Effects of Fuel Properties on Ignition Characteristics of Parallel-Bias

Oct 2, 2017 - To further understand the bias combustion behavior of new coal used for horizontal bias combustion and obtain its characteristic paramet...
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Effects of Fuel Properties on Ignition Characteristics of Parallel-Bias Pulverized-Coal Jets Yijun Zhao,† Guang Zeng,†,‡ Linyao Zhang,† Yanjun Zhang,§ Shaozeng Sun,*,† and Changhong Wei‡ †

School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China Liaoning Electric Power Research Institute, Shenyang 110006, China § Harbin Boiler Company Ltd., Harbin 6150006, China ‡

ABSTRACT: To further understand the bias combustion behavior of new coal used for horizontal bias combustion and obtain its characteristic parameters, combustion experiments were conducted in a 250-kW pilot-scale bias combustion simulator; multiple research measurements of flame spectra, combustion temperatures, and burnout rates of residual solids were used. Bituminous coal from Australia, sub-bituminous coal from Indonesia, and a blend of these coals were tested. The effects of the raw-coal equivalent moisture (RCEM), pulverized-coal fineness (PCF), and coal type on the ignition characteristics of parallelbias pulverized-coal jets in a reducing atmosphere were investigated. The results indicate that, with decreasing RCEM and PCF, the standoff distance gradually decreased, the flame stability gradually increased, the burnout rate gradually increased, and the ignition characteristics gradually improved. The RCEM had negative effects on ignition in the early stage but positive effects during subsequent combustion. Sub-bituminous coal exhibited a homogeneous−heterogeneous combined ignition mode as the RCEM increased. Bituminous coal changed to homogeneous ignition from homogeneous−heterogeneous combined ignition as the PCF increased. For the sequence bituminous coal, blended coal, sub-bituminous coal, the standoff distance gradually increased, the flame stability gradually decreased, the burnout rate gradually decreased, and the ignition characteristics gradually worsened. When the RCEM of the primary air/pulverized-coal jets reached the moisture content of each raw coal sample, the ignition mode was homogeneous for bituminous coal and a homogeneous−heterogeneous combination for sub-bituminous and blended coals. The ignition characteristics of the blended coal resembled those of sub-bituminous coal, and the burnout characteristics of blended coal resembled those of bituminous coal.

1. INTRODUCTION Much research has been performed on the ignition characteristics of pulverized coal (PC), including ignition delay, ignition mode, combustion temperature, coal flame, and burnout rate.1−6 Binner et al.7 studied the combustion of dried and wet Victorian brown (60% moisture) coal in situ; their results indicated that ignition of the PC stream was delayed and flame stability was poor if the coal was not dried before ignition, because of evaporation of the moisture in the coal. Cai et al.8 showed that PC ignited homogeneously in both O2/N2 and O2/H2O atmospheres but that PC ignition occurred sooner in O2/H2O atmospheres than in O2/N2 atmospheres with identical O2 mole fractions. Detailed simulations of the effects of the physicochemical properties of H2O on coal ignition showed that the steam shift reaction is the primary reason for coal igniting earlier in O2/H2O atmospheres than in the corresponding O2/N2 atmospheres and that the steam gasification reaction has a minor effect on ignition. Kim et al.9 studied the ignition behavior of PC particles of different ranks and sizes using a flat flame burner, with a high-speed camera to observe the ignition process. The results showed that bituminous coals with medium and high volatile contents and with particle sizes of 150−200 and 75−90 μm underwent homogeneous ignition. For particle sizes below 45 μm, highvolatile bituminous coal underwent homogeneous ignition, but medium-volatile bituminous coal underwent heterogeneous ignition. Anthracite coal with a particle size of 150−200 μm exhibited homogeneous ignition after primary fragmentation, © XXXX American Chemical Society

