NITROGEN OXIDES REDUCTION 763
air into the furnace. For multiple burner applications, it is
imperative that all burners receive equal amounts of air. One
burner that receives more excess air will produce more NO x ,
whereas the resulting burner that receives less excess air will
generate more carbon monoxide and unburned carbon.
Staged Air Burner Another type of burner is the staged
air burner, which is primarily used in forced draft liquid fuel
fired applications, although natural draft applications do
exist. The fuels used in this burner are normally such fuels
as butane, pentane, diesel, and No. 6 fuel oil. This burner
design reduces NO x by lifting the combustion air into a pri-
mary and a secondary zone. Fuel is injected into the throat
of the burner and mixed with the primary air. This zone is
fuel rich and produces partial combustion. NO x is minimized
in this zone because the nitrogen is converted into reducing
agents, which are subsequently oxidized to elemental nitro-
gen. In addition, because the generated heat in the primary
zone is rapidly dissipated, the peak flame temperature is
reduced and NO x formation is also lowered. The fuel lean
or secondary zone completes combustion by injecting air
through refractory ports, which also stabilizes the flame
profile. Although this method can lead to NO x reductions of
20–35%, staged air burners can lead to long flame profiles,
which must be closely monitored and controlled.
Staged Fuel Burner In gas fired applications, staged
fuel burners are typically used to lower NO x emissions.
Because liquid fuels can create fouling problems with the
secondary gas nozzles, this burner normally is used for
firing natural gas and other light fuels. Similar to the staged
air burner design, fuel supply is divided into primary and
secondary zones. The primary zone involves the mixing of
the combustion air with a portion of the fuel gas, resulting
in a fuel lean environment. This fuel lean combustion zone
reduces peak flame temperature and lowers the formation
of thermal NO x. Combustion is completed in the secondary
zone where nozzles inject the remaining fuel to create fuel
rich conditions. Part of the NO x formed in the first stage is
reduced by hydrogen and carbon monoxide in the secondary
zone. Staged fuel burners normally produce a flame that is
50% longer than that of normal standard gas burners.
Ultra Low NO x Burners The last type of burner design
is the ultra low NO x burner. This burner combines the staged
air or staged fuel with internal or external flue gas recircula-
tion to reduce NO x. Typically, internal FGR is utilized. In a
staged air internal FGR burner, fuel is mixed with part of the
combustion air to create a fuel rich zone. The recirculated
flue gas is developed by high pressure atomization of the
liquid or gaseous fuel. Combustion is completed by piping
the secondary air to the burner block. In a staged fuel inter-
nal FGR burner, flue gas is recirculated by the pressure of
the fuel gas. The fuel gas creates a fuel lean zone and reduces
the partial pressure of the oxygen, thereby reducing NO x
emissions.
Water / steam injection One of the most seldom used meth-
ods in controlling NO x is water/steam injection. Normally,
these techniques are applied to gas turbines. These methods
have not been used extensively because of the lower thermal
efficiency resulting from the absorption of usable energy.Both of these processes accomplish NO x reduction by low-
ering the peak flame temperature. The PFT is reduced by
directly abstracting heat from the burner flame and by dilut-
ing the oxygen concentration near the burner front. Figure 7^22
shows the approximate NO x reductions that can be expected
for different water injection rates.
Although substantial NO x reductions can be expected from
these processes, a number of operational consequences must
be considered. At low loads, carbon monoxide and unburned
hydrocarbon emissions increase. As noted earlier, usable
energy is lost, particularly due to the heat of vaporization of
water, which results in an increase in fuel consumption of up
to 5%. Further considerations include the additional wear and
tear on turbine parts and the additional complexity in control-
ling and monitoring the process.^28
Selective Catalytic Reduction (SCR) In lieu of or in addi-
tion to changing the air/fuel ratio in the combustion zone,
some facilities utilize post combustion techniques to control
NO x. There are two basic post combustion control technolo-
gies of varying types on the market today: selective cata-
lytic reduction (SCR) and selective noncatalytic reduction
(SNCR). These methods have been used extensively on an
international scale and have become a common feature on
gas-turbine cogeneration and combined cycle systems in the
United States. These systems can provide NO x reductions of
up to 90%.
One of the most popular post combustion techniques is
selective catalytic reduction. SCR works on the premise of
reacting NO x with ammonia to produce water and elemental
nitrogen. The reactions involved in the SCR process are as
follows:^294NO 4NH 3 O 3 → 4N 2 6H 2 O
6NO 4NH 3 → 5N 2 6H 2 O
2NO 2 4NH 3 O 2 → 3N 2 6H 2 O
6NO 2 8NH 3 → 7N 2 12H 2 O
NO NO 2 2NH 3 → 2N 2 3H 2 O.Performance tests indicate that the first reaction is the
dominant reaction. An SCR system consists mainly of an
ammonia injection grid, catalyst reactor and ductwork.
The first part of the SCR, the ammonia injection grid,
involves the mixing of ammonia with the flue gas stream.
Two types of ammonia are used in the process: anhydrous
and aqueous. A typical anhydrous ammonia injection system
involves the following:^29- Storage of the anhydrous ammonia in a pressurized
tank - Piping of the anhydrous ammonia to a liquid
vaporizer - Mixing of the ammonia vapor with a predetermined
amount of ambient air - Distribution of the ammonia-air mixture to the
grid for injection.
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