Off-Stoichiometric Combustion

During off-stoichiometric combustion or staged combustion, combustion occurs in two zones. In one zone, the fuel is fired with less than the

stoichiometric amount of air (stoichiometric ratio or SR <1). This creates a fuel-rich condition in the region of the primary flame. The second zone is an air-rich area where the remainder of the combustion air is introduced to complete the combustion of the fuel (SR>1). The temperature in the primary flame zone is not as high as with conventional firing because combustion is incomplete.

Off-stoichiometric combustion is an effective technique for controlling both thermal and fuel NO_X because of its ability to control the mixing of fuel with combustion air. The NO_X reduction effectiveness is limited by the same factors that limit low excess air operation, namely, for the formation of carbon monoxide and partially oxidized organic compounds, soot-related boiler tube fouling, and boiler tube fire side corrosion. The latter problem can occur when the unit is firing sulfur-containing coal or residual oil.

Overfire Air

Staged combustion can be accomplished by using overfire air (OFA) ports. These are separate air injection nozzles located above the burners as indicated in Figure 6-4. The burners are operated fuel-rich, and the overfire air ports maintain the remainder of the combustion.

Approximately 15% to 20% of the combustion air flow is diverted to the over-fire air ports.⁴

6-10 COMBUSTION MODIFICATIONS

Main fuel and air ports Main fuel and air ports

Separated

Close-coupled

Tangentially-fired flame (Top view)

Figure 6-4. OFA in tangentially-fired units

Overfire air combustion modifications require the penetration of the boiler wall by new air ducts and usually requires changes to the air handling system in order to deliver the air to the secondary combustion zone.

Furthermore, there must be sufficient space above the burners and before the heat exchange area of the boiler to provide sufficient time for the combustion reactions. Because of this limitation, this approach is not possible on some existing coal-, oil-, and gas-fired suspension-type boilers.

As with LEA, OFA may increase CO or unburned hydrocarbon emissions. It is also applicable to process heaters by using air lances rather than

changing the boiler configuration. OFA for small boilers and process heaters can be accomplished by inserting a lance through the upper furnace and injecting air through that lance. OFA provides modest NOX reductions in the range of 20%. This reduction must be balanced with the cost of additional air handling equipment and the increase in unburned carbon and CO emissions.

Approaches for OFA include Advanced OFA, which involves injecting air at higher velocities to obtain more mixing in the combustion zone. Additional fan power is required to achieve the higher air velocities. Separated OFA involves the installation of air injection ports at a considerable distance above the burners as illustrated in the left side of Figure 6-4. The approach requires that adequate burnout time (i.e., residence time) be maintained after the air is injected.

COMBUSTION MODIFICATIONS 6-11 A practical consideration in using OFA is that boiler tube corrosion may occur in the fuel-rich zone due to the reducing environment in the lower furnace of the boiler. This problem is particularly common in boilers using sulfur-containing fuels. Furthermore, as the stoichiometric ratio is reduced this problem can become very severe.

Superheaters

Pulverized fuel transported in

primary air

Secondary air from air preheater Burners operated

fuel-rich (reduced primary and secondary air) Upper furnace at SR >1.0 (fuel-lean) CO, HC burnout

Lower furnace at SR <1.0 (fuel-rich) lower temperature

Figure 6-5. Off-stoichiometric combustion

Figure 6-5 demonstrates the operating principles of the OFA method. A secondary air port or OFA injection port has been added above the primary air-fuel burner. Below this port is the fuel-rich zone (stoichiometric ratio less than 1) with peak temperatures lower than those associated with

conventional combustion (stoichiometric ratio greater than 1). The injection of OFA allows the upper zone of the furnace to achieve a stoichiometric ratio greater than 1 (fuel-lean) and promotes the burnout of CO and

hydrocarbons. If the secondary air ports are located too far from the burners, the residence time will be inadequate to allow for burnout of the CO and hydrocarbons.

6-12 COMBUSTION MODIFICATIONS

Separated Close-Coupled

Distance between burners and overfire air ports Overfire air ports

Burners

Overfire air ports

Burners

Figure 6-6. Close-coupled and separated overfire air

Figure 6-6 schematically demonstrates the difference between close-coupled OFA (CCOFA) and separated OFA (SOFA). CCOFA is the conventional OFA technique that is distinguished from SOFA by the location of the OFA ports to the burners.

The SOFA technique is illustrated on the left side of Figure 6-6, while CCOFA is illustrated on the right side of Figure 6-6. SOFA imposes a greater distance between the burners and the OFA ports than the CCOFA method does. In CCOFA the overfire air ports can be located relatively close to the burners. CCOFA reduces the residence time in the fuel-rich zone, but increases the residence time in the fuel-lean or burnout zone.

Close-coupled and separated overfire air ports can also be installed on tangentially-fired boilers, although the configuration is different. Figure 6-7 shows the top view of a tangentially-fired boiler. The fuel and air are injected at the corners to create a swirling flame in the middle of the boiler.

