A Study on the Design of Recuperative Burner(1998)

INTERNATIONAL JOURNAL OF ENERGY RESEARCH

Int. J. Energy Res., 22, 209—220 (1998)

A STUDY ON THE DESIGN OF RECUPERATIVE BURNER

BYUNG-SIK PARK1,*, DAE-HUN CHUNG1, WON-BAE KIM1 AND YOO KIM2

1 Korea Institute of Energy Research, 71-2, Jang-Dong, Yusong-Ku, Taejon 305-343, South Korea2 Chungnam National University, 220, Kung-Dong, Yusong-Ku, Taejon 305-764, South Korea

SUMMARY

Waste heat recovery from the exhaust gas of industrial furnaces and kilns that are high energy-consuming equipment isone of the effective energy conservation methods because of its high sensible heat contents. The recuperative burnerintegrated with a recuperator and burner is one of the combustion equipments with many advantages of simpleinstallation, compactness and easy control which can be applied to various fields of industry. A recuperative burner withthe capacity of 400 kW was designed using the design data from experimental results. Performance tests on this burnerwere made. The exhaust gas analysis, including NO

x, the measurement of the flame temperature, velocity, heat flux and

heat flux analysis on the recuperative burner were the main topics of hot combustion tests. Design data from theexperimental results are gas velocity, air velocity, air velocity, the tip-location of gas nozzle, the dimension of furnacesuitable to burner capacity, the dimension of recuperator and the role of cross-shaped steel plate for increasing the energyefficiency in the recuperator. For uniform temperature distribution and good thermal efficiency, it is appropriate tomaintain the furnace pressure at 2—3 mmAq. ( 1998 John Wiley & Sons, Ltd.

KEY WORDS recuperative burner; nozzle design; industrial furnace; heat exchanger

1. INTRODUCTION

In general, the chamber temperature inside the industrial furnaces is maintained to be high, and in manycases over half of the total heat input disappears in the atmosphere as a sensible heat of flue gas. For thisreason, the thermal efficiency of the cumbustion furnaces is as low as 10—40% and it is highly required tohave countermeasures for energy conservation. Even though there are many methods for the energy savingsof combustion furnaces such as improvement of combustion efficiency, heat transfer facilities and therecovery of waste heat, the method of waste heat recovery is the most commonly applied countermeasure forany kind of furnace.

However, the conventional heat recovery system constituent with burner and heat exchanger separatelyhas many spatial and economical disadvantages which originates from the construction of air supply systemand the necessity of insulation of hot air piping. To overcome these weak points the recuperative burneremerged as a compact combustion equipment which is assembled in one body with burner, heat exchangerand stack. This recuperative burner is easy to install.

The recuperative burner was applied to the combustion furnaces of the industrial field from the mid 1970swhich is already developed in 1960s. Until present, the study on the recuperative burner has been carried outin Germany, England and Japan. Schmidt and Laiquiddin (1966) studied mainly the shape of heat transferpart of the recuperative burner and many Japanese studies had focused on the NO

xreduction, air preheat

temperature and recuperation rate, Tabata et al. (1983, 1985, 1986).

*Correspondence to: B.-S. Park, Korea Institute of Energy Research, 71-2, Jang-Dong, Yusong-Ku, Taejon 305-343, South Korea

CCC 0363—907X/98/030209—12$17.50 Received 14 January 1997( 1998 John Wiley & Sons, Ltd. Revised 16 April 1997

As described above, the topics on the heat exchanger or recuperation rate of recuperative burner has beenstudied variously, but there seldom exist studies on the design of nozzle or nozzle performance. So in thisstudy it is aimed to find the appropriate design method on the nozzle and nozzle location in the burner tile forthe design of a recuperative burner of 400 kW. Besides, the change in characteristics of velocity andtemperature fields according to the variation of furnace pressure is studied. Some methods to enhance theefficiency of the radiation-type heat exchanger in which supplementary heat transfer area such as a cross-shaped steel plate and an exhaust gas distributor assembled to the recuperator has been investigated by bothexperiment and numerical analysis.

