Experimental Study on the Spectroscopy of Opposed Impinging Diesel

Mar 8, 2017 - This paper applies spectral diagnostics (SD) to study the characteristics of diesel flames in opposed burners and monitor the operation ...
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Experimental Study on the Spectroscopy of Opposed Impinging Diesel Flames Based on a Bench-Scale Gasifier Chonghe Hu, Yan Gong, Qinghua Guo,* Yifei Wang, and Guangsuo Yu* Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, Shanghai Engineering Research Center of Coal Gasification, East China University of Science and Technology, Shanghai 200237, People’s Republic of China ABSTRACT: This paper applies spectral diagnostics (SD) to study the characteristics of diesel flames in opposed burners and monitor the operation of an entrained-flow opposed multi-burner (OMB) gasifier. On the basis of a bench-scale OMB gasifier, the spectral emissions of opposed impinging diesel flames were obtained for different planes and equivalence ratios. The emission lines of OH*, CH*, C2*, Na*, Ar*, and K* radicals were found in diesel flame, and OH* was considered as a good indicator of the impinging flame height. The time-averaged and time-dependent OH* emission intensities can characterize the reaction zone and pulsation magnitude of the flame. The OH*, CH*, and C2* emission intensities of two- and four-burner impinging flames under different [O/C]e were analyzed, indicating that the strengthening effects of four-burner impinging on chemical reactions are superior to those of two-burner impinging. Moreover, whether varying the O2 velocity or diesel flow rate, the OH*/C2* intensity ratio in the impinging region is the most suitable to estimate [O/C]e for the OMB gasifier. The SD method was confirmed as a useful way to derive and monitor the flame temperature in an OMB gasifier. al.17,18 studied the chemiluminescence and structure characteristics of opposed impinging diffusion flames. On the basis of the bench-scale OMB gasifier and the optical measurement system, we systematically investigated the spectral emission characteristics in opposed impinging diesel flames. Diesel was selected as feedstock because a diesel flame is relatively clean and the obtained spectral data are more accurate. It can be better to study the impinging flame characteristics. Moreover, diesel is widely used in a combustor, an engine, a boiler, etc. The findings can be applicable to these diesel devices. Furthermore, diesel is also a kind of hydrocarbon; hence, some conclusions can be extended to a CWS flame of an OMB gasifier. In this work, the spectral emission lines of excited radicals and the peak emission intensities under different conditions were analyzed. The impinging flame height, reaction region, and equivalence ratio were effectively characterized using chemiluminescence characteristics. The feasibility of monitoring the diesel impinging flame temperature by use of SD was also estimated.

1. INTRODUCTION Coal gasification is the core technology in clean and efficient utilization of coal. Entrained-flow gasification technology can achieve high pressure and large capacity, and it is one of the most efficient ways to produce syngas.1,2 The entrained-flow opposed multi-burner (OMB) gasification technology has been widely used in recent years. More than 120 gasifiers were licensed by over 40 companies in the world.3 An impinging flame was used in the OMB gasifier, and it was formed by two four opposed burners in the same plane. The utilization of an impinging flame can enhance the mixing and mass transfer and improve the residence time and carbon conversion rate.4 Hence, much research in a bench-scale OMB gasifier has been reported. Fan et al.5 studied the impinging flame height and pulsation frequency. Gong et al.6,7 studied the behavior and deposition characteristics of coal particles. Yu et al.8,9 investigated the two-dimensional temperature distributions of diesel flames and coal−water slurry (CWS) flames. However, the study on the spectroscopic characteristics of an impinging flame in an OMB gasifier was still rare. Moreover, on the basis of spectral diagnostics (SD), the spectral emission characteristics can provide much information on flames10 and can be used to monitor and control the operation of practical equipment.11 Docquier and Candel12 reviewed the adaptability of the diagnostic technique for combustion control. Parameswaran et al.13 presented the results of temperature measurements through flame emission spectroscopy (FES) and confirmed that FES is more useful than a thermocouple (TC) for monitoring a gasifier. Zhang et al.14 studied the twodimensional CH* distributions of a diesel flame in a benchscale gasifier, and the syngas concentration can be evaluated using CH* peak intensity. Romero et al.15 provided a real-time monitoring method for combustion stoichiometry and temperature in a glass furnace based on a spectrometer system. Besides, the spectroscopy study on the gasoline combustion process was conducted by Pastor et al.16 Song and Zhang et © 2017 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Experimental Apparatus. Panels a and b of Figure 1 are the vertical and horizontal section sketchs of the bench-scale OMB gasifier, respectively. The gasifier consists of a gasification chamber and quenching chamber. The inner diameter of the refractory bricks in the gasification chamber is 300 mm, and the distance between the impinging plane (0 mm) and top refractory bricks is 600 mm. Two pairs of burners were installed oppositely, with 90° between each other, as shown in Figure 1. Diesel is transported into the central channel of the burner by gear pumps, and oxygen is supplied into the annular channel from a Dewar tank. A platinum−rhodium TC (B type) is mounted at the impinging plane to monitor the temperature of the gasifier. The impinging flame is visualized by a high-resolution Received: December 6, 2016 Revised: March 3, 2017 Published: March 8, 2017 4469

