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Oxy/Natural Gas Is Coming On Strong

The 1990s present a number of chal- lenges for the industrial operator. Increased efficiency and productivity

Environmental benefits and new burner technolo- gies increase the value of oxy/gas combustion

are needed to compete in the global marketplace, while at the same time emission levels must be reduced to

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meet heightened environmental con- cerns. For some applications, oxylnat- ural gas combustion is a way to meet these challenges. Oxy/natural gas combustion has always had a strong appeal for high tempera- ture applications such as steel melting. It produces a higher flame temperature which increases productivity and adds to available heat to raise process effi- ciency. Along with these traditional ben- efits, oxy/gas produces some important environmental advantages: it reduces NOx levels, particulate emissions, and flue gas volume. With the development of new oxylgas burners and technolo- gies, and the increased availability of lower-cost oxygen, the technology has spread into a number of other industries including nonferrous metals, glass melt- ing, and waste incineration. Although a variety of fuels can be com- busted with oxygen to produce the high temperature oxy/fuel flame, natural gas is the preferred fuel in the United States. It is widely available, inexpensive, and clean burning.

Why It Works So Well The key to oxylgas combustion is that it reduces or eliminates the nitrogen in combustion air. Air contains roughly 79% nitrogen and 21% oxygen, by vol- ume. Only the oxygen, however, is actu- ally needed for the combustion process. The nitrogen is inert and serves only as a diluent, absorbing heat from the com- bustion process and lowering flame temperature. By eliminating nitrogen and replacing it with pure oxygen in the combustion process, flame temperature increases dramatically (Figure 1). High- er flame temperature can increase pro- duction rate and reduce the size of the process equipment required. Decreasing the amount of nitrogen also increases process efficiency, defined as the amount of heat available for a process. Nitrogen in the combustion air acts as a ballast that carries energy out the exhaust. When air is replaced with oxygen, energy goes into the load instead of into heating up the nitrogen (Figure 2). If nitrogen is reduced or eliminated, flue gas volume decreases (Figure 3). Lower flue gas volume reduces the size of the ducts, fans, and other such equipment required. Also, because any harmful emissions are in higher concentrations, they are easier to remove and less

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Figure 1. Adiabatic flame temperature increases as nitrogen is replaced with oxygen.

Figure 2. By replacing air with oxygen, available heat increases and more energy goes into the load.

A4 0 NATURAL GAS APPLICATIONS IN INDUSTRY 0 SUMMER 1993

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extensive post treatment equipment is required. Particulate emissions are also reduced in those processes producing large amounts of dust. Because a lower flue gas volume results in lowered gas velocities, fewer particulates are entrained throughout the system and wind up in the exhaust stream. Further, the higher flame temperatures of oxylgas combustion generally produce fewer unburned hydrocarbons than aidfuel combustion.

Reduced NOx Emissions Reduction of NOx, or nitrous oxides, is

an increasingly important feature of oxy/gas combustion. The 1990 Clean Air Act Amendments view NOx as a pre- cursor to smog and set out strict rules for the regulation and reduction of NOx emissions from both stationary and mobile sources. Natural gas, unlike the other fossil fuels, contains no fuel- bound nitrogen and thus does not pro- duce "fuel NOx". It does, however, pro- duce thermal NOx, as do all fossil fuels. Because thermal NOx increases with flame temperature, doubling every 150" F, high temperature industrial process- es such as glass and steel making are especially concerned about reducing and regulating their NOx emissions. Oxy/gas combustion, if used carefully, can significantly reduce NOx emissions. Because the amount of nitrogen is reduced, or eliminated, from the com- bustion air, less nitrogen is available for NOx formation. However, because NOX formation increases with flame temper- ature and oxy/gas combustion pro- duces a higher temperature flame, NOX formation using any available nitrogen is enhanced. As Figure 4 indicates, ther- mal NOx formation will increase with increasing enrichment up to about 60% oxygen and then will decrease as oxy- gen percentage increases.

Big Savings For Steel Although oxy/gas burners have wide

application within the steel industry, their first and still their largest use is in electric arc furnaces (EAFs). Today, oxy/gas burners are commonly used on EAFs and can reduce tap-to-tap times by as much as 20% and electrical requirements and energy consumption by more than 10%. EAFs are used throughout the steel industry and account for over 40% of the steel pro- duced today. They are used for batch melting and as holding furnaces in iron foundries. In steel shops they are used almost exclusively for melting. In an EAF, graphite electrodes extend

Figure 3. Flue gas volume decreases as the amount of nitrogen is reduced.

