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Industrial Experiments on Anthracite Combustion and NOx Emissions with Respect to Swirling Secondary Air for a 300 MWe Deep-Air-Staged Down-Fired Utility Boiler Qingxiang Wang, Zhichao Chen, Tao Liu, Lingyan Zeng, Xin Zhang, He Du, and Zhengqi Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01237 • Publication Date (Web): 21 Jun 2018 Downloaded from http://pubs.acs.org on June 22, 2018
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Industrial Experiments on Anthracite Combustion and NOx Emissions with Respect to Swirling Secondary Air for a 300 MWe Deep-Air-Staged Down-Fired Utility Boiler Qingxiang Wang, Zhichao Chen*, Tao Liu, Lingyan Zeng, Xin Zhang, He Du, Zhengqi Li School of Energy Science and Engineering, Harbin Institute of Technology, 92, West Dazhi Street, Harbin 150001, PR China Corresponding author: Zhichao Chen E-mail:
[email protected] (Z. Chen)
ABSTRACT: A new deep-air-staging and low-NOx technology has been introduced to a 300 MWe anthracite- and down-fired boiler with swirl burners. Industrial experiments were performed at different outer secondary air vane angles (defined as β) (i.e., 20, 30, 40, and 50°) to evaluate the environmental and economic performance for the retrofitted boiler. Furthermore, combining with the previous investigations on the inner secondary air vane angle (defined as α), the influence degree of β and α on anthracite combustion and NOx emissions for the retrofitted boiler were further analyzed and compared. The experimental results revealed that, the main factors affecting the ignition and the flame fullness for the β and α are different. Compared with the α, the β had a relatively greater influence on NOx emissions for the retrofitted boiler. Compared with the orignianl boiler, a strong reducing atmosphere was formed in the primary combustion zone for the retrofitted boiler, and for the β of approximately 30°, the arithmetic mean of NOx emissions in the whole measurement range was reduced by 1073 mg/m3 at 6% O2. Taking consideration of environmental and economic effects, the optimal β for the retrofitted boiler was 20°. 1 ACS Paragon Plus Environment
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1. Introduction Anthracite accounts for 11.5% of the China's proven coal reserves, and China is one of the few countries in the world with abundant anthracite reserves. The low volatile matter and poor reactivity of anthracite cause the difficulties in the ignition and burnout.1,2 In the early years of China, the tangential- and wall-fired boilers were used to burn anthracite.3-6 In order to burn anthracite more efficiently, the down-fired boilers were introduced into China.7,8 The Babcock & Wilcox (B&W) down-fired boiler is the only type that adopts a typical swirl burner among all of the down-fired boilers.9-11 Industrial experiment results uncovered that many problems, such as late ignition12, insufficient burnout13, and especially high NOx emissions14,15 widely existed in B&W down-fired boilers. To improve economical efficiency and environmental protection of the B&W down-fired boilers, some corresponding solutions have been put forward, such as changing swirl intensity of swirl burners to advance the ignition9,16, modifying structures of combustion system to enhance the burnout17-19, and adjusting air distributions to reduce the NOx emissions20,21. NOx emissions endanger the environment and human health.22-24 The NOx from coal-fired power plants are strictly controlled by the Chinese government, and NOx emissions are not higher than 50 mg/m3 at 6% O2 by 2020. To decrease NOx emissions and enhance pulverized coal burnout for B&W anthracite- and down-fired boilers comprehensively, our group thus developed a new deep-air-staged and low-NOx technology.10,11,25 Many studies of the combustion characteristics and NOx emissions of the retrofitted boiler under different boiler loads10, inner-secondary-air (ISA) vane angles (defined as α)11 and overfire air ratios25 shows that, the stable combustion, timely ignition, and significant NOx reduction of approximately 45% without decreasing boiler thermal efficiency were achieved under the optimum conditions. To a great extent, the cost of flue gas denitration is 2 ACS Paragon Plus Environment