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Figure 1. Schematic diagram of the bench-scale OMB gasifier. −200 mm (below the impinging plane). In Figure 1, in the horizontal section, the ports are located with the angle of 45° to adjacent burners. The field of view (FOV) of the probe was 25°; therefore, the detection area is a circular area in the impinging region with the diameter of approximately 60 mm. Moreover, the cooling jacket was also designed to sufficiently cool and purge the probe through cooling water and purging gas (argon), respectively. The HR2500+ spectrometer can achieve a high spectral resolution (0.15 ± 0.01 nm) and a wide spectral region (200−900 nm). The raw intensity data collected by the spectrometer should be corrected as a

charge-coupled device (CCD) camera (JAI BB-500CL) coupled with a high-temperature endoscope (CESYCO, 60° field of view). The endoscope is protected by the cooling jacket that included cooling water and purging gas (argon). The optical signals were measured by an optical fiber and a HR2500+ spectrometer (Ocean Optics, Inc.). The fiber-optic probe can be inserted into five sampling ports, and the probe tip is close to the inner surface of the refractory bricks. As shown in Figure 1, in the vertical section, five sampling ports are located at 0 mm (the impinging plane), 100 and 200 mm (above the impinging plane), and −100 and 4470

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Energy & Fuels Table 1. Properties of Diesel molecular formula

relative molecular mass

density at 298 K (kg m−3)

caloric value (kJ kg−1)

ignition point (°C)

C (wt %)

H (wt %)

O (wt %)

CxHy

190−220

840

44400

220

86.2

13.7

0.1

Table 2. Operating Conditions of Each Burner varying diesel flow rate

varying O2 velocity condition number

[O/C]e

1 2 3 4 5 6 7

0.6 0.7 0.8 0.9 1.0 1.1 1.2

diesel flow rate (kg/h)

O2 flow rate (Nm3/h)

O2 velocity (m/s)

condition number

[O/C]e

2.00

2.90 3.38 3.86 4.34 4.83 5.31 5.79

78 91 104 117 130 143 156

8 9 10 11 12 13 14

0.6 0.7 0.8 0.9 1.0 1.1 1.2

O2 flow rate (Nm3/h)

O2 velocity (m/s)

diesel flow rate (kg/h)

4.46

120

3.08 2.64 2.31 2.06 1.85 1.68 1.54

result of some signal losses caused by some attenuation. A standard light source (deuterium−tungsten halogen lamp, Ocean Optics, Inc.) was used for the calibration of the optical system. The detailed calibration principles were described by Zhang et al.19 2.2. Experimental Conditions. The bench-scale CWS OMB gasifier was operated under atmospheric pressure. The properties of diesel are given in Table 1. The operating conditions were listed in Table 2. The equivalence ratio ([O/C]e) can be changed by varying the O2 velocity (diesel flow rate was set as 2.00 kg/h) or varying the diesel flow rate (O2 velocity was set as 120 m/s), so that both ways were considered and analyzed in section 3. [O/C]e is defined by the formula below

[O/C]e =

[O/C]a [O/C]s

(1)

where [O/C]a is the actual O/C molar ratio calculated on the basis of the feeding amount of diesel and oxygen and [O/C]s is the stoichiometric O/C molar ratio. It should be mentioned that 4 mol of hydrogen atom in diesel were regarded as 1 mol of carbon atom for the calculation of [O/C]e.

