Effect of the Primary Air Velocity on Ignition Characteristics of Bias

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Effect of the Primary-air Velocity on Ignition Characteristics of Bias Pulverized-coal Jets Guang Zeng, Shaozeng Sun, Xuefen Yang, Yijun Zhao, Zhiqiang Zhao, Zhenqi Ye, and Jianmin Gao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b03431 • Publication Date (Web): 20 Feb 2017 Downloaded from http://pubs.acs.org on February 25, 2017

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Effect of the Primary-air Velocity on Ignition Characteristics of Bias Pulverized-coal Jets Guang Zenga,b, Shaozeng Suna, Xuefen Yangc, Yijun Zhaoa,*, Zhiqiang Zhaob, Zhenqi Yeb, Jianmin Gaoa a

School of Energy Science and Engineering, Harbin Institute of Technology, Harbin

150001, China b

c

Liaoning Electric Power Research Institute, Shenyang 110006, China

Dongfang Boiler Group Company, Chengdu 611731, China

Abstract: Experiments were conducted under a hot condition of bituminous coal ignition in a 250-kW pilot-scale bias combustion simulator. The effects of the primary-air velocity (PAV) on the ignition characteristics of bias pulverized-coal jets in a reducing atmosphere were investigated for better developing of new burner for experimental bituminous coal. Multiple means of measurement and sampling were used for the axial and radial temperatures, flame spectrum, flue gas components, and residual solid inside the furnace. The standoff distance changed non-monotonically between axial distances of 90 and 330 mm with increasing PAV and was shortest for a PAV of 16 m/s. The radiation heat transfer from the hot environment had more effect on the ignition than the convection heat transfer from high-temperature flue gas in the initial stage, while the convection heat transfer from high-temperature flue gas played a greater role in the subsequent combustion of the char. At PAVs of 13 and 16 m/s, there was volatile and char homogeneous–heterogeneous combined ignition and one-mode combustion; at PAVs of 20 and 23 m/s, there was volatile-phase homogeneous ignition and two-mode combustion. The ignition of the fuel-rich jet 1

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lagged that of the fuel-lean jet at a PAV of 13 m/s. A PAV that is lower could not take advantage of bias pulverized-coal combustion technology. The ignition of the fuel-rich jet was ahead of that of the fuel-lean jet at PAVs of 16, 20, and 23 m/s. At a PAV of 13 m/s, the position of stable ignition was shortest, the temperature of stable ignition was highest and the boundary of the stable flame was smallest. The PAV of 16 m/s provided the best ignition characteristics for bituminous bias pulverized-coal jets, which is suitable to be selected as design PAV for the new burner development.

1. INTRODUCTION Ignition is an important stage in the combustion of pulverized coal (PC). There are three ignition modes, namely homogeneous ignition, heterogeneous ignition, and homogeneous–heterogeneous combined ignition.1 Howard showed that volatile homogeneous ignition and char heterogeneous ignition around the coal particle can overlap, and proposed a model that can be used to evaluate the ignition sequence of the volatile and char.2,3 Employing that model, Khatami represented the combustion of PC as two-mode or one-mode combustion, where two-mode combustion is, for example, 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.4,5 Combustion conditions, mainly the combustion mode, combustion temperature, and combustion atmosphere, are known to affect PC ignition greatly. The primary-air velocity (PAV) is the initial velocity of the primary-air (PA)/PC jet at the nozzle of the burner and is an important combustion condition that determines not only the stability but also the safety of ignition. The standoff distance is defined as the distance between the flame onset and the nozzle of the burner after the stable ignition of PC. The standoff distance is related to the flame velocity (rate of burning) and PAV. If the 2


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flame velocity matches the PAV, there is a spatially stable flame and thus a stable standoff distance. If the velocities do not match, the flame will move in the direction of the lower velocity until the velocities match, if they do. If it is not possible for the rate of burning and the imposed velocity to match at some position, the flame will either blow off or flash back.6 Therefore, the selection of optimal PAV has important practical significance on the development of rational design of boiler PC burner. Regrettably, selections of PAV during current development of new burner for PC combustion are mostly based on experience rather than experimental study. Large amounts of PC are used in tangentially fired utility boilers, which are still being widely developed and modernized worldwide. Low-NOx bias combustion technologies have been developed for the tangentially fired utility boiler because of the associated environmental and economic benefits. Horizontal bias combustion technology is widely used in tangentially fired utility boilers. In this technology, the PA/fuel stream is separated horizontally into fuel-rich and fuel-lean streams, which are then introduced separately into the furnace, to achieve strong ignition, reduce NOx emission, and prevent slagging.7–9 Stereo-stage low-NOx combustion technologies, which involve bias PC combustion and air-stage combustion, are now being used for utility boilers to achieve even lower NOx emissions.10 However, if depth-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 in the primary combustion zone.11,12 PC shows different ignition behaviors under different combustion conditions, especially like condition of PAV variation. The ignition characteristics of bias PC jets under different PAV in a reducing atmosphere need to be studied in depth, to provide 3


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practical guidelines for the burner development of clean coal combustion technologies. In modern tangentially fired utility boilers, the combustion of the PC jet is accomplished by injecting the jet into a hot furnace. Most experimental studies on coal-combustion particles use small or miniature benches equipped with external heating power supplies that maintain combustion because of the low flow rate of PC. Sufficiently high heating rates for PC can be obtained using small or miniature benches, but these heating modes are still clearly different from the heating mode used in an actual furnace, which mainly involves the convective heat transfer of high-temperature gas. Different heating modes affect the ignition characteristics; therefore, results obtained using small or miniature benches, in contrast to results obtained with a pilot-scale experimental facility, are not fully applicable to guiding industrial applications.6,13 The present study conducted experiments using a 250-kW pilot-scale bias combustion simulator (PBCS).9 The intersection and ignition of six jets were achieved for the first time, as follows. From top to bottom, two jets of bias PA/PC flow were injected into the primary combustion zone of the furnace and then mixed with two jets of high-temperature flue gas and ignited. Two jets of secondary air (SA) were then gradually involved in the combustion during the ignition process. The resultant momentum of the six jets was the same as that on the central axis of the furnace and propagated the flame down the central axis of the furnace. The flow and heat transfer behaviors of the turbulent PC jets in this study were similar to those observed in engineering practice, and the reaction environment of an actual furnace was provided. The airflow and bias PC ignition conditions in the PBCS can be adjusted flexibly and accurately to ensure stable operation during experiments and the accurate adjustment of experimental conditions. Multiple means of measurement were 4


