Effects of Bias Concentration Ratio on Ignition Characteristics of

Nov 17, 2017 - To further study the ignition characteristic of new coal used for the PC burner of HBC, we set up a 250 kW pilot-scale bias combustion ...
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Cite This: Energy Fuels 2017, 31, 14219−14227

Effects of Bias Concentration Ratio on Ignition Characteristics of Parallel Bias Pulverized Coal Jets Guang Zeng,†,‡ Shaozeng Sun,† Wenda Zhang,† Zhiqiang Zhao,‡ and Yijun Zhao*,† †

School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China Liaoning Electric Power Research Institute, Shenyang 110006, China

‡

ABSTRACT: To further advance pulverized coal (PC) combustion theory and enable the rational development of horizontal bias combustion technology, combustion experiments were conducted in a 250 kW pilot-scale bias combustion simulator; multiple research means of combustion temperatures, flame spectra, burnout rates of residual solids, and NOx formation were used. A blend of sub-bituminous coal from Indonesia and bituminous coal from Australia was tested. The effects of bias concentration ratio (BCR) on the ignition characteristics of parallel bias PC jets in a reducing atmosphere were investigated. The results indicate that with increasing BCR for parallel bias PC jets, the standoff distance gradually decreased; the peaks of subsequent combustion temperature and visible-light intensity gradually increased; the continuous flame regions became advanced and concentrated; the flame stability gradually increased; the burnout rate gradually increased; the NOx formation gradually decreased, and the ignition characteristics gradually improved. Except at a BCR of 1, the PC ignition of the fuel-rich jet was better than that of the fuel-lean jet, and the continuous flame region of parallel bias PC jets leaned obviously in the direction of the fuel-rich jet. The bias combustion changed the ignition mode of primary air PC jets. There was homogeneous ignition mode at a BCR of 1; when BCR increased to no less than 3, the ignition mode changed to homogeneous−heterogeneous combined ignition, which is beneficial to the PC ignition conditions. Based on the research results in this paper, the recommended BCR for the design of a horizontal bias combustion PC burner is 4−5.

1. INTRODUCTION China’s coal consumption accounted for 64% of its total primary energy consumption and accounted for 50% of total global coal consumption.1,2 Coal is projected to remain an important source of energy for the foreseeable future in China. Most of coal in China was used as power generation fuel; China’s coal-fired power generation accounted for 75.6% of its total power generation in 2015.1 The combustion of pulverized coal (PC) also results in serious air pollution while providing heat and power. The air pollution caused by coal combustion is restricting the sustainable development of China’s economy and society; it is also attracting attention from the international community, especially neighboring countries and regions. In this environment, horizontal bias combustion (HBC) technology has been developed because of the associated environmental and economic benefits.3 In this technology, the primary air (PA)/PC stream is separated horizontally into fuel-rich and fuel-lean streams, which are then introduced separately into the furnace and burned at a stoichiometric ratio away from normal combustion to achieve better ignition, lower NOx emission, and slagging prevention.4,5 After a long period of research, researchers from Harbin Institute of Technology have developed systematic theoretical findings on mature HBC technology. The HBC technologies are being widely applied in the tangentially fired utility boiler which are still widely developed and being modernized worldwide.6 In PC combustion, appropriate design parameters for a new coal with unknown ignition characteristic are the key to successful development of PC burners. PC shows different ignition behaviors under different combustion conditions; the ignition characteristics of bias PC jets of a new coal need to be © 2017 American Chemical Society

