Advanced Oxy-Fuel Burners and Controls
Improve Fuel Savings and Uniform Heating
A Reprint from The International Journal of Forging
Business & Technology
Figure 5. DOC installed in a “box”
reheat furnace
Advanced Oxy-Fuel Burners and Controls
Improve Fuel Savings and Uniform Heating
Larry Cates, Development Associate, Praxair, Inc.
Rick Browning, Business Development Director, Praxair, Inc.
As performance requirements for forged parts increase, so also
do the technologies used to produce them. Reheat furnaces are
one example of equipment that must meet tighter performance
specifications.
A
n advanced oxy-fuel technology called Dilute
O
Oxygen Combustion (DOC) substantially improves
t
temperature
uniformity and reduces NOx, carbon
eemissions and natural gas consumption relative to airf l burners
b
iin reheat furnaces. These pulse-fired oxy-fuel burners
fuel
have demonstrated a temperature uniformity of less than ±15˚F, with
NOx emissions of 0.01 pounds/million Btu or less, while reducing
fuel consumption by as much as 70%.
Earlier versions of this technology were developed in the late 1980s
and refined through the 1990s. Commercial combustion systems
of these advanced designs were developed and implemented under
a DOE project in the late 1990s. To date, these advanced oxy-fuel
combustion systems have been installed on continuous steel reheat
furnaces, batch reheat furnaces, batch metal melting furnaces, ladle
preheaters and glass-making furnaces.
Dilute Oxygen Combustion Concept
In DOC, the fuel gas and oxygen are injected separately via highvelocity jets, creating rapid mixing of the fuel and oxidant with hot
furnace gases (Figure 1). This mixing and dilution produces a thermally
uniform heat release with low peak flame temperatures, resulting in
more even temperature distribution throughout the furnace. The low
flame temperatures are key to inhibiting the production of thermal
NOx. Often, when observed in a hot reheat furnace, the DOC flames
are diffuse enough as to be invisible to the human eye (Figure 2).
The J/L Burner
The specific burner utilized, called the J/L Burner, incorporates
DOC technology in a “packaged” assembly. In a typical arrangement,
8
January 2011
Mixing zone
Oxygen
Flue
Fuel
Reaction zone
Figure 1. The DOC concept
• Separate fuel and oxygen injection at high velocity
• Rapid dilution of fuel and oxygen with furnace gases
• Diffuse flame results in very uniform heating
• Low peak flame temperature minimizes NOx production
O2
Fuel
Typical oxy-fuel burner, high flame
temperature, point source heating
O2
Fuel
Dilute oxygen combustion, low flame
temperature, dilute heating
Figure 2. DOC technology produces essentially “flameless” combustion.
Advanced Oxy-Fuel Burners and Controls Improve Fuel Savings and Uniform Heating
Figure 3. The J/L Burner used for DOC
Valve Skid, Burner Manifolds and Controls
A code-compliant oxy-fuel combustion system appropriate
for employing DOC technology includes a main valve rack for
pressure regulation and main shutdown control, burner manifolds
for gas control to each burner and the electrical control panel for
overall process control. For pulse firing, each burner manifold
includes long-life solenoid valves. Systems come complete with a
programmable logic controller (PLC) and a local human machine
interface (HMI). With this system, the operator has the capability
and option of employing centralized monitoring of furnace
parameters through an available communication network.
Pulse Firing
In pulse-firing technology, burners alternate between maximum
and minimum firing rates rather than modulating to intermediate
rates. To control furnace temperature, the timing of the firing
cycles is adjusted depending on the required heat input. Pulsefiring control can be thought of as frequency modulation rather
than amplitude modulation. In a typical cycle, the burner might
fire at maximum rate for five seconds and minimum rate for five
seconds for a 50% energy input to the furnace.
In its pulse-fired mode, the J/L Burner produces the same
heating characteristics and momentum each time the burner fires.
This leads to extremely uniform heating throughout the furnace.
Combustion efficiency is maximized because excess air is not
required to maintain burner momentum at low firing rates.
