advanced oxy-fuel burners and controls improve fuel
TRANSCRIPT
Advanced Oxy-Fuel Burners and ControlsImprove Fuel Savings and Uniform HeatingA Reprint from The International Journal of Forging Business & Technology
n advanced oxy-fuel technology called Dilute Oxygen Combustion (DOC) substantially improves temperature uniformity and reduces NOx, carbon emissions and natural gas consumption relative to air-
fuel burners in reheat furnaces. These pulse-fi red oxy-fuel burners 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 refi ned 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 ConceptIn DOC, the fuel gas and oxygen are injected separately via high-velocity 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 fl ame temperatures, resulting in more even temperature distribution throughout the furnace. The low fl ame temperatures are key to inhibiting the production of thermal NOx. Often, when observed in a hot reheat furnace, the DOC fl ames are diffuse enough as to be invisible to the human eye (Figure 2).
The J/L BurnerThe specifi c burner utilized, called the J/L Burner, incorporates DOC technology in a “packaged” assembly. In a typical arrangement,
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Advanced Oxy-Fuel Burners and Controls Improve Fuel Savings and Uniform HeatingLarry 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 specifi cations.
Figure 1. The DOC concept• Separate fuel and oxygen injection at high velocity• Rapid dilution of fuel and oxygen with furnace gases • Diffuse fl ame results in very uniform heating• Low peak fl ame temperature minimizes NOx production
Oxygen
FuelFlue
Reaction zone
Mixing zone
Typical oxy-fuel burner, high fl ame temperature, point source heating
Dilute oxygen combustion, low fl ame temperature, dilute heating
Figure 2. DOC technology produces essentially “fl ameless” combustion.
O2O2Fuel Fuel
8 January 2011
Figure 5. DOC installed in a “box” reheat furnace
January 2011 9
Advanced Oxy-Fuel Burners and Controls Improve Fuel Savings and Uniform Heating
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 fi eld replaceable so that gas injection velocities can be tailored to achieve a furnace’s specifi c needs. This improves burner versatility and allows parameters such as luminosity, burner momentum, fl ame intensity, fl ame length and heat-fl ux profi le to be controlled for each application. The burner also features fl ame 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 fuel-injection 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 fl ame temperature. Since combustion takes place throughout the furnace chamber instead of directly at the burner, a more diffuse fl ame is produced, resulting in more even energy distribution throughout the furnace.
Valve Skid, Burner Manifolds and ControlsA 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 fi ring, 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 FiringIn pulse-fi ring technology, burners alternate between maximum
and minimum fi ring rates rather than modulating to intermediate rates. To control furnace temperature, the timing of the fi ring cycles is adjusted depending on the required heat input. Pulse-fi ring control can be thought of as frequency modulation rather than amplitude modulation. In a typical cycle, the burner might fi re at maximum rate for fi ve seconds and minimum rate for fi ve seconds for a 50% energy input to the furnace.
In its pulse-fi red mode, the J/L Burner produces the same heating characteristics and momentum each time the burner fi res. This leads to extremely uniform heating throughout the furnace. Combustion effi ciency is maximized because excess air is not required to maintain burner momentum at low fi ring rates.
Comparison of Air-Fuel and DOC TechnologyData 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 UniformityThe combined result of DOC technology and pulse-fi ring 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 specifi cations for this furnace required only ±25˚F temperature range, the results with DOC were well within ±15˚F
Figure 3. The J/L Burner used for DOC
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Firing rate - air burnerAverage furnace temp. - air burnerFiring rate - J/L burnerAverage furnace temp. - J/L burner
Figure 4. Typical heat-up process with air-fuel compared to DOC at test conditions
0 2 4 6 8 10 12Time
Heat-up process
10 January 2011
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.
Improved Combustion Effi ciencyAir 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 fl ue. For example, in a furnace heated to 2100˚F, for an air-fi red 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 fl ue gases. In contrast, an oxy-fuel-fi red burner loses less than 0.5 million Btu to the fl ue gases.
As the furnace temperature increases, an even greater proportion of heat energy is transferred to the nitrogen in the fl ue gas, decreasing effi ciency proportionately.
Effect of Excess OxygenA measurement of 2% excess oxygen in the fl ue gas typically indicates that the furnace has been tuned for the most effi cient 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 effi ciency. Air-fuel burners are often set up to operate with excessive combustion air at low fi ring rates to provide suffi cient momentum to maintain uniform
temperature distribution throughout the furnace. This leads to ineffi cient 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.
EmissionsTypically, 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
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Typical forge furnaces
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Flue gas excess O2
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1500 1800 1900 2100 2300 2500Flue gas temperature, ˚F
Figure 8. Estimated fuel savings with DOC for typical forge furnaces
Figure 6. Temperature-uniformity data from new forge shop installation
Setpoint (˚F) Avg Temp Max Temp (+) Min Temp (-)
1550 1552.1 9 13
1900 1901.3 10 14
2300 2299.4 11 12
Figure 9. Actual emissions data from DOC-equipped forge reheat furnace
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
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Air Oxygen
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Figure 7. Estimated relationship of fuel requirement reduction and oxy-fuel
Advanced Oxy-Fuel Burners and Controls Improve Fuel Savings and Uniform Heating
January 2011 11
seal the furnace as tightly as possible. The fl ue size will be reduced by 80% or more from a conventional furnace and should be fi tted with a damper that will close at low fi ring 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 infi ltrate 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 fi ring 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.
ConclusionDOC technology has been proven in the laboratory and in commercial fur-nace installations to provide more uni-form temperature distribution, lower NOx and carbon emissions and to con-sume less fuel than conventional com-bustion technologies. The addition of pulse-fi ring 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 technol-ogy will play a greater role in industrial combustion.
Author Larry Cates, is a Development Associate with Praxair in Indianapolis, Ind. He may be reached at: tel. 317-713-2848; 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. 630-320-4165; fax 630-320-4506; or [email protected]. For additional information, visit www.praxair.com.
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