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Experimental study on the flame propagation characteristics of heavy oil oxy-fuel combustion
Journal of the Energy Institute xxx (xxxx) xxx
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Experimental study on the flame propagation characteristics of heavy oil oxy-fuel combustion Zhiqiang Wang, Yan Xiong, Xingxing Cheng*, Ming Liu School of Energy and Power Engineering, Shandong University, Jinan, Shandong Province, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 October 2018 Received in revised form 21 January 2019 Accepted 21 January 2019 Available online xxx
This paper studied the flame propagation characteristics of heavy oil oxy-fuel combustion in ignition and stable combustion. The results showed that the ignition process could be divided into three stages: the pro-ignition, mid-ignition and end-ignition. The pro-ignition, the fire core generated and evolved into spherical; the mid-ignition, the spherical fire core gradually turn into tapered structure; the end-ignition, the flame tapered structure disappeared and turn into a relative stable columnar structure. By calculating the flame propagation velocities, we found that in the same combustion atmosphere, the flame propagation velocity in 29% O2 was higher than that in 21% O2; in the same O2 concentration, the flame propagation velocity in O2/N2 atmosphere was higher than that in O2/CO2. During the stable combustion, we observed the local flame structure extinguished, distorted and grew. © 2019 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: Heavy oil Oxy-fuel combustion Turbulence Flame structure
1. Introduction Heavy oil describes the remaining crude oil after the extraction of light oil, and possesses high molecular weights, viscosities, and asphaltene contents, as well as containing ash, being difficult to burn, and being highly polluting when combusted. However, it is cheap, and there are more than twice the reserves of heavy oil as of light oil [1e3], so it has a prospect if used correctly. Oxy-fuel combustion technology means that the combustion atmosphere of air is replaced by a mixture gas of O2 and CO2, with a general O2 concentration of more than 21%, and the CO2 concentration in flue gas can reach 90e95% after dewatering and drying [4e11]. If heavy oil is applied as a fuel oil combined with oxy-fuel combustion technology to the steam injection boilers of oil field, then not only can realize the resource utilization of heavy oil, increasing combustion efficiency and controlling the pollutants emission, but also a high CO2 concentration can be captured for oil displacing [12e14], making the study of the oxy-fuel combustion characteristics of heavy oil highly significant [15,16]. In the past, scholars have done some researches on oil fuel rich-O2 combustion in O2/N2 or O2/CO2 atmosphere. In numerical simulation, S. Mei et al. [17] simulated the oxy-fuel combustion of oil in glass furnace, the results showed that when the burner was misaligned, the temperature distribution in the furnace was more uniform and also increased the heat transfer rate of the liquid glass. Y.H. Khraisha, V. Pistor, and P. Murugan et al. [18e21] studied the pyrolysis process of oil fuel, they indicated that the pyrolysis process could be divided into three stages: the first was the oil distillation and oxidation in low temperature, the second was the macromolecular decomposition, the third was the burning of residual macromolecules. But the temperature range of the three stages was different when the quality of oil fuel was different. In oxy-fuel combustion, M. Seljeskog et al. [11] investigated the combustion characteristics of heavy oil on a 250 KW boiler, the results showed that the heat exchange combusted in 30% O2/70% CO2 was similar to combusted in air. D. Ronglu [22] studied the rich-O2 combustion characteristics in O2/N2 atmosphere of diesel fuel as well as the thermal characteristics in the furnaces. In the rich-O2 combustion, the burning rates and the flame propagation velocities were higher but the flame lengths were shorter than combusted in air, and the CO concentrations were obviously lower than that in air while the thermal NOx concentrations increased with increasing the O2 concentrations. Although previous researchers have done a lot of researches on the rich-O2 combustion of oil fuel, their researches concentrated on numerical simulation, pyrolysis process, the furnace thermal characteristics, the pollution emission and the flame structures in stable
* Corresponding author. E-mail address: [email protected] (X. Cheng). https://doi.org/10.1016/j.joei.2019.01.011 1743-9671/© 2019 Energy Institute. Published by Elsevier Ltd. All rights reserved.
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combustion [11,23e27]. However, experimental studies on the flame structure of heavy oil combustion are still rare in the literature [28,29]. In this paper, we used a high speed frequency ICCD camera to investigate the flame propagation characteristics on oxy-fuel combustion of heavy oil in both ignition and stable combustion stages. 2. Experimental 2.1. Boiler combustion system The boiler combustion system was mainly composed of an air distribution system and a combustion system, and the schematic is shown in Fig. 1. The O2/N2 atmosphere was a mixture of air/pure-oxygen, and the O2/CO2 atmosphere was pure oxygen mixed with pure carbon dioxide. The air was provided by an air compressor, and the pure oxygen and carbon dioxide were provided from dewars (DPL), which stored liquid oxygen and carbon dioxide with purities of 99.9% at low temperatures. The liquid oxygen or carbon dioxide was vaporized by the vaporizer, and the gas flow was controlled by the mass flowmeters (D07-60B, Sevenstar, China). Before the experiment, the supplied gas pressures in front of the mass flowmeters were reduced to 0.25 MPa by the reducing valves, then the two gases were mixed evenly in the mixer, and finally the mixed gas was allowed into the burner with a pressure of about 0.1 MPa. The combustion system consisted of a horizontal oil-fired boiler, as well as a burner, pipeline, fuel tank, oil filter, oil heater, etc. The boiler was a small horizontal hot-water boiler with a size of 500 800 mm2 (outer diameters length). An observation window was opened on the side and top of the boiler, with a size of 120 600 mm2. The burner model was RIELLO40-G10LC (RIELLO, Italy), with a maximum output of 2.5 GPH (10 kg/h). The nozzle was a solid nozzle (Steinen S) with an output of 1.0 GPH (3.73 kg/h), and the atomization angle was 60 . The oil pump pressure was 1.2 MPa. The loop between oil valve and burner is a oil return pipe. During the experiment, the temperature field was measured using a B-type thermocouple with a temperature error of ±2.1% and a response time of 180 s to reach a stable temperature (calibrated by Shandong Academy of Measurement Sciences, China). Took the air-combustion as an example, the inner diameter of the combustion cylinder D ¼ 100 mm, the density of air at room temperature r ¼ 1.293 kg/m3, the dynamic viscosity coefficient m ¼ 1.73 105 pa.s. The expressions of the average velocity, the Reynolds number and the turbulence intensity of the flow in the tube were shown as follows:
Q ¼ V,A
(1)
rVl m
(2)
Re ¼
I ¼ 0:16,Re 8 1
(3)
In air atmosphere, the total gas flow Q ¼ 646 L/min, then according to the formula (4)e(6), the corresponding velocity was 1.37 m/s, the Reynolds number was 10216, and the turbulence intensity was 5.05%. 2.2. Fuel oil and working conditions The heavy oil for the experiment was a nonstandard fuel oil produced in the Shandong province of China consisting of a mixture of heavy and light oils, with a color of light brown, and the elemental analysis is shown in Table 1. The combustion reactions of hydrocarbon are shown as follow.
