Experimental study on ignition and combustion characteristics of

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Combustion

Experimental study on ignition and combustion characteristics of pyrolyzed char in O2-enriched atmosphere with multiple optical diagnostic techniques Hongliang Qi, Rui Sun, Jiangbo Peng, Biao Yan, Zhen Cao, Yang Yu, Guang Chang, Xiaohan Ren, and Saijie Ding Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00658 • Publication Date (Web): 02 May 2019 Downloaded from http://pubs.acs.org on May 2, 2019

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Experimental study on ignition and combustion characteristics of pyrolyzed char in O2-enriched atmosphere with multiple optical diagnostic techniques Hongliang Qia, Rui Sun a*, Jiangbo Pengb*, Biao Yanb, Zhen Caob, Yang Yub, Guang Changb, Xiaohan Renc, Saijie Dingd a School

of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China;

b National

c

Key Laboratory of Science and Technology on Tunable Laser, Harbin Institute of Technology, Harbin 150001, China;

Institute of Thermal Science and Technology, Shandong University, Jinan, 250061, PR China;

d CRCC

(Qingdao) Vehicle Inspection Station Co., Ltd, Qingdao, 266031, China

*Corresponding author: Prof. Rui Sun, [email protected]; Dr. Jiangbo Peng, [email protected]

Abstract: A flat-flame entrained-flow pulverized coal/char reactor (EFR) was designed to study the ignition and combustion characteristics of pyrolyzed bituminous (PB) char particle stream of 53–80 μm size. A highly volatile content bituminous pulverized coal (SH) was also tested for comparison. The OH planar laser-induced fluorescence (OHPLIF), CH* chemiluminescence, visible light, and three-color high-temperature pyrometer study were performed to determine the PB char ignition process, flame structure, and particle temperature. In the EFR, the heat-up and ignition of the PB char were retarded with a decrease in both ambient oxygen concentration from 30% to 5% and ambient temperature from 1800 K to 1600 K. As the O2 concentration is enriched, decreased trend in the duration of the volatile flame is observed, total combustion time and fixed carbon residual ratio decrease to about 1/2. In the range from 20% to 30% oxygen concentration, the PB char combustion was particularly intensified and numerous small fragments trailing from the char particles appeared in the flame. PB

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char particle temperatures and combustion intensity are enhanced apparently in oxygenenriched environments. At high O2 concentration, the ratios of volatile flame time in total flame time for PB char are about 1/3 to 1/2 of bituminous coal. The initial rapid volatile evolution appeared to have a little effects on PB char ignition, and the heterogeneous combustion mode is dominant in the pyrolyzed char particle flame.

Keywords: Pyrolyzed bituminous char ignition; High speed OH-PLIF; CH* chemiluminescence; Three-color pyrometer

1. Introduction As a fossil fuel, coal is an abundant reserve and has an extremely important role in power generation and industrial heating facilities. Concurrently, more efficient and cleaner utilization of coal has been research focus worldwide, which are closely related with the complex gas–solid combustion process of coal involving many successive or simultaneous reaction processes such as devolatilization, volatile combustion, and char combustion. The low-volatile coal occupy nearly 20% total coal consumption in China [1]. The primary difficulty of the low-volatile coal combustion is to achieve steady ignition front and high flame propagation velocity owing to its low reaction rate. Pyrolyzed bituminous (PB) chars are the residues of the thermal conversion process for producing liquid or gas fuels from high-volatile coal and has a large reserve in China. They are typical ultra-low-volatile coal-based solid fuels with properties of high heat value, ultra-low volatility (dry and ash-free volatile content, Vdaf < 10%), and low N