whereas lignite coal directly underwent homogeneous ignition. Levendis and co-workers10−14 performed extensive research on the ignition and combustion characteristics of PC particles. Their results showed that different coal ranks exhibited distinct ignition modes: All bituminous coals underwent homogeneous ignition and two-mode combustion. Some sub-bituminous coals underwent weaker homogeneous ignition and two-mode combustion, but others underwent heterogeneous ignition and one-mode combustion. Lignite exhibited homogeneous− heterogeneous combined ignition and one-mode combustion. Some semianthracite coals underwent homogeneous ignition and two-mode combustion, but others underwent heterogeneous ignition and one-mode combustion. High-rank anthracite exhibited heterogeneous ignition and one-mode combustion. Pedel et al.15 identified three regions in the ignition of primary air/pulverized coal (PA/PC) jets based on numerical simulations. The first region is a preheating region, in which PC particles and PA are heated by being mixed with a hightemperature gas but no flame is observed. The second region is a growing flame region, in which a small cluster of particles is first ignited and then grows into an ignited particle cloud. The third region is a continuous flame region, in which the center of the PA/PC jet is stably ignited. Yamamoto et al.16 used numerical simulations to study the ignition of a PA/PC jet by a Received: July 15, 2017 Revised: September 15, 2017 Published: October 2, 2017 A

DOI: 10.1021/acs.energyfuels.7b02055 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 1. Schematic diagram of the PBCS system.

high-temperature flue gas. Two ignition delay distances were defined, namely, the standoff distance, which is the distance from the PA nozzle exit to the starting point of the growing flame region, and the continuous flame delay distance, which is the distance from the PA nozzle exit to the starting point of the continuous flame region. In PC combustion, appropriate design parameters for a new coal used for the burner are the key to successful combustion organization. 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 tangentially fired utility boilers because of the associated environmental and economic benefits. The horizontal bias combustion (HBC) technique is widely used in tangentially fired utility boilers in large-scale boiler works in China.17 In HBC, fuel-rich and fuel-lean streams are burned in reducing and oxidizing atmospheres, respectively, in the furnace as two parallel-bias PC jets to achieve strong ignition, reduce NOx emissions, and prevent slagging. PC exhibits different ignition behaviors under different combustion conditions, so the ignition characteristics of bias PC jets with different fuel properties need to be studied in depth, to provide practical guidelines for the development of burners for clean coal combustion technologies. Most studies have concentrated on the ignition characteristics of single-atmosphere and singlejet PC streams, but such studies cannot adequately clarify the ignition process in bias combustion. In this study, to improve the understanding of bias combustion behavior and acquire the characteristic parameters for a new coal used for HBC, we set up a 250-kW pilot-scale bias combustion simulator (PBCS) with six jet intersections and performed the quantitative analysis of coal flame spectra to improve test accuracy. This

PBCS can simulate the straight-jet coal flame of a single bias PC burner in a tangentially fired utility boiler and can be used to study the ignition characteristics of parallel-bias PC jets. Airstage combustion is now being used for utility PC boilers to lower NOx emissions, but the primary combustion zone is in a reducing atmosphere,18 which decreases the PC combustion efficiency.19,20 Combustion experiments were therefore performed on parallel-bias PC jets in a reducing atmosphere in the present work. In a recent publication,21 we reported the effects of the PA velocity on the ignition characteristics of bias PC jets. In this study, using our previous research method, we focused on the effects of the fuel properties on the ignition characteristics of parallel-bias PC jets, including moisture content, pulverized-coal fineness (PCF), and coal type. The results will help to advance PC combustion theory and will also enable the rational development of HBC techniques and related numerical simulation work.

2. EXPERIMENTAL SECTION 2.1. Experimental Setup. A schematic diagram of the 250-kW PBCS is shown in Figure 1, and its main design parameters are listed in Table 1. From top to bottom, two parallel-bias PC jets are injected into the primary combustion zone of the PBCS, mixed with two jets of high-temperature flue gas, and then ignited. Two jets of secondary air then gradually become involved in the combustion at an axial distance of 540 mm. The main analysis zone is within an axial distance of 1020 mm; details are given in our previous publications.21,22 A steam generation system was added to the PBCS system; it rapidly converts drinking-quality water to steam and then injects the steam into the PA stream through an insulating pipe to regulate the moisture content in the parallel-bias PC jets. During the steam injection experiments, the steam temperature and pressure were 160 °C and 0.3 MPa, respectively, and the steam flow rate was set based on the specific needs of different experimental cases. B

