New Fuel Air Control Strategy for Reducing NOx Emissions from

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A new fuel air control strategy for reducing NOx emissions from corner-fired utility boilers at medium-low loads Sinan Zhao, Qingyan Fang, Chungen Yin, Tongsheng Wei, Huajian Wang, Cheng Zhang, and Gang Chen Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 13 Jun 2017 Downloaded from http://pubs.acs.org on June 14, 2017

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A new fuel air control strategy for reducing NOx emissions from corner-fired utility boilers at medium-low loads

Sinan Zhao1, Qingyan Fang1,2*, Chungen Yin2*, Tongsheng Wei3, Huajian Wang3, Cheng Zhang1, Gang Chen1

1

State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, China 2

Department of Energy Technology, Aalborg University, 9220 Aalborg East, Denmark 3

Xi’an Thermal Power Research Institute Co., Ltd., Shanxi 710032, China

* Corresponding authors. Phone: +86 27-87542417; fax: +86 27 87540249. E-mail: [email protected]; Phone: +45 30622577; fax: +45 98151411. E-mail: [email protected]

ABSTRACT: Due to the rapidly growing renewable power, the fossil fuel power plants have to be increasingly operated under large and rapid load change conditions, which can induce various challenges. This paper aims to reduce NOx emissions of large-scale corner-fired boilers operated at medium-low loads. The combustion characteristics and NOx emissions from a 1000 MWe corner-fired tower boiler under different loads are investigated experimentally and numerically. A new control strategy for the annular fuel air is proposed and implemented in the boiler, in which the secondary air admitted to the furnace through the air annulus around each coal nozzle tip is controlled by the boiler load, instead of being controlled by the output of the connected mill as commonly used in this kind of power plants. Both the experimental and simulation results show that the new control strategy reduces NOx emissions at the entrance of the selective catalytic reduction (SCR) system by about 20% at medium-low loads, compared to those based on the original control. The new control strategy has also been successfully applied to two other corner-fired boilers to achieve a significant NOx emission reduction at partial loads. In all the three applications, no negative 1

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effect on the combustion and steam temperature characteristics of the boilers is observed.

Keywords: Fossil fuel power plants; Flexible operation; Annular fuel air; NOx emissions

1. INTRODUCTION Globally, the share of energy produced from renewable resources is growing rapidly, among which the output of wind and solar power is highly variable [1-3] and depends on different factors such as weather conditions and time of day. The fast-growing share of renewable power, especially when gaining priority access to the grid, forces the existing fossil fuel power plants to increasingly shift their role from providing base-load power to providing fluctuating back-up power to meet unpredictable demand peaks in order to control and stabilize the grid [4]. The thermal power plants are expected to be able to run at the lowest possible load condition while still having the highest possible efficiency and the lowest possible emissions. Besides the load demand and grid stability, operation of fossil fuel power plants also needs to account for the power prices. During low power price periods, an operator must decide to either shut down and incur significant cycling damage or operate at minimum load [4]. In China, the current medium-low load operation of coal-fired power plants mainly takes place in rainy seasons due to the abundant hydropower or in late nights due to the low power need. More frequent operation under larger and faster load response conditions is expected in future due to the fast-growing wind and solar power. Such operational flexibility in terms of load changes induces significant challenges for fossil fuel power generation, by reducing reliability and efficiency of power plants, increasing emissions and accelerating wear-and-tear on plant components on plant components. The focus of this study is on pollutant emissions, especially NOx emissions, from fossil fuel power plants 2