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and S element contents. A promising utilization method for disposing PB chars is burning in large capacity utility boiler and generating electricity. The previous research of pyrolyzed char combustion is mainly in circulating fluidized bed reactor. Li et al. [2] carried out combustion experiments of pyrolyzed char in a 1MW pilot-scale circulating fluidized bed under air and 50%O2/recycle flue gas (RFG) mode. Results showed that the combustion of pyrolyzed char is very stable and the combustion efficiency is improved under O2/RFG mode with high oxygen concentration. Pulverized-coal boiler has the advantages of large capacity and is most widely employed, and the utilization of pyrolyzed char in pulverized-coal boiler is promising in China. Co-combustion of pyrolyzed char and high-volatile pulverized coal at a certain proportion is considered favorable to dispose pyrolyzed char in the practical application, and would be the primary option for large-scale utilization of pyrolyzed chars. Zhang et al. [3] conducted experiments to investigate co-combustion characteristics of pyrolyzed char and bituminous coal blends with thermogravimetric analyzer and drop tube furnace. Results indicated that the ignition and burnout temperatures of the blends decreased as the blending ratio of bituminous coal increased, and there existed significant interaction between pyrolyzed char and bituminous coal. However, the study on ignition and burnout of pulverized pyrolyzed char in O2-enriched atmosphere is scarce. The enriched oxygen (O2) combustion [4, 5] is considered to reduce ignition delay, enhance combustion intensity, and promote burnout for low volatile content solid fuels. Studying the devolatilization, ignition, and burnout characteristics of PB char particle streams in O2-enriched combustion conditions with multiple optical diagnose


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technology (as OH planar laser-induced fluorescence (OH-PLIF), visible light and CH* chemiluminescence, etc.) and traditional flue gas/char sampling technique may be beneficial for efficient and clean combustion utilization of the ultra-low-volatile coalbased fuels.

The ignition mechanism [6, 7] of coal has attracted much attention. Researchers attempt to identify whether a pulverized coal particle stream involves homogeneous or heterogeneous ignition and combustion mechanism [8]. The homogeneous ignition mode is considered to be a conventional gas-phase ignition mechanism, that first volatile matter is ignited in an envelope flame surrounding char particle [9], and then char surface is ignited. In the 1960s, Howard and Essenhigh [10] proposed a heterogeneous ignition mode, which indicated direct char surface ignition. At high heating rates or under extremely low volatile contents, coal particle surface reach a sufficiently high temperature rapidly, and it may be possible for the particle to be ignited and burnt by direct O2 attacking on solid surface before devolatilization is onset [9]. In addition, pulverized coal particles may experience a hetero–homogeneous joint ignition mode. The volatiles ignite, and the char combustion starts simultaneously. H. Jüntgen [11] provided the relationship between the ignition modes depending on the particle size and heating rate. The authors found that for small particle sizes below 50 m, the grain itself ignited heterogeneous for nearly all the heating rates (Iheterogeneous mode). For larger particle sizes and heating rates below 500°C/s, all the volatiles burnt first, followed by ignition of char (II-homogenous mode). With rising


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heating rates, these two distinct ignition steps overlaps, and therefore, at high heating rates the ignition and combustion of volatiles continuously accompany the ignition and burn-off of the produced char (III-homogenous- heterogeneous joint mode) [11]. In the past several decades, there have been abundant experiment studies [12-14] and model predictions [15-17] of the ignition of either single particle or particles stream [18] with advanced optical diagnostics techniques. The ignition mechanism depends on the rank of the coal, particle size, volatile matter, ambient gas temperature, heating rate, and O2 concentration [19-21]. Temperature and O2 concentration are two crucial reaction parameters owing to their significant influence on the ignition and flame stability.

With the rapid development of the optical diagnostic technology, various optical diagnostic techniques have been applied to fuel combustion for their advantages of owing non-invasive and high temporal and spatial resolution. With a three-color optical pyrometer and high-speed cinematography, Yiannis A. Levendis studied single-particle temperature–time history [22, 23, 24] and its ignition characteristics [25, 26]. He found that bituminous coals usually burn in a two-mode combustion with distinct volatile matters combustion and char combustion stages, whereas anthracitic and most semianthracite coals have only one char combustion stage [27]. Jan Köser [28, 29] studied the devolatilization and ignition of individual coal particles with a high-speed OH-PLIF, which is allowed for resolving the temporal volatile flame structure surrounding coal particles and determining the onset of ignition. The diffusion flame surrounding coal particles was determined by the OH radical distributions. The coal particles act as a