Heat-up process
2500
3.50
Firing rate - air burner
Average furnace temp. - air burner
Firing rate - J/L burner
Average furnace temp. - J/L burner
3.00
2.50
2000
1500
2.00
1000
1.50
1.00
500
0.50
0.00
0
2
4
6
Time
8
10
Average furnace temperature
4.00
Firing rate (MMBtu/hour)
several burners will be mounted on the sidewalls of the furnace.
The burners/lances are compactly unitized within a refractory
block requiring a single furnace penetration per burner (Figure 3).
One key aspect of the burner is that the nozzles are field
replaceable so that gas injection velocities can be tailored to achieve
a furnace’s specific needs. This improves burner versatility and
allows parameters such as luminosity, burner momentum, flame
intensity, flame length and heat-flux profile to be controlled for each
application. The burner also features flame supervision, automatic
spark ignition and air cooling for low maintenance requirements.
The J/L Burner has two main components: the J-Burner
portion and the lance portion. The J-Burner consists of a fuelinjection assembly retained within a concentric oxygen tube.
Its design makes use of a patented technology that permits the
J-Burner to be recessed within the burner refractory block for
protection from heat and without risk of damage from combustion
within the recess cavity.
The lance portion of the burner injects oxygen whenever the
furnace is above the fuel auto-ignition temperature. It is separated
from the J-Burner by several inches in order to entrain furnace
gases and prevent the mixing of oxygen and fuel close to the burner,
thereby delaying combustion. This allows the oxygen to become
“diluted” with the furnace gases, yielding a much lower flame
temperature. Since combustion takes place throughout the furnace
chamber instead of directly at the burner, a more diffuse flame is
produced, resulting in more even energy distribution throughout
the furnace.
0
12
Figure 4. Typical heat-up process with air-fuel compared to DOC at
test conditions
Comparison of Air-Fuel and DOC Technology
Data from the lab (Figure 4) indicates the faster heating, lower fuel
consumption and quicker temperature soaking of the refractory
with DOC technology compared to an air-fuel burner.
Temperature Uniformity
The combined result of DOC technology and pulse-firing control
is uniform temperature distribution. DOC technology was recently
installed in “box” reheat furnaces in a forge shop (Figure 5) and the
temperature survey results shown in Figure 6 were obtained.
Although the specifications for this furnace required only ±25˚F
temperature range, the results with DOC were well within ±15˚F
January 2011
9
Advanced Oxy-Fuel Burners and Controls Improve Fuel Savings and Uniform Heating
Figure 6. Temperature-uniformity data from new forge shop
installation
Avg Temp
Max Temp (+)
Min Temp (-)
1550
1552.1
9
13
1900
1901.3
10
14
2300
2299.4
11
12
of average temperature for all 14 thermocouples. In addition, the
system responds quickly to temperature upsets. The temperature
response after door openings was well within the required
temperature range in the two-minute window required for several
aerospace material grades.
Typical forge furnaces
80
Fuel savings, %
Setpoint (˚F)
100
Fuel requirement
2.5
Available heat
2.0
Flue loss
1900
2100
Flue gas temperature, ˚F
2300
2500
Figure 8. Estimated fuel savings with DOC for typical forge furnaces
temperature distribution throughout the furnace. This leads to
inefficient combustion.
Figure 8 shows the potential fuel savings that can result
depending on the amount of excess oxygen entering the furnace
and the furnace temperature. Replacing an air-fuel combustion
system on a well-tuned, tight furnace would result in a fuel savings
of approximately 50% at typical forging temperatures and up to
75% if high excess air had been used for uniformity purposes.