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at medium-low loads. Under such operation conditions, both the higher excess air and the lower flue gas temperature at the SCR entrance which decreases catalyst activity and NOx removal efficiency [5,6] result in higher NOx emissions. Such a side effect of flexible operation may be mitigated by various methods: (1) increasing the flue gas temperature at the SCR entrance (e.g., using economizer flue gas bypass) [7,8], which however lowers the boiler efficiency due to the reduced waste heat utilization [9]. (2) Developing low-temperature SCR catalysts, e.g., those containing transition metals such as Fe, V, Cr, Cu and Co. These catalysts not only exhibit strong low-temperature SCR activity but also are less susceptible to fly ash contamination and have a long service life. However, the catalysts are mostly still in the laboratory research stage and susceptible to be poisoned by the residual SO2 in the flue gas. Therefore, they are not yet suitable for large-scale applications [10]. (3) Optimizing boiler operation to reduce NOx concentrations at the SCR entrance, e.g., air staging [11-16] and fuel-staging [17,18]. Boiler optimization is widely used for NOx control in utility boilers. However, its effectiveness may be compromised at medium-low loads due to the higher excess air ratio. So far, NOx emissions from corner-fired tower boilers at medium-low loads have been rarely investigated, with little operation and research data available in the literature. The studies on NOx emissions from down-fired boilers at partial loads [19,20] may be of little reference value for corner-fired tower boilers because of the very different combustion modes and coal types. This paper focuses on NOx reduction at medium-low loads in a 1000 MWe corner-fired tower boiler which suffers high NOx emissions at the SCR entrance under these load conditions. A new control strategy for the secondary air admitted to the furnace through the air annulus around each coal nozzle tip is proposed and used in this boiler. In the new control, the flow rate of the annular fuel air is set as a function of the boiler load instead of the mill output. Such a strategy has not been previously used for NOx reduction at pulverized coal power plants operated at medium-low load conditions. Comprehensive experiments and

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numerical simulations are conducted for the corner-fired tower boiler to assess the effectiveness of the new fuel air control strategy. Such a new control strategy has also been implemented in two other corner-fired boilers to properly evaluate its general applicability and usefulness.

2. METHODOLOGY 2.1. The utility boiler. The object under investigation is a 1000 MWe ultra-supercritical corner-fired pulverized-coal boiler with a single tower furnace. The furnace configuration and burner arrangement at one corner are shown in Fig. 1(a) and 1(b), respectively. As shown in Fig. 1(b), pulverized coal particles are transported by the primary air (PA) from six medium-speed mills (mill A-F) to the enhanced ignition coal-burners, A1-A4, B1-B4 and C1-C4 as sketched in Fig. 1(c). The secondary air (SA) is distributed as fuel air and auxiliary air. The former is the portion of the secondary air admitted to the furnace through the air annulus around each coal nozzle tip to support combustion, stabilize flames and cool coal nozzles, and is denoted as annular fuel air. The latter is the balance of the secondary air admitted into the furnace through air nozzles located in the coal compartments above and below each coal elevation to complete the combustion, and is denoted as auxiliary air. A portion of the secondary air is also diverted to over-fire air (OFA) and secondary over-fire air nozzles located above the combustion zone, which are denoted as CCOFA (compact over-fire air) and SOFA (separated over-fire air), respectively. The coal and air streams from the four corners produce a clockwise concentric firing system (CFS) in the furnace, as depicted in Fig. 1(d). The horizontally biased SA streams form an outer concentric cycle and provide an air-rich atmosphere near the furnace walls, which protects the furnace walls from slagging. The coal fired in this boiler is Huojitu coal from Shanxi Province. The detailed properties of the coal, such as proximate and ultimate analysis, heating values, hardgrove index, ash fusion temperatures and ash

