Head: Improving Combustion Control
Deck: 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, www.us.yokogawa.com
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:
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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.