Optimization of Separated Overfire Air System for a Utility Boiler from a

Article pubs.acs.org/EF

Optimization of Separated Overfire Air System for a Utility Boiler from a 3‑MW Pilot-Scale Facility Jianwen Zhang,† Kai Chen,† Chang’an Wang,† Kun Xiao,‡ Xueyuan Xu,‡ and Defu Che*,† †

School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China Shanghai Boiler Works, Ltd., Shanghai, 200245, People’s Republic of China

‡

ABSTRACT: A 3-MW pilot-scale facility was used to study the effect of separated overfire air (SOFA) on NOx emissions from utility boilers. Experiment results showed that NOx emissions decreased as the excess air ratio in the main combustion zone first decreased, but remained unchanged when the excess air ratio was above a certain value; the carbon content in fly ash increased with the decrease of the excess air ratio in the main combustion zone; NOx emissions decreased and the heat loss due to unburned carbon increased as the residence time in the reduction zone (the zone in the furnace from the middle of combustion zone to the middle of the SOFA zone) increased. Based on the experiment results, a residence time of 2.12−2.68 s is recommended for the reduction zone. Besides, numerical simulations were conducted about the pilot-scale facility and a 600-MW boiler firing the blended coal of Shenhua coal (80 wt %) and Baode coal (20 wt %). The simulations, together with the experiments, showed that there was a critical excess air ratio with value of ∼0.8 for the minimum NOx emissions with relatively low unburned carbon. This critical value, together with the residence time of 2.35 s, which is in the range recommended by our experiments, were used in the retrofit of the 600-MW boiler by SOFA technology, leading to a reduction of NOx emissions as high as 60%. combustion zone, and SOFA position.8−11 However, in the laboratory-scale experiments, flow field is different from that in the full-scale boiler, because of different burner arrangements. The difference may lead to different NOx emissions. Thus, experimental investigation with consideration of coal type, position of SOFA, the excess air ratio in the main combustion zone and flow field should be conducted. In the present study, a 3-MW pilot-scale facility has been used to investigate the effect of SOFA on NOx emission. In the pilot-scale facility, the flow field and residence time of coal particle are set quite similar to those in a large capacity boiler. Such similarity is of crucial importance to accurately predict the effect of SOFA on NOx emissions from the large-capacity boiler. Besides, both experiments and computational fluid dynamics (CFD) have been used to predict NOx emissions from the pilot-scale facility and the large capacity boiler.

1. INTRODUCTION NOx that is released from power plants has attracted extensive attention over past years, because of its considerable harm to the environment and human health. It can react with photochemical oxidants, particulate matter, and sunlight in the air, generating photochemical smog. It is also responsible for acid rain. Therefore, it is crucial to control NOx emission. Currently, related organizations and local governments have published stricter regulations and legislation against the NOx emissions from power plants around the world. In China, State Ministry of Environmental Protection announced a new emissions standard of air pollutants for thermal power plants on July 29, 2011,1 in which the NOx emissions limit of power plants is reduced from 450 mg Nm−3 to 100 mg Nm−3 (calculated under the condition of dry flue gas and 6% oxygen by volume). Under this strict standard, numerous low-NOx combustion technologies have been extensively developed, including low NOx burners (LNB), fuel-staged combustion (reburning), flue gas recirculation (FGR), and air-staged combustion. The present study focuses on the air-staged combustion technology, especially the separated overfire air combustion (SOFA) technology. Many studies have been carried out on air-staged combustion,2−5 and it has been determined that this technology can reduce NOx emissions effectively. Over the past few decades, air-staged combustion technology has been developed from overfire air (OFA) to SOFA. The SOFA technology can reduce the NOx emissions up to 50%.6,7 Nevertheless, most previous studies were carried out with laboratory-scale equipments, such as a one-dimensional (1D) drop furnace and the furnace with one burner. Experimental results from previous studies had showed that NOx emissions were affected by coal type, coal fineness, the excess air ratio in the main © 2013 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Pilot-Scale Facility. The combustion experiments were carried out in a 3-MW pilot-scale facility, as shown in Figure 1. The experimental facility mainly consists of a raw coal crushing and drying system, a pulverizing system, a coal conveying system, a limestone conveying system, boiler proper, a forced draft fan, an induced draft fan, a flue gas recirculation fan, a natural gas ignition system, a water treatment and recycling system, desulfurization equipment, a dust removal system, a denitrification system, a flue gas online analysis system, a distributed control system, a gas supply system (O2, CO2, natural gas), an ash and slag disposal system, a compressed air system, and other auxiliary systems. Received: November 1, 2012 Revised: January 15, 2013 Published: January 16, 2013 1131

