclone, provides independent control of the air flow to the cyclone burner and barrel, and provides an instantaneous assessment of cyclone state as well as an indication of drift in cyclone performance and operation. The method also provides an independent measure of the stability of combustion in the burner and barrel. The invention does not rely soley on the measurement of air flow to the burner or barrel to determine optimum operating condition. This is important, because as fuel properties change, the air flow may have to be changed to maintain optimum operation.
In some cases, air monitors are installed on the cyclones to measure primary, secondary and tertiary air flow. These monitors can be expensive to install and maintain. Not all cyclones are equipped with quantitative indications of air flow. The invention therefore provides a basis for complementing existing air flow measurement systems or providing a semi-quantitative measure of combustion stability for those cyclones that do not have a quantitative measure of air flow.
In addition, the method is inherently self-adjusting. Optimum NOx emissions are achieved by operating the cyclone substoichiometrically. However, unburned carbon emissions and carbon monoxide emission can increase at low stoichiometries. Also, if there is insufficient heat release in the barrel due to less combustion, the slag temperature may drop causing the slag to freeze. The research leading up to this invention demonstrated that upsets in cyclone operation due to poor combustion and freezing slag are apparent in the flame scanner signal and the analysis results track the change and therefore can be used to correct the upset.
The method can be readily customized to a wide range of actuator configurations, and can be implemented on cyclone furnaces that have manually actuated dampers as well as pneumatically or electromechanically actuated dampers. The method can also be implemented in open-loop or closed-loop control.
Variations on the above-described method are also possible. For example, rather than using a single secondary air damper to control the amount of secondary air that is introduced to the cyclone barrel, some cyclones are equipped with dampers that are divided into two or three separate segments. The purpose of this design modification is to enable the operator to vary the amount of air that is introduced into the barrel along the length of barrel. Typically, a cyclone operates better by introducing more air at the end of the barrel near the re-entrant throat. This approach keeps the slag flowing freely by burning the combustion gases in an excess amount of air. Thus, the position of the split air dampers may be varied to improve the quality of the statistics. Varying the position of the split air dampers may also provide improved stability allowing a further reduction in the total secondary air flow to the cyclone and a correspondingly more substoichiometric operation resulting in lower NOx emissions. The described method also provides a direct indication that the slag is freezing.
The optimum condition and various types of instabilities can be categorized in a library of flame states for the cyclones. The current analysis results can be compared to the entries in the library of flame states to identify a match. The information can be provide to the operator.
Typically, the primary and tertiary air dampers 30, 32 are manually-adjusted dampers. These dampers 30, 32 can optionally be configured with a pneumatic or hydraulic actuator and the position thus adjusted based on guidance from the described method.
As noted above, the method described provides for open loop or advisory control i.e., the operator views the analysis results and recommendations from the analysis software and adjusts a set point bias on damper position. The method could be modified to allow for direct communication between the analysis software and the plant's control system to allow for automatic change in set point bias values in response to a command signal from the software. The method can be applied to a cyclone furnace equipped with a vortex burner or a scroll burner. The method is independent of burner type. Some minor adjustment to the library entries in the analysis software may be required.
In some instances, practice of the disclosed method may result in the analysis software generating a demand signal that will cause the primary air or secondary air to be increased or reduced beyond a desired limit. Although stable operation of the cyclone burner 16 or barrel 12 can be achieved at very low air flow rates, other factors may require that a minimum lower limit on primary, secondary or tertiary air be established. For example, since erosion of refractory materials can be worse under reducing conditions, it may be desirable to set a limit on the bias for primary air flow rate or damper position, or bias on secondary air flow rate or damper position. Since tertiary air is sometimes used to cool the front face of the cyclone burner, it may be desirable to set a limit on tertiary air damper position or flow rate to avoid thermal distortion of the burner face. Mechanical stops can be installed on manual dampers to prevent the dampers from being closed below a minimum setting.
For dampers configured with pneumatic, hydraulic or electromechanical actuators, the position of the dampers can be adjusted from a control room. Limits can be programmed into the control system that will limit either the bias on damper position or the absolute position of the damper or both.
The method can be applied equally well to a slagging combustion furnace which is a variation on the cyclone furnace design. In addition, boiler performance optimizer programs, such as neural networks, that acquire process data and adjust operating set points can be utilized to achieve and maintain optimum operation. The analysis results of the analysis software can be acquired by the neural net and be used to assess current performance and improve operation and performance.
The main flame scanner 24 and lighter scanner 26 must produce a time varying signal that is indicative of the fluctuating combustion conditions within the burner or barrel. The signals can be analyzed with the same analysis software. The analysis results can be correlated to specific operating conditions within the cyclone. Library members can be created based on the analysis results.
The method as described uses the drift of time varying value of statistics with respect to variance limits to generate a command signal to adjust the air distribution. This could lead to overcompensation in the control algorithm. The overcompensation can be constrained by using a moving average of the statistics which tends to smooth out spikes in the statistics and provide a more representative process variable to guide the control system response.
Although the method as described assumes a staged cyclone furnace 10, the method can also be applied to unstaged cyclone furnaces. The method can be used to detect upsets in slag properties due to high moisture content in the coal, coarse grind size, and change in fuel ash properties.
A method of boiler flame diagnostics and control is described above. Various details of the invention may be changed without departing from its scope. Furthermore, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation—the invention being defined by the claims.
| Number | Date | Country | |
|---|---|---|---|
| 60823568 | Aug 2006 | US |
