and Control Technologies
Principles of NOx
Flue Gas Treatment
Choosing the Best NOx Technology for the
Summary and References
NOx Burners / Methods
Ceramic Fiber Burners
Flue Gas Recirculation
Fuel Induced Recirculation
Low Excess Air (LEA)
Typically, the simplest boiler operational adjustments rely on the reduction
of excess oxygen used in combustion. The figure below illustrates how
NOx levels typically vary with oxygen content:
on image for larger view
Boilers may be operated at higher than necessary excess air conditions
for the following reasons:
- The presence of extra air
supports complete combustion at all normal firing rates and conditions,
even when air/fuel mixing is relatively poor thus minimizing the creation
of carbon monoxide, soot and other unburned hydrocarbons.
- Higher excess air levels
provide an operational safety margin that prevents inadvertent operation
at fuel-rich conditions reducing the formation of combustible vapor
mixtures and the possibility of a firebox explosion
The NOx creation rate typically
peaks at excess oxygen levels of 5 – 7% where the combination of
high combustion temperatures and the higher oxygen concentrations act
together. At both lower and higher air/fuel ratios, NOx production falls
off – due to lower flame temperature at high excess air levels and
lower oxygen at low excess air levels. Low excess air is achieved by changes
in operating procedures, system controls or both.
Boiler operation with LEA is considered an integral part of good combustion
air management that maximizes boiler efficiency. Therefore, most boilers
should be operated on LEA regardless of whether NOx reduction is an issue.
However, excessive reduction in excess air can be accompanied by significant
increases in CO. When excess air is reduced below a certain level, CO
emissions increase exponentially. This rapid increase in CO is indicative
of reduced mixing of fuel and air that results in a loss in combustion
Load reduction, when implemented, decreases the combustion intensity,
which, in turn, decreases the peak flame temperature and the amount of
thermal NOx formed. However, test results have shown that with industrial
boilers, there is only slight NOx reduction available from this technique
as the NOx reduction effect of lowering the load is often tempered by
the increase in excess air required at reduced load.
Higher excess air levels are often required with older single-burner units
because high burner velocity promotes internal gas recirculation and stable
combustion. Multiple-burner boilers generally provide a greater load turndown
Staged combustion burners, the most common type of low NOx burners (LNB),
achieve lower NOx emissions by staging the injection of either air or
fuel in the near burner region. Staged combustion burners may be further
classified as either staged air burners or staged fuel burners. The division
of combustion air reduces the oxygen concentration in the primary burner
combustion zone, lowering the amount of NOx formed and increasing the
amount of NOx reducing agents. Secondary and tertiary air complete the
combustion downstream of the primary zone, lowering the peak temperature
and reducing thermal NOx formation. The basic concept of staged combustion
is shown in the following diagram:
The deficiency of oxygen in
the first zone and the low temperatures in the second zone both contribute
to a reduction in NOx production. Staged combustion can result in NOx
reductions of up to 60% for natural gas.
Due to the staging effect of staged combustion air (SCA) burners, flame
lengths tend to be longer than those of conventional burners. This may
be of particular concern for packaged units because there is the possibility
that flame impingement will occur on the furnace walls, resulting in tube
failure and corrosion. Additionally, staged air burners are often wider
and longer than conventional burners, possibly requiring modifications
to existing waterwalls and windboxes.
Staged combustion can be accomplished external to the burner body by separate
introduction of air. External air staging techniques commonly used for
larger boilers include:
(BOOS) which is a staged combustion technique typically used
for large boilers. Introducing additional gas through operational burners
at the lower furnace zone to create fuel rich conditions controls NOx.
Additional air is supplied through registers of non-operating burners
above the lower zone to complete combustion. Since burners are taken
out-of-service, the unit capacity is typically reduced.