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used to investigate the ignition behavior of the bias PC.14 There has been much research on the ignition and combustion of PC; however, the mechanism of PC combustion in tangentially fired utility boilers is still not thoroughly understood owing to the time limit of the measuring method. It is therefore necessary to combine the pilot-scale experimental facility with a more accurate measurement means to further improve the ignition theory of PC. During the ignition process of PA/PC jets, the concentration of fuel decreases rapidly and the concentration of combustion products increases, which causes the intensity of combustion and radiation to fluctuate. The measured flame temperature thus also fluctuates. A thermocouple cannot accurately reflect the fluctuations14,15 but a spectral method can detect the fluctuation of the flame in a timely manner. A pilot-scale experimental system has stricter operating requirements for reliability and stability and requires measurements that are more difficult. There have been few studies on the ignition and combustion of PC in a pilot-scale experimental system employing an optical method. The present study, in addition to employing traditional measurement methods, measured the characteristics of the flame spectrum for PC in a 250-kW PBCS using an optical fiber spectrometer. Bituminous coal sourced from Australia was burned to study the effects of the PAV on the ignition characteristics of bias PC jets in a reducing atmosphere. The results are not only conducive to the burner development of bias PC combustion technology but also helpful to related numerical simulation work. Additionally, they provide a baseline for the future study of the characteristics of bias PC combustion under different PAV in an oxygen-enriched condition.

2. EXPERIMENTAL 2.1. Experimental Setup. Experiments were performed in a PBCS.9 Figure 1 is a schematic of the PBCS 5


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system and the main design parameters are listed in Table 1. The main components are a furnace, bias PC transportation system, air supply system, high-temperature flue gas generation system, steam generation system, tail flue gas system, measurement and sampling system, and operation-monitoring system. 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 bias PC jets, which are then injected into the furnace through two equal-dimension rectangular burner nozzles, to achieve bias feeding. Air from a forced-draft fan, after being heated in an air preheater, enters the PA and SA windboxes for temperature regulation by hot/cold air mixing, and is finally injected into the furnace as PA and SA. 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. A high-temperature flue gas generated by the complete combustion of propane passes through a special pipe into the furnace. This gas creates an ignition environment approximating that of an actual boiler, ignites the bias PC jets, and maintains stable combustion. The flue gas from the furnace finally enters the tail flue gas ducts, and successively passes through the air preheater, flue gas treatment apparatus, induced-draft fan, and chimney and into the atmosphere. The PBCS system includes a steam generation system that regulates the moisture content in the PC jets. Figure 2 is a schematic of the structural arrangement of the top of the PBCS while Figure 3 is a schematic of the high-temperature flue gas generation system of the PBCS. 2.2. Experimental Measurements. Nowadays, the development of new technology for PC combustion is mostly based on experience; the on-line 6


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measurement of properties in the combustion zone is difficult and the mechanism of PC combustion is not fully understood.15 The PBCS in the present study had a cylindrical furnace, the upper part of which is the primary combustion zone. A measurement hole placed between the two nozzles of the bias PC burner at the top of the furnace was used for the insertion of different thermocouples into the furnace along the central axis to measure the gas temperature in the axial direction within the primary combustion zone; this gas temperature is referred to as the axial temperature in the present paper. Measurement points were also arranged around the furnace vertically from top to bottom at different axial distances from the burner nozzle; i.e., 220, 380, 540, 700, 860, and 1020 mm. One side of each measurement hole was used to measure the radial temperature and gas components and to collect ash while the other side was used to obtain the flame spectrum and images, as shown in Figure 1. The measurement points on one side of the furnace were used to insert thermocouples into the furnace along the diameter, to measure the gas temperature in the radial direction at different axial distances in the primary combustion zone; this temperature is referred to as the radial temperature in the present paper. The axial and radial temperatures in each predetermined location of the primary combustion zone were measured using Inconel armored type-K thermocouples having diameters of 2 mm,5.16–18 and data were recorded on a computer. 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 7


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spectrum increased with the temperature and concentration of radiation in the furnace.14 The intensities of visible light in spectrums 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. During the experiment, the stability of PC combustion was monitored according to the change in the flue gas component. Meanwhile, the residual solid remaining from combustion of the PC was collected and the consumption rate of fixed carbon and volatile and the burnout rate of coal were analyzed off line. The residual solid and flue gas components were collected by a common sampling system, as shown in Figure 4. The flue gas component was monitored on line and ash samples were extracted and sealed for off-line analysis. During ignition stage of bias PC jets, the optimal PAV can be selected basing on the principle of faster rising rate of axial temperature, higher peak value of axial differential temperature, shorter standoff distance, stronger light intensity of flame, bigger burnout rate and smaller stable flame boundary. 2.3. Experimental Methods. Since the bituminous coal is widely used in the utility boiler, Whitehaven bituminous coal sourced from Australia was used firstly in the experiments. The proximate and ultimate analytical data for the raw coal and PC are presented in Table 2. The PC used in the study had a full particle size distribution, and the PC fineness R75 was 16% (i.e., the surplus after sieving through a 75-micron mesh was 16%). Combustion in the furnace took place in a reducing atmosphere during the 8


