Energy Fuels 2010, 24, 38–45 Published on Web 08/11/2009
: DOI:10.1021/ef900476c
Influence of Staged-Air on Combustion Characteristics and NOx Emissions of a 300 MWe Down-Fired Boiler with Swirl Burners† Zhengqi Li,* Subo Fan, Guangkui Liu, Xuehai Yang, Zhichao Chen, Wei Su, and Lin Wang School of Energy Science and Engineering, Harbin Institute of Technology, 92, West Dazhi Street, Harbin 150001, P.R. China Received May 18, 2009. Revised Manuscript Received July 21, 2009
Using an IFA300 constant temperature anemometer system, cold air experiments on a quarter-scaled burner model sited in a 300 MWe down-fired boiler were carried out to investigate the influence of various relative secondary air ratios on the flow characteristics in the burner nozzle region. With decreasing secondary air ratio, axial velocities and entrained flue gas decreased, as well as tangential velocities and rates of jet diffusion. However, the more that secondary air obstructs the diffusion of primary air, rates of primary air diffusion increased. Industrial-sized experiments were performed on a 300 MWe down-fired boiler with swirl burners. Gas temperature, concentrations of gas components (O2, CO, and NOx) in the burn region and combustible content in the fly ash were measured over a range of staged-air damper openings of 15, 25, and 35%. With openings of 15 and 25%, the level of pulverized-coal burnout is high, although NOx emissions were also a little higher. The level of burnout degree decreased for a 35% opening, and exhaust gas temperatures increased. Although the boiler efficiency decreased, NOx emissions also decreased.
1. Introduction
2. Description of the EI-XCL Burner
NOx is an extremely toxic pollutant harmful to human health and damaging to the atmosphere. Its main source derives from primary emissions of coal-fired power plants into the air. Air-staged combustion techniques are currently widely used and are a comparatively mature low-NOx combustion technology. Much research into these techniques, particularly on tangential fired and wall-arranged boilers, have been conducted and have attained significant achievements.1-4 Ren et al.5,6 have studied the influence of varying secondary air flow and vent air valve openings on the whole furnace flow field. Burdett carried out industrial tests to investigate the effects of air-staging on NOx emissions from a 500 MWe down-fired boiler unit.7 However, air-staging combustion techniques of down-fired boiler with swirl burner have been rarely studied. In this paper, cold air-flow experiments were carried out on a quarter-scale model of an enhanced ignition-axial control low-NOx (EI-XCL) burner. For various secondary air flows, data has been obtained with an IFA300 constant-temperature anemometer system in a temperature tracer test facility. To study the influence of varying staged air damper openings on NOx formation characteristics of a full-scale EI-XCL burner, industrial experiments were performed on a down-fired boiler.
Figure 1 shows the EI-XCL burner, which has a set of 14 radial vanes in both the inner and outer secondary air ducts. The swirling directions of the inner and outer secondary airflows are identical. Under the influence of a particle deflector and a conical diffuser, pulverized coal carried by the primary air diffuses along the radius and gathers in the region close to the primary air tube wall, which results in a coal-rich flow in the peripheral zone of the primary air and a coal-lean flow in the central zone. 3. Cold Air Experiments The staged air ducts of the down-fire boiler with swirl burners are arranged on the front and rear walls, which is joined with the secondary air box and directed into the furnace. The angle of these ducts with the horizontal is 30, and the air flow through it is controlled by the staged air dampers. By increasing the opening of these dampers, the staged air flow increases and secondary air flow on the arch decreases, while primary air flow remains unchanged. With an EI-XCL burner model, a quarter-scale version of the prototype, cold experiments have been carried out in the laboratory. The relative secondary air ratio is defined as the ratio of secondary air flow in the experiment to the secondary air flow with staged-air damper opening of 5%; in the test the secondary air ratio was varied while the primary air flow was kept constant. The detail test parameters are list in Table 1. 3.1. Flow Characteristics of the Burner Jet. The test facility is illustrated in Figure 2, where x is the distance to the exit of the burner along the jet flow direction, r is the radial distance from the chamber axis, and d is the diameter of the outer secondary air duct (d = 280 mm). An IFA300 constant temperature anemometer system was used to measure air velocities at various sites. Using a probe with hot-film
† Presented at the 2009 Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies. *To whom correspondence should be addressed. Telephone: þ86 451 86418854. Fax: þ86 451 86412528. E-mail:
[email protected]. (1) Tree, Dale R.; Webb, Brent W. Fuel 1997, 76, 1057–1066. (2) Costa, M.; Silva, P.; Azevedo, J. L. T. Combust. Sci. Technol. 2003, 175, 271–289. (3) Yin, C.; Caillat, S.; Harion, J.-L. Fuel 2002, 81, 103–107. (4) Li, S.; Xu, T.; Hui, S.; Wei, X. Applied Energy 2009, 86, 1797-1803. (5) Ren, F.; Li, Z.; Zhang, Y. Energy Fuels 2007, 21, 668–676. (6) Ren, F.; Li, Z.; Zhang, J. Fuel 2007, 86, 2457–2462. (7) Burdett, N. A. J. Inst. Energy 1987, LX, 103–107.