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chemical composition are given in Table 1. The coal properties data indicate that Huojitu coal is a high-volatile bituminous coal with good grindability and relatively low ash fusion temperatures. The operation conditions under different loads are listed in Table 2. At full load, the lower five mills, i.e., mill A, B, C, D and E in Fig. 1(b), are put in service, while the top mill (i.e., mill F) is out of service. The NOx emission at the SCR entrance at full load is normally low, as seen in Table 3. However, the NOx emission largely increases at partial loads, especially below 70% load, partly due to the higher overall excess air (or oxygen content) and partly due to the higher primary air/coal ratios under medium-low loads, as plotted in Fig. 2. The stoichiometric ratio in the initial combustion stage near the burner outlet is mainly decided by the primary air and annular fuel air. The air/coal ratio at the initial combustion stage in Table 2 is the primary air and annular fuel air over the total coal feeding. Compared to the annular fuel air, there is very little flexibility to adjust the primary air flowrate. The on-site experiments show that a slight decrease in the mill ventilation will lead to a rapid increase in coal particle agglomeration and sediments. 2.2. The new annular fuel air control strategy vs. the original one commonly used in power plants. The annular fuel air around each coal nozzle tip in this boiler is originally controlled by the output of the mill which is connected to the fuel/air compartment. Such a control strategy is exclusively used in this kind of power plants. When the boiler operates at high loads (>70%), the output of the five in-service mills will be adjusted based on the load, and the damper opening in the air annulus and the flowrate of the annular fuel air will be adjusted accordingly. As a result, such a control strategy is reasonable for high-load operation. However, such a commonly used control strategy does not properly address the medium-low load ( 38m) at 50% load are significantly lower than those at 70% load. It is because the burners C1 and C2 and the mill E, as indicated in Fig. 1(b), are put out of service under

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50% load, making the combustion center move downward. Figure 8 shows O2 concentration distributions on a quarter of the horizontal cross-section at the burner A3 level, from which the impact of the new annular fuel air control strategy is more clearly visualized. With the new control strategy in place, O2 concentration decays more rapidly from the burner nozzle tip to the downstream side than does it with the original control strategy, due to the reduced initial coal/air jet momentum in the former. The entire cross-section is also characterized by low O2 concentration when the new control strategy is used. It can also be found from Fig 8(a)-(d) that the O2 concentrations close to the side walls are lower for the new air control strategy. Combined with the observation that the high-temperature regions are slightly closer to the side walls, it could be disadvantageous from a corrosion point of view. However, it will not yield a serious consequence in corrosion, because the heat flux and the wall temperature will decrease with the decrease of load, which reduces the corrosion risk of water wall. Similarly, O2 concentrations on the vertical center-plane are shown in Fig. 9. Overall speaking, the O2 concentrations in the main combustion zone are very low. When the new control strategy is used, the O2 concentrations in the main combustion zone are lower than those with the original control, favoring a lower NOx formation [14,30]. After the injection of SOFA, the O2 concentrations rise rapidly. Moreover, the O2 concentrations are much higher at 50% load due to the higher excess air required for stable combustion and proper heat exchange at low loads. Figure 10 shows NOx distributions on a quarter of the horizontal cross-section at the burner A3 level. In all the cases, NOx substantially concentrates near the burner outlet and along the circle tangent, because of the high temperature and O2 concentration near the flame envelope. The use of the new annular fuel air control results in a remarkable reduction in NOx concentration in these regions. At 70% load, the average NOx over this horizontal cross-section is 252 ppm for the original control and 232 for the new control. At 11


