Energy Fuels 2010, 24, 5369–5377 Published on Web 10/04/2010
: DOI:10.1021/ef1007159
Experimental Study on the Heat Flux Distribution of a Laboratory-Scale Wall-Fired Furnace Qingwei Fan,†,‡ Shien Hui,† Qulan Zhou,*,† Xi Chen,*,‡,§ Qinxin Zhao,† and Tongmo Xu† † ‡
State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China, Department of Earth and Environmental Engineering, Columbia University, New York City, New York 10027, and § School of Aerospace, Xi’an Jiaotong University, Xi’an 710049, China Received June 8, 2010. Revised Manuscript Received August 25, 2010
We investigate the effect of different operating conditions on the combustion process of the wall-fired furnace. The flue gas temperature, furnace wall temperature, and spatial distribution of local heat flux on the wall of a laboratory-scale, gas fuel front wall-fired furnace are measured experimentally. Different combinations of operating conditions including swirl intensity (SI), air supply (AS), and burner combination (BC) are considered, and their influences on the combustion, heat-transfer, and heat flux distribution characteristics are explored. Among the operating conditions studied in this paper, the combustion process can be improved with the increase of SI as well as the inner secondary air (ISA) rate. Detailed local heat flux data are obtained and analyzed under various operating conditions. The results illustrate complicated variations over the entire furnace wall. The decrease in SI conditions significantly enhances the local heat fluxes near the bottom of the furnace (adjacent to the combustion region), where the mechanical integrity of the wall is of concern. The averaged local heat flux on the front wall is sensitive to the operating conditions. Preferable operating conditions for different BC conditions are identified on the basis of the non-uniform coefficients. The findings may help to improve the understanding of the heat flux distributions (in particular, in the wall-fired furnace) and give useful insights for the optimization, arrangement, and material selection of the heating surface.
consequences. It is estimated that 10% of all of the power plant breakdowns are caused by creep fracture of furnace tubes.2 An improved understanding of the tube temperature distribution and furnace wall heat flux distribution will be indispensable for furnace design. Many previous studies have considered the heat-transfer process of the furnace wall with diverse focuses. For example, Taler et al. have measured the tube temperature and further deduced the heat flux to the membrane wall using a tubulartype instrument.3-5 Characteristics of the metal surface, such as the damage on the metal surface, were found to deteriorate the heat-transfer processes on membrane walls; meanwhile, a local high heat flux can also accelerate the ash deposition and corrosion process on the scale.2,6-8 For laboratory- and industrial-scale pulverized coal-fired furnaces, Gill et al. and Butler et al. explored the relationship between heat flux and the local gas and particle temperatures.9-12
1. Introduction Owing to the low cost and wide availability, coal is arguably one of the most dominant energy sources for power generation and extensively used in the form of pulverized coal in thermal power plants. For example, coal-fired power plants generate almost 70% of the overall electric power in China and will continue to remain significant for the next few decades.1 From the perspectives of environmental protection and sustainability, the fundamental mechanism of the combustion process of pulverized coal needs to be understood for different working conditions. The combustion of pulverized coal in a utility furnace is a complicated process, which includes heavy turbulence and complex heat-transfer mechanisms. It is important to explore the temperature distribution and heat flux along the furnace wall from the mechanical integrity point of view. In essence, the heat-transfer process from the coal combustion flame to the steam generation tube is dominated by radiation heat transfer. The temperature of the tube, which is in general close to the creep temperature of metal, is a crucial factor for the reliability of the furnace system. Inside the steam generation tube, the high temperature and pressure of the sub- or supercritical vapor make the tube extremely vulnerable to the flame temperature excursion. If the metal temperature is within the creep regime, then creep deformation (bulging) or even fracture (longitudinal rupture) may take place with serious
(2) Jones, D. R. H. Eng. Failure Anal. 2004, 11, 873–893. (3) Taler, J. Int. J. Heat Mass Transfer 1992, 35, 1625–1634. (4) Taler, J.; Taler, D. Heat Transfer Eng. 2007, 28, 230–239. (5) Taler, J.; Duda, P.; Weglowski, B.; Zima, W.; Gradziel, S.; Sobota, T.; Taler, D. Fuel 2009, 88, 305–311. (6) Shim, H. S.; Valentine, J. R.; Davis, K.; Seo, S. I.; Kim, T. H. Fuel 2008, 87, 3353–3361. (7) Khajavi, M. R.; Abdolmaleki, A. R.; Adibi, N.; Mirfendereski, S. Eng. Failure Anal. 2007, 14, 731–738. (8) Gupta, S.; Gupta, R.; Bryant, G.; Wall, T.; Watanabe, S.; Kiga, T.; Narukawa, K. Energy Fuels 2009, 23, 2570–2575. (9) Gill, D. W.; Loveridge, D. J.; Thurlow, G. G. Symp. (Int.) Combust., [Proc.] 1969, 1239–1246. (10) Butler, B. W.; Webb, B. W. Fuel 1991, 70, 1457–1464. (11) Butler, B. W.; Webb, B. W. Energy Fuels 1993, 7, 835–841. (12) Butler, B. W.; Denison, M. K.; Webb, B. W. Exp. Therm. Fluid Sci. 1994, 9, 69–79.