Figure 4. NOX formation drops off dramatically when oxygen concentrations reach about 65%.

into the furnace from a dome-shaped roof. An electric arc forms between the electrodes and the scrap metal to pro- duce intense heat. That heat allows for the melting of the charge. By installing oxy/gas burners, EAF

operators can benefit substantially from the additional energy input. Oxy/natural gas combustion can input approximate- ly 25% of the total energy needed to melt the scrap and is especially efficient during the early melt cycle of each scrap charge. More heat is released from the oxy/gas combustion: up to 13 million Btu/hr/fL3 of combustion space, which is substantially more than the 300,000 Btu/hr/fL3 provided by standard natural gas combustion in air. Oxy/gas burners can also help eliminate the cold spots that normally occur during melting between the electrodes. By focusing a high temperature oxy/gas flame at the cold spots during meltdown, melting is faster, more efficient, and more uniform. Oxy/gas burners for EAFs are offered by many manufacturers in the U. S. includ-

ing AGA Gas Inc., Air Products, and American Combustion, Inc., and are available in a number of configurations. For smalJer EAF installations, a single burner is commonly suspended from a supporting beam and aimed through the slag door. Larger installations may have a three-burner system installed in the sidewalls or roofs. Two newer sys- tems are the Split-Fire" burner manu- factured by Air Products and the Pyre- ton'" combustion systems produced by American Combustion and marketed by a number of natural gas equipment manufacturers. The Split-Fire burner (Figure 5) is a

water-cooled burner designed to effi- ciently transfer heat to furnace cold spots while minimizing operator involve- ment and capital costs. A single Split- Fire burner installed in the sidewall directs two hot oxy/gas flames at two predetermined cold spots. Oxygen and natural gas flow are separated between two burner ports, creating two distinct flames that can be aimed at cold spots.

ciinnnnFR 1041 .I h l ~ r I l R B 1 G n s nPPl ICnTIONS IN INDUSTRY A 5

APPLIED GAS TECHNOLOGY %I

Figure 6. Th Pyretron combustion system has been specially adapted for ladle preheating.

For smaller installations under 50 tons, this burner is a less costly alternative to the three-burner system and can be easily installed. Savings and production increases from the Split-Fire burner have been carefully monitored by Air Products and vary by furnace size and scrap type. Production increases range from a low of 5% to highs well in the 30% range. Energy cost savings per ton vary from 43 to 85 kWH/ton. Further, reduced electrical use extends elec- trode life. The Pyretron combustion system uses air in addition to oxygen and natural gas to produce a burner with high versatility. Because the introduction of air, oxygen, and natural gas are each controlled separately, ratios can be modulated to

produce a wide range of flame charac- teristics and flame temperatures from 3600" F to 5000" F. This versatility is especially useful in EAF scrap melting where scrap mix, and the surface area available, influences heat transfer effec- tiveness. In one 180-ton capacity, 120 MVA EAF, a Pyretron combustion sys- tem produces a 16% increase in tons per hour and a 12% decrease in tap-to- tap time. Electric consumption and elec- trode usage was reduced 7%. In anoth- er 90-ton capacity EAF, productivity was increased overall by 20% and elec- trical consumption reduced 71 KWH per charged ton. Other oxy/gas burner applications in

the steel industry include ladle preheat- ing and continuous reheat furnaces.

Figure 5. The Split- Fire"' burner allows EAF operators tu use efficient natural gas instead of all electric steel melting.

Ladles are used to carry and pour molten metal and must be preheated to minimize thermal shock to the ladle lin- ing. By using oxylgas burners, the pre- heating times for ladles have been reduced up to 30%. The Pyretron sys- tem has also been specially adapted for ladle preheating (Figure 6) applications and is gaining increased popularity. Reheat furnaces consume a lot of ener- gy and time because steel is traditional- ly reheated several times before reach- ing its final configuration. Oxy/gas burners can substantially reduce these expenditures. Two new oxylnatural gas technologies

for the steel industry are also being developed by Air Products under GRI sponsorship. The first is the rapid steel reheater which uses an innovative burn- er to produce heating rates of 200" F to 400" F per minute (see Gas Technolo- gy, Spring 1993). Plans are being made for field testing of the heater in a thin- slab edge heating application. The sec- ond project is a Combined Energy Steelmaking technology for augmenting EAFs. Both hearth-mounted, sub- merged combustion tuyeres and side- wall-mounted oxy/gas burners are used to produce increased scrap-melting rates. A full-scale field experiment with a steel industry partner is planned.

