AUTHORS: Uwe Bonnet, Hans Kaczor and Joachim Georg WünningWS Wärmeprozesstechnik GmbH and Buderus Edelstahlwerke AG

The special steelworks of Buderus Edelstahlwerke AGproduces hot and cold rolled strip, rolled semi-

products and pieces for die casting and free forging, withan annual production of about 85,000t. The forgingblocks are reheated in gas-fired automated car bottom orbogie furnaces. The maximum length of the piece to beforged in the car bottom furnace is 15m. Forgings canthen be annealed, tempered and shaped by a variety ofCNC machines. The production portfolio of free forgedpieces encompasses worked pieces for general machinery,energy generating equipment and small forgings. Thelargest weight is about 80t.

INITIAL SITUATIONCar bottom furnaces are designed for the heat treatmentof particularly large pieces. The furnace at issue isapproximately 10m long and 4.5m wide and wasequipped with 18 high velocity re-circulating burnersrated 250kW each, corresponding to a nominal, naturalgas maximum throughput of 450Nm3/h. Both side wallswere equipped with nine such burners firing straight, hardjet flames. The cars are loaded apart, pushed into positionand then sealing between the furnace bottom and the carbottom is automatically provided. The flame and the hotcombustion gases were driven through the bottom carand the forging pieces as far as the opposite wall, fromwhere they were forced upwards to the roof, eventuallybeing re-entrained by the original jet issuing from theburner (see Figure 1). Because of this design, the pieces tobe reheated for forging could not just lie on the bottomcar, but had to be positioned in an accurate pattern onheavy steel supports measuring 400x400mm. The latterhad to be put in exactly the right position in advance ofloading otherwise the flame would directly impinge onthe stock surface, causing additional oxidation andinterrupting the heat distribution pattern. In these faultypassageways, because of the flame deflection, the

supports and the stock would be locally overheated andthe process of reheating the whole stock would beinterrupted or take longer. Furthermore, the tips of theinvolved burners could be damaged by excessiveoverheating. The maximum total fuel throughput was210Nm3/h natural gas and was limited by the timeinterval required by the on–off control technique wherebyevery 60 seconds the burners of one side wall wereinterchanged with those of the opposite wall, so that only50% of the installed 18 burners were available at anytime. The furnace was divided into three zones and airpreheating and low-NOx techniques were not adopted.

This was unsatisfactory in terms of productivity,maintenance, environmental pollution and overall cost. WSWärmeprozesstechnik was given the contract to remedythis situation by revamping the furnace.

FLAT FLAME REKUMAT® BURNERSFigure 2 shows the scheme after revamping. NineRekumat burners, each rated at 180kW and provided withintegrated air preheaters, were installed in the samelongitudinal positions as the original burners on each sidewall, but placed centrally. The total maximum gas flowafter revamping was 315Nm3/h with all 18 burnersfiring, distributed in three zones and fired in a circularpattern. Thanks to the increased overall power input,faster reheating of the stock could be achieved and agreater furnace capacity realised, as well as highertemperature reheat cycles. The new flat flame burners are

Heating of forging ingots in bogie hearth furnaces can be greatly improved by a specially designed ceramic nozzle burner that distributes heat in a more uniform pattern while minimisingscale formation, energy consumption and NOx emissions, and maximising productivity.

Flat flame burners for bogie hearth furnace

FORMING PROCESSES

r Fig.1 Cross-section of the bogie hearth furnace

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FLAMELESS FIRING IN HIGH TEMPERATURE FURNACESThe present developments of burners and heatingsystems focus on reduction of flue gas losses and noxiousemissions. In particular for high-temperature processesthe well-known technique of air preheating is used. Therelevant energy saving potential is, however, only partiallyexploited. One important reason is the increase in flametemperature which produces excessive thermal NOformation. However, it may be possible to reduce theflame temperature and therefore the NOx emission byinertisation of the flame. The process relies on theprinciple of mixing large quantities of flue gases into the

further away from the bottom edge of the car and, as canbe seen in Figure 3, the heat flow is no longer directedagainst the stock but, because of the arrangement of theburner nozzles, uniformly spread through the furnace. InFigure 3 the flat flame burners are firing in flame modeand the distribution in side firing jet flames is clearlyvisible. Figure 4 gives a detailed view of the burner nozzleinstalled in the middle of the furnace wall lagged withceramic fibres. As the flame is no longer directed underthe stock, the steel supports could also be substantiallysmaller (by a factor of almost 4). This provides a fasterand more economic reheating process thanks to thereduced ballast to be reheated with the stock.

