NITROGEN OXIDES REDUCTION 761
through internal or external flue gas recirculation (FGR).
In FGR, a portion of the hot inert flue gas (300F–500F)
is recirculated from the stack back to the burner box. The
recirculation rate is normally limited to 15–25%. Figure 5^22
shows the NO x reductions that can be expected from dif-
ferent amounts of flue gas recirculation. In external FGR,
flue gas can be mixed with combustion air or can be
injected into the primary combustion zone through the use
of a recirculation fan. Internal FGR utilizes the pressure
energy of the combustion air, fuel gas or steam or recir-
culate the flue gas. Although internal FGR eliminates the
need for fans and controls, it does necessitate larger burner
sizes.
Flue gas recirculation reduces NO x because the flue
gas acts as a diluent. The additional flue gas reduces the
peak flame temperature, cutting thermal NO x formation.
In addition, the partial pressure of oxygen is reduced, also
limiting NO x formation. In industrial applications, FGR is
typically used in forced draft gas fired units which burn
low nitrogen fuels. As noted above, external FGR requires
substantial equipment for installation: (1) recirculationTABLE 15^
NO x Control methods and reduction 22,23Control method NOx reduction (%)Low excess air 1–15
Off-stoichiometric combustion 30–60
Low-NOx burner
Staged-air burner 25–35
Staged-fuel burner 40–50
Low excess air burner 20–25
Burner w/external FGR 50–60
Burner w/internal FGR 40–50
Air or fuel-gas staging w/internal FGR 55–75
Air or fuel-gas staging w/external FGR 60–80
Flue gas recirculation 40–80
Waster/stream injection 40–70
Selective catalytic reduction 70–90
Selective noncatalytic reduction—Urea 25–50
Selective noncatalytic reduction—Ammonia 25–509080706050403020100
0246 81 0121416182022
Flue Gas Recirculation, %NOReduction, %xFIGURE 5 NOx reduction as a function of % flue gas recirculation.^22C014_002_r03.indd 761C014_002_r03.indd 761 11/18/2005 1:26:55 PM11/18/2005 1:26:55 PM