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reduced, and the boiler economic benefit is enhanced. The swirl intensity of the burner has significant effects on the ignition, burnout and NOx emission characteristics, which directly affects the safety, economic, and environmental protection operation of the boiler unit.26 Li’s group has long studied research on the effects of the swirl intensity of swirl burner on combustion and NOx emissions.27,28 For a centrally-fuel-rich swirl burner with the blended coal of anthracite and lean coal, industrial experiments indicated that, with increasing the outer secondary air (OSA) vane angle (defined as β) from 25 to 40°, arithmetic means of NOx concentrations and temperature gradients in the whole measurement range of burner outlet region were reduced from 1124 to 888 mg/m3 at 6% O2 and from 3095 to 2450 °C/m, respectively.27 For a double swirl coal combustion burner with burning bituminous coal, the industrial experiments indicated that, with increasing β from 35 to 90°, pulverized coal burnout and temperature gradients in the whole measurement range of burner outlet region decreased from 53 to 37% and from 560 to 446 °C/m, respectively. What was different from the above investigation was that the arithmetic mean of NOx concentrations increased from 491 to 674 mg/m3 at 6% O2 in the whole measurement range of burner outlet region.28 These investigations showed that, for the same swirl burner, the β has significant effects on ignition, burnout and NOx emissions. In addition, there is a great difference on the influence rules of the β on NOx formation for different swirl burners with different structures. After the application of the new deep-air-staged and low-NOx technology in the B&W down-fired boiler, the operation and structure parameters of swirl burner are both changed, so the effects of the β of the retrofitted burners on the safety, burnout and NOx emissions for the retrofitted boiler need to be newly and comprehensively evaluated. Fan et al.9,16 experimentally investigated the influence of α and β of a swirl burner installed in 3 ACS Paragon Plus Environment
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an original B&W down-fired boiler on temperature distributions and NOx formation in the outlet region of the burner. With increasing β from 25 to 50°, the variation range of temperature gradients in the whole measurement range of burner outlet and the arithmetic means of NOx concentrations of all measurement points near the sidewall in the lower furnace varied from 27 to 275 °C/m and from 443 to 1388 mg/m3 at 6% O2, respectively;9 with increasing α from 42 to 60°, the variation range of temperature gradients in the whole measurement range of the burner outlet and the arithmetic means of NOx concentrations of all measurement points near the sidewall in the lower furnace varied from 31 to 89 °C/m and from 227 to 426 mg/m3 at 6% O2, respectively.16 It can be summarized from the above studies that, compared with the α, the the β has a greater impact on the pulverized coal ignition and NOx formation. If the unstable ignition or NOx emissions exceeding the limit standard (In particular, the boiler load varies substantially.) occur in the actual boiler operation, the safety and environmental performance of the boiler is rapidly improved by adjusting the β. Accordingly, the overall evaluation of the influence degree of β and α on the combustion characteristics and NOx emissions is necessary, and has important guide significance for the actual boiler operation adjustments. It is worth noting that, for a 300 MWe B&W down-fired boiler (same with the aforementioned furnace type) after the application of new deep-air-staged and low-NOx technology, when the α increased from 35 to 55°, the variation range of temperature gradients in the whole measurement range of the burner outlet and the arithmetic means of NOx concentrations of all measurement points near the sidewall in the lower furnace varied from 446 to 971 °C/m and from 451 to 675 mg/m3 at 6% O2, respectively.11 For the down-fired boiler after the introduction of overfire air, the influence degree of the α on pulverized coal ignition and NOx formation increased compared with the original B&W down-fired boiler. Accordingly, what influence the β has on 4 ACS Paragon Plus Environment
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anthracite combustion and NOx emissions is also worth exploring and analyzing for the B&W down-fired with overfire air. In the present paper, industrial experiments by adjusting the β (i.e., 20, 30, 40, and 50°) were carried out to study effects of β on anthracite combustion and NOx emissions of the B&W down-fired boiler retrofitted by the new deep-air-staged and low-NOx technology. Furthermore, combining with previous investigations, influence degree of β and α on the temperature distribution in the outlet region of the burner and furnace temperature distribution, flame fullness in the furnace, carbon content in fly ash, and NOx emissions of the retrofitted boiler were further analyzed and compared. This research can not only validate NOx reduction and combustion improvements after the application of the new deep-air-staged and low-NOx technology, but also favors the operation and design of the similar boilers.