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source of gaseous combustible hydrocarbons, and OH-PLIF is employed to visualize intermediate radicals distribution such as OH during volatile combustion process. OH is mainly used to represent the combustion intensity of the volatile matters. Numerous products of volatile pyrolysis correspond to severe volatile combustion at high O2 concentrations, and consequently, a high OH fluorescence intensity is observed. However, OH-PLIF is usually limited owing to its narrow field-of-view, which is difficult for the investigation on the entire flame range from ignition to the end of single particle volatile combustion [29] of coal particle streams [28]. Various optical diagnostic techniques (OH-PLIF, CH*, C2* chemiluminescence, two-color or threecolor pyrometer, shadow Doppler particle analyzer (SDPA), and laser Doppler velocimetry (LDV)) were employed to comprehensively study the ignition and combustion of particle streams [30, 31, 32]. With an optical particle-sizing pyrometer, Christopher R. Shaddix from Sandia Laboratory [4] measured the joint temperature– size statistics of size-classified burning pulverized coal chars. He indicated that O2enriched combustion significantly increased the char surface temperature and reduced the char burnout time. Using CH* chemiluminescence and a three-color pyrometer Yuan Ye [33-35] characterized the pulverized coal devolatilization, volatile homogeneous ignition, and char heterogeneous ignition processes. The normalized visible light signal intensity was determined to characterize the ignition delay of a coal particle stream. As the ambience temperature increased from 1200 K to 1500 K, a transition from the heterogeneous ignition mode to a hetero–homogeneous mode occurred. The visible light signal at 1200 K exhibited a monotonous increment before


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the appearance of the peak. However, in the cases of high temperatures from 1500 K to 1800 K, an apparently incandescent flame owing to the occurrence of coherent volatile combustion was observed. The author explained this phenomenon based on the competition between volatile matter evolution and heterogeneous surface reaction. Using CH* chemiluminescence, Mie scatter, and simultaneous OH-PLIF diagnostic technology Seungmin Hwang [36, 37] evaluated the combustion and investigated the detailed flame structure of a methane (CH4)-assisted pulverized coal flame. The measurement results were expected to be helpful for the assessment of the numerical simulations of a two-phase combustion. CH* chemiluminescence was utilized to characterize the ignition regions and duration of the volatile combustion, which was better than using the blackbody radiation light from soot or hot char. However, the CH* signal was so weak that it was difficult to clearly detect and had a strong spectral overlap with the blackbody light. OH-PLIF was applied successfully to visualize the reaction zones in gaseous flames and reveal the global structure of gas-assisted pulverized coal flames [29, 38].

In this work, pyrolyzed bituminous (PB) chars, as the main tested samples, were obtained from the coal pyrolysis process in an industrial pyrolysis furnace at temperatures between 500–700 °C. The ignition and combustion characteristics of the PB chars were studied using the experimental system of a pulverized coal/pyrolyzed char reactor under high-temperature flow conditions. 100Hz and 500Hz OH-PLIF with a large view field laser sheet (view field maximum height 80 mm and maximum width


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100 mm), a three-color pyrometer, visible lights, and CH* chemiluminescence were employed to study the ignition and combustion characteristics of the PB char particles stream. The effects of the O2 concentration and ambient temperature on the ignition delay distance of the pulverized fuels flame and combustion mode of the PB chars were investigated with OH-PLIF and CH* chemiluminescence.

2．Experimental setup 2.1. Flat-flame entrained-flow pulverized coal/char optical reactor Based on the flat-flame burner designed by Hartung [39], a flat-flame entrained-flow pulverized coal/char reactor was installed for optical diagnosing the PB char or pulverized coal streams [40]. Fig.1 shows a schematic of the reactor [41]. Char/coal particles are injected through an inner diameter 2-mm-stainless-steel tube at the reactor centerline. Two annular nozzle zones surround the central circular hole. The inner annular zone produces a CH4/oxidants-premixed flat flame, which has more than 2000 round holes with a diameter of 0.5 mm (at 293 K, the quenching diameter of CH4/air is 2.5 mm). According to a certain mixing ratio, CH4, N2, and O2 mix in the premixing tank and burn outside the flat-flame nozzle holes. This forms a several millimeterthickness blue premixed flat flame, which will simulate high-temperature flue gas ambience. The heating rate for injected particles is about 1.1×105~2.2×105 K/s, which is calculated with the temperature downstream the feeding tube nozzle and heating time of injected particles. The outer annular nozzle zone provides a protective gas. The protective gas of N2 and O2 is premixed and then injected from the round holes of the