studied in depth on the basis of an appropriate experimental study, to provide practical guidelines for the parameter design of PC burner of HBC technologies, which can make full use of the technical advantages of HBC. There are many experimental means for the study of ignition and combustion theories of PA/ PC stream in the existing literature, but the flow and heattransfer behaviors in these experiments differ from the bias PC combustion in the furnace of an actual tangentially fired boiler. A large number of drop tube furnaces (DTFs) and entrained flow reactors (EFRs) were used by researchers to study the ignition characteristics of PC. However, the heating mode of PC in the electrical heating furnace of DTFs or EFRs is mainly radiative heat transfer,7−12 which is quite different from the convection heat transfer between the PA/PC jets and the hightemperature flue gas jets in the furnace of an actual tangentially fired boiler; moreover, the PA/PC stream is heated by only the temperature field of one-dimensional distribution in the flat flame EFRs,13−16 which is also different from mixing and heat transfer between PA/PC flows and the high-temperature flue gas flows in the furnace of an actual tangentially fired boiler. In addition, the vertical or horizontal reactors in which PC was ignited by the high-temperature flue gas were also used by the researchers to conduct the PA/PC stream combustion experiment;17−20 these reactors can simulate only the ignition and combustion of single PA/PC jet in a single atmosphere but cannot flexibly and accurately adjust the flow and mixing of multiple jets of PA/PC jets and high-temperature flue gas jets Received: September 14, 2017 Revised: October 23, 2017 Published: November 17, 2017 14219

DOI: 10.1021/acs.energyfuels.7b02410 Energy Fuels 2017, 31, 14219−14227

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Energy & Fuels in the furnace. Obviously, the conclusions obtained by the above experimental devices are helpful to understand and analyze the ignition behavior of the conventional PA/PC stream but unsuitable for the study of bias PC combustion in the actual tangentially fired boiler. To further study the ignition characteristic of new coal used for the PC burner of HBC, we set up a 250 kW pilot-scale bias combustion simulator (PBCS) in this paper which can simulate the ignition process of one bias PC straight jet burner in actual tangentially fired boilers21,22 and acquire the design parameters for a new coal used for the PC burner of HBC technologies. The common utilization of sub-bituminous coal is blending with other coals and use as power generation fuel. If the blended coal is configured properly and good combustion conditions are created, it will make full use of the advantages of each coal type and improve the safety and economy of the boiler; if blended coal is configured improperly, it will lead to unstable combustion, lower efficiency, worse slagging, and even boiler shutdown. Some boiler explosion accidents have occurred when power plants burned the blended coal of Indonesian sub-bituminous coal.23 The combustion technology of low-rank coal, represented by Indonesian sub-bituminous coal, needs to be further developed.24 The study of the ignition characteristics of blended coal of sub-bituminous coal can expand the coal adaptability of boilers and ensure the safe and economical operation of power plants. The initial pulverized coal concentration (PCC) is defined as the mass ratio of PC to PA in the initial PA/PC jets, and the bias concentration ratio (BCR) is defined as the concentration ratio of PC in the fuel-rich jet to that in the fuel-lean jet.25,26 The PCC in the fuel-rich jet of HBC technology is increased, which makes the ignition of bias PC jets easier and more stable.22 The variation of BCR will change the combustion atmosphere and temperature significantly; therefore, the ignition characteristics of bias PC combustion are affected.27,28 Air stage combustion is now being used for utility PC boilers with applied HBC technologies to lower NOx emissions, but if deep air stage combustion is used, the excess air coefficient in the primary combustion zone will be much less than 1, and PC ignition will occur in an oxygen-deficient atmosphere. Although this inhibits NOx formation, the change in combustion atmosphere and temperature will decrease the combustion efficiency of HBC technologies in the primary combustion zone.29 Therefore, we studied the ignition characteristics of bias PC jets in a reducing atmosphere through a series of publications.21,22 In this study, using our previous research method,22 we focused on the effect of the BCR on the ignition characteristics of parallel bias PC jets based on the blend of subbituminous coal and bituminous coal, and the better ignition characteristics correspond to a shorter standoff distance, greater peaks of subsequent combustion temperature and visible-light intensity, stronger flame stability, higher PC burnout rate, and lower NOx formation under combined homogeneous− heterogeneous ignition.22 The analytical method schematic for judging better ignition characteristics is shown in Figure 1. The study of this paper will help to guide the parameter design of the PC burner of HBC technologies, advance PC combustion theory, and enable related numerical simulation work.