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the criterion for determining the standoff distance in each variable experiment. First, the emission intensities of hydrocarbons3,25−29 and hot soot25,27,30−32 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.21 Next, the axial differential temperature of the parallel-bias PC jets was obtained as the difference between the axial temperature and the axial blank temperature, that is, axial differential temperature = axial temperature after coal feeding − axial blank temperature before coal feeding,21 which reflects the trend in heat release during bias PC combustion.33,34 Based on the heatrelease 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),14 the ignition mode of the parallel-bias PC jets was determined.21 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.15,16,28 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;5,15,16,24,27 this definition was used to determine the boundary of the continuous flame region. During ignition of the parallel-bias PC jets, better ignition characteristics correspond to a shorter standoff distance, stronger flame stability, and higher PC burnout rate under combined homogeneous−heterogeneous ignition. In the experiments performed in this study, PBCS combustion took place in a reducing atmosphere; the air-to-coal stoichiometry35 was 0.75;21,22 the PA velocity was 16 m/s; the PA temperature was 85 °C; the initial PC concentration,36,37 which is defined as the mass ratio of PC to PA in the PC/PA jets, was 0.33; and the bias concentration ratio,36,37 which is defined as the mass ratio of PC in the fuel-rich jet to that in the fuel-lean jet, was 4.21,22 Variable RCEM experiments were performed using RCEMs of 28.1%, 25.4%, and 22.7%, which were achieved by quantitative steam injection into the PA stream of 100%, 50%, and 0%, respectively, of the moisture released from raw coal containing 28.1% moisture to PC containing 22.7% moisture. Variable PCF experiments were performed using R75 values of 12%, 16%, and 26% (i.e., the surpluses after 75-μm sieving were 12%, 16%, and 26%, respectively).21 The experiments using various coal types were performed using bituminous, sub-bituminous, and blended coals, and all of the RCEMs of the PA/PC jets in these experiments were adjusted to the respective moisture contents of the raw coals. When all of 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 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.

Table 1. Main Design Parameters of the 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 temperatures maximum furnace operating temperature 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

Whitehaven bituminous coal sourced from Australia, Trafigura subbituminous coal sourced from Indonesia, and a 1:1 blend of these coals were used in the experiments; these three coals are referred to as bituminous coal, sub-bituminous coal, and blended coal, respectively, in the rest of this article. The proximate and ultimate analytical data for the raw and pulverized coals are presented in Table 2. The data show that the amount of moisture in the PC was much lower than that in the raw coal. This is because the raw coal was dried in a coal mill during PC preparation, and the released moisture was vented to the atmosphere; therefore, the moisture content of the PC decreased. The PC for the directly fired pulverizing system in the utility PC boiler was also dried in a coal mill, but the released moisture was kept in the PA/ PC stream; therefore the moisture content in the PA/PC stream at the outlet of the coal mill was unchanged. In this study, steam was injected into the PA stream based on the differences between the moisture content of the raw coal and that of the PC; the sum of the amounts of moisture in the PA jets and the PC was equal to the amount of moisture in the raw coal. The total moisture content of the PA/PC jets in the present work is referred to as the raw-coal equivalent moisture (RCEM); that is, RCEM = moisture of PA jets + moisture of PC. The RCEMs in the experiments using various moisture contents and various coal types were adjusted using the steam generation system. 2.2. Research Methodology. A combination of flame spectra and the temperatures and burnout rates of the residual solids in the 250kW PBCS was used to obtain accurate information on the PC ignition characteristics. Details of the procedure for recording flame spectra and determining temperatures and residual solid burnout rates can be found in our previous publication.21 The standoff distance is the distance from the flame onset to the nozzle of the burner after stable ignition of the PC.16,23 In this study, the standoff distance is defined as the position at which the intensity of the axial visible light in the coal flame spectrum reaches 10% of the maximum peak intensity.21,24 The average value of 10% of the maximum peak intensity of visible light in different cases was used as