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outer annular nozzle zone. It prevents the optical window and gasket from being overheated and char particles fouling. The concentration of O2 in the protective gas is adjusted consistent with main flue gas downstream flat-flame zone. A 90 mm × 90 mm quartz chimney isolates the reactor from the surrounding air and allows optical diagnosing through the optical windows. The reactor is 210 mm high. The optical windows are JGS1 quartz windows coated with 200 nm–550 nm of an anti-reflection film, whose average transmission rate is > 96 %.

MFC MFC

N2 Reflector

Dye laser

CH4 O2 N2

Filter

O2 N2

SHG

Nd：YAG laser

Digital signal generators

Aperture Reflector

MFC MFC MFC

Pulverized coal feeder

Premixed tank

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Rota Rota

Lens1

Optical collector Concave lens Lens2 Optical filter

Flat flame Coal flame

ICMOS

Exhaust gas Flue gas cooling system

Drying bottle

Fig.1 Entrained-flow pulverized coal/char reactor and associated optical diagnostics instrument

2.2 Experiment operation conditions


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O2-enriched/N2 combustion [4, 5], which is beneficial for enhancing flame stability and combustion intensity of the ultra-low-volatile fuels in the furnace, was tested. Table 1 presents the composition flow rates of the premixed gas mixture of CH4, O2, and N2. The total flow of CH4, O2, and N2 are kept unchanged. The flue gas background at different temperatures and different O2 concentrations in the downstream flat flame is achieved by adjusting the proportion of CH4, O2, and N2 in the premixed gas mixture. CH4 and oxidants are perfectly mixed and complete CH4 burnout is ensured. Here, the gas concentration was obtained from CH4 complete combustion theoretical calculation of plat flame. O2 and CO2 concentration in flue gas downstream plat flame were measured by flue gas analyzer and found to reach the theoretical concentration calculation approximately. While the flow rate of CH4 remains constant for the target temperature, four different O2 concentrations of 5%, 10%, 20%, and 30% are achieved by adjusting the flow rates of O2 and N2. By adjusting the CH4 flow rate, three different temperatures of 1600 K, 1700 K, and 1800 K are achieved and verified with computational fluid dynamics (CFD) modeling. The flow rate of the shielding gas is 300mL/min (298 K, 101.325 Pa), where the proportion of O2 and N2 is adjusted according to the O2 concentration in flue gas downstream flat flame. The gas for entraining the particles through the center pipe is N2 at a flow rate of 200 mL/min. PB char or SH pulverized coal feeding rate is 1.9 g/h. The gas temperature profile without char/coal injection on the centerline of the reactor was measured to confirm preset reaction conditions with a 500-μm type B thermocouple and calculated with radiative loss correction, as shown in Fig.2. The temperature near the feeder tube nozzle is


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slightly lower when the entraining flow N2 is injected in, mainly owing to its cooling effect. The gas temperature remains approximately at the target values within the range of 0–90 mm downstream the center nozzle.

2000

1600K 1700K 1800K

1800

Temperature (K)

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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1600

1400

1200

1000 0

5

10

15

Burner height (cm)

Fig.2. Measured centerline gas temperature distribution in the reactor

Table 1 Operation parameters of combustion tests Background Case

Premixed gas

flue gas

Flue gas after plat flame

Temperature

O2

CH4

N2

(K)

(L/min)

(L/min)

(L/min)

Q11

1600

2.81

1.03

Q12

1600

3.52

Q13

1600

Q14

N2 (%)

O2 (%)

CO2 (%)

H2O (%)