r 2009 American Chemical Society
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Energy Fuels 2010, 24, 38–45
: DOI:10.1021/ef900476c
Li et al.
Figure 1. EI-XCL burner and position of the monitoring pipe (dimensions in mm): (1) primary air duct, (2) secondary air valve, (3) inner secondary air duct, (4) inner secondary air duct vanes, (5) adjustable outer secondary air vanes, (6) fixed outer secondary air vanes, (7) outer secondary air duct, and (8) monitoring pipe.
Figure 3. Spatial profiles of axial mean velocities for various relative secondary air ratios.
Table 1. Parameters for the Cold Air Experiments parameter relative secondary air ratio air velocity primary (m/s) air inner secondary air outer secondary air air temperature (C)
case 1 100%
case 2 90%
case 3 80%
case 4 70%
16.89
16.89
16.89
16.89
13
12
11
10
26
24
22
20
20
Figure 4. Decay of the maximum axial velocity.
secondary air ratio of 100%. As the flow nears cross-section x/d=0.5, the maximum axial velocity value drops to 34.9 m/s and drops further to 16.3 m/s as the flow reaches cross-section x/d=1.5. Subsequently, the velocity decays slowly as the flow moves toward x/d = 3, and finally the peak velocity value disappears in secondary zone. The jet velocity tends to unity, and the average velocity is about 10 m/s. With decreasing secondary air ratios, the radial velocity difference in the primary air (r/d = 0-0.2) and inner secondary air zone (r/d = 0.2-0.4) is small, but the maximum axial velocity clearly decays in the secondary air zone. When the secondary air ratio is set at 90%, the peak velocity value is 54.7m/s and falls to 45.5m/s when the secondary air ratio is 80%. The decay of axial velocities at these two ratios are similar to that at 100%; the velocity decays rapidly in the early section of the jet flow (x/d = 0-1.5) and then slows (x/d = 1.5-3). Near cross-section x/d = 3, there is no obvious peak value, and the average velocity is about 10 m/s. With a secondary air ratio of 70%, the peak velocity value in the outer secondary air zone is 34 m/s, when the flow reaches cross-section x/d = 0.5, the maximum axial velocity has decayed to 24.1 m/s, from where the maximum velocity decays linearly. At cross-section x/d = 0.5, the axial velocity is similar to other ratios. As seen in Figure 4, the decay rate of the maximum velocity decreases in the initial jet flow with decreasing secondary air ratio, but increases in the latter jet flow. Comparison of the maximum axial velocities at x/d = 3 for the four secondary air ratios shows that they are nearly equal
Figure 2. Cold air experiment systems.
sensors, we measured the three-dimensional (3D) flow field at the exit of the burner.8 The error for all velocity measurements was less than 5%. Figures 3 and 4 show, respectively, spatial profiles of the average axial velocity and maximum axial velocity given fixed relative secondary air ratios of 100, 90, 80, and 70%. As can be seen in Figure 3, there was no center recirculation zone for any of the four secondary air ratios. Axial velocities have two obvious peak values along the radial direction between cross sections x/d=0 and 0.5. The peak near the center is formed by primary air flow, and the peak further away from the center is formed by secondary air flow. The peak value in the outlet secondary air zone (r/d = 0.4-0.5) is 59.4 m/s with a (8) Mueller, U. R. Fluid Mech. 1982, 119, 155–172.