Table 2. Proximate and Ultimate Analyses of Experimental Coals sub-bituminous

a

bituminous

parametera

raw

pulverized

Mar Var Aar FCar

28.10 32.61 4.99 34.30

22.7 35.63 5.06 36.61

Car Har Oar Nar Sar

48.89 3.45 12.91 0.9 0.76

NCV

18.43

raw

blended pulverized

Proximate Analysis (wt %, As-Received) 9.70 6.80 25.45 27.96 19.13 11.78 45.72 53.46 Ultimate Analysis (wt %, As-Received) 51.77 57.56 67.06 3.7 3.48 3.92 15.5 8.63 8.89 0.85 1.19 1.24 0.42 0.31 0.31 Net Calorific Value (MJ/kg, As-Received) 19.58 22.03 25.74

raw

pulverized

18.90 29.03 12.06 40.01

14.75 31.79 8.42 45.04

53.23 3.47 10.77 1.05 0.54

59.42 3.81 12.20 1.05 0.37

20.23

22.66

M, moisture; V, volatiles; A, ash; FC, fixed carbon; ar, as-received. C

DOI: 10.1021/acs.energyfuels.7b02055 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Because of the normal fluctuations in coal combustion, the average values of multiple measurements was used in the experiments to minimize errors. The temperature measurements and average values deviated in the range of 0−12 °C; the deviation for the recorded spectra and their averages was less than 10%. Details of the experimental method can be found in our previous publication.21

steam and char. This shows that only the appropriate amount of RCEM had positive effects on subsequent combustion. The standoff distances of sub-bituminous parallel-bias PC jets at different RCEMs are shown in Figure 3; each standoff

3. RESULTS AND DISCUSSION 3.1. Effects of RECM on the Ignition Characteristics of Parallel-Bias PC Jets. The axial visible-light intensities of subbituminous parallel-bias PC jets at different RCEMs are shown in Figure 2. The axial visible-light intensity gradually increased

Figure 3. Standoff distances at different RCEMs of sub-bituminous coal.

distance is presented with a relative error bar to show the uncertainty in the standoff distance.21 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 could actually peak within 80 mm on 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.21 The standoff distance gradually decreased with decreasing RCEM because the heat consumed by moisture in the initial stage of ignition decreased; therefore, the ignition heat of the parallel-bias PC jets gradually decreased. The standoff distances of parallel-bias PC jets gradually decreased when the ignition energy was constant.7,27 The axial absolute irradiances of hydrocarbons and hot soot in sub-bituminous parallel-bias PC jets at different RCEMs are shown in Figure 4. The axial absolute irradiances of hydrocarbons and hot soot gradually increased with increasing axial distance up to 700 mm and then gradually decreased. Panels a−c of Figure 4 show that, at 220 mm, near the standoff distances for RCEMs of 22.7%, 25.4%, and 28.1%, the axial absolute irradiance of hydrocarbons was lower than that of hot soot. These results show that the volatile combustion reaction was not dominant in the ignition stage for sub-bituminous parallel-bias PC jets and that the ignition of volatiles was accompanied by the ignition of char; that is, the volatiles and char ignited simultaneously. The preliminary results, based on the spectra, show that the ignition modes of sub-bituminous parallel-bias PC jets at RCEMs of 22.7%, 25.4%, and 28.1% involved the homogeneous−heterogeneous combined ignition of volatiles and char.11,13,21 The axial differential temperatures of parallel-bias PC jets at different RCEMs of sub-bituminous coal are shown in Figure 5. At RCEMs of 22.7%, 25.4%, and 28.1%, the axial differential temperature exhibited one peak prior to an axial distance of 540 mm, but there was a constant value in the increase in the axial

Figure 2. Axial visible-light intensities at different RCEMs of subbituminous coal.

and then gradually decreased with increasing axial distance. Up to an axial distance of 470 mm, the axial visible-light intensity at an RCEM of 25.4% was smaller than that at an RCEM of 22.4%, which means that the flame at an RCEM of 25.4% was weaker. This is because more heat was consumed by evaporation and superheating of moisture in the initial stage of ignition as the RCEM increased. These results indicate that the RCEM had negative effects on ignition in the early stages for parallel-bias PC jets.7 After an axial distance of 470 mm, the axial visible-light intensity at an RCEM of 25.4% was greater than that at an RCEM of 22.4%, which means that the flame at an RCEM of 25.4% was stronger. This is because combustion in the PBCS furnace took place in a reducing atmosphere during the experiments, the moisture had already been converted into steam in the subsequent combustion stage, and the steam promoted char combustion in the hightemperature and low-oxygen environment. Sufficient steam enabled char gasification (C + H2O = CO + H2), although the gasification reaction was endothermic, and the resultant CO and H2 were combustible. The gasification reaction changed the gas−solid two-phase chemical combustion reaction on the char surface into a gas−gas homogeneous chemical combustion reaction; this accelerated combustion and shortened the char burnout time.8,39 The combustible products underwent a steam shift reaction (CO + H2O = CO2 + H2), which is an exothermic reaction. Both of the above steam reactions promote PC combustion,8,39 and the steam shift reaction is the primary reason for advanced ignition.8 These results indicate that the RCEM had positive effects on subsequent combustion of parallel-bias PC jets.38 The axial visible-light intensity at an RCEM of 28.1% was consistently the smallest, which means that the flame at this RCEM was the weakest. This is because the excess RCEM consumed too much heat in the initial ignition stage, which affected the promotion reactions between D