Figure 1. Analytical method schematic for judging better ignition characteristics. components are a furnace, bias PC transportation system, air supply system, high-temperature flue gas generation system, tail flue gas system, measurement and sampling system, and operation-monitoring system. From top to bottom, two parallel bias PC jets are injected into the primary combustion zone of the PBCS and then mixed with two jets of high-temperature flue gas and ignited. Two jets of secondary air then gradually become involved in the combustion at an axial distance of 540 mm; the PA is used to carry PC and to provide the oxygen required for ignition; the SA replenishes oxygen sequentially and maintains a reasonable stoichiometry in the reaction between air and coal. The flows and temperatures of PA and SA are maintained constant separately under variable BCR.21,22 The main analysis zone is within an axial distance of 1020 mm. The bias PC transportation system consists mainly of two variable-frequency adjustable-screw feeders, two PA ducts of equal dimensions, and a bias PC burner. Two streams of PC from two silos, after continuous stable bias feeding, are separately mixed with PA to form two parallel bias PC jets, which are then injected into the furnace through two equal-dimension rectangular burner nozzles, to achieve bias feeding. The details of other components and operation of PBCS were given in our previous publications.21,22 Multiple means of measurement and sampling were used to acquire accurate information on the bias PC ignition characteristics in the 250 kW PBCS, including the axial and radial temperatures, flame spectrum, NOx formation concentration, and residual solid inside the furnace.21,22 The present study determined the emission intensity of the flame in the furnace employing a miniature fiber-optic spectrometer and acquired two kinds of two-dimensional spectral images. One type of image was of the original spectrum, also referred to as the scope mode spectrum, while the other type was of the spectrum after absolute irradiance calibration. The emission intensities of the two kinds of spectra increased with the temperature and concentration of radiation in the furnace.22 The intensities of visible light in spectra of different experimental cases were analyzed using the original spectrum to obtain a more accurate standoff distance of PC ignition. The intensities of light rays of different wavelengths in the same spectrum of one experimental case were analyzed using the spectrum obtained after absolute irradiance calibration to obtain emission characteristics of the volatile and char during PC ignition.22 In the experiments performed in this study, PBCS combustion took place in a reducing atmosphere. The air-to-coal stoichiometry30 was 0.75, and the total fuel feeding flow and total air supply flow were kept constant for the variable BCRs experiments.21,22 The PA velocity was 16 m/s, the PA temperature 85 °C, and the initial PCC 0.33 kg/kg, which were applied and performed well in the our previous studies on ignition characteristics of bias PC jets.21,22 Different experimental cases of variable BCR were realized through the bias PC feeding based on the same basis of PA and secondary air, and the variable BCR experiments were performed using BCR = 1:1, 3:1, 4:1, and 5:1. Here, BCR = 1:1 means no bias regular ignition; BCR = 3:1, 4:1, and 5:1

2. EXPERIMENTAL METHODOLOGY A schematic diagram of the 250 kW PBCS is shown in Figure 2,21,22 and its main design parameters are listed in Table 1. The main 14220

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Figure 2. Schematic diagram of PBCS. A 1:1 blend of Whitehaven bituminous coal sourced from Australia and Trafigura sub-bituminous coal sourced from Indonesia was used in the experiments, which is referred to as blended coal in the rest of this paper. The proximate and ultimate analytical data for the blended coal is presented in Table 2. The pulverized coal used in the study had a full particle size distribution, and the PC fineness was R75 = 16% (i.e., the surplus after 75 μm sieving was 16%).21,22

Table 1. Main Design Parameters of PBCS parameter

value

rated total thermal power rated coal fired thermal power rated gas fired thermal power coal feeding flow rate range gas feeding flow rate range primary air/secondary air temperature maximum operation temperature of furnace primary combustion zone height furnace inner diameter

250 kW 200 kW 50 kW 3−60 kg/h 2−10 N m3/h 85/250 °C 1500 °C 1280 mm 800 mm

Table 2. Proximate and Ultimate Analyses for Experimental Coal Proximate Analysis (%, as received)

were selected based on the actual BCR limitation of industrial louver coal concentrator and its recommended optimal BCR range by previous study.31 When all the parameters reached the target values during the pilot-scale hot-condition experiments, the combustion was adjusted according to the combustion temperature, flame spectrum in the primary zone, and the O2 content at the furnace outlet. Data were acquired only if the PBCS was operated in a continuous and stable state that could be repeated to ensure data accuracy and repeatability. Because of the normal fluctuations in coal combustion, the average value of multiple measurements was used in the experiments to minimize errors. Axial and radial temperature data were acquired for 10 min, and average values were used as the axial and radial temperatures; the temperature measurements and average value deviated in the range 0−12 °C. The two types of PC flame spectrum data were acquired online, and the final spectrum of each type was the average of 100 spectra; the deviation for the recorded spectra and their average was less than 10%. The measurements of NOx concentration data were acquired for 10 min, and average values were used as the NOx concentration; the measurements of NOx concentration and its average value deviated less than 1%. Details of the experimental method can be found in our previous publication.22