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experiments. The air-to-coal stoichiometry was 0.7519 and the PC concentration (PCC), which is defined as the mass ratio of PC to PA in the PC/PA jets was 0.33.20,21 The bias concentration ratio of PC in the fuel-rich jet to that in the fuel-lean jet was 4.20,21 The variable PAV experiments were performed using PAVs of 13, 16, 20, and 23 m/s to analyze the ignition characteristics of bituminous bias PC jets. The main parameters for each set of experiments are given in Table 3. The PC was ignited by high-temperature flue gas that was generated by the complete combustion of propane. The high-temperature flue gas was also used to maintain the stable combustion of PC jets owing to the poor combustion stability of the straight-through PC jet.22,23 The thermal power of high-temperature flue gas used for the ignition and stable combustion of PC was fixed in the experiment. Experiments were performed as follows. The induced-draft fan and forced-draft fan were started up, the high-temperature flue gas generation systems on both sides were put into service, and each side was kept running at thermal power of 50 kW. Two streams of the bias PC were then fed into the system at an appropriate flow rate. The flow rates of bias PC jets were increased until they corresponded to thermal power of 200 kW and the ignition became stable. The output of each high-temperature flue gas generator was gradually reduced to thermal power of 25 kW, the total thermal power of the PBCS was 250 kW. The above process took approximate one hour to reach stable combustion state of formal experiment, and the furnace exit pressure was maintained around −100 Pa during the experiment.24 Data were acquired only if the PBCS operated in a continuous and stable state that could be repeated, to ensure data accuracy and repeatability. Continuous and stable operation was determined as follows. (1) The calibration of bias PC variable-frequency screw feeders under a cold 9


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condition showed that the feeding flow rate was stable, if the transporting lines were not plugged during the experiment, then the pulverized coal feeding rates at same feeder frequency were equal for cold and thermal condition. (2) The combustion stability of bias PC jets was monitored according to the negative pressure of the furnace exit during the experiment. (3) The flue gas components of O2, CO2, CO, and NOx fluctuated in a relatively stable range during the experiment. (4) The axial and radial temperatures measured in the furnace fluctuated in a relatively stable range during the experiment. (5) The flame spectrum collected in the furnace fluctuated in a relatively stable range during the experiment. Prior to the PC being fed into the furnace, an axial temperature (referred to as the axial blank temperature of ignition) was acquired when the high-temperature flue gas of 100-kW thermal power used for ignition developed fully and formed a relatively stable temperature profile in the furnace and an axial temperature (referred to as the axial blank temperature of combustion) was acquired when high-temperature flue gas of 50-kW thermal power used to maintain stable combustion developed fully and formed a relatively stable temperature profile in the furnace. The axial blank temperature of ignition and axial blank temperature of combustion were universal in all experimental cases for the PBCS, as shown in Figure 5. Both curves of the axial blank temperature fluctuate as the axial distance along the jet direction increases from 60 to 180 mm. The fluctuations were caused by the mixing of two low-temperature PA jets and two high-temperature flue gas jets before the PC was fed. The four jets started mixing after an axial distance 60 mm. The two PA jets intersected with each other first on the central axis of the furnace at an axial distance 120 mm and then 10


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intersected with the two high-temperature flue gas jets on the central axis of the furnace at an axial distance of 180 mm.25 With increasing axial distance, both axial blank temperatures increased monotonically from an axial distance of 180 to 540 mm and decreased monotonically from 540 to 1020 mm owing to the low-temperature SA jets gradually mixing but without any fuel being added after combustion. 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 the average value deviated in the range 0–12 °C owing to the normal fluctuation of coal combustion. The two types of PC flame spectrum data were acquired using a fiber-optic spectrometer. The final spectrum of each type was the average of 100 spectra. The deviation of the measured spectra and their average due to the normal fluctuation and pulse features of the flame was less than 10%. In the analysis of the collected residual solid, the consumption rates of the fixed carbon and volatile were calculated using formulas (1) and (2) to analyze the consumption of the volatile and fixed carbon during the PC combustion process.2,18,26– 28

The burnout rate was calculated using formula (3) to analyze the combustion

efficiency of bituminous coal.6,24,29,30

CR = 1 − × × 100

CR = 1 − × × 100

(1)

(2)

Here CR and CR are, respectively, the consumption rates (%) of the volatile and fixed carbon of the dry coal; V , FC , and A (%) are, respectively, the contents of the volatile, fixed carbon, and ash of the dry collected residual solid; and V , FC , and A (%) are, respectively, the contents of the volatile, fixed carbon, and ash of the dry coal. 11


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BR = 1 − ×

× 100

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(3)

Here BR (%) is the burnout rate of the bituminous coal; A is the ash content of the collected residual solid; and A is the ash content of the dry coal.

3. RESULTS AND DISCUSSION 3.1. Effect of the PAV on the standoff distance of the bituminous bias PC jet ignition. During PC ignition experiments, the ignition characteristics can be reflected by changes in temperature in the axial direction within the primary combustion zone of the furnace.16,29,31–33 Axial temperatures of bituminous bias PC jet combustion at different PAVs are shown in Figure 6. With increasing axial distance, the change trends of the axial temperature 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 90 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 90 to 330 mm owing to the heating of the two high-temperature flue gas jets. The standoff distance of bituminous bias PC jets ignition at different PAVs should be an axial distance of 90–330 mm. Further observation reveals that the axial temperature increased most sharply before an axial distance of 150 mm when the PAV was 13 m/s because the heat source of PC jet ignition mainly included the radiation heat from the hot environment, convection heat from high-temperature flue gas entrained by the PC jet, and heat released from burned coal. It took longer for PA travelling at lower velocity to travel the same axial distance, and more radiation heat was absorbed from the hot environment.6 The turbulence intensity was lower at lower PAV, which does not 12


