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Optimization of Separated Overfire Air System for a Utility Boilerfrom a 3‑MW Pilot-Scale FacilityJianwen 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 fromutility boilers. Experiment results showed that NOx emissions decreased as the excess air ratio in the main combustion zone firstdecreased, but remained unchanged when the excess air ratio was above a certain value; the carbon content in fly ash increasedwith the decrease of the excess air ratio in the main combustion zone; NOx emissions decreased and the heat loss due tounburned carbon increased as the residence time in the reduction zone (the zone in the furnace from the middle of combustionzone to the middle of the SOFA zone) increased. Based on the experiment results, a residence time of 2.12−2.68 s isrecommended for the reduction zone. Besides, numerical simulations were conducted about the pilot-scale facility and a 600-MWboiler firing the blended coal of Shenhua coal (80 wt %) and Baode coal (20 wt %). The simulations, together with theexperiments, showed that there was a critical excess air ratio with value of ∼0.8 for the minimum NOx emissions with relativelylow unburned carbon. This critical value, together with the residence time of 2.35 s, which is in the range recommended by ourexperiments, were used in the retrofit of the 600-MW boiler by SOFA technology, leading to a reduction of NOx emissions ashigh as 60%.

1. INTRODUCTION

NOx that is released from power plants has attracted extensiveattention over past years, because of its considerable harm tothe environment and human health. It can react withphotochemical oxidants, particulate matter, and sunlight inthe air, generating photochemical smog. It is also responsiblefor acid rain. Therefore, it is crucial to control NOx emission.Currently, related organizations and local governments havepublished stricter regulations and legislation against the NOxemissions from power plants around the world. In China, StateMinistry of Environmental Protection announced a newemissions standard of air pollutants for thermal power plantson July 29, 2011,1 in which the NOx emissions limit of powerplants is reduced from 450 mg Nm−3 to 100 mg Nm−3

(calculated under the condition of dry flue gas and 6% oxygenby volume). Under this strict standard, numerous low-NOxcombustion technologies have been extensively developed,including low NOx burners (LNB), fuel-staged combustion(reburning), flue gas recirculation (FGR), and air-stagedcombustion. The present study focuses on the air-stagedcombustion technology, especially the separated overfire aircombustion (SOFA) technology.Many studies have been carried out on air-staged

combustion,2−5 and it has been determined that this technologycan reduce NOx emissions effectively. Over the past fewdecades, air-staged combustion technology has been developedfrom overfire air (OFA) to SOFA. The SOFA technology canreduce the NOx emissions up to 50%.6,7 Nevertheless, mostprevious studies were carried out with laboratory-scaleequipments, such as a one-dimensional (1D) drop furnaceand the furnace with one burner. Experimental results fromprevious studies had showed that NOx emissions were affectedby coal type, coal fineness, the excess air ratio in the main

combustion zone, and SOFA position.8−11 However, in thelaboratory-scale experiments, flow field is different from that inthe 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 combustionzone and flow field should be conducted. In the present study, a3-MW pilot-scale facility has been used to investigate the effectof SOFA on NOx emission. In the pilot-scale facility, the flowfield and residence time of coal particle are set quite similar tothose in a large capacity boiler. Such similarity is of crucialimportance to accurately predict the effect of SOFA on NOx

emissions from the large-capacity boiler. Besides, bothexperiments and computational fluid dynamics (CFD) havebeen used to predict NOx emissions from the pilot-scale facilityand the large capacity boiler.

2. EXPERIMENTAL SECTION2.1. Pilot-Scale Facility. The combustion experiments were

carried out in a 3-MW pilot-scale facility, as shown in Figure 1. Theexperimental facility mainly consists of a raw coal crushing and dryingsystem, a pulverizing system, a coal conveying system, a limestoneconveying system, boiler proper, a forced draft fan, an induced draftfan, a flue gas recirculation fan, a natural gas ignition system, a watertreatment and recycling system, desulfurization equipment, a dustremoval system, a denitrification system, a flue gas online analysissystem, 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, 2012Revised: January 15, 2013Published: January 16, 2013

