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Combustion Characteristics and NO x

Emissions of Two Kinds of Swirl Burnersin a 300-MWe Wall-Fired Pulverized-CoalUtility BoilerZhengqi Li a , Jianping Jing a , Zhichao Chen a , Feng Ren a , Bin Xu a

, Hongda Wei a & Zhihong Ge aa School of Energy Science and Engineering, Harbin Institute ofTechnology, Harbin, China

Available online: 12 Jun 2008

To cite this article: Zhengqi Li, Jianping Jing, Zhichao Chen, Feng Ren, Bin Xu, Hongda Wei & ZhihongGe (2008): Combustion Characteristics and NO x Emissions of Two Kinds of Swirl Burners in a 300-MWe

Wall-Fired Pulverized-Coal Utility Boiler, Combustion Science and Technology, 180:7, 1370-1394

To link to this article: http://dx.doi.org/10.1080/00102200802043318

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COMBUSTION CHARACTERISTICS AND NOX EMISSIONSOF TWO KINDS OF SWIRL BURNERS IN A 300-MWe

WALL-FIRED PULVERIZED-COAL UTILITY BOILER

Zhengqi Li, Jianping Jing, Zhichao Chen, Feng Ren, Bin Xu,Hongda Wei, and Zhihong GeSchool of Energy Science and Engineering, Harbin Institute of Technology,Harbin, China

Measurements were performed in a 300-MWe wall-fired pulverized-coal utility boiler.

Enhanced ignition-dual register (EI-DR) burners and centrally fuel rich (CFR) swirl coal

combustion burners were installed in the bottom row of the furnace during experiments.

Local mean concentrations of O2, CO, CO2 and NOx gas species, gas temperatures, and

char burnout were determined in the region of the two types of burners. For centrally fuel

rich swirl coal combustion burners, local mean CO concentrations, gas temperatures and the

temperature gradient are higher and mean concentrations of O2 and NOx along the jet flow

direction in the burner region are lower than for the enhanced ignition-dual register burners.

Moreover, the mean O2 concentration is higher and the gas temperature and mean CO con-

centration are lower in the side wall region. For centrally fuel rich swirl coal combustion

burners in the bottom row, the combustion efficiency of the boiler increases from 96.73%

to 97.09%, and NOx emission decreases from 411.5 to 355 ppm @ 6% O2 compared to

enhanced ignition-dual register burners and the boiler operates stably at 110 MWe without

auxiliary fuel oil.

Keywords: NOx; Pulverized coal; Swirl burner; Utility boiler

INTRODUCTION

Coal plays an important role in world energy production. In China, coal con-stitutes approximately 75% of the primary energy resources and is the predominantenergy resource for the power industry. For pulverized-coal-fired boilers in powerplants, high NOx emissions, flame stability at low load and high-temperature

Received 1 December 2006; accepted 18 January 2008.

We thank M. Costa and J. L. T. Azevedo for the support to this work. This work was supported by

Hi-Tech Research and Development Program of China (Contract No. 2007AA05Z301), the Ministry of

Education of China via the 2004 New Century Excellent Talents in University (Contract No. NECT-

04-0328), Heilongjiang Province via 2005 Key Projects (Contract No. GC05A314), Key Project of the

National Eleventh-Five Year Research Program of China (Contract No. 2006BAA01B01), the National

Basic Research Program of China (Contract No. 2006CB200303), and the Post-doctoral Foundation of

Heilongjiang Province LRB07-216.

Address correspondence to Zhengqi Li, School of Energy Science and Engineering, Harbin Insti-

tute of Technology, 92, West Dazhi Street, Harbin 150001, P.R. China. E-mail: green@hit.edu.cn

(Z. Q. Li)

1370

Combust. Sci. and Tech., 180: 1370–1394, 2008

Copyright # Taylor & Francis Group, LLC

ISSN: 0010-2202 print/1563-521X online

DOI: 10.1080/00102200802043318

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corrosion on the water-cooled walls of the furnace are the main concerns in designand operation. Low-NOx pulverized-coal burner technologies present an efficientmethod to solve these problems, of which the enhanced ignition-dual register burnerhas been widely applied (Larue and Cioffi, 1988). Li proposed a new low-NOx

pulverized-coal burner technology, the centrally fuel rich swirl coal combustionburner, based on the radial bias combustion burner and enhanced ignition-dualregister burner.

Industrial experiments performed on full-scale boilers reveal the coal combus-tion characteristics and mechanism of NOx formation. Costa et al. (1997, 2003, and2007) measured local mean gas species concentrations (O2, CO, CO2, NOx), gas tem-peratures, and char burnout at several ports in a 300-MWe, front-wall-fired, pulver-ized-coal utility boiler. Vikhansky et al. (2004) measured heat fluxes in a 550-MWe,opposite-wall-fired, pulverized-coal utility boiler. Experiments have been performedin pulverized-coal, tangentially fired and wall-fired boilers (Butler and Webb, 1991;Butler et al., 1992; Li et al., 2004; Bonin and Queiroz, 1991, 1996; Black andMcQuay, 1996; Tree and Webb, 1997; Fan et al., 1999), but few detailed measure-ments have been performed in the burner region.

In this work, measurements were performed in a 300-MWe, wall-fired, pulver-ized-coal utility boiler. An enhanced ignition-dual register and centrally fuel richburners were installed in the bottom row of the furnace during experiments. Datawere recorded for local mean concentrations of O2, CO, CO2, NOx, gas temperaturesand the temperature gradient, char burnout and release of C, H and N from coal atpositions in the region of the two burners. A comparison was made between Costaet al. (2007) in port 3.6 and the data measured along the side wall in this paper forthe concentrations of O2, CO, CO2, NOx, and gas temperatures. The influence of theenhanced ignition-dual register and centrally fuel rich burners on the efficiency ofcoal combustion, NOx emissions and flame stability without auxiliary fuel oil atlow load is also presented.