11.17

74.5

4.8

6.9

13.8

1.03

10.45

69.6

9.7

6.9

13.8

5.07

1.03

8.90

59.3

20.0

6.9

13.8

1600

6.62

1.03

7.34

49.0

30.3

6.9

13.8

Q21

1700

3.11

1.18

10.75

71.7

4.7

7.9

15.7

Q22

1700

3.90

1.18

9.92

66.2

10.2

7.9

15.7

Q23

1700

5.43

1.18

8.39

55.9

20.5

7.9

15.7

Q24

1700

6.73

1.18

7.09

47.3

29.1

7.9

15.7

Q31

1800

3.41

1.33

10.22

68.2

5.3

8.8

17.7

Q32

1800

4.12

1.33

9.56

63.7

9.7

8.8

17.7

Q33

1800

5.58

1.33

8.10

53.9

19.5

8.8

17.7

Q34

1800

7.04

1.33

6.64

44.2

29.2

8.8

17.7


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2.3. Fuel properties Table 2 Ultimate analysis and proximate analysis of pulverized PB char and SH coal samples Ultimate analysis（d,wt%）

Proximate analysis（d,wt%）

Samples C

H

N

S

O

Volatile

CFixed

Ash

Moisture

PB char

78.86

1.48

0.99

0.34

4.93

9.47

77.13

11.13

2.27

SH coal

71.11

3.92

0.8

0.28

12.34

30.25

58.2

6.93

4.62

PB char and Shenhua bituminous (SH) coal were used in the experiments. Pyrolyzed bituminous (PB) chars, as the main tested samples, were obtained from the coal pyrolysis process in an industrial pyrolysis furnace at temperatures between 500– 700 °C. The samples were ground and sieved to a 53–80 μm size fraction. All the samples were dried at 105 °C for half an hour before the tests, and the ultimate and proximate analysis data of the pulverized fuels on dry basis are listed in Table 2.

2.4. Visible light, CH* chemiluminescence, and OH-PLIF diagnostics system For coal/char flame images detection, the digital camera, Nikon D7100 with the maximum aperture of f3.5 and shutter speeder of 30–1/8000s, were employed. F5.6/0.1s/ISO320 were chosen for the digital camera to prevent saturation and suppress background noises.

The OH-PLIF diagnostics system employed to investigate the ignition and combustion


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characteristics of the pulverized SH coal and PB char is shown in Fig.1. The laser is a tunable Dye laser pumped by a Nd:YAG solid-state laser at 532 nm. A Second Harmonic Generation (SHG) frequency multiplication module is mounted at the exit of the Dye Laser in order to reach the wavelength of 283 nm. The final laser beam passes through a beam shaping system including reflector and optical lens, the point laser beam is shaped into a two-dimensional sheet laser beam. The sheet laser injected into flame to excite the OH* radical, the absorbed photons are excited from a low energy level to a high energy level, but the excited OH* is unstable, returns to the ground state by emitting fluorescence. The two-dimensional fluorescence signal is captured by the intensified complementary metal-oxide semiconductor (ICMOS) camera. The lifetime of free radical fluorescence is very short, only about a few tens of nanoseconds. Therefore, the laser excitation process and the ICMOS high-speed camera acquisition process must be synchronized. The DG645 digital signal generator is used to transmit pulse signals to the laser and the DG535 digital signal generator which controlled the ICMOS camera. The laser and the ICMOS camera acquire the free radical fluorescence signal synchronously. The camera uses an ultraviolet lens (Coastal Optics, f = 105 mm, f/4.5) with a gate width of 100 μs. A Semrock band-pass filter centered at 310 nm and Schott UG11 filter are installed in front of the camera lens (carbon yellow green noise is removed). The camera resolution is 1280 × 1024. A sheet laser 80-mm-high and 500μm-thick is used for diagnosing the particle stream flame. The width of vision is 100 mm. Limited by the laser energy, the camera frame rate is only 100 Hz and gate width is 100 μs. The spatial resolution is approximately 78 μm/pixel. Two frame rates 100 Hz


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and 500 Hz were employed for the measurement. 100Hz PLIF was employed to study the ignition and combustion characteristics of particle stream. 500Hz PLIF was employed to study the volatile evolution of single particle in the particle stream. To distinguish the ignition and combustion characteristics of individual particle more clearly, a sheet laser 22.8-mm-high and 300-μm-thick is used to diagnose the single particle flame characteristics. The camera frame rate is 500 Hz and gate width is 100 μs. The spatial resolution is approximately 22.27 μm/pixel. First, the ICMOS camera take 1000 pictures without coal injected as the background to eliminate the OH* signal from CH4 flat-flame. An ICMOS camera continuously captures 1000 pictures with coal injected to average the signal information to reduce the deviation. The transient twodimensional OH-PLIF images of the PB char combustion flame are obtained by image processing. The OH* signal intensity without background noise as a function of the height below the burner is obtained from the gray image by MATLAB. Typical single particle evolution images are selected from 500Hz OH-PLIF images. The OH* signal intensity around single particle are obtained from the pixels values of gray pictures by MATLAB.

