Effect of Air Distribution on Aerodynamic Field and Coal Combustion

Sep 8, 2010 - In this paper, the influence of air distribution on aerodynamic field and combustion performance in a. 0.9 MW arch-fired furnace has bee...
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Energy Fuels 2010, 24, 5514–5523 Published on Web 09/08/2010

: DOI:10.1021/ef1006935

Effect of Air Distribution on Aerodynamic Field and Coal Combustion in an Arch-Fired Furnace Ruwei Liu, Shi’en Hui,* Zhanying Yu, Qulan Zhou, Tongmo Xu, Qinxin Zhao, and Houzhang Tan State Key Laboratory of Multiphase Flow in Power Engineering, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China Received June 4, 2010. Revised Manuscript Received August 16, 2010

In this paper, the influence of air distribution on aerodynamic field and combustion performance in a 0.9 MW arch-fired furnace has been investigated by analyzing the momentum ratio of air flows and the air stoichiometric ratio in the preceding stage combustion zone. It is found that the momentum ratio of air-fuel flows directly affects the arch air penetration length and the position of the flame center, which is critical to the temperature distribution in the furnace and on the furnace walls. The best combustion performance in the experimental range occurs when the momentum ratio of arch air to secondary air equals 1.34 and that of arch air to D&E-layer secondary air equals 4.42. In addition, it is found that the heat loss due to incomplete combustion, also called combustible loss, and NOx emission in the flue gas are related to the air stoichiometric ratio (SR). The minimum values of unburned carbon in fly ash, unburned carbon in the slag and NOx emission at the furnace outlet are attained when SR = 0.67, 0.63, and 0.59, respectively. Furthermore, both combustible loss and NOx emission obtain proper values simultaneously when SR = 0.634, which could be considered as the optimum operation condition.

However, many problems, such as improper temperature distribution, high combustible loss, and high NOx emission, are widely present in the practical operation.7-10 For example, uncontrolled NOx emission can approach 1300 mg/Nm3 in some W-shaped boilers. Additionally, it is very difficult to reduce the NOx emission and the combustible loss simultaneously because most methods of reducing NOx emission lead to combustion instability and poor burn-out characteristics. Therefore, it is quite important to find an optimum operation condition under which proper temperature distribution, relatively low NOx emission, and relatively low combustible loss could be obtained. We believe that the effect of air distribution is very critical to the problems of W-shaped boilers. In recent years, much work has been devoted to understanding the effects of air distribution on the swirl or tangentially fired furnace boilers,11-14 but only a few relevant studies are reported on the performance of arch-fired boilers. Burdett investigated the effect of air staging on NOx emission from a 500 MW (electrical) W-shaped boiler.15 Wei and Yan measured the velocity profile at room temperature in a modeling bench experiment and mainly discussed the influence of arch air on the aerodynamic field.16,17 Xu studied the effects of the tertiary air on the flow

1. Introduction Fuels such as lean coal, anthracite coal, and petroleum cokes are abundant and are widely used as fuel for power generation in China. These fuels are hard to be ignited and are hard to be burned out because of their low volatile content and low reactivity. It is considered that the arch-fired boiler, also called a W-shaped boiler, has an advantage over horizontally fired boilers to burn these fuels, since it creates a “W” shaped flame to provide sufficient residence time for char combustion and admits secondary air through openings along the vertical wall underneath the arches as the flame develops to achieve flame stability through a wide load range without support fuels.1-6 Therefore, it is very important to develop the archfired combustion technology in China. Arch-fired boilers were first introduced into China in the 1990s, and tens of such boilers have been put into operation. *To whom correspondence should be addressed. Phone: þ86 29 82668784. Fax: þ86 29 82668703. E-mail: [email protected]. (1) Goidich, S. J.; Garcia-Mallol, J. A.; Seltzer, A. H.; Wagner, D. E. Integration of the Arch Furnace and BENSON Vertical OTU Technologies for High Efficiency Low Volatile Fuel Combustion. In Proceedings of the Power-Gen International Conference, Orlando, November 28-30, 2006. (2) Garcia-Mallol, J. A.; Steitz, T.; Chu, C. Y.; Jiang, P. Z. Ultra-low NOx Advanced FWArch Firing: Central Power Station Applications. In Proceedings of the 2nd U.S.-China NOx and SO2 Control Workshop, Dalian, China, August 25-28, 2005. (3) John, S. E.; Garcia-Mallol, J. A.; Simmerman, R. N. Advanced FW Arch Firing: NOx Reduction in Central Power Station. In Proceedings of the 19th Annual International Coal Conference, Pittsburgh, PA, September 23-27, 2002. (4) Brower, P.; Winkin, J. P.; Ge, C. Q. Anthracite Firing - Largest Steam Generators. In Proceedings of Power-Gen Europe Conference, Milan, Italy, June 9-11, 1998. (5) Winkin, J. P.; Garcia-Mallol, J. A. Mod. Power Syst. 1997, 17, 57–61. (6) Winkin, J. P.; Garcia-Mallol, J. A.. Anthracite Firing in Large Utility Arch Fired Boilers. In Proceedings of 59th Annual American Power Conference, Chicago, USA, 1997; Vol. 2, pp 1166-1174. r 2010 American Chemical Society