dx.doi.org/10.1021/ef301779c | Energy Fuels 2013, 27, 1131−1140

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Figure 1. Schematic of a 3-MW experimental facility for coal combustion.

Figure 2. Configuration and dimensions of 3-MW experimental facility.

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Dimensions of the experimental facility are illustrated in Figure 2 in detail. The locations of three layers of SOFA are shown in Figure 2a. Under each experimental condition, only the SOFA ports of one layer are opened and the others are shut off. The zone between the middle of combustion zone and the SOFA port is known as the reduction zone. The zone between the SOFA port and the furnace exit is known as the burn-out zone. Boundary of the reduction zone and the burnout zone may change under different conditions, because of the change of the location of SOFA port put into operation. The arrangement of burners and OFA ports are shown in Figure 2b and 2c. Eight SOFA ports are included in each layer. Four SOFA ports are located at walls and the others are located at four corners, as shown in Figure 2b. Under each experimental condition, only the SOFA ports located at walls (corners) are opened and the others are shut off. A portable multifunction flue gas measurement system (Testo 350XL), with the accuracy of ±5.0 ppm (0.0−99.0 ppm) and ±5.0% (100.0−2000.0 ppm), was used to obtain the concentrations of NO and NO2 in the flue gas. The flue gas for measurement is from the flue pass before the bag filter, which is numbered 11 in Figure 1. Fly ash sampling points were arranged in the horizontal flue gas pass. Pitot tube with parallel dust sampling (PTP-III-type) was employed to sample fly ash. The sampling instrument consists of host, sampling gun, filter cartridge, trap, U-shaped manometer, rubber tube and pump, etc. 2.2. NOx Formation Mechanism. Thermal NOx, fuel NOx, and prompt NOx are three main mechanisms for NOx formation. The contribution of prompt NOx is usually negligible in coal-fired furnaces.12 Therefore, the other two (thermal NOx and fuel NOx) are taken into account. 2.2.1. Thermal NOx. Thermal NOx is generated by oxidation of the atmospheric nitrogen. The reactions governing the formation of thermal NOx, as presented in the extended Zeldovich mechanism, are as follows:

HCN + NO → N2 + ... k6

NH3 + O2 → NO + ... k7

NH3 + NO → N2 + ...

kchar

char + NO ⎯⎯⎯→ N2 + ...

− k 7[NO][NH3] − kcharCchar

k3

N + OH XoooY H + NO

(R3)

k −3

Therefore, the net rate of thermal NOx formation is given by the following expression:

d[NO] = k1[O][N2] + k 2[N][O2 ] + k 3[N][OH] dt − k −1[NO][N] − k −2[NO][O] − k −3[NO][H] (1) In eq 1, the rate constants (k±1, k±2, and k±3) are selected based on the evaluation of Hanson et al.13 Assuming that the rate of consumption of N atoms is equal to its formation, which is reasonable in this study, the thermal NOx formation rate given by eq 1 can be simplified as follows:

( (

1− d[NO] = 2k1[O][N2] dt 1+

k −1k −2[NO]2 k1[N2]k 2[O2 ]2 k −1[NO] k 2[O2 ]k 3[OH]

) )

(2)

In eq 2, local concentrations of O and OH radicals are estimated by adopting the partial equilibrium approach. 2.2.2. Fuel NOx. Fuel NOx is generated by oxidation of the nitrogen bound in the coal. In the generation process, HCN and NH3 are assumed to be nitrogen-bearing intermediates. These nitrogen-bearing intermediates are competitively oxidized and reduced through following reactions: k4

HCN + O2 → NO + ...