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Energy Fuels 2010, 24, 38–45
: DOI:10.1021/ef900476c
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Figure 7. Spatial profiles of the maximum content of primary air distribution for various relative secondary air ratios.
Radial velocities at the jet flow boundary are large, and the position of the positive peak value moves to the inside as the secondary air ratio decreases. At cross-section x/d = 1, the peak radial velocity values for secondary air ratios of 100 and 90% are nearly at r/d = 0.8; when the secondary air ratio decreases to 80 and 70%, these peak values are at r/d = 0.7. This indicates that the inner jet flow is relatively constant and independent of any change in secondary air ratios, although diffusion velocities decrease as secondary air ratios decrease. Figure 6 shows that at cross-section x/d = 0 the tangential velocity has two peak values for all four air ratios. The smaller peak value near the burner center is formed by the inner secondary air flow; the greater lateral peak value is formed by the outer secondary air flow. Comparing tangential velocity distributions of each cross-section, the overall profiles are little affected by changing the secondary air ratio, but values do decrease slightly with decreasing secondary air ratio. At a secondary air ratio of 100%, the maximum tangential velocity value is higher than the other ratios, but the decay rate is rapid in the initial jet flow. Between cross-sections x/d = 0.5-1.5, the tangential velocity values at 100% are close to those at 90%, although their values are higher downstream from cross-section x/d = 2. Tangential velocity values are similar for both 90 and 80% ratios with the decay rates being nearly the same as well. Tangential velocity curves are almost coincident except at cross-section x/d = 1, where the velocity value of 90% is obviously higher than 80%. At cross-section x/d = 2, tangential velocities of the two ratios decay almost completely with only weak values. For a secondary air ratio of 70%, the maximum tangential velocity at cross-section x/d = 0 is 8.11 m/s but falls to 2.94 m/s at cross-section x/d=1.5. The maximum tangential velocity value at this ratio is lower than for the other three air ratios. Tangential velocities vanish as the jet flow reaches cross-section x/d=2. 3.2. Investigation of Mixing Characteristics. Mixing characteristics of primary and secondary air were investigated with various settings of outer secondary-air vane angles using a particular temperature indication method.9,10 This method involves heating the primary air to 50 C while maintaining the secondary air at ambient temperature. A Cu50 thermocouple was used to measure gas temperature, the error being (0.8 C.
Figure 5. Spatial profiles of radial mean velocities for various relative secondary air ratios.
Figure 6. Spatial profiles of tangential mean velocities for various relative secondary air ratios.
at about 15 m/s. The result indicates that the secondary air ratio has some effect on the burner outlet nearby, but has little effect on the jet formation in the flow. Figures 5 and 6 show the dependence on relative secondary air ratios of radial profiles of the average radial and tangential velocities. Figure 5 shows that the radial velocity distributions for given cross-section x/d are very similar across all secondary air ratios. At cross-section x/d = 0, radial velocities vanish in the primary air zone but are negative in the inner secondary air zone (r/d=0.2-0.4), while the peak velocity value forms in the inner secondary air zone, indicating that the airflow is toward the center line in the inner secondary air zone. Radial velocities have positive value in the outer secondary air zone (r/d = 0.4-0.5), signifying that the jet flow diffuses to both sides; radial velocities near the jet boundary are negative. At cross-section x/d = 0.5, radial velocities in primary air zone vanish, but become negative at the boundary, which indicates that the inner secondary air is starting to compress the primary air. At cross-section x/d=1, radial velocities in the jet flow zone are small, with the velocities of primary air and secondary air mixing within the jet flow decrease.
(9) Truelove, J. S.; Wall, T. F.; Dixon, T. F.; Mac Stuart, I. 19th Symposium (International) on Combustion 1982, 1181–1187. (10) Richter, Oliver; Hoffmann, Heike; Kraushaar-Czarnetzki, Bettina Chem. Eng. Sci. 2008, 63, 3504–3513.
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Energy Fuels 2010, 24, 38–45
: DOI:10.1021/ef900476c
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Figure 9. Extended boundary of primary air for various relative secondary air ratios. Figure 8. Spatial profiles of the maximum radial mixture velocities distribution for various relative secondary air ratios.
