Improving Combustion Control

Tunable diode laser analyzers optimize combustion through direct measurement of O₂, CO and methane

By Don Wyatt, Business Unit Manager, Laser Analysis Division, Yokogawa Corporation of America

Combustion control has been evolving over the years due to demands for greater efficiency, increased capacity, reduced emissions and safer operation. As a result, simply measuring the amount of oxygen and/or carbon monoxide (CO) at a single point in the duct work often doesn't provide the required level of measurement needed for optimal control.

In many combustion control applications, oxygen is measured at a single point only with a zirconia probe analyzer (Figure 1), a simple and rugged analyzer typically installed directly in the ductwork area downstream of the firebox. For some applications, this type of analyzer is an adequate solution, but not when improved measurement and control is required.

Single point measurement of oxygen and no other parameters is problematic and insufficient for several reasons. Larger furnaces exhibit stratification, causing significant variations in the oxygen content within the furnace, causing measurement errors of true oxygen concentration.

A zirconia probe analyzer can't be mounted directly in the firebox of the furnace due to elevated temperatures, so the probe is subject to errors caused by air infiltration from leaks in the ductwork, often referred to as tramp air. Tramp air leads to artificially high readings, often masking a low level of oxygen in the burner zone that can result in uncombusted fuel which causes inefficient and hazardous operation.

It's difficult to measure true values of oxygen with a zirconia probe analyzer, and even an accurate measurement of oxygen doesn't provide all of the information needed for precise control. The optimal amount of excess oxygen will change as the furnace load changes, so what's needed is a second measurement that can be used by the automation system to continually determine the optimum excess oxygen level.

Optimizing Excess Oxygen

As seen in Figure 2, the optimum level of excess oxygen minimizes both the uncombusted fuel, directly related to the CO level, and the NOx emissions. But due to boiler load changes, fouling of burners, changing humidity of the burner air, and other conditions—the optimum level of excess oxygen changes constantly.

Consequently, many furnaces are set to a high level of excess oxygen. Unfortunately, this results in drastically reduced furnace efficiency as some of the furnace heat is used to heat up the excess nitrogen present in the combustion air. Not only is efficiency reduced, but emissions are increased, specifically of NOx.

The key to maintaining the optimum amount of excess oxygen is precise measurement of not only oxygen, but also CO. By monitoring for rises in the level of CO, the combustion automation system can determine the ideal level of oxygen needed under any scenario.

As seen in Figure 3, as the level of excess oxygen is reduced for more efficient operation, the level of CO will remain low until the ideal level of oxygen is reached. At that point, the level of CO will rise quickly.

Measuring both oxygen and CO within the furnace provides the data needed for the combustion control system to operate at peak efficiency while minimizing dangerous unburned fuel conditions and NOx emissions. Another benefit of CO measurement is detection of process heater safety issues such as fuel rich burner conditions or burner flame outs. The challenge is to continually and accurately measure both oxygen and CO with a reliable and easy-to-use analyzer.

CO is typically measured with a Non-Dispersive Infrared (NDIR) analyzer mounted downstream from the firebox (Figure 1). This downstream measurement location isn't optimal, but it's required due to the same elevated temperature issues that plague zirconia probes.

The CO level variance increases as the distance from the firebox in the ductwork increases due to continued reaction of the CO with residual oxygen. Placing the NDIR analyzer downstream from the firebox also causes a lag in measurement, a particular problem as the amount of CO can change quickly as the level of oxygen reaches the optimum level (figure 3). Fortunately, a new measurement technology has emerged over the past 10 years that addresses these measurement issues.

Lasers to the Rescue

A new class of analyzer was developed as part of the NASA atmospheric and planetary monitoring programs. These analyzers used tunable diode lasers to measure components in the Earth's upper atmosphere as well as in the Martian atmosphere.

Since then, a number of commercial applications have been developed including the use of tunable diode laser spectrometers (TDLS) to monitor oxygen, CO and other chemical compounds. The lasers used in a TDLS analyzer are able to make measurements extremely quickly over long path distances (Figure 4).

This allows the lasers to be mounted outside the firebox, alleviating issues with elevated temperatures, and also allows TDLS analyzer to measure oxygen and CO directly in the firebox of the furnace. Measurement in the firebox eliminates the aforementioned issues caused by measuring oxygen and CO downstream of the actual combustion area.

Shining the laser beam across the firebox over the burners at distances up to 30 meters yields a composite measurement, avoiding spot measurement problems due to oxygen stratification. And since a TDLS is laser based, the speed of measurement is quick, typically around 5 seconds.

Application Specifics

For a combustion measurement application, two TDLS analyzers are mounted as shown in Figure 4. The first analyzer measures oxygen and the second measures CO measurement. The second analyzer can also be set up to measure methane. The addition of methane measurement is important during furnace start up and shut down to ensure there is no excess fuel in the firebox, a significant safety issue. Methane measurement also helps flag when a burner might be fouling or has gone out entirely.

The success of TDLS has lead to a new API-556 standard that is soon to be released by the American Petroleum Institute. This standard will recommend the use of TDLS analyzers for the operation of many of fired heaters and steam generators in petroleum refinery, hydrocarbon-processing, petrochemical and chemical plants.

Measuring oxygen, CO and methane with TDLS analyzers allows the automation system to improve combustion control of furnaces and fired heaters. The benefits of this improved control include:

• Reduced excess oxygen and lower operating costs because the fuel-to-air mixture is precisely controlled;
• Unsafe fuel rich combustion is prevented because fuel is limited to the available air;
• Unburned fuel is detected readily, avoiding unsafe combustion;
• NOx emissions are cut because excess oxygen levels are reduced.

At one recent installation site, two TDLS analyzers were combined with modern combustion control hardware and have been operational since June 2010. The analyzers have been operating reliably with no maintenance to date. Plant operators have been able to reduce excess oxygen by 1 to 1.5 percent, thus making the heater operation more efficient. And with the measurement of CO and methane, the optimum oxygen set point is available and used during all operating conditions.

This type of performance has been demonstrated in sites around the world on a wide range of process heaters and furnaces burning a variety of fuels. According to research firm ARC (www.arcweb.com), "Second only to raw material costs, energy is the leading cost pressure currently affecting manufacturers. New analysis techniques, such as tunable diode laser spectroscopy, can improve efficiency, maximize throughput, reduce emissions, and improve safety and reduce energy in combustion process."

References:

1. Application and Benefits of Combustion Management to Fired Heaters, a white paper produced by Yokogawa Corporation of America, http://www.advertasmarketing.com/case_studies/CombustionONE_White_Paper_ 13Oct10.pdf.
2. Instrumentation, Control, and Protective Systems for Fired Heaters; API Recommended Practice 556, http://ballots.api.org/cre/soics/ballots/docs/556reballot0710.pdf.

Figure 1: Measuring oxygen and CO in the ductwork area downstream of the firebox with conventional analyzers presents problems due to stratification, air infiltration and time lags.

Figure 2: The optimum level of excess oxygen minimizes both the uncombusted fuel, directly related to the carbon monoxide (CO) level, and the NOx emissions

Figure 3: As the level of excess oxygen is reduced for more efficient operation, the level of CO will remain low until the ideal level of oxygen is reached, then CO will rise quickly.

Figure 4: The lasers used in a tunable diode laser spectrometer analyzer are able to make measurements extremely quickly over long path distances.