Article

pubs.acs.org/EF

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

pubs.acs.org/EF

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

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

Energy & Fuels Article

dx.doi.org/10.1021/ef301779c | Energy Fuels 2013, 27, 1131−11401132

Dimensions of the experimental facility are illustrated in Figure 2 indetail. The locations of three layers of SOFA are shown in Figure 2a.Under each experimental condition, only the SOFA ports of one layerare opened and the others are shut off. The zone between the middleof combustion zone and the SOFA port is known as the reductionzone. The zone between the SOFA port and the furnace exit is knownas the burn-out zone. Boundary of the reduction zone and the burn-out zone may change under different conditions, because of the changeof the location of SOFA port put into operation. The arrangement ofburners and OFA ports are shown in Figure 2b and 2c. Eight SOFAports are included in each layer. Four SOFA ports are located at wallsand the others are located at four corners, as shown in Figure 2b.Under each experimental condition, only the SOFA ports located atwalls (corners) are opened and the others are shut off.A portable multifunction flue gas measurement system (Testo 350-

XL), 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 NOand NO2 in the flue gas. The flue gas for measurement is from the fluepass before the bag filter, which is numbered 11 in Figure 1. Fly ashsampling points were arranged in the horizontal flue gas pass. Pitottube with parallel dust sampling (PTP-III-type) was employed tosample fly ash. The sampling instrument consists of host, samplinggun, filter cartridge, trap, U-shaped manometer, rubber tube andpump, etc.2.2. NOx Formation Mechanism. Thermal NOx, fuel NOx, and

prompt NOx are three main mechanisms for NOx formation. Thecontribution of prompt NOx is usually negligible in coal-firedfurnaces.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 ofthermal NOx, as presented in the extended Zeldovich mechanism, areas follows:

+ ⇄ +O N N NOk

k

2

1

(R1)

+ +−X YoooN O O NOk

k2

2

2

(R2)

+ +−X YoooN OH H NOk

k

3

3

(R3)

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

= + +

− − −− − −

tk k k

k k k

d[NO]d

[O][N ] [N][O ] [N][OH]

[NO][N] [NO][O] [NO][H]

1 2 2 2 3

1 2 3

(1)

In eq 1, the rate constants (k±1, k±2, and k±3) are selected based on theevaluation of Hanson et al.13 Assuming that the rate of consumption ofN atoms is equal to its formation, which is reasonable in this study, thethermal NOx formation rate given by eq 1 can be simplified as follows:

=−

+

− −

−

( )( )t

kd[NO]

d2 [O][N ]

1

1

k kk k

kk k

1 2

[NO]

[N ] [O ]

[NO][O ] [OH]

1 22

1 2 2 22

1

2 2 3 (2)

In eq 2, local concentrations of O and OH radicals are estimated byadopting 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 areassumed to be nitrogen-bearing intermediates. These nitrogen-bearingintermediates are competitively oxidized and reduced throughfollowing reactions:

+ → +HCN O NO ...k

24

(R4)

+ → +HCN NO N ...k

25

(R5)

+ → +NH O NO ...k

3 26

(R6)

+ → +NH NO N ...k

3 27

(R7)

In the four reactions R4−R7, N2 and NO are included in theproducts. In addition, according to Levy et al.,14 the NO is reduced byheterogeneous reaction on the char surface. The reaction is shownbelow:

+ ⎯ →⎯⎯ +char NO N ...k

2char

(R8)

Therefore, the net rate of fuel NOx formation, according to reactionsR4−R8, is given by the following expression:

= + −

− −

α α

tk k k

k k C

d[NO]d

[HCN][O ] [NH ][O ] [NO][HCN]

[NO][NH ]

4 2 5 3 2 6

7 3 char char (3)

where α is the oxygen-reaction order, determined by local oxygenmole fraction,15 and Cchar (m

2 m−3) is the char surface density pervolume unit.