(7) Zhang, H.; Lu, J. F.; Xu, X. Q.; Zeng, R. L.; Yue, G. X. Power Eng. 2005, 25, 628–632. (8) Che, G..; Hao, W. D.; Guo, Y. Q. Power Syst. Eng. 2004, 20, 38–40. (9) Luo, S. Y.; Chen, D. K. Power Syst. Eng. 2004, 20, 27–28. (10) Huang, W.; Li, W. J. Power Eng. 2005, 25, 813–819. (11) Kurose, R.; Ikeda, M.; Makino, H. Fuel 2001, 80, 1447–1455. (12) Spliethoff, H.; Greul, U.; Rudiger, H.; Klaus, R. S. Fuel 1995, 75, 560–564. (13) Coda, B.; Kluger, F.; F€ ortsch, D.; Spliethoff, H. Energy Fuels 1998, 12, 1322–1327. (14) Mana, C. K.; Gibbins, J. R.; Witkampb, J. G.; Zhang, J. Fuel 2005, 84, 2190–2195. (15) Burdett, N. A. J. Inst. Energy 1987, 60, 103–107.

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Figure 1. Scheme of the experimental facility for the 0.9 MW arch-fired furnace.

field in a small-scale test bench.18 Wang et al. investigated the aerodynamic experiments at room temperature on a 300 MW (electrical) W-shaped boiler and proposed the optimum air distribution.19 Ren et al. carried out an experimental study on the influence of the secondary air-box damper opening on airflow and combustion characteristics from a 300 MW (electrical) W-shaped boiler.20 Li et al. discussed the influence of declivitous secondary air on combustion characteristics of a down-fired 300 MW (electrical) utility boiler.21 Fan et al. simulated the combustion processes as well as NOx emission characteristics in an arch-fired boiler furnace.22,23 Zhou et al. worked on the measurements on flame temperature and its 3D distribution in a 660 MW (electrical) arch-fired coal combustion furnace by visible image processing and verification by using an infrared pyrometer.24 However, most experimental investigations mentioned above were carried out at room temperature or in utility boilers, and few combustion experiments were performed on a bench-scale arch-fired system where the air distribution could be changed in a wider range and where more operation conditions could be evaluated. On the other hand, few studies have been published on both NOx emission and combustible

loss simultaneously. Moreover, the effects of staged secondary air distribution on aerodynamic characteristics and combustion performance have been rarely studied. In this paper, the aerodynamic characteristics and combustion process with different air distribution schemes in a 0.9 MW arch-fired furnace are experimentally investigated. Influences of air distribution on flame penetration length, temperature distribution in the central furnace and on the wall of the furnace, combustible loss, and NOx emission are studied under different operation conditions. The characteristic parameters for the optimization of air distribution have been proposed, which may be of benefit for the design and operation of arch-fired boilers. 2. Experimental Details Figure1 is the schematic diagram of the test facility for a 0.9 MW (electrical) (heat input power) arch-fired furnace, which is actually a pilot model of a real arch-fired boiler in operation manufactured by Dongfang Boiler Group Co., LTD (DG). At the furnace throat, the furnace is divided into two parts, referred to as lower furnace and upper furnace, respectively. The lower furnace is the main combustion zone with height of 1.75 m and cross sectional area of 1.00  0.60 m2. The upper furnace is the burning-out zone with height of 1.80 m and cross sectional area of 0.50  0.60 m2. The furnace is built with refractory bricks inside and red bricks outside. Four pulverized coal feeders are installed on the feeder platform, which could feed 4 burners symmetrically located on the two arches. These feeders are calibrated using the measuring weight method for various operating conditions, and the pulverized coal feeding rate has an uncertainty of (3.5% of the measured value. Two burners are set on the front arch of the furnace and two on the rear. Each burner has two primary air nozzles and two vent air nozzles from which the primary air and vent air were injected into the furnace, respectively, but secondary air pipes as well as cyclones are not installed on the arches for the sake of simplification. All of the secondary air is gradually admitted through four

(16) Wei, X. L.; Xu, T. M.; Hui, S. E. Power Eng. 1994, 14, 27–32. (17) Yan, X.; Xu, W. J.; Sun, X. G.; Hui, S. E.; Xu, T. M. J. Eng. Therm. Energy Power 2001, 16, 263–266. (18) Xu, W. J.; Yan, X.; Sun, X. G.; Hui, S. E.; Xu, T. M. J. Xi’an Jiaotong Univ. 2001, 35, 108–110. (19) Wang, D. K.; Zeng, H. C. Cent. China Electr. Power 2001, 14, 18–21. (20) Ren, F.; Li, Z. Q.; Zhang, Y. B.; Sun, S. Z.; Zhang, X. H.; Chen, Z. C. Energy Fuels 2007, 21, 668–676. (21) Li, Z. Q.; Ren, F.; Chen, Z. C.; Chen, Z.; Wang, J. J. Fuel 2010, 89, 410–416. (22) Fan, J. R.; Liang, X. H.; Chen, L. H.; Cen, K. F. Energy 1998, 23, 1051–1055. (23) Fan, J. R.; Zha, X. D.; Cen, K. F. Fuel 2001, 80, 373–381. (24) Wang, H. J.; Huang, Z. F.; Wang, D. D.; Luo, Z. X.; Sun, Y. P.; Fang, Q. Y.; Lou, C.; Zhou, H. C. Meas. Sci. Technol. 2009, 20, 1–12.

