Silicon carbide has the necessary strength, temperature resistance and high thermal conductivity for radiant-tube applications at high operating temperatures.

Fig 1 Alloy metal single-end radiant tube burner

Historically, applying recuperator technology to radiant tubes usually meant placing a recuperator in the discharge of a "U" or "W" tube because straight through tubes with recuperators at the discharge end require complex hot-air return ducting at a higher installation cost. However, the development several years ago of a radiant tube within a tube eliminated the need for a separate recuperator. In this design, the gases discharging from the inner tube are returned to the inlet side by passing between the inner and outer tubes. The outer tube then receives radiation from both the inner tube and the hot return gas stream and discharges that radiation to the furnace and the load. The first burners to apply these designs were fabricated using tubes made of high-temperature alloys (Fig. 1).

A metal-matrix SiSiC-a new type of silicon-carbide (SiC) material-developed by Schunk-INEX, Holland, N.Y., has further advanced radiant-tube technology and is more cost effective than previous materials under high-temperature operating conditions. This ceramic material is used in the design of Hauck's SERamic single-end radiant-tube burners. Burner performance was evaluated in an installation at Ispat/Inland Steel's galvanizing line in East Chicago, Ill., and delivered a reliable solution to operational problems with existing radiant tubes at the facility.

Fig 2 Failed radiant tubes removed from No.5 galvanizing line

Unacceptable radiant-tube failure rate

The No. 5 galvanizing line at the Ispat/Inland Flat Products plant was rebuilt and commissioned in 1996. The line's 15 heating zones contained 422 fabricated single-end alloy radiant tube burners from another manufacturer. Furnace design operating temperature is 1750F (955C).

The first tube failure occurred in zone 8 in August 1997. By the end of 1999, 105 additional tubes (25% of the total number of tubes) had failed. During the line shutdown in March 2000, 148 (35%) tubes were replaced and an additional 225 (53%) were replaced in January 2001, leaving only 49 of the original 422 tubes intact mostly in zones 1 to 5. All tubes in zones 7 through 15 were replaced (Fig. 2).

The single-end radiant-tube burner fires through an alloy center tube on the inside of, and concentric to, the alloy outer tube. Products of combustion return along the gap between the inner and outer tube, then pass through a recuperator built into the return path, transferring heat to the incoming combustion air. Combustion products are then collected in a manifold and exhausted. Combustion air is fed to the burners under a positive pressure.

The failure mode of the tubes in the galvanizing line appeared to be in the inner tube, about 39 in. (990 mm) from the burner mounting flange. The inner tube failure appeared to cause a failure of the outer tube, allowing products of combustion into the furnace chamber, which required shutting down the burner.

Fig 3 Radiant tube thermocouple locations (LR-2000-02); dimensions in inches; Fig 4 Tube temperature vs. tube length (3% O2 high-fire and 12% O2 low-fire settings, push-through set up)

Pinpointing the cause of tube failure

At the request of the steelmaker, Hauck agreed to help identify the cause of the premature tube failures by conducting tests at its in-house research lab equipped with a number of instrumented and load-regulated test furnaces. A test furnace installation using one of the Ispat/Inland tubes was used to pinpoint the exact nature of the problem and find a solution. The test installation included a good tube and burner in a furnace with full instrumentation including an array of thermocouples on both the inner and outer tubes.

The burner was installed to fire per the galvanizing line furnace design specifications. Furnace temperature was held constant at 1750F, and the tube was exhausted through a system that maintained an exhaust pressure of -0.3 in. WC. The burner was fired high-low; 190,000 Btu/hr at 3% O2 at high fire and 20,000 Btu/hr at 12% O2 at low fire. Four thermocouples were placed on the inner tube (one specifically at the suspect failure location) and five on the outer tube (Fig. 3). Testing also was performed using a push only operation because the Inland furnace operation did not have a negative pressure on the exhaust.

Operating with the push-pull system, the exhaust temperature at high fire was 1180F (640C) with 0% CO and 451 ppm (0.541 lb/ MMBtu) NOx. The thermocouple located on the inner tube 39 in. from the flange measured a temperature of 2050F (1120C). The temperature differential across the outer tube was about 50F (30C); that is, 1825 to 1875F (995 to 1025C). Low-fire temperatures were not a problem.

When operating in the push only arrangement at the same high fire conditions of 190,000 Btu/hr and 3% O2, exhaust temperature was 1140F (615C) with 0% CO, and 335 ppm (0.402 lb/MMBtu) NOx. The hot thermocouple (39 in. from the flange) measured a temperature of 2070F (1130C), with an outer-tube temperature differential 50F (1825 to 1875F). Tube temperatures are shown in Fig. 4. At these operating conditions, test results indicate that inner tube temperatures in excess of 2000F (1095C) exceeded the design limit of the alloy tube, leading to a premature tube failure.