*To whom correspondence should be addressed. E-mail: qlzhou@ mail.xjtu.edu.cn (Q.Z.);
[email protected] (X.C.). (1) Li, Z. Q.; Sun, R.; Sun, S. Z.; Chen, L. Z.; Wu, S. H.; Qin, Y. K. Energy 2002, 27, 1119–1130. r 2010 American Chemical Society
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Energy Fuels 2010, 24, 5369–5377
: DOI:10.1021/ef1007159
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These previous efforts are very useful for understanding the mechanism of heat transfer as well as refining the radiation and combustion models. However, the detailed measurement of the spatial distribution of heat flux under complicated working conditions is rare. Besides the aforementioned importance to mechanical integrity of the furnace, the knowledge of the spatial distribution of temperature and heat flux may also contribute to optimizing the performance of the furnace under various working conditions (which is critical for the purposes of environmental protection/NOx reduction and accommodation of various types of coal, including low-rank coal). Among the limited previous studies in this area, Li et al. studied the furnace temperature and heat flux distributions on a 1 MW tangentially fired furnace with coal over coal reburn, where the effects of the positions of the reburn nozzle and the reburn fuel fraction were considered.13 Jing et al. measured the gas species, temperature, coal burnout, and wall heat flux in a 200 MW lignite-fired furnace, focusing on the influence of overfire air damper opening.14 For the important wall-fired type of furnace, to our knowledge, the only work in the literature was carried out by Doroschuk et al., who investigated the performance of 500 and 800 MW units.15 The results were used as the validation by Kouprianov et al. on a 550 MW opposite wall-fired furnace under several operating conditions, and the measured heat flux on the furnace wall was in reasonable agreement with simulation results.16 However, a systematic experimental investigation on the wall-fired furnace is still lacking. In other words, thus far, there is no adequate and detailed experimental data that bridge the operating variables and the heat flux distribution, especially in the wall-fired pulverized coal furnaces. An extensive experimental investigation may also complement numerical simulations of the combustion and heat-transfer processes inside the furnace (e.g., that in a 500 MW wall-fired furnace17 and utility tangentially fired furnaces18-20). In the present paper, experimental measurements of the temperature of flue gas and membrane furnace wall are performed and the local heat flux distributions are obtained on a laboratory-scale wall-fired gas fuel furnace under varying operating conditions. Our main objective is to investigate the effect of different operating conditions, including the air supply method, the swirl intensity, and the burner arrangement, on the combustion process (via the measurement of the gas temperature, furnace wall temperature, and heat flux). The findings may provide useful insights for optimizing the combustion process.