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Figure 2. Measuring points distribution. Table 1. Proximate Analysis and Ultimate Analysis of Zhang Ze Coal (wt %, air-dry basis)

rows of different openings on the vertical walls underneath the arches, which are called D-layer air openings, E-layer air openings, tertiary air openings, and F-layer air openings, respectively. In the cold aerodynamic experiments, an IFA300 constanttemperature anemometer system with a 1240-type probe is used to measure the air velocity at different height and width of the furnace. The error for the velocity measurements is less than 2%. Both central temperature and wall temperature profiles are measured throughout the combustion experiments. The central temperatures along the height of the furnace are measured by water-cooled standard PtRh10-Pt thermocouples, and the wall temperatures along the height and width of the furnace are measured by NiCr-NiSi thermocouples. The measurement accuracy is (1.5 °C in the measurement range. All the temperature measurement points are shown in Figure 2, panels a-c, where panels a and b are the left wall view, and panel c is the front wall view. In Figure 2, h0 is the relative furnace height and defined as: h h ¼ H 0

b1 B1

ð1Þ

b2 B2

Car (%) Har (%) Oar (%) Nar (%) Sar (%) Aar (%) Mar (%) Vdaf (%) Qar.net kJ/kg

64.38 3.24 3.37 2.93 0.43 24.85 0.8 16.3 23 390

deformation temperature (DT)

softening temperature (ST)

fluid temperature (FT)

1450 °C

1490 °C

1500 °C

Figure 2c. After removal of the fly ash by a fly ash filter, the gas sample is heated up to about 180 °C, which is above the dewpoint temperature of flue gas. Concentrations of gases (NO, NO2, CO, CO2, SO2, H2O, etc.) are continuously determined by a GASMET FTIR Dx4000 flue gas analyzer, and the measurement accuracy is 0.1%; O2 concentration is determined by MSI compact flue gas analyzer instead of the GASMET FTIR Dx4000, and the measurement accuracy is 0.3%. On the basis of compositions of the coal, the flue gas concentrations, and the unburned carbon in fly ash and furnace slag, the total mass balance and carbon balance under various conditions are calculated by mass balance method. Zhang ze lean coal was used throughout the combustion experiments. The proximate and ultimate analysis data are given in Table 1. The ash slagging characteristics are shown in Table 2.

ð2Þ

where b1 is the distance of certain measurement point on the left wall from the central line of the wall (where b1 = 0), and B1 is the depth of the furnace, as shown in Figure 2, panels a and b. b0 2 is the relative furnace width and is defined by: b20 ¼

value

Table 2. Ash Slagging Characteristics

where h is the height of measurement points above the bottom of the furnace (where h = 0), and H is the total height of the furnace. b10 is the relative furnace depth and is defined as: b20 ¼

name

ð3Þ

where b2 is the distance of certain measurement point on the front wall from the central line of the wall (where b2 = 0), and B2 is the width of the furnace, as shown in Figure 2c. To measure the unburned carbon in the fly ash, an isokinetic sampling system is established to take fly ash from the exit of furnace. Fly ash is collected by a filter and combustible loss is measured by burning weight-loss method. The fly ash sampling point is shown in Figure 2c. In the flue gases measurement, a water-cooled stainless probe is used to take flue gases from a sampling point as shown in

3. Results and Discussion 3.1. Influence of Air Distribution on the Penetration Length of the Arch Air. As one of the most important aerodynamic characteristics, the penetration length of arch air is critical to the formation of a proper aerodynamic field in an arch-fired furnace and determines the flame penetration length in combustion conditions, which influences the position of the flame center and the mixing degree of combustion air 5516

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and fuel. Short penetration length can lead to earlier turning of flame, superheater overheating, high combustible loss, excessive NOx emission and so on, which is also called flame short circuit and should be avoided. In this section, we study the influence of air distribution on the penetration length of the arch air in several cold state experiments, which is useful for the further investigation into the effect of air distribution on the flame penetration length in combustion conditions. The velocity of the arch air decreases as it develops downward. We consider that the attenuation of the arch air finishes when the velocity of the main air stream reduces to 5 m/s. The penetration length of the arch air (PL) can then be defined as: L ð4Þ PL ¼ H where L is the distance from the furnace throat to the position where the velocity of the main air stream reduces to 5 m/s, and H is the height of the lower furnace. The value of PL should be kept in a certain range to obtain proper aerodynamic field in the furnace. It is reported that the optimum PL value lies between 0.375 and 0.58.17,18 The air flows could be considered as several cross jets. Therefore, the effects of air distribution on PL can be studied by analyzing the momentum ratio among jets. Nine different cases are studied in the cold state experiments, as shown in Table 3. The tertiary air is helpful to the adjustment of the aerodynamic field in the lower furnace but its effect is not considered in this paper. The velocity of tertiary air is thus kept at 12.5 m/s in all these cases. In the first six cases, the flow rate of the arch air is changed while the air distribution ratio of the secondary air is kept constant. The effect of these air distribution schemes on PL is studied by analyzing the momentum ratio of arch air to total secondary air, which is expressed as: . . m1k v1k þ mf vf ð5Þ M1 ¼ . . . . m2kD vD þ m2kE v2kE þ m3k v3k þ m2kF v2kF