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50% load, the average NOx is 261 ppm vs. 228 ppm for the original and new control, respectively. Figure 11 plots NOx formation on the burner A3 level along the line L1 which is from the burner nozzle tip to the furnace center as indicated in Fig. 1(c). Thermal NOx is almost zero at the burner nozzle tip, but increases rapidly at a distance of about 1.5 m where the temperature starts to rise rapidly. After peaking at about 2m downstream of the burner tip, thermal NOx formation decreases sharply due to the low O2 concentration, which shows the similar formation characteristics of thermal NOx with another kind of low-NOx pulverized coal burner [31]. The use of the new annular fuel air control strategy not only results in a much attenuated thermal NOx formation but also brings forward the thermal formation somehow closer to the burner nozzle tip, which also verifies the advance of the ignition point. In the fuel NOx plots, there are two opposite peaks, as seen in Fig. 11(b). The positive peak corresponds to the rapid release of the volatile N and its conversion to NO, while the negative peak corresponds to the partial reduction of the generated NOx by HCN, NH3 and other reducing substances into N2. It also shows the similar formation characteristics of fuel NOx with the low-NOx pulverized coal burner [31]. The use of the new control strategy slightly decreases and brings forward the fuel NOx formation and remarkably enhances the fuel NOx reduction. Figure 12 shows the NOx profile along the furnace height. In overall, the NOx distributions are in good accordance with the temperature and O2 distributions. The NOx concentration exhibits a sharp increase above the height of 20m where the coal particles are injected into the furnace. The use of the new control for the annular fuel air leads to a remarkable decrease in NOx concentration in the main combustion zone. Table 3 shows the calculated and measured NOx emission (6% O2) at the outlet of the air preheater under various loads. For the 100% load, the measured NOx emission is 187 mg/Nm3, which is much lower than the emission of over 350 mg/Nm3 from another similar 1000 MW corner-fired tower boiler co-firing a bituminous coal with a lean coal recently reported by Zheng et al. [4] and is also much lower than the values

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of 271 and 530 mg/Nm3 from a 500 MW corner-fired tower boiler with only CCOFA [32] and a 600 MW corner-fired tower boiler without any OFA [12]. For the 70% load, the measured NOx emission for the new control strategy is 262 mg/Nm3, lower than that from the 1000 MW corner-fired tower boiler in [4] which is above 300 mg/Nm3. Furthermore, both the experiment and simulation results show a remarkable reduction in NOx emissions when the new control strategy is used, e.g., by 20.61% (measured) at 70% load and by 23.0% (measured) at 50% load. Such decrease in NOx emissions can be explained from two aspects: (1) the reduced annular fuel air attenuates the combustion intensity at the initial combustion stage, which decreases the local gas temperature and then thermal NOx; and (2) the reduced air/coal ratio at the initial combustion stage keeps the pulverized coal under a low stoichiometric ratio condition. Hence, the intermediate species, such as HCN and NH3 which are favorable for reducing the generated NOx to N2, are restrained from converting fuel-N to NOx, and the fuel NOx eventually decreases. Table 3 not only summarizes the simulated but also the measured impacts of the new annular fuel air control strategy on the overall performance of the 1000 MWe utility boiler at various loads. The use of the new control strategy remarkably reduces the NOx emissions at medium-low loads, while not compromising the burnout condition of the coal particles. The carbon content in fly ash is always lower than 0.5%. Long-term operation data also show that the steam temperature of the boiler is maintained in a normal range. Moreover, the cooling effect on the burner nozzles is not remarkably affected. The onsite measurement on the burner A1 shows that the burner nozzle wall temperature is below 403 K under the new annular air control strategy, indicating the new control strategy is completely feasible. 3.3. General feasibility and effectiveness of the new control strategy. The new annular fuel air control strategy is also applied to another two corner-fired tower boilers (one rated 1000 MWe and another 660 MWe) which have the same operation problems. The onsite measurements show that the use of the new control

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strategy reduces NOx emissions (6% O2) by 10.8% at 70% load and by 43.4% at 50% load for the 1000 MWe boiler, and reduces NOx emissions by 15.8% at 70% load and by 23.8% at 50% load for the 660 MWe boiler. In addition, no negative effect on combustion and steam temperature is observed either from the two boilers. Some of the key measured results from the two boilers are summarized in Table 4. The successful demonstration of the new annular fuel air control strategy in all the three utility boilers well verifies its general feasibility and effectiveness.