3. RESULTS AND DISCUSSION 3.1. Characteristics of Spectral Emissions in Different Planes. 3.1.1. Spectral Emission Lines in Two- and FourBurner Impinging Diesel Flames. When condition 10 is taken as an example, the spectral emission lines of the diesel impinging flame at different planes were shown in Figure 2. The emission peaks at 309 and 431 nm arise from OH* and CH*, respectively. The emission peaks at 463 and 516 nm are both from C2* emission. The peak at 554 nm could be from C2* emission as well or the Ba* excitation because the diesel contains barium salt as an additive. To confirm the origination of the 554 nm peak, we added enough amount of additional organic barium salt (naphthenic acid barium) to diesel, investigating the effects of added barium salt on 554 nm peak intensity using the same operating conditions. As shown in Figure 3, additional barium salt has less influence on the emission intensity at 554 nm, indicating that the 554 nm peak is not from Ba*. Hence, it can be concluded that the emission peak at 554 nm arises from C2*. Besides, because alkali elements (Na and K) exist in diesel, the emission line at 589 nm is Na* and the double-peak emission lines at 766 and 769 nm are K*. The emission line at 671 nm is Ar* because argon is the purging gas. The continuum background emissions also can be observed in the visible and infrared regions. The above radicals are only detected in the impinging plane, and no emission spectra are detected in the rest of the four

Figure 2. Spectral emission lines of different planes in two-burner impinging flames.

planes, because the impinging flame height (L) cannot reach 100 or 200 mm in the two-burner impinging flame. Moreover, alkali radicals (Na* and K*) and inert gas radicals (Ar*) are excited by thermal excitation as a result of the high flame temperature. Non-metallic radicals (OH*, CH*, and C2*) are formed by chemical excitation that results from the chemical 4471

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Figure 3. Spectral emission lines of the impinging plane in four-burner impinging flames (condition 3).

reactions. This paper mainly focused on the chemiluminescence characteristics of OH*, CH*, and C2* emissions. It should be noted that, when there is no flame, the emissions of hot refractory bricks in the furnace will not influence the spectral emissions of impinging flames. Figure 4 shows the emission spectra of ultraviolet and visible regions in the four-burner impinging flame (under condition 10). In comparison to the two-burner impinging flame, spectral emissions were detected in any plane, because the impinging flame height reached ±100 and ±200 mm. Especially, OH* emission lines (309 nm) and continuum background emissions can be measured in different planes, whereas CH* (431 nm) and C2* (463, 516, and 554 nm) emission lines can be measured only in the impinging plane, because CH* and C2* radicals are superimposed by the intense background emissions in L = ±100 and ±200 mm planes. Hence, OH* can be considered as a good indicator of the impinging flame height. To obtain the actual intensities of OH*, CH*, and C2*, the measured intensities should be corrected as a result of the interference of background emissions. In diesel diffusion flames, the background emissions mainly include the CO2* emission bands at 310−600 nm and the blackbody emissions emitted by soot production. Notice that OH* needs no correction because it is free from both CO2* and blackbody emissions, which is also the reason why OH* can be used to characterize the impinging flame height. However, the background emissions have to be subtracted from the measured CH* and C2* intensities based on eq 2, and a typical subtraction method mentioned in ref 20 was used to correct the emission intensities in the current work *

Iλcorrection = Iλmeasurement − (IλCO2 + Iλblackbody )

Figure 4. Spectral emission lines of different planes in four-burner impinging flames. (R2)

H + O2 → O + OH*

(R3)

From Figure 5a, for varying the O2 velocity, the OH* intensity has a maximum at [O/C]e = 0.7 in different planes. When [O/ C]e < 0.7, the OH* intensity is enhanced with increasing [O/ C]e. Because increasing O2 promotes reactions R1−R3, more OH* is generated. Increasing the O2 velocity improves atomization effects of diesel, making more diesel react with O2 in the impinging region. However, when [O/C]e > 0.7, the OH* intensity decreases with increasing [O/C]e. Continuing to increase the O2 velocity has less effect on diesel atomization but causes diesel and O2 to react mainly near the burner exits, and then less reactions occur in the impinging region. Thus, the OH* intensity decreases with [O/C]e. As shown in Figure 5b, for varying the diesel flow rate, the OH* intensity in the impinging plane decreases with increasing [O/C]e. For one thing, decreasing the diesel flow rate leads to the reduction of hydrocarbons in reactions R1 and R2, preventing the formation of OH*. For another, less diesel flow to the impinging region leads to the decrease of the OH* intensity. Moreover, the intensity trends with [O/C]e in other planes are similar to that in the impinging plane under both varying the O2 velocity and diesel flow rate.

