Effects of particle concentration variation in the primary air duct on combustion characteristics and NOx emissions in a 0.5-MW test facility with pulverized coal swirl burners

Effects of particle concentration variation in the primary air duct on combustion characteristics and NOx emissions in a 0.5-MW test facility with pulverized coal swirl burners

Applied Thermal Engineering 73 (2014) 859e868 Contents lists available at ScienceDirect Applied Thermal Engineering journal homepage: www.elsevier.c...

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Applied Thermal Engineering 73 (2014) 859e868

Contents lists available at ScienceDirect

Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng

Effects of particle concentration variation in the primary air duct on combustion characteristics and NOx emissions in a 0.5-MW test facility with pulverized coal swirl burners Zhengqi Li*, Song Li, Qunyi Zhu, Xiqian Zhang, Guipeng Li, Yong Liu, Zhichao Chen, Jiangquan Wu School of Energy Science and Engineering, Harbin Institute of Technology, 92, West Dazhi Street, Harbin 150001, PR China

h i g h l i g h t s  Reactive flow experiments of different coal concentration variation were conducted.  Coal concentration has great impact on the flame type, gas temperature & components.  Larger coal concentration of inner primary air reduces NOx emission significantly.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 May 2014 Accepted 19 August 2014 Available online 27 August 2014

The effects on combustion characteristics and NOx formation of the particle concentration variation in the primary air duct were investigated on a 0.5-MW pulverized-coal combustion test facility with swirl burners. The gas temperature, components and fly ash in the near-burner region and furnace center were sampled and measured. It shows that the pulverized-coal concentration variation in the primary air duct is of great importance as for defining the flame structure, gas temperature profile and gas components. With the burner changing from general type to inner-lean-outer-rich to inner-rich-outer-lean types I and II, the flame diffusion zone shrinks, its color less bright, the O2 concentration in the flame zone decreases and the CO concentration increases significantly. Additionally, the NOx concentration decreases, with NOx emissions at the furnace exit dropping from 1182 to 861 mg/m3 (a reduction of 27.15%). Finally, the release rates of carbon, hydrogen and nitrogen and the burnout rate of carbon all decrease. The gas temperatures observed in the inner-rich-outer-lean types I and II burners are lower than those of the general and the inner-lean-outer-rich type burners in the near-burner region. The release rates of elements and the burnout rate of carbon are close to 100% for all four burners. © 2014 Elsevier Ltd. All rights reserved.

Keywords: Pulverized coal combustion NOx emission Flame pattern

1. Introduction In recent years, the contribution of NOx generated from coal combustion to air pollution has become of concern to the international community [1,2] and the allowable NOx emissions from coal-fired utility boilers are consequently being reduced. As an example of new restrictions, the permissible emission limit for power plants over 500 MWe in the EU as of 2008 was 500 mg NO2/ Nm3 at 6% O2, while the limit as of 2016 for such power plants will be lowered to 200 mg NO2/Nm3 at 6% O2 [3]. Since 2012, the

* Corresponding author. Tel.: þ86 451 8641 8854; fax: þ86 451 8641 2528. E-mail address: [email protected] (Z. Li). http://dx.doi.org/10.1016/j.applthermaleng.2014.08.041 1359-4311/© 2014 Elsevier Ltd. All rights reserved.

allowable NOx emissions from over 300 MWe coal-fired power plants operating in China have been 100 mg NO2/Nm3 at 6% O2 in major cities and 200 NO2/Nm3 at 6% O2 in other locations. Therefore, it is both necessary and desirable to further study the combustion characteristics of pulverized coal and the associated mechanisms of NOx formation, to allow the development of new combustion technologies which lower NOx emissions. Coal combustion tests are an important means of studying the characteristics of combustion and the mechanisms of NOx formation in a swirl burner. Abbas et al. measured gas temperature, O2, CO and NOx concentrations, as well as a variety of stable nitrogen species and solids originating from two distinct pulverized coal fired burners in a laboratory furnace. They found a significant decrease in NOx emissions within the near burner region when the

