758 NITROGEN OXIDES REDUCTION
Stationary Source ControlLow excess air operation (LEA) NO x emissions are a func-
tion of the amount of available oxygen. Thus, one simple
method of reducing NO x emissions is by reducing the excess
air level to the burners. Low excess air operation is effective
in reducing fuel NO x formation, but is limited in decreasing
thermal NO x emissions. Normally, this level is set to some
constraint such as flame length, flame stability or carbon
monoxide formation. As noted in Table 15, low excess air
operation does not result in substantial NO x reductions.
Off stoichiometric combustion (OS) In off stoichiometric
combustion techniques, NO x reduction is achieved by alter-
ing the fuel/air ratio in the primary combustion zone.
Burners-out-of-Service (BOOS) One such technique
is known as burners-out-of-service. As the name implies,
this operational control method involves taking one or more
burners out of service, in a multiburner unit, by terminat-
ing fuel supply to the selected burners but leaving the air
registers open. NO x is reduced by lowering the peak flame
temperature (PFT) in the remaining operating burners. As
the temperature decreases in the combustion zone, the NO x
emissions will also decrease. The temperature decreases as
a result of the remaining burners operating in a fuel rich
environment, which corresponds to lower oxygen availabil-
ity; thus, the peak flame temperature is lowered. In addi-
tion to the fuel/air ratio, the peak flame temperature is alsodependent on the radiative heat effects in the boiler.^24 Each
burner is in radiative exchange with adjacent burners; there-
fore, if the number of burners in-service are reduced, then
the radiative effects are reduced along with the peak flame
temperature.
Overfire Air (OA) One other method of changing the
fuel/air ratio to the burners is by installing overfire air ports
above the burner zone. A controlled portion of the combus-
tion air, normally 10–20%, is redirected above; flames to the
OFA ports.^25 Effective implementation this control method
relies on a number of parameters, most notably adequate
mixing of the overfire air with the primary combustion pro-
duction. In addition, OFA is a function of the location and
number of ports, ports spacing and geometry, and on the fan
capacity and furnace dimensions. By itself, OFA can yield
15–30% reductions in NO x emissions. However, there are
certain advantages of OFA which have been noted in several
cases. Because NO x reduction requires a separation of the
OFA ports from the primary combustion zone, poor temper-
ature distribution in the convective zone and deterioration in
carbon burnout has been observed.
One variation of the overfire air control method is called
lance air. This method involves the installation of air tubes
around the periphery of each burner to supply staged air.^22
Flue gas recirculation (FGR) One of the most effective
methods of reducing NO x emissions for gas fired units isTABLE 12^
Properties of selected solid fuels 18,21
Percent by weight
Proximate analysis Ultimate analysisCarbonVolatile
matter MoistureAsh
CHNO SHHV
(MJ/kg)“K”
factor
Meta-anthracite (RI) 65.3 2.5 13.3 18.9 64.2 0.4 0.2 2.7 0.3 21.7 790.60
Anthracite (PA) 77.1 3.8 5.4 13.7 76.1 1.8 0.6 1.8 0.6 27.8 815.63
Semianthracite (PA) 78.9 8.4 3.0 9.7 80.2 3.3 1.1 2.0 0.7 31.3 838.32
Bituminous (PA) 70.0 20.5 3.3 6.2 80.7 4.5 1.1 2.4 1.8 33.3 856.67
High-volatile bituminous
(PA) 58.3 30.3 2.6 9.1 76.6 4.9 1.6 3.9 1.3 31.7 850.68
(CO) 54.3 32.6 1.4 11.7 73.4 5.1 1.3 6.5 0.6 30.7 861.10
(KY) 45.3 37.7 7.5 9.5 66.9 4.8 1.4 6.4 3.5 28.1 850.04
(IL) 39.1 40.2 12.1 8.6 62.8 4.6 1.0 6.6 4.3 26.7 855.34
Subbituminous (CO) 45.9 30.5 19.6 4.0 58.8 3.8 1.3 12.2 0.3 23.6 862.60
Lignite (ND) 30.8 28.2 34.8 6.2 42.4 2.8 0.7 12.4 0.7 16.8 —
Brown coal (Australia) 15.3 17.7 66.3 0.7 ———— 0.18.6—
Wood (Douglas fir) 17.2 82.0 35.9 0.8 52.3 6.3 0.1 40.5 0.0 21.0 —
August Victoria (Germany) NA 33.5 NA 5.5 85.5 5.2 1.5 — 1.1 — —
Prosper (Germany) NA 34.3 NA 7.5 88.7 2.8 1.6 — 1.2 — —
Göttleborn (GB-Germany) NA 36.5 NA 10.2 79.5 4.9 1.5 — 1.0 — —
Emil Mayrisch (EM-Germany) NA 14.3 NA 8.5 89.3 4.2 1.5 — 0.9 — —
Blends
40% GB-60%EM NA 23.6 NA 9.1 85.3 4.5 1.5 — 1.0 — —
70% GB-30%EM NA 30.1 NA 9.5 83.4 4.6 1.5 — 1.0 — —C014_002_r03.indd 758C014_002_r03.indd 758 11/18/2005 1:26:54 PM11/18/2005 1:26:54 PM