(3)

where α is the oxygen-reaction order, determined by local oxygen mole fraction,15 and Cchar (m2 m−3) is the char surface density per volume unit. 2.3. Similarity Modeling Analysis for NOx Emission. NOx emissions from a coal-fired boiler is subject to the following factors: coal type, furnace temperature, atmosphere in combustion zone, residence time of coal particles in reduction zone, and coal fineness.8−11 The impact of coal type on NOx emissions is reflected mainly by the volatile content and by the nitrogen content in coal. Higher volatile content leads to larger proportion of volatile nitrogen in fuel nitrogen,16,17 and the volatile nitrogen is more likely to be converted to N2; hence, lower NOx emissions are attained. The higher nitrogen content usually produces more NOx under the same combustion condition. Temperature mainly affects the formation of thermal NOx. The thermal NOx is ignorable when the temperature is lower than 1500 °C. However, when the temperature is higher than 1500 °C, thermal NOx generation rate increases by a factor of 6 as the temperature increases by 100 °C. Under typical circumstances, thermal NOx accounts for ∼10% of the total NOx emissions for a practical boiler.18,19 Flue gas composition has an important impact on the formation of fuel NOx. Fuel nitrogen first generates intermediate products (HCN and NHi) at high temperature, and then these intermediates produce NO or N2, according to reactions R4−R7.20 These two reactions are competitive. Under an oxidizing atmosphere, reactions R4 and R6 are dominant, while the leading reactions are R5 and R7 for a reducing atmosphere. The principle of low NOx combustion technology generally involves the ceration of a reductive atmosphere to ensure that the intermediate products react according to reactions R5 and R7. The atmosphere near the burner is primarily dependent on the total excess air ratio and the stoichiometric ratio of the main combustion zone. A longer residence time can induce more reduction of NO to N2. The longer distance between SOFA and upper primary air nozzle can be adopted to increase the residence time and reduce NOx emissions. Generally, NOx emissions first decrease as the coal particle size decreases. Nevertheless, when the fineness of coal particle is smaller than a specific value, further decreases in coal particle size no longer reduce NOx emissions.21 The five factors above also affect the combustion efficiency of a boiler.20 Therefore, some requirements should be satisfied to ensure that the experimental results in the 3-MW pilot-scale facility can be employed to predict the practical performance of the 600-MW utility boiler. The coal type, the coal fineness, the excess air ratio in combustion zone, and the residence time of pulverized coal in both combustion zone and NOx reduction zone in the 3-MW facility should be the same as those in the 600-MW utility boiler. The temperature distribution in the 3-MW facility should be similar to that in the 600MW boiler.

(R2)

k −2

(R8)

d[NO] = k4[HCN][O2 ]α + k5[NH3][O2 ]α − k6[NO][HCN] dt

k2

N + O2 XoooY O + NO

(R7)

Therefore, the net rate of fuel NOx formation, according to reactions R4−R8, is given by the following expression:

(R1)

k2

(R6)

In the four reactions R4−R7, N2 and NO are included in the products. In addition, according to Levy et al.,14 the NO is reduced by heterogeneous reaction on the char surface. The reaction is shown below:

k1

O + N ⇄ N + NO

(R5)

(R4) 1133


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2.4. Experimental Conditions. 2.4.1. Coal Properties. A blended coal, comprised of Shenhua coal (80 wt %) and Baode coal (20 wt %), was used in the pilot-scale experiments. The coal is the same as that used in the 600-MW utility boiler. The proximate and ultimate analyses of experimental coal are shown in Table 1.