UTILITY BOILER

A B&W B-1025=16.8-M type boiler with a 300-MWe unit was made byBabcock & Wilcox Beijing Co. Ltd. The opposite-wall-fired, pulverized-coal boilerwith a dry-ash type furnace is equipped with 20 enhanced ignition-dual registerburners. There are 12 enhanced ignition-dual register burners arranged in three rowson the front wall of the furnace. The other eight burners are arranged in two rows onthe rear wall, opposite the eight burners in the top and bottom of the three rows onthe front wall. Five medium-speed mills and a positive-pressure direct-fired systemare used to supply pulverized coal to the burners.

Figure 1 shows the enhanced ignition-dual register burner, which has eightradial vanes in the inner secondary air duct and 12 tangential vanes in the outer sec-ondary air duct. The swirling directions of the inner and outer secondary airflows areidentical. Under the influence of the particle deflector and conical diffuser, pulver-ized coal carried by the primary air diffuses along the radius and gathers in theregion close to the primary air tube wall, which results in a coal-rich flow in the per-ipheral zone of the primary air and a coal-lean flow in the central zone.

EMISSION CHARACTERISTICS OF WALL-FIRED COAL BOILER 1371

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Figure 1 Enhanced ignition-dual register burner and position of the monitoring pipe (dimensions in m):

(1) primary air duct, (2) inner secondary air duct, (3) outer secondary air duct, (4) water-cooled wall,

(5) tangential vanes, (6) radial vanes, (7) monitoring pipe, and (8) conical diffuser.

Figure 2 Centrally fuel rich burner and position of the monitoring pipe (dimensions in m): (1) primary air

duct, (2) monitoring pipe, (3) cone separators, (4) radial vanes, (5) inner secondary air duct, (6) tangential

vanes, and (7) outer secondary air duct.

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When the primary air is emitted into the furnace through the burner nozzle, thefuel-rich flow in the peripheral zone of the primary air premixes with the secondaryair, which moves most pulverized coal into the low-temperature secondary air. Theflow that loses coal mass in the peripheral zone of the primary air and the fuel-leanflow in the central zone both enter the central recirculation zone. Thus, there is a lowconcentration of pulverized coal in the high-temperature central recirculation zone(Larue et al., 1988).

Figure 2 shows the centrally fuel rich burner, which has 16 bent-shaft vanes inthe inner secondary air duct and 12 tangential vanes at the entrance of the outer sec-ondary air duct. Compared to the enhanced ignition-dual register burner, the cen-trally fuel rich burner has cone separators in the primary air tube to affect thedistribution of pulverized coal in the primary air instead of a particle deflectorand conical diffuser. Under the influence of the cone separators, pulverized coal car-ried by the primary air is concentrated into the central zone of the primary air, whichresults in a coal-rich flow in the central zone of the primary air and a coal-lean flowin the peripheral zone.

To compare the combustion characteristics and NOx emission of the enhancedignition-dual register and centrally fuel rich burners, experiments were carried out inthe utility boiler. Table 1 shows the design parameters for the two burners.

IN SITU INDUSTRIAL COLD AIR FLOW EXPERIMENTS

Characteristics of the flame can be analyzed and predicted from in situ indus-trial cold air flow experiments. Table 2 shows the experimental parameters. In thecold flow experiments, a coordinate frame was set at the outlet of the burner. A thincloth was fixed for each grid of the frame. The traverse distance between two mea-surements was 0.1m. We estimate the uncertainties in establishing the location of thecentral recirculation zone border were 0.1m. From the flow direction of the cloth,the jet borders and the central recirculation zone boundary of the burner weremeasured.

Figure 3 shows the aerodynamic field of the two burners, L is the distance to theexit of the burner along the jet flow direction, r is the distance to the chamber axisalong radius direction, and D is the burner outer diameter of the outer secondary

Table 1 Design parameters of the two kinds of swirl burners in the utility boiler

Quantity

Enhanced ignition-dual

register burner

Centrally fuel

rich burner

Exit area of the primary air (m2) 0.1979 0.2597

Exit area of the inner secondary air (m2) 0.5648 0.4979

Exit area of the outer secondary air (m2) 0.6677 0.6677

Temperature of the primary air (�C) 75 75

Temperature of the secondary air (�C) 353 353

Mass flow rate of the primary air (kg s�1) 5.80 5.80

Mass flow rate of the inner secondary air (kg s�1) 3.59 3.59

Mass flow rate of the outer secondary air (kg s�1) 8.37 8.37

EMISSION CHARACTERISTICS OF WALL-FIRED COAL BOILER 1373

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air duct (D ¼ 1.495m). For the centrally fuel rich burner, the divergent angle is 90�

and the maximum length and diameter of the central recirculation zone are 1.20Dand 1.27D respectively. For the enhanced ignition-dual register burner, the divergentangle is 67� and the maximum length and diameter of the central recirculation zoneare 1.07D and 0.74D, respectively.

The results show the centrally fuel rich swirl coal burner created a larger centralrecirculation zone and divergent angle than did the enhanced ignition-dual registerburner. The inner secondary air vanes of enhanced ignition-dual register burnerare adjustable. To ensure the vanes can be flexibly adjusted, there is a gap betweenvanes and pipes. Some of the inner secondary air ejects into the furnace withoutpassing the vanes, which results in lower swirl ability of the enhanced ignition-dualregister burner. For the centrally fuel rich burner, the inner secondary air vanes arefixed on the pipes. Thus, the centrally fuel rich swirl coal burner creates a larger cen-tral recirculation zone and divergent angle than does the enhanced ignition-dualregister burner.

