Furnace Improvements Services

www.heatflux.com 1

Specifying the right requirements for low

NOx burners can significantly reduce

nitrogen oxides (NOx) emissions from a

furnace. Ultra Low NOx burners that can

meet even the most stringent emission

control limits imposed by some states,

are now available and offer a very

attractive route to NOx reduction.

However, burner selection and

specification should be done very

carefully, because burner operation has a

direct effect on furnace performance.

This article describes the various types of

low NOx burners and outlines the main

design parameters that must be

considered when selecting a burner

system.

Low NOx burners generally modify

the means of introducing air and fuel to

delay the mixing, reduce the availability

of oxygen, and reduce the peak flame

temperature. Whether for a new furnace

or a retrofit application, these burners

must meet five major requirements.

Operation with lower NOx formation:

A flame pattern compatible with

furnace geometry; Easy maintenance and

accessibility; A stable flame at turndown

conditions; and the ability to handle a

wide range of fuels.

Burner Types

Table 1 lists the types of burners

currently in use in chemical process

industries (CPI) plants and petroleum

refineries. Figure 1 compares staged-

combustion burners with standard gas

and oil burners.

Staged-Air Burners. Combustion air is

split and directed into primary and

secondary zones, thus creating fuel-rich

and fuel-lean zones.

These burners are most suitable

for forced-draft

liquid-fuel-fired

applications.

Combustion air

pressure energy

lends itself to

better control of

the staging air

flows. It ensures

a high enough air

velocity to

produce good air-

fuel mixing and a

good flame.

Staged-air burners lend themselves very

well to external flue gas recirculation

(FGR). In such designs, flue gas is

generally introduced into the primary

combustion zone.

Staged Fuel Burners. The fuel gas is

injected into the combustion zone in two

stages, thus creating a fuel-lean zone and

delaying completion of the combustion

process. The fuel supply is divided into

primary fuel and secondary fuel in a ratio

that depends on the NOx level required.

The flame length of this type of burner is

about 50% longer than that of a standard

gas burner.

Staged-fuel burners are ideal for

fuel gas fired natural draft applications.

Low Excess Air Burners. These burners

reduce NOx emissions by completing

combustion with the lowest amount of

excess air possible, usually no more than

5-8%. Increases in excess air result in

increases in NOx formation (Figure 2a).

AIR POLLUTION CONTROL

Specify Better Low NOx Burners For Furnaces

Ashutosh Garg,

Kinetics Technology

International Corp.

Air staging, fuel staging, and

internal flue gas recirculation

are among the design fea-

tures that help reduce NOx

emissions

Originally appeared in: January 1994 issue, pgs 46-49

CHEMICAL ENGINEERING PROGRESS Reprinted with publisher’s permission.

Furnace Improvements Services

www.heatflux.com 2

Most forced-draft burners have the

ability to operate at very low levels of

excess air. In a multiple-burner

installation, it is essential that all burners

receive equal amounts of air. This can be

achieved by simulating the flow profiles

in the ducts and burners. Flow

deficiencies and other irregularities can

then be detected and corrected using

splitters and vanes, ensuring equal air

distribution within +1%.

Flue Gas Recirculation Burners. In

these burners, 15-25% of the hot (300-

500oF) flue gas is circulated along with

combustion air. The flue gas acts as a

diluents, reducing flame temperature and

suppressing the partial pressure of

oxygen, thus reducing NOx formation.

Flue gas can be injected into burners

through a separate scroll into the primary

zone or mixed with incoming air.

External FGR can be used with natural

draft burners, although it is mostly used

with forced-draft preheated air burners.

In some new burner designs, flue gas

is internally re-circulated using the

pressure energy of fuel gas, combustion

air, or steam. This makes the operation of

burners simple and eliminates the FGR

fan and its controls, although burner size

becomes large.

Ultra Low NOx Burners. Several

designs are available today that combine

two NOx reduction steps into one burner

without any external equipment. These

burners typically inappropriate staged air

with internal FGR or staged fuel with

internal FGR. In the former design, fuel

is mixed with part of the combustion air,

creating a fuel-rich zone. High pressure

atomization of liquid fuel or fuel gas

creates FGR. The secondary air is routed

by means of pipes or ports in the burner

block to complete combustion and

optimize the flame profile.