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so-called central tube in a single annular orifice burner jet was replaced by a single central orifice burner jet [4]. Zhao et al. showed that some of the pulverized coal particles leaving the burner can directly enter the radial recirculation zone behind the petal flame stabilizer, where they immediately ignite and burn. This process is advantageous in terms of lowering NOx emissions [5]. Abbas et al. summarized the existing research into NOx formation during pulverized coal combustion and examined reduction mechanisms [6]. Smart et al. measured the NOx concentrations obtained with five types of re-burn fuels and different process parameters and investigated the effectiveness of an internally fuel-staged burner for NOx reduction [7]. Godoy et al. acquired burnout data for pulverized coal fired cylindrical combustion to examine variations in the extent of burnout, which is influenced by the particle residence time in the inner recirculation zone [8]. Truelove et al. measured the flow fields, temperatures and species distributions obtained with volatile contents varying from 19 to 40% dry-ash-free (DAF) in the near-burner zone and suggested that flame stability was primarily determined by the burner aerodynamics with little influence from coal volatile content [9]. In reactive and non-reactive flow experiments, Lyle et al. measured axial and tangential velocities in pulverized coal flames using laser Doppler anemometry (LDA) and studied the influence of swirl number on the distributions of coal particles, turbulence characteristics, flame structures and NOx emissions [10,11]. Xue et al. investigated NOx emissions and carbon burnout rates as functions of the thermal state of a 1 MW test facility with an enhanced ignition-dual register burner [12,13]. Weber et al. provided scaling properties of swirling pulverized coal flames with thermal inputs ranging from 180 kW to 50 MW [14]. Van der Lans et al. studied the effects of particle size, residence time, gas temperature, sliding speed, swirl number, burner geometry and fuel properties on NOx formation. They determined that it is difficult to predict the mixing of the coal flame based on the structure of the burner, the coal properties and the operating parameters. However, they were able to determine that mixing of the coal flame directly affected NOx formation, demonstrating that many of the furnace phenomena can be explained but not be predicted [15]. Costa et al. measured local mean concentrations of O2, CO, CO2 and NOx in the flue gas as well as gas temperatures and char burnout using a number of observation ports in a 300 MWe, front-wall-fired, pulverized coal utility boiler [16e18]. Li et al. measured the same parameters in a 300 MWe wall-fired pulverized coal utility boiler with two swirl burners [19]. The primary air in a swirl burner can be divided into inner and outer portions. The primary air near the center of the burner is considered the inner primary air, while the outer primary air is located further from the center of the burner, as shown in Fig. 1. Swirl burners can exhibit three distinct types of particle concentration variation in the primary air duct: general, inner-leanouter-rich and inner-rich-outer-lean. A general burner is defined as one in which the pulverized coal concentration in the inner primary air is the same as that in the outer primary air. An innerlean-outer-rich burner, however, exhibits a pulverized coal concentration in the inner primary air which is less than that in the outer primary air, and both the volute and enhanced ignition-dual register burners belong to this type. Finally, an inner-rich-outerlean burner is defined as one in which the pulverized coal concentration in the inner primary air is greater than that in the outer primary air, and includes centrally fuel rich swirl burners [20]. Zeng et al. studied the effects of primary air duct pulverized coal concentration distribution on slagging near the burner nozzle by numerical simulation [21]. The results showed that the particle adhesion number density on the water-cooled tube wall in an inner-rich-outer-lean type burner was significantly lower than in other types of burners.

Fig. 1. Diagram showing inner and outer primary air flows.

