Numerical Simulation of Flow and Combustion Characteristics in a

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Numerical Simulation of Flow and Combustion Characteristics in a 300 MWe Down-Fired Boiler with Different Overfire Air Angles Feng Ren, Zhengqi Li,* Guangkui Liu, Zhichao Chen, and Qunyi Zhu School of Energy Science and Engineering, Harbin Institute of Technology, 92 West Dazhi Street, Harbin 150001, People’s Republic of China ABSTRACT: A computational fluid dynamics (CFD) model of a 300 MWe down-fired furnace equipped with overfire air (OFA) has been developed using Fluent 6.3. A level of confidence in the current CFD model has been established by performing meshindependence tests and verified by comparing furnace temperature simulations with actual furnace data. The validated CFD model is then applied in the investigation of the effects of several operating conditions at full load with different OFA nozzle angles. In the down-fired furnace, the primary air/fuel was found unable to penetrate the horizontal secondary air flow zone and reach the furnace hopper; thus, no combustion occurs in the furnace hopper zone. The peak-temperature zone appears in the upper furnace, contrary to the original design concept. Simulation results also indicate that an OFA nozzle angle set below 30° moves the mixing point of the OFA flow and the up-flowing gas downward, further aiding combustion in the upper furnace. However, when the angle increases above 30°, OFA is unable to reach the furnace center, weakening combustion in the furnace, because no reaction with coal can take place. This study provides a basis to assess in-depth future operations of down-fired boilers and help in designing OFA equipment.

1. INTRODUCTION In China, reserves of low-volatile coals, such as anthracite and lean coal, are abundant. These coal types account for 25% of the total coal consumption in electricity generation. Down-fired combustion technology, which is employed in burning lowvolatile coals, has developed quickly in the past 20 years. This technology increases the coal burnout rate by increasing particulate residence times in the furnace. In practice, extremely high NOx emissions persist. The uncontrolled NOx emissions exceed 1700 mg/Nm3 (at 6% O2 dry) and are at times as high as 2100 mg/Nm3, almost the highest reported for any type of boiler.1
3 To better understand the combustion and NOx emission characteristics of down-fired boilers, several experimental and numerical investigations have been conducted.1
7 For much of this work, the research has focused on combustion and NOx emission characteristics of the current types of down-fired boilers; few have proposed methods to lower NOx emissions. In general, there are two categories of NOx control technologies: primary control technologies that reduce the amount of NOx produced in the coal combustion zone and secondary control technologies that reduce the NOx present in the flue gas away from the coal combustion zone. Much research has revealed that the former are generally more cost-effective within a certain level of NOx removal.8
10 Dependent upon fuel and also the design of the furnace and combustion system, diverse primary control technologies are available: low NOx burners, fuel/air staging, overfire air (OFA), reburning, and flue gas recirculation. Of these methods, OFA is the more mature and widely used.11
21 From among the literature, Fan and colleagues performed laboratory experiments in a one-dimensional furnace burning anthracite and showed that OFA is useful in NOx removal.19 Li and co-workers have applied the OFA method on a 300 MWe down-fired boiler and verified that this method can greatly reduce NOx emissions in down-fired boilers.20,21 However, the resulting low-NOx combustion environment usually leads to high levels of carbon produced in fly ash, which is economically r 2011 American Chemical Society

disadvantageous for power plants.13
16,19,20 It is thus necessary to determine how to lower carbon content in fly ash after the introduction of OFA. This paper describes preliminary investigations by numerical simulations of the OFA mixing behavior and final coal combustion in the upper furnace of the down-fired furnace. We have focused on the influence of the OFA nozzle angle and finding an optimal angle that lowers the unburnt loss caused by OFA. By comparing O2 concentrations and gas temperatures within the furnace, temperatures of the furnace outlet, and combustible material content in the fly ash, we can then perform a comparative analysis of the combustion air distribution at different OFA angles under actual operations and provide a theoretical basis for the operation of utility boilers in real situations.