Table 2. Comparison of Key Parameters between Pilot-Scale Facility and the Utility Boiler pilot-scale facility

parameter

Table 1. Proximate and Ultimate Analyses of Experimental Coal (As-Received Basis) coal property proximate analysis (wt %) moisture volatile matter fixed carbon ash ultimate analysis (wt %) hydrogen carbon sulfur nitrogen oxygen higher heating value, HHV (kJ kg−1)

Shenhua coal

Baode coal

blended coal

13.46 31.58 50.11 4.85

3.81 27.35 43.58 25.26

11.53 30.73 48.80 8.93

6.13 66.86 0.44 0.62 7.64 29380

4.64 58.16 0.39 0.74 6.99 24600

5.83 65.12 0.43 0.64 7.51 25156

flue gas temperature in the main combustion zone (°C) flue gas temperature at furnace exit (°C) heat input (MW) residence time in reduction zone (s) residence time in burn out zone (s) total residence time (s)

utility boiler, heat output = 600 MW

heat output = 3.0 MW



1450

1550

1500

1350

888

1077

912

765

1559 2.35

3.0 1.79

2.0 2.68

1.0 5.36

2.59

1.17

1.75

3.50

4.94

2.96

4.43

8.86

Table 3. Experimental Runs of the Pilot-Scale Facility experimental group Gr 1 Gr 2 Gr 3

Laser particle size analyzer (Malvern, Mastersizer 2000 type) was used to analyze the particle size distribution of the blended coal used in the pilot-scale experiments and the data are shown in Figure 3. The analysis results indicated that the coal fineness (R90) is 16%, and the average particle size is 55.87 μm, which is similar to the coal used in the 600-MW boiler, whose R90 value is 18% and the uniformity exponent is 1.1. 2.4.2. Determination of Heat Output. Although the pilot-scale facility can achieve a maximum output of 3 MW, some experiments were conducted in the pilot-scale facility to determine an appropriate heat output, at which the temperature distribution in the pilot-scale facility is similar to that in the 600-MW utility boiler. Table 2 shows the temperature distributions in both the pilot-scale facility and the 600-MW boiler. Experimental results show that the temperature distribution in the pilot-scale facility with heat output of 2 MW is in general agreement with that in the 600-MW boiler. Hence, the heat output of pilot-scale facility was kept at 2 MW during the subsequent experiments. 2.4.3. Experimental Runs. Six groups of experimental runs, classified by the difference in SOFA port arrangement, are included in the present study. Table 3 shows the detailed experimental runs on the pilot-scale facility. For each group, six experimental runs with different SOFA flow rates are included. The mass flow ratios of SOFA to the total air are 0, 10%, 20%, 30%, 35%, and 40%. The corresponding excess air ratios in main burner zone are 1.20, 1.08, 0.96, 0.84, 0.78, and 0.72, respectively. The stoichiometric ratio of primary air is fixed at 28% to ensure the delivery of pulverized coal. In

Gr 4 Gr 5 Gr 6

SOFA location low position, four-corners arrangement low position, four-walls arrangement middle position, four-corners arrangement middle position, four-walls arrangement high position, four-corners arrangement high position, four-walls arrangement

ratios of SOFA to total air 0, 10%, 20%, 35%, 40% 0, 10%, 20%, 35%, 40% 0, 10%, 20%, 35%, 40% 0, 10%, 20%, 35%, 40% 0, 10%, 20%, 35%, 40% 0, 10%, 20%, 35%, 40%