(2)

where I means emission intensity and λ means wavelength. 3.1.2. OH* Emission Intensities in Four-Burner Impinging Diesel Flames. Because OH* is treated as the indicator of the impinging flame height, the time-averaged and time-dependent OH* intensity in different planes was investigated, as shown in Figures 5 and 6, respectively. Moreover, for hydrocarbon flames, it is generally considered that OH* radicals are formed via the routes21,22 below. CH + O2 → CO + OH*

HCO + O → CO + OH*

(R1) 4472

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rate. This indicates that the impinging region is the core reaction zone. The OH* intensities in L = ±100 mm planes are higher than those in L = ±200 mm planes, showing that the vertical jet region is also an important reaction zone and the reaction intensity decreases with the propagation of upward and downward streams. When condition 3 is taken as an example, the timedependent OH* intensities are shown in Figure 6. In the impinging plane, the OH* intensity fluctuation is the largest, indicating that the flame pulsation is the strongest in the impinging region. In L = ±100 mm planes, the OH* intensity fluctuations are the smallest, indicating that the flames in the vertical jet region are relatively stable. With the development of the vertical jet flame, the OH* intensity fluctuations in L = ±200 mm planes become larger, because flames close to the end have a stronger pulsation and become more unstable. Moreover, the fluctuation magnitude in the L = 100 mm plane is similar to that in the L = −100 mm plane, and the L = 200 mm plane and L = −200 mm plane are similar as well in fluctuation magnitude. From the perspective of mean intensity shown by the horizontal lines in Figure 6, the mean OH* intensities are 763 and 760 in the L = 100 mm plane and L = −100 mm plane, respectively, and 737 and 732 in the L = 200 mm plane and L = −200 mm plane, respectively. Furthermore, the OH* intensities under different conditions in Figure 5 illustrate that they is also similar between the L = 100 mm plane and L = −100 mm plane and between the L = 200 mm plane and L = −200 mm plane. Therefore, through the analysis of fluctuation magnitude and mean intensity, it can be inferred that the upward and downward streams are similarly symmetrical. 3.2. Characteristics of Chemiluminescence Emissions in the Impinging Plane. 3.2.1. Emission Intensities of Twoand Four-Burner Flames in the Impinging Plane. Because the impinging region is the core reaction zone, the chemiluminescence emissions of two- and four-burner flames in the impinging plane were discussed in this section. Besides, the C2* emission at 554 nm was chosen because the emission peak is clearly observable and the intensity is the strongest. All C2* in section 3.2 refer to the spectral emission at 554 nm. In comparison to time-averaged OH* intensity trends, the timeaveraged CH* and C2* intensities of four-burner impinging have the same trends. For varying the O2 velocity, CH* and C2* intensity peaks at [O/C]e = 0.7. For varying the diesel flow rate, the intensities keep decreasing with [O/C]e. Generally, CH* and C2* in hydrocarbon flames are mainly formed through the following pathways:23,24

Figure 5. Time-averaged OH* intensity in different conditions and planes.

Figure 6. Time-dependent OH* intensity in different planes under condition 3.

C2 + OH → CH* + CO

(R4)

C2H + O → CH* + CO

(R5)

C2H + O2 → CH* + CO2

(R6)

CH 2 + C → H 2 + C2*

(R7)

C2 H + H → H 2 + C2 *

(R8)

Analogous to the analysis of OH* intensity trends in section 3.1.2, the increase of CH* and C2* intensities is due to the promotion impacts on reactions or the improvement of atomization effects with increasing O2 velocity. The decrease with [O/C]e results from more diesel reacting with O2 near the burner exits instead of the impinging region when continuing to increase the O2 velocity or decrease the diesel flow rate.