Table 2 Cold air flow experimental parameters of the two burners

Quantity

Enhanced ignition-dual

register burner

Centrally fuel

rich burner

Mass flow rate of the primary air (kg s�1) 4.15 4.38

Mass flow rate of the inner secondary air (kg s�1) 2.95 3.12

Mass flow rate of the outer secondary air (kg s�1) 6.88 7.40

The vanes angle of the inner secondary air (see Figs.1 and 2) (�) 60 60

The vanes angle of the outer secondary air (see Figs.1 and 2) (�) 35 35

Air temperature (�C) 20 20

Figure 3 Jet border and the central recirculation zone boundary of the two burners.

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MEASUREMENTS OF GAS TEMPERATURE, GAS SPECIESCONCENTRATION AND CHAR BURNOUT IN THE BURNER REGION

Data Acquisition Techniques

Because a swirl burner works independently, there is little variation in tempera-ture and velocity field. Data were obtained for the gas temperature, gas species con-centration and char burnout in the region of burner No. 4, which was at the bottomon the rear wall (see Fig. 4). Data which were measured at positions along the Xdirection near the burner region using a water-cooled stainless steel probe insertedthrough monitoring ports (see Figs. 1 and 2) in the burner. The initial measurementposition along the X direction was on the rear wall. Data which were measured atpositions along the direction from the side wall to the burner were measured througha monitoring port in the side wall, as shown in Fig. 4. Measurements were made inthe enhanced ignition-dual register burner region when the four burners in the bot-tom row on the front wall were not operating. Thus, to maintain consistent experi-mental conditions throughout the measurements, these 4 burners were also stoppedwhen measurements were performed in the centrally fuel rich burner region. Toavoid the water-cooled stainless steel probe being distorted by hot gas, the distance

Figure 4 Schematic top view of the burners and the monitoring ports at the bottom of the furnace

(dimensions in m).

EMISSION CHARACTERISTICS OF WALL-FIRED COAL BOILER 1375

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between the measured point and side wall is no less than 1.5m. Thus, the measuredpoint is never in the center of the burner. The study on the center of the burner isdone by numerical simulation.

Gases were sampled using a water-cooled stainless steel probe and analyzedonline on a Testo 350M instrument. Gas temperature was measured using a nickelchromium-nickel silicon thermocouple placed inside a water-cooled stainless steelprobe. Char sampling was also performed using a water-cooled stainless steel probe.

The water-cooled stainless steel probe is shown in Figure 5 and consists of awater-in pipe, water-out pipe, sampling pipe, outer pipe and so on. The high pressurecool water coming from the water-in pipe cools the sampling pipe and the water afterthe heat change outflows from the water-out pipe. The gas was sampled by a sam-pling pipe. When smoke entered the sampling pipe, the temperature deceased rapidlyand the pulverized coal stopped burning. The samples passed through filtratingdevices into a Testo 350M gas analyzer to be analyzed. The coke sample wasobtained with a vacuum pump and sampling pipe, between which there is a cycloneseparator, coke collector, flow meter and other devices.

The accuracy of the Testo 350M gas analyzer for each species measurementis 1% for O2 and CO2, 5% for CO, and 5 ppm for NO and NO2. Calibration wascarried out on each sensor before measurement.

The CO2 concentration that cannot be measured directly by the Testo 350Mgas analyzer was calculated from the O2 concentration using

CO2 ¼ CO2;max20:94�O2

20:94ð1Þ

where CO2,max is the largest CO2 percentage of fuel combustion, which rangesbetween 18.4% and 18.7% for bitumite coal. CO2,max is 18.5% in this experiment.

Char Burnout

Char burnout was calculated using

w ¼ ½1� ðxk=xxÞ�=ð1� xkÞ ð2Þ

where w is the char burnout, x is the ash weight fraction, and the subscripts k and xrefer to the ash contents in the input coal and char sample, respectively.

Figure 5 Water-cooled stainless steel probe.

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The percentage release of components (C, H and N) was calculated using

b ¼ 1� ½ðxix=xikÞðxak=xaxÞ� ð3Þ

where xi is the weight percentage of the species of interest, xa is the ash weightpercentage and the subscripts k and x refer to different contents in the input coaland char sample, respectively (Costa et al., 2003).

During the experimental campaign, the utility boiler was operated stably with afull load. Table 3 shows characteristics of the coal used in the experiments. Table 4summarizes the boiler design and operation parameters. The values given in Table 4are averaged over the duration of the experimental campaign. Tables are presentedto summarize the results for each burner in the appendix.

Table 3 Characteristics of the coal used in the experiments

Quantity

Enhanced ignition-dual

register burner

Centrally fuel

rich burner

Proximate analysis (as received, wt.%)

Ash 25.18 27.13

Volatiles 32.26 33.15

Fixed carbon 39.78 40.82

Moisture 16.1 11.8

Net heating value (kJ kg�1) 16920 17790

Ultimate analysis (as received, wt.%)

Carbon 47.05 48.05

Hydrogen 2.29 2.51

Nitrogen 0.62 0.54

Sulfur 0.82 1.23

Oxygen 7.94 8.74

Table 4 Boiler design and operation parameters for two burners

Quantity

Enhanced ignition-dual

register burner

Centrally fuel

rich burner

Design

parameters

Flow rate of the main steam (ton=h) 973.2 974.3 1025.0

Pressure of the main steam (MPa) 16.3 16.7 16.8

Temperature of the main steam (�C) 536.2 537.8 540.0

Reheat steam outlet temperature (�C) 537.5 538.5 540.0

Reheat steam outlet pressure (MPa) 3.2 3.2 3.4

Coal feed rate (ton=h) 147.3 148.1 154.2

Primary air flow rate (ton=h) 243.7 251.7 285.3

Secondary air flow rate (ton=h) 656.1 663.6 731.3

Primary air temperature (�C) 74.0 73.0 75.0

Secondary air temperature (�C) 369.0 362.0 353.0

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RESULTS AND DISCUSSION

Before examining the data, it is necessary to show distribution of particle vol-ume flux for two burners. A three-component particle-dynamics anemometer is usedby Chen (2007) for measurements, in the near-burner region of the characteristics ofgas=particle two-phase flows with a centrally fuel rich swirl coal combustion burnerand enhanced ignition-dual register burner, in a gas=particle two-phase test facility.