Mar

Var

14.75

31.79

Aar

FCar

low heat value (MJ/kg)

8.42 45.04 Ultimate Analysis (%, as received)

22.66

Car

Har

Oar

Nar

Sar

59.42

3.81

12.20

1.05

0.37

3. RESULTS AND DISCUSSION 3.1. Effect of BCR on the Standoff Distance of Bias PC Jet Ignition. Axial temperatures of parallel bias PC jets ignition at different BCRs are shown in Figure 3. With increasing axial distance, the change trends of the axial temperatures were similar in that the axial temperature gradually increased to an axial distance of 60 mm because the two bias PC jets had not yet intersected on the central axis of the furnace. The axial temperature gradually decreased as the axial distance increased from 60 to 120 mm owing to the two bias PC jets starting to mix on the central axis of the furnace and starting to absorb heat. The axial temperature sharply increased as the axial distance increased from 120 to 360 mm 14221

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the intensity of the axial visible light in the coal flame spectrum reaches 10% of the maximum peak intensity.15,22 The average value of 10% of the maximum peak intensity of visible light at different BCRs was used as the criterion for determining the standoff distance in each variable experiment. The standoff distances of parallel bias PC jets ignition at different BCRs are shown in Figure 5. Each standoff distance is presented with a

Figure 3. Axial temperatures at different BCRs.

owing to the heating of the two high-temperature flue gas jets. The standoff distance of parallel bias PC jets ignition at different BCRs should be an axial distance of 90−360 mm.22 Further observation reveals that the increasing trend of axial temperature became more intense before an axial distance of 360 mm as BCRs increased, but the axial temperatures were quite close for the BCRs of 4 and 5. The axial temperature continuously increased as the axial distance increased from 360 to 540 mm. The increasing trend of axial temperature was more intense for the lower BCR because the ignition of PC for the lower BCR was weaker and consumed less oxygen in the initial stage, which made the subsequent combustion of PC became more intense because of relatively large amounts of oxygen in the subsequent stage. The axial temperature became stable at axial distances ranging from 540 to 1020 mm for different BCRs, which means the subsequent combustion of parallel bias PC jets became stable. Burning was more intense at BCRs of 5 and 4, followed by 3 and 1. This is because with increasing BCR, the PCC in the fuel-rich jet gradually increased, which caused more heat release during PC combustion. The axial visible-light intensities of parallel bias PC jets ignition at different BCRs are shown in Figure 4. With

Figure 5. Standoff distances at different BCRs.

relative error bar to show the uncertainty in the standoff distance.22 The relative error bar indicates the error ratio of the distance between two adjacent measuring points and the axial distance of the measured peak spectral intensity of visible light in the primary combustion zone of the PBCS. The intensity of the visible light was measured at different measuring points along the axial direction. Adjacent measuring points were separated by 160 mm. The spectral intensity of visible light may actually peak within 80 mm either side of the measuring hole through which the peak spectral intensity was measured. Therefore, the relative error bar has no impact on the comparative study on the standoff distance under different cases.22 The standoff distance gradually decreased as BCR increased, but there was a small difference between the standoff distances of the BCRs of 4 and 5. This is because although the initial PCC remains the same, with increasing BCR, the PCC in the fuel-rich jet gradually increased and more volatile was easier to release in the fuel-rich jet, which is beneficial to advance ignition of PC jets.33 However, when BCR increased to a relatively higher BCR, although enough volatile was released, the oxygen content was relatively insufficient in a reducing atmosphere of PBCS, which caused the small difference between the standoff distances of BCRs of 4 and 5. 3.2. Effect of BCR on the Ignition Mode of Bias PC Jets. First, the emission intensities of hydrocarbons14,18,34−37 and hot soot14,34,38−40 in the flame spectrum were used to investigate the intensities of volatile and char combustion, and the ignition mode of the parallel bias PC jets was determined. Next, the axial differential temperature of the parallel bias PC jets was obtained from the axial temperature and axial blank temperature,22 which reflects the trend in heat release during bias PC combustion.41,42 The combustion of PC can be represented as two-mode or one-mode combustion, where twomode combustion is the gas-phase or homogeneous combustion followed by heterogeneous combustion while one-mode combustion is the simultaneous combustion of the volatile and char or only combustion of char after the devolatilization of coal. Therefore, two heat-release peaks appeared during two-mode combustion while one heat-release