The opposed impinging flames can be divided into the jet region, impinging region, and vertical jet region. The vertical jet region includes upward and downward streams. From Figure 5, it is also found that the OH* intensities in the impinging plane are the highest for both varying the O2 velocity and diesel flow 4473

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Energy & Fuels Moreover, the OH*, CH*, and C2* intensities of two-burner impinging have the same trends as four-burner impinging. On the basis of the theoretical analysis, the amount of diesel and O2 of four-burner impinging doubles that of two-burner impinging; therefore, the emission intensities of four-burner impinging could be presumed to also double those of twoburner impinging. However, from Figures 7, 8, and 9, it is

Figure 8. Time-averaged CH* intensities in the impinging plane.

monotonic relations with [O/C]e, while the OH*/C2* ratio exhibits exponential dependence upon [O/C]e. Interestingly, for varying the diesel flow rate, OH*/C2* also exhibits exponential dependence upon [O/C]e and OH*/CH* and CH*/C2* ratios exhibit non-monotonic relations with [O/C]e. The relationships between the OH*/C2* ratio and [O/C]e can be fitted by the following curves:

Figure 7. Time-averaged OH* intensities in the impinging plane.

OH* = exp(1.077 − 1.066[O/C]e + 1.738[O/C]e 2 ) C2 *

found that OH*, CH*, and C2* emission intensities of fourburner impinging are actually stronger than double intensities of two-burner impinging under different conditions, respectively. Impinging flames can make more reactant gather in the impinging region, improving the fuel residence time, strengthening the chemical reactions. From the viewpoint of radical intensities, the strengthening effects of four-burner impinging on chemical reactions are superior to those of twoburner impinging. 3.2.2. Chemiluminescence Emissions in the Impinging Plane for [O/C]e Measurements. In the present study, the chemiluminescence emissions in the impinging region are only dependent upon [O/C]e; therefore, the relationships between them were investigated in this section. Figures 10 and 11 show the intensity ratios under different conditions, and the error bars represent the fluctuations of different ratios. For varying the O2 velocity, OH*/CH* and CH*/C2* ratios exhibit non-

for varying the O2 velocity

(3)

OH* = exp(1.875 − 2.267[O/C]e + 2.113[O/C]e 2 ) C2 * for varying the diesel flow rate

(4)

The fitting degrees of eqs 3 and 4 are 0.977 and 0.991, respectively. According to eqs 3 and 4, for the given OH*/C2* ratios, the uncertainties on [O/C]e measurements are within 8 and 5%, respectively. Therefore, the OH*/C2* intensity ratio in the impinging region is most suitable to estimate [O/C]e for the OMB gaisifier. 3.3. Flame Temperature in the Impinging Region. SD is a non-intrusive method to measure the flame temperature. In this section, the diesel flame temperatures in the impinging 4474

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Figure 9. Time-averaged C2* intensities in the impinging plane.

region were calculated through two-color pyrometry, and the feasibility of applying SD to temperature measurements of the OMB gasifier was also assessed. The spectral radiation of refractory bricks has no influence on that of the diesel flame, because the data were collected when the temperature of refractory bricks was quite low. It can be confirmed by Figure 2, in which the spectral radiation of refractory bricks was almost non-existent. The principle of the two-color method is based on Planck’s law, which describes the relationship between the emission intensity and temperature for a blackbody Ib(λ , T ) =

5

λ (e

C1 C 2 / λT

− 1)

(5)

2

where C1 = 2hc , C2 = hc/k, h is Planck’s constant, k is Boltzmann’s constant, and c is the speed of light. Ib(λ, T) is the blackbody intensity for a given wavelength λ and a temperature T. For an actual body, the spectral emissivity ελ is the ratio of the actual intensity I(λ, T) and Ib(λ, T) for a given wavelength. I(λ , T ) = ελIb(λ , T )

Figure 10. Chemiluminescence intensity ratios with [O/C]e under varying O2 velocities.