Table 3. Operating Conditions in the Cold State Experiments secondary air velocity (m/s) primary vent air air velocity velocity case (m/s) (m/s) 1 2 3 4 5 6 7 8 9

32 28 25 25 18 15 28 28 28

15 10 7.1 7.1 5 3 10 10 10

D layer

E layer

F layer

5 6 6.5 7 7.5 7.8 9 4 2

5 6 6.5 7 7.5 7.8 10 5 3

5 6 6.5 7 7.5 7.8 3.3 7.8 9.1

M1

M2

2.63 12.54 1.53 6.20 1.06 4.05 0.96 3.50 0.45 1.57 0.29 0.98 1.36 2.46 1.33 10.79 1.20 33.77

Figure 3. PL change trend as M1 changes in cases 1-6.

where m_ 1k, m_ f, m_ 3k, m_ 2kD, m_ 2kE, and m_ 2kF represent the mass flow rates of primary air, vent air, tertiary air, D-layer secondary air, E-layer secondary air, and F-layer secondary air, respectively; v1k, vf, v3k, v2kD, v2kE, and v2kF denote the velocities of primary air, vent air, tertiary air, D-layer secondary air, E-layer secondary air, and F-layer secondary air, respectively; In the cases 2, 7, 8, 9, the volumetric flow rates of arch air and the total combustion air are kept constant while the secondary air distribution ratio is changed. The momentum ratio of arch air to D-layer and E-layer secondary airs is proposed to figure out the influence of different secondary air distribution on PL, which is expressed as: . . m1k v1k þ mf vf ð6Þ M2 ¼ . . m2kD vD þ m2kE v2kE

Figure 4. PL change trend as M2 changes in cases 1-6.

The relationship between PL and M1 in cases 2, 7, 8, 9 is shown in Figure 5, and the relationship between PL and M2 in cases 2, 7, 8, 9 is shown in Figure 6. It is shown in Figure 5 that PL no longer increases as the increase of M1 in these four cases, which is quite different from that in Figure 3. Therefore, the changing trend of PL could not be judged only by M1 value when the volumetric flow rate of arch air is kept constant and the secondary air distribution ratio is changed. On the other hand, it can be clearly seen from Figure 6 that the PL value first increases rapidly and then increases smoothly as M2 increases. Obviously, the PL value has a closer relationship with M2 than M1 under these conditions.

The relationship between PL and M1 in cases 1-6 is shown in Figure 3, and the relationship between PL and M2 in cases 1-6 is shown in Figure 4. It is indicated that PL increases with the increase of M1 or M2. The increasing rate is rapid when M1 or M2 is small, but slow when M1 or M2 is large. The maximum increasing rate happens when M1 = 1.1. The proper PL value, 0.375-0.58 as mentioned above, could be obtained within the range of 0.9-2.3 for M1 and 3-10 for M2. 5517

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Liu et al. Table 4. Operating Conditions during the Combustion Experiments secondary air velocity (m/s) primary air vent air velocity velocity case (m/s) (m/s) D layer 1 2 3 4 5 6

21 16 26 14 21 21

10 15 5 7 10 10

5 5 5 10 7.3 2

E layer

F layer

7 7 7 11 7.3 4

9 9 9 6.4 7.3 11.5

M01

1.34 4.42 1.08 3.56 1.75 5.76 0.53 1.17 1.40 3.10 1.23 16.20

SR ¼ ðQ1k þ Qf þ Q2kD þ Q2kE Þ=Qo Figure 6. PL change trend as M2 changes in cases 2, 7, 8, and 9.

0.67 0.67 0.67 0.81 0.75 0.52

ð9Þ

where Q1k, Qf, Q2kD, Q2kE, and Qo denote the volumetric flow rates of primary air, vent air, secondary air of layer D and layer E, and the theoretical volumetric flow rate of the combustion air, respectively. In the combustion experiments, six different cases are arranged, as shown in Table 4. The coal feeding rate is kept at 2.31 kg/min. The total volumetric flow rate of the combustion air is 0.28 N m3/s. The overall air stoichiometric ratio at the furnace exit is 1.15, and the gauge pressure in the experiments is kept at about -20 Pa. In this paper, we consider the tertiary air as a portion of secondary air, and the effect of different volumetric flow rates of tertiary air on the combustion is not investigated. The velocity of tertiary air is thus kept at 12 m/s in all these cases. 3.2.1. Effect of Air Distribution on the Central Temperature Profile. It is quite difficult to track one coal particle or one coal stream in the microscopic scale during the combustion process to learn the combustion characteristics of coal particles. However, the ignition and burn-out characteristics of coal particles could be indirectly studied in the macroscopic scale by measuring the central temperature along the furnace height. Therefore, the central temperature is of great importance. In this section, the effects of M01, M02, and SR on the central temperature distribution are studied respectively. The first four cases in Table 4 are arranged to figure out the effect of M01 on the central temperature profile. Secondary air distribution is kept unchanged in cases 1-3. Flame short circuit occurs in case 4, which is an inappropriate operating condition and is presented here for comparison with other cases. The experimental results are shown in Figure 7.