Figure 4. Axial visible-light intensities at different BCRs.

increasing axial distance, the axial visible-light intensity gradually increased before 700 mm and gradually decreased after 700 mm. With increasing BCR, the peak of visible-light intensity gradually increased, which indicates the flame stability gradually increased, but the flame stabilities were quite close for the BCRs of 4 and 5. The standoff distance is the distance from flame onset to the nozzle of the burner after stable ignition of the PC.32 In this study, the standoff distance is defined as the position at which 14222

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Figure 6. Axial absolute irradiances of hydrocarbons and soot at different BCRs.

peak appeared during one-mode combustion.22 Based on the heat-release features of one heat peak for one-mode combustion (i.e., simultaneous burning of volatiles and char) and two heat peaks for two-mode combustion (i.e., sequential burning of volatiles and char),11 the ignition mode of the parallel bias PC jets was determined finally.22 The axial absolute irradiances of hydrocarbon and hot soot of parallel bias PC jets ignition at different BCRs are shown in Figure 6. With increasing axial distance, the axial absolute irradiances of hydrocarbon and hot soot gradually increased before 700 mm and gradually decreased after 700 mm. Figure 6 also shows that with BCR increasing, the axial absolute irradiances of hydrocarbon and hot soot increased, which means the combustion intensity of volatile and char of parallel bias PC jets increased, but the combustion intensity differences were quite close for the BCRs of 4 and 5. Figure 6a shows that at 220 mm, near the standoff distance for a BCR of 1, the axial absolute irradiance of hydrocarbons was greater than that of hot soot. This shows that the volatile combustion reaction was dominant in the ignition stage for parallel bias PC jets, and that volatile and char ignitions occurred successively. The preliminary results, based on spectra, show that the ignition mode for a BCR of 1 was volatile-phase homogeneous ignition.8,10,22 Figure 6b−d shows that at 220 mm, near the standoff distances for BCRs of 3, 4, and 5, the axial absolute irradiance of hydrocarbons was lower than that of hot soot. The result shows that the volatile combustion reaction was not dominant in the ignition stage of parallel bias PC jets and that the ignition of the volatile was

accompanied by that of the char, which means the volatile and char ignited simultaneously. The preliminary results, based on spectra, show that the ignition modes for BCRs of 3, 4, and 5 are volatile and char homogeneous−heterogeneous combined ignition.8,10,22 This is because with increasing BCR, the PCC in the fuel-rich jet gradually increased and the coal particle numbers in the fuel-rich jet increased; the ignition conditions of the volatile and char were fulfilled at the same time.43 The axial differential temperatures of parallel bias PC jets ignition at different BCRs are shown in Figure 7. It is seen that at BCRs of 3, 4, and 5, the axial differential temperature has one obvious peak prior to an axial distance of 540 mm, but there is a pause in the rise of the single peak of axial differential

Figure 7. Axial differential temperatures at different BCRs. 14223

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Figure 8. Radial flame temperature distribution at different axial distances for different BCRs.