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encourage convection heat transfer between PA/PC jets and high-temperature flue gas.32,34 The increase in the axial temperature was therefore most sharp after an axial distance of 150 mm when the PAV was 16 m/s, because there was not only enough time to absorb the radiation heat from the hot environment but also the good ability to entrain high-temperature flue gas, which would heat the PC jets to the ignition state quickly through the best combination of radiation and convection heat transfer.35 The axial temperature continuously increased as the axial distance increased from 330 to 540 mm. The increase in the axial temperature was sharper for a PAV of 20 or 23 m/s and more gradual for a PAV of 13 or 16 m/s because the greater turbulence intensity at a higher PAV increased convection heat transfer between high-temperature flue gas and PA/PC jets, which played a greater role in promoting the subsequent combustion of char after the ignition of bituminous coal. The axial temperature became stable at axial distances ranging from 540 to 1020 mm, which means the combustion of bituminous bias PC jets became stable. Burning was most intense at a PAV of 16 m/s, intense at a PAV of 22 m/s, and less intense at PAVs of 13 and 23 m/s. The standoff distance of bituminous bias PC jets where the velocity of PC jets equaled the flame propagation velocity is known to be between 90 and 330 mm. The specific standoff distance can be obtained from the intensity of visible light in the flame spectrum of the ignition of bituminous coal. The axial visible-light intensities of bituminous coal bias combustion at different PAVs are shown in Figure 7. With increasing axial distance, the change trends of the axial visible-light intensities are similar; the axial visible light intensity gradually increased before 700 mm and gradually decreased after 700 mm, although the decrease between 700 and 860 mm was less for a PAV of 20 m/s. After the combustion at different PAVs became stable in the subsequent stage, the axial visible-light intensity was the most intense at a PAV of 13


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16 m/s followed PAVs of 20, 13, and 23 m/s. This result is consistent with the previous finding for the axial temperature. The present work defines the standoff distance as the position where the intensity of visible light reaches 10% of the maximum peak intensity.36 The definition was validated well with the test conclusions of temperature and burnout rates, which was most accord with the observation results of visual and camera check on the pulverized coal flame during the experiment. The standoff distances of bituminous bias PC jets combustion at different PAVs are shown in Figure 8. Each standoff distance is presented with a relative error bar to show the uncertainty in the standoff distance.37–39 The relative error bar in Figure 8 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.38 Therefore the relative error bar has no impact on the comparative study on the standoff distance under different PAV. Figure 8 shows that the standoff distance changed non- monotonically as the PAV increased. The standoff distance was shortest for a PAV of 16 m/s followed by PAVs of 13, 23, and 20 m/s. The bituminous bias PC jets were heated to the ignition state within the shortest distance for a PAV of 16 m/s owing to the best combination of radiation and convection heat transfer for this PAV. The ignition at a PAV of 13 m/s occurred in advance of that at a PAV of 20 m/s and that at a PAV of 23 m/s, which indicates that the radiation heat transfer between the hot environment and PA/PC jets 14


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has more effect on the ignition than the convection heat transfer between high-temperature flue gas and PA/PC jets in the initial stage.40 As known previously, the combustion at a PAV of 20 m/s was more intense than that at a PAV of 13 or 23 m/s, but the standoff distance was longest at a PAV of 20 m/s. The ignition was delayed as the PAV increased to 20 m/s because the increase in convection heat transfer from high-temperature flue gas did not make up for the reduction in radiation heat transfer from the hot environment. 3.2. Effect of the PAV on the ignition mode of bituminous bias PC jets. The ignition characteristics of PC can be studied according to the emission intensities at different wavelengths in the PC flame spectrum; e.g., the emission intensity of hydrocarbon at 431 nm (full width at half-maximum of 10 nm) has been used to analyze the characteristics of volatile combustion15,23,37,38,41–43 and the emission intensity of hot soot at 550 nm (full width at half-maximum of 20 nm) has been used to analyze the characteristics of char combustion.19,25,37,38,44 The present study investigated emission characteristics of the volatile and char during PC ignition according to the trends of hydrocarbon and hot soot in the flame spectrum after absolute irradiance calibration and then analyzed the ignition mode of bituminous bias PC jets at different PAVs. The axial absolute irradiances of hydrocarbon and hot soot of bituminous bias PC jets combustion at different PAVs are shown in Figure 9. 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 8 showed the standoff distance was, respectively, 190, 179, 256, and 195 mm when the PAV was 13, 16, 20, and 23 m/s. Figure 9a and b show that prior to the standoff distances of 190 and 179 mm for PAVs of 13 and 16 m/s, respectively, the axial absolute irradiance of 15


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hydrocarbon was lower than that of hot soot. The result shows that the volatile combustion reaction was not dominant in the ignition stage of bituminous 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. Analysis based on previous studies 5,45 shows that the ignition modes for PAVs of 13 and 16 m/s are volatile and char homogeneous–heterogeneous combined ignition. Figures 9c and d show that prior to standoff distances of 256 and 195 mm for PAVs of 20 and 23 m/s, respectively, the axial absolute irradiance of hydrocarbon was greater than that of hot soot. The result shows that the volatile combustion reaction was dominant in the ignition stage of bituminous bias PC jets and that the ignitions of volatile and char occurred successively. Analysis based on previous studies 5,45 shows that the ignition modes for PAVs of 20 and 23 m/s are volatile-phase homogeneous ignition. Figure 9 also shows that the axial absolute irradiances of hydrocarbon and hot soot were greatest and the combustion reaction of volatile and char of bituminous bias PC jets was thus most intense at a PAV of 16 m/s, which is consistent with the previous findings. According to analysis of the residual solid collected at different axial distances, the consumption rate of the volatile and fixed carbon was calculated to analyze the ignition mode of bituminous bias PC jets.2,18,26–28 The consumption rates of the volatile and fixed carbon during bituminous bias PC jet combustion at PAVs of 16 and 20 m/s are shown in Figure 10. It is seen that at a PAV of 16 m/s, the consumption rate of the volatile and fixed carbon gradually increased with increasing axial distance. Especially prior to a standoff distance of 179 mm, the consumption rates of the volatile and fixed carbon sharply increased and remained close to each other, which indicates that the volatile and char ignited simultaneously. Analysis based on previous 16


