Data processing: measuring – calibrating – or testing – Measurement system – Remote supervisory monitoring
Reexamination Certificate
2001-11-14
2004-08-10
Assouad, Patrick (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system
Remote supervisory monitoring
C700S274000, C702S032000
Reexamination Certificate
active
06775645
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates, in general, to methods and apparatus for boiler flame diagnostics and control. More particularly, the present invention provides methods and apparatus for monitoring the operating state of burner flames using temporal irreversibility and symbol sequence techniques.
DESCRIPTION OF THE RELATED ART
Economic pressures and increasingly restrictive environmental regulations have contributed to an increasing need for advanced management systems that efficiently regulate utility boilers. Inefficient boiler control is responsible for wasting large amounts of fuel heating value and releasing nitrogen oxide pollutants into the atmosphere.
Monitoring systems that accurately reflect burner-operating states are essential to advanced boiler management. Accurate monitoring of burner-operating states is more important for advanced low-NO
x
burners than conventional burners because low-NO
x
burners are more sensitive to changes in operating parameters and feed system variations. Conventional combustion monitoring systems provide information that has been averaged over many burners and long time scales (e.g., measurements of excess air, coal feed, or NO
x
emissions at time scales of several minutes or hours). However, large NO
x
and carbon burnout fluctuations can occur in individual burners over short time scales (i.e., between about 10 seconds to fractions of a second). These fluctuations produce widely different boiler performance for operating conditions that otherwise are indistinguishable. Accordingly, combustion diagnostics should reflect both long and short time-scale transients for more reliable boiler optimization.
A key variable in the combustion of fossil fuels, such as oil, gas and pulverized coal, is the air/fuel (“A/F”) ratio. The A/F ratio strongly influences the efficiency of fuel usage and the emissions produced during the combustion process (especially, for low-NO
x
burners). The A/F ratio also affects slagging, fouling and corrosion phenomena that typically occur in the combustion zone. In current steam generators fired with fossil fuel, the A/F ratio is controlled by measurement of oxygen and/or carbon monoxide (“CO”) concentration in the stack gases or at the economizer outlet. In either case, the gas measurement is taken at a location removed from the actual location of the combustion process. Unfortunately, in multiburner, steam generator furnaces the A/F ratio differs from burner to burner and accordingly may vary significantly with burner location. Since both combustion efficiency and NO
x
generation levels depend on the localized values of the A/F ratio (i.e., the distribution and mixing within each flame), measurement and control of the global A/F ratio produced by the entire furnace of the steam generator does not necessarily optimize performance.
A number of factors can change the A/F ratio during normal boiler operation. These variables include coal pulverizer wear, which may lead to a change in the size distribution of the coal particles, change in the overall fuel flow rate from the pulverizer, change in the distribution among burners of the fuel flow, change in the distribution of fuel within the flame due to erosion/corrosion of the impeller or conical diffuser, change in the overall air flow rate change in the distribution of air among individual burners and change in the distribution of air among individual burners due to change in the position of air registers.
All burners (especially, burners with staged air and/or fuel injection) undergo characteristic transitions in dynamic stability (i.e., bifurcations) as the above parameters are varied. The most important burner bifurcations are caused by the nonlinear dependence of flame speed on the relative amounts of fuel and air present. In particular, flame speed (i.e., combustion rate) drops exponentially to zero when the A/F ratio approaches either fuel-lean or fuel-rich flammability limits. Fuel-lean refers to conditions where excess air (i.e., oxygen) is present and fuel-rich refers to conditions where excess fuel is present. Local variation in the A/F ratio creates some zones adjacent to the burner that sustain combustion and other zones that do not sustain combustion. These zones may interact through complex mechanisms that depend on the details of turbulent mixing imposed by burner design, specific operating settings and the relative amounts and spatial distribution of incoming fuel and air. In coal-fired burners, the complexity of the process is further increased by the presence of both solids and volatile components in the fuel, which mix and burn at characteristically different rates. The details of the distribution and interaction of combusting and non-combusting zones is critical in determining the efficiency of fuel conversion and the levels of pollutants emitted (such as oxides of nitrogen and carbon monoxide).
Although the dynamics of coal-fired burners are complex, certain global bifurcations in flame structure typically occur. These global bifurcations represent conditions under which the dominant structure of the flame (e.g., the global flame shape, size, or location) suddenly changes from stable to unstable or vice-versa. These stability shifts are driven by changes in the relative A/F ratios in the primary and secondary combustion zones, changes in the gas velocity profile, and/or the rate of mixing between these zones. A typical operating condition for low NO
x
coal-fired burners involves fuel-rich combustion in the primary zone and fuel-lean combustion in the secondary zone. Primary zone combustion becomes unstable and flickers on and off in repeated ignition and extinction events, when conditions in the primary zone are too rich or the flow velocity is too high. Under extreme conditions, primary zone combustion may be completely extinguished.
Extinction of combustion at the base of the primary zone represents a bifurcation in which the “attached” flame state is no longer stable (i.e., the initial flame front is no longer supported in the vicinity of primary air and fuel exit pipes). When the initial flame front is no longer supported in the vicinity of the fuel exit pipes, the flame front may shift axially downstream from the face of the burner and can assume a detached “lifted” condition. A lifted flame represents an alternate stable flame state that can persist even though the attached flame is unstable. In a lifted flame, the distance from the burner face to the flame boundary and the stability of that boundary depends on many factors such as the primary air exit velocity, the A/F ratio in the secondary zone and the detailed air flow velocity profile. Under some conditions, stable lifted and attached flame states may co-exist, so that the burner can assume either condition depending on the initial burner state. External perturbations to the burner (e.g., air or fuel flow disturbances) may cause transitions between these two states.
Extinction of combustion in the primary zone can also occur if there is an excessive amount of oxygen present. This can happen in coal-fired burners when the release of volatile matter from the fuel is too slow to keep the gas mixture above the lean flammability limit. Whether caused by high air velocity or excessively rich or lean primary zone conditions, lifted flames are an undesirable operating condition typically associated with excessive emissions of pollutants.
Bifurcations and associated flame front shifting can also occur in the radial direction due to excessively high or low rates of mixing between primary and secondary zones. These types of bifurcations can produce axial shifts in flame shape and symmetry that result in helical and/or side-to-side motions. In some cases, flame size may also undergo large expansion and contraction. Large variations in the amount of visible and infrared light emissions from the flame are observed during such events. Like axial flame shifting, radial flame shifts are associated with excessive emissions of pollutants and reduced fuel utilization. As is well k
Daw Charles Stuart
Finney Charles E. A.
Flynn Thomas J.
Fuller Timothy A.
Assouad Patrick
Electric Power Research Institute Inc.
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