3.3. Effect of BCR on the Flame Temperature Distribution of Bias PC Jets. Radial flame temperature distribution at different axial distances of parallel bias PC jets ignition for different BCRs are shown in Figure 8. With increasing axial distance, the temperatures at the center of parallel bias PC jets (−20 and +20 mm) are first lower than the temperatures at the two sides and then gradually become higher than the peripheral temperatures, indicating that the ignition of the parallel bias PC jets started at the edges of the jets and then gradually spread into the center of the jets, finally resulting in burning throughout the jets.18,19,22 Figure 8a shows that for parallel bias PC jets at a BCR of 1, the temperatures at radial positions of 20 and 40 mm were same as those at −20 and −40 mm for an axial distance of 220 mm. The result shows that the ignitions of the fuel-rich jet and the fuel-lean jet at a BCR of 1 occurred simultaneously. Panels b, c, and d of Figure 8 show that for parallel bias PC jets at BCRs of 3, 4, and 5, respectively, the temperatures at radial positions of 20 and 40 mm were higher than those at radial positions of −20 and −40 mm for an axial distance of 220 mm. The result shows that the ignition of the fuel-rich jet was better than that of the fuel-lean jet at BCRs of 3, 4, and 5. This is because the fuel-rich jet at higher BCRs ignited rapidly and released large amounts of heat. Meanwhile, the flame of fuellean jet was weaker because of bigger coal particle spacing, and the advanced ignition of the fuel-rich jet is the key to bias PC combustion technology.22 With increasing axial distance, the radial temperatures gradually increased at different BCRs, but the radial temperatures in the fuel-rich jet began to be lower than that in the fuellean jet from an axial distance of 700 mm. The reason is that most oxygen was consumed in the ignition stage of the fuel-rich

temperature at a BCR of 3. Meanwhile, at a BCR of 1, the axial differential temperature has two obvious peaks prior to an axial distance of 540 mm. The axial differential temperature prior to an axial distance of 540 mm was used to determine the ignition mode because it reflects the heat release trend in the ignition stage of parallel bias PC jets at different BCRs, because the main flow of secondary air is not involved in the PC ignition prior to an axial distance of 540 mm. Figure 7 also shows that the peak value of the axial differential temperature in the initial ignition stage gradually increased as BCR increased, which indicates that the combustion of parallel bias PC jets became more intense.12 The first of the two peaks in the axial differential temperature distribution at a BCR of 1 was caused by the release of heat from combustion of the volatile, and the second peak is caused by the release of heat from combustion of the char. These results show that the ignition mode of parallel bias PC jets at a BCR of 1 was volatile homogeneous ignition,7,9,10,22,44 which is consistent with previous findings based on the flame spectrum. The single peak in the axial differential temperature distributions at BCRs of 3, 4, and 5 arise from the release of heat from simultaneous combustion of the volatiles and char, but the PCC in the fuel-rich jet was not high enough at a BCR of 3; therefore, its axial differential temperature had one peak with a pause. These results show that the ignition modes of parallel bias PC jets at BCRs of 3, 4, and 5 were volatile and char homogeneous−heterogeneous combined ignition,7,9,10,22 which is consistent with previous findings based on the flame spectrum. These results show that the ignition mode of parallel bias PC jets changed from homogeneous to homogeneous− heterogeneous combined ignition when the BCR increased to 3 from 1. 14224

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Energy & Fuels jet at higher BCRs owing to the combustion being most intense, resulting in a relative lack of oxygen in the reducing atmosphere of the furnace at the subsequent combustion stage.22 Previous studies have shown that the center of a PA/PC jet burns stably and enters the continuous flame region when the combustion intensity reaches 50% of the maximum peak intensity.36,45,46 The continuous flame delay distance is therefore defined as the position at which the intensity of visible light reaches 50% of the maximum peak intensity.14−16,45,46 This was used to determine the boundary of the continuous flame region combined with the radical temperature distribution.47 The continuous flame regions of parallel bias PC jets ignition at different BCRs are shown in Figure 9. The figure shows that the continuous flame region

Figure 10. Burnout rates of residual solids sampled at 540 m for different BCRs.

intensity at different experimental conditions. With increasing BCR, the burnout rate of parallel bias PC jets gradually increased. This is because the combustion reaction of parallel bias PC jets became more intense as BCR increased, which is consistent with previous findings. The NOx formation concentrations48 and percentage of NOx decrease at different BCRs are shown in Figure 11. With

Figure 9. Continuous flame regions at different BCRs.