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studies 2,18,26–28,46 shows that the ignition mode of bituminous bias PC jets was volatile and char homogeneous–heterogeneous combined ignition. Figure 10b shows that at a PAV of 20 m/s, the consumption rates of the volatile and fixed carbon gradually increased with increasing axial distance. Especially prior to a standoff distance of 256 mm, the consumption rates of the volatile and fixed sharply increase, but the consumption rate of the volatile was far greater than that of the char, which indicates that the volatile ignited in advance of the char. Analysis based on previous studies 2,18,26–28,46

shows that the ignition mode of bituminous bias PC jets was volatile-phase

homogeneous ignition. The ignition mode concluded from the consumption rate of the volatile and fixed carbon is the same as that concluded from the flame spectrum. Figure 11 shows the burnout rate of bituminous bias PC jets at PAVs of 16 and 20 m/s. At both PAVs, the burnout rates gradually increased with increasing axial distance. Especially prior to the standoff distance, the burnout rates sharply increased, but the burnout rate was consistently greater at a PAV of 16 m/s than at a PAV of 20 m/s, which indicates that the combustion efficiency at a PAV of 16 m/s was consistently far greater than that at a PAV of 20 m/s. Accordingly, the burnout rate of residual solid sampled at the same measuring hole after the standoff distance was used to analyze the combustion efficiency at different PAVs. The burnout rates sampled at an axial distance of 380 mm at different PAVs are shown in Figure 12. It is seen that the combustion efficiency was greatest at a PAV of 16 m/s followed by PAVs of 13, 20, and 23 m/s, which is consistent with previous findings. 3.3. Effect of the PAV on the combustion mode of bituminous bias PC jets. The study of PC ignition considers not only the ignition temperature, ignition delay, and ignition mode but also the combustion mode, such as one-mode combustion (i.e., the burning of the volatile and char simultaneously) and two-mode combustion (i.e., the 17


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burning of the volatile and char successively).47 The axial differential temperature of combustion can be obtained from the axial temperature and axial blank temperature of bituminous bias PC jets combustion and reflects the condition of heat release during PC combustion.29,32 The experimental conditions of PC combustion affect the exothermic reaction and axial differential temperature, and each axial differential temperature represents the heat release rate. The axial differential temperature was therefore used to quantitatively analyze the combustion of bituminous bias PC jets and to reveal the characteristics of ignition and combustion. The axial differential temperatures of bituminous bias PC jets combustion at different PAVs are shown in Figure 13. It is seen that at PAVs of 16 and 13 m/s, the axial differential temperature has one obvious peak prior to an axial distance of 540 mm. There is a pause in the rise of the axial differential temperature at a PAV of 13 m/s. Meanwhile, at PAVs of 20 and 23 m/s, the axial differential temperature has two obvious peaks prior to an axial distance of 540 mm, with the second peak being larger than the first. The axial differential temperature increases with the axial distance beyond an axial distance of 540 mm. The axial differential temperature prior to an axial distance of 540 mm was used to analyze the combustion mode because it reflects the heat release trend in the ignition stage of bituminous bias PC jets at different PAVs, owing to the fact that the main flow of SA does not mix into the PC ignition prior to an axial distance of 540 mm. The single peak of the axial differential temperature curves at PAVs of 13 and 16 m/s is due to the release of heat from simultaneous combustion of the volatile and char. Analysis based on previous studies 4,45,48 shows that the combustion modes of bituminous bias PC jets at PAVs of 13 and 16 m/s were one-modes, which is consistent with the previous finding that the ignition modes at PAVs of 13 and 16 m/s 18


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were volatile and char homogeneous–heterogeneous combined ignition. The first of the two peaks in the axial differential temperature curves at PAVs of 20 and 23 m/s is 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. Analysis based on previous studies 4,45,48,49 shows that the combustion modes of bituminous bias PC jets at PAVs of 20 and 23 m/s were two-modes, which is consistent with the previous finding that the ignition mode at PAVs of 20 and 23 m/s was volatile-phase homogeneous ignition. The ignition of PC includes the combustion reactions of the volatile and char, and the reactions of the volatile and char can occur simultaneously but differ in intensity. The volatile and char will burn simultaneously if the ignition conditions of the volatile and char are fulfilled at the same time. The heat release rate of the combustion reaction will then be much higher, and the char will continue to burn while the reaction of the volatile ends. The axial differential temperature of combustion will therefore continue to increase or there will be a pause in the rise. The previous section revealed that at a PAV of 16 m/s, the combustion reaction was most intense and the volatile and char of bituminous coal simultaneously reached the ignition condition within the shortest distance. There was thus one peak with an obvious upward trend for the axial differential temperature of combustion at a PAV of 16 m/s. Meanwhile, at a PAV of 13 m/s, more radiation heat from the hot environment was absorbed to promote the ignition of bituminous bias PC jets.6,40 The volatile and char of bituminous coal then also simultaneously reached an ignition condition, but the turbulence intensity at a PAV of 13 m/s was relatively low and not conducive to convection heat transfer between high-temperature flue gas and PA/PC jets. The curve of the axial differential temperature of combustion therefore had one peak with a pause. 19


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If the ignition condition of the volatile is fulfilled, the volatile will ignite first but char will still not burn at that moment and the rate of heat release of the combustion reaction will be lower. If the char has still not begun to burn when the reaction of the volatile ends, the axial differential temperature of combustion will decrease. The first relatively small peak of the axial differential temperature then appears. As the bituminous bias PC jets continue to be heated afterward, the char will reach the ignition condition and burn, and the second relatively large peak of the axial differential temperature appears. The previous section revealed that at PAVs of 20 and 23 m/s, it took the PA less time to travel the same axial distance and the radiation heat absorbed from the hot environment was limited. This resulted in the volatile igniting first. However, the greater turbulence intensity at PAVs of 20 and 23 m/s increased convection heat transfer between high-temperature flue gas and PA/PC jets, which promoted the subsequent combustion of char after bituminous coal ignition. Two peaks thus appeared in the curve of the axial differential temperature of combustion at PAVs of 20 and 23 m/s. The axial differential temperature was highest at the position where the combustion reaction of bituminous bias PC jets was most intense; i.e., the peak of the axial differential temperature represented the most intense point of the PC combustion reaction, and the combustion was more intense for a larger peak.50 Figure 13 shows that the combustion reaction of bituminous bias PC jets was most intense at a PAV of 16 m/s, which is consistent with previous findings. 3.4. Effect of the PAV on the stable ignition of bituminous bias PC jets. In addition to the axial temperature, the radial temperature can also be used to analyze the characteristics of PC ignition and combustion.13,29 Radial temperatures at different axial distances of bituminous bias PC jets combustion are shown in Figure 14 for 20