2. DESIGN OF BURNER NOZZLE AND EXPERIMENTAL METHOD

Figure 1 is an assembly diagram of the burner used in the experiment. The part of gas nozzle wasmanufactured with heat-resistant steel alloy and the burner tile for flame stabilization was made withhigh-temperature castable and other parts including the main body are made with mild steel.

The design of the gas-nozzle is emphasized in the aspects of gas/air mixing and stability. The nozzleconsists of gas nozzle and flame stabilizer, which are one body and a gas ejection ring was attached to thenozzle body by the fitting method of heat setting. The fuel gas is ejected out through the circumferential holesin the radial direction. The air flows in the axial direction and mixed with fuel gas by means of flowrecirculation due to the flame stabilizer.

In Figure 2 the size and mixing process is shown for the conceptually designed nozzle. Air bulk velocity ofnozzle exit is designed to be 15—20 m s~1 at the preheated temperature of 300°C and fuel gas velocity ofnozzle holes to be 20—25 m s~1. In the case of recuperative burner, the axial momentum of the mixed gasshould be large enough for the exhaust gas to reach the end wall. As a result, the full path of exhaust gas in thefurnace can be longer length so as to achieve the higher convection heat transfer between the exhaust gas andfurnace wall. Some portion of fuel gas ejected in the nozzle holes is premixed with air and makes a flame justafter the nozzle exit and another portion of fuel gas flows with air far down the furnace as diffusion flametype. The burner tile made with castable is designed to have exhaust gas path holes which are inclined 30° tothe axial direction of the gas nozzle.

Figure 1. Schematic of recuperative burner

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Int. J. Energy Res., 22, 209—220 (1998) ( 1998 John Wiley & Sons, Ltd.

Figure 2. Test gas nozzle

Figure 3. Experimental furnace

To test the performance of the designed recuperative burner, an experimental furnace of Figure 3 wasdesigned and manufactured for various parameter studies. Fuel gas was town gas which was composed ofLPG and air having lower heating value of 15 000 kcal (Nm3)~1. Temperature was measured with suctionpyrometer of Land Combustion Co. Ltd, England and the velocity field inside the furnace was measured withwater-cooling five-hole pitot tube. Among the contents of exhaust gas, oxygen and carbon monoxide wasdetected with a paramagnetic-type gas analyser of Servomex, England and NO

xwas measured with the

chemiluminescent-type gas analyser of Thermotechnik, U.S.A.

DESIGN OF RECUPERATIVE BURNER 211

( 1998 John Wiley & Sons, Ltd. Int. J. Energy Res., 22, 209—220 (1998)

Table 1. Heat exchanger design data

Combustion air Flow rate (kg h~1) 480Exit temp (°C) 500

Pressure drop (mmAq) Below 300

Exhaust gas Flow rate (kg h~1) 528Entrance temp (°C) 850

Pressure drop (mmAq) Below 5Heat flux (kcal h~1) 48000

Fuel used City gas

3. HEAT EXCHANGER DESIGN AND EXPERIMENTAL METHOD

The heat exchanger applied to the recuperative burner is based on the design specifications of Table 1. Theshape and size of the heat exchanger for the design specifications is shown in Figure 1. Heat transfer area ofthe designed heat exchanger is 2)0 m2 for the air side and 1)8 m2 for the exhaust gas side. From the viewpointof heat transfer supplementary heat transfer area of about 2)8 m2 for the air side has to be added and for theexhaust gas side the supplementary heat transfer area of about 0)7 m2 has to be added for the cross-shapedsteel plate and a distribution for uniform exhaust. The pressure drop of air can be maintained under300 mmH

2O for the case of 1)5 times the normal operation, by accurate design of air flow passages. In the

heat exchanger design of this type it is focused on the heat transfer area maximization through thesupplementary heat transfer area along with the flow passage optimization to meet the limitation of air-sidepressure drop.