We employed reactive flow experiments using a specially constructed 0.5-MW coal combustion test facility to study the influence of primary air duct pulverized coal concentration variation on coal combustion characteristics and NOx formation. 2. Test facility The 0.5-MW test facility was composed of a coal feeding system, an ignition system, a swirl burner, the furnace, a cooling system, a gas temperature and gas composition measurement system, an air pre-heater and a flue gas dust removal system (as shown in Fig. 2.). The furnace was positioned vertically and had a diameter of 800 mm and a height of 5760 mm. The apparatus was insulated with corundum-mullite and employed a labyrinth seal. The operating gas temperature of the furnace was 0e1600  C, the gas residence time in the furnace was approximately 5.0 s and the internal gas pressure was in the range of 100e150 Pa. Both the volume and temperature of the primary and secondary air could be adjusted. Two coal feeders provided pulverized coal to the inner and outer primary air ducts separately. 3. Experimental conditions and parameters 3.1. Experimental conditions When the maximum gas temperature along the center line of the furnace reached 1100  C, the pulverized coal swirl burner was capable of working independently, at which point the firing gun was turned off and removed. The burner operating parameters, such as the velocities and the temperatures of the inner and outer primary and secondary airs and the feed rates of the inner and outer primary airs, were maintained as stably as possible, so that fluctuations in these parameters were less than 10% of the mean values. When the measured gas temperature fluctuation at the center line of the furnace was below 10  C for ten consecutive minutes, and the gas pressure was in the range of 50 to 50 Pa at the furnace exit, the experiment trial was begun. During each trial, thermocouples were used to measure gas temperature, and the gas composition and fly ash were both sampled using a water-cooled collecting probe. The water-cooled stainless steel probe applied in this work (shown in Fig. 3.) consisted of a water feed pipe, water outlet pipe, sampling pipe and outer pipe. The gas was sampled via the sampling pipe. When the flue gas entered the sampling pipe, its temperature dropped rapidly and the pulverized coal stopped burning. The resulting samples were passed through filters into a Testo 350 M gas analyzer. The coke samples were obtained using a vacuum pump in conjunction with a sampling pipe, between which there was a cyclone separator, coke collector and flow meter. The accuracy of the Testo 350 M gas analyzer when measuring gas species was within 1% for O2, 3% for CO and 5 ppm for NO and NO2. Each sensor was calibrated before measurements were made. The repeatability of the flue gas data was on average within 5%. Char burnout was calculated using the equation.

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Fig. 2. Schematic diagram of the test facility.

j ¼ ½1  ðuk =ux Þ=ð1  uk Þ;

(1)

where j is the char burnout, u is the ash weight fraction and the subscripts k and x refer to the ash contents in the input coal and the char sample. The percentage releases of chemical components (C, H and N) were calculated from the equation.

b ¼ 1  ½ðuix =uik Þðuak =uax Þ;

Fig. 3. Water-cooled stainless steel probe.

(2)

where ui is the weight percentage of the species of interest, ua is the ash weight percentage and the subscripts k and x refer to the content in the input coal and char sample [16]. The repeatability of char burnout and component release data was on average within 10%. 3.2. Experimental parameters Four different experiments were performed, using a general type, an inner-lean-outer-rich type and inner-rich-outer-lean type I and II burners. Fig. 4 presents the specific dimensions of the swirl burner used in this work. There are 16 bent-shaft vanes in the inner secondary air duct and 12 tangential vanes at the entrance of the outer secondary air duct. The diameter of the apparatus was 160 mm and the area ratio of the inner primary air to outer primary air was 1:4. The pulverized coal feed rate during the trials was 40 kg/h. Throughout the experiments, the temperatures of the primary and secondary airs, the air velocities and the excess air ratio were maintained at constant values, and only the feed rate of

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Fig. 4. Details of the experimental burner (all dimensions are in mm): a. inner primary air duct b. outer primary air duct c. inner secondary air duct d. outer secondary air duct e. tangential vanes f. bent-shaft vanes.

the pulverized coal to the inner and outer primary air was varied. The experimental parameters are summarized in Table 1. The coal combustion characteristics were measured at sections where x/d ¼ 0.5, 1.5, 2.5 and 3.5 and r/d ¼ 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.8, 1.0, 1.5, 2.0 and 2.5, where x is the distance from the measurement point to the burner nozzle, d is the maximum diameter of the burner and r is the radius (see Fig. 5.). At each measurement point, the gas temperature and gas composition were measured using an online probe and the fly ash was sampled. Bituminous coal with an average particle size of 39.94 mm was used in this study; the properties of the coal are provided in Table 2 and Fig. 6. 4. Results and discussion 4.1. Coal combustion characteristics and NOx formation in the near burner region Fig. 7 shows radial profiles of gas temperatures and gas species concentrations (O2, CO and NOx) along the sections where x/d ¼ 0.5e3.5. The peak values of the gas temperature, CO

Fig. 5. Experimental furnace with measurement points indicated (all dimensions are in mm).