(6)

Kλ = Kλ−α

For the diesel flame, ελ can be usually estimated by the empirical correlation below25 ελ = 1 − e−KλL

(8)

where K is the absorption coefficient, L is the length of the optical path, and α is an empirical parameter whose value is

(7) 4475

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Energy & Fuels ⎛1 1⎞ C 2⎜ λ − λ ⎟ ⎝ j i⎠ T ( i , j) = ⎡ ⎛ Ii Cj ⎞ ⎛ 1 − e−Kλj−αL ⎞⎤ ⎛ λi ⎞ ⎢⎣ln⎜⎝Φm Ij Ci ⎟⎠ + 5 ln⎜⎝ λj ⎟⎠ + ln⎝⎜ 1 − e−Kλi−αL ⎠⎟⎥⎦

(9)

where Ii and Ij are the measured emission intensities for λi and λj, respectively, Ci and Cj are the correction factors of the fiberoptic spectrometer using a standard light source (tungsten halogen lamp), and Φm is an attenuation coefficient that mainly depends upon the optical path length. The spectral region for calculation was chosen as 675−760 nm, because it is free from Ar* (at 671 nm) and K* (at 766 and 769 nm) emission lines. In addition, if λi and λj are close enough, the spectral emissivity can be removed from eq 7 to simplify the equation.26 Generally, the wavelength interval |λi − λj| ≥ 20 nm for the diesel combustion flame;27 therefore, 20 nm was selected as the wavelength interval in this paper. To verify the SD temperatures, the TC was used to record the temperatures in the impinging region. As shown in Figure 12, on the premise of no damaging the TC and the flame

Figure 12. TC position in the OMB gasifier.

structure, it was mounted in the impinging plane and inserted as close as possible to the impinging region. The maximum inserting length obtained by experiments was 7 cm to the refractory bricks. The SD and TC temperature measurements under different conditions were presented in Figure 13. For varying the O2 velocity and diesel flow rate, the calculated SD temperatures are much higher than the TC temperatures. Because the TC has a significant distance to the impinging region, the measured temperatures are unavoidably lower than the actual temperatures. However, the temperature changes with different [O/ C]e derived from SD are consistent with those measured by TC. This indicates that SD temperatures can truly reflect the temperature changing trend of the impinging region. As shown in Figure 13, the SD temperatures reach a maximum at [O/C]e = 0.9 and the maximum temperatures are 2150.38 and 2185.52 K for varying the O2 velocity and diesel flow rate, respectively. This phenomenon corresponds with the study28 that the adiabatic flame temperatures could tend to occur at [O/C]e < 1 in the actual combustion. For one side, the generated heat of combustion increases but the specific heat of

Figure 11. Chemiluminescence intensity ratios with [O/C]e under varying diesel flow rates.

1.39 for the diesel flame in the visible region.25 When eqs 3−6 are combined, for λi and λj that are two chosen wavelengths, the flame temperature equation can be obtained as follows: 4476

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4. CONCLUSION An experimental study on the spectral emission characteristics in an opposed impinging diesel flame has been carried out in a bench-scale OMB gasifier. On the basis of SD, the impinging flame characteristics and the application of SD to the OMB gasifier were investigated. Specifically, the main findings can be summarized as follows: (1) The spectral emission lines of OH* (309 nm), CH* (431 nm), C2* (463, 516, and 554 nm), Na* (589 nm), Ar* (671 nm), and K* (766 and 769 nm) were found in the diesel flame. Among these radicals, only OH* can be detected in different planes; hence, OH* can be considered as a good indicator of the impinging flame height. (2) The time-averaged OH* emission intensities are the highest in the impinging plane and decrease with the propagation of upward and downward streams. This indicated that the impinging region is the core reaction zone and the vertical jet region is also an important reaction zone. (3) Time-dependent OH* intensity fluctuation is the largest impinging plane, and the fluctuations in L = ±100 mm planes are the smallest and become larger in L = ±200 mm planes. The results can reflect the flame pulsation magnitude in different planes. Through the analysis of intensities and fluctuations, it can be inferred that the upward and downward streams are similarly symmetrical. (4) With increasing [O/C]e (varying the O2 velocity), OH*, CH*, and C2* intensities have a maximum at [O/C]e = 0.7. With increasing [O/C]e (varying the diesel flow rate), the intensities always decrease. Moreover, the intensities of fourburner impinging are actually stronger than double intensities of two-burner impinging under different conditions, indicating that the strengthening effects of four-burner impinging on chemical reactions are superior to those of two-burner impinging. (5) The chemiluminescence emissions were used to characterize [O/C]e. Whether varying the O2 velocity or diesel flow rate, the OH*/C2* intensity ratio in the impinging region exhibits an exponential dependence upon [O/C]e. Hence, we consider that the OH*/C2* ratio is the most suitable to estimate [O/C]e for the OMB gasifier. (6) SD was applied to derive the flame temperatures in the OMB gasifier. The SD temperature presented a maximum at [O/C]e = 0.9, and the maximum temperatures are 2150.38 and 2185.52 K for varying the O2 velocity and diesel flow rate, respectively. Moreover, in comparison to the temperature changes measured by TC and the results from the previous literature, the SD method is useful to derive and monitor the flame temperature in an OMB gasifier.