For the verification of the designed recuperative burner, the heating test to simulate the real industrialheating process, an experimental furnace which is made with 10 sectional parts and joined with flanges isutilized to measure the heat flux for each zone. The flowrate of cooling water is measured with float-typeflowmeter outside the furnace in two ways, that is, some of which flows in and out the bottom wall and theother of which flows in and out the top and two side walls. T-type thermocouples were also installed in theinlet and outlet of the cooling water. Thus, the water-cooling furnace equipped with the above can be utilizedas a calorimeter for heat transfer analysis.

4. PERFORMANCE TEST AND ANALYSIS

The gas ejection velocity of the recuperative burner was determined as 20—25 m s~1. In case the combustiongas flows in the furnace chamber as the ejection velocity of some 80—100 m s~1 and an axial momentum canbe reached in the favourable state for the combustion gas to reach the rear part of the furnace. If the gasejection velocity becomes over 25 m s~1, flame lift and instability can occur. And if the gas ejection velocitybecomes low under 25 m s~1, the axial momentum is also small and the combustion gas cannot reach therear part, causing the unfavorable heat transfer. To understand the flame structure, combustion gas velocityand temperature in the furnace chamber were measured. Figure 4 shows the velocity and temperaturedistribution of the combustion gas.

The measured maximum temperature was 1650°C, which is located at x"120 mm in the direction of thefurnace length. Combustion gas velocity was measured simultaneously at the three location of x"285, 700,1115 mm and the maximum velocity of 50—55 m s~1 occurred at x"285 mm.

Figure 5 shows the furnace chamber temperature in the axial direction. In the figure, the wall temperatureis measured simultaneously at x"665, 1850, 3450 mm. The wall temperature shows evenly 1100—1300°C inthe relatively wide range up to x"2500 mm, which may be attributed to the high convection velocity.Especially in the rear part of the furnace there exists a stagnation zone of convection. This can be explained

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Figure 4. Temperature field in furnace

Figure 5. Temperature variation with axial distance

from the fact that the furnace length is too long compared to the sectional area for the combustion gas toreach the rear region.

The temperature of the rear stagnation region is maintained at 800—900°C, which may be result of solidwall radiation. The temperature difference between the wall and the chamber space 50 mm apart from thewall shows almost the same 80°C along the furnace length. In the result of temperature distribution, thereexists fairly homogeneous temperature from the burner tile section to furnace length of 2500 mm, but afterthe length of 2500 mm the temperature descends slowly. So the appropriate furnace length with the sectionalarea of 800]800 mm is evaluated as 2500 mm for the designed recuperative burner of 400 kW.

Figure 6 shows the comparison of temperature in the conventional and recuperative burners. The exittemperature of exhaust gas in the recuperative burner is kept almost 1000°C and the temperature distribu-tion near the wall is fairly homogeneous. But the conventional burner shows rapid temperature decrease inthe stack direction and results to be 800—900°C. From this figure it is seen that the recuperative burner has



Figure 6. Comparison of temperature field between recuperative burner and conventional gas burner

more merit than the conventional burner in maintaining the furnace chamber temperature high andhomogeneous.

To distinguish the relation between the flame ejection diameter (d) (cf. Figure 1 s d"150 mm) and thedistance (l) from burner (Figure 1 s wall to gas nozzle (Figure 1 r is experimented on the flamecharacteristics of Figure 7 by changing the ratio of l/d. In the case of l/d"0)7, the combustion little in theburner tile and the reaction was not complete. In addition, the velocity distribution was not so good insymmetry and flame instability occurred. In the case of l/d'1, radial direction component of the jet ejectedfrom nozzle collided with the burner tile wall and converted to the axial direction component. But in the caseof l/d"1)3, the combustion is made to occur fully in the burner tile region and the flow-field moves towardthe burner entrance. As a result, the combustion proceeds fast, like in the case of premixed flame and thecombustion gas after reaction cannot penetrate into the rear part of the furnace and the residence time ofcombustion gas in the furnace chamber becomes short and the phenomena of shot path is apt to occur, whichis harmful for good thermal efficiency. Thus, it is evaluated to be favorable that l/d is designed to be 1)0 inorder to have both combustion in the burner tile region and diffusion combustion after the burner tile exit.