and NOx concentrations and the lowest measured value of the O2 concentration are given in Table 3. In the radial zone defined by 0  r  50 mm and x/d ¼ 0.5, as the burner type is changed from general to inner-lean-outer-rich to inner-rich-outer-lean types I and II, the peak value of the gas temperature decreases from 1384 to 1261  C. The lowest value of the O2 concentration decreases from 7.43 to 2.65%, and the peak value of the NOx concentration decreases from 1413.5 to 882.4 mg/ m3. In addition, the peak value of the CO concentration increases from 7007 to 75,016 ppm. In the case of the general burner, the pulverized coal concentration in the inner primary and outer primary air is the same. However, the inner primary air pulverized coal concentration is lower in the inner-lean-outer-rich type burner and higher in the inner-rich-outer-lean types I and II burners. Increases in the pulverized coal concentration in the inner primary air duct result in a lack of O2 as the coal burns, leading to increases in CO formation and reduced coal combustion rates, even though the

Table 1 Experimental parameters. Case

1

2

3

Type of the burner

General type

Inner-lean-outer-rich type

Inner-rich-outer-lean type

Feed rate of pulverized coal into the inner primary air (kg/h) Feed rate of pulverized coal into the outer primary air (kg/h) Concentration of pulverized coal in the inner primary air (kgc/kga) Concentration of pulverized coal in the outer primary air (kgc/kga) Temperature of the primary air ( C) Temperature of the secondary air ( C) Ratio of the primary air (%) Ratio of the inner secondary air (%) Ratio of the outer secondary air (%) Mass flow rate of the primary air (kg/h) Velocity of the primary air (m/s) Mass flow rate of the inner secondary air (kg/h) Velocity of the inner secondary air (m/s) Mass flow rate of the outer secondary air (kg/h) Velocity of the outer secondary air (m/s) Excess air ratio at the exit of the furnace

8 32 0.34 0.34 275 300 27.5 29 43.5 117.24 14.26 123.64 15.27 185.46 18.13 1.15

0 40 0 0.425

4

Type I

Type II

20 20 0.85 0.213

40 0 1.7 0

Z. Li et al. / Applied Thermal Engineering 73 (2014) 859e868 Table 2 Coal characteristics. Ultimate analysis (wt %, ar)

Proximate analysis (wt %, ar)

C

V

H

O

N

S

66.12 3.89 12.47 0.79 0.82 32.02

A

M

FC

7.46

8.45

52.07

Qnet,ar (kJ/kg)

25,160

distance over which heat is released increases. A high CO concentration also produces a reduction in NOx and a decrease in peak gas temperature. In the radial zone defined by 50  r  125 mm, as the burner is transitioned from general to inner-lean-outer-rich to inner-rich-outer-lean types I and II, the O2 concentration increases while the CO and NOx concentrations and the gas temperature decrease. This zone corresponds to the secondary air jet region of the burner. In the case of the inner-rich-outer-lean types I and II burners, the coal concentration is lower in the outer primary air duct and thus it is difficult for pulverized coal to diffuse into the secondary air jet region. For this reason, the O2 concentration is greater while the CO and NOx concentrations are lower. In the radial zone defined by 125  r  400 mm, corresponding to the external recirculation zone of the burner jet, the gas temperature decreases rapidly in all four burners, while O2, CO and NOx concentrations are relatively constant. In the radial zone where 0  r  50 mm and x/d ¼ 1.5, the peak values of the gas temperature and the CO concentration are higher while the lowest value of O2 concentration and the peak value of the NOx concentration are lower, compared to the value obtained at x/d ¼ 0.5 for all four burner types. As the burner type is changed from general to inner-lean-outer-rich to inner-rich-outer-lean types I and II, the gas temperature peak value decreases from 1415 to 1323  C. The lowest O2 concentration decreases from 3.31 to 1.03%, and the peak NOx concentration decreases from 1170.8 to 773.1 mg/m3. In addition, the peak CO concentration increases from 53,398 to an apparent value of 100,000 ppm, representing the upper limit of the Testo 350 M CO concentration measurement range. As the combustion process continues and the heat release continues to rise, the peak value of the gas temperature increases. Additionally, a greater quantity of O2 is consumed, which results in a decrease in the lowest measured O2 concentration. More CO is also generated, which leads to decreased NOx emissions. In the radial zone where 50  r  150 mm, the gas temperature increases, the O2 concentration decreases and the CO and NOx concentrations

Fig. 6. Distribution of coal particle sizes.