2. Methodology 2.1. Original boiler and swirl burner As shown in Figure 1, the combustion system of the original boiler consists of 16 swirl burners, 16 vent air nozzles, 32 staged air nozzles, 2 secondary air box and so on. The pulverized coal of swirl burners from the arches is ejected down to form a "W" type of flame. Corresponding to the swirl burners, there are the vent and staged air nozzles located in the lower furnace. For the original boiler, the air from the secondary air box is divided into two parts, the burner secondary and staged air. The three-level air staged combustion (i.e. burner secondary, vent and staged air) is formed along the furnace height. Figure 2a shows the structure diagram of the swirl burners. The minimum and maximum α are 20 and 70°, respectively. The minimum and maximum β are 10 and 50°. The ISA and OSA rotation directions of the swirl burner are the same, and the swirl intensities of the 5 ACS Paragon Plus Environment
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burner can be changed by adjusting the α and β. The detailed schematics and operational principles of the down-fired boiler and swirl burner can be found in the literature.11 platen final superheater superheater reheater 2175 64
2550 150
2963 246
2175
I
a half of rear wall symmetry axis
10950
left sidewall
(b) overfire air port layout on the rear wall primary coal/air flow 35°
inner secondary air
II
outer secondary air
30°
baffle rings
upper furnace
235
swirl burner
8400
25 °
80
46815
I II overfire air
A
28°
vent air
11 36 x1 95 2 4
inspection port 2
staged air
880
x8
15600
a
primary coal/air flow inner secondary air outer secondary air
540
55°
12180
368x14
lower furnace 10377
15060
18370
(c) schematics of the retrofitted burner
inspection port 1
20° 19555
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baffle rings
(d) section view of "A" direction
(a) vertical furnace section
Figure 1. Combustion system of the down-fired 300 MWe boiler with deep-air-staged and low-NOx technology.
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primary air
primary air
vent air
adjustable inner secondary air vane
vent air
baffle ring of inner secondary air
inner secondary air outer secondary air
inner secondary air outer secondary air
baffle ring of outer secondary air
80
235
adjustable outer secondary air vane
368× 14 880× 8 1136 ×12
540 954
(a) original swirl burner
(b) retrofitted swirl burner adjustable outer secondary air vane
adjustable inner secondary air vane
outer secondary air β
inner secondary air
α
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(c) principle schematic of ISA vane
(d) principle schematic of OSA vane
Figure 2. Schematic diagram of original and retrofitted swirl burner (mm). 2.2. New combustion system The retrofitted 300 MWe down-fired boiler with the deep-air-staged and low-NOx technology differs from the aforementioned original boiler in two aspects: (i) The slit-type overfire air nozzles are newly added to the upper furnace near the throat at a declination angle of 25° (shown in Figure 1). (ii) To guarantee that the OSA and ISA velocities of the burners for the retrofitted boiler with overfire air are not reduced compared with the original boiler and to guarantee the penetration depth of pulverized coal, the baffle rings are newly added to the ISA and OSA duct nozzles (shown in Figure 2b) to reduce the flow areas of the ISA and OSA ducts. Along the direction of the airflow jet, the ISA baffle ring is designed as a cone ring structure. Due to the short distance between OSA vane and OSA duct nozzle, the OSA baffle ring is designed as equal diameter annular structure. The flow areas of original ISA and OSA ducts of all burners are reduced by approximately 25%. For the 7 ACS Paragon Plus Environment