In staged-fuel gas burners with

internal FGR, fuel gas pressure induces

recirculation of flue gas, creating a fuel

lean zone and a reduction in oxygen

partial pressure.

The former design can be used with

the liquid fuels, whereas the latter design

is used mostly for fuel gas applications.

Design Parameters

The following parameters require

attention during system design.

Fuel Specification. Correct and accurate

fuel specifications are essential for

predicting NOx emissions.

For gaseous fuels, the complete

analysis listing all the constituents is

required, as well as any possible

variations in gas composition. Major

components affecting NOx emissions are

hydrogen and hydrocarbons in the C3-C4

range. Other physical properties, such as

pressure, temperature, and heating value

are required for burner design.

For liquid fuels, the most important

parameter is the fuel’s nitrogen content –

about 40-90% of the fuel nitrogen shows

up as NOx in the flue gas. Other liquid

fuel parameters required by burner

vendors are pressure, temperature,

viscosity, and heating value.

Atomization Medium. For low NOx

burners, steam is preferred as the

atomization medium over compressed

air, because higher quantities of steam

decrease the amount of NOx in the flue

gas. Increases in steam temperature

increase NOx emissions.

Fuel Filters. Staged fuel gas burners

have more gas tips and rises than

standard burners, and the fuel gas flow

per tip is reduced to as low as one-fourth.

It is important that these burners be used

with clean fuel gases. To accomplish this,

installation of fuel gas filters and

knockout pots to remove particulates and

condensate is recommended.

Figure 1: Off-stoichiometric combustion can be achieved by air staging or fuel staging.

Courtesy of John Zink Co

Furnace Improvements Services

www.heatflux.com 3

Some plants have opted for low fuel

gas pressures or double orifice designs

for the gas tips to keep the tip size large

enough to avoid plugging when firing

dirty gases. These options generally do

not give good results, and they also

produce longer flames (flame length is

discussed later).

Heat Release And Turndown. Plant

engineers have typically specified a

margin on the heat release rate as high as

30-50% over the design heat release.

Furthermore, on most standard burners,

the turndown for gas fuel is generally 5:1

and for oil it is 3:1. These two parameters

offered virtually unlimited flexibility to

overfire and underfire the furnaces.

However, to ensure optimum

performance of low NOx burners, it is

important to limit the overdesign margin

to only 10%. In most cases, the turndown

should be limited to 3:1 for gas and 2:1

for oil. This will require more attention

from the operators and minimization of

burner outages. The result, though, will

be better performance from the low NOx

burners.

Heater Draft. The available heater draft

is a very important design parameter,

especially for natural-draft burners,

because it directly indicates the air

pressure energy available for air/fuel

mixing. It is, therefore, important that

available draft be specified correctly.

In some cases, it may be

advantageous to increase the available

draft by increasing the stack height or

diameter. Increased draft availability can

reduce the size of the burners. However,

increased draft at the hearth also

increases the likelihood of air leakage

into the furnace, so the furnace should be

made leak-tight to prevent such air

infiltration.

Firebox Temperature- In the past,

standard burners specified independent of

the furnace design parameters. However,

the performance of low NOx burners is

closely linked with furnace design and

firing arrangements. NOx formation is

dependent on firing density and firebox

temperature. The burner vendor needs the

firebox temperature and geometry to

predict NOx emissions correctly. Higher

firebox temperature leads to higher NOx

formation, as depicted in Figure 2b.

Combustion Air Temperature.

Combustion air temperature has a direct

bearing on flame temperature, and the

higher the flame temperature, the more

thermal NOx is formed, as shown in

Figure 2c. If the heater is already

equipped with an air preheater, then

burners utilizing flue gas recirculation

offer a good degree of NOx reduction. In

new heaters, alternative methods of waste

heat recovery should be investigated.

Flame Length- This parameter has the

most important effect on the operation of

the furnace.