n n Cm Hn þ m þ O2 /mCO2 þ H2 O þ DH 4 2
(4)
Fig. 1. Experimental oxy-fuel combustion system.
Please cite this article as: Z. Wang et al., Experimental study on the flame propagation characteristics of heavy oil oxy-fuel combustion, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.01.011
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Table 1 Elemental analysis results and calorific value of the heavy oil. Element
Car
Har
Oar
Nar
Sar
Calorific value(kJ/kg)
Fuel oil (wt.%)
79.9
12.7
5.04
0.36
0.25
41406
S þ O2 ¼ SO2
(5) 3
According to combustion theory, the theoretical oxygen consumption (m /kg) required to completely combust 1 kg of fuel was calculated using the following formula:
VO2 ¼ ð1:866Car þ 5:56Har þ 0:7Sar 0:7Oar Þ=100
(6)
Without regard to the leakage of the boiler, the theoretical oxygen consumption was 135 L/min by calculation according to the Eq. (3), the burner parameter and the fuel elemental analysis results. Before burning, oil fuel was evaporated firstly and then mixed with the combustion-supporting gas O2, and the burning rate depended on the mixing rate between O2 and oil vapors. If supplied with the theoretical O2, part of fuel will not burn completely, so we generally provided more O2 than the theoretical values. In our previous studies, we found that the flame characteristics, the temperature distributions and the pollution emissions in O2/CO2 atmosphere were similar to combust in air, so we compared the flame propagation characteristics of the follow three conditions: 21% O2/79% N2, 29% O2/71% N2, 29% O2/71% CO2, different excess air coefficients a. 3. Results and discussion 3.1. The vertical size of the observation window Fig. 2 was the background image before the heavy oil burning. The two solid yellow lines in the horizontal direction were respectively corresponded the upper and the lower edges of the observation window. The blue solid short line in the vertical direction was the distance between the upper and lower edges of the observation window, the pixel length of the blue line was 203 and the actual length was 105 ± 1 mm, so each unit pixel distance corresponded 0.517 mm. 3.2. The ignition process The burner was high-voltage electric ignition with a voltage reached 8 kv, and the high-voltage electric was alternating current, so there will be an ignition time gap. The high-speed camera has a high shooting frequency (5000 Hz), which is much larger than the alternating current frequency, so it can capture the flame structure change image in the ignition process more completely. As the mixture moves forward as a whole, there will be an ignition gap before the formation of a new nucleus after the ignition process, and the ignition time gap was shown as a narrow dark area in the images, the ignition process was very short. From the images obtained by ICCD high-speed camera, we observed the whole ignition process clearly. For any successful ignition process, it could be divided into three stages according to the change of the flame structures: the pro-ignition, the fire core generated and evolved into spherical; the mid-ignition, the spherical fire core gradually turn into tapered structure; the end-ignition, the flame tapered structure disappeared and turn into a relative stable columnar structure. 3.2.1. The pro-ignition process Fig. 3 was the flame structures evolution in the pro-ignition process, took the first image of the fire core that could be observed as the “initial time” 0, and the interval time between the adjacent images was 0.2 ms. The solid yellow line in Fig. 3b-(1) indicated the exit position of the burner. The heavy oil was mixed with the supply gas after it ejected from the injection nozzle, and the oil-air mixtures were ignited while they were flowing through the high voltage electrode. Since the electric charges of the high-voltage electrode were mainly distributed in a small range near the tip of the electrode, it could be assumed that the ignited oil-gas was a small spherical fire core, then the flame front propagated spherically with the same velocity, and at the same time the fire core also moved downstream with the airflow. Compared the images (1)e(3) of the three conditions in Fig. 3, found that the positions of the fire core are similar, and the volume of the fire core increased gradually. In the three conditions, all the fire cores of (1) were similar to dim scattered agglomerates with an irregular shape, and the fire core of the images (2) and (3) were spherical and they were brighter than (1). The fire core grew slowly in the initial time, and it developed rapidly to a bright sphere after it could be observed at the first time. For a successful ignition, after the fire core leaved the ignited electrode area, whether the fire core could grow continuously depended on the relative values between the exothermic of the fire core and the heat Table 2 Working conditions. NO.
O2 concentration (%)
a
Composition of gas supply (L/min) air
O2
CO2
Total gas flow
A1 B1 B2
29 21 29
1.452 1.05 1.455
0 676 607
196 0 69
480 0 0
676 676 676
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Fig. 2. The vertical size of the observation window.