p Fig.2Cross-sectionthrough the

revampedfurnace r Fig.3 Flat flame burners in the forge furnace

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FORMING PROCESSES

combustion air before the reaction with fuel takes place.The entrainment of the inert flue gases (that act aschemical and thermal ballast) is carried out by means ofthe high momentum of the air jets injected into thecombustion space. Thanks to a special design of theFLOX® burner nozzles, controlled and completelyflameless combustion takes place without pulsations orvisible flames, and without the typical flame noise. Inorder to carry out this process, the fuel is mixed with thecombustion air in a place where the required premixingwith flue gases has already taken place, but where thereis still enough turbulent mixing energy left to fulfil thecombustion reactions. This flameless combustion mode isonly possible above the ignition temperature of the fuel(850°C). Because flameless combustion is not visible, aburner safety control such as an ionisation or a UVdetector will not work, so a furnace temperature above850°C is the only safety method available.

The burners described here are fitted to fire both inflame mode (which is also used for start-up from cold)and in FLOX mode. Switching is easy and no separate

burners are required for the two firing modes below andabove 850° C. When the furnace is heated above theFLOX threshold, flameless combustion occurs and noise issignificantly reduced. Most importantly, the NOxemissions are drastically reduced to 10–20% of the valuecorresponding to flame mode, and the problem ofincreasing NO formation with increasing air preheating issolved. In a conventional system, NOx formation increasesexponentially with temperature while it drops by almostone order of magnitude in flameless conditions. Here,virtually unlimited high temperature preheated air can beadopted with very significant energy saving and hencewith significant economic advantages.

q Fig.4Flat flameburner nozzlefrom inside the furnace

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stock and the furnace walls. The convective heat transfer tothe wall zone adjacent to the burner overheats the wall andthe latter is then capable of transferring energy by radiationto the stock. In contrast to the pre-existing solution, the useof flat flame burners no longer involves direct contactbetween flame and stock and so avoids local stockoverheating. Furthermore, the temperature uniformity hasbeen improved thanks to the division of the each flame intofour jets. Figures 6 and 7 were taken in a test furnace at the

Gas Wärme Institute (Essen). In Figure 6 theburner is in flame mode and in Figure 7 is inflameless or FLOX mode. It is evident that thewall temperature close to the burner tip isoverheated in flame mode (Figure 6) while auniform temperature distribution can beinferred from Figure 7.

The expected reduction in specific gasconsumption is within calculated limits(20–35%, depending on temperature), justcomparing efficient air preheating with no

preheating even without taking into account thereduction of the mass of the supports that carry the stock(see Figure 8). This expectation has been confirmed inpractice. Furthermore, total available power has increasedby a factor approaching two, allowing higher productivityand/or higher temperature cycles, which makes thefurnace better available for high-performance operation.

CONCLUSIONSThe advantages of the flat flame burner applied to theforging furnace can be summarised as follows:

` An increase of more than 30% in the thermalefficiency without air preheat has been easily achieved

` Thermal NO formation is substantially abated evenwith very high air preheat

` The whole temperature span can be covered by asingle burner in either flame or FLOX mode

` Troubles caused by oxidation and safety control can be minimised

` The four nozzle distribution heats the stock indirectly,so minimising oxidation

` The ceramic construction of the combustion chambersignificantly reduces maintenance MS

FLOX® and REKUMAT® are registered trademarks of WSWärmeprozesstechnik

Hans Kaczor is head of the Energy and ControlDepartment in the Buderus Edelstahlwerke, Wetzlar,Germany. Joachim Wünning is General Manager andUwe Bonnet is in the Technical Office Nord/West of WSWärmeprozesstechnik, GmbH, Renningen, Germany.

To embody these results and to preheat air efficiently,self-recuperative burners are provided with an integratedair pre-heater shaped as a counter current corrugated heatexchanger (see Figure 5). A burner of the REKUMAT serieswith a specially designed primary combustion chamberwas selected for the present case and produces either fouraxial flame or four flameless jets at 90° to the long axis ofthe burner. Every combustion air jet has such a high exitvelocity that enough flue gases are entrained from withinthe furnace before the reaction with fuel takes place and acontrolled flameless pattern is established.

The outlet momentum of the burner jets and the flowpattern so generated, produce a heat transfer to the forging

r Fig.5 Burner with pre-heater andcombustion chamber

r Fig.6 Flat flame burner in flame mode

r Fig.7 Flat flame burner in flameless mode

r Fig.8 Increase in thermal efficiency with air preheating

FORMING PROCESSES

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