Therefore, M1 has the dominant influence on the penetration length of the arch air if the air distribution ratio of the secondary air is kept constant. PL increases as M1 increases. On the other hand, M2 has the dominant influence on PL if the volumetric flow rate of the arch air is kept constant and M2 < 10. However, the effect of M2 is negligible when M2 is greater than 10. The maximum value of PL will be obtained when the box dampers of D-layer and E-layer secondary air are closed completely. 3.2. Effect of Air Distribution on the Combustion Process. As mentioned above, the air distribution directly affects the penetration length of arch air, which is critical to the formation of a W-shaped flame in the furnace. To take into account the effect of coal particulates on the momentum of gas-solid flow in the combustion, eqs 5 and 6 can be rewritten to analyze the flame penetration length with different air distribution, which can be expressed as follows: . . ð1 þ KμÞm1k v1k þ mf vf ð7Þ M 10 ¼ . . . . m2kD vD þ m2kE v2kE þ m3k v3k þ m2kF v2kF . . ð1 þ KμÞm1k v1k þ mf vf . . m2kD vD þ m2kE v2kE

SR

On the other hand, the air distribution affects the air stoichiometric ratio in the furnace, which determines the chemical reaction atmosphere. During the combustion process, the pulverized coal is generally ignited near the D-layer and E-layer openings, as shown in Figure 1, where the air stoichiometric ratio is less than 1. As a result, a relatively low-temperature, fuel-rich, and oxygen deficient chemical atmosphere is formed in this zone, which helps reduce the formation of NOx. This combustion zone is referred to as preceding stage combustion zone in this paper. Unburned coal from the preceding stage combustion zone is then combusted near the T-layer and F-layer openings where the air stoichiometric ratio is greater than 1 and the combustion is in air-rich atmosphere. This area is referred to as primary combustion zone in this paper. According to the air staging combustion technology, the air stoichiometric ratio in the preceding stage combustion zone has a critical effect on the combustion performance and the NOx emission, which is represented by SR. Its expression is shown as eq 9:

Figure 5. PL change trend as M1 changes in cases 2, 7, 8, and 9.

M 20 ¼

M02

ð8Þ

where μ is the mass concentration of the fuel in the primary air; and K represents the velocity slip ratio between the coal particulates and the air stream, which was set as 0.8 in the experiments.17,18 5518

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Figure 7. Central temperature profile along furnace height in cases 1-4.

Figure 8. Central temperature profile along furnace height in cases 1 and 4-6.

It can be seen from Figure 7 that the central temperature increases first and then decreases. The position with the maximum central temperature is usually consistent with the flame center, so the flame center is defined in this paper as the point where the maximum central temperature is measured. The temperature changing trend is similar to that in the wall-fired furnaces, which reflects the general combustion process of coal particles. Furthermore, it is also illustrated that the temperature decreases more quickly in the lower furnace than that in the upper furnace. The reason for this phenomenon is that part of the recirculation flue gas from bottom of the lower furnace would preheat the fresh air and pulverized coal injected into the furnace from the arches except for the heat transfer to furnace walls. The main heat loss in the upper furnace is only the heat dissipation. The heat transferred from the recirculation flue gas is very conducive to the ignition of the coal. As mentioned before, flame penetration length is a very important parameter for the arch-fired combustion. If it is too large, the flame may scour the furnace hopper and lead to slagging. On the contrary, too small flame penetration length may result in the superheater overheating and slagging at the exit of the lower furnace. It is found in Figure 7 that the flame center moves upward and that the outlet temperature increases as the value of M01 decreases, which means that the flame penetration length decreases with the decrease of M01. In particular, we observed slagging at the furnace hopper in case 3 because of a larger M01, and the flame center moves to the exit of the lower furnace because of too small M01 in case 4, which leads to flame short circuit and higher outlet temperature. Both case 3 and case 4 should be avoided in practical operation. Additionally, although the ignition temperature decreases as the concentration of pulverized coal increases by reducing the velocity of the primary air and M01, the flame penetration length will be shortened and the volumetric flow rate of hot recirculation flue gas will be reduced like that in case 4, which is a great disadvantage to the combustion. We do not recommend reducing the ignition temperature with much lower velocity of the primary air. Therefore, sufficient momentum of arch air should be ensured in the combustion process and M01 should be adjusted into a rational range. The height of flame center in case 1 is approximately equal to that of the middle of F-layer opening, which is widely accepted as an ideal height.