Figure 8 shows the flame characteristics when the furnace pressure is changed. In the negative furnacepressure, the flame temperature and velocity shows little decreasing tendency relative to the case of thepositive furnace pressure. This means that the flow pattern is distributed by the air inflow and the O

2in the

exhaust gas is increased. In this case, the temperature distribution is affected by the air inflow andhomogeneous heating of product can be injured. In the case of positive furnace pressure, the hot combustiongas goes out through the furnace wall holes or cracks and results in a great heat loss. Therefore it isappropriate that the furnace pressure is kept at 2—3 mmH

2O.

Figure 9 shows the NOxincreasing trend according to the increase of air preheat temperature and burner

capacity. Among the exhaust gases NOxcontent reaches a favorable level of about 100 ppm (based on 11%

O2) at air preheat temperature of 500°C. This is the reason why the combustion proceeds in the partly

premixed flame with high fuel/air mixing and the temperature distribution shows smooth tendency due to theburner tile effect and results to avoid the locally high-temperature region. Figure 10 shows the measured heatflux distribution. Inside the experimental furnace walls were installed with a 80 mm castable for the top andtwo side walls and 80 mm ceramic fibre for the bottom wall. Outside the furnace wall 20 water-cooled jacketswere installed in similar two ways like the above.

As is seen in Figure 10(a), the heat flux distribution by the recuperative burner is relatively smooth in thebegining of heating process, which means that the furnace is not heated enough to show stabilized heattransfer without any more thermal storage process in the furnace construction materials such as castable,

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Figure 7. The variations of temperature and velocity field with furnace pressure

ceramic fibre and steel. In Figure 10(a) it is noted that the heat flux to the bottom wall is higher than that tothe upper walls. This is the reason why upper walls installed with castable has much more heat resistancethan the bottom wall installed with ceramic fibre. But Figures 10(b) and 10(c) show inverse phenomenacompared with the case of Figure 10(a). This can be explained from the difference of thermal diffusivity



Figure 8. The variations of temperature and velocity field with l/d

Figure 9. NOx

variation with preheated air temperature

between the two materials. If the heating time exceeds 1 h, the thermal storage for the low thermalconductivity material is thought to be almost complete to balance the heat transfer between each fluidboundaries, i.e., the so-called steady-state condition is thought to be almost reached.

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Figure 10. Heat flux distributions according to the heating time

The phenomena is distinct with the time elapse and the heat flux difference between the bottom zone andupper zones is made greatly. This difference can be explained from the different material emissivities, differentthermal diffusivities and temperature differences between exhaust gas and different furnace inner walls. With



Figure 11. Heat flux distributions according to the heat input

the time elapsed, the zones near the right- and left-end sides of the furnace axial direction have more heat fluxrather than the other zones. This phenomena may be different from the conventional furnace operation,especially at the right-side phenomena. The phenomena can be analyzed from the axial conduction in fluxfrom the fully stored heat of bulky thermal storage material and the solid wall radiation effect.

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Figure 12. Temperature variation with time

Figure 13. The characteristics of pressure drop in heat exchanger

Figure 11 shows the result of different power operation of the same recuperative burner. In the figure theheat flux distribution shows the same trend and it can commonly be seen as high heat flux in the rear part ofthe furnace. This can be explained by the existence of end-wall effect, that is, from the end wall there emittedmuch solid radiation heat and there exists direct heat conduction from the much heat-stored end wall. Asa whole, the heat-flux distribution can be regarded relatively homogeneous like a stirred reactor although theratio of furnace length to width is quite long, compared to the ratio of conventional furnace length to width.