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retrofitted boiler with the deep-air-staged and low-NOx technology, the secondary air from a secondary air box is divided into three parts, the burner secondary, staged and overfire air, and the four-level air staged combustion (i.e. burner secondary, vent, staged and overfire air) was formed along the furnace height. The detailed structures and principles of the new deep-air-staged and low-NOx technology have been described in the literature.25 2.3. Full-scale industrial measurements The experiments were carried out at a load of 300 MWe. To guarantee the accuracy of measurement results, all instruments, including the optical pyrometers, thermocouples and flue gas analyzers, are required to be calibrated in advance before the experiments. The specifications of various experiment instruments are shown in Table S1. During the experiments, it is necessary to maintain the stability of coal characteristics and avoid any interference operations such as the pollution discharge and soot blowing. Table 1 shows the proximate and ultimate analyses of Jincheng anthracite used in the experiments, and presents results of proximate and ultimate analyses, as well as tests for total sulfur content and net heating value performed in compliance with standards ISO 17246:2005, ISO 17247:2005, ISO 19579:2006 and ISO 1928:2009, respectively. The operation parameters (All the parameters of the instruments had been calibrated ahead of time.) in the centralized control room were recorded every 10 minutes, and the arithmetic means of operation parameters were recorded in Table 2. During the experiments, the following measurements were made: (i) Flue gas temperature distribution in the outlet region of the burner. (ii) Flue gas temperature distributions in the furnace. (iii) Flue gas species concentrations in the zone near the sidewall. (iiii) Flue gas species concentrations, exhaust temperature and carbon content in fly ash at the furnace exit. The additional 8 ACS Paragon Plus Environment
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measurement details and accuracies can be given in the Supporting Information and found in the literatures.10,29-32 Figure 3 shows the aforementioned measurement locations. Noted that, compared with the β of 20, 30 and 40°, the negative pressure fluctuation in the furnace was relatively larger for the β of 50°, (shown in Table 2), and the combustion stability in the furnace is relatively poor. For the β of 50°, for the safe operation of the boiler, the less time-consuming measurements (including the measurements (i), (ii), and (iiii)) were only completed, and the longer time-consuming measurement (including the measurement (iii)) was no longer carried out. Table 1. Proximate and ultimate analyses of coals used in industrial-size measurements. Proximate analysis, wt.% (as dry basis) Volatile matter
Ash
Fixed carbon
Net heating value (kJ/kg)
8.34a
30.22a
61.44a
21108a
6.96b
27.69b
65.35b
21170b
Ultimate analysis, wt.% (as dry basis) Carbon
Hydrogen
Nitrogen
Oxygen
Sulfur
62.94a
2.37a
0.77a
3.05a
0.65a
64.91b
2.28b
0.78b
2.85b
1.49b
Coal (a) and (b) correspond to the boiler before and after the retrofit, respectively.
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Table 2. Main operation parameters and selected experimental results.
Item
β (before retrofit)
β (after retrofit)
20°
20°
30°
40°
50°
Capacity (MWe)
300
300
300
300
300
Flow rate of the main steam (t/h)
997
961
962
963
960
Pressure of the main steam (MPa)
16.1
16.4
16.2
16.3
16.3
Temperature of superheat steam (°C)
542
537
539
539
538
Temperature of reheat steam (°C)
539
538
539
540
539
Total flux of the primary air (t/h)
230
243
245
243
244
Temperature of the primary air (°C)
88
92
91
90
91
Flux of the inner secondary air (t/h)
501
493
494
494
493
Flux of the outer secondary air (t/h)
508
500
502
501
501
Temperature of the secondary air (°C)
368
366
364
366
365
Burner secondary air ratio (%)
65.5
49.2
49.1
49.3
49.4
Staged air ratio (%)
15.9
13.3
13.4
13.2
13.2
Overfire air ratio (%)
—
17.9
17.8
17.9
17.7
Exhaust gas temperature (°C)
147
144
145
150
158
NOx emissions (mg/m3 at 6% O2)
1287
674
746
790
836
O2 concentration at the furnace exit (%)
4.5
3.3
3.2
3.4
3.3
CO concentration at the furnace exit (ppm)
15
16
18
28
44
11.10
11.4
11.5
11.9
12.9
3.5
0.2
0.4
0.7
1.1
89.01
90.03
89.94
89.52
88.68
-30 to -96
-30 to -63
-47 to -89
-57 to -97
-34 to -180
18.0
8.0
10.9
15.7
24.1
Carbon content in fly ash (%) Carbon content in bottom slag (%) Boiler thermal efficiency (%) Negative pressure in the lower furnace (Pa) De-superheating water flow rate (t/h)
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48620 overfire air
39780
48620 21
90
32700
0
26355
inspection port 1
vent air
overfire air fr o
nt
39780 wa
23500
ll
swirl burner
ht rig
w
all
32700
vent air
staged air
staged air 29
22
63
40
21860
inspection port 2
23500
13 88
19400 19400 15040 18980
z y
15040
x o
Figure 3. Measurement locations during the industrial experiments (mm).
