Efficient Gas Burner with Reduced NOx Emissions
|Fig. 1. Ecothal system components|
The Ecothal burner system was developed with a focus on high efficiency simultaneously combined with very low emissions. The burner design is non-complex and robust, assuring long operating time and low maintenance needs.
The system is best used in demanding carburizing and hardening furnaces (typically automotive manufacturers) at high power and/or process temperatures. It was introduced selectively for batch furnaces in Germany starting in early 2004. As of today, a total of about 20 furnaces have been re-equipped using the concept.
The system is based on the Kanthal APM radiation and flame-tube concept, which is proven to give long service intervals even at high power densities. The system can be used in heat-treatment systems up to 1000-1050°C (1832-1922°F) process temperatures, depending upon power densities.
The principal construction of the system is conventional (i.e. the burner, a flame tube, end cross, etc.). It is assembled into a radiation/protective tube, sealing off the combustion from the process chamber (Fig. 1). Existing control and regulation systems/principles can still be used when changing over to Ecothal burner systems.
|Fig. 2. Temperature profile for a 5-inch Ecothal system|
The burners feature an efficiency of 75-80% at a furnace temperature of 950°C (1742°F). This is achieved by an optimized design of the burner body and the flame shape. The optimized burner design makes it possible to run the burner as low as at a lambda of 1.05 (approximately 1% oxygen remaining in the exhaust gas) with no risk of forming carbon monoxide (CO). Using a lower lambda value reduces energy losses due to excess air passing through the burner system. The higher power capability together with the increased efficiency make it possible to use smaller sizes for the radiation tubes when converting to Ecothal burners. The higher efficiency can also be used to reduce the number of burners, but the temperature uniformity requirements would need to be considered. The reduction in size renders lower thermal mass and lower cost for material.
The burner design has been optimized for uniform flame-tube temperature to maximize use of material. Therefore, a very uniform temperature is achieved along the protective tube. This tube uniformity improves process uniformity. The temperature profile for the 5-30 system is shown in Figure 2.
|Fig. 3. Emissions vs. power|
The German standard “TA-Luft” is normally used in Europe. It indicates NOx levels are to be kept lower than 175ppm (ref. 3% O2), which is equivalent to 70mg/MJ. The development work for Ecothal focused on lowering the emission of NOx without cooling the combustion. This was done by increasing the excess of combustion air (lambda value) in order to maintain the high combustion efficiency. The result is that emissions have been reduced by up to 90% compared with other similar burners on the market. The 4-inch system manages to maintain emissions well below 50ppm/20mg/MJ (ref. 3% O2), at 20kW (68,000 BTU), at a process temperature of 950°C (1742°F) and fueled by natural gas (Fig. 3). This low emission is even possible at a lambda of 1.05, which means that efficiency gain by lower excess of combustion air is around 10%. Typically, burners are tuned to run at lambda = 1.15.
Since combustion of hydrocarbons aims at maximizing transformation into carbon dioxide (CO2), the emissions of CO2 can only be reduced by an increase of combustion efficiency. This is why it is so important to minimize the excess of combustion air (lambda).
Typical emissions/properties (achievable simultaneously, lambda = 1.05) when fired with natural gas and air are:
NOx - 50 ppm (ref. 3% O2)
NOx - 20mg/MJ
CO2 - <75g/MJ
CO - 0 ppm
Noise level - <80dBA
Efficiency - 75-80%
Field tests have reported 20-40% savings in fuel at the same production level. The only maintenance necessary is a yearly visual inspection, which is mandatory by safety legislation in most markets.
Control and Regulation
The burners can be run at a wide power range and use UV surveillance for flame monitoring. The surveillance system does not need to be adjusted for different power levels and is insensitive to normal fluctuations in air and gas supply (including calorific heat value).
The burners use a rich premix of gas with air. The combustion principle uses high speed. Therefore, for highest efficiency and lowest emissions it is recommended to use a high/low/off control for the burners. Nevertheless, the burners can be operated using any type of control and regulation principle.
Burners are ignited by internal ignition spark and are designed to use conventional air and gas supply pressures. A 20kW (68,000 BTU) burner needs approximately 35mbar at the burner intake. It is possible to replace old burner systems without expensive rebuilding of the gas train – just “bolt on.”
|Fig. 4. Burner certificate for a 4-inch system|
Installation, Handling and Maintenance
The robust design, enhanced by using only metallic materials in the burner, renders easy, trouble-free handling during installation and maintenance. No measurements need to be set or adjusted at installation because everything is given by the design.
The burners are comprised of three parts. The burner body and the gas lance are held together by four bolts and a patented detection and ignition electrode unit is inserted concentrically into the gas lance. For this reason, the risk of erratic assembling after mandatory inspection intervals is eliminated.
The burner concept and construction is identical for all burner sizes, and the 4-inch (6-inch pending) size is tested and approved by the TÜV Munich in compliance with EN746-2 (Fig. 4).
|Fig. 5. Creep strength comparison for 1%/1,000 hours|
Kanthal APM Material
This material is an iron-chromium-aluminum alloy produced using powder metallurgy. The aluminum oxide (alumina) formed on the surface by the reaction of the bulk aluminum with oxygen from the process atmosphere renders an extremely protective coating. This oxide coating provides resistance toward further oxidation/corrosion and prohibits pickup of elements from the atmosphere, which cannot react with the alloy. Therefore, this material will not be susceptible to carbon or sulfur pickup in carburizing and hardening atmospheres.
Pitting in the passage from furnace process chamber into the insulation is eliminated. The protective alumina also allows for higher operating temperatures, either as increased process temperatures or as a result of increased power densities, without increased corrosion/oxidation/spallation rates. The mechanical properties have been improved by alloying and production techniques. The latest material development is APMT, in which the hot strength has been improved by the addition of molybdenum. The corrosion and oxidation properties are the same.
In the creep diagram (Fig. 5), a comparison with typical nickel-chromium-(iron) alloys is made. With the APMT material, strength levels are about the same but with the important difference that the APM/APMT materials can be used at much higher temperatures. The nickel-chromium materials cannot be used at really high temperatures because they will lose their strength dramatically due to dissolution of precipitates.
In addition to radiation and protective tubes, APM/APMT materials are also used in other applications, such as transportation rollers and construction material in general. The materials are available in wire, rod/bar, strip, tubes and tailor-made roller dimensions (heavy wall thicknesses). IH
For more information: Chris Clowes, business development manager, SandvikWire and Heating Technology Corp., Kanthal Heating Systems, P.O. Box 281, Bethel, Connecticut 06801; tel: 203-744-1440; fax: 203-743-2547; e-mail: firstname.lastname@example.org; web: www.kanthal.com
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: high efficiency, temperature uniformity, low emissions of NOx and CO2, all metallic solution