Cost, Safety and Performance Issues Associated with Ultra Low NOx Burners

Presented at the ABMA Technical Conference, West Palm Beach, FL

November 6-7, 1996

Thomas E. de Haan, Coen Company, Inc.


Boiler owners and operators are concerned about the problems and limitations associated with the latest low NOx technology to meet today’s stringent emission requirements. Most of these concerns stem from the trend toward higher rates of flue gas recirculation (FGR) which can increase operating cost, reduce safety, and limit boiler performance.

Specifically, this paper will review the operating cost of FGR to meet typical NOx regulations of less than 30-80 ppm and lower levels of 10 ppm. The safety and performance factors associated with using very high rates of FGR (> 30%) to reach levels of < 10 ppm will be presented. The paper will show how high rates of FGR can reduce the limits of flammability (LOF), cause flame instability, and limit boiler performance. New technology and methods will be introduced that helps mitigate this trend.


Are today’s boiler operators considering all the operating cost and risk factors associated with meeting today’s emission requirements? Are regulators in their zeal to lower emissions at any cost forcing suppliers and users to use technology sooner than they should? To meet the Clean Air Act Amendments (CAAA) of 1990, many operators are rushing to install low NOx equipment without considering all the facts. Regulators are implementing ever lower emission requirements without knowing the full impact of their decisions.

The CAAA has classified nearly 100 major urban areas as non-attainment for ozone; i.e. not meeting National Ambient Air Quality Standards (NAAQS). Each state must submit a State Implementation Plan (SIP) outlining how they intend to reach attainment by reducing NOx.

Non-Attainment – Existing Sources

For existing sources in non-attainment areas, each state must define and implement Reasonably Available Control Technology (RACT). RACT for existing boilers varies from state to state depending on the severity of non-attainment. In California, NOx is generally regulated to 30 ppm, Texas is looking at 50 ppm, and the Ozone Transport Region (OTR) recommends a NOx limit of 0.1 lb/MMBtu or about 80 ppm. All RACT limits can be met with current burner technology using moderate rates of FGR or new premix type burners that use no FGR.

Non-Attainment – New Sources

For new sources in non-attainment areas, the lowest Achievable Emission Rate (LAER) must be implemented regardless of cost of compliance. For boilers, this translates to 9 ppm NOx which is achievable by Selective Catalytic Reduction (SCR). New burner technology using very high rates of FGR can now also meet this limit. Alternatively, a combination approach of SCR and burner technology can be used.

Major Issues and Problems

The use of SCR technology can double the cost of a new boiler installation. The use of moderate and especially high rates of FGR can increase operating cost, reduce safety and compromise performance of the boiler.

The major Issues facing boiler owners today are:

· regulatory trends

· capital cost of implementing low NOx rules

· operating cost of low NOx equipment

· safety and risk factors

· performance limitations of low NOx equipment

· future unknown regulations

Boiler owners need to be aware of these major issues and select equipment that provides maximum safety with minimum cost.


Air or Fuel Staging

Nearly all low NOx burners use some method of staged air or staged fuel technique. Staging creates a longer time for the combustion process and lowers the peak flame temperatures. Since NOx is highly dependent on peak flame temperature, this is an effective method to moderate thermal NOx reductions. NOx reductions below 0.1 lb/MMBtu (80 ppm) without FGR can be achieved using this approach. Advantages of staging is its simplicity and low cost. Because staging lengthens the combustion time, it puts limits on flame volume, excess air and combustibles.

Flue Gas Recirculation

For lower levels of NOx reduction, FGR has been used extensively in low NOx burner design with or without staging. Moderate amounts of FGR can substantially lower peak flame temperature. An air or fuel staged burner with 5% to 10% FGR can easily reduce NOx below 0.1 lb/MMBtu, and 10% to 20% FGR can reduce NOx emissions below 30 ppm to meet all state RACT requirements. FGR is very effective for moderate amounts of NOx reduction with minimal effect on excess air and combustibles. The major drawbacks are very high operating cost, boiler efficiency loss, and high systems losses due to increased mass flow through the boiler. Some boiler derating may be required depending on the NOx limits and the Space Hear Release (SHR) rate (Btu/hr/ft3) in the packaged boiler.

Premixing without FGR

To avoid the disadvantages of FGR, Coen developed a new burner concept that premixes fuel and air. Premixing reduces both thermal and prompt NOx to meet RACT limits without the use of FGR. This is now a proven technology with over 100 installations worldwide. The principle advantage of this design is the elimination of FGR. The only limitation is that boiler length and SHR may be affected.

Premixing with FGR

To achieve ultra-low levels of NOx emissions <10 ppm to meet LAER, premixing is again utilized but with very high rates of FGR in the range of 30% to 40%. High operating cost, reduced limits of flammability (LOF), and limitations on burner performance are even more pronounced with these levels of FGR. The only alternative is to install a Selective Catalytic Reduction system which can double boiler cost, but doesn’t affect flame stability and burner performance.


The use of FGR places a significant burden on the boiler owner from additional brake horsepower required to pump FGR through the system. Also, FGR will cause the stack temperature to rise, yielding reduced unit efficiency from thermal losses. To meet typical RACT emission rules of 30 ppm generally requires 10%-20% FGR. New premix burner designs that meet LAER levels of <10 ppm generally require 30% FGR or more.

FGR Horsepower Cost

For a typical 100,000 PPH boiler operating with 15% FGR, the annual operating cost for fan BHP is $48,000 per year. For 35% FGR, the additional cost is $112,000 per year.

FGR Boiler Efficiency Cost

Again, for a typical 100,000 PPH boiler with 15% FGR, the annual operating cost from stack losses is $17,000 per year. For 35% FGR, the cost is $40,000 per year.