863

increase. As the coal particles penetrate this zone and mix with the secondary air, sufficient O2 becomes available for combustion while the inhibiting and reducing effects of CO on NOx are reduced so that the NOx concentration in this region increases. In the radial zone defined by 150  r  400 mm, the gas temperature increases in all four burners as compared to the x/d ¼ 0.5 location, while the gas composition remains constant. This is due to increases in the gas temperature within the external recirculation zone as coal combustion proceeds. In the radial zone where 0  r  100 mm and x/d ¼ 2.5, gas temperature increases are observed in all four cases. The lowest O2 concentration and the peak CO concentration both decrease and the peak NOx concentration increases with the general and the inner-lean-outer-rich type burners. In contrast, the lowest O2 concentration decreases to a greater extent while the peak CO concentration increases, and the peak NOx concentration decreases with the inner-rich-outer-lean types I and II burners, compared to the values acquired at x/d ¼ 1.5. As the burner type changes from general or inner-lean-outer-rich to inner-rich-outer-lean types I and II, the peak gas temperature decreases from 1431 to 1336  C. The lowest O2 concentration decreases from 2.8 to 0.06% and the peak NOx concentration drops from 1176.4 to 567.5 mg/m3, whereas the peak CO concentration increases from 42,925 to 100,000 ppm. The fuel concentration is high in the center zone of the inner-richouter-lean types I and II burners and the coal flame diffusion region is reduced, while the flame length is increased. There is insufficient O2 in this region, which leads to the observed increase in CO concentration. This elevated CO concentration not only catalyzes the heterogeneous reduction of NOx on the surfaces of coal particles but also accelerates the reduction rate based on direct interaction between CO and NOx, which in turn lowers the conversion of nitrogen in the fuel to NOx. Furthermore, concentrations of reducing radicals in the gasdsuch as H2, CH, C2, NH and CNdare increased by the lack of O2, which enhances the NOx reduction reaction. All these effects result in a decrease in the NOx concentration in this region. In the radial zone where 100  r  250 mm, as the burner type transitions from general to inner-lean-outer-rich to inner-richouter-lean types I and II, the gas temperature decreases, the CO concentration increases and the NOx concentration decreases. As the gas flows radially outward, the gas composition remains similar to that in the combustion region, with the exception of the O2 concentration, which increases slightly with secondary air mixing. In the radial zone where 250  r  400 mm, the gas temperature increases in all four burners, compared to the values obtained at x/ d ¼ 1.5, while the gas composition remains constant. At x/d ¼ 3.5, the gas temperature profiles in all four burners are relatively constant along the radial direction. As the burner type changes from general to inner-lean-outer-rich to inner-rich-outerlean types I and II, the gas temperature gradually declines. With the general and inner-lean-outer-rich burners, the O2 concentration is approximately 2.5% in the radial zone defined by 0  r  125 mm and approximately 3.8% in the radial zone where 125  r  400 mm. With the inner-rich-outer-lean types I and II burners, the O2 concentration is approximately 1.3% in the radial zone where 0  r  125 mm and approximately 4% in the radial zone where 125  r  400 mm. In the case of the general burner, the CO concentration is relatively constant along the radial direction. The CO concentration in the inner-lean-outer-rich burner is higher than that in the general burner within the radial zone demarcated by 0  r  125 mm, while the CO concentrations of both burners are similar within the radial zone where 125  r  400 mm. Within the radial zone where 0  r  125 mm, the CO concentrations of the inner-rich-outer-lean types I and II burners vary from 82,592 to 13,289 ppm and 95,711 to 14,243 ppm, respectively. In the radial zone defined by 125  r  400 mm, the

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Fig. 7. Radial profiles of gas temperatures and gas species concentrations (O2, CO and NOx) at the sections where x/d ¼ 0.5e3.5 for the general type (  ), inner-lean-outer-rich type (B), inner-rich-outer-lean type I (;) and inner-rich-outer-lean type II (D) burners. *The Testo 350 M useable range for CO concentration is from 0 to 100,000 ppm.