Figure 6 shows profiles of the particle volume flux in the range from 0 to100 mm in different cross-sections of the two burners, where particle volume flux isthe particle volume passing per unit area per unit time, where d is the outer diameterof the outer secondary air duct. Compared with the enhanced ignition-dual registerburner, in the same cross-section, the particle volume flux peak value near the cham-ber axis of the centrally fuel rich burner is much closer to the chamber axis. In thecross-section L=d ¼ 0.5, the maximum particle volume flux of the centrally fuel richburner is three times that of the enhanced ignition-dual register burner.

In six cross-sections from L=d ¼ 0.3 to 2.5, the particle volume flux in the cen-tral recirculation zone of the centrally fuel rich burner is much larger than that of theenhanced ignition-dual register burner. In the three sections from L=d ¼ 0.3 to 0.7,the maximum particle volume flux in the central recirculation zone of the centrallyfuel rich burner is five times that of the enhanced ignition-dual register burner.

Due to the centrally fuel rich burner structure and particle inertia, particles inthe fuel-rich primary air duct are ejected directly into the chamber center and formthe peak for the particle volume flux near the chamber axis. For the enhancedignition-dual register burner, when particles eject into the primary air duct, becauseof either collision with the conical diffuser or guidance of the conical diffuser, theparticles form a zone of peak particle volume flux near the chamber axis. Particles

Figure 6 Particle volume flux profiles for the two burners.

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mix with the secondary air, and some are taken by the secondary air to form anotherother peak zone separate to that near the chamber axis.

Figure 7 shows profiles of the gas temperature near the burner region for thetwo burners. For both burners, the gas temperature along the X direction firstincreased and then decreased, with high increase rates in the early stage. The rate ofgas temperature increase was 238�C=m at positions between 0 and 0.2m from thecentrally fuel rich burner and 145�C=m at positions between 0 and 0.2m from theenhanced ignition-dual register burner. Thus, the gas temperature and its rate ofincrease were higher in this region for the centrally fuel rich burner. For both burners,gas temperatures increased sharply and then remained at a high level at a distancefrom the burner because of rapid combustion of the pulverized coal in the high-temperature gas. At positions away from the burner, the gas temperature graduallydecreased due to fuel consumption and mixing of the primary and secondary airflows.

The profiles in Figure 7a show the centrally fuel rich burner could maintain agas temperature up to 1000�C at a distance of 0.2–0.9m from the burner, but theenhanced ignition-dual register burner could only maintain a gas temperature upto 1000�C at a distance of 0.3–0.4m from the burner. Thus, the centrally fuel rich

Figure 7 Profiles of gas temperature measured (a) along the X direction and (b) in the radial direction near

the burner.

EMISSION CHARACTERISTICS OF WALL-FIRED COAL BOILER 1379

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burner has a larger high-temperature zone, which is more advantageous for pulver-ized coal combustion and burnout. As observed in Figure 7b for both burners, gastemperature increased from the side wall to the burner center.

Thus, gas close to the high-temperature central recirculation zone is at a highertemperature than that near the water-cooled side wall. The gas temperature near thewater-cooled side wall was higher for the enhanced ignition-dual register burner thanfor the centrally fuel rich burner (see Fig. 7b). Figure 7b shows the gas temperaturetrends measured in this work and by Costa et al. are similar. However, the tempera-tures measured by Costa et al. are larger. This is because Costa et al. measured thetemperature at the 3rd row of burners whereas we measured the temperature at the1st row, and the types of burners and coal character were also different.

Figure 8 shows O2 concentration profiles near the burner region for the twoburners. For both burners, the O2 concentration along the X direction first decreasedsharply and then slowly decreased (see Fig. 8a). The minimum O2 concentrationalong the X direction was 0.84% for the centrally fuel rich burner at 0.4m fromthe rear wall and 8.51% for the enhanced ignition-dual register burner at 0.6m from

Figure 8 Profiles of O2 concentration measured (a) along the X direction and (b) in the radial direction

near the burner.

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the rear wall. Thus, the O2 concentration in the central zone was lower for the cen-trally fuel rich burner than for the enhanced ignition-dual register burner.

The reason for the initial sharp decrease in O2 concentration along the X direc-tion is that the pulverized coal combusts rapidly and consumes a great deal of oxy-gen. At positions away from the rear wall, the O2 concentration increases becauseoxygen in the secondary air is supplied to the primary air. As observed inFigure 8b, the O2 concentration from the side wall to the burner changed slightlyfor both burners. Furthermore, the O2 concentration near the side wall was higherfor the centrally fuel rich burner than for the enhanced ignition-dual register burner.As observed in Fig. 8b, the O2 concentration measured by Costa et al. is much lowerthan that in the enhanced ignition-dual register and centrally fuel rich burners. Thisis because Costa et al. performed experiments adopting the over fire air system forthe boiler, which decreased the air supply in the main burning zone.

Figure 9 shows CO concentration profiles near the burner region for the twoburners. For both burners, CO concentrations along the X direction first increasedand then decreased (see Fig. 9a). Figures 8a and 9a show that, along the X direction,

Figure 9 Profiles of CO concentration measured (a) along the X direction and (b) in the radial direction

near the burner.