Traditionally, furnace operators are

accustomed to short, crisp flames, which

prevent flame impingement damage to

the furnace tubes. The key to getting a

short flame has been to increase excess

air until the flames are blue sand short.

This practice has been curbed to some

extent by the installation of oxygen

analyzers.

The basic design principle of low

NOx burners calls for staged combustion

and cooler flames. This is in direct

conflict with the good mixing of air and

fuel required for efficient combustion.

Thus, a balance needs to be struck

between the two requirements so as to

achieve acceptable NOx levels and flame

dimensions.

A typical low NOx burner has a

Figure 2. NOx emissions are

a function of various

furnace parameters.

Furnace Improvements Services

www.heatflux.com 4

flame that is about 50-100% longer than

the flame in a standard burner (when

operated design conditions). Any

variation in operating condition tends to

increase the flame length in low NOx

burners, thereby increasing the chance of

flame impingement.

The expected flame length must be

kept in mind when specifying the heat

release rate and the total number of

burners. It is also recommended that the

maximum heat release rate per burner be

limited to 10 MMBtu/h. Furthermore, the

burner flame length should be kept to a

third of the firebox height for low-roof

cabin heaters. Typical clearances for low

NOx burners are recommended in Table 2.

Burner Size. Today’s low NOx burners are

much larger than standard burners for several

reasons:

Air staging has led to the use of

secondary and tertiary air controls.

Fuel staging had led to the segregation of

gas tips and, thus, larger diameter

burners. It also requires more gas piping

and separate gas controls.

Recirculation of flue gases requires

separate gas tubes, and the increased

volumes of gas and air require larger

burner throats.

Internal flue gas recirculation calls for

larger burner tiles and re-circulation flue

gas ports.

Thus, it is becoming very difficult to fit the

new low NOx burner in an existing heater

floor without sacrificing some degree of

operational and maintenance flexibility. It is

essential that the engineering contractor be

given drawings of the general arrangement of

the heater and the steelwork to work out the

installation details.

Burner Testing

Burner design is mostly empirical and

predicted design and operating conditions can

only be verified through performance tests.

Thus, burner testing is strongly recommended

for all new low NOx burners.

Testing of these burners should be

handled with care. The flue gas flow and the

expected temperature profile in the furnace

usually cannot be reproduced exactly in the

test furnace. For this reason, emission test

results should be considered estimates, and

actual emission calculations should

incorporate a margin to account for this.

Low NOx burners have been installed in a

variety of applications in both new facilities

and in revamped plants. Table 3 summarizes

several installations.

Acknowledgement

The author is thankful to KTI manage-ment for permission to publish this arti-cle. Thanks are also due to Rose Wil-liams for repeatedly typing the manu-script.

Further Reading Bell, C.T., and S. Warren, “Experience with Burner NOx Reduction,” Hydrocar-bon Processing, 62(9), pp.145-147 (Sept. 1983). Garg A., Trimming NOx from Fur-naces”, Chem. Eng. 99 (11), pp.122-130 (Nov. 1992) Johnson, W.M., and R.R. Martin, “Staged Fuel Burners for NOx Control in Fired Heaters”, presented at the 1984 Winter National Meeting of AIChE, Atlanta, GA (Mar. 1984) Kunz, R.G., et al., “Control NOx from Gas Fired Hydrogen Reformer Fur-naces” presented at the National Petro-leum Refiners Association. Waibel, R., et al., “Fuel Staging Burn-ers for NOx Control”, presented at the 1986 Symposium on Industrial Com-bustion Technology, sponsored by Gas Research Institute, U.S. Dept. of En-ergy, American Flame Research Com-mittee, and American Society for Met-als (now ASM International), Chicago, IL (Apr. 1986)

A. Garg is manager of thermal

engineering at Kinetics Technology International Corp. (KTI), Houston, TX (713/974 5581; 713/974 6691). He has more than 19 years of ex-perience in process design, sales, and commissioning of fired heaters and combustion systems. Previ-ously, he worked for Engineers India Ltd., and for KTI in India. He received a B.Tech in chemical engi-neering from the Indian Institute of Technology. He is a registered pro-fessional engineer in California and a member of AIChE.