Non-Ferrous Metals Also Benefit The benefits of oxy/gas combustion

extend into non-ferrous melting applica- tions. To date, the aluminum industry has been the largest user of the tech- nology, although oxy/natural gas com- bustion is spreading into the copper, brass, and lead sectors. Direct-fired aluminum melters fitted with the Pyretron combustion system have recorded productivity gains ana reduced NOx levels. Because the atmosphere is controlled, loss rate due to oxidation also drops. In one circular top-charged melting furnace, melting rate increased overall by 20% while fue utilization decreased by almost 10%. Ir conjunction with American Combustion the Institute of Gas Technology mea sured NOx emissions from anothe Pyretron installation on a well-charge(

A6 0 NATURAL GAS APPLICATIONS IN INDUSTRY SUMMER 1993

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aluminum melting furnace. Analysis showed that a 35% increase in furnace throughput capacity could be achieved without exceeding the existing annual NOx emission levels produced with a cold-air burner.

Glass Melting Is Hot The glass industry, which requires high melting temperatures up to 3000” F, is turning to oxylgas as a means to cost- effectively reduce NOx and particulate emissions while enhancing productivity. Available alternatives include preheat- ed air, which is energy efficient but increases NOx levels, and electric boost, which is costly to operate. Equip- ment manufacturers, such as AGA Gas, Inc., Air Products, and Maxon, are responding to the needs of the glass industry and producing new burners specially designed for glass melting. “We went out and interviewed glass

manufacturers to find out what they wanted in an oxy/fuel burner,” explained Buddy Eleazer, Project Manager at Air Products. “The traditional oxylfuel burn- ers are high momentum, high velocity burners designed for the steel industry. Glass manufacturers, on the other hand, wanted low maintenance, non- water-cooled burners with a slow flame that didn’t pick up dust.” The result of this research was the new Cleanfire@ low NOx burner which has a soft and more luminous flame. Because velocity is slower, the need for water cooling is eliminated. The burner uses oxygen rather than air, which reduces NOx and particulate emissions. One of the first installations of the

Cleanfire burner system was in a borosilicate melting furnace, replacing a recuperative aidfuel furnace. The man- ufacturer was specifically looking for a cost-effective way to reduce particulate and NOx emissions. With the Cleanfire system, NOx emissions dropped almost 70%, particulate emissions decreased by about 60%, furnace fuel efficiency increased by 50%, and glass quality imp roved. Maxon has also developed a new burn- er for glass melting. The OxyTherm@ (Figure 7) reduces NOx levels substan- tially while lowering the volume of com- bustion products in the furnace. It is also nonwater-cooled and has a short, low-

incinerators require elevated tempera- tures with high excess air levels and long residence times a combination which also produces high NOx levels. By using oxylgas combustion, waste combustors can operate at high- er temperatures and longer residence times while produc- ing less NOx. Fur- ther, the lower flue gas volume and velocity produced with oxy/natural gas allows the downsiz- ing of post-gas treat- ment while reducing particulate carryover through the incinerator. In several recent applications imple-

mented by Air Products, oxylgas com- bustion has produced excellent results. In one Superfund site using a mobile, high temperature incinerator, NOx emissions dropped by two-thirds with oxy/gas and cleanup was completed

Figure 7 Designed for high tempera- ture, glass melting applications, the oxy-Therm@ burner can substantially reduce ~0~ levels.

one month ahead of schedule. In anoth- er, which processes creosote-contami- nated soil through counter-current kilns, NOx emissions per ton of material dropped up to 35%. Sandy soil and par- ticulate emissions were the problem at a third site. When new regulations cut permissible emission levels in half, the switch to oxylgas combustion was made and emissions were kept below the new limits, average soil processing increased 23%, and clean-up was com- pleted 60 days ahead of schedule. Recent studies by both GRI and the

Department of Energy predict a sharp increase in the use of oxy/gas combus- tion by American industry-and it’s easy to see why.

Portions of this article are adapted from “Oxygen Enrichment Enhances Combustion,” by C.E. Baukal, P.B. Eleazer, and L.K. Farmer, in Technol- oav Impact, Air Products and Chemicals, Inc.,

velocity flame. Because it leads to less - Nimbe; 1, October 1991. turbulence and grain-producing batch dusting, it produces cleaner and better quality glass. This article was compiled with the assistance of

Charles Baukal and Buddy Eleazer, Air Products and Chemicals, Inc., Allentown, PA; Chris Ebeling, AGA Gas, Cleveland, OH; Robert Levinson and

TO achieve high destruction and Terrv Pulliam. American Combustion. Norcorss. Cleaner Waste Incineration

removal efficiencies (DREs), waste GA;’and Doug Sugai, Maxon Corp., Muncie, IN

More information on oxy/gas burn- ers is available from:

AGA Gas Inc. Circle 603 Air Products and Chemicals Circle 604

American Combustion Inc. Circle 605

Gas Research Institute Circle 606 Maxon Corporation Circle 607

CIInInnFR 1091 NATllRAl GAS A P P l ICATIONS IN INDIJSTRY 0 A 7