EMISSION CHARACTERISTICS OF WALL-FIRED COAL BOILER 1381

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CO concentration decreases with increasing O2 concentration. When pulverized coalbegins to burn it does so quickly and this requires much O2. At this time, there is littlesecondary airmixingwith primary air, which results inmuchCOwithout sufficient O2.

With the distance between measured point and water-cooled wall increasing,more secondary air mixes with primary air and CO transforms into CO2. Costaet al. (2003) obtained the same result in their experiments. The maximum CO con-centration along the X direction was 39448 ppm for the centrally fuel rich burnerat 0.5m from the rear wall and 20685 ppm for the enhanced ignition-dual registerburner at 0.6m from the rear wall. The CO concentration in the central zone washigher for the centrally fuel rich burner than for the enhanced ignition-dual registerburner.

As observed in Figure 9b, the CO concentration from the side wall to the bur-ner first increased and then decreased for the enhanced ignition-dual register burner;the CO concentration was very low for the centrally fuel rich burner. Furthermore,the CO concentration near the side wall was lower for the centrally fuel rich burnerthan for the enhanced ignition-dual register burner. As observed in Fig. 9b, the CO2

concentration measured by Costa et al. were larger than the data for the enhancedignition-dual register burner and centrally fuel rich burner, because the O2 concen-tration in the main burning zone was lower.

Figure 10 shows CO2 concentration profiles near the burner region for the twoburners. The CO2 concentration along the X direction increased and remained at ahigh level for both burners (see Fig. 10a). The CO2 concentration in the central zonewas higher for the centrally fuel rich burner than for the enhanced ignition-dualregister burner. The CO2 concentration first increases because combustion of thepulverized coal leads to a rapid decrease in O2 concentration; the CO2 concentrationthen remains at a high level while the CO concentration gradually decreases.

As observed in Figure 10b, for both burners the CO2 concentration changedslightly because the O2 concentration changed slightly. Furthermore, the CO2 con-centration in the region near the side wall was higher for the enhanced ignition-dualregister burner than for the centrally fuel rich burner. Figure 10b shows the CO2 con-centrations measured by Costa et al. were higher than the data for the enhancedignition-dual register burner and centrally fuel rich burner, because the O2 concen-tration in the main burning zone was lower.

Figure 11 shows char burnout near the burner region for the two burners. Theaverage char burnout measured along the X direction was higher for the centrallyfuel rich burner than for the enhanced ignition-dual register burner (see Fig. 11a).Char burnout measured from the side wall to the burner was high near the side wallfor both burners (see Fig. 11b).

Figure 12 shows the release of C, H and N from coal near the burner region forthe two burner types. Hydrogen release was fastest and carbon release was slowest,which agrees with the laboratory results of Ismail (1989) and Smoot et al. (1984), andthe industrial results of Costa et al. (2003). As observed in Fig. 12a, release of C, Hand N from coal along the X direction was faster for the centrally fuel rich burnerthan for the enhanced ignition-dual register burner, indicating the coal combustionrate was higher for the centrally fuel rich burner. The release of C, H and N fromcoal measured from the side wall to the burner wall was high near the side wallfor both burners (see Fig. 12b).

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From the aerodynamics and the distribution of particle volume flux for theenhanced ignition-dual register burner, the central recirculation zone is small andparticle volume flux is low in the burner center zone. It is difficult to form a hightemperature, high pulverized coal concentration in the burner center zone. The gastemperature is low and the ignition of pulverized coal is delayed, which is not infavor of burn-out. Thus, the local mean O2 concentration near the enhancedignition-dual register burner was higher compared to the centrally fuel rich burner,while local mean concentrations of CO and CO2, char burnout, and release of C, Hand N were lower. For the centrally fuel rich burner, the central recirculation zone islarge enough to inhale much hot gas and pulverized coal is concentrated in the burnercenter. The majority of the coal carried by the primary air entered the center of thecentral recirculation zone near the burner, so the concentration of pulverized coal inthe central recirculation zone increased and the residence time of the coal in the cen-tral recirculation zone was prolonged.

Figure 10 Profiles of CO2 concentration measured (a) along the X direction and (b) in the radial direction

near the burner.

EMISSION CHARACTERISTICS OF WALL-FIRED COAL BOILER 1383

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This led to a high-temperature and fuel-rich atmosphere in the central recircu-lation zone, which increased the gas temperature, accelerated pulverized coalignition, and increased the coal combustion rate, which is advantageous for burnout.Compared to the enhanced ignition-dual register burner, local mean concentrationsof CO and CO2, char burnout, and release of C, H and N were higher near the cen-trally fuel rich burner, but the local mean O2 concentration was lower.

For enhanced ignition-dual register burner, particle volume flux in the second-ary air is larger than that in other position. Thus, near the side wall, the gas tempera-ture, CO concentration and CO2 concentration are high and O2 concentration is low.For the centrally fuel rich burner, pulverized coal in the primary air duct is ejecteddirectly into the burner center and a little pulverized coal is carried into secondaryair due to the burner structure and particle inertia. Thus, near the side wall, thegas temperature, CO concentration and CO2 concentration are low and O2 concen-tration is high. The collision probability between pulverized coal and the side wall isreduced. This is advantageous for the formation of an oxidizing atmosphere near the

Figure 11 Char burnout measured (a) along the X direction and (b) in the radial direction near the burner.

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Figure 12 Release of carbon, hydrogen, and nitrogen from the coal measured (a) along the X direction and

(b) in the radial direction near the burner.

EMISSION CHARACTERISTICS OF WALL-FIRED COAL BOILER 1385

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water-cooled wall, which increases the ash fusion point, and for resisting slaggingand high-temperature corrosion (Huffman et al., 1981; You and Zhou, 2006).