30%, 30%, 30%, 30%, 30%, 30%,

addition, the excess air ratio at the outlet of pilot-scale facility is 1.2, which is the same as that in the 600-MW boiler. 2.5. Results and Discussion. 2.5.1. Effect of the Excess Air Ratio in the Main Combustion Zone. Figure 4 shows the effect of the excess air ratio in the main combustion zone on the NOx emissions for various SOFA port arrangements on the pilot-scale facility with tangentially fired burners. The NOx emissions at the furnace exit are almost kept unchanged when the excess air ratio in the main combustion zone increases from 0.72 to 0.78. Nevertheless, the NOx emissions increase significantly when the excess air ratio is increased from 0.78 to 1.2. This trend is in accordance with previous studies.9,10,22 In this paper, a critical excess air ratio is defined, below which NOx emissions are hardly varied with the excess air ratio in the main combustion zone. The critical ratio is ∼0.8 in this study, which is quite close to previous study results.9,10 It can also be observed from Figure 4 that if the position of SOFA port in the vertical direction is fixed, the SOFA arrangement type (corner or wall) has a limited influence on NOx emission. Higher location of SOFA port leads to lower NOx emissions at the furnace exit.

Figure 3. Size distribution of the blended coal used in the pilot-scale experiments. 1134


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Figure 4. Experimental results of NOx emissions at the furnace exit for different excess air ratios in the main combustion zone.

Figure 6. Experimental results of carbon content in fly ash and NOx emissions at the furnace exit for different residence times in the reduction zone.

The influence of the excess air ratio in the main combustion zone on carbon content in fly ash is shown in Figure 5. Experimental results

As shown in Figure 6, there is insignificant difference of NOx emissions between the runs of “SOFA-wall” and “SOFA-corner”. An increase in the residence time in the reduction zone leads to a decrease in NOx emissions. An increase in the residence time in reduction zone means that more fuel nitrogen is released in the oxygen-deficient zone; hence, lower NOx emissions are achieved. In the reduction zone, the residence time has a weak effect on the burnout, because the fuel is rich and the amount of oxygen is insufficient for complete combustion. However, for a given furnace, a larger reduction zone means a smaller burnout zone. In a retrofit situation, the increased residence time in the reduction zone therefore leads to greater unburned carbon loss.9 Figure 6 indicates the influence of residence time of the coal particle in the reduction zone on the carbon content in fly ash when the excess air ratio in main burner zone is 0.78. The increase of residence time leads to higher unburned carbon loss for both SOFA arrangements (SOFA-wall and SOFA-corner). Nevertheless, unburned loss has no considerable change when the residence time is increased from 1.56 s to 2.12 s for SOFA-wall or from 2.12 s to 2.56 s for SOFA-corner, which implies that the residence time in the reduction zone is not the unique parameter that determines the carbon content in fly ash. The carbon content can also be affected by SOFA arrangement. Figure 7 shows the relationship between NOx emissions and carbon content in fly ash. The data in Figure 7 are from the experiments in the 3-MW pilot facility. SOFA employment on the tangentially fired furnace can provide a NOx reduction of 50%, with a moderate (25− 35%) increase in the carbon content in fly ash, which is in good agreement with previous studies.7 It can be obviously observed from

Figure 5. Experimental results of carbon content in fly ash for different excess air ratios in the main combustion zone. show that the carbon content in fly ash decreases as the excess air ratio increases for different SOFA arrangements. The arrangement of SOFA ports has an unimportant influence on the carbon content in fly ash. For high SOFA arrangement, the decrease of the excess air ratio in the main combustion zone will not give rise to a sharp increase of the unburned carbon loss when the excess air ratio in the main combustion zone is >0.96. The same trends can be obtained for middle and low SOFA arrangements when the excess air ratio in the main combustion zone is >0.78. Nevertheless, when the carbon content in fly ash is increased from a minimum of 0.75% to a maximum of 2.11%, the corresponding heat loss due to unburned carbon is increased from 0.05% to 0.15%. Obviously, the corresponding boiler thermal efficiency will have a reduction of 0.1%, which is acceptable for a large-capacity utility boiler. Based on the discussion above, for the tangentially fired boiler, the excess air ratio of 0.78 in the main combustion zone can provide the lowest NOx emissions with the least sacrifice of fuel burnout. 2.5.2. Effect of the Residence Time in Reduction Zone. For a largevolume furnace, it is a common practice to determine the residence time by assuming plug flow between specific elevations.9,23 The residence time in a specific zone is the ratio of calculation space volume and estimated flue gas volume flow rate.23 When the excess air ratio in the main combustion zone is fixed, the residence time can reflect the height of SOFA ports.9 The reduction zone is defined as the zone in the furnace from the middle of the combustion zone to the middle of the SOFA zone. Figure 6 exhibits the influence of residence time on reduction zone on NOx emissions, when the excess air ratio is 0.78. Residence times of 1.56, 2.12, and 2.68 s correspond to low, middle and high SOFA arrangements, respectively.