Let f1 denote the relative concentration of primary air to total air comprising primary air, secondary air and surrounding air. It is related to air flow temperatures as follows: f1 ¼
tm - t2 100% t1 - t2
ð1Þ
where t1 is the temperature of primary air, t2 the temperature of the outlet secondary air, and tm the temperature of the measuring point. From measuring errors in gas temperatures, the percentage errors in f1 are less than 3%. Let ΔC denote the relative mixing rate of primary air with secondary air and surrounding air per axial distance. It is defined as follows: fðn -1, 1Þ - fðn, 1Þ ð2Þ ΔC ¼ xn xn -1 d - d
Figure 10. Spatial profiles of the content of primary air distribution for various relative secondary air ratios.
where n indexes the measuring points and f(n,1) is the relative concentration of primary air at the nth point. The error in ΔC is the same as that of f1. Figure 7 shows the variation of the maximum relative concentration of primary air along axial distance at different cross sections. At cross-section x/d = 0.5, maximum relative concentrations are similar for all secondary air ratios at each sections. Between cross sections x/d = 1-3, the decay rate of the maximum relative concentration increase with decreasing ratios. Overall trends in the maximum primary air relative concentrations against ratios of 100, 90, and 80% are essentially similar at each cross-section, although a slight decrease in the maximum primary air relative concentration with decreasing secondary air ratio can be observed. As the jet flow reaches cross-section x/d = 4, the primary air maximum concentration has dropped to about 20% for these three air ratios. With respect to the secondary air ratio of 70%, the maximum primary air relative concentration decreases to less than 60% at cross-section x/d = 1, whereas the maximum primary air concentration for the other three ratios is about 70%. Before cross-section x/d = 4, the maximum primary air concentration for the 70% ratio is lower by about 10% of those for the other three ratios. At crosssection x/d = 4, the maximum primary air concentration for the 70% ratio also has dropped to around 20%. Figure 8 shows the maximum radial mixing rate of primary air ΔCmax along the radial direction at different cross sections. Between cross sections x/d = 0 and 1 the maximum
mixing rate clearly decreases, and the mixing rates decreases with decreasing air ratio. With a secondary air ratio of 100%, the mixing rates decay rapidly out to cross-section x/d = 1; after which the mixing rates fall off linearly. With 90% and 80 ratios, the maximum mixing rate remains unchanged in the region between cross sections x/d = 1 and 2, but thereafter this rate decays quickly. As the ratio decreases to 70%, the maximum radial mixing rates also decrease rapidly in the region between the cross sections x/d = 1 and 2 and slows thereafter. At cross-section x/d = 4, the maximum radial mixing rates of each of the four ratios are basically uniform. Figure 9 shows the diffusion boundary of the primary air at various cross sections, that is, the radial distance where the primary air concentration reduces to 0%. As can be seen in Figure 9, for fixed cross-section the primary air boundary expands with decreasing secondary air ratio. Along the axial distance for each of the four ratios the primary air boundary constricts and then expands. Comparing with Figure 4, the cause lies with the fact that the inner secondary air radial velocity is negative and therefore centrally directed. Thus, secondary air compresses the primary air in this region. At a ratio of 100%, the minimum radius of the primary air boundary is at cross-section x/d = 0.5, which at 90, 80, and 70% ratios shifts to cross-section x/d = 1, with radius increasing with decreasing secondary air ratios. Figure 10 shows the variation of primary air concentration along the radial direction for various cross sections. The 41
Energy Fuels 2010, 24, 38–45
: DOI:10.1021/ef900476c
Li et al.