Figure 13 shows NOx concentration profiles near the burner region for the twoburners, where the NOx reduced to 6% O2. The NOx concentration along the Xdirection first increased and then decreased near the centrally fuel rich burner. Forthe enhanced ignition-dual register burner, the NOx concentration along the X direc-tion first increased and then remained at a high value. The NOx concentration in thecenter of the burner region was much higher than for the centrally fuel rich burnerregion. From the side wall to the burner, the NOx concentration increased for bothburners, with fluctuations. The NOx concentrations for both burners were almost thesame. Figure 13b shows the mean concentrations of NOx measured by Costa et al.were lower than that of enhanced ignition-dual register and centrally fuel richburners. This is because an over fire air system was used in the boiler.

Compared to the enhanced ignition-dual register burner, the NOx concen-tration in the center of the centrally fuel rich burner region was much lower. Whenthe pulverized coal exits the outlet of the centrally fuel rich burner, devolatilization

Figure 13 Profiles of NOx concentration measured (a) along the X direction and (b) in the radial direction

near the burner.

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takes place and the release of volatiles from coal increases with increasing gas tem-perature. At the same time, the release of N from coal and NOx emission increases.With the distance between the measured point and water-cooled wall increasing, thegas temperature becomes high and there is an increase in the release of volatiles fromthe coal.

The pulverized coal concentration and volatile concentrations are high in thecentral recirculation zone with a low amount of O2. In this reducing environment,NOx formation is low and most of the reactive nitrogen is converted to N2

(van der Lans et al., 1997). For the enhanced ignition-dual register burner, the rapidburning of pulverized coal is also the reason for the initial increase in NOx concen-tration. The pulverized coal concentration, gas temperature and release ratio of vola-tiles from coal are low in the central recirculation zone of the enhanced ignition-dualregister burner. Because volatiles burn in an oxidizing atmosphere, NOx emissionincreases.

With increasing distance from the side wall to the burner center in the high-temperature central recirculation zone, the gas temperature and pulverized coal con-centration increased, resulting in an increase in NOx concentration for both burners.

COMPARISON OF BOILER PERFORMANCE FOR THE ENHANCEDIGNITON-DUAL REGISTER AND CENTRALLY FUEL RICH BURNERS

Combustion Efficiency

Combustion efficiency gives the degree of pulverized coal burn-out and iscalculated using

g ¼ 1� q4 ð4Þ

q4 ¼32700� Aar

100

Qr

aslagCslag

100� Cslagþ aashCash

100� Cash

� �ð5Þ

where g is the combustion efficiency (%), q4 is the incomplete loss (%), Aar is theproduced ash (%), Qr is the heat entering the furnace (KJ=kg) that is almost equalto the net heating of the fuel, aslag ¼ 0.1 and aash ¼ 0.9 are ash mass fractions in slagand fly ash, Cslag and Cash are carbon fractions in slag and fly ash, 32700 is the giveby per kilogram carbon burns completely. The error of in the carbon content analy-sis apparatus is �0.1%.

With eight centrally fuel rich burners on the bottom row, the boiler could runstably at a load of 300MWe. The test showed the negative furnace pressure, mainsteam pressure, main steam temperature, reheated steam pressure and reheatedsteam temperature all met the design requirements. Compared to the enhancedignition-dual register burners, combustible material content decreased from 6.54%to 5.86% in the fly ash and from 3.19% to 2.87% in the slag. The efficiency ofpulverized coal combustion increased from 96.73% to 97.09%.

EMISSION CHARACTERISTICS OF WALL-FIRED COAL BOILER 1387

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NOx Emission Experiments

With eight centrally fuel rich burners on the bottom and the boiler operating atthe rated load of 300MWe, NOx emission was measured at the outlet of the air pre-heater. NOx emission was 411.5 ppm @ 6% O2 for the enhanced ignition-dual regis-ter burner and 355 ppm @ 6% O2 for the centrally fuel rich burner, a decrease of56.5 ppm (13.74%). The NOx emission measured by Costa et al. was below243.9 ppm @ 6% O2, which was lower than data for the centrally fuel rich burnerand enhanced ignition-dual register burner.

Influence on the Boiler Minimum Load Without Auxiliary Fuel Oil

A low-load experiment was performed with random coal using eight centrallyfuel rich burners on the bottom row. Proximate analysis of the experimental coalyielded Vdaf ¼ 35.12%, Mar ¼ 15.07%, Aar ¼ 19.9%, and Qnet,ar ¼ 19080 kJ=kg.With only eight centrally fuel rich burners on the bottom row running, the load sta-bilized at 110MWe (36.7% rated load) for 2 h. Boiler steam parameters were in thenormal range. Furnace pressure fluctuation was �50 Pa, which implies combustionwas stable in the furnace. The flame scanners showed a steady signal rather than anintermittent signal, and the boiler ran well. For the enhanced ignition-dual registerburners, the minimum load was up to 180MWe (60% rated load) without auxiliaryfuel oil. Table 5 shows the operation parameters at minimum loads without auxiliaryfuel oil for the two kinds of burners.

CONCLUSIONS

The results indicate the local mean concentrations of CO and CO2 gas species,the gas temperatures and the temperature gradient, char burnout and the release ofC, H and N measured at positions along the X direction near the centrally fuel richburner region were all higher than the values obtained near the enhanced ignition-dual register burner region. The mean concentrations of O2 and NOx measured atpositions along the X direction near the centrally fuel rich burner region were lower.