We recommend in this paper that the value of M01 be set at about 1.3, as in case 1 under combustion conditions. Cases 1, 4, 5, and 6 in Table 4 are arranged to investigate the effect of M02 on the central temperature profile. The volumetric flow rate of the primary air and vent air is kept constant in cases 1, 5, and 6 while the volumetric flow rate of the secondary air is changed. Similarly, case 4 is arranged for comparison with other cases, in which flame short circuit happens. The experimental results are shown in Figure 8. It is illustrated in Figure 8 that the changing trend of the temperature along the furnace height is very similar to what we observed in Figure 7. However, some differences still exist. M01 in case 5 is greater than that in case 1, but the flame center is higher in case 5 than that in case 1. M01 in case 6 is smaller than that in case 1, but the flame center is lower in case 6 than that in case 1. Therefore, M01 is not the only influencing factor here and it can be concluded that the distribution of the secondary air is critical to the position of flame center if the volumetric flow rate of the arch air is constant. It could be found that the position of flame center moves downward as M02 increases, which means that the flame penetration length increases. Obviously, the flame penetration length will be dominantly influenced by M01 again if the volumetric flow rate of the secondary air is very small, and M02 increases up to a certain value as in the analysis of cold aerodynamic field. In the practical operation, we recommend that the value of M02 be set at about 4.4 as that in case 1 for better central temperature distribution. Except for different positions of temperature peaks, the values of temperature peaks are also different, as shown in Figures 8. We think that the maximum temperature values are closely related to SR. In cases 1 and 4-6, different SR values indicate different air staging extents because the volumetric flow rate of the arch air is kept constant in the experiments. Figure 9 shows the change trend of maximum temperature values in these cases, as SR increases. It can be seen from Figure 9 that the maximum temperature value first increases and then decreases as SR increases. The maximum value occurs when SR = 0.67 in the experimental range. The reason for these phenomena can be explained based on air staging technology. If SR is smaller, just like in case 6, insufficient oxygen is supplied to the coal combustion after ignition, which is a disadvantage to the combustion. Too much unburned coal then enters the primary combustion zone, and part of them may not participate in the combustion. Therefore, the maximum temperature is 5519

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Figure 9. Maximum temperature values as SR increases. Figure 11. Temperature distribution on the central-cross section in case 4.

the lower furnace. The maximum temperature in the experiments is 1454 °C where h0 = 0.43 and b0 1 = 0. Flame short circuit happens. Under this condition, the temperature level in the lower furnace decreased significantly, about 130170 °C below the normal temperature level. Severe slagging occurred on the two arches and at the exit of the lower furnace. It can be concluded by analyzing the aerodynamic experiments and the central temperature distribution under combustion conditions that the optimum value of M01 is about 1.3 and M02 is about 4.4. 3.2.2. Effect of Air Distribution on the Inner Wall Temperature Distribution. It is well-known that the heat flux distribution on the water wall is critical to the optimum design and reliable operation of heat-absorbing surface. Because the measurement of inner wall temperature is much easier than that of heat flux and the heat flux distribution could be evaluated from the inner wall temperature distribution theoretically, inner wall temperature instead of heat flux is measured during the experiments to investigate the distribution characteristics of heat flux on the furnace wall. Test results in case 1 and case 4 are analyzed here, which represent the normal and the worse operation conditions, respectively. The arrangement of the measuring points is shown in Figure 2b. It is indicated in Figure 12 that the temperature distribution on the left wall in case 1 is nonuniform along furnace depth, which is nearly bilateral symmetry. Temperature on the central section of the wall is much higher than that on the two sides. The maximum temperature is 1380.5 °C where h0 = 0.28, in accordance with the height of the flame center. It is shown in Figure 13 that the temperature distribution on the front wall is nearly uniform along furnace width on the lower furnace wall, but is slightly nonuniform on the upper furnace wall. The wall temperature distribution depends on the aerodynamic field formed in the combustion. Because the primary air nozzles and vent air nozzles are established on the arches in equal distance and the secondary air openings are installed underneath the two arches in the same spacing as that of primary air nozzles, the air flow distribution near the front wall and the temperature distribution on the front wall are nearly uniform along furnace width. However, the uniformity will be destroyed by upward flue gas in the upper furnace.

Figure 10. Temperature distribution on the central cross-section in case 1.

lower. If SR is greater, just like in cases 4 and 5, too much air is injected into the preceding stage combustion zone and the flame penetration length is shortened. Therefore, lots of coal could not enter the primary combustion zone but goes up to the upper furnace directly, which leads to lower maximum temperature in the lower furnace. Therefore, it is illustrated in Figures 7 and 8 that the central temperature distribution in case 1 is reasonable whereas that in case 4 should be avoided. In order to compare the characteristics of the temperature distribution in case 1 and case 4, temperatures on the central cross-section where b0 2 = 0 are measured in the experiments and the twodimensional temperature profile in these two cases are shown in Figures 10 and 11, respectively. It is shown in Figure 10 that temperature distribution across the central cross-section in case 1 is nearly bilateral symmetry. Temperatures near the furnace side walls are lower, while temperatures in the furnace center are much higher, which is a typical temperature distribution in archfired boilers. The maximum temperature in the experiments is 1525 °C where h0 = 0.25 and b0 1 = 0.1. The height of the flame center is between the central line of F-layer opening and the upper side of the furnace hopper, and it is considered appropriate. It is shown in Figure 11 that temperature distribution on the central cross-section in case 4 is still bilateral symmetry like that in case 1. However, the flame penetration is insufficient, and the flame center moves up to the exit of 5520

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Figure 12. Temperature distribution on the left wall in case 1.