2. METHODOLOGY 2.1. Boiler Geometry and Operating Conditions. The commercial package, Gambit 4.0, has been chosen to generate a digital model of the boiler configuration. The investigated 300 MWe boiler unit generates 285 kg/s of steam, at 17.5 MPa and 540 °C, when operated at full load. The boiler is a II-type layout with double arches and a single furnace, which is 38.2 m high. Figure 1 shows the geometric configuration of the computational fluid dynamics (CFD) model. In the original boiler, 24 pulverized coal burners are set on the arches to stabilize the W-shaped flame. The zone below the arches is called the lower furnace, while that above the arches is the upper zone. Each burner on the arches has two fuel-rich and two fuel-lean nozzles, through which the pulverized anthracite particles are fed into the furnace. Coal properties are listed in Table 1. As depicted in Figure 1, secondary-air stream A feeds fuel-nozzle air to the furnace through the port concentric with each fuel-rich nozzle. Another stream B feeds air to the furnace through the port concentric Received: December 8, 2010 Revised: March 5, 2011 Published: March 22, 2011 1457

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Figure 1. Geometry of the CFD model for the FW-type 300 MWe down-fired boiler.

Table 1. Coal Properties proximate analysis (wt %, as received)

ultimate analysis (wt %, air-dried)

volatile

ash

moisture

fixed carbon

net heating value (kJ/kg)

C

H

S

N

O

8.24

27.39

7.0

57.37

21740

62.72

1.84

1.47

0.96

2.23

with each fuel-lean nozzle. A third stream C feeds secondary air to ports concentric with the oil igniter, igniter flame detector, and sight port behind the fuel-rich nozzles on the arch. The remainder, which is also supplied from the secondary air duct, comprises most of the secondary air required for combustion. The compartmentalized wind box of A distributes the combustion air to the furnace through three tiers of slots, D, E, and F, between the vertical waterwall tubes in the front and rear walls under the arches. Normally, the dampers of D and E tiers are closed, and only a very small quantity of secondary air leaks into the furnace. A more detailed description can be found elsewhere.1
3 To investigate the influence of the OFA angle on the combustion and NOx emission characteristics, several OFA ports were situated in the upper furnace when creating the computer model. Because of the mirror symmetry of the furnace, only half of the furnace along its width was simulated to conserve resources and reduce computational costs. According to experience garnered from previous simulations, a simplification of the configured domain can reduce the number of cells and shorten computing times but has little effect on actual distributions of the main variables, such as temperature and gas component concentration. Under full-load operations, all burners normally supply 114 tons of coal particles/h through the corresponding fuel-rich and fuel-lean nozzles. The quantities of primary air and secondary air are 50.3 and 209.8 kg/s, respectively. In this study, both the coal and combustion air flow rates are assumed to be evenly distributed between the nozzles and ports of the same type. Air temperatures at the inlet are based on power plant data. Here, at a fixed OFA ratio of 25%, five distinct simulations are studied, distinguished by their settings of the OFA nozzle angle at 0°, 10°, 20°, 30°, and 40°. The specific boundary conditions for the inlets

Table 2. Furnace Parameters Associated with Different OFA Nozzle Angles parameter

value

parameters for the fuel-rich flow coal feed rate (kg/s)

0.612

air velocity (m/s)

16.4

parameters for the fuel-lean flow coal feed rate (kg/s)

0.049

air velocity (m/s)

3.4

primary air temperature (°C)

105

velocity of secondary air A (m/s) velocity of secondary air B (m/s)

9.8 9.8

velocity of secondary air D (m/s)

0

velocity of secondary air E (m/s)

2

velocity of secondary air F (m/s)

9.8

OFA velocity (m/s)

23

secondary air temperature (°C)

320

are listed in Table 2. The particle size of the coal at each fuel-rich and fuel-lean nozzle follows the Rosin
Rammler distribution, with the minimum diameter of 5 μm and the maximum diameter of 80 μm. 2.2. Numerical Simulation Method. A CFD program, FLUENT (version 6.3.26), has proven to be quite a powerful and effective tool in simulating control operations of coal-fired utility boilers and 1458