3. Results and discussion 3.1. Pulverized coal ignition 1200
1000 O
flue gas temperature ( C)
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800 o
20 600
o
30
o
40
o
50
400
o
20 (before retrofit) 200 0.0
0.5
1.0
1.5
2.0
2.5
3.0
distance from the burner nozzle (m) Figure 4. Temperature distribution in the outlet region of the burner at different β. In Figure 4, to facilitate the comparison between different conditions, the ignition temperature of pulverized coal is defined as 1000 °C.33,34 In the whole measurement range, the temperature in 11 ACS Paragon Plus Environment
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the outlet region of the burner increased fastest all the time with increasing measurement distance for the β of 20°, and the ignition distance was 0.8 m from the burner nozzle; for β of 30, 40 and 50°, the temperature in the outlet region of the burner increased rapidly first, then increased slowly, and the ignition distance were 1.0, 1.4 and 2.4 m from the burner nozzle, respectively. The ignition distance increased constantly with increasing β. In particular, when the β increased from 40 to 50°, the ignition distance increased significantly. For the boiler before the retrofit with the β of 20°, the temperature in the outlet region of the burner increased constantly first and reached a peak (only 904 °C) at a distance of 1.0 m from the burner nozzle, then decreased constantly with increasing the measurement distance, which indicated that the coal was not ignited in the whole measurement range. Generally speaking, compared with the boiler before the retrofit, the ignition distance was significantly shortened for the retrofitted boiler. The most fundamental reason is that, the burner secondary air ratio reduced from 65.5 to approximately 49%, and the thermal resistance between primary air and the high-temperature flue gas (HTFG) in the furnace decreased. 1500
O
temperature gradient ( C/m)
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1350 1200 1050 900 750 OSA vane angle β ISA vane angle α OSA vane angle β (Fan et al. 2010)
600 450 300 15
20
25
30
35 40 45 o vane angle ( )
50
55
60
Figure 5. Flue gas temperature gradients at a distance between zero point and 1.2 m from the burner outlet at different α and β. The amount of HTFG entrained by the burner increases with an increase in the swirl intensity 12 ACS Paragon Plus Environment
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of burners, which is beneficial to the ignition. For the down-fired boiler with swirl burners, the recirculation region under the arch is larger with a decrease in the swirl intensity of burners, and the amount of HTFG of recirculation region under the arch back to the outlet region of the burner increases, which also favors the ignition.9,11,20 The ignition of the down-fired boiler with swirl burners is affected by the above two aspects (including the HTFG entrained by the burner and the HTFG of recirculation region under the arch). Figure 5 shows the temperature gradients at a distance between zero point and 1.2 m from the burner outlet (hereinafter referred to as the flue gas temperature gradients) at different α and β. When the α increased from 35 to 45°, the flue gas temperature gradient decreased from 1445 to 913 °C/m, and the HTFG of recirculation region under the arch had a greater influence on the ignition than the HTFG entrained by the burner. The temperature gradient increased from 913 to 963 °C/m with the α increased from 45 to 55°, and compared with the HTFG of recirculation region under the arch, the HTFG entrained by the burner had a greater influence on the ignition.11 Different from the α, the temperature gradient decreased from 1445 to 732 °C/m continuously and the ignition distance from burner nozzle increased with the β increased from 20 to 50°, which indicated that, with the β increased from 20 to 50°, compared with the HTFG entrained by the burner, the HTFG of recirculation region under the arch always had a greater influence on the pulverized coal ignition. For a 300 MWe original B&W down-fired boiler, Fan et al.9 investigated the influence of β on the ignition, as shown in Figure 5. The temperature gradient decreased from 830 to 444 °C/m when the β increased from 25 to 32.5°. With the β increased from 32.5 to 50°, the highest temperature in the outlet region of the burner increased from 444 to 1050 °C in the whole measurement range. As a consequence, the β of the original burner and the α of the retrofitted burner had the same effect rule on the ignition characteristics. That is, after 13 ACS Paragon Plus Environment
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the angle increased to a certain value, the HTFG entrained by the burner had a greater influence on the ignition than the HTFG of recirculation region under the arch. By the comparisons and analyses of the influence of the β on the ignition characteristics before and after the retrofits, it can be concluded that, for the swirl burner after retrofit, the OSA rotation characteristics were weakened to some extent, and on the contrary, the OSA mainly showed the jet characteristics. The reasons are as follows: (i) The operation parameters of the swirl burner were changed. For the boiler after the retrofit, a part of secondary air (accounting for 18% of the total air mass flow rate) was delivered to the overfire air, which caused that the burner secondary air amount was reduced substantially and the swirl intensity of the burner decreased. (ii) The secondary air duct nozzle structures of the swirl burner were changed. For the boiler after the retrofit, the OSA baffle ring was added to the OSA duct nozzle of the burner. In particular, due to the shorter distance between the OSA vane and the OSA duct nozzle, the OSA baffle ring was designed as an equal diameter annular structure instead of a cone ring structure like the ISA baffle ring. The rotating OSA would collide directly with the OSA baffle ring, which caused that the OSA tangential velocity attenuated rapidly, and the OSA swirl intensity was greatly reduced.