Figure 13. SD and TC temperature measurements of the impinging region.

combustion products decreases with increasing [O/C]e. For another, the diesel cannot burn completely as a result of burner atomization effects and other influential factors. Furthermore, the values of the temperature also correspond with the previous study about the diesel flame temperature in the OMB gasifier. Yu et al.8,9 investigated the temperature distribution using the filtered back-projection method. They found that the temperature range is 1923−2373 K under gasification conditions, which is similar to our results in Figure 13. Gong et al.29 found that the temperature of the impinging region reaches 1950 K using the two-color method, when the diesel flow rate is 2.0 kg/ h and the O2 flow rate is 3.0 Nm3/h (approximately to [O/C]e = 0.6 under varying O2 velocities in our work). The SD temperature of the impinging region is 1933.71 K at [O/C]e = 0.6, which is only 16.29 K lower than that in the literature. Therefore, it can be confirmed that the SD method is useful to derive the flame temperature in an OMB gasifier and also monitor the temperature change during the operation of an OMB gasifier.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Chonghe Hu: 0000-0003-0002-5075 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

This research was supported by the National Natural Science Foundation of China (Grants 51406056 and 21676091) and the Fundamental Research Funds for the Central Universities (Grant 222201414030). 4477

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(22) Benvenutti, L. H.; Marques, C. S. T.; Bertran, C. A. Chemiluminescent emission data for kinetic modelling of ethanol combustion. Combust. Sci. Technol. 2004, 177, 1−26. (23) Nori, V. N.; Seitzman, J. M. CH* chemiluminescence modeling for combustion diagnostics. Proc. Combust. Inst. 2009, 32, 895−903. (24) Smith, G. P.; Park, C.; Schneiderman, J.; Luque, J. C2 Swan band laser-induced fluorescence and chemiluminescence in low-pressure hydrocarbon flames. Combust. Flame 2005, 141, 66−77. (25) Zhao, H.; Ladommatos, N. Optical Diagnostics for Soot and Temperature Measurement in Diesel Engines. Prog. Energy Combust. Sci. 1998, 24, 221−255. (26) Keyvan, S.; Rossow, R.; Romero, C.; Li, X. Comparison between visible and near-IR flame spectra from natural gas- fired furnace for blackbody temperature measurements. Fuel 2004, 83, 1175−1181. (27) Fu, T.; Wang, Z.; Cheng, X. Temperature Measurements of Diesel Fuel CombustionWith Multicolor Pyrometry. J. Heat Transfer 2010, 132, 051602. (28) Turns, S. R. An Introduction to Combustion: Concepts and Applications; McGraw-Hill: New York, 1996. (29) Gong, Y.; Guo, Q.; Liang, Q.; Zhou, Z.; Yu, G. ThreeDimensional Temperature Distribution of Impinging Flames in an Opposed Multiburner Gasifier. Ind. Eng. Chem. Res. 2012, 51, 7828− 7837.