curves show that at cross-section x/d = 0, the primary air concentration varies slightly with changing ratio. In the secondary air flow region, the primary air concentration increases with decreasing secondary air ratios. Because the relative secondary air ratio decreases, the relative primary air
flow increases. The difference in the primary air concentration within the central region is small. 4. In Situ Industrial Experiments In situ experiments were carried out in a 300 MWe boiler manufactured by Babcock and Wilcox Beijing Company Limited (see Figure 11 for a schematic diagram). The characteristics of this type of boiler are subcritical pressure, natural circulation, P-type layout, double arches, and single furnace. Pulverized coal is injected by hot air. Sixteen EI-XCL burners are symmetrically arranged on the arches (shown in Figure 1). Vent air from the coal mill enters the furnace from the front and rear walls. The angle between the gas vent pipe and the horizontal is 28. A series of pipelines is connected with the secondary air box under the air vent pipe. The angle between this pipe and the horizontal is 30. An air flow called staged air is sent into the furnace from the front and rear walls (Figure 1). The staged air flow is controlled by the damper opening. Gas temperatures near the left side wall and in the burner region and gas concentrations near the left side wall and at the exit of the air preheater were measured. Temperatures were measured by a thermocouple through monitoring ports. Figure 11 shows the measurement locations on the sidewall. The water-cooled stainless-steel probe, shown in Figure 12, includes a water-in pipe, a water-out pipe, a sampling pipe, and outer pipe. The high-pressure cooling water entering from the water-in pipe cools the sampling pipe and after heat exchange exits via the water-out pipe. The gas was sampled by the sampling pipe. If smoke enters the sampling pipe, temperatures decease rapidly and the pulverized coal stops burning. The samples pass through filtering devices into a Testo 350 M gas analyzer to be analyzed. The coke sample was obtained with a vacuum pump and sampling pipe, between which there is a cyclone separator, coke collector, flow meter, and other devices. The accuracy of the Testo 350 M gas analyzer for each species measurement is 1% for O2 and CO2, 5% for CO, and 5 ppm for NO and NO2. Calibration was carried out on each sensor before measurement. During the experimental procedures, the utility boiler was operating stably at the rated load. Table 2 shows characteristics of the coal used in the experiments. Table 3 summarizes the boiler design and the operation parameters; the operation parameters are averaged over the duration of the experimental procedures.
Figure 11. Schematic of burner showing locations of key features (dimensions in mm).
Figure 12. Water-cooled stainless steel probe.
Figure 13. Gas temperature distribution in the burner region with different staged air damper opening.
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Energy Fuels 2010, 24, 38–45
: DOI:10.1021/ef900476c
Li et al.
Figure 13 shows the profiles of gas temperature in the burner nozzle region. The temperature in the region of No. 1 burner is higher than that of No. 2 burner (Figure 11) which is closer to the side wall. The staged air damper opening was set at either 15 or 25%. The temperature in the region of the two burners initially increased to a maximum, because of the influence of the external recirculation. Subsequently, gas temperatures gradually decreased for damper opening of 15%. From the cold air experiments, the extended boundary of the primary air is small in this case, and with the primary air jet flow diffusing outward toward the measuring area temperatures are brought down. For a damper opening of 25%, gas temperatures in the burner closer to the center line of the furnace at first decrease slightly and then increase to a peak value. This is because with decreasing secondary air flow hightemperature gas entrained by the external recirculation and the primary air mix rapidly, pulverized coal is ignited, and hence temperatures in this area increase rapidly. However, closer to the side wall, the temperature of the gas is lower, pulverized coal does not ignite, and therefore temperatures increase only slightly, maintaining a steady level. With the staged air damper opening increased to 35%, tangential velocities of secondary air decrease, and
Table 2. Coal Characteristics Proximate Analysis (air-dried wt %) volatile matter
ash
8.98
28.96
moisture
fixed carbon
net heating value (kJ/kg)
0.78
59.72
21125
Ultimate Analysis (air-dried wt %) carbon
hydrogen
sulfur
nitrogen
oxygen
60.05
2.58
1.52
0.88
3.65
Table 3. Boiler Design and Operation Parameters quantity flow rate of the main steam (ton/h) pressure of the main steam (MPa) temperature of the main steam (C) reheat steam outlet temperature (C) reheat steam outlet pressure (MPa) coal feed rate (ton/h) primary air temperature (C) secondary air temperature (C) vent air temperature (C) staged air temperature (C)
operation parameters
design parameters
974.3
935
16.8
17.5
530.8
540.6
535.4
540.0
3.6
3.84
119.4 215
116 215
362.0
341
113
95
362.0
341
Table 4. Gas Components and Boiler Efficiency O2 content at the furnace exit (%) NOx content at the furnace exit (mg/m3 at 6% O2 dry) solid unburned loss (%) exhaust gas temperature (C) boiler efficiency spray water flux of superheater (ton/h)
15%
25%
35%
5.84 1437
5.55 1439
5.62 1274
3.06 126 90.4 53
3.82 128 89.59 55.7
4.13 134 89.45 67.9
Figure 14. Gas temperature distribution and gas species concentrations in the zone near the water-cooled wall of monitoring port No.1 with different staged air damper opening.
