Reburn NOx Control

The Clean Air Act Amendment of 1990 (CAAA) established the framework for NOx emission regulations to mitigate ozone non-attainment areas and acid rain. The U.S. EPA has since developed specific NOx regulations.

For applications where they can meet the NOx reduction requirements, low-NOx burners generally are the technology of choice, based on cost. However, in many applications low-NOx burners are not the answer to meeting CAAA requirements. Reasons range from the unavailability of low-NOx burners (such as for cyclones), performance problems with low-NOx burners (such as carbon loss or tube-wall wastage), or insufficient NOx reduction.

However, reburn can be applied to virtually any unit firing any fuel to achieve NOx reductions typically in the range of 50% to 70%. Moreover, it can be integrated with nitrogen-agent injection and other technologies for even greater NOx reduction.

Reburn NOx Control Technology
The concept of NOx reduction via reactions with hydrocarbon fuels has been recognized for some time. During the past 20 years or so, EER has developed a considerable reburn database. It consists of extensive pilot-scale tests, a number of full-scale applications, and a design methodology that can be used to apply reburn and project performance to a wide range of applications.

Basic Reburn Process
Reburn is a control technology whereby NOx is reduced by reaction with hydrocarbon-fuel fragments. The reburn process is illustrated in Figure 1 for a front-wall-fired boiler.

In applying reburn, no physical changes to the main burners or cyclones are required. The burners simply are turned down and operated with the lowest excess air commensurate with acceptable lower furnace performance considering such factors as flame stability, carbon loss, slag tapping and ash deposition.

Reburn fuel is injected above the main combustion zone to produce a slightly fuel-rich reburn zone where most of the NOx reduction occurs. Maximum NOx reduction performance typically occurs with the reburn zone operating in the range of 90% theoretical air. Above the reburn zone, over-fire air is injected to complete combustion.

Reburn Design Considerations
Due to the substantial design differences among existing boilers and furnaces and NOx control requirements that vary with local requirements, reburn should be custom designed, preferably using analytical and physical models, to match site-specific factors.

Firing configuration. Because reburn does not require modifications to the main combustion system, it can be applied to virtually any combustion system including boilers with wall-, tangential-, and cyclone-firing configurations. Reburn also can be applied to such industrial furnaces as glass and steel-reheating types.

Applying reburn to hot furnaces with high baseline NOx levels is particularly attractive since both factors speed up the NOx reduction reactions. This reduces the amount of reburn fuel and the size of the reburn zone required to achieve a specific NOx goal.

Main- and reburn-fuel characteristics. Since reburn involves no physical changes to the main combustion system, it can be applied to furnaces fired with any fuel (coal, oil, gas, Orimulsion, and others.).

Except possibly for its cost, gas is the preferred reburn fuel. It produces the greatest NOx reduction per unit of reburn fuel injected, has no ash or sulfur, and requires no pulverization or atomization.

The cost and availability of gas are the key factors that encourage consideration of other reburn fuels. For example, on coal-fired units, the use of pulverized coal as the reburn fuel avoids any cost penalty of the reburn fuel over the main fuel. Other fuels of interest include oil (with oil-fired units) and Orimulsion.

Furnace volume. There must be sufficient space above the burners or cyclones to install the reburn components and to produce adequate residence time in the reburn and burnout zones. By designing the reburn fuel and over-fire-air injectors for rapid mixing, space requirements are in the range typically available on full-scale utility boilers.

EER has yet to find a commercial system where the residence time was inadequate. EER has achieved NOx reduction as high as 70% in a cyclone application with an effective reburn-zone residence time of only 0.25 second. Although such applications are feasible, longer residence times reduce the amount of reburn fuel required to achieve a specific NOx control goal while improving carbon loss, particularly with coal reburn.

Reburn fuel injectors. The reburn fuel injectors should be located close to the upper firing elevation, leaving enough space above the burners to achieve essentially complete combustion in the flames from the main burners prior to introduction of the reburn fuel.

For maximum NOx reduction, injection of the reburn fuel should penetrate across the furnace depth and mix rapidly with the furnace gases. Since the amount of reburn fuel injected is small compared with the furnace gas-flow rate, achieving penetration and rapid mixing is a challenge, especially with large furnaces, but achievable by increasing the momentum of the injected stream via a carrier gas or by high-velocity injection.

The design of the reburn fuel injectors should minimize (ideally, eliminate) any oxygen introduced with the reburn fuel. This oxygen must be consumed by additional reburn fuel to achieve the desired reburn-zone stoichiometry. (EER's second-generation systems [so far applied to two units—wall- and tangentially-fired] use the pressure available in the gas pipeline to produce high-velocity reburn jets, totally eliminating additional oxygen.)

Coal as the reburn fuel requires a gaseous carrier for pneumatic transport. Flue gas is preferred over air to minimize the reburn-fuel quantity.

Over-fire-air ports. Most of the primary-fuel char oxidation occurs in the oxygen-rich primary combustion zone. The burnout zone completes combustion of the reburn fuel. For gas reburn, CO oxidation is the primary combustible. For fuels that contain fixed carbon, such as coal and Orimulsion, the combustibles includes CO as well as the carbon in the flyash.

The location of the over-fire-air ports should allow for balancing the NOx reduction performance of the reburn zone with the combustion efficiency of the burnout zone—generally done by locating the over-fire-air ports substantially higher in the furnace than for conventional over-fire air applications but well below the furnace exit.

As for the gas injectors, the design of the over-fire-air ports should provide for rapid and complete mixing. EER uses dual concentric-zone over-fire-air ports with swirl control, which permits control of the injection velocity independently of flow rate—a key convenience when varying the reburn fuel to control NOx emission level.

Flame sensing and controls. EER integrates the reburn system with the normal boiler controls for fully automated operation. Depending on the NOx control goal, the reburn fuel injection can be fixed or varied in response to boiler operating conditions and/or NOx emissions.

The fuel-injection controls include both permissives and trips to ensure safe operation. The primary permissive/trip for reburn-system operation is furnace temperature since gas injection does not produce a visible flame and conventional scanners are ineffective. This approach has been fully effective in five reburn applications and has been reviewed and approved by Factory Mutual and Hartford Steam for them.

About the authors:
Blair A. Folsom, Donald A. Engelhardt, Roy Payne, and Todd M. Sommer are with the Energy and Environmental Research Corporation, 18 Mason, Irvine, CA 92618.

Sandy S. Chang and Dennis T. O'Dea work for New York State Electric and Gas, Corporate Drive, Kirkwood Industrial Park Binghamton, NY 13902.

Lloyd E. Riggs and Robert G. Rock are employees of the Eastman Kodak Company, Kodak Park Site, Rochester, NY 14652.

The previous article was adapted from the Introduction to the technical paper, "Field Experience—Reburn NOx Control," developed on behalf of the Energy and Environmental Research Corporation, 1345 North Main St., PO Box 153, Orville, OH 44667. Tel: 330-682-4007; Fax: 330-684-2110.