Figure 1. Schematic of the experimental setup.
together eight steel plates (2 mm in thickness) and seven steel tubes (12 mm in outer diameter and 2 mm in thickness) one by one, and then the four membrane walls are assembled by pin connection. The cooling water is pumped into the tubes from the bottom of the furnace wall and heated up (by fuel combustion) as it flows upward and eventually returns to the water tank from the top of the tubes. Figure 2 shows the arrangements of the measurement points, as well as the swirl burner. We measure the temperature of the cooling water on both the front and the right membrane walls using T-type thermocouples. Along the furnace height direction (z direction), every tube contains six thermocouples, enabling five local heat flux regions. We also measure the temperature on the inner furnace wall using an assembly of thermocouples, which are welded on the left wall. To measure the flue gas temperature, K-type shielded thermocouples are used, which are inserted into the furnace chamber from the six test ports arranged on the top wall of the furnace. Four swirl burners are installed on the front wall of the furnace, two of which are clockwise swirl, while the other two are counterclockwise. For full load, the furnace can sustain four burners running, and thus, different burner combinations (BCs) are achieved, as illustrated in Figure 3. The spaces between two neighboring burners are set to be 2 D and 4 D in horizontal and vertical directions, respectively, where D is the outer diameter of the burner. The height of the first row of the burner is 210 mm, and the height of the second row is 330 mm (from z = 0). The swirl burner adopted here has a primary air (PA) duct (6 mm in inner diameter and 1 mm in thickness) in its central part, where the fuel and the primary air enter the furnace. An inner secondary air (ISA) duct (14 mm in inner diameter and 1 mm in thickness) is fixed outside the PA duct, where the ISA is swirled by the axial vanes with an angle of 50° along the axial direction. Different swirl intensities (SIs) are achieved by changing the distance between the vanes and the burner exit. Three swirl intensities, referred to as SI-1, SI-2, and SI-3, are chosen, with the distance between the vanes and the burner exit being 0 R, 1 R, and 2 R, respectively (where R is the outer radius of the PA duct). Under the SI-1 condition, the distance from the vanes to the burner exit is 0 R, which means that the swirl flow can directly enter the furnace chamber; thus, the swirl intensity is the largest. With the SI-2 and SI-3 conditions, the respective distances are 1 R and 2 R, which imply that the swirl flows must pass a certain distance inside the inner secondary air duct before they enter the furnace, and thus, their swirl intensities at the burner exit are smaller compared to the SI-1 condition. This is referred to as SI-1 > SI-2 > SI-3 in terms of the swirl intensity. The outer secondary air (OSA) is injected straight into the furnace from the OSA duct (20 mm in inner diameter and 2 mm in thickness) without swirl.
2. Experimental Section The schematic diagram of the experimental setup is shown in Figure 1. The furnace is 1.37 m in height and 0.32 m in both width and depth. Each membrane wall is constructed by welding (13) Li, S.; Xu, T. M.; Zhou, Q. L.; Tan, H. Z.; Hui, S. H. Int. J. Therm. Sci. 2010, 49, 225–234. (14) Jing, J. P.; Liu, G. K.; Chen, Z. C.; Liu, C. L. Energy Fuels 2009, 23, 3573–3585. (15) Doroschuk, V. E.; Rubin, V. B. Steam Boilers and Turbines of 500 MW and 800 MW Units: Development and Implementation; Energiya: Moscow, Russia, 1979. (16) Vikhansky, A.; Barziv, E.; Chudnovsky, B.; Talanker, A.; Eddings, E.; Sarofim, A. Int. J. Energy Res. 2004, 28, 391–401. (17) Kouprianov, V. I. Energy 2001, 26, 839–853. (18) Yin, C. G.; Caillat, S.; Harion, J. L.; Baudoin, B.; Perez, E. Fuel 2002, 81, 997–1006. (19) Kumar, M.; Sahu, S. G. Energy Fuels 2007, 21, 3189–3193. (20) Hwang, Y. L.; Howell, J. R. J. Energy Resour. Technol. 2002, 124, 56–66.