absorption of the oil-gas mixture around the fire coredonly if the exothermic heat of the fire core was greater than the amount of heat required to evaporate the oil droplets in the oil-gas mixture could the fire core grew and the oil combusted continuously. In generally, the higher the amount of heat released, the faster the oil droplets evaporated as well as the burning rate. During the fire core moved downstream along with the gas flow, the heat released by the fire core diffused rapidly into the unburned oil-gas mixture due to high turbulence intensity, so the temperature around the fire core rose slowly and the oil droplet evaporation rate was also small, which led the fire core to grow slowly before burning in the burner. As shown in Fig. 3a, the first image was the fire core which had been leaving the burner, it was dim and irregular in the shape, however, the fire core rapidly grew into a bright spherical fire core after 0.2 ms. It can be seen that during the formation of the fire core, its initial growth was relatively slow, but it burned drastic and grew rapidly when reached a certain scale. From the images (4)e(8) in Fig. 3a, we can see that the fire core did not remain a solid sphere after it grew to a bright spherically fire core, instead, a black void appeared inside the fire core. As the time evolved, the black void gradually expanded, and the front of the fire core turned into a bright meniscus shape, at the same time, the backend of the fire core turned to be dim. It may be the reason that the discharge ignition of the high-voltage electrodes was not continuously, so there would be a non-lighting time gap with the end of an ignition process, which led to be a spatial gap inside the fire core, and the spatial gap expanded continuously as time evolved until the next ignition process begins. As shown in Fig. 3a-(3) and Fig. 3a-(8), took the fire core as a regular sphere, the centroid pixel coordinates of them were 848 and 836 in the horizontal direction, and the pixel distance between the two was 12, so the actual distance was L2 ¼ 12 0.517 ¼ 6.204 mm, with a time interval t2 ¼ 0.2 ms 5 ¼ 1.0 ms, and the fire core propagation velocity of the centroid was V2 ¼ L2/t2 ¼ 6.204 mm/1.0 ms ¼ 6.204 m/s. By comparing Fig. 3a and b, the fire core volume combusted in 29% O2/71%N2 was obviously larger that in air(21% O2/79%N2), which showed that it was beneficial to heavy oil combustion to increase O2 concentration in the same inert gas atmosphere. By comparing Fig. 3a and c, we could find that in the same O2 concentration, the fire core in O2/N2 atmosphere grew more rapidly than in O2/CO2, which indicated that heavy oil was more easily to burn in O2/N2 than in O2/CO2. From Fig. 3a and c, it was obviously that the fire core of the two were similarly to each other, which indicated that heavy oil combusted in 29% O2/71% CO2 was similarly to that in air. The displacement-velocity in the normal direction of the flame front was defined as the propagation velocity of the flame. As shown in Fig. 4, in order to calculate the flame propagation velocity in the pro-ignition process, we extracted the flame edge by MATLAB to get the flame front locations,the specific operation is as follows: The image was read by imread function, and the original rgb image was transformed into a grayscale image by rgb2gray, and then the gray image was binarized by the domain value function. Then the image contour was scanned to get the flame front locations. Fig. 5 was the locations of the tip of fire core front according to the images in Fig. 3, with the abscissa of time and ordinate of pixel. As shown in Fig. 5, the locations of the fire core in 21% O2/79% N2 was nearly the same as that in 29% O2/71% CO2, which indicated that they had the same rates in growing and evolving, while it grew obviously faster in 29% O2/71%N2 after 0.2 ms. In the time of 0e0.2 ms, the position of the three was nearly the same. From the images of Fig. 3, it could be observed that the fire cores were dim and irregular, which meant a “moderate” burning, and the combustion of the flame had little effects on the flame turbulence as well as its propagation velocity, so the flame propagation velocity mainly depended on gas flow field. During the time that the oil-gas mixture leaving from the ignition electrode to 0 ms, we assumed the gas flow velocity was constant in the horizontal, and the gas flow velocity could be obtained by the displacement-velocity in the normal direction of the flame front:
v¼
ds dt
(7)
vdthe propagation velocity, dsdthe moving distance of the flame-front, dtdthe moving time of the flame-front. 1 pixel unit corresponded to actual distance 0.51724 mm, then the flame propagation velocity was:
v¼
Ds 0:51724 mm Dt
(8)
In the time of 0e0.2 ms:
nA1 ¼ nB1 ¼ nB2 ¼
3 0:51724 mmz7:76m=s 0:2ms
(9)
vA12dthe propagation velocity of flame on the fire core surface in 29% O2/71% CO2,vB11dthe propagation velocity of flame on the fire core surface in21%O2/79%N2,vB13dthe propagation velocity of flame on the fire core surface in 29% O2/71%N2.
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Fig. 3. The flame structures evolution in the pro-ignition process (interval time: 0.2 ms).
In the time of 0.2e2.0 ms:
nA1 ¼ nB1 ¼
40 0:51724 mmz11:49m=s 1:8ms
(10)
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Fig. 4. The flame front extraction by MATLAB.
Fig. 5. The location of the tip of fire core front in pro-ignition process.
nB2 ¼
64 0:51724 mmz18:39m=s 1:8ms
(11)
The results show that the propagation velocity of flame on the fire core surface in 29% O2/71%N2 was obviously larger than that of 21% O2/ 79% N2 and 29% O2/71% CO2. 3.2.2. The mid-ignition process Fig. 6 was the flame structures evolution in the mid-ignition process. As shown in the figure, when the fire core evolved a certain degree, its spherical structure turned to be a taper structure, and the black void inside the fire core was gradually filled with the next fire core, which resulted in hierarchical structures in the flame. Comparing Fig. 6b and c, in the mid-ignition stage, the flame in 21% O2/79% N2 evolved similarly to that in 29% O2/71% CO2, but the former was brighter than the later. As the flame continuously evolved with time, the taper top upturned and the flame structure turned to be camber from taper. As shown in Fig. 6a, the first fire core burned about 2 ms after the first time it could be observed, during this period, a new fire core generated inside the old one, and the new fire core also evolved to be a taper structure (Fig. 6a-(3)dFig. 6a-(5)). Both the old taper structure and the new kept for some time to propagate downstream with the gas flow, which was different from the evolution process of in 21% O2/79% N2 and 29% O2/71% CO2. In actual, the results in Fig. 6a showed that the ignition process was not continuously, the gap in the adjacent taper structures could be considered as an ignition time gap. Fig. 7 showed the locations of the tip of fire core front in mid-ignition process. In 29% O2/71% N2, the flame front location had a mutation and lagged behind that in 21% O2/79% N2 and 29% O2/71% CO2. The reason was mainly that the flame front in 29% O2/71% N2 was from the first fire core before 2 ms but from the second fire core after 2 ms, however, it was from the first fire core for the later two, as shown in Fig. 6. In mid-ignition process, the total flame propagation velocity in 29% O2/71% N2 was larger than that in
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Fig. 6. The flame structures evolution in the mid-ignition process (interval time:1.0 ms).
Fig. 7. The location of the top of fire core front in mid-ignition process.
21% O2/79% N2 and 29% O2/71% CO2, but as the fire core further evolved, the velocities of the three were closed, and the value was about 10.60 m/s after 8 ms. During the combustion, the high-voltage ignition electrode was AC discharge, assuming u ¼ 0 while t ¼ 0, then the current and voltage could be expressed as follow:
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Fig. 8. The instantaneous active power.