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Energy & Fuels analyzing processes that occur within them. This program was used to conduct numerical simulations here. The calculation procedure includes numerical solution of the time-averaged conservation equations for the gas and particle phases, using the Eulerian description for the former and the Lagrangian description for the latter. Gas turbulence was specifically taken into account by the so-called standard k
ε model, which has been found to perform well in a preliminary study of non-swirl flames. A stochastic tracking model was applied to analyze pulverized-coal flows, while calculations of gas/particle two-phase coupling employed the particle-source-in-cell method. Radiation was described using the P-1 model, and devolatilization was modeled with the two-competing-rate Kobayashi model. The combustion of volatiles was modeled by employing probability density function theory, and char combustion was modeled by employing a diffusion/kinetics model. More detailed descriptions can be found in the literature.22 NOx formation includes thermal NOx and fuel NOx but hardly any prompt NOx. Here, only NO production was taken into account because the NOx emitted into the atmosphere from combusting fuels consists mostly of NO, accompanied by much lower concentrations of NO2 and N2O. The concentration of thermal NOx was calculated using the extended Zeldovich mechanism (specifically, N2 þ O f NO þ N, N þ O2 f NO þ O, and N þ OH f NO þ H).23 The fuel NOx concentration was calculated using De Soete’s model.24 The nitrogen in the volatile evolves via intermediates HCN and NO, and nitrogen in char converts to NO directly. The advanced chemical percolation devolatilization model with the prediction of nitrogen release was used to estimate the mass fraction of the nitrogen release in the volatile and the percentage of nitrogen in the volatile evolving via HCN. The conversion ratio of nitrogen in char is set at 0.5. The formation of prompt NOx was neglected in calculations. A “no-slip” boundary condition is employed along the wall for the gas phase. Heat transfer on the wall boundary is calculated by setting the temperature outside the furnace wall. Here, the temperature outside the furnace is assumed to be the temperature of the water and steam in the water wall tubes, 360 °C. The emissivity of the wall set at 0.7. The mass, momentum, chemical species, and energy equations are each discretized using the finite volume approach. The SIMPLEST algorithm for the pressure correction was applied to couple the velocity and pressure fields. Using the first-order finite difference method, the conservation equations of the gas phase were solved with successive underrelaxation iterations until the solution satisfied a pre-specified tolerance.

3. RESULTS AND DISCUSSION 3.1. Grid Independence Test and Validation. A meshindependence test is conducted without OFA. An initial mesh with about 1 221 623 nodes was first created in the computational domain. The number of mesh nodes was then decreased to 1 043 256, 796 580, and 614 720. Mesh independence was checked by comparing the gas-phase vertical velocity component w and the temperature along the “grid-testing line” (shown in Figure 1) in the furnace. Figure 2 plots the vertical gas velocity w for each of the four mesh densities for easy comparison. The finest mesh (1 221 623 nodes) and medium-density mesh (1 043 256 nodes) yield similar results. For this reason, we chose a mesh density of 1 043 256 nodes and have applied it to all further work reported in this study. A comparison of the predicted temperature and O2 concentration without OFA with experimental measurements is shown in Figure 3. The experimental measurement was performed on the 300 MWe down-fired boiler, from which the geometry of this numerical simulation is modeled. At the time of the measurements, the operating conditions of the unit were almost the same as those set for the CFD simulations; i.e., the unit was operating at full load. In the experimental measurement, several temperature values of the furnace gas were measured with a 3i hand-held

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Figure 2. Mesh-independence test based on gas vertical velocities.

pyrometer (manufactured by Raytek, Santa Cruz, CA) inserted through monitoring ports in the front, rear, and side walls (shown in Figure 1). Also, gases were sampled through three monitoring ports at several points of the furnace at three different levels (measured monitoring ports shown in Figure 1) using a 2.5 m long water-cooled stainless-steel probe for analysis of local mean O2 concentrations. Gases were withdrawn and analyzed online using a Testo 350M system. More detailed information of the actual boiler structure and the measurement have been described in earlier reports.1
3 In the comparison of the experimental and numerical data, both of the temperatures were obtained using the same calculational procedure. For both, the temperature at each height of the boiler was determined by averaging the temperatures from all ports at the same height along each of the four walls. Simultaneously, the measured and modeled O2 concentrations were compared point by point through the three monitoring ports shown in Figure 1. It could be found in Figure 3 that, except for the specific values, the measured temperature and O2 concentrations are a little different from the modeled temperature and O2 concentrations. The overall trend in the measured values are reflected in the modeled values; good agreement between measurement and simulation is obtained. More importantly, here, peak temperatures for both appear in the upper furnace. This indicates that most of the combustion is occurring in the upper furnace, although this is inconsistent with the design concept. Table 3 shows the O2 and NOx concentrations at the furnace exit for the experimental and the numerical simulation cases in the down-fired furnace without OFA. It also shows the differences between the measurement and simulation results. However, both cases give the same characteristics in the down-fired furnace that the combustion in the furnace is good, while the NOx emission is high. It also means that the results from the numerical simulation can be used for the analysis for the actual conditions. 3.2. Analysis of the Flow Field. The ratio of unburnt carbon in fly ash depends upon the combustion characteristics of the furnace. To attain a full understanding of these characteristics, accurate aerodynamic fields within the furnace are needed first. Figure 4 shows a typical flow field in the down-fired furnace. The primary air/fuel streams flow rapidly down into the furnace at high speeds. That flow is blocked and diverted by horizontally injected secondary air originating from the slot-type ports on the front and rear walls. Two large recirculation zones form under the horizontal secondary air. An updraft of primary air/fuel and 1459