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Figure 2. Measurement point arrangements as well as the swirl burner. Table 1. Composition of the Gas Fuel composition
value
composition
value
CH4 (%) C3H8 (%) C4H8 (%) density (kg/m3)
3.15 36.88 34.18 2.177
C2H6 (%) C4H10 (%) C5H12 (%) lower calorific value (MJ/m3)
1.95 23.72 0.12 97.7
addition, the relatively lower heat load in the laboratory-scale furnace is not sufficient for pulverized coal ignition. Therefore, we employ gas fuel with good inflammability (fuel composition is given in Table 1) to replace the pulverized coal. Nevertheless, the results of heat flux may still provide useful insights for the combustion of pulverized coal. The power entering an individual burner is 3.7 kW, and the corresponding air supply conditions are summarized in Table 2. An attempt is made to keep these variables at near constant levels throughout the test period, so that the system is at a steady state. The stoichiometric ratio is 1.2 at the burner exit and 1.6 at the furnace exit because of the air leakage from the fire observation hole in the burner region. The local heat flux entering the furnace wall is represented by the steady-state energy absorption of the cooling water. The flow rate of the cooling water is kept at a relatively high level to eliminate the effect of the convection heat-transfer resistance (which is associated with the heat transfer from the tube wall to
Figure 3. Schematic of the combinations of burners that correspond to different BCs.
In previous laboratory-scale studies, the pulverized coal is mostly used in one dimension and a single burner furnace. However, for the present multi-burner and upward flow furnace under investigation, the fuel feed and ignition may be difficult because of the small dimension. As we know, the pneumatic transmission of pulverized coal is very difficult in the small-sized tube (just like the one used in our experiment), because of the collision of coal particles with the tube and among the particles themselves; therefore, blockages are more likely to happen. In 5371
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where the subscript i denotes the heat flux region in the height (z) direction, the subscript j denotes the tube index on the furnace wall, and ha is the average heat flux of the furnace wall. The relative (dimensionless) height, width, and depth are defined as
the cooling water) on the heat flux distribution, and also, the water temperature is kept below the boiling point, so that the single-phase heat-transfer status is satisfied. Therefore, the total heat flux h (kW/m2) in each heat flux region is calculated as h ¼ mcp ðTout - Tin Þ=A
X ¼ x=a Y ¼ y=a
ð1Þ
σ ¼
AS-2
AS-3
PA ratio (%) ISA ratio (%) OSA ratio (%) PA flow rate (L/min) ISA flow rate (L/min) OSA flow rate (L/min)
10 50 40 7.17 35.84 28.67
10 60 30 7.17 43.01 21.51
10 40 50 7.17 28.67 35.84
ð4Þ
X ðvi - va Þ2
ð5Þ
where vi is the measured variable, va is the average of vi, and σ is the standard deviation. The “non-uniformity” is a statistical variable, which is used to represent the uniformity of the heat flux and gas temperature distributions along the surface. For the non-uniformity coefficient ξ calculated via eqs 4 and 5, it is always larger than 1, and if it is closer to 1 (i.e., smaller), then the distribution of the variable is more uniform.
Table 2. Air Supply Conditions AS-1
va þ 3σ va
ξ ¼
ð2Þ
name
ð3Þ
where x, y, and z are the actual values in the Cartesian coordinate system and a (=0.32 m) denotes the width of the furnace. To analyze the spatial non-uniformity of the measured variables, such as the gas temperature and the local heat flux, a dimensionless number ξ is defined on the basis of statistics as
where m is the mass transport rate of the cooling water (kg/s), cp is the specific heat capacity of the cooling water (kJ kg-1 K-1), Tout and Tin are the cooling water temperatures on the outlet and inlet surface of each heat flux region, respectively (K), and A is the heat-transfer area, which is specified as half of the outer surface of each heat flux region (m2). The relative heat flux Rh is defined to reflect the spatial distribution feature of the heat flux as Rh ¼ hij =ha
Z ¼ z=a
3. Results and Discussion The combustion process changes significantly under varying operating conditions. This dominates the flue gas temperature, the wall temperature, and the local heat flux distribution. Therefore, these parameters are measured and analyzed in the present paper. Figure 4 shows the flue gas temperature distribution along the burner axis. In this example, the temperature is measured on the right two burners with burner arrangement BC-1 (see Figure 3) operating conditions SI-1 and AS-1. As the axis distance increases from Y = -0.5 to -0.3, the flue gas temperature reaches the peak value and then begins to attenuate as the axis distance increases. This is a typical temperature profile in the swirl burner, which includes the ignition, the intensively combustion reaction, and the burnout processes. The gas temperature of the center of burner is 250 °C higher than that on the side of the burner in terms of the
Figure 4. Flue gas temperature on the burner axis for BC-1, AS-1, and SI-1.