Table 3 The top taper location of the outermost flame and sub-outer. NO.
6a-4 6a-5 6a-6
Local flame propagation velocity(m/s)
Pixel location Level I
Level II
Pixel distance
830 806 779
845 823 794
15 17 15
10.60
u ¼ Um sin ut
(12)
i ¼ Im cosðut fÞ
(13)
the transient power was:
P ¼ u,i ¼ ðUm sin utÞ,ðIm cosðut fÞÞ ¼ Um Im sin ut cosðut fÞ
(14)
1 P ¼ Um Im cos f sin2 ut Um Im sin f sin 2 ut ¼ pðtÞ þ qðtÞ 2
(15)
in the formula (15), p (t) was active power,
pðtÞ ¼ Um Im cos f sin2 ut ¼
Um Im cos f ð1 cos 2 utÞ 2
(16)
Fig. 9. The flame structures evolution in the end-ignition process(interval time: 2.0 ms).
Please cite this article as: Z. Wang et al., Experimental study on the flame propagation characteristics of heavy oil oxy-fuel combustion, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.01.011
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Set the phase angle of voltage and current was 0, then according to the burner technical parameters the active power p(t) was written as follow:
pffiffiffi pffiffiffi 2Un 2In cos f ð1 cos 2 utÞ ¼ Un In ð1 cos 2 utÞ pðtÞ ¼ 2 pðtÞ ¼ 8000V 16mA ð1 cos 200ptÞ ¼ 128ð1 cos 200 ptÞ ðJ=sÞ
(17) (18)
The ignition of unburned oil-gas mixture need an external energy input, only with enough energy inputted the oil-gas mixture may be ignited (see Table 2). In this study, the external energy was the spark generated by the discharge of the high voltage electrode, and the instantaneous active power was shown in Fig. 8, with a range of 0e256 J/s. When the instantaneous active power was less than a certain value, the oil-gas mixture could not be ignited at the moment, so the actual ignition process was not continuously, and the ignition time gap could be calculated by the gap in the adjacent taper structures in Fig. 6. According to Fig. 6a-(4), (6), the top taper location of the outermost flame (the first fire core, Level I) and sub-outer (the second fire core, Level II) were shown in Table 3. According to Fig. 7, here the local flame propagation velocity was almost a constant of 10.60 m/s, the average pixel distance was 15.67, so the ignition time gap was:
t¼
s 15:67 0:51724 mm ¼ z0:765 ms v 10:60 m=s
(19)
As shown in Fig. 8, in a completely ignition circle (50 Hz, 20 ms), the ignition time gap was (10e0.382 ms, 10 þ 0.382 ms), and the
Fig. 10. The change of the flame structure during stable combustion (interval time: 2.0 ms).
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corresponding instantaneous active power was p(t) ¼ 3.68 J/s, which was similar to the value of gasoline under spark ignition obtained by Ma Jian et al. [30,31].(0e15 mJ/ms). According to the calculation results, both the ignition time gap and its corresponding instantaneous active power were small, so the combustion loss caused by the ignition gaps could be neglected. 3.2.3. The end-ignition Fig. 9 was the flame structures evolution in the end-ignition process. The flame had been fully evolved, at this time the flame had a great effect on the flow field, which increased the turbulence intensity a lot, so the flame taper structure disappeared and the flame presented irregularly. 3.3. The change of the flame structure during stable combustion As shown in Fig. 10, during the stable combustion, the flame shape was irregular, and the flame structure changed with time evolved. Inside the yellow circle of the first image, there was a narrow flame in the upper left, this narrow flame had a tendency to separate from the main flame zone; after 2 ms (the second image), the narrow flame had escaped from the main flame zone and the shape changed to be bend; after 4 ms, the flame changed to be small, and the size was no more than half of the initial one of the first image; the flame could not be distinguished after 8 ms. In a high turbulent flow field, the flame structure changed rapidly, there were mainly two reasons for the changing of the flame structure: on the one hand, some oil-gas mixture in the flame had been exhaust, which presented a local flame extinction; on the other hand, the flame was irregularly jitter, this kind of irregularly jitter not only changed the flame spatial distribution, but also may ignite the remaining oil-gas mixture in the flow field, and the flame structure presented as the distort or growth in local.In further research, we will explore that would the change in the flame structure affect combustion efficiency and emission of pollutants. 4. Conclusion This paper studied the flame propagation characteristics of heavy oil oxy-fuel combustion in ignition and stable combustion. The results showed that the ignition process could be divided into three stages d the pro-ignition, mid-ignition and end-ignition. In the pro-ignition, the fire core generated and evolved into spherical; in the mid-ignition, the spherical fire core gradually turn into tapered structure; in the end-ignition, the flame tapered structure disappeared and turn into a relative stable columnar structure. By calculating the flame propagation velocities, we found that in the same combustion atmosphere, the flame propagation velocity increased with the increase of oxygen concentration; in the same O2 concentration, the flame propagation velocity in O2/N2 atmosphere was larger than that in O2/CO2. During the stable combustion, we observed the local flame structure extinguished, distorted and grew. Acknowledgments This work was supported in part by the National Major Scientific Instruments and Equipment Development Project: Diagnosis of Combustion Process of Heavy Oil O2/CO2 Diffusion Flame (2012YQ04016410). References [1] M. Meratizaman, S. Monadizadeh, A. Ebrahimi, H. Akbarpour, M. 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[26] J. Xie, Combustion Characteristics and Pollutant Emission Characteristics of Biomass Mixed Coal under O2/CO2 atmosphereO2/CO2, Shandong University, 2012. [27] E.J. Miklaszewski, Y. Zheng, S.F. Son, Oxy-fuel combustion: laboratory experiments and pilot scale tests, Fuel 104 (2013) 452e461. [28] Y. Qian, J. Zhan, D. Lai, M. Li, X. Liu, G. Xu, Primary understanding of non-isothermal pyrolysis behavior for oil shale kerogen using reactive molecular dynamics simulation, Int. J. Hydrogen Energy 41 (2016) 12093e12100. [29] J. Gomez, J.P. Mmbaga, R.E. Hayes, M. Toledo, F. Gracia, Modelling hydrogen production by the rich combustion of heavy fuel oil, Int. J. Hydrogen Energy 41 (2016) 17933e17943. [30] Jian Ma, Qiyang Yan, Zishu Yao, et al., Comparative study on ignition process between laser-induced spark ignition and spark ignition, Intern. Combust. Engine Eng. 37 (4) (2016) 113e118. [31] C. Xu, D. Fang, Q. Luo, et al., Comparative study of gasoline laser induced spark ignition and spark ignition combustion process, J. Zhejiang Univ. 16 (10) (2015) 830e838. http://dict.youdao.com/search?q¼%20%20%0D%0A&keyfrom¼fanyi.smartResult.