- Over-fire-air (OFA)
which typically involves the injection of secondary air into the furnace
through OFA ports above the top burner level, coupled with a reduction
in primary airflow to the burners. The fuel rich air-fuel mixture is
fed to the normal burners reducing flame temperature and oxygen concentration.
In staged fuel burners, combustion air is introduced without separation
and instead the fuel is divided into primary and secondary streams. Despite
the high oxygen concentration in the primary combustion zone, thermal
NOx formation is limited by low peak flame temperatures which result from
the fuel-lean combustion. Quenching of the flame by the high excess air
levels also occurs, further limiting the peak flame temperatures and providing
active reducing agents for NOx reduction. Inerts from the primary zone
then reduce peak flame temperatures and localized oxygen concentration
in the secondary combustion zone, thereby reducing NOx formation. An advantage
of staged fuel burners over staged air burners is that they tend to have
shorter flame lengths, decreasing the likelihood of flame impingement.
The following sections summarize the
performance, applicability, and availability of the various methods of
implementing SCA on the major types of natural-gas or oil-fired boilers.
SCA is not considered a primary NOx control method for existing firetube
boilers because of the major modifications required to retrofit staged
air to these boilers. Side-fired air application is difficult as retrofit
requires penetration of the firetube boiler water shell.
Packaged Watertube Boilers
Packaged watertube boilers generally use only one burner, so BOOS is
not applicable as a means of achieving staged combustion. As with firetube
boilers, retrofit of SCA to smaller packaged watertube units is often
impractical due to the difficulty of retrofitting equipment.
Field-erected Watertube Boilers
For field-erected watertube boilers equipped with more than one burner,
staged combustion can be achieved by using OFA, BOOS, or biased burner
firing. Biased burner firing consists of firing certain burners fuel-rich
while other burners are fired fuel-lean. This may be accomplished by
maintaining normal air distribution to the burners while adjusting fuel
flow so that more fuel is sent to desired burners. Usually, the upper
row of burners is fired fuel-lean, but this varies from boiler to boiler.
Although operation with BOOS can measurably reduce NOx, the operating
performance of the boiler can be somewhat degraded because of the need
to increase excess air in order to control CO, hydrocarbon, and smoke
emissions. Adjustments to the airflow controls, such as burner registers,
may be required to achieve the desired burner stoichiometry without
increasing these emissions. Also, operation with BOOS usually requires
that the unit be derated unless modification to the fuel delivery system
Generally, OFA is applicable only to large furnaces with sufficient
volume above the burners to allow complete combustion and steam temperature
Many retrofits have utilized combinations of the above combustion modification
methods. The combination of LNB with FGR is used by many vendors.
An LNB type known as a cyclonic burner has been developed by AESYS/York-Shipley
for packaged boilers. In cyclonic combustion, high tangential velocities
are used in the burner to create a swirling flame pattern in the furnace.
This causes intense internal mixing as well as recirculation of combustion
gases, diluting the temperature of the near-stoichiometric flame and lowering
thermal NOx formation. The tangential flame causes close contact between
combustion gases and the furnace wall, adding a convective component to
the radiant heat transfer within the furnace. The increased heat transfer
and low excess air operation of the cyclonic burner result in increased
boiler efficiency. To achieve ultra-low NOx levels, a small quantity of
low-pressure steam is injected into the burner, which further reduces
the local flame temperature and NOx formation.
Ceramic Fiber Burners
Corporation has developed a patented fully pre-mixed surface stabilized
combustion technology to achieve single digit NOx emissions. The technology
has been used in firetube, watertube and process heater applications.
In the burner, fuel gas is premixed with combustion air before entering
the burner. The surface material is cooled by the incoming air/fuel mixture
and the low combustion temperature limits thermal NOx formation.