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different PAVs. With increasing axial distance, the temperatures at the center of bituminous 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 bituminous 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 jets15,22. There was plenty of high-temperature flue gas and adequate oxygen in the upstream of the bituminous bias PC jets, which resulted in the most intense exchange of heat and mass in the boundary of the jets. Meanwhile, the PCC continually decreased from the center to the boundary of the cross section of jets, and the ignition therefore started at the edge in the upstream. As temperature increased in the downstream of the jets, the devolatilization of PC gradually strengthened, the bituminous coal and surrounding air became more fully mixed, and the combustion reaction therefore occurred in the interior of the jets subsequently.22 Figure 14a shows that for bituminous coal bias jet combustion with a PAV of 13 m/s, the temperatures at radial positions of 20 and 40 mm were lower than those at −20 and −40 mm for an axial distance of 220 mm. The result shows that the ignition of the bituminous PC fuel-rich jet lagged that of the fuel-lean jet at a PAV of 13 m/s, which was due to less mixing between PA/PC jets and high-temperature flue gas at the lower PAV. However, the advanced ignition of the fuel-rich jet is the key to bias PC combustion technology, and a lower PAV therefore cannot take advantage of bias PC combustion technology. The radial temperature gradually increased with increasing axial distance at a PAV of 13 m/s, but the radial temperature in the fuel-rich jet began decreasing at an axial distance of 860 mm. The previous section showed that more oxygen was consumed in the ignition stage for a PAV of 13 m/s owing to the more intense combustion, resulting in a relative lack of oxygen in the reduced 21


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atmosphere of the furnace (having a stoichiometric ratio of 0.75) at the subsequent combustion stage. Figure 14b, c, and d show that for bituminous bias PC jet combustion at 16, 20, and 23 m/s, 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 bituminous PC fuel-rich jet was ahead of that of the fuel-lean jet at PAVs of 16, 20, and 23 m/s, which was due to the greater mixing between PA/PC jets and high-temperature flue gas at the higher PAV. The radial temperature gradually increased with increasing axial distance at PAVs of 16, 20, and 23 m/s, and only the radial temperature at a PAV of 16 m/s in the fuel-rich jet began decreasing at an axial distance of 700 mm. For the same reason stated above, most oxygen was consumed in the ignition stage at a PAV of 16 m/s owing to the combustion being most intense, resulting in a relative lack of oxygen in the reduced atmosphere of the furnace at the subsequent combustion stage. The radial temperatures at PAVs of 20 and 23 m/s constantly increased until an axial distance of 1020 mm owing to the lower oxygen consumption in the ignition stage. During the combustion of PC, the PC burns stably when the intensity of the visible light of the flame spectrum reaches 50% of the maximum peak intensity. 34,36 The position of the stable ignition of bituminous bias PC jets combustion under different PAVs is thus defined at the position where the intensity of visible light reaches 50% of the maximum peak intensity. Following previous studies, 28,44,51 the temperature of stable ignition was obtained from the axial temperature in Figure 6, as shown in Table 4. The position of stable ignition was shortest at a PAV of 16 m/s, followed by PAVs of 13, 23 and 20 m/s, which is consistent with the previous finding for the standoff distance. The temperature of stable ignition was highest for a PAV of 16 m/s and 22


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lowest for a PAV of 23 m/s, which is also consistent with the previous findings. In this paper, according to the stable-ignition temperature and radial-temperature profile at different axial distances, the temperature boundary of the stable ignition of bituminous bias PC jets, which is referred to as the stable flame boundary, is obtained for different PAVs.52 Figure 15 shows that the radial distance in this study ranged from −320 to +320 mm owing to the dimensional limit of the inner diameter of the furnace. The stable flame boundary was smallest and the intensity of combustion therefore most intense at a PAV at 16 m/s. The stable flame boundary was smaller and therefore the intensity of combustion more intense at a PAV of 20 m/s than at other PAVs. These results are consistent with the previous findings. It can be also found from figure 15 that the PAVs had distinct effect on the stable flame shape of bias PC jets, Except at a PAV of 13 m/s, the stable flame boundary of bituminous bias PC jets leans in the direction of the fuel-rich jet, as what mentioned before, the ignition of the bituminous PC fuel-rich jet lagged that of the fuel-lean jet at a PAV of 13 m/s, and the turbulence intensity of 13m/s was lower, therefore the leaning of the stable flame boundary of bituminous bias PC jets in the direction of the fuel-rich jet was smallest; the ignition of the bituminous PC fuel-rich jet was ahead of that of the fuel-lean jet at PAVs of 16, 20, and 23 m/s, and the turbulence intensity of jets increased gradually as the PAV increased, therefore the stable flame boundary of bituminous bias PC jets leans more obviously in the direction of the fuel-rich jet, verifying again that a lower PAV cannot take advantage of bias PC combustion technology.