REFERENCES

(1) Mondal, P.; Dang, G. S.; Garg, M. O. Syngas production through gasification and cleanup for downstream applications - Recent developments. Fuel Process. Technol. 2011, 92, 1395−1410. (2) Tremel, A.; Haselsteiner, T.; Kunze, C.; Spliethoff, H. Experimental investigation of high temperature and high pressure coal gasification. Appl. Energy 2012, 92, 279−28. (3) Guo, Q. Proceedings of the 2016 Gasification and Syngas Technologies Conference; Vancouver, British Columbia, Canada, Oct 16−19, 2016. (4) Yu, Z. Multi-burner Gasification Reactor for Gasification of Slurry or Pulverized Hydrocarbon Feed Materials and Industry Applications Thereof. U.S. Patent 20080134578 A1, June 12, 2008. (5) Fan, P.; Gong, Y.; Zhang, Q.; Guo, Q.; Yu, G. Experimental Study of the Impinging Flame Height in an Opposed Multi-burner Gasifier. Energy Fuels 2014, 28, 4895−4904. (6) Gong, Y.; Yu, G.; Guo, Q.; Zhou, Z.; Wang, F.; Liu, Y. Experimental study on particle characteristics in an opposed multiburner gasifier. Chem. Eng. Sci. 2014, 117, 93−106. (7) Gong, Y.; Yu, G.; Guo, Q.; Zhou, Z.; Wang, F.; Liu, Y. Experimental study of the particle deposition characteristics in an entrained flow gasifier. Chem. Eng. Sci. 2015, 138, 291−302. (8) Yan, Z.; Liang, Q.; Guo, Q.; Yu, G.; Yu, Z. Experimental investigations on temperature distributions of flame sections in a bench-scale opposed multi-burner gasifier. Appl. Energy 2009, 86, 1359−1364. (9) Yu, G.; Yan, Z.; Liang, Q.; Guo, Q.; Zhou, Z. The investigations of temperature distributions in an opposed multi-burner gasifier. Energy Convers. Manage. 2011, 52, 2235−2240. (10) Orain, M.; Hardalupas, Y. Effect of fuel type on equivalence ratio measurements using chemiluminescence in premixed flames. C. R. Mec. 2010, 338 (5), 241−254. (11) Ballester, J.; García-Armingol, T. Diagnostic techniques for the monitoring and control of practical flames. Prog. Energy Combust. Sci. 2010, 36, 375−411. (12) Docquier, N.; Candel, S. Combustion control and sensors: a review. Prog. Prog. Energy Combust. Sci. 2002, 28, 107−150. (13) Parameswaran, T.; Hughes, R.; Gogolek, P.; Hughes, P. Gasification temperature measurement with flame emission spectroscopy. Fuel 2014, 134, 579−587. (14) Zhang, Q.; Gong, Y.; Guo, Q.; Song, X.; Yu, G. Experimental Study on CH* Chemiluminescence Characteristics of Impinging Flames in an Opposed Multi-Burner Gasifier. AIChE J. 2016, 7, 405− 410. (15) Romero, C.; Li, X.; Keyvan, S.; Rossow, R. Spectrometer-based combustion monitoring for flame stoichiometry and temperature control. Appl. Therm. Eng. 2005, 25, 659−676. (16) Pastor, J. V.; García-Oliver, J. M.; García, A.; Micó, C.; Durrett, R. A spectroscopy study of gasoline partially premixed compression ignition spark assisted combustion. Appl. Energy 2013, 104, 568−575. (17) Zhang, T.; Yu, G.; Guo, Q.; Wang, F. Experimental Study on the Characteristics of Impinging Reaction Region with OH* Chemiluminescence in Opposed Impinging Diffusion Flames. Energy Fuels 2013, 27, 7023−7030. (18) Song, X.; Guo, Q.; Hu, C.; Gong, Y.; Yu, G. OH* Chemiluminescence Characteristics and Structures of the Impinging Reaction Region in Opposed Impinging Diffusion Flames. Energy Fuels 2016, 30, 1428−1436. (19) Zhang, T.; Guo, Q.; Liang, Q.; Dai, Z.; Yu, G. Distribution Characteristics of OH*, CH*, and C2* Luminescence in CH4/O2 Coflow Diffusion Flames. Energy Fuels 2012, 26, 5503−5508. (20) Hu, C.; Gong, Y.; Guo, Q.; Song, X.; Yu, G. An experimental study on the spectroscopic characteristics in coal-water slurry diffusion flames based on hot-oxygen burner technology. Fuel Process. Technol. 2016, 154, 168−177. (21) Haber, L. C.; Vandsburger, U. A global reaction model for OH* chemiluminescence applied to a laminar flat-flame burner. Combust. Sci. Technol. 2003, 175, 1859−1891. 4478

DOI: 10.1021/acs.energyfuels.6b03239 Energy Fuels 2017, 31, 4469−4478