Table 5 Operation parameters at their minimum load without auxiliary fuel oil for the two burners

Quantity

Enhanced ignition-dual

register burner

Centrally fuel

rich burner

Minimum load without auxiliary fuel oil (MW) 179.5 110.0

Flow rate of the main steam (ton=h) 743.0 624.0

Pressure of the main steam (MPa) 10.5 6.8

Temperature of the main steam (�C) 518.0 514.0

Reheat steam outlet temperature (�C) 518.0 506.7

Negative pressure of the furnace (Pa) �75.7 �95.4

Primary air flow rate (ton=h) 184.2 136.5

Primary air=fuel temperature (�C) 69.0 64.0

Secondary air temperature (�C) 378.0 297.0

Secondary air flow rate (ton=h) 291.3 231.0

Coal feed rate (ton=h) 94.0 63.2

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Compared to the enhanced ignition-dual register burners, at positions near the sidewall, the mean O2 concentration was higher and the gas temperature and mean con-centrations of CO and CO2 were lower for the centrally fuel rich burners.

The analysis results for C, H and N release indicate that hydrogen release ismost rapid and carbon is released slowly. For the enhanced ignition-dual registerburners, the efficiency of pulverized coal combustion was 96.73%, NOx emissionwas 411.5 ppm @ 6% O2, and the minimum load was up to 180MWe without auxili-ary fuel oil. For the centrally fuel rich burners, the efficiency of pulverized coal com-bustion was 97.09%, NOx emission was 355 ppm @ 6% O2 and the minimum loadwas up to 110MWe without auxiliary fuel oil.

REFERENCES

Black, D.L. and McQuay, M.Q. (1996) Particle size and velocity measurements in the radiantsection of an industrial-scale, coal-fired boiler: The effect of coal type. Combust. Fire, 328,19–27.

Bonin, M.P. and Queiroz, M. (1991) Local particle velocity, size, and concentration measure-ments in an industrial pulverized coal fired boiler. Combust Flame, 85, 121–133.

Bonin, M.P. and Queiroz, M. (1996) A parametric evaluation of particle-phase dynamics in anindustrial pulverized-coal-fired boiler. Fuel, 75, 195–206.

Butler, B.W. and Webb, B.M. (1991) Local temperature and wall radiant heat flux measure-ments in an industrial scale coal fired boiler. Fuel, 70, 1457–1464.

Butler, B.W., Wilson, T., and Webb, B.W. (1992) Measurement of time-resolved local particlecloud temperature in a full-scale utility boiler. Proc. Combust. Instit., 24, 1333–1339.

Chen, Z.C. (2007) Studies on characteristics of gas-particle two phase flows with centrally fuelrich swirl coal combustion burner and its application. Ph.D. Dissertation, Harbin Insti-tute of Technology (in Chinese).

Costa, M., Azevedo, J.L.T., and Carvalho, M.G. (1997) Combustion characteristics of a front-wall-fired pulverized-coal 300MWe utility boiler. Combust. Sci. Technol., 129, 277–293.

Costa, M., Silva, P., and Azevedo, J.L.T. (2003) Measurements of gas species, temperature,and char burnout in a low-NOx pulverized-coal-fired utility boiler. Combust. Sci.Technol., 175, 271–289.

Costa, M. and Azevedo, J.L.T. (2007) Experimental characterization of an industrialpulverized coal-fired furnace under deep staging conditions. Combust. Sci. Technol.,179, 1923–1935.

Fan, J.R., Sun, P., Zheng, Y.Q., Ma, Y.L., and Cen, K.F. (1999) Numerical and experimentalinvestigation on the reduction of NOx emission in a 600MW utility furnace by usingOFA. Fuel, 78, 1387–1394.

Huffman, G.P., Huggins, F.E., and Dunmyre, G.R. (1981) Investigation of the high-temperature behaviour of coal ash in reducing and oxidizing atmospheres. Fuel, 60,585–597.

Ismail, M. (1989) Char burnout and flame stability in pulverized fuel furnace. Ph.D. disser-tation, Department of Mechanical Engineering, University of London.

Larue, A.D. and Cioffi, P.L. (1988) Low NOx burner development in the USA. Mod. PowerSyst., 8, 42–47.

Li, Z.Q., Yang, L.B., Qiu, P.H., Sun, R., Chen, L.Z., and Sun, S.Z. (2004) Experimental studyof the combustion efficiency and formation of NOx in an industrial pulverized coalcombustion. Int. J. Energy Res., 28, 511–520.

EMISSION CHARACTERISTICS OF WALL-FIRED COAL BOILER 1389

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Smoot, L.D., Hedman, P.O., and Smith, P.J. (1984) Pulverized-coal combustion research atBrigham Young University. Prog. Energy Combust. Sci., 10, 359–441.

Tree, D.R. and Webb, B.W. (1997) Local temperature measurements in a full-scale utilityboiler with over fire air. Fuel, 76, 1057–1066.

Van der Lans, R.P., Glarborg, P., and Dam-Johansen, K. (1997) Influence of process para-meters on nitrogen oxide formation in pulverized coal burners. Prog. Energy Combust.Sci., 23, 349–377.

Vikhansky, A., Bar-Ziv, E., Chudnovsky, B., Talanker, A., Eddings, E., and Sarofim, A.(2004) Measurements and numerical simulations for optimization of the combustion pro-cess in a utility boiler. Int. J. Energy Res., 28, 391–401.

You, C. and Zhou, Y. (2006) Effect of operation parameters on the slagging near swirl coalburner throat. Energy Fuels, 20, 1855–1861.