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Figure 4. Axial absolute irradiances of hydrocarbons and soot at different RCEMs of sub-bituminous coal.

differential temperature of combustion therefore continues to increase, or there is a constant value in the increase. Much heat was consumed in the initial ignition stage of sub-bituminous coal because of its relatively high moisture content; therefore, its axial differential temperature had one peak with a constant value. These results show that the ignition modes of subbituminous parallel-bias PC jets at RCEMs of 22.7%, 25.4%, and 28.1% involved the homogeneous−heterogeneous combined ignition of volatiles and char,10,12,13,21 which is consistent with the previous findings based on the flame spectrum. Figure 5 shows that the peak value of the axial differential temperature in the initial ignition stage gradually increased with decreasing RCEM, which indicates that the combustion of subbituminous coal became more intense.40 The burnout rates of residual solids sampled at an axial distance of 540 mm of the sub-bituminous parallel-bias PC jets at different RCEMs are shown in Figure 6. The figure shows that the burnout rate of sub-bituminous coal gradually increased with decreasing RCEM. This is because the combustion reaction of subbituminous parallel-bias PC jets became more intense as the RCEM decreased. The continuous flame regions of sub-bituminous parallel-bias PC jets at different RCEMs, based on the continuous flame delay distance and radial temperature profile of the flame at different axial distances, are shown in Figure 7.41 The figure shows that the boundaries of the continuous flame region at RCEMs of 22.7%, 25.4%, and 28.1% all started at an axial distance of 540 mm. The continuous flame region of the subbituminous coal gradually became smaller with decreasing RCEM, and the flame stability increased,42 consistent with

Figure 5. Axial differential temperatures at different RCEMs of subbituminous coal.

differential temperature. 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 sub-bituminous parallel-bias PC jets at different RCEMs, owing to the fact that the main flow of secondary air is not involved in PC ignition prior to 540 mm. The single peak in the axial differential temperatures at RCEMs of 22.7%, 25.4%, and 28.1% corresponds to the release of heat from simultaneous combustion of volatiles and char. If the volatiles and char fulfill the ignition conditions at the same time and burn simultaneously, the heat release rate of the combustion reaction is much higher, and the char continues to burn once the reaction of volatiles has ended. The axial E

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decreased. The peak value of the axial visible-light intensity gradually increased with decreasing PCF; that is, the flame stability gradually increased.43 The standoff distances of bituminous parallel-bias PC jets at different PCFs are shown in Figure 9. The standoff distance gradually decreased with

Figure 6. Burnout rates of residual solids sampled at 540 m at different RCEMs of sub-bituminous coal.

Figure 9. Standoff distances at different PCFs of bituminous coal.