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Journal of the Energy Institute journal homepage: http://www.journals.elsevier.com/journal-of-the-energyinstitute
Experimental study on the flame propagation characteristics of heavy oil oxy-fuel combustion Zhiqiang Wang, Yan Xiong, Xingxing Cheng*, Ming Liu School of Energy and Power Engineering, Shandong University, Jinan, Shandong Province, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 October 2018 Received in revised form 21 January 2019 Accepted 21 January 2019 Available online xxx
This paper studied the flame propagation characteristics of heavy oil oxy-fuel combustion in ignition and stable combustion. The results showed that the ignition process could be divided into three stages: the pro-ignition, mid-ignition and end-ignition. The pro-ignition, the fire core generated and evolved into spherical; the mid-ignition, the spherical fire core gradually turn into tapered structure; the end-ignition, the flame tapered structure disappeared and turn into a relative stable columnar structure. By calculating the flame propagation velocities, we found that in the same combustion atmosphere, the flame propagation velocity in 29% O2 was higher than that in 21% O2; in the same O2 concentration, the flame propagation velocity in O2/N2 atmosphere was higher than that in O2/CO2. During the stable combustion, we observed the local flame structure extinguished, distorted and grew. © 2019 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: Heavy oil Oxy-fuel combustion Turbulence Flame structure
1. Introduction Heavy oil describes the remaining crude oil after the extraction of light oil, and possesses high molecular weights, viscosities, and asphaltene contents, as well as containing ash, being difficult to burn, and being highly polluting when combusted. However, it is cheap, and there are more than twice the reserves of heavy oil as of light oil [1e3], so it has a prospect if used correctly. Oxy-fuel combustion technology means that the combustion atmosphere of air is replaced by a mixture gas of O2 and CO2, with a general O2 concentration of more than 21%, and the CO2 concentration in flue gas can reach 90e95% after dewatering and drying [4e11]. If heavy oil is applied as a fuel oil combined with oxy-fuel combustion technology to the steam injection boilers of oil field, then not only can realize the resource utilization of heavy oil, increasing combustion efficiency and controlling the pollutants emission, but also a high CO2 concentration can be captured for oil displacing [12e14], making the study of the oxy-fuel combustion characteristics of heavy oil highly significant [15,16]. In the past, scholars have done some researches on oil fuel rich-O2 combustion in O2/N2 or O2/CO2 atmosphere. In numerical simulation, S. Mei et al. [17] simulated the oxy-fuel combustion of oil in glass furnace, the results showed that when the burner was misaligned, the temperature distribution in the furnace was more uniform and also increased the heat transfer rate of the liquid glass. Y.H. Khraisha, V. Pistor, and P. Murugan et al. [18e21] studied the pyrolysis process of oil fuel, they indicated that the pyrolysis process could be divided into three stages: the first was the oil distillation and oxidation in low temperature, the second was the macromolecular decomposition, the third was the burning of residual macromolecules. But the temperature range of the three stages was different when the quality of oil fuel was different. In oxy-fuel combustion, M. Seljeskog et al. [11] investigated the combustion characteristics of heavy oil on a 250 KW boiler, the results showed that the heat exchange combusted in 30% O2/70% CO2 was similar to combusted in air. D. Ronglu [22] studied the rich-O2 combustion characteristics in O2/N2 atmosphere of diesel fuel as well as the thermal characteristics in the furnaces. In the rich-O2 combustion, the burning rates and the flame propagation velocities were higher but the flame lengths were shorter than combusted in air, and the CO concentrations were obviously lower than that in air while the thermal NOx concentrations increased with increasing the O2 concentrations. Although previous researchers have done a lot of researches on the rich-O2 combustion of oil fuel, their researches concentrated on numerical simulation, pyrolysis process, the furnace thermal characteristics, the pollution emission and the flame structures in stable
* Corresponding author. E-mail address: [email protected] (X. Cheng). https://doi.org/10.1016/j.joei.2019.01.011 1743-9671/© 2019 Energy Institute. Published by Elsevier Ltd. All rights reserved.