Figure 5. Influences of SI and AS conditions on gas temperature distribution under BC-3.
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Figure 7. Influences of SI on the furnace wall temperature under BC-1.
highest gas temperature and AS-2 results in the lowest ones in ports 1-3, especially in the combustion region. In ports 4-6, the opposite tendency can be found. To understand the characteristic of the flue gas temperature distribution, one needs to figure out the flow, combustion, and heat-transfer processes first. The fuel and the PA are injected to the furnace and then mixed with the hot flue gas, which is constrained by the ISA. In other words, ignition and combustion occur just a short distance away from the burner exit, as illustrated in Figure 4. Under the restriction of the OSA, the entire flame can reach a certain depth inside the furnace before it turns upward and downward because of the pressure gradient created by the OSA constraint. Two outer recirculation regions (outside the combustion flame) are formed near the furnace front wall region, just above and below the burners. In the up recirculation region, the flue gas flows downward to the burner region near the front wall, and thus, the temperatures in test ports 4-6 are dominated by the recirculation flue gas in most sections. However, with BC-3 conditions, two burners are installed in the second row of the burner region. As a result, the combustion status affects the gas temperature significantly in this section. When we decrease the SI, the mixing process becomes worse; thus, the combustion process is delayed, and the flame length is prolonged. Subsequently, the high-temperature region moves toward ports 4-6. Upon the AS-2 condition, more air is supplied, which leads to the increase in the swirl momentum; consequently, the flame length is shortened, and the flame spread angle is increased. Thus, the gas temperatures in ports 4-6 are higher than that of AS-1 as a result of the high spread angle and good mixing (the situation is similar for ports 1-3). For the AS-3 condition, a prolonged flame length is created as more air is taken as the OSA, and thus, the results have an opposite trend.
Figure 6. Non-uniform coefficients of the flue gas temperature.
peak value. However, near the rear wall of the furnace, they exhibit about the same value of 320 °C. Because the center burner is embraced by the hot flue gas produced by the nearby burners, its heat-transfer rate from the flame to the ambient environment is relative slower compared to that of the side burner (which is adjacent to the cooling water tube), and thus, a higher gas temperature profile occurs. Figure 5 shows the representative effects of the SI and AS conditions on the flue gas temperature distribution under the illustrative example of BC-3. It can be seen that the gas temperatures in ports 4-6 are somewhat lower than that in ports 1-3 (which are nearer the burner), and they reduce to the same values of about 300-320 °C at the furnace exit. For the swirl burner used in our experiment, one can expect a spreading flame, where the heat release rate from the flame to the ambient gas is restricted by the OSA near the burner exit region, and thus, the gas temperatures in ports 4-6 are low. Meanwhile, the momentum of the OSA decreases away from the burner, which makes the flame spread wider with a higher heat release rate, and this results in the higher temperature in ports 1-3. When we examine the effect of SI conditions, a decrease tendency in the gas temperature of ports 4-6 is found as SI decreases; however, the trend is almost opposite in ports 1-3. The effect of the AS condition on the flue gas temperature is also opposite for ports 1-3 and 4-6, where AS-3 exhibits the 5373
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Figure 8. Influences of SI on the local heat flux distribution.