Typically, low-NOx air-fuel burners are rated to produce about 0.1
pounds/million Btu. The amount of NOx produced by DOC will
depend on the amount of nitrogen available in the furnace. Since
nitrogen enters the furnace through leaks, it is very important to
Figure 9. Actual emissions data from DOC-equipped forge
reheat furnace
1.5
1.0
0.5
Air
Oxygen
Figure 7. Estimated relationship of fuel requirement reduction and
oxy-fuel
Effect of Excess Oxygen
A measurement of 2% excess oxygen in the flue gas typically
indicates that the furnace has been tuned for the most efficient
stoichiometric combustion whether using air or oxygen as the
oxidant. A higher value indicates that excess oxidant is entering
the furnace, which, in the case of air, means additional nitrogen
is entering the furnace and lowering its efficiency. Air-fuel burners
are often set up to operate with excessive combustion air at low
firing rates to provide sufficient momentum to maintain uniform
10
1800
Emissions
3.0
0.0
20
1500
2%
4%
6%
8%
10%
Flue gas
excess O2
40
Improved Combustion Efficiency
Air contains nearly 80% nitrogen, which does not contribute to
combustion. If the combustion air is not preheated, this nitrogen
will absorb a substantial amount of the energy produced by
combustion before being exhausted through the flue. For example,
in a furnace heated to 2100˚F, for an air-fired burner to provide 1
million Btu of heat energy to the furnace load, about 2.5 million Btu
of energy must be provided by the fuel (Figure 7). The remaining
1.5 million Btu of energy produced by combustion of the fuel is lost
to flue gases. In contrast, an oxy-fuel-fired burner loses less than 0.5
million Btu to the flue gases.
As the furnace temperature increases, an even greater proportion
of heat energy is transferred to the nitrogen in the flue gas,
decreasing efficiency proportionately.
60
January 2011
Firing Rate %
CO
NO
CO2
O2
NOx (lb/mmBtu)
100
11
13
84
0.9
0.0019
90
18
15
83
1.2
0.0022
80
13
18
83
1.1
0.0026
70
16
14
82
1.1
0.0021
60
15
12
81
1.2
0.0018
50
14
14
80
1.3
0.0021
40
13
9
79
1.4
0.0014
30
13
14
77
1.4
0.0022
20
11
5
61
1.8
0.0010
10
11
5
60
2.1
0.0010
Average
14
12
77
1.4
0.0018
seal the furnace as tightly as possible.
The flue size will be reduced by 80%
or more from a conventional furnace
and should be fitted with a damper
that will close at low firing rates. Due
to the very low volume of natural gas
and oxygen entering the furnace, a low
positive pressure will be maintained, so
atmospheric nitrogen can infiltrate the
furnace through any opening.
Recently, emissions measurements
were made on a commercial “box” reheat
furnace at a forging facility equipped
with DOC technology. Figure 9 shows
the results of these measurements. The
measured NOx emissions, averaged
over a full range of firing rates, were
0.0018 pounds NOx/million Btu.
It should also be noted that because
the fuel consumption will be reduced
by more than 50%, the actual pounds
of NOx will also be reduced by this
percentage. Carbon emissions likewise
will be reduced due to the lower fuel
consumption.
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Heating > Welding > Annealing > Heat Treating > Hardening > Coating > Stirring
Conclusion
FORGING AHEAD
DOC technology has been proven in
the laboratory and in commercial furnace installations to provide more uniform temperature distribution, lower
NOx and carbon emissions and to consume less fuel than conventional combustion technologies. The addition of
pulse-firing technology provides even
greater capabilities. This technology is
reliable to operate and easy to install
and maintain. As control of emissions
becomes more essential, DOC technology will play a greater role in industrial
combustion.
At Elotherm, we could have used the recession as a reason for cutting back…
but we didn’t.
Author Larry Cates, is a Development
Associate with Praxair in Indianapolis,
Ind. He may be reached at: tel. 317713-2848; fax 317-713-2828; or
[email protected]. Author Rick
Browning is Business Development
Director with Praxair in Burr Ridge, Ill.
He may be reached at: tel. 630-3204165; fax 630-320-4506; or rick_
[email protected]. For additional
information, visit www.praxair.com.
Instead, we accelerated product development, improved our manufacturing and
engineering capabilities, and expanded
services in the USA and worldwide.
Elotherm really is committed to
manufacturing excellence in any
economic climate. As a result, we
are prepared to help you achieve your
strategic goals with:
Innovative Products
– iZoneTM intelligent billet and bar
heating technology
– Moduline,TM our new modular
system architecture
– Patented workpiece energy monitoring
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724.553.3471
412.202.3313 (fax)
[email protected]
www.sms-elotherm.com
877 7ELOTHERM (877.735.6843)
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e-mail [email protected]
or visit www.sms-elotherm.com
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Elotherm Induction Tech. Co. Ltd.
2F, No.2200
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P.R. CHINA
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+ 86 (0) 21 2408 6468
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[email protected]
January 2011
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