4. CONCLUSIONS A new annular fuel air control strategy, in which the flow rate of the secondary air admitted to the furnace through the air annulus around each coal nozzle tip is controlled by the boiler load, instead of being controlled by the output of the connected mill, is proposed to reduce NOx emissions from corner-fired utility boilers at medium-low loads. Full-scale industrial measurements and comprehensive numerical simulations are performed to evaluate the impacts of the new control strategy. The measurements and simulations show good agreement in the results: the use of the new control strategy largely reduces NOx emissions from corner-fired utility boilers at medium-low loads while without compromising the combustion efficiency. For the corner-fired utility boilers under study, the use of the new control strategy reduces NOx concentrations at the SCR entrance by about 20% at both 70% and 50% load while maintains the carbon content in fly ash and steam temperature characteristics, compared to those with the original control strategy.

Acknowledgments This work was sponsored by the National Natural Science Foundation of China (NO. 51676076, No. 51390494) and the State Scholarship Fund from China Scholarship Council (No.201606165023). 14


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References (1) BP Statistical Review of World Energy 2016. http://www.bp.com/en/global/corporate /energy-economics /statistical -review-of-world-energy.html#BPstats. (2) Liu, T., Xu, G., Cai, P., Tian, L., Huang, Q. Renewable Energy 2011, 36, 1284-1292. (3) Panwar, N., Kaushik, S., Kothari S. Renew. Sust. Energ. Rev. 2011, 15, 1513–1524. (4) Zheng, Y., Gao, X., Sheng, C. Korean J. Chem. Eng. 2017, 34, 1273-1280. (5) Wan, Y.; Zhao, W.; Tang, Y.; Li, L.; Wang, H.; Cui, Y. Appl. Catal. B: Environ. 2014, 148, 114-122. (6) Uddin, M.; Shimizu, K.; Ishibe, K.; Sasaoka, E. J. Mol. Catal. A: Chem 2009, 309, 178-183. (7) Albrecht, M.; Rogan, J. System for controlling flue gas exit temperature for optimal SCR operations: US, US 6609483[P], 2003. (8) Albrecht, M.; Bloss, J. Frascello S. Multiple pass economizer and method for SCR temperature control: US, US 20070261646 A1[P], 2007. (9) Peña, D.; Uphade, B.; Smirniotis, P. J. Catal. 2004, 221, 421-431. (10) Li, J.; Chang, H.; Ma, L.; Hao, J.; Yang, R. Catal. Today 2011, 175, 147-156. (11) Staiger, B.; Unterberger, S.; Berger, R.; Hein, K. Energy 2005, 30, 1429-1438. (12) Díez, L.; Cortés, C.; Pallarés, J. Fuel 2008, 87, 1259-1269. (13) Li, S.; Xu, T.; Hui, S.; Wei, X. Appl. Energy 2009, 86, 1797-1803. (14) Zhou, H.; Mo, G.; Si, D.; Cen, K. Energy Fuels 2011, 25, 2004-2012. (15) Kuang, M.; Li, Z.; Xu, S.; Zhu, Q. Environ. Sci. Technol. 2011, 45, 3803-3811. (16) Ren, F.; Li, Z.; Chen, Z.; Fan, S.; Liu, G. Environ. Sci. Technol. 2010, 44, 6510-6516. (17) Zhang, X.; Zhou, J.; Sun, S.; Sun, R.; Qin, M. Fuel 2015, 142, 215-221. (18) Kuang, M.; Li, Z.; Zhang, Y.; Chen, X.; Jia, J.; Zhu, Q. Energy 2012, 37, 580-590. (19) Xu, H.; Smoot, L.; Hill, S. Energy Fuels 1999, 13, 411-420. (20) Li, Z.; Liu, G.; Zhu, Q.; Chen, Z.; Ren, F. Appl. Energy 2011, 88, 2400-2406. (21) Yin, C.; Yan, J. Appl. Energy 2016, 162, 742-762. (22) Badzioch, S.; Hawksley P. Ind. Eng. Chem. Process Des. Dev. 1970, 9, 521-530. (23) Hurt, R.; Sun, J.; Lunden, M. Combust. Flame 1998, 113, 181-197. (24) Sheng, C.; Moghtaderi, B.; Gupta, R.; Wall, T. Fuel 2004, 83, 1543–1552. (25) Hanson, R.; Salimian, S. Survey of rate constants in H/N/O systems. Combustion Chemistry. Springe 1984: 15