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Figure 3. Comparison of calculated and measured temperatures and O2 concentrations in the furnace (MP is short for monitoring port).

Table 3. Simulation Results of Gas Temperatures and Various Concentrations at the Furnace Outlet NOx concentration parameter

O2 concentration (%)

(mg/m3, at 6% O2)

measurement

2.82

2101

simulation

1.64

1753

Figure 5. Simulated particle trajectories in a 300 MWe down-fired boiler.

Figure 4. Simulated flow field in a 300 MWe down-fired boiler.

secondary air flows into the central zone through the lower and upper furnaces. As a consequence, most of the coal particles concentrate in the center of the furnace. In the upper furnace, OFA flows are added to and mix with the up-flowing gases; the mixing dynamics here directly determine coal combustion in the upper furnace. Figure 5 shows the particle trajectories of two

different sizes. The majority of particles follow the air, first flowing down into the lower furnace and then reversing up. They form the “W”-shaped flow type. It could be found that the trajectories of the particles with the diameter of 20 μm are somewhat disorganized and random, while those with the diameter of 50 μm are more orderly because of the larger inertia. Figure 6 presents a sequence of flow fields around the outlet area of a single OFA nozzle, each with different OFA nozzle angles. This figure indicates that the OFA initially flows downward and toward the furnace center with an initial momentum. As this air begins to mix with upward-flowing gas, a steady rate of collisions stops the OFA from its initial course and, instead, redirects its flow vertically upward. Apart from the air ratio, the level and depth OFA able to reach in the upper furnace are the more 1460

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Figure 6. Flow fields at the outlet of an OFA nozzle with different nozzle angles.

Figure 7. Relative drop in OFA along the height of the furnace at different nozzle angles.

Figure 8. Decay curves for the dimensionless horizontal velocity along the OFA with different OFA nozzle angles.

important OFA parameters. The depth determines how much of the coal mixes with OFA and burns in the oxygen atmosphere in the final combustion, while the level determines how long the coal can burn in that atmosphere. The five flow fields of Figure 6 indicate that, as the OFA nozzle angle increases, the OFA level descends further into the furnace. As a consequence, the mixing point of OFA and gas flow starts lower down in the furnace, and this is advantageous in burnout of the coal particles. Figure 7 shows the relative drop (
z direction) that OFA descends along the height of the furnace. As the OFA nozzle angle increases, the variation of this drop quickens, leading to the conclusion that the OFA nozzle should be set at a large angle. However, closer inspection of the simulation sequence given in Figure 6 shows that, as the OFA nozzle angle increases, the ability for OFA to penetrate into the center of the upper furnace weakens significantly; this is clearly obvious with a nozzle angle of 40°. Because the concentration of particles is at its highest in the center, no OFA mixing is disadvantageous for coal burnout. Figure 8 shows the variation of the OFA horizontal velocity Vx along the breadth (x direction) of the furnace. Here, Vmax represents the horizontal velocity at the outlet of the OFA nozzle; the origin is set at the outlet of the OFA nozzle. The figure indicates that the horizontal velocity decays as the OFA pushes toward the furnace center. Greater nozzle angles hasten the decay of this velocity; at 40°, the OFA stops at a distance of 2.8 m from the OFA nozzle, signifying that the OFA flow was not able to reach the center. With smaller nozzle angles, the OFA