decreasing PCF. This is because, with decreasing PCF, the heating rate and chemical reaction rate of smaller coal particles increased. The ignition conditions of larger coal particles were achieved slower because of their greater thermal resistance; therefore, there was a significant ignition delay at a PCF of 26%.44 The axial absolute irradiances of hydrocarbons and hot soot for bituminous parallel-bias PC jets at different PCFs are shown in Figure 10. The axial absolute irradiances of hydrocarbons and hot soot gradually increased with increasing axial distance up to 700 mm and then gradually decreased. Figure 10a shows that, at 220 mm, near the standoff distance for a PCF of R75 = 26%, the axial absolute irradiance of hydrocarbons was the same as that of hot soot. This shows that the volatile combustion reaction was dominant in the ignition stage for bituminous parallel-bias PC jets and that the volatile and char ignitions occurred successively. The preliminary results, based on spectra, show that the ignition mode for a PCF of R75 = 26% was volatile-phase homogeneous ignition.11,13,21 This could be because a higher volatile concentration around larger coal particles was easier to reach under homogeneous ignition conditions.45 Panels b and c of Figure 10 show that, at 220 mm, near the standoff distances for PCFs of R75 = 16% and R75 = 12%, respectively, the axial absolute irradiance of hydrocarbons was lower than that of hot soot. This shows that volatiles and char were ignited simultaneously in the ignition stage for subbituminous parallel-bias PC jets. The preliminary results, based on spectra, show that the ignition modes for PCFs of R75 = 12% and R75 = 16% are volatile and char homogeneous− heterogeneous combined ignition.11,13,21 This could be because smaller coal particles released lower amounts of volatiles, which were more easily carried away by the jets, making it difficult to reach the volatile concentration needed for homogeneous ignition.45 The heating rate of the coal particles increased with decreasing PCF, which is conducive to combined ignition of volatiles and char.44 The axial differential temperatures of bituminous parallel-bias PC jets at different PCFs are shown in Figure 11. The figure shows that, up to an axial distance of 540 mm, two clear peaks were observed at a PCF of R75 = 26%, with the second peak being larger than the first. One clear peak was observed for the

Figure 7. Continuous flame regions at different RCEMs of subbituminous coal.

previous findings for axial visible-light intensities. In agreement with previously reported results, the continuous flame regions of sub-bituminous parallel-bias PC jets at different RCEMs also leaned in the direction of the fuel-rich jet, as previously reported. This is because ignition of the fuel-rich jet was better than that of the fuel-lean jet when the PA velocity was 16 m/s for each RCEM.21 3.2. Effects of PCF on the Ignition Characteristics of Parallel-Bias PC Jets. The axial visible-light intensities of bituminous parallel-bias PC jets at different PCFs are shown in Figure 8. The axial visible-light intensity gradually increased with increasing axial distance up to 700 mm and then gradually

Figure 8. Axial visible-light intensities at different PCFs of bituminous coal. F

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Figure 10. Axial absolute irradiances of hydrocarbons and soot at different PCFs of bituminous coal.

volatile homogeneous ignition,10,12,13,21,46 which is consistent with previous findings based on the flame spectrum. The single peaks in the axial differential temperature curves at PCFs of R75 = 12% and R75 = 16% arise from the release of heat from the simultaneous combustion of volatiles and char. These results show that the ignition modes of bituminous parallel-bias PC jets at PCFs R75 of 12% and 16% were volatile and char homogeneous−heterogeneous combined ignition,10,12,13,21 which is consistent with previous findings based on the flame spectrum. These results show that the ignition mode of bituminous coal changed from homogeneous to homogeneous−heterogeneous combined ignition with decreasing PCF. Figure 11 shows that the peak axial differential temperature in the initial ignition stage gradually increased with decreasing PCF, which indicates that the combustion reaction of bituminous coal became more intense.40 The burnout rates of residual solids sampled at an axial distance of 540 mm for bituminous parallel-bias PC jets at different PCFs are shown in Figure 12. The figure shows that the burnout rate of bituminous coal gradually increased with decreasing PCF. This is because the combustion reaction of bituminous parallelbias PC jets became more intense with decreasing PCF. The continuous flame regions of bituminous parallel-bias PC jets at different PCFs are shown in Figure 13.41 The figure shows that the continuous flame region boundary at a PCF of R75 = 26% started at an axial distance of 540 mm, but those at PCFs of R75 = 12% and R75 = 16% started at an axial distance of 380 mm. This indicates that the continuous flame boundary of parallel-bias PC jets advanced with decreasing PCF. With decreasing PCF, the continuous flame region of bituminous

Figure 11. Axial differential temperatures at different PCFs of bituminous coal.