The images of CH* chemiluminescence are captured by the same camera used for OHPLIF with a front camera lens of a Semrock 10-nm narrow-band interference filter centered at 427 nm. An ICMOS camera continuously captures 1000 pictures at the frame rate of 500 Hz to average the signal information to reduce the deviation. The average signal is used as the emission profile for the CH* chemiluminescence. The


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height of ICMOS camera view field is 150 mm, covering the whole flame. Average and variance can be obtained from the 1000 pictures. The uncertainty (ε) of measurements is defined as the ratio percent of variance and average. The uncertainty (ε) of measurements captured by the camera is less than 1% in all experiments. 2.5. Three-color pyrometer measurement system Based on the three-color optical pyrometer system of Northeastern University [23], a three-color pyrometer was designed to measure the pulverized coal/char gas particle flame temperature. Fig.3 shows a schematic of the three-color pyrometer. The light from the burning PB char particles is focused by the lens group and collimating lens on one end of the optical fiber. This 1.2-m-long, 390 nm–1100 nm single fiber optic has the advantage of maximizing the light transmission efficiency because its entire crosssection is being utilized. The output parallel light is divided into three beams by two dichroic edge filters. Three medium-bandwidth interference filters with full width at half maximum (FWHM) of 80 nm are used to define the measured characteristic wavelengths of 640 nm, 810 nm, and 990 nm. The three wavelengths avoid the absorption bands of water vapor and carbon dioxide. The light through the interference filters is then focused by the focusing lens onto a photodetector (Thorlabs PDA-36A). The photodetector converts the optical signal into a voltage signal after amplification. A NI USB-6341 data acquisition card is used to collect the voltage signals. The software LabVIEW Signal Express is used to perform data acquisition of the photodetector voltage signals. The three-color pyrometer projects the flame image onto a 3-mmdiameter collimating lens, and the temperature measured is not at one point but


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measured of a specific small measuring volume. The measured temperature is mainly of char particles but will be interfered by signals from other various components as soot and gas-phase products [42]. To avoid measuring deviation, the three-color pyrometer is calibrated by a high-temperature blackbody furnace. The temperature range of the blackbody furnace is 1073–2573 K. The interval is 100 K between two calibration points. Relative error of the solid phase particle temperature measurement with threecolor pyrometer is less than 10%.

Photodetector(Si) Focusing lens

Photodetector housing

Interference filter 810nm 990nm

Amplifier A/Dconvertion

Dichroic edge filters

Collimators Pinhole

Interference filter Focusing lens

Fused silica Optical fiber plano-convex lens 640nm

Photodetector(Si)

PC

(a) Blackbody calibration points Linear fit (3238×1/T-1.429)

1.5

ln[(S990/ε990)/(S810/ε810)]

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1.0 0.5 0.0 0 -0.5

2

4

6

8

1/T×104(K)

-1.0 -1.5

(b) Fig.3. Three-color pyrometer system. (a) schematic of three-color pyrometer; (b) pyrometer


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calibration charts combining data from blackbody furnace.

3. Results and discussion 3.1. Ignition characteristics of pyrolyzed bituminous char and bituminous coal based on OH-PLIF and CH* chemiluminescence diagnose PB char samples were dried and ground to particle size as 53–80 μm before the experiment, and vibrators and stepping motors were used in particle feeding system. Low feeding rate 1.9 g/h ensured a dispersed not agglomerated char particle flame downstream plat flame burner in the reactor without any particle interaction. There are two typical methods to define the ignition point distance. One is the position corresponding to 20% of the maximum lights signal intensity in flame initial part, and the other is the corresponding distance point of the maximum slope of signal profile. Both the methods have similar results on the ignition delay in the tests. For consistency and statistical convenience [38], the former method is adopted for OH* and CH* signals and the later method is employed for temperature profiles measured by three-color optical pyrometer.