Total Additional Cost Due to FGR

Many new installations for low NOx burners are 200-300,000 PPH and FGR rates of 20% to meet RACT and 35% or more for LAER are not uncommon. A simple calculation will show that the annual operating cost for these large sizes is extremely high. Meeting the federal government’s goals for reducing emissions can be a very costly proposition.

In an effort to control emissions to the lowest level possible, regulators are ignoring the other important national goal of improving fuel efficiency. When high FGR rates are used, operating cost increases and boiler efficiency decreases. Regulators need to know the facts if they are to resolve these seemingly cross purpose national goals.


Boiler operators are not commonly aware that low NOx burners utilizing FGR are not operationally as reliable or controllable as non-low NOx burners. Increasing amounts of FGR will compound the affect. Operating at or near LOF can result in considerable furnace rumble, vibration, and flame stability problems.

Minimize FGR

With 5% to 10% FGR used by many burner suppliers to meet 0.1 lb/MMBtu, the LOF can be reduced by 40%. To meet typical RACT requirements of 30 ppm, up to 20% FGR may be needed which can reduce LOF by 70%. For ultra low NOx operation below 10 ppm the 30% to 40% FGR rates required can reduce LOF by 90% or more. FGR and excess air must be controlled tightly at all firing rates to avoid going beyond LOF. When converting to low NOx firing, boiler owners should look for burners that utilize minimum FGR or preferably no FGR at all.

Fuel/Air Ratio

Operating at the correct fuel/air ratio is of paramount importance. Slight variations in stack excess air or fuel pressure can cause the burner to operate beyond LOF. Many low NOx burner designs require higher levels of excess air for good NOx control, especially at reduced loads. This reduces efficiency and must be minimized as much as possible. With high levels of FGR, variations in excess air can cause operation outside the LOF. Even small variations in air temperature and humidity can cause major upsets. The fuel/air ratio must be controlled more precisely at very high rates of FGR to remain within the LOF. There is a fine line between excess air for good NOx control and that required for flame stability. At high FGR rates, excess air must be controlled accurately at all firing rates. It is essential that an O2 trim system be employed and monitored.

Combustion Controls

The operating control system should be simple, reliable, and repeatable. Many combustion experts feel that expensive metering type control systems are the best for low NOx operation. Unfortunately, metering controls are not very dependable at low firing rates. Also, fuel and air are being controlled separately and a failure of either system can be serious. With FGR, it is very important to have repeatable and reliable control of the combustion process.

One of the best control systems is still single point positioning (SPP). With SPP, the air and fuel are linked together through mechanical linkage via a common steel jackshaft. This physical linkage helps prevent any variations from initial settings. A characterized fuel valve, Coen “AC” valve, is set in 20 increments to match air flow at all firing rates. Any small variations in fuel quality or air conditions can be handled with an O2 trim system. A variable frequency drive fan is a worthwhile option to improve efficiency and control at low loads.

A variation of the SPP system is the Coen Differential Control (CDC) system. The CDC system is safe and simple because it is also based on the concept of SSP. A control loop that maintains constant pressure drop across the air flow control damper and burner is added to SSP. This constant pressure drop is maintained by a Variable Frequency Drive (VFD) controller on the forced draft fan motor. If air flows varies from set point, the VFD will maintain the set differential pressure assuring the same air flow for a given firing rate. Air density and temperature differences are controlled by an oxygen trim system that modifies the set point of the CDC controller. Main benefits of this system are:

· Safe simple control based on SPP

· VFD requires less BHP at lower firing rates

· Controls O2 levels accurately

· Maintains burner control within LOF

Performance Limitations

Whenever a low NOx burner is installed, the burner and boiler performance requirements need to be addressed. High rates of FGR along with staged air/fuel techniques can compromise the operational limits of a boiler.

To meet RACT, burner designs generally can achieve a 8-10 to 1 turndown range which is adequate for most boiler applications. When using high rates of FGR to meet LAER, turndown ratios may be limited to 4-5 to 1. If your process requires high levels of turndown, you will need to get a permit to operate at higher levels of NOx at lower firing rates or go to a multiple burner arrangement.

Operating conditions play an important part in the applications of low NOx burners. Exceeding turndown limitations can result in operating outside the LOF range and causing flame instability.


For moderate amounts of NOx reduction to meet RACT rules of 30 to 80 ppm, boiler operators should use a burner that utilizes little or no FGR. Burner designs are available to meet 30 ppm without the use of FGR. Fig. No. 8 shows a premix burner design that has been used in over 100 installation worldwide to meet 30 ppm without FGR.

To meet ultra low NOx levels of below 10 ppm, burner designs are available; however, the controllability of these burner designs still needs improvement. Precise control of FGR rate and fuel/air ratio is essential. These burner designs are being enhanced to extend LOF allowing the burner to operate with today’s control technology, safely and reliably.

The most critical requirement for any ultra low NOx project, is to make sure that the burner, FGR system, primary air fan, ductwork, and combustion controls are all supplied or designed by a single source.


To assure the best, most reliable and safe low NOx system, you must do your homework. Review the operating parameters with several boiler and burner suppliers. Ask for similar installations especially for ultra low NOx applications. Inquire if any vibration, rumble or control problems are evident. Check the operating conditions as to turndown, capacity, O2 levels, and the amount of FGR. Calculate your five year operating cost.

In the zeal to lower emissions, regulators are unknowingly pushing boiler operators into systems with high operating cost, while energy efficiency, also high on our national agenda is being ignored. Worst is the costly push towards unsafe operation at high FGR rates with inadequate controls.

Rather than forcing expensive SCR systems or marginal burner packages on industrial boiler owners, states should review their implementation strategy in tandem with the latest technology available along with the implementation cost.