Chemical Engineering May 2011 - 47

Figure 2. With a series of vanes at exit angles of about 35-40 deg, the low-swirl
burner splits the flow of incoming air into a non-swirling center surrounded by a
swirling annular flow
CO2 + 2H2O), twice as much oxygen
is consumed as methane on a mole or
volume basis. The equivalence ratio
of a system is defined as the ratio of
the experimental fuel-to-oxidizer ratio
to the stoichiometric fuel-to-oxidizer
ratio. The equivalence ratio may be expressed
as either: phi (fuel-to-oxidizer)
or lambda (air-to-fuel).
In the methane combustion reaction
above, there is no residual oxygen and
no residual fuel, so the system has an
equivalence ratio of one both as fuelto-oxidizer
ratio or air-to-fuel ratio.
The equivalence ratio phi will be used
in this article when discussing fuelair
mixtures. When there is less fuel
than air available for the flame (phi
<1), the flame is lean and burns at a
lower temperature than a flame at
stoichiometric conditions. A rich flame
with more fuel than available air (phi
>1) also burns at a lower temperature
than a stoichiometric flame, but generates
partially oxidized species from
the fuel, including carbon monoxide,
formaldehyde and other aldehydes.
Since these species can contribute
significantly to poor air quality, rich
flames are generally avoided.
NOx production paths
NOx are regulated pollutants, and
consist primarily of nitric oxide (NO)
and nitrogen dioxide (NO2). In a flame,
NOx can be formed by three different
processes to generate thermal NOx,
fuel NOx and prompt NOx.
Thermal NOx is produced at high
temperatures by elementary reactions
of the Zeldovich mechanism:
N2 + O -> NO + N
N + O2 -> NO + O
(1)
(2)
N + OH -> NO + H
(3)
Fuel NOx is produced from combustion
of nitrogen-containing fuel components.
Fuel-generated NOx can be
avoided by using fuels without significant
quantities of nitrogen-containing
compounds.
Prompt NOx is produced by radical
reactions in the flame. However,
this is a relatively small channel for
NOx production, and generally not as
important to overall NOx emissions
as thermal NOx. In most combustion
systems, thermal NOx is the dominant
production channel, and NOx
emissions can be controlled by limiting
the flame temperature.
The dependence of NOx and carbon
monoxide concentrations on flame
temperature are shown for lean flames
in Figure 1. At very lean conditions,
the flame temperature is not sufficiently
high to completely burn out
CO, and CO levels rise rapidly as the
flame temperature decreases. As the
fuel-air mixture is made increasingly
lean, it approaches the lean blowout
limit where the flame does not release
sufficient heat to sustain itself. On the
other side of the graph, flame temperature,
and NOx, increase as the fuelto-oxidizer
ratio increases.
Low-NOx burner design
Low-NOx burner design involves several
factors. One factor is to establish
mixing of the air and fuel before they
flow into the flame zone, so the flame
burns at a uniform and well-defined
temperature. If air and fuel are poorly
mixed, there will be fuel-rich regions
and fuel-lean regions within the flame
zone. The fuel-rich regions will burn
Figure 3. The interaction of the flows at
the exit of the swirler assembly creates a
flow profile that expands radially outward
with a high flame temperature, generating
excess NOx, and possibly burning
incompletely due to excess fuel.
The fuel-lean regions will not generate
much NOx, but may burn at too low a
temperature for complete combustion.
An effective gas-mixing system
should be integrated into a low-emissions
burner design, but the creation of
a volume of mixed fuel and air in the
burner upstream of the flame zone can
introduce some safety concerns. Proper
control of the flame position must be
maintained to avoid the flame flashing
back to the fuel injection zone. The fuel
injection system should be insensitive
to pressure fluctuations in the flame
zone to avoid combustion oscillations.
Premixed burners can often operate
effectively over a range of lean equivalence
ratios. For maximum efficiency,
the burner should be set to operate at
the highest flame temperature that
will produce acceptable NOx emissions
and will not damage the downstream
section of the combustion system.
One advanced premixed burner design
that is capable of very low-emission
operation is the low-swirl burner
(Figure 2). The low-swirl burner is a
scalable design that has demonstrated
excellent operating stability while
achieving low-emissions performance.
The concept was developed as a flame
research tool by one of the authors
(Robert Cheng), and is used for industrial
heating applications.*
*Note: The low-swirl burner design has been
licensed by Honeywell's Maxon Corp. (Muncie,
Ind., www.maxoncorp.com) for process heating
over a specific heat-output range and was commercialized
for natural gas operation. The lowswirl
burner technology is available for licensing
for other applications.
ChemiCal engineering www.Che.Com may 2011 45
http://www.maxoncorp.com http://www.Che.Com
Chemical Engineering May 2011

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