Plant operators have used combustion flue gas analysis for decades as a method of optimizing boiler combustion fuel/air ratios. By measuring the amount of excess oxygen and/or carbon monoxide (CO) in the flue gases, the plant can be tuned to operate at the best heat rate and lowest nitrogen oxide (NOx) levels. Operation at best efficiency also inherently produces the least amount of carbon dioxide (CO2). This article presents considerations for sensor selection and placement in the furnace to maximize performance and balance furnace operation.
Before discussing sensors and their application, some discussion of combustion essentials is required.
The theoretical ideal, or stoichiometric combustion, occurs when all of the fuel is reacted with just the right amount of oxygen in the combustion air so that no fuel is left unburned and no oxygen (O2) remains in the flue gas (Figure 1). Any air that is not used to combust the fuel is called "excess air."
1. Key flue gas measurements determine ideal combustion stoichiometry. Source: Rosemount Analytical
Operating furnaces never attain this stoichiometric ideal due to the practical limitations of continuously mixing large quantities of fuel and air and burning the mixture to completion (Figure 2). Operators usually find that the best operating point lands in the 1% to 4% excess air range with zero to 200 ppm of CO formed. This optimum operating point is different for every furnace and changes with differing boiler loads and firing rates. A higher firing rate induces greater turbulence through the burner(s), providing better mixing of fuel and air, thereby leaving less excess air (or O2) before unburned fuel (represented by CO) appears or "breaks through" (Figure 3). A curve, or function generator is typically developed from test data to assign the ideal O2 control point based upon an index of firing rate, such as fuel flow or steam flow, which is usually determined during tuning of the boiler controls.
2. A computational fluid dynamic simulation shows the turbulent mixing of fuel and air through a boiler burner. Courtesy: Rosemount Analytical
3. A typical distributed control system (DCS) trend report depicts the relationship of O2 and CO and indicates a CO breakthrough point. Source: Rosemount Analytical
This ideal curve should be reestablished from time to time as burners wear and other furnace conditions change over time. The curve for burners using natural gas and light oil fuels will tend to remain valid for long periods of time—even years. Burners firing solid fuels such as coal, petroleum coke, or pelletized biofuels will experience more frequent pluggage and other degradation of the burners and fuel delivery systems and will benefit from more frequent testing to confirm the firing rate/O2 curve.
Large furnace operators will typically dynamically control excess oxygen to the optimal level using the plant distributed control system (DCS). Control of CO is more difficult because target levels are usually in the parts per million range, and making fan or damper adjustments small enough to control at these low levels is difficult. Many operators will make manual adjustments based upon the CO signal or use the measurement as a feedforward signal to adjust the O2 control setpoint upward or downward.
Reevaluate Your Combustion Goals
The traditional goal of striving for the best boiler combustion efficiency often must be reevaluated when NOx production rises and slag begins to form on the boiler tubes.
Minimize Production of Thermal NOx. Oxygen levels in the furnace section of the boiler and flame temperatures are key indicators of NOx production (Figure 4). One operating strategy to produce less NOx is to use staged combustion whereby cooler fuel-rich combustion is established at the burner. Overfire air is later added higher in the furnace to complete combustion. This process results in less heat and oxygen passing through the burner and produces less NOx. Advanced control strategies using neural nets like Emerson’s Ovation Smart Process are often installed to continuously find the optimum combustion air settings to minimize thermal NOx production.
4. The relationship of NOx produced in a boiler as a function of flue gas excess O2 for a typical utility boiler. Source: Rosemount Analytical
Another NOx reduction strategy is flue gas recirculation, where a certain amount of flue gas is mixed with the incoming combustion air. An O2 probe mounted after this mixing damper can be used to control final O2 going to the burner, resulting in a cooler flame that produces less NOx.
Reduce Slag Generation. Flux sensors provide good information about soot and slag buildup, but close attention to combustion analyzers can provide another indication of slag formation. Fly ash fusion temperatures are usually affected by the amount of excess O2 in the flue gases for a particular coal, and some operators run with an O2 setpoint that has been established to prevent slag (Figure 5).
5. Slag formation on boiler tubes. Courtesy: Rosemount Analytical
Technologies for Measuring Combustion Flue Gases
Each significant constituent found in boiler flue gases can be best measured by a specific technology sensor, particularly oxygen and carbon monoxide sensors.