Figure 6 shows the non-uniform coefficients of the flue gas temperature along the furnace height direction under varying operating conditions. It can be seen that ξ is larger in the burner region, which attenuates rapidly as the height increases and then slowly reduces with Z. The minimum and maximum non-uniform coefficients are 1.4 and 1.96, respectively. The fluctuation of ξ is major between 1.0 and 1.4, implying relatively uniform heat-transfer capability. On the basis of this information, we can further estimate the spatial distribution characteristic of the flue gas temperature. Figure 7 shows the effect of the SI condition on the furnace inner wall temperature distribution along the furnace height and depth directions under BC-1. In the Z direction, the wall temperature peaks at Z = 0.6, which corresponds to the combustion region, and then reduces to a relatively constant value from Z = 1.0 to 2.5. In general, the wall temperature along the height direction is maintained in the range of 60-100 °C, which means that a relative uniform temperature distribution can be expected in the wall-fired furnace. When
we decrease the swirl intensity to SI-2, the wall temperature increases somewhat, owing to the delayed combustion, which makes the high-temperature combustion region closer to the center of the furnace. As we further decrease the SI to SI-3, the high-temperature region may be beyond the center of the furnace, which reduces the wall temperature but is still higher than that of SI-1. Another evidence for the exceeded flame is that a peak temperature point is observed at Y = 0.06 in the SI-3 condition in the depth (Y) direction, which implies a relatively strong combustion reaction occurring in this area because of the delayed combustion. Along the Y direction, the wall temperature is high at the burner exit and then, in general, decreases as Y increases. In the SI-2 condition, a higher wall temperature can be found compared to SI-1, and thus, the further reduction of SI results in the lower wall temperature near the burner exit region. Figure 8 shows the relationship between the SI condition and the spatial distribution (contour plot) of heat flux (which accounts for both the convection and radiation heat transfer) 5374
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Figure 9. Influences of AS on the local heat flux distribution.
on the front and right walls. The relative heat flux fluctuates significantly in our experiments, which is coincident with previous studies.16 In general, one can observe a gradual decrease tendency along the furnace height direction for all cases studied in this paper. With BC-1 and SI-1 conditions, for example, two small regions with prominent heat flux appear on both sides of the front wall from Z = 1.2 to 2.0, while at the same height (Z), a larger flux is found on the right wall. These results imply the existence of the high-temperature flue gas regions in the corner of the wall, where flue gas flows upward without well mixing and adequate heat release. Fortunately, the maximum relative heat flux is just 1.5 and, thus, not significant compared to other cases discussed later. Meanwhile, as SI decreases, the local relative heat flux increases significantly on the bottom of front and right walls (from Z = 0.6 to 1.2), especially for the SI-3 condition. This may be a consequence of the poor mixing and delayed combustion at reduced SI, and thus, the resistance time of the combustion flame in the burner region is prolonged.
Because the burners are separated in the BC-2 condition, the heat flux distribution is relatively more uniform compared to that of BC-1 under the same operating condition, whereas the relative heat fluxes on the right wall are much more nonuniform than that on the front wall in this example of BC-2. The heat flux still increases with the reduction of SI (on the basis of the same reason stated above). In addition, the most prominent heat flux region moves toward the far side burner region on the right wall as the SI decreases. In the BC-3 condition, the burners are concentrated near the center of the furnace, and thus, the heat flux characteristics are similar for the front and right walls. The region of high relative heat flux is located in the middle of the front wall, whereas for the right wall, the prominent heat flux region is near the rear side. These features are contrary to the situations that occur in BC-1 and BC-2 conditions. Figure 9 shows the effect of the AS condition on the spatial distribution of local heat flux. Upon BC-1 setup, the short and widespread flame occurring in the AS-2 condition leads to the 5375
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Figure 11. Average non-uniform coefficients for local heat fluxes.
bottom panel of Figure 10, the average local heat flux decreases as the SI decreases, because the high-temperature flame moves farther away from the front wall. Figure 11 illustrates the average non-uniform coefficient of the local heat flux on both the front and right walls, where the lower coefficient indicates the uniform heat absorption of the different cooling-water tubes and, thus, a better performance. In our experiments, all non-uniform coefficients vary in the range of 1.2-2.1, most of which are within 1.4-1.6. In the BC-1 condition, a decreasing tendency of the averaged ξ is found as SI decreases. The working condition with AS-2 and SI-1 exhibits the lowest averaged ξ, and thus, a better performance can be expected in this case. In the BC-2 condition, the operating condition of AS-3 and SI-1 seems to be advantageous. The averaged ξ is similar in BC-3, where the conditions with AS-1 and SI-2 and AS-2 and SI-1 have better performance.