2.3. Similarity Modeling Analysis for NOx Emission. NOxemissions 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 coalfineness.8−11

The impact of coal type on NOx emissions is reflected mainly by thevolatile content and by the nitrogen content in coal. Higher volatilecontent leads to larger proportion of volatile nitrogen in fuelnitrogen,16,17 and the volatile nitrogen is more likely to be convertedto N2; hence, lower NOx emissions are attained. The higher nitrogencontent usually produces more NOx under the same combustioncondition.

Temperature mainly affects the formation of thermal NOx. Thethermal NOx is ignorable when the temperature is lower than 1500 °C.However, when the temperature is higher than 1500 °C, thermal NOxgeneration rate increases by a factor of 6 as the temperature increasesby 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 offuel NOx. Fuel nitrogen first generates intermediate products (HCNand NHi) at high temperature, and then these intermediates produceNO or N2, according to reactions R4−R7.20 These two reactions arecompetitive. Under an oxidizing atmosphere, reactions R4 and R6 aredominant, while the leading reactions are R5 and R7 for a reducingatmosphere.

The principle of low NOx combustion technology generally involvesthe ceration of a reductive atmosphere to ensure that the intermediateproducts react according to reactions R5 and R7. The atmosphere nearthe burner is primarily dependent on the total excess air ratio and thestoichiometric 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 canbe adopted to increase the residence time and reduce NOx emissions.

Generally, NOx emissions first decrease as the coal particle sizedecreases. Nevertheless, when the fineness of coal particle is smallerthan a specific value, further decreases in coal particle size no longerreduce NOx emissions.

21

The five factors above also affect the combustion efficiency of aboiler.20 Therefore, some requirements should be satisfied to ensurethat the experimental results in the 3-MW pilot-scale facility can beemployed to predict the practical performance of the 600-MW utilityboiler. The coal type, the coal fineness, the excess air ratio incombustion zone, and the residence time of pulverized coal in bothcombustion zone and NOx reduction zone in the 3-MW facility shouldbe the same as those in the 600-MW utility boiler. The temperaturedistribution in the 3-MW facility should be similar to that in the 600-MW boiler.



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

Laser particle size analyzer (Malvern, Mastersizer 2000 type) wasused to analyze the particle size distribution of the blended coal usedin the pilot-scale experiments and the data are shown in Figure 3. Theanalysis results indicated that the coal fineness (R90) is 16%, and theaverage particle size is 55.87 μm, which is similar to the coal used inthe 600-MW boiler, whose R90 value is 18% and the uniformityexponent is 1.1.2.4.2. Determination of Heat Output. Although the pilot-scale

facility can achieve a maximum output of 3 MW, some experimentswere conducted in the pilot-scale facility to determine an appropriateheat output, at which the temperature distribution in the pilot-scalefacility is similar to that in the 600-MW utility boiler. Table 2 showsthe temperature distributions in both the pilot-scale facility and the600-MW boiler. Experimental results show that the temperaturedistribution in the pilot-scale facility with heat output of 2 MW is ingeneral agreement with that in the 600-MW boiler. Hence, the heatoutput of pilot-scale facility was kept at 2 MW during the subsequentexperiments.2.4.3. Experimental Runs. Six groups of experimental runs,

classified by the difference in SOFA port arrangement, are includedin the present study. Table 3 shows the detailed experimental runs onthe pilot-scale facility. For each group, six experimental runs withdifferent SOFA flow rates are included. The mass flow ratios of SOFAto the total air are 0, 10%, 20%, 30%, 35%, and 40%. Thecorresponding 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 ofprimary air is fixed at 28% to ensure the delivery of pulverized coal. In

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 Ratioin the Main Combustion Zone. Figure 4 shows the effect of the excessair ratio in the main combustion zone on the NOx emissions forvarious SOFA port arrangements on the pilot-scale facility withtangentially fired burners. The NOx emissions at the furnace exit arealmost kept unchanged when the excess air ratio in the maincombustion zone increases from 0.72 to 0.78. Nevertheless, the NOxemissions increase significantly when the excess air ratio is increasedfrom 0.78 to 1.2. This trend is in accordance with previousstudies.9,10,22

In this paper, a critical excess air ratio is defined, below which NOxemissions are hardly varied with the excess air ratio in the maincombustion zone. The critical ratio is ∼0.8 in this study, which is quiteclose to previous study results.9,10 It can also be observed from Figure4 that if the position of SOFA port in the vertical direction is fixed, theSOFA arrangement type (corner or wall) has a limited influence onNOx emission. Higher location of SOFA port leads to lower NOxemissions at the furnace exit.