4. CONCLUSIONS Bituminous coal sourced from Australia was burned in a 250-kW PBCS to investigate the effect of the PAV on the ignition characteristics of bituminous bias PC jets in a 23


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reducing atmosphere. The following are the main conclusions drawn from the results of the study. (1) With increasing axial distance, the axial temperature increases and the axial intensity of visible light gradually increases before 700 mm and gradually decreases after 700 mm. With increasing PAV, the standoff distance changes non-monotonically between 90 and 330 mm, with the standoff distance being shortest for a PAV of 16 m/s. The radiation heat transfer from the hot environment has a stronger effect on the ignition than the convection heat transfer from high-temperature flue gas in the initial stage, while convection heat transfer from high-temperature flue gas plays a greater role in the subsequent char combustion. (2) With increasing axial distance, the axial absolute irradiance of hydrocarbon and hot soot gradually increase before 700 mm and gradually decrease after 700 mm. At PAVs of 13 and 16 m/s, there is volatile and char homogeneous–heterogeneous combined ignition and one-mode combustion. At PAVs of 20 and 23 m/s, there is volatile-phase homogeneous ignition and two-mode combustion. The ignition mode concluded from the consumption rate of the volatile and fixed carbon is the same as that concluded from the flame spectrum. (3) With increasing axial distance, the radial temperature gradually increases and the ignition of bituminous bias PC jets starts from the edges and then gradually spreads into the center of the jets, finally resulting in burning throughout the jets. The ignition of bituminous PC fuel-rich jet lags that of the fuel-lean jet at a PAV of 13 m/s. A lower PAV cannot take advantage of bias PC combustion technology. Ignition of the fuel-rich jet is ahead of that of the fuel-lean jet at PAVs of 16, 20, and 23 m/s. At a PAV of 16 m/s, the position of stable ignition was shortest, the temperature of stable ignition was highest, and the stable flame boundary was smallest. 24


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(4) For bituminous bias PC jets at different PAVs, multiple means of measurement find that the combustion reaction is most intense and the ignition characteristics are best at a PAV of 16 m/s.

■ AUTHOR INFORMATION *Corresponding Author Tel: +86-451-86413231; Fax: +86-451-86412528 Email: [email protected] Notes The authors declare they have no competing financial interest.

■ ACKNOWLEDGMENTS The authors gratefully acknowledge funding from the Collaborative Innovation Center of Clean Coal Power Plant under the Poly-generation, National Science and Technology Supporting Program (2014BAA02B03) and the National Natural Science Foundation under the Heat Transfer and Flow Control innovation research group (Grant No. 51421063). The authors are grateful to Prof. Sun Rui for invaluable advice. 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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Energy Resour.Technol. 2000, 123(1), 32-38. (45) Riaza, J.; Khatami, R.; Levendis, Y.A. Álvarez, L.; Gil, M.V.; Pevida, C.; Rubiera, F.; Pis, J.J. Single particle ignition and combustion of anthracite, semi-anthracite and bituminous coals in air and simulated oxy-fuel conditions. Combust. Flame 2014, 161(4), 1096-1108. (46) Zou, C.; Cai, L.; Zheng, C. Numerical research on the homogeneous/heterogeneous ignition process of pulverized coal in oxy-fuel combustion. International Journal of Heat and Mass Transfer 2014, 73, 207-216. (47) Khatami, R.; Levendis, Y. A. An over view of coal rank influence on ignition and combustion phenomena at the particle level．Combust. Flame 2016, 164(2), 22-34. (48) Levendis, Y. A.; Joshi, K.; Khatami, R. Sarofim, A.F. Combustion behavior in air of single particles from three different coal ranks and from sugarcane bagasse. Combust. Flame 2011, 158(3), 452-465. (49) Riaza, J.; Khatami, R.; Levendis, Y. A. Álvarez, L.; Gil, M.V.; Pevida, C.; Rubiera, F.; Pis, J.J. Combustion of single biomass particles in air and in oxy-fuel conditions. Biomass and bioenergy 2014, 64, 162-172. (50) Zhu, M.; Zhang, H.; Tang, G.; Liu, Q.; Lu, J.; Yue, G.; Wang, S.; Wan, S. Ignition of single coal particle in a hot furnace under normal- and micro-gravity condition. Proc. Combust. Inst. 2009, 32(2), 2029-2035. (51) Zhou, K.; Lin, Q.; Hu, H.; Hu, H.; Song, L. The ignition characteristics and combustion processes of the single coal slime particle under different hot-coflow conditions in N2/O2 atmosphere. Energy 2016, 41, 1-12. 32


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(52) Liu, B.; Wu, Y.; Cui, K.; Zhang, H.; Matsumoto, K.; Takeno, K. Improvement of ignition prediction for turbulent pulverized coal combustion with EDC extinction model. Fuel 2016, 181, 1265-1272.

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Captions Table 1 Main design parameters of the PBCS Table 2 Proximate and ultimate analyses of bituminous coal Table 3 Experimental parameters Table 4 Parameters of stable ignition at different PAVs Figure 1 Schematic diagram of the PBCS system. Figure 2 Schematic of the structural arrangement of the top of the PBCS. Figure 3 Schematic of the high-temperature flue gas generation system of the PBCS. Figure 4 Schematic of the flue gas and ash sampling system of the PBCS. Figure 5 Axial blank temperatures of ignition and combustion. Figure 6 Axial temperatures at different PAVs. Figure 7 Axial visible-light intensities at different PAVs. Figure 8 Standoff distances at different PAVs. Figure 9 Axial absolute irradiances of hydrocarbon and hot soot at different PAVs. Figure 10 Consumption rates of the volatile and fixed carbon at PAVs of 16 and 20 m/s. Figure 11 Burnout rates of residual solid at PAVs of 16 and 20 m/s. Figure 12 Burnout rates of residual solid sampled at 380 mm for different PAVs. Figure 13 Axial differential temperatures at different PAVs. Figure 14 Radial temperatures at different axial distances for different PAVs. Figure 15 Stable flame boundaries at different PAVs.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Table 1. Main design parameters of the PBCS Parameter