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APPENDIX

Table

A1

Experim

entalresultsofcentrallyfuel

rich

burner

alongtheX

direction

X(m

)

Gas

temperature

(�C)

O2(dry

volume%)

CO

(dry

volumeppm)

CO

2(dry

volume%)

NO

x(dry

volume

ppm

@6%

(3%

)O2)

Char

burnout(%

)

Carbon

release

(%)

Hydrogen

release

(%)

Nitrogen

release

(%)

0574

19.95

774

0.87

371.2

(445.5)

——

——

0.1

825

13.71

3050

6.39

464.4

(557.3)

——

——

0.2

1050

4.55

4440

14.48

431.2

(517.5)

——

——

0.3

1066

1.52

16933

17.16

342.4

(410.9)

——

——

0.4

1078

0.84

25791

17.76

301.0

(361.2)

——

——

0.5

1080

1.00

39448

17.62

315.6

(378.7)

——

——

0.6

1067

1.23

38821

17.41

320.5

(384.6)

——

——

0.7

1055

1.51

37084

17.17

357.6

(429.1)

——

——

0.8

1051

1.72

36586

16.98

359.0

(430.8)

53

50.4

78.8

61.7

0.9

1006

1.98

36095

16.75

395.6

(474.7)

——

——

1.0

995

2.13

30912

16.62

374.6

(449.6)

——

——

1.2

960

2.88

26169

15.96

365.9

(439.0)

62

54.1

77.3

65.4

1.4

880

2.88

17213

15.96

345.4

(414.4)

——

——

1.6

838

3.05

7741

15.81

335.6

(402.7)

66

49.6

89.3

48.2

1.9

—5.72

3267

13.45

362.4

(434.9)

——

——

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Table

A2

Experim

entalresultsofenhancedignition-dualregisterburner

alongtheX

direction

X(m

)

Gas

temperature

(�C)

O2(dry

volume%)

CO

(dry

volumeppm)

CO

2(dry

volume%)

NO

x(dry

volume

ppm

@6%

(3%)O

2)

Charburn

out(%

)

Carbon

release

(%)

Hydrogen

release

(%)

Nitrogen

release

(%)

0568

20.23

59

0.63

97.6

(117.1)

——

——

0.1

722

19.60

471

1.18

267.8

(321.4)

——

——

0.2

894

17.51

1561

3.03

455.6

(546.7)

——

——

0.3

1004

11.28

3366

8.53

464.4

(557.3)

——

——

0.4

1015

9.37

8624

10.22

549.3

(659.1)

32.3

29.5

48.4

40.1

0.5

972

9.30

16546

10.28

610.2

(732.3)

——

——

0.6

950

8.51

20825

10.98

553.7

(664.4)

——

——

0.7

802

8.87

19825

10.66

587.3

(704.8)

——

——

0.8

745

9.40

18336

10.20

586.8

(704.2)

45.7

31.2

51

47.5

0.9

704

10.13

16235

9.55

615.6

(738.7)

——

——

1.0

—11.32

8667

8.50

531.7

(638.0)

——

——

1.2

—12.06

5223

7.85

557.1

(668.5)

——

——

1.4

—12.62

4850

7.35

545.9

(655.0)

43.2

25.8

42.8

39.4

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Table

A3

Experim

entalresultsin

theradialdirectionnearcentrallyfuel

rich

burner

Distance

from

the

sidewall(m

)

Gas

temperature

(�C)

O2(dry

volume%)

CO

(dry

volume

ppm)

CO

2(dry

volume%)

NO

x(dry

volume

ppm

@6%

(3%)O

2)

Char

burnout(%

)

Carbon

release

(%)

Hydrogen

release

(%)

Nitrogen

release

(%)

0675

11.61

18

8.24

300.5

(360.6)

——

——

0.1

732

10.57

16

9.16

285.9

(343.0)

——

——

0.2

762

10.3

14

9.40

270.2

(324.3)

——

——

0.3

782

10.16

12

9.52

279.5

(335.4)

——

——

0.4

794

10.65

15

9.09

287.3

(344.8)

96.7

94.1

97.5

98.8

0.5

804

10.08

12

9.59

290.7

(348.9)

——

——

0.6

805

9.17

12

10.40

275.6

(330.7)

——

——

0.7

821

9.44

12

10.16

288.8

(346.5)

——

——

0.8

819

9.42

10

10.18

294.6

(353.6)

96.1

93.5

99.5

97.6

0.9

—9.6

11

10.02

293.7

(352.4)

——

——

1.0

828

10.27

10

9.43

312.7

(375.2)

——

——

1.2

841

10.3

99.40

—96.2

96.7

99.5

97.7

1.4

852

10.55

10

9.18

——

——

—

1.5

860

11.35

98.47

——

——

—

1.6

871

——

——

——

——

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Table

A4

Exp

erim

entalresultsin

theradialdirectionnearenhan

cedignition-dualregisterburner

Distance

from

thesidewall(m

)

Gas

temperature

(�C)

O2(dry

volume%)

CO

(dry

volumeppm)

CO

2(dry

volume%)

NO

x(dry

volume

ppm

@6%

(3%

)O2)

Char

burnout(%

)

Carbon

release

(%)

Hydrogen

release

(%)

Nitrogen

release

(%)

0780

7.65

82

11.74

271.7

(326.0)

——

——

0.1

835

7.36

176

12.00

262.9

(315.5)

——

——

0.2

884

6.95

208

12.36

274.1

(329.0)

——

——

0.3

900

6.78

257

12.51

286.3

(343.6)

——

——

0.4

937

6.63

542

12.64

283.9

(340.7)

——

——

0.5

960

6.3

571

12.93

298.0

(357.7)

——

——

0.6

966

6.21

571

13.01

290.2

(348.3)

——

——

0.7

997

5.83

627

13.35

291.7

(350.0)

——

——

0.8

1007

5.88

623

13.31

291.7

(350.0)

——

——

0.9

—5.79

615

13.38

282.0

(338.3)

——

——

1.0

1017

5.97

603

13.23

318.5

(382.2)

97.6

97.3

99.5

98.5

1.2

1057

6.42

604

12.83

302.4

(362.9)

98.4

98.6

99.2

99.1

1.4

1059

7.02

520

12.30

302.4

(362.9)

98.4

97.7

99.7

99.2

1.5

1037

7.41

434

11.95

299.0

(358.8)

——

——

1.6

1024

——

——

——

——

1394

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