Figure 7. The relationship between NOx emissions at the furnace exit and the carbon content in fly ash. 1135


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Figure 7 that NOx reduction due to SOFA is achieved via a greater loss of unburned carbon. High NOx reduction and low unburned carbon loss should be traded off in practical boiler retrofit. A residence time of 2.12−2.68 s is recommended in the present study. 2.5.3. Gas Temperature at the Furnace Exit. Figure 8 shows the temperature measurements for experimental group Gr 3. It can be seen

Method for Pressure-Linked Equations) algorithm was used to solve the Navier−Stokes equation and the continuum equation. 3.1. Models of Numerical Simulation. The conservation equations for mass, species, momentum, and energy in the Reynolds-averaged forms are solved. The realizable κ−ε model was adopted to model the turbulent flow. The P-1 radiation model is used to calculate radiation heat transfer.24 The coal particle motion is calculated by Newton’s second law. The particle motion is influenced by the drag force.25 The process of volatile matter release from the coal particle can be modeled by the one-step model or a two-step model. In the present study, the coal devolatilization is modeled by a one-step model, by which satisfactory results can be obtained.26 The homogeneous combustion of volatile matter is described by probability density function (PDF) model, which is recommended by Khalil.27 The char combustion can be calculated by the model presented by Field et al.28 NOx formation simulation is carried out as a post-processing procedure. The NOx formation calculation is based on the flow field, temperature distribution, and species concentration. In this paper, the prompt NOx formation is not considered because it is negligible in a practical pulverized-coal boiler. The model presented by Zeldovich in 194629 is employed to describe the formation of thermal NOx. Fuel NOx is generated from both the volatile matter and the char in the coal particle.30,31 Both HCN and NH3 are assumed to be intermediate species in the formation of the fuel NOx, as shown in Figure 9. When HCN is assumed as the intermediate species, schemes A and B are included in formation mechanism of fuel NOx. When NH3 is assumed as the intermediate species, schemes C and D are included in the formation mechanism of fuel NOx.32 3.2. Simulation Objects and Conditions. First, a 3-MW pilot-scale facility was numerically simulated to validate the models mentioned above. The height, horizontal width, and depth of this facility are 11.5, 1.2, and 1.2 m, respectively. Air is injected into the furnace through the burners, second air ports, and the SOFAs, which are located 2.65, 3.68, and 4.72 m, respectively, from the middle of the combustion zone, as shown in Figure 2. Eight burners, each with a coal combustion capacity of 24 kg h−1, are installed in the 3-MW facility. During the simulation, 31 runs, which are the same as experimental runs, are included in the present study..

Figure 8. Experimental results of flue gas temperatures at the furnace exit for different air ratios in the main combustion zone in runs of experimental group Gr 3.

that increasing the excess air ratio in the main combustion zone from 0.78 to 1.2 will not lead to a significant change of the furnace exit gas temperature (FEGT) for the pilot-scale furnace. As SOFA ratio is increased, the maximum temperature in the furnace moves to the upper section of the boiler, and then the time for cooling the gas is shortened. However, different from a utility boiler, the pilot-scale furnace is tall and thin. Therefore, for the pilot-scale furnace, the time required to cool the gas is long enough. Movement of the maximum temperature to the upper section of the pilot-scale furnace does not affect the FEGT much.