axial differential temperature at PCFs of R75 = 12% and R75 = 16%. The first of these two peaks at PCFs of R75 = 26% corresponds to the release of heat from volatile combustion, and the second peak at a PCF of R75 = 26% corresponds to the release of heat from char combustion. If the volatiles ignite before the char has started to burn, the rate of heat release from the combustion reaction will be lower. If the char has still not begun to burn when the reaction of the volatiles ends, the first relatively small axial differential temperature peak appears. With continued heating of the parallel-bias PC jets, the char begins to burn, and the second relatively large axial differential temperature peak appears. These results show that the ignition mode of bituminous parallel-bias PC jets at a PCF of R75 = 26% was G

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largest axial visible-light intensity, sub-bituminous coal gave the smallest, and the value for blended coal was between the other two. This indicates that bituminous coal gave the most stable flame, sub-bituminous coal gave the least stable flame, and the value for blended coal was intermediate.43 The standoff distances for parallel-bias PC jets of different coal types are shown in Figure 15. The standoff distance for bituminous coal

Figure 12. Burnout rates of residual solids sampled at 540 m at different PCFs of bituminous coal.

Figure 15. Standoff distances for different coal types.

was shortest, that of sub-bituminous coal was longest, and the value for blended coal was between those of the other two coals. This is because the ignition of sub-bituminous coal showed the greatest delay under the same ignition energy because it had the highest RCEM content.7,27 These results show that the ignition characteristics of blended coal parallelbias PC jets resemble those of sub-bituminous coal. The axial absolute irradiances of hydrocarbons and hot soot of parallel-bias PC jets of different coal types are shown in Figure 16. The axial absolute irradiances of hydrocarbons and hot soot gradually increased with increasing axial distance up to 700 mm and then gradually decreased. Figure 16a shows that, at 220 mm, near the standoff distance for bituminous coal, the axial absolute irradiance of hydrocarbons was the same as that of hot soot. This shows that the ignitions of volatiles and char occurred successively in the ignition stage for bituminous parallel-bias PC jets. The preliminary results, based on spectra, show that the ignition mode of bituminous coal was volatilephase homogeneous ignition.11,13,21 Panels b and c of Figure 16 show that, at 220 mm, near the standoff distances for subbituminous and blended coals, respectively, the axial absolute irradiance of hydrocarbons was lower than that of hot soot. These results show that the volatiles and char ignited simultaneously in the ignition stage for parallel-bias PC jets. These results suggest that the ignition modes for subbituminous and blended coals were volatile and char homogeneous−heterogeneous combined ignition.11,13,21 The axial differential temperatures of parallel-bias PC jets of different coal types are shown in Figure 17. The figure shows that up to an axial distance of 540 mm, bituminous coal gave two clear axial differential temperature peaks, with the second peak being larger than the first. Subcontinuous coal gave one axial differential temperature peak, but there was a constant value in the rise of the axial differential temperature. Blended coal gave one clear axial differential temperature peak. The first of the two axial differential temperature peaks for bituminous coal corresponds to the release of heat from volatile combustion and the second peak corresponds to the release of heat from char combustion. These results show that the ignition mode for bituminous parallel-bias PC jets was volatile

Figure 13. Continuous flame regions at different PCFs of bituminous coal.

coal became gradually smaller, and the flame became more stable, which is consistent with previous findings for axial visible-light intensities. In agreement with previously reported results, the continuous flame regions of bituminous parallel-bias PC jets at different PCFs leaned in the direction of the fuel-rich jet because the ignition of the fuel-rich jet was better than that of the fuel-lean jet at a PA velocity of 16 m/s for each PCF.21 3.3. Effects of Coal Type on the Ignition Characteristics of Parallel-Bias PC Jets. The axial visible-light intensities of parallel-bias PC jets of different coal types are shown in Figure 14. The axial visible-light intensity gradually increased with increasing axial distance up to 700 mm and then gradually decreased. In all cases, bituminous coal gave the

Figure 14. Axial visible-light intensities for different coal types. H

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Figure 16. Axial absolute irradiances of hydrocarbons and soot for different coal types.

volatile released from sub-bituminous coal and bituminous coal was nonuniform during the ignition process of blended coal, and the sub-bituminous coal ignited first then followed by bituminous coal. Figure 17 shows that combustion of bituminous coal was most intense, that of sub-bituminous coal was weakest, and that of the blended coal was medium.40 The burnout rates of residual solids sampled at an axial distance of 540 mm for parallel-bias PC jets of different coal types are shown in Figure 18. The figure shows that the burnout rate of bituminous coal was highest, that of sub-bituminous coal was lowest, and that of the blended coal was medium. This indicates that the burnout characteristics of blended coal parallel-bias PC jets resembled those of bituminous coal.