Figure 14. Temperature distribution on the left wall in case 4.

Figure 15. Temperature distribution on the front wall in case 4.

Figure 13. Temperature distribution on the front wall in case 1.

On the other hand, the nonuniform temperature distribution on the left wall is primarily results from the W-shaped flame formed in the furnace, as shown in Figures 10 and 11. Furthermore, because the flame usually scours the central section of the left wall and the lower furnace outlet, slagging was observed at these positions during the experiments, and the related wall temperatures were relatively higher. Figures 14 and 15 are the temperature distribution on left wall and front wall in case 4. The maximum temperature is 1355.7 °C where h0 =0.28, in accordance with the height of the flame center. The maximum wall temperature decreases but its height moves upward in comparison with that in case 1. Besides, the temperature level on the upper furnace walls increases and is much higher than that in case 1. We define tAV to denote the average wall temperature at certain height, which is expressed as: n X tAV ¼ ti =n ð10Þ

Figure 16. Average wall temperatures along furnace height.

The changing trends of ti and δ on the left wall are illustrated in Figures 16 and 17, respectively. It is found in Figure 16 that tAV in case 4 is much lower than that in case 1 on the lower furnace wall, but is much higher on the upper furnace wall, especially near the upper furnace inlet. We believe that these phenomena are resulted from the flame short circuit and higher flame center in case 4. In this case, the temperature on the upper wall rises and the outlet gas temperature goes up, which induces superheater overheating and is harmful to the operation reliability. Besides, severe slagging happens near the furnace throat in this case.

i¼1

where ti is the wall temperature at the measuring point i; n denotes the number of measuring points. δ is defined to represent the nonuniformity coefficient of wall temperatures at certain height. It could be written as: δ ¼ ðtmax - tmin Þ=tAV

ð11Þ

where tmax and tmin are the maximum and minimum wall temperatures at certain height, respectively. 5521

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Figure 17. Nonuniformity coefficient of wall temperatures along furnace height.

Figure 18. Combustible loss and NOx emission with different SR.

It is shown in Figure 17 that δ on the left wall first increases and then decreases in both the normal operation condition like in case 1 and the worse one like in case 4. The maximum δ appears at the outlet of the lower furnace. Moreover, δ in case 4 is larger near the furnace bottom and is smaller on the upper furnace wall than that in case 1. Additionally, δ in case 4 is much larger near the outlet of the lower furnace than that in case 1. 3.3. Effect of Air Distribution on the Combustible Loss and NOx Emission. Air staging combustion is one of the useful technologies in the reduction of NOx emission in the flue gas. In arch-fired boilers, air staging combustion is generally carried out by gradually admitting the secondary air through air openings on the vertical walls underneath the arches. Therefore, the effect of air distribution, especially the secondary air distribution, is quite important to the NOx emission as well as the combustible loss. In this section, we introduce the parameter SR to evaluate the air staging extent and investigate the effect of air staging with different secondary air distribution on the combustible loss and NOx emission. The operating conditions are cases 1 and 4-6, as shown in Table 4. The combustible loss in pulverized coal boilers mainly consists of two parts: carbon loss in the fly ash and carbon loss in the furnace slag, both of which are measured in our combustion experiments. Figure 18 illustrates the change trend of carbon loss in the fly ash and in the slag. It can be seen that the carbon content both in the fly ash and in the slag first descends and then ascends with the increase of SR in the experimental range. The reason for these phenomena is related to the effect of SR on the combustion process. Neither the ignition nor the devolatilization process of the coal is finished completely in the preceding stage combustion zone. If SR is too large, just like in cases 4 and 5, more air is sent from D&E openings into the preceding stage combustion zone. The flame penetration length is shortened as shown in Figures 8, 10, and 11, and more unburned coal could not enter the primary combustion zone but goes up to the upper furnace directly, which leads to an increase of combustible loss. Additionally, the maximum temperature in the furnace decreases, as shown in Figures 8-11, which is harmful to the coal burnout. Therefore, the combustible loss increases when SR is too large. On the contrary, if SR is too small just like in case 6, insufficient air is supplied to the combustion after the ignition and the combustion process is delayed. Moreover, the maximum