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364-421. (26) De Soete, G. Symposium on Combustion 1975, 15, 1093-1102. (27) Perry, S.; Fletcher, T.; Solum, M.; Pugmire, R.J. Energy Fuels 2000, 14, 1094-1102. (28) Genetti, D.; Fletcher, T. Energy Fuels 1999, 13, 1082-1091. (29) Glarborg, P.; Jensen, A.; Johnsson, J. Prog. Energy Combust. Sci. 2003, 29, 89–113. (30) Belosevic, S.; Beljanski, V.; Tomanovic, I.; Crnomarkovic, N.; Tucakovic, D. Energy Fuels 2012, 26, 425 −442. (31) Shi, L.; Fu, Z.; Duan, X.; Cheng, C.; Shen, Y.; Liu, B. Appl. Therm. Eng. 2016, 98, 766–777. (32) Choi, C.; Kim, C. Fuel 2009, 88, 1720–1731.

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Table 1. Coal properties Proximate analysis, (wt%), as received (ar) Volatile matter

26.20

Moisture

14.00

Ash

7.04

Fixed carbon

52.76

Lower heating value, (kJ/kg)

23.39

Higher heating value, (kJ/kg)

24.50

Ultimate analysis, (wt%), as received (ar) Carbon

63.25

Hydrogen

3.40

Oxygen

11.18

Nitrogen

0.64

Sulphur

0.50

Hardgrove index

62

Ash fusion temperature, (oC) Deformation temperature

1080

Softening temperature

1120

Hemisphere temperature

1180

Flow temperature

1220

Ash chemical composition, (wt%) SiO2

26.31

Al 2O3

12.66

TiO2

0.48

Fe2O3

20.66

CaO

18.09

MgO

1.08

K2O

0.70

Na2O

0.43

SO3

16.20

MnO2

0.06

Others

3.33

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Table 2. Operation conditions of the five cases tested and simulated

Case

1

2

3

4

5

Load

100%

70%

70%

50%

50%

ABCDE

ABCDE

ABCDE

ABCD

ABCD

74

52

52

47

47

Annular fuel air control strategy

Original

Original

New

Original

New

Annular fuel air damper opening

90%

55%

40%

55%

20%

Total coal feeding rate [kg/s]

103.3

73.0

73.0

52.7

52.7

Primary air (temperature 349.7K) [kg/s]

179.1

160.0

160.0

124.4

124.4

Annular fuel air (a portion of secondary air, 616K) [kg/s]

69.8

66.3

48.9

50.2

25.2

Auxiliary air (a portion of secondary air, 616K) [kg/s]

401.2

297.0

314.4

258.1

283.1

CCOFA (a portion of secondary air, 616K) [kg/s]

32.1

26.6

26.6

23.9

23.9

Leaking air (300K) [kg/s]

36.3

26.5

26.5

21.6

21.6

SOFA (a portion of secondary air, 616K) [kg/s]

198.0

135.7

135.7

103.3

103.3

Air/fuel ratio at initial combustion stage [kg/kg]

2.41

3.10

2.86

3.31

2.84

Mills in service Output of each mill [t/h]

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Table 3. The measured and simulated results of the 1000 MWe corner-fired tower boiler

Case

1

2

3

4

5

Load

100%

70%

70%

50%

50%


Original

Original

New

Original

New


90%

55%

40%

55%

20%

Measured

2.90

4.13

4.11

6.32

6.29

Simulated

2.78

4.09

4.07

6.23

6.21

Measured

0.85

0.45

0.33

0.35

0.46

Simulated

0.76

0.42

0.48

0.32

0.36

Measured

187

330

262

390

300

Simulated

180

335

278

397

325

Measured

94.07

94.70

-

95.59

-

O2 content (vol %)