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Energy Fuels 2010, 24, 38–45
: DOI:10.1021/ef900476c
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Figure 15. Gas temperature distribution and gas species concentrations in the zone near the water-cooled wall of monitoring port No. 2 with different staged air damper opening.
entrained high-temperature gas concentrations decrease, yielding very low temperatures within the initial measuring area. With increasing measuring depth, high temperature gas and the main airflow gradually mix, initiating a temperature increase. Figure 14 shows gas temperature and gas composition near the sidewall measured through monitoring port No. 1. With increasing staged air damper opening, gas temperatures in the region decrease, O2 content increases, and CO and NOx content decrease. With the staged air damper opening at 15%, tangential velocities of secondary air are higher, the external recirculation is large, and high-temperature entrained gas accumulates in the region, wherein temperatures are high, O2 content is low, and combustible gas and NOx content are high. With the staged air damper opening increased to 25 and 35%, tangential velocities of secondary air decrease, so hightemperature entrained gas decreases. This causes temperatures in the region to be low, O2 content high, and combustible gas and NOx content low. Figure 15 shows gas temperature and gas composition profiles near the sidewall measured through monitoring port No. 2 (see Figure 11). As shown in Figure 15, with increasing staged air damper opening, temperatures increase, O2 content fluctuates between 5 and 10%, and combustible gas and NOx content decrease. With an opening of 15%, primary air diffuses slowly mixing later with high-temperature gas. The coal precipitate is therefore largely volatile in the region, and as a consequence, the coal burns violently, and combustible gas and NOx content are high. With the opening increased to 25 and 35%, the coal and the high temperature flue gas mix earlier, combustible gas has already burned away at this measuring point, and air staged
combustion reduces the formation of NOx; hence, combustible gas and NOx content are low. Temperatures increase because the large supply of staged air intensifies the burning violently. Table 4 shows O2 and NOx content at the exit of air preheaters. At three different openings of the staged air damper, O2 content at the exit of air preheater is basically stable. By increasing the opening from 15 to 25%, unburnt solid losses, exhaust gas temperatures, and de-superheated water levels increase slightly, but the boiler efficiency and NOx emissions decrease slightly. At the larger opening of 35%, the boiler efficiency decreases a little, but NOx emissions decreases by 11.5%. Thus, in accordance with the results obtained above, the optimal setting for the opening of the staged air damper during boiler operations is 35%. 5. Conclusion 1 For the different secondary air ratios of 100, 90, 80, and 70% employed in this study, no central recirculation zone developed. With decreasing relative secondary air ratio, the axial velocity of the jet flow decreases initially, and as the jet flow develops, the magnitude of the velocity tends to be uniform, and the jet flow depth is essentially constant. Radial velocities are also basically uniform over the center region of the jet, but at the edge of the jet, it decreases as the secondary air ratio and the diffusion velocity decrease. Tangential velocities decrease as the secondary air ratio decreases, therefore the amount of entrainment of the jet flow decreases. 2 Staged air damper openings of 15 and 25% benefits diffusion of primary air and speeds up the mixing of the 44
Energy Fuels 2010, 24, 38–45
: DOI:10.1021/ef900476c
Li et al.
damper opening of 35%, NOx emissions decrease significantly. It can be concluded from the above that the optimal opening of the staged air damper during boiler operation is 35%.
primary air with the high-temperature gas. At the wider opening of 35%, the rate of temperature rise within the jet flow becomes slower, which hinders ignition since the entrained flue gas decreases. 3 For staged air damper openings of 15 and 25%, unburnt solid losses, exhaust gas temperatures, and de-superheated water from the superheater increase, while NOx emissions decrease slightly. With the wider staged air
Acknowledgment. This work is sponsored by the Hi-Tech Research and Development Program of China (863 program) (Contract No. 2006AA05Z321).
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