Table 1. Proximate and Ultimate Analyses of ExperimentalCoal (As-Received Basis)

coal propertyShenhuacoal

Baodecoal

blendedcoal

proximate analysis (wt %)moisture 13.46 3.81 11.53volatile matter 31.58 27.35 30.73fixed carbon 50.11 43.58 48.80ash 4.85 25.26 8.93

ultimate analysis (wt %)hydrogen 6.13 4.64 5.83carbon 66.86 58.16 65.12sulfur 0.44 0.39 0.43nitrogen 0.62 0.74 0.64oxygen 7.64 6.99 7.51

higher heating value, HHV(kJ kg−1)

29380 24600 25156

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

Table 2. Comparison of Key Parameters between Pilot-ScaleFacility and the Utility Boiler

pilot-scale facility

parameter

utility boiler,heat output =600 MW

heatoutput =3.0 MW

heatoutput =2.0 MW

heatoutput =1.0 MW

flue gas temperature inthe main combustionzone (°C)

1450 1550 1500 1350

flue gas temperature atfurnace exit (°C)

888 1077 912 765

heat input (MW) 1559 3.0 2.0 1.0residence time inreduction zone (s)

2.35 1.79 2.68 5.36

residence time in burnout zone (s)

2.59 1.17 1.75 3.50

total residence time (s) 4.94 2.96 4.43 8.86

Table 3. Experimental Runs of the Pilot-Scale Facility

experimentalgroup SOFA location

ratios of SOFA tototal air

Gr 1 low position, four-cornersarrangement

0, 10%, 20%, 30%,35%, 40%

Gr 2 low position, four-wallsarrangement

0, 10%, 20%, 30%,35%, 40%

Gr 3 middle position, four-cornersarrangement

0, 10%, 20%, 30%,35%, 40%

Gr 4 middle position, four-wallsarrangement

0, 10%, 20%, 30%,35%, 40%

Gr 5 high position, four-cornersarrangement

0, 10%, 20%, 30%,35%, 40%

Gr 6 high position, four-wallsarrangement

0, 10%, 20%, 30%,35%, 40%



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

show that the carbon content in fly ash decreases as the excess air ratioincreases for different SOFA arrangements. The arrangement of SOFAports has an unimportant influence on the carbon content in fly ash.For high SOFA arrangement, the decrease of the excess air ratio in themain combustion zone will not give rise to a sharp increase of theunburned carbon loss when the excess air ratio in the maincombustion zone is >0.96. The same trends can be obtained formiddle and low SOFA arrangements when the excess air ratio in themain combustion zone is >0.78. Nevertheless, when the carboncontent in fly ash is increased from a minimum of 0.75% to amaximum of 2.11%, the corresponding heat loss due to unburnedcarbon is increased from 0.05% to 0.15%. Obviously, thecorresponding boiler thermal efficiency will have a reduction of0.1%, 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 thelowest NOx emissions with the least sacrifice of fuel burnout.2.5.2. Effect of the Residence Time in Reduction Zone. For a large-

volume furnace, it is a common practice to determine the residencetime by assuming plug flow between specific elevations.9,23 Theresidence time in a specific zone is the ratio of calculation spacevolume and estimated flue gas volume flow rate.23 When the excess airratio in the main combustion zone is fixed, the residence time canreflect the height of SOFA ports.9 The reduction zone is defined as thezone in the furnace from the middle of the combustion zone to themiddle of the SOFA zone. Figure 6 exhibits the influence of residencetime on reduction zone on NOx emissions, when the excess air ratio is0.78. Residence times of 1.56, 2.12, and 2.68 s correspond to low,middle and high SOFA arrangements, respectively.

