Encyclopedia of Environmental Science and Engineering, Volume I and II

762 NITROGEN OXIDES REDUCTION

fan, (2) additional ductwork, (3) flow control dampers, and

(4) carbon monoxide and oxygen analyzers.^26 The disad-

vantages of FGR include flame instability and a decrease

in net thermal output at high recirculation rates; thus, the

reasoning for limiting the recirculation rate.

Low NO x Burners (LNB) Another way to achieve the

staging effects similar to that of OS combustion is to inter-

nally design the effect into the burner. A variety of low NO x

burners have been developed to achieve the desired fuel/air

ratio required for NO x reduction in different applications.

Figure 6^23 displays some of the burner types used in industry.

Table 15 lists some of the burner types and the NO x reduc-

tions that can be expected. Retrofitting a facility with low

NO x burners offers many advantages and include the follow-

ing: (1) requires no added instrumentation for controlling

the fuel/air ratio, (2) needs no additional ductwork or flue

gas recirculation fans, as discussed in the previous section,

(3) minimal long-term capital investment and maintenance

requirements, and (4) cost effective compliance with the

regulatory standards.

Low Excess Air Burner The first burner type listed in

Table 15 is the low excess air burner, which reduces NO x

emissions by combusting the fuel with the lowest amount of

excess air possible, usually 5–8%. Typically, boilers oper-

ate at 18–30% excess air at full load depending on the fuel.

Case studies have shown that a reduction of excess air from

30% to 10% can cut NO x emissions by 30%. The NO x is

reduced by the same principles as noted in LEA operations.

LEA burners are most commonly of a forced-draft design

and obviously have the ability to operate at low levels of

excess air. Furnaces can operate in two modes: (1) forced

draft; (2) natural draft.^27 Forced draft operation involves the

pressurization of the air by a fan into the furnace, whereas the

natural draft mode requires the stack to draft the combustion

Burner

Tile

Heater

Floor

Flame

Stabilizer

Air

Inlet

Gas Gun

Pilot

Standard Gas Burner

Oil Gun

Primary Air

Secondary

Air

Standard Oil Burner

Pilot

Secondary

Combustion

Secondary

Fuel

High Air-to-Fuel

Ratio in Primary

Combustion Zone

Combustion

Air

Secondary

Fuel Connection

Primary

Fuel Connection

Pilot

Staged air is mixed

with the combustion

products from the

primary zone,

which lowers the

peak flame

temperature.

Sub-stoichiometric

conditions in primary

zone increase the

amount of

reducing agents

(H 2 and CO).

Staged

Air

Secondary

Air

Staged Air Burner

Oil Gun

Primary Air

Staged Fuel Gas Burner

FIGURE 6 Burner configurations.^23

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