Oxygen. The ubiquitous technology for measuring combustion flue gases has been the zirconium oxide (ZrO2) fuel cell oxygen analyzer (Figure 6). All automobiles now use one or more of these sensors for controlling the fuel/air ratio, and small engines, such as those used by lawn mowers and chain saws, will soon be using them in response to new emissions reduction rules. Although not the focus of this article, details of Nernstian behavior that governors the operation of zirconia oxygen sensors can be found here.
6. A zirconium oxide sensor can be mounted on the end of a probe up to 6 meters long. Courtesy: Rosemount Analytical
Zirconium oxide sensing technology is ideally suited for measuring combustion flue gases for the following reasons:
- The sensing cell generates its own millivolt signal, similar to the way a thermocouple works.
- This raw millivolt signal is inverse and logarithmic, and its accuracy actually improves as O2 levels decrease, making it a good match with O2 levels found in combustion processes.
- The sensor operates at temperatures in the 700C to 750C range, so the flue gas temperatures inside a furnace usually do not present a problem.
- The robust design of the ZrO2 sensor permits survival in the corrosive constituents commonly found in flue gases.
- No sampling system is required. The sensor can be placed directly into the flue gas stream on the end of probe that can be from half a meter to 6 meters long. Because flue gases enter the sensor via passive diffusion, even applications with heavy particulate content are possible with a low rate of filter pluggage. The in-situ ZrO2 probe results in a point measurement within the flue gas duct, so several probes of different lengths may be required in order to get a representative average across large flue gas ducts.
- Sensors can be calibrated online and in place. Automated calibration triggered by an online "calibration recommended" diagnostic is possible.
Carbon Monoxide. CO is usually the first combustible gas component to appear when the combustion fuel/air ratio begins to become too rich. Desired CO levels in combustion flue gases are typically less than 200 ppm, and infrared (IR) spectroscopy is well suited to measuring at these low levels. In today’s state-of-the-art sensors, repeatability better than ¬±5 ppm is possible, with low interference from H2O and CO2. Instruments are usually found in one of two configurations: extractive systems where the flue gases are removed from the duct and cleaned before being sent to a rack-mounted analyzer, or a line-of-sight sensor that uses an IR source mounted on one side of the duct and a receiver or detector mounted on the opposite side (Figure 7).
7. The typical cross-duct IR measuring system. Courtesy: Rosemount Analytical
Measuring CO concentrations using the line-of-sight method produces an inherent average across the entire duct, so fewer analyzers are required to measure across a large duct. Conversely, the granularity of the data collected is lower when fewer averaged signals are used. A dual-pass probe, another measurement option, directs IR energy to a mirror at the end of a hollow pipe that is reflected back to the source end for analysis. The flue gases are permitted to fill the probe tube through holes or filters.
Problems with Line-of-Sight Measurements
Any optical technology presents application challenges that need to be considered when selecting either an O2 or CO analyzer:
- An extractive system involves transporting and filtering the sample flue gases, removing the moisture, and returning the sample to the process or to a safe vent. This approach adds considerable cost to the system and increased system maintenance if there is particulate matter in the flue gases.
- A cross-duct line-of-sight system cannot be placed where temperatures are much above 600C, nor will the system endure high levels of particulates. Thermal growth of the ductwork and vibration can negatively affect alignment of the source and receiver sides. Also, this type of arrangement cannot undergo a true calibration because this would involve filling the entire duct with calibration gases. The U.S. Environmental Protection Agency does not certify line-of-sight analyzers.
- A dual-pass probe system must contend with soiling of the reflecting mirror at the end of the probe. It is possible to conduct an online calibration by filling the optical path inside the pipe with calibration gases.
Tunable diode lasers (TDL) have recently come onto the scene, again using spectroscopy technology, but with a laser source and a diode-sensing array. These systems typically use the line-of-sight arrangement across the duct or the dual-pass probe method. This technology is also capable of measuring O2 in the overtone range and NOx. Again, much information is available about the underlying technology, so this article does not cover the basics.
As with the traditional IR systems, a TDL in a cross-stack line-of-sight arrangement will inherently average across the entire furnace volume, requiring fewer instruments to cover a large duct, but it also provides less definition of the measured flue gases within the duct. Again, TDL analyzers configured as line-of-sight cannot be verified with known calibration gases.
New Applications in Large Power Boilers
Each of the 20 or more burners in a large boiler can be considered as a separate process, each producing its own flue gases. The flue gases in furnaces utilizing burners in a single- or opposed-wall firing configuration tend to form up into "columns" that often tend to stay stratified throughout the furnace.