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combustion [11,23e27]. However, experimental studies on the flame structure of heavy oil combustion are still rare in the literature [28,29]. In this paper, we used a high speed frequency ICCD camera to investigate the flame propagation characteristics on oxy-fuel combustion of heavy oil in both ignition and stable combustion stages. 2. Experimental 2.1. Boiler combustion system The boiler combustion system was mainly composed of an air distribution system and a combustion system, and the schematic is shown in Fig. 1. The O2/N2 atmosphere was a mixture of air/pure-oxygen, and the O2/CO2 atmosphere was pure oxygen mixed with pure carbon dioxide. The air was provided by an air compressor, and the pure oxygen and carbon dioxide were provided from dewars (DPL), which stored liquid oxygen and carbon dioxide with purities of 99.9% at low temperatures. The liquid oxygen or carbon dioxide was vaporized by the vaporizer, and the gas flow was controlled by the mass flowmeters (D07-60B, Sevenstar, China). Before the experiment, the supplied gas pressures in front of the mass flowmeters were reduced to 0.25 MPa by the reducing valves, then the two gases were mixed evenly in the mixer, and finally the mixed gas was allowed into the burner with a pressure of about 0.1 MPa. The combustion system consisted of a horizontal oil-fired boiler, as well as a burner, pipeline, fuel tank, oil filter, oil heater, etc. The boiler was a small horizontal hot-water boiler with a size of 500 800 mm2 (outer diameters length). An observation window was opened on the side and top of the boiler, with a size of 120 600 mm2. The burner model was RIELLO40-G10LC (RIELLO, Italy), with a maximum output of 2.5 GPH (10 kg/h). The nozzle was a solid nozzle (Steinen S) with an output of 1.0 GPH (3.73 kg/h), and the atomization angle was 60 . The oil pump pressure was 1.2 MPa. The loop between oil valve and burner is a oil return pipe. During the experiment, the temperature field was measured using a B-type thermocouple with a temperature error of ±2.1% and a response time of 180 s to reach a stable temperature (calibrated by Shandong Academy of Measurement Sciences, China). Took the air-combustion as an example, the inner diameter of the combustion cylinder D ¼ 100 mm, the density of air at room temperature r ¼ 1.293 kg/m3, the dynamic viscosity coefficient m ¼ 1.73 105 pa.s. The expressions of the average velocity, the Reynolds number and the turbulence intensity of the flow in the tube were shown as follows:
Q ¼ V,A
(1)
rVl m
(2)
Re ¼
I ¼ 0:16,Re 8 1
(3)
In air atmosphere, the total gas flow Q ¼ 646 L/min, then according to the formula (4)e(6), the corresponding velocity was 1.37 m/s, the Reynolds number was 10216, and the turbulence intensity was 5.05%. 2.2. Fuel oil and working conditions The heavy oil for the experiment was a nonstandard fuel oil produced in the Shandong province of China consisting of a mixture of heavy and light oils, with a color of light brown, and the elemental analysis is shown in Table 1. The combustion reactions of hydrocarbon are shown as follow.
n n Cm Hn þ m þ O2 /mCO2 þ H2 O þ DH 4 2
(4)
Fig. 1. Experimental oxy-fuel combustion system.
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Table 1 Elemental analysis results and calorific value of the heavy oil. Element
Car
Har
Oar
Nar
Sar
Calorific value(kJ/kg)
Fuel oil (wt.%)
79.9
12.7
5.04
0.36
0.25
41406
S þ O2 ¼ SO2
(5) 3
According to combustion theory, the theoretical oxygen consumption (m /kg) required to completely combust 1 kg of fuel was calculated using the following formula:
VO2 ¼ ð1:866Car þ 5:56Har þ 0:7Sar 0:7Oar Þ=100
(6)
Without regard to the leakage of the boiler, the theoretical oxygen consumption was 135 L/min by calculation according to the Eq. (3), the burner parameter and the fuel elemental analysis results. Before burning, oil fuel was evaporated firstly and then mixed with the combustion-supporting gas O2, and the burning rate depended on the mixing rate between O2 and oil vapors. If supplied with the theoretical O2, part of fuel will not burn completely, so we generally provided more O2 than the theoretical values. In our previous studies, we found that the flame characteristics, the temperature distributions and the pollution emissions in O2/CO2 atmosphere were similar to combust in air, so we compared the flame propagation characteristics of the follow three conditions: 21% O2/79% N2, 29% O2/71% N2, 29% O2/71% CO2, different excess air coefficients a. 3. Results and discussion 3.1. The vertical size of the observation window Fig. 2 was the background image before the heavy oil burning. The two solid yellow lines in the horizontal direction were respectively corresponded the upper and the lower edges of the observation window. The blue solid short line in the vertical direction was the distance between the upper and lower edges of the observation window, the pixel length of the blue line was 203 and the actual length was 105 ± 1 mm, so each unit pixel distance corresponded 0.517 mm. 3.2. The ignition process The burner was high-voltage electric ignition with a voltage reached 8 kv, and the high-voltage electric was alternating current, so there will be an ignition time gap. The high-speed camera has a high shooting frequency (5000 Hz), which is much larger than the alternating current frequency, so it can capture the flame structure change image in the ignition process more completely. As the mixture moves forward as a whole, there will be an ignition gap before the formation of a new nucleus after the ignition process, and the ignition time gap was shown as a narrow dark area in the images, the ignition process was very short. From the images obtained by ICCD high-speed camera, we observed the whole ignition process clearly. For any successful ignition process, it could be divided into three stages according to the change of the flame structures: the pro-ignition, the fire core generated and evolved into spherical; the mid-ignition, the spherical fire core gradually turn into tapered structure; the end-ignition, the flame tapered structure disappeared and turn into a relative stable columnar structure. 3.2.1. The pro-ignition process Fig. 3 was the flame structures evolution in the pro-ignition process, took the first image of the fire core that could be observed as the “initial time” 0, and the interval time between the adjacent images was 0.2 ms. The solid yellow line in Fig. 3b-(1) indicated the exit position of the burner. The heavy oil was mixed with the supply gas after it ejected from the injection nozzle, and the oil-air mixtures were ignited while they were flowing through the high voltage electrode. Since the electric charges of the high-voltage electrode were mainly distributed in a small range near the tip of the electrode, it could be assumed that the ignited oil-gas was a small spherical fire core, then the flame front propagated spherically with the same velocity, and at the same time the fire core also moved downstream with the airflow. Compared the images (1)e(3) of the three conditions in Fig. 3, found that the positions of the fire core are similar, and the volume of the fire core increased gradually. In the three conditions, all the fire cores of (1) were similar to dim scattered agglomerates with an irregular shape, and the fire core of the images (2) and (3) were spherical and they were brighter than (1). The fire core grew slowly in the initial time, and it developed rapidly to a bright sphere after it could be observed at the first time. For a successful ignition, after the fire core leaved the ignited electrode area, whether the fire core could grow continuously depended on the relative values between the exothermic of the fire core and the heat Table 2 Working conditions. NO.