flows could all reach the furnace center, implying that smaller OFA angles would be more advantageous. Thus, an optimal OFA angle setting should be neither too large nor too small. 3.3. Analysis of Combustion. Figure 9 presents a sequence of gas-temperature field simulations for a longitude cross-section of the furnace passing through the OFA nozzle (the indicatory section in Figure 1). When the OFA nozzle angle is varied, the temperature distribution throughout the furnace shows similar regularities. Gas temperatures are low in the furnace hopper because the primary air/fuel is unable to penetrate the horizontal secondary air flow zone, and thus, almost no combustion occurs here. Two high-temperature zones appear in the furnace, one of which appears in the zone not far from the primary air/fuel nozzles caused by the release of volatile material from the coal. These zones react with the surrounding oxygen and generate a great deal of heat, thus raising the temperature of the primary air/fuel. However, the quantity of volatile is too little that the heat generated cannot assist further combustion. The large quantity of relatively cold secondary air from the front and rear walls also does not assist the primary air/fuel combustion but rather weakens it; here, the temperatures fall quickly. Only when the gas flow reaches the area above the arches does the second high-temperature zone appear. Most of the char combustion is completed in the upper furnace, which is not in accordance with the initial design concept of down-fired boilers. The reason can be pinpointed to the low reactivity of anthracite. 1461

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Figure 9. Calculated gas temperature (K) over the indicatory section with different OFA nozzle angles.

Figure 10. Distributions of the average gas temperature along the furnace height with different OFA nozzle angles.

Figure 10 presents, for different OFA nozzle angles, the average gas temperature distributions along the furnace height. Generally, from the furnace bottom to the furnace exit, temperatures initially rise and then gently fall; overlaying this trend are three cooling areas (valleys of the curves). The average temperature in the furnace hopper zone (levels z = 0
5 m) is low because no combustion occurs in this area. At level z = 6 m, cooling is caused by the mixing of cold horizontal secondary air. Above the secondary air flow zone, the temperature begins to rise along the furnace height because of the combustion of volatile matter and char. A second cooling area appears at level z = 13 m that is caused by the injection of primary air from the arches. Temperatures continue to rise above this level until its peak value again above the arches. With the introduction of OFA, a cooling area arises at level z = 17 m. After this peak zone, gas temperatures fall. Also noticeable in Figure 10 is the fact that, independent of the OFA nozzle angle, temperature distributions are very similar in

the lower furnace below level z = 14 m. Clearly, the OFA nozzle angle has no influence over combustion in the lower furnace. However, after the injection of OFA at level z = 17 m, the five curves begin to separate. Above the third cooling area, average temperatures raise as the OFA nozzle angle increases from 0° to 30°, signaling an enhancement in combustion in the upper furnace. As mentioned in the flow field analysis, a rise in the OFA nozzle angle lowers the mixing point of OFA and upflowing gas. Coal residence times are prolonged, so that more coal burns in the oxygen atmosphere, thereby raising the temperature of the upper furnace. However, at the 40° angle, the temperature is apparently lower than for the lower settings. From the flow field in Figure 6, it was noted that OFA is not able to reach the center of the upper furnace and mix with the particles there. Therefore, with low combustion, the average temperature of the upper furnace falls, affecting then coal burnout. Figure 11 shows the simulation results of O2 concentration fields with different OFA nozzle angles for the indicated longitudinal cross-section shown in Figure 1. In general, the O2 distributions are similar in all five settings. It can be seen that, except for the exit area of air ports, at each level of the furnace, the highest O2 concentrations appear in the center part of the furnace. O2 concentrations near the wall of the arch and the upper furnace are low. Considering the high-temperature environment of this area, this may cause problems in the form of wall slagging and high-temperature corrosion; some special measure, such as introducing small amounts of air near the wall, may need to be taken into consideration. Figure 12 presents the OFA nozzle angle dependence of the distribution of the average O2 concentration along the furnace height. The average O2 in the furnace hopper zone (levels z = 0
5 m) and F-tier air flow zone (levels z = 5
7 m) is around 0.20, indicating that O2 has not been consumed in this area. This confirms the fact that the primary air/fuel stream could not penetrate the F-tier air flow zone and that no combustion occurs in this area. Above this zone, O2 concentrations fall quickly because of the intense combustion in the furnace. Only at level 1462

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Figure 11. Calculated O2 concentrations over the indicatory section with different OFA nozzle angles.

