Flameless oxidation burners for heating strip lines - …

1 REPORTSHEAT PROCESSING October 20031 Radiant tube conceptsTwo main groups of radiant tubeconcepts can differentiated. One groupis called non-recirculating radiant tubesand the other one the recirculatingradiant tubes. As it can be seen in Fig. 1with the exception of the nonrecirculating single ended radiant tube,all the non-recirculating radiant tubeshave a burner at one end of the tube andthe flue gas outlet at the other end of thetube. Usually, slow mixing burners areused to distribute the heat as good aspossible over the length of the radiant tubes are usinghigh velocity burners to circulate thecombustion gases within the radianttube thereby distributing the heat evenlyover the radiant tube surface.

2 Thetemperature differences on the tubesurface can be minimize to a fraction,compared to non-recirculating recirculating tubes are still widelyused, especially U-tubes in the heattreating industry and U-and W-tubes inthe steel industry, but within the lastyears, most of the major burnercompanies included recirculating singleended radiant tubes in their tube materialIn the present time, three differentmaterial classes are used:1. cast alloy tubes2. fabricated alloy tubes3. ceramic tubesThere are various pros and cons for castand fabricated alloy tubes, but in generalboth types can be used under similarconditions.

3 Ceramic tubes gainedimportance since the introduction ofsilicon carbide, especially reactionbonded silicon carbide SiSiC. Thesematerials overcame the problem ofthermal shock sensitivity from tubesmade from mullite and others. The material properties of SiSiC at hightemperatures are superior to alloy,especially regarding strength, creepstress and thermal oxidation , allowingfor net heat fluxes of up to 110 BTU/hrin2 and can be used in furnaces withzone temperatures up to 2250 F. Alloytubes are generally used for heat fluxesof 60 BTU/hr in2 and less and forfurnace temperatures of maximal2000 F.

4 On the other hand, the ceramicproduction process permits only certainshapes and the possibilities formachining and joining (welding, brazing)are very limited. The mechanicalbrittleness must be also , proper design and sometraining of the operators can overcomethese difficulties and many thousandceramic radiant tubes are in operationnow, providing reliable high performanceto their users. In strip lines , ceramicradiant tubes open the opportunity forconsiderably increased production of 30% and more. However, the furnace andthe process have to be checkedthouroughly to avoid problems like tubedamage caused by strip breakage andalso problems which can result fromhigher furnace temperatures, which canbe well above the final strip temperature( strip breakage at reduced strip speed, Flameless oxidation burners for heatingstrip linesNatural gas is the most widely used fuel for heating strip in furnaces due to itsavailability and clean combustion and is enabling much lower energy costs,compared to electrical heating systems.)

5 Provided that an efficient system is energy costs issues, heating systems play an important role for theperformance of a furnace, especially regarding temperature uniformity / productquality, net heat input / productivity, energy efficiency / operating costs,maintenance / operating costs, tube life / operating costs, flue gas emissions /pollution, furnace downtime, ease of operation and investment costs. Thefollowing reports shows that all these aspects have to be considered to make adecision for a new heat treating furnace or a retrofit Joachim G.

6 W nningWS W rmeprozesstechnikGmbH, Renningen(Germany)Phone: +49 (0)7159 / 1:Recirculating and non-recirculating radiant tubesFig. 5:Calculated (CFD) temperatures for flame and FLOX -modeHEAT PROCESSING October 20032 REPORTS rollers, furnace insulation). Alloy tubesremain a good choice for manyapplications due to their greaterflexibility in shape and in applicationswhere mechanical damage cannot beavoided, strip breakage in a verticalstrip line, strong vibrations, shocks orderailed efficiency of fuel fired systems isoften defined by the term available available heat is the heat, availableto the furnace and the workload and isequal to the gross input minus the fluegas losses [1].

7 Rising flue gastemperatures are leading to higher fluegas losses and thereby to loweravailable heat (Fig. 2). For example: atflue gas temperatures of 1800 F, withoutair preheating???=0), only about half ofthe energy, provided in the form of fuel isavailable to heat up the furnace, the restis lost with the flue gases, even when theburners are adjusted properly. Thiseffect is even more important for radianttube systems compared to direct firedfurnaces since the flue gas temperaturecan be substantially higher than thefurnace temperature (Fig.

8 3).For example: For a furnace temperatureof 1300 F, the tube temperature is1700 F and the internal gas temperature(flue gas temperature prior to an optionalheat exchanger) is 2000 F and more, if aceramic radiant tube with a net heat fluxof 110 BTU/hr in2 is , without proper heat recoverymethods, efficiency can be very poor(available heat < 50%) even at relativelylow furnace temperature. For evenhigher furnace temperatures, theavailable heat can drop even further,eliminating the cost advantage of naturalgas over electricity most effective way to reduce fluegas losses is to use heat recoverysystems to preheat the combustion heat recovery concepts areused.

9 Plug in recuperators and central heatexchangers provide moderate airpreheat temperatures self recuperative burners providehigher air preheat temperatures byeliminating transport losses since theheat exchanger is integrated into theburner and placed within the furnacewall regenerative burners enable largeheat transfer areas in a compactdesign, obtaining air preheattemperatures which are close to theflue gas temperature (prior to the heatexchanger)Fig. 4:Recuperative burner in flame and FLOX - modeFig. 2:Available heatFig. 3: Exhaust temperatures from radiant tubesREPORTSHEAT PROCESSING October 20033 Central heat exchangers and plug inrecuperators are used in combinationwith non- recirculating radiant recuperative burners are generallyused for recirculating radiant burners were used in non-recirculating tubes but the challengescoming from the high air preheattemperatures often lead to difficulties.

10 Anewly developed regenerative burnerdesign in combination with A-typeradiant tubes will overcome thesedifficulties and is ready for the marketintroduction. It will be mainly depend onthe future energy prices how long it willtake until this concept gains aconsiderable market the most common gaseous fuels likenatural gas, NOx-emissions are thegreatest concern. High temperatures,confined flow conditions and high airpreheat temperatures contribute tostrongly rising , very effective low NOx-combustion concepts must be high velocity concept was a firststep, but not enough as the air qualitystandard started to became moresevere.