O2 concentration (%)
a
Composition of gas supply (L/min) air
O2
CO2
Total gas flow
A1 B1 B2
29 21 29
1.452 1.05 1.455
0 676 607
196 0 69
480 0 0
676 676 676
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Fig. 2. The vertical size of the observation window.
absorption of the oil-gas mixture around the fire coredonly if the exothermic heat of the fire core was greater than the amount of heat required to evaporate the oil droplets in the oil-gas mixture could the fire core grew and the oil combusted continuously. In generally, the higher the amount of heat released, the faster the oil droplets evaporated as well as the burning rate. During the fire core moved downstream along with the gas flow, the heat released by the fire core diffused rapidly into the unburned oil-gas mixture due to high turbulence intensity, so the temperature around the fire core rose slowly and the oil droplet evaporation rate was also small, which led the fire core to grow slowly before burning in the burner. As shown in Fig. 3a, the first image was the fire core which had been leaving the burner, it was dim and irregular in the shape, however, the fire core rapidly grew into a bright spherical fire core after 0.2 ms. It can be seen that during the formation of the fire core, its initial growth was relatively slow, but it burned drastic and grew rapidly when reached a certain scale. From the images (4)e(8) in Fig. 3a, we can see that the fire core did not remain a solid sphere after it grew to a bright spherically fire core, instead, a black void appeared inside the fire core. As the time evolved, the black void gradually expanded, and the front of the fire core turned into a bright meniscus shape, at the same time, the backend of the fire core turned to be dim. It may be the reason that the discharge ignition of the high-voltage electrodes was not continuously, so there would be a non-lighting time gap with the end of an ignition process, which led to be a spatial gap inside the fire core, and the spatial gap expanded continuously as time evolved until the next ignition process begins. As shown in Fig. 3a-(3) and Fig. 3a-(8), took the fire core as a regular sphere, the centroid pixel coordinates of them were 848 and 836 in the horizontal direction, and the pixel distance between the two was 12, so the actual distance was L2 ¼ 12 0.517 ¼ 6.204 mm, with a time interval t2 ¼ 0.2 ms 5 ¼ 1.0 ms, and the fire core propagation velocity of the centroid was V2 ¼ L2/t2 ¼ 6.204 mm/1.0 ms ¼ 6.204 m/s. By comparing Fig. 3a and b, the fire core volume combusted in 29% O2/71%N2 was obviously larger that in air(21% O2/79%N2), which showed that it was beneficial to heavy oil combustion to increase O2 concentration in the same inert gas atmosphere. By comparing Fig. 3a and c, we could find that in the same O2 concentration, the fire core in O2/N2 atmosphere grew more rapidly than in O2/CO2, which indicated that heavy oil was more easily to burn in O2/N2 than in O2/CO2. From Fig. 3a and c, it was obviously that the fire core of the two were similarly to each other, which indicated that heavy oil combusted in 29% O2/71% CO2 was similarly to that in air. The displacement-velocity in the normal direction of the flame front was defined as the propagation velocity of the flame. As shown in Fig. 4, in order to calculate the flame propagation velocity in the pro-ignition process, we extracted the flame edge by MATLAB to get the flame front locations,the specific operation is as follows: The image was read by imread function, and the original rgb image was transformed into a grayscale image by rgb2gray, and then the gray image was binarized by the domain value function. Then the image contour was scanned to get the flame front locations. Fig. 5 was the locations of the tip of fire core front according to the images in Fig. 3, with the abscissa of time and ordinate of pixel. As shown in Fig. 5, the locations of the fire core in 21% O2/79% N2 was nearly the same as that in 29% O2/71% CO2, which indicated that they had the same rates in growing and evolving, while it grew obviously faster in 29% O2/71%N2 after 0.2 ms. In the time of 0e0.2 ms, the position of the three was nearly the same. From the images of Fig. 3, it could be observed that the fire cores were dim and irregular, which meant a “moderate” burning, and the combustion of the flame had little effects on the flame turbulence as well as its propagation velocity, so the flame propagation velocity mainly depended on gas flow field. During the time that the oil-gas mixture leaving from the ignition electrode to 0 ms, we assumed the gas flow velocity was constant in the horizontal, and the gas flow velocity could be obtained by the displacement-velocity in the normal direction of the flame front:
v¼
ds dt
(7)
vdthe propagation velocity, dsdthe moving distance of the flame-front, dtdthe moving time of the flame-front. 1 pixel unit corresponded to actual distance 0.51724 mm, then the flame propagation velocity was:
v¼
Ds 0:51724 mm Dt
(8)
In the time of 0e0.2 ms:
nA1 ¼ nB1 ¼ nB2 ¼
3 0:51724 mmz7:76m=s 0:2ms
(9)
vA12dthe propagation velocity of flame on the fire core surface in 29% O2/71% CO2,vB11dthe propagation velocity of flame on the fire core surface in21%O2/79%N2,vB13dthe propagation velocity of flame on the fire core surface in 29% O2/71%N2.
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Fig. 3. The flame structures evolution in the pro-ignition process (interval time: 0.2 ms).
In the time of 0.2e2.0 ms:
nA1 ¼ nB1 ¼
40 0:51724 mmz11:49m=s 1:8ms
(10)
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Fig. 4. The flame front extraction by MATLAB.
Fig. 5. The location of the tip of fire core front in pro-ignition process.
nB2 ¼
64 0:51724 mmz18:39m=s 1:8ms
(11)
The results show that the propagation velocity of flame on the fire core surface in 29% O2/71%N2 was obviously larger than that of 21% O2/ 79% N2 and 29% O2/71% CO2. 3.2.2. The mid-ignition process Fig. 6 was the flame structures evolution in the mid-ignition process. As shown in the figure, when the fire core evolved a certain degree, its spherical structure turned to be a taper structure, and the black void inside the fire core was gradually filled with the next fire core, which resulted in hierarchical structures in the flame. Comparing Fig. 6b and c, in the mid-ignition stage, the flame in 21% O2/79% N2 evolved similarly to that in 29% O2/71% CO2, but the former was brighter than the later. As the flame continuously evolved with time, the taper top upturned and the flame structure turned to be camber from taper. As shown in Fig. 6a, the first fire core burned about 2 ms after the first time it could be observed, during this period, a new fire core generated inside the old one, and the new fire core also evolved to be a taper structure (Fig. 6a-(3)dFig. 6a-(5)). Both the old taper structure and the new kept for some time to propagate downstream with the gas flow, which was different from the evolution process of in 21% O2/79% N2 and 29% O2/71% CO2. In actual, the results in Fig. 6a showed that the ignition process was not continuously, the gap in the adjacent taper structures could be considered as an ignition time gap. Fig. 7 showed the locations of the tip of fire core front in mid-ignition process. In 29% O2/71% N2, the flame front location had a mutation and lagged behind that in 21% O2/79% N2 and 29% O2/71% CO2. The reason was mainly that the flame front in 29% O2/71% N2 was from the first fire core before 2 ms but from the second fire core after 2 ms, however, it was from the first fire core for the later two, as shown in Fig. 6. In mid-ignition process, the total flame propagation velocity in 29% O2/71% N2 was larger than that in
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Fig. 6. The flame structures evolution in the mid-ignition process (interval time:1.0 ms).