3. NUMERICAL SIMULATIONS Computational fluid dynamics (CFD) simulations of combustion in both a pilot-scale facility and a large-capacity power plant boiler have been carried out. The CFD models have been validated by comparing the simulation results with experimental data of pilot-scale facility. The purpose of numerical simulation is to optimize the arrangement of SOFA in the 600-MW utility boiler. During the simulation, the finite-volume method was used to discretize the differential equations and SIMPLE (Semi-Implicit

Figure 9. Schemes of NOx formation mechanism. Schemes A and B assume HCN as an intermediate species, whereas schemes C and D assume NH3 as an intermediate species. 1136


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After the validation of the models, the 600-MW boiler had been numerically simulated to optimize the parameters of SOFA. The height, horizontal width, and depth of the utility boiler are 66.2, 19.5, and 16.9 m, respectively. The burner nozzles at different levels of the furnace are illustrated in Figure 10. Under operation, five primary air (PA) nozzles, numbered

Table 4. Operating Information of the 600-MW Boiler parameter air flow rates for each nozzle (kg s−1)

air inlet temperature (K)

location

value

PA (nozzle: A, B, C, D, E) SA (nozzle: AB, BC, CD, DE, EF) SA (nozzle: FF) SA (nozzle: AA) SA (perimeter of nozzle: A, B, C, D, E) CCOFA (nozzle: CCOFA-1, CCOFA2) SOFA (nozzle: SOFA1, SOFA2, SOFA3, SOFA4, SOFA5)

7.28 8.56 2.02 4.74 2.56 7.03

primary air secondary air CCOFA SOFA

350 594 594 594

9.52

furnace wall temperature (K)

750

coal mass flow rate (kg s−1)

64.21

excess air coefficient

1.2

water-wall is measured by experiment. The furnace center gas temperature by numerical simulation is consistent with experimental results in the 3-MW pilot-scale facility, as shown in Figure 11. Therefore, the boundary conditions are

Figure 10. The configuration and dimensions of a 600-MW boiler.

from A to E, are put into use and the PA nozzle of F is shut down. Between two PA nozzles, there is one secondary air (SA) nozzle. Two narrow SA nozzles are installed at both the top and bottom of the burners. Under operation, the upper narrow SA nozzle (SA FF) is shut down. Two close-coupled over-fire air (CCOFA) units are installed at the top of the burner zone. The SOFA ports are located 8.15, 8.75, 9.35, 9.95, 10.55, and 11.15 m above the middle of the combustion zone. Twenty burners, each with a coal combustion capacity of 11 000 kg h−1, are installed. The detailed performance data of the full-scale boiler are shown in Table 4. 3.2.1. Grid. The test of grid independence has been carried out before numerical simulations. The test results of the 3-MW pilot-scale facility show that, when the total amount of grids increase from 740167 to 1192437, the simulation results are almost unchanged. Therefore, the geometrical model with 740167 grids is employed in the simulation of the 3-MW facility, while a geometrical model with 1126676 grids is employed in the simulation of the 600-MW utility boiler, which also satisfies the requirement of grid independence. 3.2.2. Boundary Conditions. The fixed-temperature boundary condition was adopted in the numerical simulation of the pilot-scale facility. The wall temperature distribution of the

Figure 11. Comparison of the temperature in the center of the furnace between experimental and numerical simulation results.

reasonable. Table 5 shows the detailed boundary conditions of the 3-MW experimental facility. For a utility boiler, the wall Table 5. Simulated Boundary Conditions of a 3-MW Experimental Facility

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zone

temperature (°C)

burner zone burnout zone connection zone water cooling jacket water cooling door at burner zone water cooling door at connection zone horizontal flue zone hopper zone