Combustion analyzers are typically mounted in the "back pass" of the furnace, just ahead of, or after, the economizer, and it is common for operators to see this stratification when using multiple O2 probes in these large ducts (Figure 8). It’s also common to see differences of 1% O2 or more across a large furnace. To accommodate this gradient, an arithmetic average of multiple probes is often calculated in the DCS, and that average is what is then sent as an input to the O2 control loop.
8. An array of oxygen probes mounted vertically downstream of an economizer. Source: Rosemount Analytical
Furnaces firing solid fuels (particularly coal) can have high levels of fly ash carried with the combustion flue gases. Separate schedule 40 pipes are often used as "abrasive shields" to protect in-situ oxygen probes from fly ash erosion. Some operators have discovered, however, that fly ash is often much less abrasive in the hotter zones of the furnace (500C to 700C) above the economizer. As the combustion flue gases are cooled through the economizer and air heater, the ash often agglomerates into larger "popcorn ash" that is far more abrasive. A location higher in the furnace can not only minimize abrasion but also detect stratification better. Rosemount Analytical has developed a heavy-wall probe body that is more cost-effective than traditional abrasion shields but that endures fly ash erosion well in high areas of coal-fired boilers.
Differential readings from several O2 probes in the flue gas can be used as a diagnostic tool for identifying furnace problems, such as:
- Fouled burners
- Sticking sleeve dampers
- Induced draft fan imbalances
- Roping in coal pipes
- Classifier pluggage or coal fineness problems
- Coal mill imbalances
The flue gases passing through a tangential-fired furnace do not experience as much stratification of the flue gases as in a wall-fired boiler, but some operators claim the ability to detect corner-to-corner variations with the O2 probes in tangential furnaces.
Oxygen probes are provided in a wide range of lengths, as discussed earlier. So, how does a plant engineer determine the optimum placement of a probe? A variable insertion capability permits placing a given probe into the "sweet spot" of the flue gas duct, where its readings will be most representative of a given burner, or column of burners (Figure 9).
9. Variable insertion O2 probes can be installed in the most convenient orientation. Courtesy: Rosemount Analytical
New Sensor Developments
Continued research into the ZrO2 fuel cell technology is yielding new sensor capabilities. The inverse logarithmic characteristic of these sensors was discussed earlier. In a new development, a sensing cell has been developed that will continue to produce an increasing voltage signal as flue gas O2 levels pass through zero and into reducing conditions (Figure 10). Typical O2 probes will sit at zero during reducing events, and operators may wonder if the zero readings are an indication that the analyzer has failed or if the reading is actually at zero. The new stoichiometry feature permits setting the lower O2 level at 1% or 2% O2, providing certainty that a reducing event is present and giving an indication of the level of oxygen deficiency. An operator can see if his initial control actions have had the desired effect: preventing the overcontrol and cycling that often results during recovery. Reducing events don’t happen often, but most operators will admit experiencing them.
10. Millivolt signal output from a new ZrO2 sensing cell can operate in oxidizing and reducing conditions, such as those often found in boiler furnaces. Source: Rosemount Analytical
Continuing research into the ZrO2 fuel cell technology are also yielding new CO measuring opportunities. Although the work has only recently begun, eight test sites established at North American power plants have produced some promising early results (Figure 11). This sensor will provide very useful data to operators trying to keep their boilers functioning as efficiently as possible to identify trouble spots caused by imperfect combustion. Continued research into fuel cell sensing technology has yielded a new sensor for the measurement of CO in ppm levels.
11. A DCS trace depicts the concentration of CO in a furnace operating under reducing conditions using a Rosemount Analytical Alpha CO probe. Source: Rosemount Analytical
New installation locations are being attempted in hotter zones ahead of the economizer, an area of the boiler with less abrasive fly ash. Variable insertion probe mounts also afford the ability to find the ideal location within a flue gas duct. Innovative users have installed flue gas analyzers to detect leaks in air heaters or duct transitions to obtain a more accurate estimate for their heat rate calculation of air in-leakage. Regardless of the application, the maximum benefit gained from flue gas analyzers results from close collaboration between instrument suppliers, plant instrument engineers who install them in the plant, and operations personnel who use them on a daily basis.
—Douglas E. Simmers is the worldwide product manager for combustion analyzers at Rosemount Analytical, a division of Emerson Process Management.