When the laser excite the OH* in the pyrolyzed char flame to induce fluorescence, it also excite the OH* in the CH4 flat-flame. However, OH* fluorescence intensity from the CH4 flat-flame were very weak compared with pyrolyzed char flame OH-PLIF images. So, CH4 flat-flame OH-PLIF signal can be eliminated from pyrolyzed char flame OH-PLIF images together with OH* spontaneous emission and laser scattering


Energy & Fuels 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

signals as background noise when pyrolyzed char was injected in. The spontaneous light of OH* and laser scattering effect may have noise effects on OH LIF image. To avoid OH* spontaneous noise, a Semrock 310-nm central band-pass filter and Schott UG11 filter were equipped to remove the carbon yellow–green noise. When the laser excites OH* in the PB char flame, it also has a laser scattering effect. Using another laser wavelength off from OH* exciting wavelength to repeat measurement, it was found that the effects of the laser scattering and OH* spontaneous radiation intensity were very weak compared to fluorescence signals of OH* in the PB char flame. The above three noises were eliminated from the OH* fluorescence signal as background noise for more reliable and accurate fluorescence intensity.

Fig.4. Ignition delay distance of PB char particle streams at different ambient temperatures

Fig.4 shows the OH* intensity curve of the PB char at different temperatures. The zero point of the x-axis is the nozzle exit position of the char feeder tube. The intersection abscissa of the curve and 20% of the signal maximum is the ignition delay distance. When temperature raised from 1600K to 1800K, the maximum OH signal intensity


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increased from ~13000 AU to 15000 AU and ignition delay distance decreased ~55%, from 12.32mm to 5.53mm. AU represents fluorescence intensity, which shows intensity of flame is strengthened with the increasing of ambient temperature.

According to the thermal explosion theory [43], the autoignition time of PB particles is closely related to the reactivity of the fuel–oxidant mixture. Fig.5 shows the OH* signal intensity at different O2 concentrations at 1600K. Increased O2 concentration leads to a steeper OH* intensity curve and more intense combustion. The apparent enhancement of the combustion with higher oxygen content probably results from two factors: the closer proximity of the volatile flame to the coal particle and higher char surface temperature. As the O2 concentration increases from 20% to 30%, the peak value of the OH* signals increases greatly, more than twice of 5%O2, 10%O2 cases, which indicates that O2-enriched atmosphere enhances the PB char combustion intensity significantly. The combustion tests of SH coal stream showed similar behaviors, which are consistent with Murphy and Bejarano study [4, 5]. However, the violent burst-like flames of the PB char at 20% and 30% O2 concentration are more obvious than SH coal.

An ICMOS camera has employed an image intensifier to provide high-magnification for flame CH* chemiluminescence signals. Different exposure times are set to maximize the CH* signal without saturation for all cases. Fig.6 shows the effects of O2 concentration and temperature on the PB char ignition delay distance and CH* peak value by CH* chemiluminescence measurement. As the temperature or O2


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concentration increases, ignition delay distance decreases. When the temperature increases, it will lead to higher heating rate and higher fuel reaction rate to enhance combustion intensity of PB char and O2. Therefore, the ignition point moves upstream, whereas the ignition delay distance decreases. The relation between the onset of the ignition and O2 concentration in local mixture is not simply linear. A slight change of O2 concentration in the local mixture causes a significant change of ignition point. This is verified by the measured OH* signal intensity distribution (fig.5). The apparent enhancement of CH* peak value with higher O2 concentration indicates that the combustion rate is significantly enhanced and ignition delay distance is greatly shorten. Thus O2 concentration increasing apparently plays more important role on flame ignition distance and combustion intensity than ambient temperature.

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OH signal intensity (AU)

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Oxygen concentration (%)

Fig.5. OH signal intensity of PB char at

Fig.6. Ignition delay distance and CH* peak

different O2 concentrations

value of PB char by CH* chemiluminescence (Average of 1000 pictures, ε

Experimental study on ignition and combustion characteristics of

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