As shown in Figure 6, there is insignificant difference of NOxemissions between the runs of “SOFA-wall” and “SOFA-corner”. Anincrease in the residence time in the reduction zone leads to a decreasein NOx emissions. An increase in the residence time in reduction zonemeans 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 theburnout, because the fuel is rich and the amount of oxygen isinsufficient for complete combustion. However, for a given furnace, alarger reduction zone means a smaller burnout zone. In a retrofitsituation, the increased residence time in the reduction zone thereforeleads to greater unburned carbon loss.9

Figure 6 indicates the influence of residence time of the coal particlein the reduction zone on the carbon content in fly ash when the excessair ratio in main burner zone is 0.78. The increase of residence timeleads to higher unburned carbon loss for both SOFA arrangements(SOFA-wall and SOFA-corner). Nevertheless, unburned loss has noconsiderable change when the residence time is increased from 1.56 sto 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 theunique parameter that determines the carbon content in fly ash. Thecarbon content can also be affected by SOFA arrangement.

Figure 7 shows the relationship between NOx emissions and carboncontent in fly ash. The data in Figure 7 are from the experiments in the3-MW pilot facility. SOFA employment on the tangentially firedfurnace can provide a NOx reduction of 50%, with a moderate (25−35%) increase in the carbon content in fly ash, which is in goodagreement with previous studies.7 It can be obviously observed from

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

Figure 5. Experimental results of carbon content in fly ash for differentexcess air ratios in the main combustion zone.

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

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



Figure 7 that NOx reduction due to SOFA is achieved via a greater lossof unburned carbon. High NOx reduction and low unburned carbonloss should be traded off in practical boiler retrofit. A residence time of2.12−2.68 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

that increasing the excess air ratio in the main combustion zone from0.78 to 1.2 will not lead to a significant change of the furnace exit gastemperature (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 forcooling the gas is shortened. However, different from a utility boiler,the pilot-scale furnace is tall and thin. Therefore, for the pilot-scalefurnace, the time required to cool the gas is long enough. Movementof the maximum temperature to the upper section of the pilot-scalefurnace does not affect the FEGT much.

3. NUMERICAL SIMULATIONSComputational fluid dynamics (CFD) simulations of combus-tion in both a pilot-scale facility and a large-capacity powerplant boiler have been carried out. The CFD models have beenvalidated by comparing the simulation results with experimentaldata of pilot-scale facility. The purpose of numerical simulationis to optimize the arrangement of SOFA in the 600-MW utilityboiler.During the simulation, the finite-volume method was used to

discretize the differential equations and SIMPLE (Semi-Implicit

Method for Pressure-Linked Equations) algorithm was used tosolve the Navier−Stokes equation and the continuum equation.

3.1. Models of Numerical Simulation. The conservationequations for mass, species, momentum, and energy in theReynolds-averaged forms are solved. The realizable κ−ε modelwas adopted to model the turbulent flow. The P-1 radiationmodel is used to calculate radiation heat transfer.24

The coal particle motion is calculated by Newton’s secondlaw. The particle motion is influenced by the drag force.25 Theprocess of volatile matter release from the coal particle can bemodeled by the one-step model or a two-step model. In thepresent study, the coal devolatilization is modeled by a one-stepmodel, by which satisfactory results can be obtained.26 Thehomogeneous combustion of volatile matter is described byprobability density function (PDF) model, which is recom-mended by Khalil.27 The char combustion can be calculated bythe model presented by Field et al.28

NOx formation simulation is carried out as a post-processingprocedure. The NOx formation calculation is based on the flowfield, temperature distribution, and species concentration. Inthis paper, the prompt NOx formation is not consideredbecause it is negligible in a practical pulverized-coal boiler. Themodel presented by Zeldovich in 194629 is employed todescribe the formation of thermal NOx. Fuel NOx is generatedfrom both the volatile matter and the char in the coalparticle.30,31 Both HCN and NH3 are assumed to beintermediate species in the formation of the fuel NOx, asshown in Figure 9. When HCN is assumed as the intermediatespecies, schemes A and B are included in formation mechanismof fuel NOx. When NH3 is assumed as the intermediate species,schemes C and D are included in the formation mechanism offuel NOx.