3.2. Flue gas temperature in the furnace Figure 6 shows the temperature distribution along the furnace height direction at different β. The swirl intensity increased, and OSA axial velocity attenuation increased, as well as the penetration depth of pulverized coal decreased. At the same time, the ignition distance increased constantly with increasing β. As a result, with increasing β, the pulverized coal combustion path decreased, and the heat released from the pulverized coal combustion reduced, so the temperature decreased constantly at the same height of lower furnace. With increasing β, the unburned carbon in 14 ACS Paragon Plus Environment
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flue gas from the lower furnace increased. Under the sufficient mixture between the overfire air and the flue gas from the lower furnace, the more unburned carbon was burned and released more heat in the upper furnace. Consequently, the furnace temperature increased continuously at the height of 48.6 m with increasing β, which was consistent with the rule that the exhaust temperature increased with increasing β (shown in Table 2). It was worth noting that the temperature at the same height of lower furnace for the β of 50° was much lower than that under the other three β. The furnace temperature was much higher for the β of 50° than that under the other three β at the furnace height of 48.6 m. From the analyses of ignition and furnace temperature distribution, compared with the β of 20, 30 and 40°, the utilization space in the lower furnace was relatively smaller and the combustion condition was relatively poor for the β of 50°. In addition, the negative pressure in the furnace fluctuated from -34 to -180 Pa (shown in Table 2), and the negative pressure fluctuation was relatively larger and the combustion stability in the furnace was relatively poor for the β of 50°. Compared with the boiler before the retrofit, for the β of 20°, the furnace temperature at the height of 15 m was higher, and the furnace temperature at the height of 48.6 m was lower for the retrofitted boiler, which indicated that the new combustion system could promote the penetration depth and the burnout of pulverized coal.
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50 o
45 height from the ground (m)
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40 35 30
upper furnace
20 o 30 o 40 o 50 o 20 (before retrofit)
arch
25 20
lower furnace
15 furnace hopper
10 800
900
1000 1100 1200 1300 1400 1500 1600 flue gas temperature (OC)
Figure 6. Furnace temperature distribution at different β. Figure 7 clearly and intuitively shows that the furnace temperatures at the height of 15 m and at the height of 48.6 m at different β and α, respectively. At the furnace height of 15 m, the β and α roughly had the same influence rules on furnace temperature. The furnace temperature decreased constantly from 1140 to 1041 °C with increasing the β and from 1140 to 1065 °C with increasing α at the height of 15 m. The β and α had the different influence rules on furnace temperature at the height of 48.6 m. The furnace temperature increased more and more quickly from 843 to 973 °C with increasing β. The furnace temperature increased from 843 to 892 °C first, and then decreased to 844 °C at the height of 48.6 m with increasing α. The penetration depth of pulverized coal decreased with the α increased from 45 to 55° (The conclusion could be drawn from Figure 6 that the furnace temperature at the upper part of hopper decreased with increasing α.), and the pulverized coal was ignited in advance (shown in Figure 4), which eventually caused the temperature decreased at the upper part of upper furnace.11 It could be concluded that, with the α increased from 45 to 55°, compared with the penetration depth of pulverized coal, the pulverized coal ignition played a more important role in the temperature at upper part of upper furnace. The 16 ACS Paragon Plus Environment
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pulverized coal ignition was delayed with increasing β (shown in Figure 4), and the penetration depth of pulverized coal decreased (In Figure 6, the conclusion could be drawn that the temperature at upper part of furnace hopper decreased with increasing β.). The above reasons caused together that, with increasing β, the pulverized coal residence time in the lower furnace decreased, and the
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800