temperature is relatively lower as shown in Figure 9, which is also not conducive to the combustion. Therefore, more combustible loss is caused when SR is small. The relationship between NOx emission and SR during coal combustion is also illustrated in Figure 18. It can be seen that the NOx emission first decreases and then increases promptly with the increase of SR in the experimental range. Some explanations are as follows. On the basis of the air staging combustion theory, the temperature in the preceding stage combustion zone is related to the air stoichiometric ratio in this zone, which is represented by SR in the paper. When SR is smaller, less oxygen is sent into the preceding stage combustion zone. Therefore, coal combustion in this zone is restrained and the temperature level in this zone is lower. When SR is greater, oxygen is relatively sufficient in the preceding stage combustion zone and the temperature level in this zone is higher. The same results could be obtained by comparing the temperature distribution on the central cross-section in Figure 10 and 11. The temperature level in the preceding stage combustion zone is higher in Figure 11 than that in Figure 10, because the SR value in case 4 is greater than that in case 1. The temperature level in the preceding stage combustion zone then affects the NOx emission. When SR is too small, strongly reducing atmosphere is formed in the preceding stage combustion zone, which is helpful to the NOx emission reduction. However, more coal is burnt incompletely because of the lower temperature level in this zone and fuel-N in the coal is not released sufficiently. Large amount of unburned carbon with high fuel-N content then enters the primary combustion zone below and reacts with the rich oxygen in this area, which finally results in more NOx emission at the exit of the furnace. On the contrary, if SR is too large, the temperature level is higher and a weakly reducing atmosphere is formed in this zone, which abates NOx reduction and results in higher NOx emission. It can be seen from Figure 18 that values of both combustible loss and NOx emission first decline and then go up. There must be an optimum SR for better combustion performance if other variables are kept constant. In order to find out the optimum values of SR, we fit three polynomial curves to the data points by nonlinear least-squares method to illustrate the changing trend of NOx emission, unburned carbon in the fly ash, and unburned carbon in the slag when 5522

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SR is changed. Meanwhile, three equations for the curves are derived as shown in eqs 12-14. NOx ðSRÞ ¼ 9932:9ðSRÞ2 - 11755:9ðSRÞ þ 3902:0

air distribution on the combustible loss and NOx emission in the same furnace. In the cold-state experiment, if the air distribution ratio of the secondary air is kept constant, the penetration length of arch air (PL) increases as M1 increases. The proper PL value could be obtained when M1 = 0.9-2.3 and M2 = 3-10. If the volumetric flow rate of arch air and the total combustion air is unchanged but the air distribution of secondary air changes, M2 determines PL and PL increases with an increase of M2. However, the effect of M2 is negligible when M2 is greater than 10. The combustion experiments show that the position of the flame center and the temperature distribution in the central furnace is directly affected by the air distribution. If air distribution of the secondary air is kept constant, the flame center descends and the outlet temperature decreases as M01 or M02 increases. If the volumetric flow rate of arch air is unchanged, the position of the flame center decreases with the increase of M02. Furthermore, the inner wall temperature distribution shows that the temperature profile on the left wall has bilateral symmetry along furnace depth and that the temperature profile on the front wall is nearly uniform along furnace width in both the lower furnace and the upper furnace. The maximum nonuniformity coefficient of inner wall temperature (δ) appears at the lower furnace outlet. On the other hand, the inner wall temperature on the upper wall and the outlet gas temperature go up, and severe slagging happens near the furnace throat when flame short circuit occurs, which induces superheater overheating and is harmful to the operation reliability. In the experimental range, the optimum operation condition appears when M01 = 1.34 and M02 = 4.42, in which the temperature distribution is proper and severe slagging is avoided. The effect of air distribution, especially the secondary air distribution, on the combustible loss and NOx emission has been analyzed. It is illustrated that both combustible loss and NOx emission first decrease and then increase as SR increases. The minimum values of unburned carbon in fly ash, unburned carbon in the slag, and NOx emission are obtained when SR = 0.67, 0.63, and 0.59, respectively. In addition, both the combustible loss and NOx emission attain proper values simultaneously when SR = 0.634.

ð12Þ

FaðSRÞ ¼ 1320:7ðSRÞ3 - 2305:2ðSRÞ2 þ 1307:8ðSRÞ - 228:0 ð13Þ SaðSRÞ¼ - 636:9ðSRÞ3 þ 1465:0ðSRÞ2 - 1087:3ðSRÞ þ 271:3 ð14Þ where NOx(SR), Fa(SR), and Sa(SR) are functions of the parameter SR, which represent values of NOx emission, unburned carbon in the fly ash, and unburned carbon in the slag, respectively. We calculate the first derivatives of NOx(SR), Fa(SR), and Sa(SR) with respect to SR and find that NOx(SR), Fa(SR), and Sa(SR) reach minimal values when SR=0.59, 0.67, and 0.63, respectively. Furthermore, we take values of NOx(SR), Fa(SR), and Sa(SR) in case 1 as the base value and then calculate the first derivative of the sum of relative values of NOx(SR), Fa(SR), and Sa(SR). Obviously, the minimal value could be acquired when the derivative equals zero as defined in eq 15.   d NOx ðSRÞ FaðSRÞ SaðSRÞ þ þ ¼ 0 ð15Þ dðSRÞ 453 10:6 8:9 By solving eq 15, we find that the total value of combustible loss and NOx emission reaches a minimal when SR = 0.634. Obviously, eq 15 does not have a clear physical meaning, but we could keep both combustible loss and NOx emission in an acceptable range by solving it, which could be considered as an optimum value for the air distribution in arch-fired boilers. 4. Conclusion In the experiments, two parameters M1 (M01) and M2 (M02) have been presented to illustrate the influence of air distribution on the aerodynamic field in an arch-fired furnace, and a parameter SR has been presented to illustrate the influence of

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