Figure 10. Average local heat flux along horizontal direction over the front wall under different operation conditions.
increase of the relative heat flux on both the front and right walls; however, for BC-2 and BC-3 setup, the heat fluxes are reduced under the same AS-2 condition. In essence, for BC-2 and BC-3 conditions, the flame can spread into any direction with little restriction of the adjacent burners, which leads to better mixing and a shorter flame. Thus, the flue gas temperature and heat release rate are more uniform. However, upon the BC-1 condition, the side burner can restrict the heat release of the center burner; thus, the temperature at the furnace center is higher (also see Figure 4), which subsequently leads to the higher heat flux region when the flame is short. With the AS-3 condition, a similar combustion status to the case of reducing SI occurs; thus, the relative heat fluxes on the bottom of the wall are expected to increase. Figure 10 illustrates the average local heat flux (calculated from the seven cooling-water tubes on the front wall), whose variation tendency along the furnace height direction agrees well with a previous investigation.11 The average heat flux varies in the range of 8-25 kW/m2, which is much lower than that measured in the utility furnace (100-500 kW/m2 in ref 11 and 210-370 kW/m2 in ref 14) but is close to the laboratoryscale experimental data (50-110 kW/m2 in ref 10 and 33-60 kW/m2 in ref 12). The effects of the BC, AS, and SI conditions on the average local heat flux distribution are also shown in Figure 10. It can be seen that the average local heat flux gradually increases from BC-1 to BC-3 (the top panel), which is because the burners are gradually raised and concentrated toward the furnace center, which improves the combustion status. From the middle panel of Figure 10, the average heat flux for AS-2 is a little higher than that of AS-1, which is expected, owing to the better combustion and the shorter flame length. In the AS-3 condition, an extremely high heat flux value exists in the section of Z = 1.0, while low heat flux values appear in the other sections, because the prolongation of the flame resistance time in the combustion region can significantly increase the local heat flux. According to the
4. Conclusion Experimental studies of the flue gas temperature, the furnace wall temperature, and the local heat flux are carried out in a laboratory-scale, gas fuel front wall-fired furnace. On the basis of the experimental data, the combustion processes inside the furnace are reconstructed and the influences of different working conditions, including the SI, the AS, and the BC conditions, are analyzed in detail. The operating conditions have significant effects on the combustion region, and the furnace of the swirl burners shows a nearly uniform flue gas temperature distribution. Inside the combustion region, the first row of burners tends to improve the combustion process of the second row of burners. The non-uniform coefficient for the flue gas temperature is calculated from the experimental data, which shows a decrease tendency along the furnace height direction. These results are valuable in estimating the uniformity inside the furnace. The complex distributions of the local heat flux are obtained in the present paper for different working conditions, which show significant variations in both the horizontal and vertical directions. Near the bottom of the furnace walls (where mechanical reliability may be critical), the increase in the local heat flux is related to the prolongation of the flame resistance time. The averaged local heat flux on the front wall is found to be strongly influenced by the operating conditions, which increases under conditions of an increasing SI, an increasing ISA, and a rising burner height. 5376
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The non-uniform coefficient of the heat flux varies intensely in our experiments. However, the optimized operating condition can still be figured out on both the front and right walls. To ensure the safety of the furnace, attention should be given along the vertical direction, where extremely high heat flux may appear in the combustion region when the AS and SI conditions are appropriately chosen. The findings in this study will serve as the first step toward understanding the heat flux distribution, especially in a wall-fired furnace, and give valuable insights for the design, arrangement, and material selection of the heating surface of the furnace.
m = water mass flow rate cp = water specific heat capacity T = water temperature Rh = relative heat flux X and x = relative and actual width Y and y = relative and actual depth Z and z = relative and actual height a = actual width (depth) A = actual heat-transfer area Greek Symbols ξ = non-uniform coefficient ν = statistics variable σ = standard deviation
Acknowledgment. The work is supported by the National Basic Research Program of China (Contract 2005CB221206), National Key Technology R&D Program of China (Contract 2006BAK02B03), National Natural Science Foundation of China (50928601), National Science Foundation (CMMI0643726), Changjiang Scholar Program of the Ministry of Education of China, and China Scholarship Council.
Subscripts in = water inlet surface out = water outlet surface i = index of the heat flux region j = index of the tube wall a = average value
Nomenclature D = burner outer diameter h = total heat flux
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