Gas Recirculation (FGR)
involves recycling a portion of the combustion gases from the stack to
the boiler windbox. Introducing some of the flue gas back into the combustion
air can reduce peak flame temperatures. The incoming air is diluted and
the oxygen concentration in the combustion zone is reduced. During combustion,
the recirculated flue gases also absorb some of the heat and thereby reduce
the peak combustion temperatures. In order to retrofit a boiler with FGR,
the major additional equipment needed are a gas recirculation fan, dampers
The figure shows a typical
can increase the flame size which may cause impingement on heat transfer
surfaces thus limiting firing rate. The energy efficiency may be negatively
impacted due to reduced heat transfer as a result of lower flame temperatures.
Usually, boiler thermal efficiency reductions resulting from FGR are limited
to around 1% or lower.
For a given firing rate, FGR increases the throughput of gases in the
burner and firebox. The higher gas flow may require a larger forced draft
fan or an additional flue gas recirculation fan.
FGR has little effect on fuel bound NOx emissions. FGR is used on a number
of watertube and firetube boilers firing natural gas. Boilers are usually
not operated with more than 20 percent FGR due to flame stability considerations.
Overall NOx reductions are typically in the 40 – 70% range. In general,
thermal NOx reductions from distillate-oil-fired boilers with FGR are
somewhat less than from natural-gas-fired units. This is due to the greater
potential for flame instability and emissions of unburned combustibles
from distillate-oil-fired units, which limits the practical rate of FGR
that can be used.
Fuels containing sulphur can create sulphur oxides in the flue gas. In
addition, soot can buildup on the fan and burner passages. Therefore,
FGR is typically not used when burning heavy fuel oils.
Fuel Induced Recirculation (FIR)
Fuel induced recirculation is a control technology for natural-gas-fired
boilers. FIR involves the recirculation of a portion of the boiler flue
gas and mixing it with the gas fuel at some point upstream of the burner.
The primary difference between FIR and FGR is that in FIR the flue gas
is mixed with the fuel stream, whereas in FGR the flue gas is recirculated
into the combustion air. By diluting the fuel prior to combustion, which
lowers the volatility of the fuel mixture, FIR reduces the concentration
of hydrocarbon radicals that produce prompt NO. Additionally, FIR reduces
thermal NOx in the same manner as FGR, by acting as a thermal diluent.
Thus, one of the main benefits of FIR technology is that it impacts both
prompt NO and thermal NOx formation in gas-fired boilers.
Flue gas recirculation is induced using the natural gas dynamics of the
burner flow streams, without additional equipment such as recirculation
Steam/Water Injection (S/WI)
When water or steam are injected in the flame, they reduce the
peak flame temperature and the oxygen concentration. The quenching of
the flame reduces the NOx by as much as 75 percent, depending on the amount
of water or steam injected. Less water than steam is needed to achieve
the same quenching effect because of the heat of vaporization required
to change water into steam. Water/Steam can also be directly injected
into the combustion air just prior to the flame.
Because of low initial cost, the technique is considered particularly
effective for small single-burner packaged boilers operated infrequently.
In these applications, the oil gun positioned in the center of the natural
gas ring burner is used to inject the water at high pressure. The amount
of water injected normally varies between 25 and 75 percent of the natural
gas feedrate, on a mass basis. However, the technique has some important
environmental and energy impacts. For example, CO emissions may increase
because of the quenching effect on combustion, and the thermal efficiency
of the boiler decreases because the moisture content of the flue gas increases,
contributing to greater thermal losses at the stack. Another concern related
to the technique is its potential for unsafe combustion conditions that
can result from poor feedrate control.
One vendor of steam injection technology offers an advanced design based
on “hypermixing”. In this concept, the injected steam acts
as more than just a heat sink. By injecting high pressure steam directly
at the burner head instead of simply adding it into the air or fuel steam,
steam injection can become a significant source of mixing power and it
becomes possible to increase the mixing rates with the products of combustion
in the furnace. The mixing power boost provided by the high velocity steam
greatly improves the temperature uniformity within the furnace reducing
peak temperatures which results in lower NOx formation. The improved mixing
uniformity can also reduce excess air requirements.