Value

Rated total thermal power

250kW

Rated coal fired thermal power

200kW

Rated gas fired thermal power

50kW

Coal feeding flow rate range

3–60 kg/h

Gas feeding flow rate range

2–10 Nm3/h

Primary Air/Secondary Air temperature

85/250°C

Maximum operation temperature of furnace

1500°C

Heating rate of pulverized coal jets

105°C/s

Single section furnace body length

640mm

Primary combustion zone height

1280mm

Furnace inner diameter

800mm

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Table 2. Proximate and ultimate analyses of bituminous coal Coal type

Items Proximate analysis (wt %, as received) Mar

Var

Aar

FCar

Raw coal

9.70

25.45

19.13

45.72

Pulverized coal

6.80

27.96

11.78

53.46

Ultimate analysis (wt %, as received) Car

Har

Oar

Nar

Sar

Raw coal

57.56

3.48

8.63

1.19

0.31

Pulverized coal

67.06

3.92

8.89

1.24

0.31

Low calorific value (MJ/kg,as received) Raw coal

22.03

Pulverized coal

25.74

36


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Energy & Fuels

Table 3. Experimental parameters Coal

Coal Pulverized

flow Coal type

moisture

PAV

coal fineness rate

PCC BCR

content

(m/s)

(kg/kg)

R75 (%) (kg/h)

(%)

Bituminous

28

16

6.8

13

4

0.33

Bituminous

28

16

6.8

16

4

0.33

Bituminous

28

16

6.8

20

4

0.33

Bituminous

28

16

6.8

23

4

0.33

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Table 4. Parameters of stable ignition under different PAV Items

Value

Primary air velocity (m/s)

13

16

20

23

Stable ignition position(mm)

427

408

485

448

Stable ignition temperature (°C)

1060

1150

1120

1045

38


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Figure 1. Schematic diagram of the PBCS system.

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Figure 2. Schematic of the structural arrangement of the top of the PBCS.

40


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Figure 3. Schematic of the high-temperature flue gas generation system of the PBCS.

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Figure 4. Schematic of the flue gas and ash sampling system of the PBCS.

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1000

Blank temperature (℃ )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Blank temperature of ignition Blank temperature of combustion

800

600

400

200

0 0

200

400

600

800

1000

Axial Distance (mm)

Figure 5. Axial blank temperatures of ignition and combustion.

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1300

Axial temperature (℃ )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1100

900

700

PAV of 13m/s PAV of 16m/s PAV of 20m/s PAV of 23m/s

500

300 60

220

380

540

700

860

1020

Axial distance (mm)

Figure 6. Axial temperatures at different PAVs.

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800

Visible light intensity (counts)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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600

400

200

0 60

220

380

540

700

860

1020

Axial distance (mm)

Figure 7. Axial visible-light intensities at different PAVs.

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Energy & Fuels

400

Sandoff distance (mm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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300

200

100

190

179

256

195

13

16

20

23

0


Figure 8. Standoff distances at different PAVs.

46


Soot at PAV of 13m/s Hydrocarbon at PAV of 13m/s

0.4

0.2

0.0 220

Figure 9a. PAV of 13 m/s


0.45 0.30 0.15 0.00 60

220

380 540 700 860 1020 Axial distance (mm)

Figure 9c. PAV of 20 m/s


0.8 0.6 0.4 0.2 0.0 60

220

380

540

700

860

1020

Axial distance (mm)

Figure 9b. PAV of 16 m/s

0.75 0.60

1.0

380 540 700 860 1020 Axial distance (mm)

Absolute irrandiance (µW/(cm2·nm))

0.6


0.8

60


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0.35 Soot at PAV of 23m/s Hydrocarbon at PAV of 23m/s

0.28 0.21 0.14 0.07 0.00 60

220

380

540

700

860 1020

Axial distance (mm)

Figure 9d. PAV of 23 m/s

Figure 9. Axial absolute irradiances of hydrocarbon and hot soot at different PAVs.

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100

Volatile at PAV of 16m/s Fixed Carbon at PAV of 16m/s

80 60 40 20 0

Consumption rate (%)

100

Consumption rate (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Volatile at PAV of 20m/s Fixed Carbon at PAV of 20m/s

80 60 40 20 0

60

220

380

540

700

Axial distance (mm)


860

1020

60

220

380

540

700

860

1020

Axial distance (mm)


Figure 10. Consumption rates of the volatile and fixed carbon at PAVs of 16 and 20 m/s

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80

PAV of 16m/s Burnout rate (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

PAV of 20m/s

60

40

20

0 60

220

380

540

700

860

1020

Axial distance (mm)

Figure 11. Burnout rates of residual solid at PAVs of 16 and 20 m/s.

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Energy & Fuels

60

46.2

Burnout rate (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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45

41.5

40.1

38.7

20

23

30

15

0

13

16


Figure 12. Burnout rates of residual solid sampled at 380 mm for different PAVs.

50


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Axial different temperature (℃ )

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950

800

650


500

350 60

220

380

540

700

860

1020

Axial distance (mm)

Figure 13. Axial differential temperatures at different PAVs.

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1300

Radial temperature (℃ )


1300

1100

900

700

500

Axial 220mm Axial 380mm Axial 540mm Axial 700mm Axial 860mm Axial 1020mm

300 -400 -300 -200 -100

0

100

200

300

1100

900

700

500

100

200

300

400


1300


1300

1100

900

500

0

Radial distance (mm)


700


300 -400 -300 -200 -100

400



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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300 -400 -300 -200 -100

0

100

200

300

400

1100

900

700


500

300 -400 -300 -200 -100

0

100

200



Figure 14c. PAV of 20 m/s

Figure 14d. PAV of 23 m/s

300

400

Figure 14. Radial temperatures at different axial distances for different PAVs.

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1020

Axial distance (mm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

860

700

540

380

PAV of 20m/s PAV of 23m/s

PAV of 13m/s PAV of 16m/s

-400 -300 -200 -100

0

100

200

300

400


Figure 15. Stable flame boundaries at different PAVs.

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Effect of the Primary Air Velocity on Ignition Characteristics of Bias

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