1400 1220 1173 510 650 600 600 900


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ratio on NOx emissions in the main combustion zone of a 3MW pilot-scale facility. The NOx emissions decreased dramatically when the excess air ratio in the main combustion zone decreased from 1.2 to 0.78. Nevertheless, when the excess air ratio in the main combustion zone is reduced from 0.78 to 0.72, NOx emissions show only negligible variations. The simulation results show that the variation of NOx emissions with the excess air ratio is in accordance with that obtained from the 3-MW pilot-scale facility experiments. Figure 13 shows the distributions of NOx emissions along the furnace height of a 3-MW facility at the middle plane for different SOFA proportions. It can be observed from Figure 13 that when SOFA proportion is >0.3, there is no significant difference for the NOx distributions in the combustion facility. Considering low NOx emissions at the furnace exit, the recommended excess air ratio in main combustion is 0.78. It means that the mass flow rate of air in SOFA zone occupies 35% of the total air when the excess air ratio at the furnace exit of the combustion facility is 1.2. In this study, both HCN and NH3 are assumed to be intermediate species in the fuel NOx formation mechanism. Figure 14 shows a comparison of the NOx emissions at the furnace exit between experimental and numerical simulation results when the ratio of HCN/NH3 is 9:1.33 This figure shows that the NOx emissions obtained from numerical simulations

temperature of water-wall is 30−70 K higher than the temperature of water in the water-wall. In the present study for the 600-MW boiler, the wall temperature of the water-wall is assumed to be uniform and the water in the water-wall is saturated. Therefore, the wall temperature of the water-wall was fixed at 750 K for the 600-MW boiler. 3.3. Analysis of Numerical Results. 3.3.1. 3-MW PilotScale Facility. Figure 12 shows the influence of the excess air

Figure 12. Numerical simulation results of NOx emissions at the furnace exit for different excess air ratios in the main combustion zone.

Figure 13. Distributions of NOx emissions along the height of a 3-MW facility at the middle plane for different SOFA proportions. 1138


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Figure 14. Comparison of NOx emissions between experimental and numerical simulation results at the furnace exit.

Figure 16. Experimental and simulation results of NOx emissions at the furnace exit of the 600-MW boiler.

are in good agreement with those from experiments. As shown in Figure 15, under the condition of high SOFA-wall

simulations in the 3-MW pilot-scale facility. A 60% reduction in NOx emissions has been achieved in the 600-MW boiler by using SOFA.

4. CONCLUSIONS Experiments in a 3-MW pilot-scale facility and numerical simulations of both a pilot-scale facility and a 600-MW boiler have been carried out, and the following conclusions can be drawn: (1) For the tangentially fired boiler using separated overfire air (SOFA), NOx emissions can be reduced dramatically with the decrease of the excess air ratio in the main combustion zone first. But, when the excess air ratio in the main combustion zone is reduced to a certain extent, further decrease of the excess air ratio will not give rise to a decrease in NOx emissions. The critical ratio is ∼0.8 for the tangentially fired boiler, which is less than that obtained from one-dimensional furnace experiments. (2) For the tangentially fired boiler, the carbon content in fly ash decreases as the excess air ratio in the main combustion zone increases. For high SOFA arrangement, the loss due to unburned carbon decreases slowly as the excess air ratio increases when the excess air ratio in the main combustion zone is >0.96. Similar variation is obtained for middle and low SOFA arrangements when the excess air ratio in the main combustion zone is >0.78. The heat loss due to unburned carbon is still acceptable, even if the excess air ratio in the main combustion zone is reduced from 1.2 to 0.72. (3) The increase of residence time in reduction zone leads to a decrease in NOx emissions, but an increased loss of unburned carbon, so a tradeoff must occur before practical application of SOFA for the tangentially fired boiler. (4) Both experimental and simulation results show that a residence time of 2.12−2.68 s in the reduction zone is reasonable. A residence time of 2.35 s in the retrofit of the 600-MW tangentially fired boiler is employed, and a 60% reduction of NOx emissions has been achieved.

Figure 15. Comparison of NOx emissions between experiment and numerical simulation results at the furnace exit under high SOFA-wall conditions.

arrangement, they agree quite well when the excess air ratio in the main combustion zone is

Optimization of Separated Overfire Air System for a Utility Boiler from a

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