32

3.2. Simulation Objects and Conditions. First, a 3-MWpilot-scale facility was numerically simulated to validate themodels mentioned above. The height, horizontal width, anddepth of this facility are 11.5, 1.2, and 1.2 m, respectively. Air isinjected 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 shownin Figure 2. Eight burners, each with a coal combustion capacityof 24 kg h−1, are installed in the 3-MW facility. During thesimulation, 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 furnaceexit for different air ratios in the main combustion zone in runs ofexperimental group Gr 3.

Figure 9. Schemes of NOx formation mechanism. Schemes A and B assume HCN as an intermediate species, whereas schemes C and D assumeNH3 as an intermediate species.



After the validation of the models, the 600-MW boiler hadbeen numerically simulated to optimize the parameters ofSOFA. The height, horizontal width, and depth of the utilityboiler are 66.2, 19.5, and 16.9 m, respectively. The burnernozzles at different levels of the furnace are illustrated in Figure10. Under operation, five primary air (PA) nozzles, numbered

from A to E, are put into use and the PA nozzle of F is shutdown. Between two PA nozzles, there is one secondary air (SA)nozzle. Two narrow SA nozzles are installed at both the top andbottom of the burners. Under operation, the upper narrow SAnozzle (SA FF) is shut down. Two close-coupled over-fire air(CCOFA) units are installed at the top of the burner zone. TheSOFA ports are located 8.15, 8.75, 9.35, 9.95, 10.55, and 11.15m above the middle of the combustion zone. Twenty burners,each with a coal combustion capacity of 11 000 kg h−1, areinstalled. The detailed performance data of the full-scale boilerare 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-MWpilot-scale facility show that, when the total amount of gridsincrease from 740167 to 1192437, the simulation results arealmost unchanged. Therefore, the geometrical model with740167 grids is employed in the simulation of the 3-MWfacility, while a geometrical model with 1126676 grids isemployed in the simulation of the 600-MW utility boiler, whichalso satisfies the requirement of grid independence.3.2.2. Boundary Conditions. The fixed-temperature boun-

dary condition was adopted in the numerical simulation of thepilot-scale facility. The wall temperature distribution of the

water-wall is measured by experiment. The furnace center gastemperature by numerical simulation is consistent withexperimental results in the 3-MW pilot-scale facility, as shownin Figure 11. Therefore, the boundary conditions are

reasonable. Table 5 shows the detailed boundary conditionsof the 3-MW experimental facility. For a utility boiler, the wall

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

Table 4. Operating Information of the 600-MW Boiler

parameter location value

air flow rates for eachnozzle (kg s−1)

PA (nozzle: A, B, C, D, E) 7.28SA (nozzle: AB, BC, CD, DE, EF) 8.56SA (nozzle: FF) 2.02SA (nozzle: AA) 4.74SA (perimeter of nozzle: A, B, C, D, E) 2.56CCOFA (nozzle: CCOFA-1, CCOFA-2)

7.03

SOFA (nozzle: SOFA1, SOFA2,SOFA3, SOFA4, SOFA5)

9.52

air inlet temperature (K) primary air 350secondary air 594CCOFA 594SOFA 594

furnace wall temperature(K)

750

coal mass flow rate(kg s−1)

64.21

excess air coefficient 1.2

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

Table 5. Simulated Boundary Conditions of a 3-MWExperimental Facility

zone temperature (°C)

burner zone 1400burnout zone 1220connection zone 1173water cooling jacket 510water cooling door at burner zone 650water cooling door at connection zone 600horizontal flue zone 600hopper zone 900



temperature of water-wall is 30−70 K higher than thetemperature of water in the water-wall. In the present studyfor the 600-MW boiler, the wall temperature of the water-wallis assumed to be uniform and the water in the water-wall issaturated. Therefore, the wall temperature of the water-wall wasfixed at 750 K for the 600-MW boiler.3.3. Analysis of Numerical Results. 3.3.1. 3-MW Pilot-