The physics of radiant-tube technology requires that the inner tube operate at a temperature several hundred degrees higher than that of the outer tube to provide a net heat flux from the burner to the load. A thermal gradient must exist between each of the elements to gain a net heat flux to the furnace. For example, if the furnace temperature is 1700F (930C), the outer-tube temperature must be at least 100 to 150F (55 to 80C) higher. Similarly, the inner-tube temperature also must be 100 to 150F higher than the outer tube. Therefore, a furnace temperature of 1700F requires that the inner tube operate at 1900 to 2000F (1040 to 1095C), which is very close to the maximum practical temperature limits for most available alloys. Even the most exotic alloys will have very little strength or creep resistance at these temperatures. In a horizontal position, few will have enough strength to resist creep under their own weight.

Fig 5 Sicaflex reaction-bonded SiC inner tube; Fig 6 SERamic radiant-tube burner assembly

SERamic burner

An SER 115 burner was installed in the same 7-1?in. OD by 7-1?in. ID by 8-1?ft long (190 by 184 mm by 2.6 m) alloy tube used in the previous test. The center tube consists of six Sicaflex(r) (reaction-bonded SiC) tube segments inserted into the alloy outer tube. The tube segments are designed with clearance when connected together to allow the assembled inner tube to flex enough so as not to stress the inner tube assembly when the outer tube deflects during high-temperature operation (Fig. 5). The SER is designed with a gap of several inches between the burner nozzle and the entrance to the inner tube, which allows products of combustion to be drawn back into the flame, reducing flame temperature and NOx formation. The SER recuperator, combustor and nozzle sections are made of a metal SiSiC matrix (Fig 6).

Tests were conducted at the same furnace temperature of 1750F in the push only mode at 190,000 Btu/hr. At 3% O2 in the exhaust, the exhaust temperature was 1400F (760C) with 0% CO and 154 ppm (0.185 lb/MM Btu) NOx. There were no thermocouples on the inner tube assembly. The outer tube temperature differential was 40F (25C); that is, 1835 to 1875F (1000 to 1025C). Low fire was at 25,000 Btu/hr with 14% O2 in the exhaust.

The outer tube temperature (Fig. 7) indicates that this design delivers the same heat flux to the load as the original SER, but with a slightly lower thermal gradient across the tube. The lower thermal gradient is a positive step toward achieving a longer outer tube life without a failure of the inner tube.

Silicon carbide has the strength, temperature resistance and high thermal conductance needed for a radiant-tube application. Advances in the manufacturer of SiC have produced materials having temperature limits to 2400F (1315C), which can be applied to radiant-tube burner construction.

The metal-matrix SiSiC material is less expensive than reaction-bonded SiC, but has similar strength and temperature limitations. Tubes up to 6 in. in diameter by 8 ft long (152 mm by 2.4 m) can be produced from the material. In addition developments in bonding technology have led to the design of heavy-duty flanges, nozzles and end caps, which can survive the same temperature and thermal stresses as the base materials, and which can be bonded to the tubes to form more complex shapes required in burner applications. Burners made of this material are cost competitive with metal burners but have the high-temperature performance of SiC materials.

Fig 7 Outer tube temperature vs. tube length (Hauck SER-115 vs. existing burners) at operating conditions of 190,000 Btu/h at 3% O2; Fig 8 SERamic burner assembly without the tube

Galvanizing-line installation

The nine burners in west zone 13 of the Ispat/Inland No. 5 galvanizing line (burners 191 to 199) were replaced with SER 115 burners having an alloy outer tube in January 2001 (Fig. 8). The installation was made using a ratio regulator cross connected to the main air line at each burner. An orifice meter was installed downstream of the regulator sensing connection to reduce the air pressure to the burner to the required 6.4 in. WC for 160,000 Btu/hr for operation in zone 13. The required gas pressure is higher (8.5 in. WC), which required that the regulator sense line be upstream of the pressure-reducing air orifice meter.

After several months of operation with the furnace running at a temperature of 1750F and a line speed of 263 fpm, the burners still were firing at an average of around 160,000 Btu/hr. Tube temperatures on a number of the original burners were being monitored with thermocouples. The average NOx level for the 9 SER burners was about 12% higher than those observed in the lab tests (173 ppm corrected to 3%, or 0.208 lb/MMBtu), but the burners were running at a higher excess air with an average O2 in the exhaust of 5.56%. Burner installation in zone 13 is shown in Fig. 9.

Conclusions

Metal alloy SERs can experience premature failure at furnace operating temperatures of 1700F and higher because the inner tube temperature has to run at a considerably higher temperature. The new SiC materials have good heat resistance at temperatures to 2450F (1345C) and are optimal materials for use in these types of burner applications. They have been used in SERs at furnace operating temperatures in excess of 2000F (1095C). Ceramic SERs can operate at flux rates well in excess of 125 Btu/in.2/h, whereas alloy tubes are limited to 40 to 50 Btu/in.2/h in high-temperature furnaces. Ceramic SERs also potentially could replace electric heating elements in many furnace applications.

The cost of the SERamic SER design using metal-matrix SiSiC is relatively low compared with that using reaction-bonded SiC materials in other ceramic SER designs. Compared with alloy-tube designs, the SERamic design reduces NOx levels by as much as 50%. In the case of the Ispat/Inland Steel galvanizing line, a 100% SERamic installation would result in a NOx reduction of about 40 to 50 tons per year. The energy-efficient burner, operating with a fuel efficiency in the 60 to 70% range in high temperature furnaces, offers a very rapid payback.