Figure 12 shows the preheat temperature result according to the types of heat exchangers. The 3 casesshow the effect by the distributor and/or cross-shaped steel plate. Case A shows the effect when the twosupplementary equipment is used. Case B shows the effect for the cross-shaped steel plate only and case C, forthe distributor only. The material is used as high heat resistant alloy. As is seen in the figure, the effect ofcross-shaped steel plate is much more than that of distributor.

Air-side pressure drop is presented in Figure 13. As the figure shows, the pressure drop across the total airflow passage in the heat exchanger is designed relatively small compared to the conventional commercial



great exchangers. This is possible by adopting the simulation technique of flow-passage optimization. Curve(A) is the result of pressure drop when the furnace is operated with the peeping holes opened. And in the casemost of the operating air can be leaked through the peeping holes, but curve (B) is obtained with the peepingholes tightly closed. For the case of (A) the pressure in the furnace chamber shows almost the same as theatmosphere pressure. But for the case of (B), the furnace pressure shows curve (C) in the figure. In detail, at anair flow rate of 516 Nm3h~1 the furnace pressure can be kept at 5 mmH

2O and the exhaust gas cannot be

extracted to the atmosphere by the natural draft system, but by the forced draft system like an eductorsystem.

With the high flow rate, the phenomena becomes more severe, and at the flow rate of 1140 Nm3h~1 thefurnace pressure reaches 28 mmH

2O. The operating range of the recuperative burner is 300—500 Nm3h~1

based on the total exhaust pressure drop reaches only 1)5—5)0 mmH2O. Thus, the recuperative burner can be

regarded to be designed enough to meet the pressure drop conditions.

5. CONCLUSION

For the recuperative burner of 400 kW, it is recommended to design the gas ejection velocity to be20—25 m s~1 in the radial direction and the air velocity to be 15—20 m s~1 in the axial direction. This isconsidered to be favourable for the flame to be maintained partly in the premixed type and mainly in thediffusion type.

The maximum air preheat temperature reaches 500°C and NOx

content shows 100 ppm (based on 11%O

2) in this case. The furnace size adapted for the recuperative burner of 400 kW was evaluated as

800]800]2500 mm and the burner tile holes diameter for exhaust gas extraction should be appropriatelydetermined for the exhaust gas velocity to be maintained as 80—100 ms~1.

The gas nozzle should be located in the same distance as the ejection diameter just after the burner tile. Thefurnace pressure of 2—3 mmH

2O is appropriate in the point of homogeneous temperature distribution in the

furnace and thermal efficiency. Heat exchange performance with the combustion air and the exhaust gas canbe enhanced with the aid of the cross-shaped steel plate and distributor.

In the design it is favorable for the sectional area of the recuperator to be determined as the same area ofthe total extraction holes area of the burner tile and the recuperator height can be selected as 3—5 times thediameter considering the heat exchange amount and installing space.

ACKNOWLEDGEMENTS

The authors would like to express their thanks for the financial support to this research by MOST of Korea.

REFERENCES

Schmidt, Th. and Laiquddin, S.S. (1966). ‘Burner with high momentum flow of the combustion products’, Gaswarme Nr. 8, pp. 271—279.Tabata, A., Hosoi, K. and Tanaka, H. (1983). ‘Development of self-recuperative burner(X)’, ¹echnical Report No. 27, Tokyo Gas Co.,

pp. 183—190.Tabata, A., Hosoi, K. and Tanaka, H. (1985). ‘Development of self-recuperative burner(XIII)’, ¹echnical Report No. 29, Tokyo Gas Co.,

pp. 133—140.Tabata, A., Hosoi, K. and Tanaka, H. (1986). ‘Development of self-recuperative burner(IX)’, ¹echnical Report No. 30, Tokyo Gas Co.,

pp. 71—79.

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A Study on the Design of Recuperative Burner(1998)

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