CO concentration of both burners is slightly higher than that measured in the general type burner. The NOx concentrations of the general and inner-lean-outer-rich burners change very little along the radial direction. The NOx concentrations of the inner-richouter-lean types I and II burners are quite low within the radial zone of 0  r  125 mm and constant in the radial zone where 125  r  400 mm. Moving from the general or inner-lean-outerrich to inner-rich-outer-lean type I and II burners, the gas temperature decreases, the CO concentration increases and the NOx concentration decreases. The majority of coal combustion is complete at this stage in the general and inner-lean-outer-rich burners and so the gas temperature and the composition remain constant, with only slight variations along the radial direction. However, a large number of coal particles are concentrated in the central zone of the inner-rich-outer-lean type I and II burners. In this coal combustion zone, because of the lack of O2, the coal combustion rate decreases and the length of the coal flame increases, resulting

in an elevated CO concentration. This high CO concentration in turn leads to a decrease in NOx, owing to the catalytic effect of CO on the NOx reduction reaction, and so NOx concentrations in the radial zone where 0  r  125 mm are much lower. Fig. 8 summarizes the release rates of carbon, hydrogen and nitrogen as well as the carbon burnout rates at four axial locations (x/d ¼ 0.5e3.5). The release rate of hydrogen is the highest while carbon exhibits the lowest. The hydrogen in coal is present primarily in the form of low molecular weight alkanes and hydroxyl groups, which exhibit poor thermal stability and readily decompose as the coal is heated. The combustion of the coal initiates with the burning of volatile compounds and the majority of the nitrogen is found within these species. For this reason, the observed release rate of nitrogen is greater than that of carbon. The release rates of carbon, hydrogen and nitrogen in the radial zone defined by 0  r  100 mm are less than the rates in the radial zone where r  100 mm. This occurs because the coal must move through a

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Table 3 Peak values of gas temperature, CO and NOx concentrations and the lowest value of O2 concentration at sections where x/d ¼ 0.5e3.5. Case

1

2

3

Type of the burner

General type

Inner-lean- outer-rich type

Inner-rich-outer-lean type Type I

Type II

1384 1415 1431 1420 7.43 3.31 2.8 2.64 7007 53,398 42,925 9741 1413.5 1170.8 1176.4 1180.5

1362 1397 1414 1399 6.34 2.21 2.16 2.35 11,936 71,452 63,737 28,342 1291.3 1087.2 1091.6 1103.8

1318 1345 1360 1372 4.01 1.85 0.97 1.21 32,931 93,650 100,000 82,592 1075.6 906.9 734.7 936.8

1261 1323 1336 1353 2.65 1.03 0.06 0.9 75,016 100,000a 100,000 95,711 882.4 773.1 567.5 748.5

The peak values of the gas temperature ( C)

The trough value of the O2 concentration (%)

The peak values of the CO concentration (ppm)

The peak values of the NOx concentration (mg/m3)

a

x/d x/d x/d x/d x/d x/d x/d x/d x/d x/d x/d x/d x/d x/d x/d x/d

¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼

0.5; 1.5; 2.5; 3.5; 0.5; 1.5; 2.5; 3.5; 0.5; 1.5; 2.5; 3.5; 0.5; 1.5; 2.5; 3.5;

(0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0 (0

               

r r r r r r r r r r r r r r r r

               

50 mm) 50 mm) 100 mm) 125 mm) 50 mm) 50 mm) 100 mm) 125 mm) 50 mm) 50 mm) 100 mm) 125 mm) 50 mm) 50 mm) 100 mm) 125 mm)

4

The range of Testo 350 M for CO concentration is from 0 to 100,000 ppm.