Figure 17. Axial differential temperatures for different coal types.

homogeneous ignition,10,12,13,21,46 which is consistent with previous findings based on the flame spectrum. The single peak in the axial differential temperature curves of sub-bituminous and blended coals corresponds to the release of heat from simultaneous combustion of volatiles and char. These results show that the ignition modes for parallel-bias PC jets of subbituminous and blended coals were volatile and char homogeneous−heterogeneous combined ignition,10,12,13,21 which is consistent with previous findings based on flame spectra. The RCEM of sub-bituminous parallel-bias PC jets was relatively high, therefore the axial differential temperature had one peak, with a constant value. These results show that the ignition characteristics of blended coal parallel-bias PC jets resembled those of sub-bituminous coal. This is because the

Figure 18. Burnout rates of residual solids sampled at 540 m for different coal types. I

DOI: 10.1021/acs.energyfuels.7b02055 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels The continuous flame regions of parallel-bias PC jets of different coal types are shown in Figure 19.41 The figure shows

ignition characteristics gradually deteriorated in the sequence bituminous, blended, and sub-bituminous coals. When the RCEMs of the PA/PC jets reached the respective moisture content of each raw coal, the ignition mode for bituminous coal was homogeneous, and those for sub-bituminous and blended coals were a homogeneous−heterogeneous combination. The ignition characteristics of the blended coal resembled those of sub-bituminous coal, and the burnout characteristics of blended coal resembled those of bituminous coal.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-451-86412238. Fax: +86-451-86412528. E-mail: [email protected]. ORCID

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

Figure 19. Continuous flame regions for different coal types.

that the continuous flame regions for bituminous, subbituminous, and blended coals all started at an axial distance of 540 mm. When the RCEMs of the PA/PC jets were adjusted to the respective moisture contents of the raw coals, for the sequence bituminous, blended, and sub-bituminous coals, the continuous flame region became larger, and the flame stability gradually decreased. In agreement with previously reported results, the continuous flame regions of parallel-bias PC jets of different coal types leaned in the direction of the fuel-rich jet. This is because ignition of the fuel-rich jet was better than that of the fuel-lean jet at a PA velocity of 16 m/s for each coal type.21

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. We thank Helen McPherson, Ph.D., from Liwen Bianji, Edanz Group China, for editing the English text of a draft of this manuscript.

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4. CONCLUSIONS Combustion experiments were conducted in a 250-kW PBCS. The effects of RCEM, PCF, and coal type on the ignition characteristics of parallel-bias PC jets in a reducing atmosphere were investigated based on a combination of flame spectra, combustion temperatures, and burnout rates of residual solids. The main conclusions drawn from the results of the study are as follows: (1) With decreasing RCEM of parallel-bias PC jets, the standoff distance gradually decreased, the peak visible-light intensity from subsequent combustion gradually increased, the flame stability gradually increased, the burnout rate gradually increased, and the ignition characteristics gradually improved. The RCEM had negative effects on the ignition in the early stages but had positive effects during subsequent combustion. At RCEMs of 22.7%, 25.4%, and 28.7%, the ignition mode of sub-bituminous coal was a homogeneous−heterogeneous combination. (2) With decreasing PCF of parallel-bias PC jets, the standoff distance gradually decreased, the peak visible-light intensity from subsequent combustion gradually increased, the flame stability gradually increased, the burnout rate gradually increased, and the ignition characteristics gradually improved. At PCFs of R75 = 12% and R75 = 16%, the ignition mode of bituminous coal was a homogeneous−heterogeneous combination. The ignition mode was homogeneous at a PCF of R75 = 26%. (3) For different coal types, the standoff distance gradually increased, the peak visible-light intensity from subsequent combustion gradually decreased, the flame stability gradually decreased, the burnout rate gradually decreased, and the

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DOI: 10.1021/acs.energyfuels.7b02055 Energy Fuels XXXX, XXX, XXX−XXX