Fig. 7. The location of the top of fire core front in mid-ignition process.
21% O2/79% N2 and 29% O2/71% CO2, but as the fire core further evolved, the velocities of the three were closed, and the value was about 10.60 m/s after 8 ms. During the combustion, the high-voltage ignition electrode was AC discharge, assuming u ¼ 0 while t ¼ 0, then the current and voltage could be expressed as follow:
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Fig. 8. The instantaneous active power.
Table 3 The top taper location of the outermost flame and sub-outer. NO.
6a-4 6a-5 6a-6
Local flame propagation velocity(m/s)
Pixel location Level I
Level II
Pixel distance
830 806 779
845 823 794
15 17 15
10.60
u ¼ Um sin ut
(12)
i ¼ Im cosðut fÞ
(13)
the transient power was:
P ¼ u,i ¼ ðUm sin utÞ,ðIm cosðut fÞÞ ¼ Um Im sin ut cosðut fÞ
(14)
1 P ¼ Um Im cos f sin2 ut Um Im sin f sin 2 ut ¼ pðtÞ þ qðtÞ 2
(15)
in the formula (15), p (t) was active power,
pðtÞ ¼ Um Im cos f sin2 ut ¼
Um Im cos f ð1 cos 2 utÞ 2
(16)
Fig. 9. The flame structures evolution in the end-ignition process(interval time: 2.0 ms).
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Set the phase angle of voltage and current was 0, then according to the burner technical parameters the active power p(t) was written as follow:
pffiffiffi pffiffiffi 2Un 2In cos f ð1 cos 2 utÞ ¼ Un In ð1 cos 2 utÞ pðtÞ ¼ 2 pðtÞ ¼ 8000V 16mA ð1 cos 200ptÞ ¼ 128ð1 cos 200 ptÞ ðJ=sÞ
(17) (18)
The ignition of unburned oil-gas mixture need an external energy input, only with enough energy inputted the oil-gas mixture may be ignited (see Table 2). In this study, the external energy was the spark generated by the discharge of the high voltage electrode, and the instantaneous active power was shown in Fig. 8, with a range of 0e256 J/s. When the instantaneous active power was less than a certain value, the oil-gas mixture could not be ignited at the moment, so the actual ignition process was not continuously, and the ignition time gap could be calculated by the gap in the adjacent taper structures in Fig. 6. According to Fig. 6a-(4), (6), the top taper location of the outermost flame (the first fire core, Level I) and sub-outer (the second fire core, Level II) were shown in Table 3. According to Fig. 7, here the local flame propagation velocity was almost a constant of 10.60 m/s, the average pixel distance was 15.67, so the ignition time gap was:
t¼
s 15:67 0:51724 mm ¼ z0:765 ms v 10:60 m=s
(19)
As shown in Fig. 8, in a completely ignition circle (50 Hz, 20 ms), the ignition time gap was (10e0.382 ms, 10 þ 0.382 ms), and the
Fig. 10. The change of the flame structure during stable combustion (interval time: 2.0 ms).
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corresponding instantaneous active power was p(t) ¼ 3.68 J/s, which was similar to the value of gasoline under spark ignition obtained by Ma Jian et al. [30,31].(0e15 mJ/ms). According to the calculation results, both the ignition time gap and its corresponding instantaneous active power were small, so the combustion loss caused by the ignition gaps could be neglected. 3.2.3. The end-ignition Fig. 9 was the flame structures evolution in the end-ignition process. The flame had been fully evolved, at this time the flame had a great effect on the flow field, which increased the turbulence intensity a lot, so the flame taper structure disappeared and the flame presented irregularly. 3.3. The change of the flame structure during stable combustion As shown in Fig. 10, during the stable combustion, the flame shape was irregular, and the flame structure changed with time evolved. Inside the yellow circle of the first image, there was a narrow flame in the upper left, this narrow flame had a tendency to separate from the main flame zone; after 2 ms (the second image), the narrow flame had escaped from the main flame zone and the shape changed to be bend; after 4 ms, the flame changed to be small, and the size was no more than half of the initial one of the first image; the flame could not be distinguished after 8 ms. In a high turbulent flow field, the flame structure changed rapidly, there were mainly two reasons for the changing of the flame structure: on the one hand, some oil-gas mixture in the flame had been exhaust, which presented a local flame extinction; on the other hand, the flame was irregularly jitter, this kind of irregularly jitter not only changed the flame spatial distribution, but also may ignite the remaining oil-gas mixture in the flow field, and the flame structure presented as the distort or growth in local.In further research, we will explore that would the change in the flame structure affect combustion efficiency and emission of pollutants. 4. Conclusion This paper studied the flame propagation characteristics of heavy oil oxy-fuel combustion in ignition and stable combustion. The results showed that the ignition process could be divided into three stages d the pro-ignition, mid-ignition and end-ignition. In the pro-ignition, the fire core generated and evolved into spherical; in the mid-ignition, the spherical fire core gradually turn into tapered structure; in the end-ignition, the flame tapered structure disappeared and turn into a relative stable columnar structure. By calculating the flame propagation velocities, we found that in the same combustion atmosphere, the flame propagation velocity increased with the increase of oxygen concentration; in the same O2 concentration, the flame propagation velocity in O2/N2 atmosphere was larger than that in O2/CO2. During the stable combustion, we observed the local flame structure extinguished, distorted and grew. Acknowledgments This work was supported in part by the National Major Scientific Instruments and Equipment Development Project: Diagnosis of Combustion Process of Heavy Oil O2/CO2 Diffusion Flame (2012YQ04016410). References [1] M. Meratizaman, S. Monadizadeh, A. Ebrahimi, H. Akbarpour, M. 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Please cite this article as: Z. Wang et al., Experimental study on the flame propagation characteristics of heavy oil oxy-fuel combustion, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.01.011
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Please cite this article as: Z. Wang et al., Experimental study on the flame propagation characteristics of heavy oil oxy-fuel combustion, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.01.011