greater distance to get to the r  100 mm zone, which results in a longer coal combustion period and thus a greater release rate of all the elements. As the burner is changed from general to inner-leanouter-rich to inner-rich-outer-lean types I and II, the release rates of carbon, hydrogen and nitrogen and the burnout rate of carbon all decrease. Because the gas temperature of the coal flame decreases in the inner-rich-outer-lean types I and II burners the coal combustion rate is also reduced. Fig. 9 presents photographs of coal flames obtained when using all four burners, taken through the first furnace window. In the general and inner-lean-outer-rich type burners the pulverized coal burns with a very bright, light yellow flame (in web version) which exhibits a large diffusion zone but is relatively short. In contrast, the flames generated in the inner-rich-outer-lean types I and II burners are also yellow (in web version) but are less bright and exhibit a reduced flame zone. In each case, a long flame is evident in the central zone of the furnace. Abbas [4] divided swirl burners into two kinds: single annular orifice (SAO) and single central orifice (SCO). The flames of the general and inner-lean-outer-rich burners are similar to those of SAO burners, while the inner-rich-outer-lean types I and II burners have flame characteristics matching those of SCO burners. The gas temperature profiles as well as the distributions of the O2, CO and NOx concentrations along the center lines of the furnaces are given in Fig. 10. In all four burners, gas temperatures are observed to rapidly increase, plateau and then quickly decline before finally reaching a constant value. The distance from the burner nozzle to the point at which a gas temperature of over 1300  C was measured was approximately 1500 mm. After rapid ignition of the pulverized coal, the resulting coal combustion releases a great quantity of heat which increases the gas temperature and results in a peak temperature value. Following burnout of the pulverized coal, the gas temperature declines quickly at first and then drops more slowly to a constant value. As the burner type changes from general to inner-lean-outer-rich to the inner-richouter-lean types I and II, the gas temperatures decrease over the range of 0e1500 mm while the maximum gas temperatures gradually drop to 1422, 1403, 1360 and 1334  C, respectively. The gas temperatures also increase over the range of 1500e5760 mm. In both the general and inner-lean-outer-rich type burners, the pulverized coal is able to mix efficiently with the secondary air in the coal combustion zone so that coal particles are surrounded by sufficient O2. Coal particles under these conditions ignite quickly

and release extensive heat over a short distance. Hence, the gas temperature and the peak value of the gas temperature remain at a relatively high level over the distance range of 0e1500 mm, although the gas temperature falls rapidly after coal combustion is complete. This results in a decline in the gas temperature from 1500 to 5760 mm. The pulverized coal concentration is higher in the inner primary air in the case of the inner-rich-outer-lean types I and II burners and this leads to a lack of O2 as the coal burns, resulting in a decrease in the coal combustion rate. Therefore, the gas temperature and the peak gas temperature decrease from 0 to 1500 mm. As the secondary air mixes with the coal, the O2 concentration again becomes sufficient to support combustion and so the gas temperature increases from 1500 to 5760 mm. The distribution of O2 concentrations can be considered to exist in three stages: sharp decline, slow increase and essentially constant. Under conditions in which coal particles ignite quickly, a large amount of O2 is consumed and so the O2 concentration exhibits a sharp initial decrease. As the secondary air mixes with the primary air-coal mixture, however, the O2 concentration slowly increases. Finally, as coal particles burn out, the concentration of O2 tends to reach a constant value. As the burner type is changed from general to inner-lean-outer-rich to inner-rich-outer-lean types I and II, it is evident that the minimum O2 concentration values decrease and the range over which these values occur widens. This takes place because higher coal concentrations lead to more pronounced O2 deficiency in the coal combustion zone. The CO concentrations in all four burners rise rapidly to a peak value and then quickly drop before trending to zero. When a large amount of pulverized coal ignites, the coal combustion reaction is very fast in the early stage and hence a large amount of O2 is consumed. CO is produced rapidly under these conditions and achieves its peak value early in the combustion process. As the secondary air mixes with the primary air-coal mixture, this CO is able to react with O2 to produce CO2 and so the CO concentration exhibits a rapid decline. As the burner type changes from general to inner-lean-outer-rich type to inner-rich-outer-lean types I and II, the maximum CO concentrations are seen to decrease, essentially following the opposite trend to the O2 distributions. The NOx concentration distributions in all four cases may be considered to have four stages: rapid rise, decline, slight increase and nearly constant. Following rapid ignition of the pulverized coal, a large quantity of volatile compounds are formed by carbon, hydrogen and nitrogen released by the combustion reactions. The

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Fig. 8. Radial profiles of carbon, hydrogen and nitrogen release rates and carbon burnout rates in the sections where x/d ¼ 0.5e3.5 for: general type (  ), inner-lean-outer-rich type (B), inner-rich-outer-lean type I (;) and inner-rich-outer-lean type II (D) burners.