This is accomplished with a tower of fuel and air ports in each corner of the boiler. A profile of the primary burners and the OFA ports is shown on the left side of Figure 6-7 for SOFA and on the right side for CCOFA. With CCOFA, the close proximity of the OFA ports to the burners can be seen, while the SOFA method provides a distinct physical separation between the OFA ports and the burners. The SOFA method provides additional

residence time in the fuel-rich zone and less residence time in the burnout zone.

Figure 6-8 shows the configuration of the use of OFA in a stoker grate boiler.

In normal operation, the undergrate air is fed underneath the fuel bed to maintain combustion on the bed. The upper grate air has been traditionally used for burnout. In NOX control using OFA, some of the undergrate air and/or the upper grate air are diverted to the OFA port. The gases coming off the bed are incomplete combustion products. The OFA provides for a fuel-lean zone that can complete burnout of the combustion products.

COMBUSTION MODIFICATIONS 6-13

Main fuel and air ports Main fuel and air ports

Separated

Close-coupled

Tangentially-fired flame (Top view)

Figure 6-7. Overfire air port arrangements

Steam

Figure 6-8. OFA arrangement in a stoker-grate boiler

6-14 COMBUSTION MODIFICATIONS

= Air Only

= Primary Air and Fuel

Lower Furnace Furnace

Exit

Figure 6-9. Burner firing condition using burners-out-of-service approach

OFA is not highly effective when applied to spreader stoker boilers, overfeed stokers, and undergrate stokers. In these types of boilers, combustion occurs on the grate surface and the units are already equipped with overfire air systems. Additional air ducts above the grate and additional overfire air flow do not significantly reduce the NOX

emission levels.

Burners-Out-Of-Service

When some burners are operated on air only, this modification is called burners-out-of-service (BOOS), as shown in Figure 6-9. Burners out of service is typically performed with no more than 25% to 30% of the burners.⁵

BOOS is similar to OFA, but does not require the installation of new OFA ports. The approach is to reduce air to several of the lower burners and to eliminate fuel in several upper level burners. This arrangement simulates an OFA air system because the reduced air in the lower burners creates a fuel-rich zone and the reduction of fuel in the upper ports creates a fuel-lean zone.

Using BOOS on an existing boiler can result in a steam load reduction if the active fuel burners do not have the capacity to supply fuel for a full load.

Therefore, BOOS is typically used on wall-fired units and other units that have the ability to operate at less than full load conditions.

Most utility boilers installed since 1971 have been designed with overfire air ports so that all fuel burners are active during the staged combustion operation.⁶ Using staged combustion modifications on oil- and gas-fired boilers reduces NOX emissions by approximately 30% to 40%.⁶ Modifying existing coal boilers has reduced NOX emissions 30% to 50%.³

COMBUSTION MODIFICATIONS 6-15 Staged combustion can also be accomplished by careful control of air and fuel mixing in the burner.

6-16 COMBUSTION MODIFICATIONS

Biased Firing

In some boilers, a number of the burners are operated fuel-rich, and others are operated air-rich in a staggered configuration called biased firing, as shown in Figure 6-10. Rather than use some burners exclusively for air flow (as is done in BOOS), bias firing involves adjusting the stoichiometry in each burner by reducing combustion air in some burners and increasing

combustion air flow to other burners. Fuel flow to all burners is maintained so that the stoichiometry ratio varies among burners, but an adequate overall stoichiometry is maintained. This approach decreases the peak flame temperature, similar to BOOS. However, because all burners are firing air and fuel in biased firing, there is less load penalty.

= Low SR

= Higher SR

Some burners operate at low stoichiometric ratio (SR) and some burners at higher SR.

Lower Furnace Furnace

Exit

Figure 6-10. Burner firing conditions using the biased firing approach

Practical Considerations of Off-Stoichiometric Combustion

A number of practical considerations exist in using off-stoichiometric firing techniques to reduce NOX emissions. This is particularly true of older units, where it may be difficult to identify locations for the installation of the OFA ports. For example, furnace volume can be an issue from the standpoint of having enough overall volume in the boiler to ensure adequate burnout.

Fan capacity can be another issue resulting from the use of OFA. A fan is required that can generate a high pressure drop to inject the combustion air into the flue gas stream. Inadequate velocity will cause inadequate mixing in the unit and produce high amounts of unburned hydrocarbons and CO.

Another concern is the extent to which the boiler control system must be modified in order to control critical parameters. Controlling the different amounts of fuel and air that are required to be delivered to the burners and controlling OFA dampers may require substantial installations of

equipment. Air infiltration can cause problems in controlling the amount of O2 that is entering the boiler system. The ability to control air flow at both high and low load conditions and the ability to physically locate new air

COMBUSTION MODIFICATIONS 6-17 ducts on the outside of the unit are major factors in deciding if an OFA approach is appropriate for reducing NOX emissions, particularly in older units. Air infiltration problems in older boilers can also complicate the achievement of appropriate overfire air conditions.

Finally, flue gas residence time is another major consideration in using off-stoichiometric firing, particularly OFA. Specifically, the temperature and the amount of time between the injection of the primary air and the OFA must be adequate for complete burnout.