Scale Facility. Figure 12 shows the influence of the excess air

ratio on NOx emissions in the main combustion zone of a 3-MW pilot-scale facility. The NOx emissions decreaseddramatically when the excess air ratio in the main combustionzone decreased from 1.2 to 0.78. Nevertheless, when the excessair ratio in the main combustion zone is reduced from 0.78 to0.72, NOx emissions show only negligible variations. Thesimulation results show that the variation of NOx emissionswith the excess air ratio is in accordance with that obtainedfrom the 3-MW pilot-scale facility experiments. Figure 13shows the distributions of NOx emissions along the furnaceheight of a 3-MW facility at the middle plane for differentSOFA proportions. It can be observed from Figure 13 thatwhen SOFA proportion is >0.3, there is no significantdifference 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. Itmeans that the mass flow rate of air in SOFA zone occupies35% of the total air when the excess air ratio at the furnace exitof 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 thefurnace exit between experimental and numerical simulationresults when the ratio of HCN/NH3 is 9:1.

33 This figure showsthat the NOx emissions obtained from numerical simulations

Figure 12. Numerical simulation results of NOx emissions at thefurnace 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.



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

arrangement, they agree quite well when the excess air ratioin the main combustion zone is <1. This regularity is also foundunder other conditions with different SOFA arrangements.Figure 14 shows the relative error of the NOx emissionsconcentration. The error is controlled to be <25%. Therefore,the assumption of NOx formation mechanism in the presentstudy is reasonable and is adopted in the numerical study of the600-MW boiler.3.3.2. 600-MW Boiler. According to experimental results in

the 3-MW pilot-scale furnace, a residence time of 2.35 s in thereduction zone was adopted in the retrofit of the 600-MWboiler,. Simulations under different excess air ratios wereconducted. Figure 16 shows the influence of the excess air ratioon NOx emissions in the main combustion zone of the 600-MW boiler. NOx emissions from numerical simulations underdifferent excess air ratios are in good agreement with thosefrom experiments in the 600-MW boiler. The relative deviationis <10%. Both numerical and experimental results indicate thatthe NOx emissions decrease dramatically when the excess airratio in the main combustion zone is reduced from 1.08 to 0.78.Nevertheless, the NOx emissions show negligible variationswhen the excess air ratio decreases from 0.78 to 0.72. Similarresults have been obtained by experiments and numerical

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

4. CONCLUSIONSExperiments in a 3-MW pilot-scale facility and numericalsimulations of both a pilot-scale facility and a 600-MW boilerhave been carried out, and the following conclusions can bedrawn:

(1) For the tangentially fired boiler using separated overfireair (SOFA), NOx emissions can be reduced dramaticallywith the decrease of the excess air ratio in the maincombustion zone first. But, when the excess air ratio inthe main combustion zone is reduced to a certain extent,further decrease of the excess air ratio will not give rise toa decrease in NOx emissions. The critical ratio is ∼0.8 forthe tangentially fired boiler, which is less than thatobtained from one-dimensional furnace experiments.

(2) For the tangentially fired boiler, the carbon content in flyash decreases as the excess air ratio in the maincombustion zone increases. For high SOFA arrangement,the loss due to unburned carbon decreases slowly as theexcess air ratio increases when the excess air ratio in themain combustion zone is >0.96. Similar variation isobtained for middle and low SOFA arrangements whenthe 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 zoneis reduced from 1.2 to 0.72.

(3) The increase of residence time in reduction zone leads toa decrease in NOx emissions, but an increased loss ofunburned carbon, so a tradeoff must occur beforepractical application of SOFA for the tangentially firedboiler.

(4) Both experimental and simulation results show that aresidence time of 2.12−2.68 s in the reduction zone isreasonable. A residence time of 2.35 s in the retrofit ofthe 600-MW tangentially fired boiler is employed, and a60% reduction of NOx emissions has been achieved.

■ AUTHOR INFORMATIONCorresponding Author*Tel.: +86-29-82665185. Fax: +86-29-82668703. E-mail:[email protected].

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

Figure 15. Comparison of NOx emissions between experiment andnumerical simulation results at the furnace exit under high SOFA-wallconditions.

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



mailto:[email protected]

NotesThe authors declare no competing financial interest.

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