Figure 7. Furnace temperatures at heights of 15 and 48.6 m at different β and α. 3.3. Pulverized coal combustion and NOx formation In Figure 8, the O2 concentration decreased continuously at the same measurement distance with decreasing β in the whole measurement range of inspection port 1, which indicated pulverized 17 ACS Paragon Plus Environment
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coal burned more intensely with decreasing β. At inspection port 1, for the β of 20°, the O2 concentration increased rapidly with increasing measurement distance from 1400 to 2200 mm, which indicated that the water-cooled probe entered the burner secondary air effect zone at beginning of the measurement distance of 1400 mm. The O2 concentration decreased continuously when the measurement distance increased from 2200 to 2800 mm, which indicated that, at beginning of the measurement distance of 2200 mm, the water-cooled probe entered the pulverized coal combustion zone. Similarly, for the β of 30 and 40°, the water-cooled probe entered the burner secondary air effect zone at beginning of the measurement distance of 1000 and 1200 mm, respectively, and both entered the pulverized coal combustion zone at beginning of the measurement distance of 2200 mm. With decreasing β, the OSA tangential velocity decreased and the outward diffusion ability of the airflow became weaker,27,28 so with decreasing β, the water-cooled probe delayed entering the burner secondary air effect zone. At different β, the water-cooled probe entered pulverized coal combustion zone at the same measurement distance, and the flame fullness was basically the same at inspection port 1. Different from the β, the water-cooled probe all started entering the burner secondary air effect zones at beginning of the measurement distance of 1400 mm under different α, and the size of the burner secondary air effect zone was not changed.11 The reason was that the condition that ISA was tightly wrapped and squeezed by the OSA was changed by adjusting the α. Same with the β, at the beginning of the measurement distance of 2200 mm, the water-cooled probe also entered pulverized coal combustion zone at different α.11 At inspection port 1, at different β, the NOx concentrations near the sidewall with increasing the measurement distance increased first, and then decreased. In the whole measurement range of inspection port 1, the NOx concentrations decreased constantly at the same 18 ACS Paragon Plus Environment
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measurement distance with decreasing β. For the β of 20, 30 and 40°, the arithmetic means of NOx concentrations of all measurement points at inspection port 1 were 623, 727 and 794 mg/m3 at 6% O2. The maximum variation of NOx concentrations was 171 mg/m3 at 6% O2 by adjusting the β. For the α of 35, 45 and 55°, the arithmetic means of NOx concentrations of all measurement points at inspection port 1 were 623, 618 and 632 mg/m3 at 6% O2.11 The maximum variation of NOx concentrations was 14 mg/m3 at 6% O2 by adjusting the α. It could be seen that the β had greater influence on the NOx formation in the initial pulverized coal combustion stage than the α. Same with the inspection port 1, the CO concentration increased and the O2 concentration decreased continuously at the same measurement distance with the decrease in β at inspection port 2. The reason was that the pulverized coal penetration depth and residence time increased with decreasing β, and the pulverized coal burned more sufficiently, so the oxygen consumption of pulverized coal combustion and the CO generation increased. At inspection port 2, for the β of 20 and 30°, the O2 concentration was reduced continuously with increasing measurement distance, which indicated that the entire furnace cross section was filled with coal flame. However, for the β of 40°, the O2 concentration increased from 2.1 to 5.2% and the CO concentration fluctuated approximately 400 mg/m3 at 6% O2 with the measurement distance increased from 1800 to 2800 mm, which indicated that the entire furnace cross section was not filled with the coal flame. Compared with the original B&W down-fired 300 MWe boiler with the β of 32.5° (approximately 30°)9, the arithmetic means of O2 and CO concentrations of all measurement points decreased 8.8% and increased 6073 mg/m3 at 6% O2 for the retrofitted boiler with the β of 30°, respectively. A strong reducing atmosphere were formed in the primary combustion zone. The NOx formation was suppressed effectively, and arithmetic mean of NOx concentrations of all measurement points 19 ACS Paragon Plus Environment
Energy & Fuels
decreased 1073 mg/m3 at 6% O2. o
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2000 1800 1600 1400 1200 1000 800 600 400 200 400