concentration of nitrogen compounds increases quickly and, because the O2 concentration is higher in the initial stage of coal combustion, these nitrogen compounds are oxidized to NOx and the NOx concentration rises rapidly. With rapid coal combustion, the O2 concentration quickly declines in the coal combustion zone and the CO concentration increases, producing a reducing atmosphere. This increases the rate of the catalytic reaction between CO and NOx and the NOx concentration declines. As coal combustion moves into the coke burning stage, part of the char nitrogen is converted to NOx and thus the NOx concentration in the gas shows a small increase. As coal burns out, there is little change in the level of NOx. As the burner changes from general to inner-lean-outer-rich to inner-richouter-lean types I and II, the NOx concentrations decline. In a general type burner, the pulverized coal concentration in the inner

primary air is the same as that in the outer primary air. The measured data show that the gas temperature and the peak value of the gas temperature are relatively high in this burner and the resulting oxidizing atmosphere favors NOx formation. For this reason, the NOx emissions of the general burner were the highest among the four burners examined. In contrast, with the inner-richouter-lean types I and II burners, the O2 concentration is relatively low while the CO concentration is relatively high in the coal combustion zone and thus the catalytic reaction between CO and NOx is enhanced. The presence of reducing radicals such as H2, CH, C2, NH and CN also increases the NOx reduction rate. In addition, the gas temperature and the peak value of the gas temperature in these burners are relatively low and so the NOx emissions of the innerrich-outer-lean type II burner are the lowest among the four

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Fig. 9. Photographic images of four flame types acquired through the initial window of the furnace.

Fig. 10. Profiles of gas temperatures and gas species concentrations (O2, CO and NOx) along the center line of the furnace for: general type (  ), inner-lean-outer-rich type (B), inner-rich-outer-lean type I (;) and inner-rich-outer-lean type II (D) burners.

Fig. 11. Profiles of carbon, hydrogen and nitrogen release rates and carbon burnout rates along the center line of the furnace for: general type (  ), inner-lean-outer-rich type (B), inner-rich-outer-lean type I (;) and inner-rich-outer-lean type II (D) burners.

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burners. The NOx concentrations measured at the exit of the combustor for the general, inner-lean-outer-rich and inner-richouter-lean types I and II burners were 1182, 1104, 923 and 861 mg/m3, respectively. Hence the NOx concentration was decreased by 27.15% moving between burners. This shows that the variation of the particles in the primary air plays a significant role in NOx formation. The release rates of carbon, hydrogen and nitrogen and the carbon burnout rates are given in Fig. 11. In all four burners, the carbon, hydrogen and nitrogen release rates as well as the carbon burnout rates rise rapidly before entering a period of slower increase and then eventually plateauing near 100%. The coal combustion rate is very high during the initial combustion period and increases rapidly. As combustion progresses, however, the combustion rate is reduced and the rate of rise slows. When the coal burns out, the burnout rate is close to 100% in the distance range between 2500 and 5760 mm. In all four burners, the highest release rate is that of hydrogen, followed by nitrogen and carbon. As the burner type changes from general to inner-lean-outer-rich to innerrich-outer-lean types I and II, the carbon, hydrogen and nitrogen release rates and the carbon burnout rates all decrease over the range from 0 to 2500 mm. When the burner is changed to the inner-rich-outer-lean types I and II, the gas temperature is low along the center line of the furnace during the initial period of combustion and the coal combustion rate is decreased. 5. Conclusions Reactive flow experiments were performed in a 0.5-MW pulverized coal combustion test facility incorporating swirl burners, applying bituminous coal as the fuel. We draw the following conclusions from measurements of particle concentration variation in the primary air duct. The particle concentration variation in the primary air duct evidently plays an important role in defining both the flame type and the gas composition in the near burner region, as well as the level of NOx emissions at the furnace exit. As the burner type is changed from general to inner-lean-outer-rich to inner-rich-outerlean types I and II, the flame diffusion zone is reduced, the flame becomes less bright, the concentration of O2 in the flame zone decreases, the concentration of CO increases significantly and both the overall NOx concentration and its peak value decrease. The NOx emissions at the furnace outlet are also significantly reduced although the gas temperature and O2 and CO concentrations tend to be uniform. The particle concentration variation in the primary air duct clearly affects the gas temperature in the near burner region. As the burner type changes from general to inner-lean-outer-rich to innerrich-outer-lean types I and II, both the overall gas temperature and its peak value are lower in the initial combustion stage. As the secondary air combines with the primary air-coal mixture the gas temperature is increased. The particle concentration variation in the primary air duct also has an impact on the release rates of carbon, hydrogen and nitrogen, as well as on the carbon burnout rate in the initial stage of coal combustion, but has little effect on burnout in the final combustion stage. As the burner type transitions from general to inner-leanouter-rich type to inner-rich-outer-lean types I and II, the release

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