boundary at BCRs of 1 and 3 started at an axial distance of 540 mm, but those at BCRs of 4 and 5 started at an axial distance of 380 mm. This indicates that the continuous flame region boundary of parallel bias PC jets became advanced as BCR increased. With increasing BCR, the continuous flame region of parallel bias PC jets became gradually concentrated, which means the flame stability became stronger, which is consistent with previous findings based on axial visible-light intensities. But because of the normal fluctuations and pulse features of the flame in coal combustion, there was a small overlap between the continuous flame region boundaries of BCRs of 1 and 3 at the fuel-rich side, but it had no impact on the comparison result of the trends of continuous flame region as BCR increased. It can be also found from Figure 9 that the BCRs had distinct effect on the continuous flame region boundary of parallel bias PC jets. The continuous flame regions of parallel bias PC jets at a BCR of 1 located in the center of the fuel-rich jet and fuellean jet, but the continuous flame regions of parallel bias PC jets leaned obviously in the direction of the fuel-rich jet as BCR increased to no less than 3, which is due to the ignition of fuelrich jet being better than that of the fuel-lean jet at a PA velocity of 16 m/s for each BCR.22 3.4. Effect of BCR on NOx Formation Characteristics during Bias PC Ignition Process. The burnout rates of residual solids sampled at an axial distance of 540 mm for parallel bias PC jets ignition at different BCRs are shown in Figure 10. Basing on our previous study,22 the burnout rates of residual solid gradually increased with increasing axial distance, and the burnout rate of better ignition was consistently greater than that of the relative poor ignition; therefore, the burnout rate of residual solid sampled at the same measuring hole after the standoff distance was used to analyze the combustion

Figure 11. NOx formation concentrations and percentage of NOx decrease at different BCRs.

increasing BCR, the NOx formation concentrations gradually decreased. The results of Figures 10 and 11 indicate that the better ignition characteristics for the parallel bias PC jets, the lower the NOx formation. The reason is that if the ignition characteristics of parallel bias PC jets in a reducing atmosphere were better, the combustion in the burning zone of PBCS was more intense, and more oxygen was consumed in the area near the nozzle of PC burner. Thus, when the air-to-coal stoichiometry in PBCS was 0.75, the reaction between oxygen and precursor of nitrogen oxides in the released volatiles was more limited during PC initial ignition process; therefore, the NOx formation concentrations gradually decreased,49 which is not same as the NOx formation trend of normal PC combustion.

4. CONCLUSIONS Combustion experiments were conducted in a 250 kW PBCS. The effects of BCR on the ignition characteristics of parallel bias PC jets in a reducing atmosphere were investigated based on a combination of flame spectra, combustion temperatures, burnout rates of residual solids, and NOx formation. The main conclusions drawn from the results of the study are as follows. 14225

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Article

Energy & Fuels

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(1) With increasing BCR for parallel bias PC jets, the standoff distance gradually decreased; the peaks of subsequent combustion temperature and visible-light intensity gradually increased; the continuous flame regions became advanced and concentrated; the flame stability gradually increased; the burnout rate gradually increased; the NOx formation gradually decreased, and the ignition characteristics gradually improved. (2) The ignition of the parallel bias PC jets started at the edges of the jets and then gradually spread into the center of the jets. Except at a BCR of 1, the ignition of PC fuel-rich jet was better than that of the fuel-lean jet, and the continuous flame region of parallel bias PC jets leaned obviously in the direction of the fuel-rich jet. (3) The bias combustion changed the ignition mode of PA/ PC jets, while BCR was at 1(no bias combustion), there was homogeneous ignition mode; while BCR increased to no less than 3 (bias combustion), the ignition mode changed to homogeneous−heterogeneous combined ignition, which is beneficial to the PC ignition conditions. (4) Based on the research results in this paper, the recommended BCR for the design of a PC burner for HBC is 4−5.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86-451-86413231. Fax: +86-451-86412528. E-mail: [email protected]. ORCID

Guang Zeng: 0000-0002-7918-9592 Shaozeng Sun: 0000-0003-2793-5925 Yijun Zhao: 0000-0002-2461-562X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge funding from the National Key R&D Program of China (2017YFB0602002). Technical assistance with experiments and analysis from the National Engineering Laboratory for Reducing Emissions from Coal Combustion of China is acknowledged.

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REFERENCES

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DOI: 10.1021/acs.energyfuels.7b02410 Energy Fuels 2017, 31, 14219−14227

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DOI: 10.1021/acs.energyfuels.7b02410 Energy Fuels 2017, 31, 14219−14227