boilers burning natural gas represent a significant source of stationary NOx emissions
when aggregated, most of the focus on controlling boiler emissions has been on
NOx. The two general categories of NOx controls for boilers are:
A general overview
of each follows. More detailed information is available in the Gas Technology
AdvisorTM module on Boilers.
Control Techniques for Reducing NOx
The most common methods of reducing NOx emissions from the combustion
of natural gas in a boiler include low excess air firing, burner modification
or replacement, and flue gas recirculation.
Excess Air Firing
the excess air in a boiler limits the amount of nitrogen available for NOx formulation.
This can be accomplished with the installation of an oxygen trim system, resulting
in NOx reductions in the 5-10% range. There is a limit to how far the excess air
can be reduced (usually 3% O2) until carbon monoxide (CO) formulation and unburned
hydrocarbons become a concern.
Modification or Replacement
Burner modifications or replacement (referred to as low-NOx burners)
are used to stage the combustion in the boiler furnace. Instead of producing a
very robust full flame, the burner with staged combustion delays the combustion
process. This results in a cooler flame, which suppresses thermal NOx formulation.
NOx emission reductions of 40-85% can be achieved with staged combustion, which
is used in conjunction with reduced excess air.
Gas Recirculation (FGR)
reductions in NOx beyond those achievable with low-NOx burners, flue gas recirculation
(FGR) is used. In a FGR system, a portion of the flue gas is recycled from the
stack into the burner wind box. Upon entering the wind box, the recirculated gas
is mixed with combustion air prior to being fed to the burner. The recycled flue
gas consists of combustion products which act as inerts during combustion of the
The FGR system
reduces NOx emissions by two mechanisms. Primarily, the recirculated gas acts
as a dilutant to reduce combustion temperatures, thus suppressing the thermal
NOx mechanism. The amount of FGR can be increased up to about 25% for increased
NOx reduction to the desired level, with the limitation being the impact on flame
stability. FGR is usually used in combination with low-NOx burners and reduced
excess air to achieve reductions in NOx emissions of 60-90% from uncontrolled
levels. The application of FGR usually requires the replacement of the boiler
burner, fan and wind box.
Combustion Control Techniques
Other combustion control techniques used to reduce NOx emissions are
water or steam injection and gas reburning.
or Steam Injection
Water or steam injection is similar to the use of overfire in combustion
staging. Water or steam can be injected into the burner flame to reduce flame
temperatures and reduce thermal NOx formation. This method can result in NOx reductions
in the 50-70% range. However there is a significant efficiency penalty associated
with the level of injection and this method of control has become less popular
with the advances made in low-NOx burner technology coupled with FGR.
Gas reburning injects additional amounts of natural gas in the upper
furnace, just before the overfire air ports, to provide increased reduction of
NOx. This method of gas injection for reburning has been used with some success
to reduce NOx emissions from coal boilers.
Internal Recirculation (FIR) Burner
The Forced Internal Recirculation (FIR) burner, a new low-NOx burner
developed by The Gas Technology Institute, has demonstrated a capability to meet
or exceed the lowest NOx levels achievable through other combustion methods. The
main advantage of this burner is that FGR is not required.
uses premixed substoichiometric combustion and a two-stage design to force internal
recirculation of combustion products. This unique design brings secondary air
through the primary combustion zone and enhances internal recirculation of gaseous
products of partial combustion resulting from lower flame temperatures. Successful
development of the FIR burner will meet an unfulfilled market need for a cost-effective,
low-emission boiler burner. The basic technology provides a solid foundation for
further advancements in single digit NOx emission burners (< 10 ppm NOx) to
meet future environmental regulations across many applications.