Fly ash carbon content (%)

NOx emission (mg/m3, 6% O2)

Boiler efficiency (%)

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Table 4. Use of the new control strategy in another two boilers: the results

Boiler

1000 MWe corner-fired boiler

Load

70%

660 MWe corner-fired boiler

50%

70%

50%


Original

New

Original

New

Original

New

Original

New


50%

35%

55%

15%

65%

20%

50%

20%

O2 content (vol %)

4.28

4.31

6.31

6.33

4.47

4.50

6.18

6.16

Carbon content in fly ash (%)

1.38

1.26

0.925

1.03

0.73

0.83

0.42

0.39

NOx emission (mg/m3, 6% O2)

370

330

560

320

265

223

265

202

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Energy & Fuels

(a)

(c)

(b)

(d)

Figure 1. Overview of the 1000MWe boiler: (a) schematic diagram; (b) arrangement of burners at each corner and their connection with mill; (c) enhanced ignition burner; (d) low-NOx concentric firing system.

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Figure 2. Operation parameters of the 1000 MWe boiler: (a) Overall excess oxygen content; (b) primary air/coal ratio; (c) original and new annular fuel air control strategies.

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Energy & Fuels

Figure 3. Grid scheme: (a) on a vertical cross-section; (b) on a horizontal cross-section in the main combustion zone.

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Figure 4. CFD results along a same line in the main combustion zone (x=0–23.16m, y=11.58m, z at the layer of primary air A3) based on four different meshes: (a) velocity component Vy, and (b) gas temperature

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Figure 5. Gas velocity on the horizontal cross-section at the coal burner A3 level: (a) 70% boiler load with the original control strategy for the annular fuel air, (b) 70% load with the new control strategy, (c) 50% load with the original control strategy, and (d) 50% load with the new control strategy.

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Figure 6. Gas temperature on the horizontal cross-section at the coal burner A3 level: (a) 70% boiler load with the original control strategy for the annular fuel air, (b) 70% load with the new control strategy, (c) 50% load with the original control strategy, and (d) 50% load with the new control strategy.

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Energy & Fuels

Figure 7. Gas temperature along furnace height: (a) 70% boiler load with the original control strategy for the annular fuel air, (b) 70% load with the new control strategy, (c) 50% load with the original control strategy, (d) 50% load with the new control strategy, and (e) average temperature along furnace height.

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Figure 8. O2 mole fraction on the horizontal cross-section at the coal burner A3 level: (a) 70% boiler load with the original control strategy for the annular fuel air, (b) 70% load with the new control strategy, (c) 50% load with the original control strategy, and (d) 50% load with the new control strategy.

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Energy & Fuels

Figure 9. O2 mole fraction along furnace height: (a) 70% load with the original control strategy for the annular fuel air, (b) 70% load with the new control strategy, (c) 50% load with the original control strategy, (d) 50% load with the new control strategy, and (e) average O2 mole fraction along furnace height.

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Figure 10. NOx concentration on the horizontal cross-section at the coal burner A3 level: (a) 70% boiler load with the original control strategy for the annular fuel air, (b) 70% load with the new control strategy, (c) 50% load with the original control strategy, and (d) 50% load with the new control strategy.

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Figure 11. NOx concentration profiles from the burner A3 nozzle top to the furnace center: (a) Thermal NOx and (b) Fuel NOx.

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Figure 12. NOx concentration along furnace height: (a) 70% load with the original control strategy for the annular fuel air, (b) 70% load with the new control strategy, (c) 50% load with the original control strategy, (d) 50% load with the new control strategy, and (e) average O2 concentration along furnace height.

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New Fuel Air Control Strategy for Reducing NOx Emissions from

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