Table 4. Simulation Results of Gas Temperatures and Various Concentrations at the Furnace Outlet OFA nozzle flue gas O2 NOx combustible angles temperature concentration concentration material content (deg) (K) (%) (mg/m3, at 6% O2) in the fly ash (%) 0 10 20 30 40

Figure 12. Distributions of the O2 concentration along the furnace height with different OFA nozzle angles.

z = 17 m near the OFA ports does the O2 concentrations rise slightly. From Figure 12, we find that, similar to the average temperature distribution along the furnace height, all five curves representing the different OFA angle settings coincide with one another in the lower furnace (below level z = 14 m). Only after the addition of OFA do differences occur. At low OFA nozzle angles from 0° to 30°, the increase of the OFA nozzle angle leads to a fall in O2 concentrations in the upper furnace. The cause is the previously mentioned lower mixing point, thus bringing forward ignition of coal and O2 in OFA. At the larger 40° angle, OFA fails to reach the furnace center and cannot react with coal there; therefore, as shown in Figure 12, O2 concentrations are at their highest, thereby not favoring coal burnout. From the point of view of combustion, 30° provides an optimal value for the OFA nozzle angle. 3.4. Distribution Characteristics of Gas Temperature and Concentrations at the Furnace Outlet. Table 4 presents simulation data of the gas temperature at the furnace outlet

1340 1352 1375 1423 1287

2.63 2.25 1.95 1.74 2.87

1425 1422 1430 1442 1436

2.82 2.52 2.32 1.57 3.44

together with distributions of O2 and NOx concentrations and combustible material content in the fly ash for each of the OFA nozzle angle settings. It shows that NOx emissions rise as OFA nozzle angles increase, except for the 40° setting for which NOx emissions are the least among the five. However, this variation is very slight, and we can consider that these emissions are essentially constant; the OFA nozzle angle has little influence on the final NOx emission of a down-fired furnace. For the temperature and O2 concentration at the furnace outlet, similar to the distribution in the upper furnace, the 30° setting produces the highest temperatures and the lowest O2 content, while in contrast, the 40° setting produces the lowest temperatures and the highest O2 content. The significance is that, with the same quantity of coal and boundary conditions, the 30° setting consumes the most oxygen and releases the most heat; the 40° setting has the reverse effect. From the combustible material content in the fly ash, the 30° setting leaves the least unburnt carbon at the furnace outlet, while the 40° setting leaves the most. It may be a little confusing that the OFA angle has little influence on the NOx emission but affects the coal combustion more largely. The reason could be explained that, in the furnace combustion, most NOx generations are completed in the early stage of the coal combustion. In the later combustion, the main reaction for NOx is the reduction reaction between the generated NOx and the char. This reaction is of low reactivity and may be influenced little by the surrouding 1463

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Energy & Fuels combustion conditions. Therefore, among these cases with different OFA supply angles in the latter combustion but with the same air and coal supply conditions in the early combustion, NOx varies little. All of these parameters at the furnace outlet indicate that, by regulating the OFA nozzle angle, better combustion can be achieved in the furnace without any accompanying rise in NOx emissions. With the OFA angle at 30°, the down-fired boiler operates at its best combustion efficiency, a setting that should be selected for if environmental impact of coal-fired burners is to be mitigated.

4. CONCLUSION A CFD model of a 300 MWe down-fired furnace has been developed. A level of confidence in the current CFD models has been established by mesh-independence testing and validation against actual power plant data. The validated CFD model was then applied to predict combustion behaviors of anthracite coal in the furnace at different OFA nozzle angles while operating at full load, from which the following conclusions are drawn: (1) In the down-fired furnace, the primary air/fuel was unable to penetrate the horizontal secondary air flow zone and reach the furnace hopper. Thus, no combustion could occur in the furnace hopper zone, which amounts to a waste of the furnace volume. (2) As the OFA angle increased from 0° to 30°, the mixing point of the OFA and the up-flowing gas in the furnace is lowered, thus prolonging residence times for coal in the oxygen atmosphere, which is advantageous toward coal burnout in the furnace. Given the range of variation in the angle, the OFA flows all reached the center part of the upper furnace and reacted with the coal there. (3) At a nozzle angle of 40°, OFA was unable to reach the furnace center and react with coal particles, thereby lowering the total reaction temperature in the upper furnace and suppressing coal burnout. (4) Changing OFA nozzle angles has little influence on the final NOx emission at the furnace outlet. (5) The suggested OFA nozzle angle for boiler operation is concluded to be 30°.

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’ AUTHOR INFORMATION Corresponding Author

*Telephone: þ86-451-8641-8854. Fax: þ86-451-8641-2528. E-mail: [email protected].

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