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Figure 8. Flue gas species concentrations in the region near sidewall at inspection ports of 1 and 2. 3.4. NOx emissions and carbon content in fly ash 20 ACS Paragon Plus Environment
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As shown in Table 2, for the boiler after retrofit, with increasing β from 20 to 50°, NOx emissions basically increased linearly from 674 to 836 mg/m3 at 6% O2; carbon content in fly ash increased from 11.4 to 12.9%, in particular, when the β increased from 40 to 50°, carbon content in fly ash increased significantly and increased one percentage point. Similarly, with increasing β, the de-superheating water flow rate and exhaust gas temperature also increased constantly. For the boiler after retrofit, with increasing α from 35 to 55°, the NOx emission concentration increased from 674 to 765 mg/m3 at 6% O2 first, and then decreased to 741 mg/m3 at 6% O2; carbon content in fly ash increased from 11.4 to 12.6% first and then decreased to 11.7%, and de-superheating water flow rate increased from 8 to 22.9 ton/h first and then decreased to 16.7 ton/h.11 The influence of α and β on NOx emission concentration and carbon content in fly ash were corresponding to aforementioned influence of α and β on the ignition and the furnace temperature distribution. At the same time, for the retrofitted boiler, the comparisons showed that the β had a relatively greater influence on the NOx emissions than the α. Compared with the other three β, for the β of 20°, the timely ignition, the highest temperature in the lower furnace and boiler thermal efficiency, and the lowest carbon content in fly ash and NOx emissions were achieved. Compared with the boiler before the retrofit, de-superheating water flow rate decreased obviously and exhaust temperature decreased slightly; NOx emissions decreased from 1287 to 674 mg/m3 at 6% O2 and reduced by 47.6% with increasing boiler thermal efficiency slightly for the retrofitted boiler with the β of 20°. After the adoption of the new deep-air-staged and low-NOx technology, the NOx emissions were reduce greatly with the boiler thermal efficiency increasing slightly under the optimal β of 20°.
4. Conclusions 21 ACS Paragon Plus Environment
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By carrying out industrial measurements in a 300 MWe anthracite- and down-fired utility boiler with the new deep-air-staged and low-NOx technology, the anthracite combustion and NOx emissions were investagated at different β (i.e., 20, 30, 40, and 50°, respectively). Furthermore, combining with previous investigations on the α, the influence degree of β and α on anthracite combustion and NOx emissions of the retrofitted boiler were further analyzed and compared. The main conclusions are as follows: (1) Different from the influence of α on the ignition, the pulverized coal was ignited in advance with decreasing β, and the HTFG of recirculation region under the arch always had a greater influence on the ignition than that entrained by the burner. Compared with the original swirl burner, OSA rotation characteristics for the retrofitted burner were weakened to some extent. (2) Compared with the α, the β had a relatively greater influence on NOx emissions for the retrofitted boiler. Compared with the original B&W down-fired boiler that burned the coal with relatively higher volatile content, a strong reducing atmosphere was formed in the primary combustion zone of the retrofitted boiler. The NOx formation was suppressed effectively, and the arithmetic mean of NOx concentrations of all measurement points at inspection port 2 decreased 1073 mg/m3 at 6% O2. (3) Different with the α, the burner secondary air effect region decreased continuously with decreasing β at inspection port 1. Compared with β of 40°, the flame fullness in the lower furnace was larger for the β of 20 and 30° at inspection port 2. (4) Compared with the other three β, for the β of 20°, the timely ignition, the highest temperature in the lower furnace and boiler thermal efficiency and the lowest NOx emission concentration were achieved. For the retrofitted boiler with the optimal β of 20°, the NOx emission 22 ACS Paragon Plus Environment
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concentration decreased from 1287 to 674 mg/m3 at 6% O2 and reduced by 47.6% with increasing boiler thermal efficiency slightly.
Acknowledgments This work is supported by the National Key Research and Development Program of China (Grant No. 2016YFC0203702).
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