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Study on the characteristics of evaporation–atomization–combustion of biodiesel
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Journal of the Energy Institute journal homepage: http://www.journals.elsevier.com/journal-of-the-energyinstitute
Study on the characteristics of evaporationeatomizationecombustion of biodiesel Q1 Q8
Yicheng Shen a, b, Fashe Li a, b, *, Zuowen Liu a, b, Huage Wang a, b, Jiaxu Shen a, b a b
State Key Laboratory of Complex Nonferrous Metal Resources Cleaning Utilization in Yunnan Province, Kunming, 650093, China Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming, 650093, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 May 2018 Received in revised form 6 August 2018 Accepted 29 August 2018 Available online xxx
A great deal of research is being carried out on renewable diesel fuels. The number of raw materials (especially waste, animal, and vegetable oils), production technologies, and additives of biodiesel is increasing. In our work, a evaporationeatomizationecombustion system consisting of a biomass liquid fuel was designed to produce a laminar premixed flame for studying the combustioneemission characteristics of biodiesel. The combustion characteristics of biodiesel including flame height, flame front area, flame speed, and OH total signal intensity were studied by planar laser-induced fluorescence of OH (OH-PLIF). The emission characteristics of biodiesel (CO, CO2, and NO) were studied with a flue gas analyzer. The experimental results showed that the flame height, flame front area, flame speed, and the OH total signal intensity changed with the equivalence ratio (F). The relationship between the OH radical intensity and the emission of CO/CO2 was obtained from the OH-PLIF average signal intensity. The [CO]/ [CO2] ratio decreased with the OH-PLIF average signal intensity. Finally, we obtained the relationship between the OH-PLIF average signal intensity and the NO emissions. © 2018 Energy Institute. Published by Elsevier Ltd. All rights reserved.
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Keywords: Laminar premixed Combustion flame OH-PLIF Equivalence ratio [CO]/[CO2] NO
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1. Introduction Fossil fuels have remained the main source of energy for our societal development for a long time. However, the increasing demand for energy is putting additional pressure on fossil fuel resources, leading to early depletion [1]. The ever-increasing stringent emissions regulations, rising oil prices, and the finite supply of fossil fuels have made it necessary to search for sustainable and environmentally friendly energy sources [2]. As a renewable biofuel, biodiesel can be a promising alternative for meeting energy demands [3]. Among the production routes currently available for transforming oils or fats into biodiesel, transesterification has become increasingly advanced. Biodiesel is a liquid with composition and properties similar to those of fossil/mineral diesel. However, unlike fossil-derived diesel, biodiesel is an oxygenated fuel and does not contain aromatics and cycloalkanes. In addition, when mixed with regular diesel, biodiesel can effectively reduce CO, HC, and soot emissions from diesel engines and reduce greenhouse gas emissions [4,5]. Thus, in order to use fuels more efficiently and rationally and to reduce pollutant emissions, it is necessary to understand the combustion characteristics of these liquids. Biodiesel combustion research includes combustion mechanism, combustion stability, and emission characteristics, among other aspects. OH radicals are important combustion intermediates that allow identifying combustion reaction areas more accurately. These species are commonly used to study flame characteristics such as structure, propagation velocity, turbulence, heat release rates, local extinguishing, and OH radical concentration distributions [6e10]. The distribution of OH radicals as determined by planar laser-induced fluorescence (PLIF) can be used to study flame characteristics and combustion mechanisms [11,12]. When burned, hydrocarbon fuels readily generate OH radicals [13]. Since biodiesel fuels contain hydrocarbons, it is desirable to study the combustion of biodiesel by OH-PLIF. Basic research on the combustion characteristics of biodiesel fuels from the perspective of free radical evolution is scarce in the literature. Love Norman et al. [14] used PLIF to measure the concentrations of CH and OH radicals in laminar flames using soy methyl ester, canola methyl ester, and methyl stearate. Alviso et al. [15] used experimental and numerical methods to study biodiesel combustion mechanisms
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* Corresponding author. State Key Laboratory of Complex Nonferrous Metal Resources Cleaning Utilization in Yunnan Province, Kunming, 650093, China. E-mail address: [email protected] (F. Li). https://doi.org/10.1016/j.joei.2018.08.005 1743-9671/© 2018 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Please cite this article as: Y. Shen et al., Study on the characteristics of evaporationeatomizationecombustion of biodiesel, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2018.08.005
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using a laminar counter-flow spray premixed flame. These authors used OH-PLIF to experimentally analyze the structure of flames generated by biodiesel and MD. The combustion characteristics of several fuels in engines (e.g., neat biodiesel, biodiesel, diesel-biodiesel mixtures, and ethanol blended fuels) has been widely studied [16e18]. Therefore, in order to study the characteristics of biodiesel and its combustion of from the perspective of OH radical evolution, a biodiesel laminar premixed flame was produced by using an evaporation and atomization combustion biomass liquid fuel system by OH-PLIF. These results can provide reference values for the characteristics of biodiesel flames and the relationship between OH radicals and combustion emissions. 2. Experimental setup and procedures 2.1. Experimental facility In this study, a biodiesel laminar premixed flame was produced by using an evaporation and atomization combustion biomass liquid fuel system. As shown in Fig. 1, this system included a gas supply carrier system, a fuel supply system, an evaporation and atomization system, and a burner. The fuel supply system comprised a fuel syringe pump and a liquid heater. The gas carrier system was formed by an air path (composed of an air compressor and a flow-meter), an oxygen gas path (composed of an oxygen cylinder and a flow-meter), a gas mixing bottle, and a gas heater (1, Fig. 1). The gas carrier system included an air cylinder, a flow-meter, and a gas heater (2, Fig. 1). The evaporation and atomization system included liquid and gas spray (1 and 2) needles, an evaporation and atomization chamber, a ceramic heater, and a thermocouple. The burner outlet was a stainless-steel tube with an internal diameter of 8 mm. In this system, refined biodiesel was used as the combustion agent, while air was used as the atomization medium. During the test, the air was first heated to 200 C in the gas supply system and injected into the evaporation and atomization chamber through the gas spray needle 2 in order to preheat the system. The gas spray needle 2 has a large diameter of 2 mm. At the same time, a ceramic heater was wrapped outside the chamber as auxiliary heating and insulation material up to 200 C. Second, the biodiesel was heated to 60 C in the fuel supply system and injected into evaporation and atomization chamber through the liquid spray needle. Meanwhile, the air was heated to 200 C in the gas carrier system and injected into the evaporation and atomization chamber through the gas spray needle 1, and the air pressure was adjusted to 0.7 MPa. The biodiesel was sprayed, atomized into small droplets with air, and rapidly evaporated at high temperature after which it entered into the burner. Finally, the air and oxygen were mixed in the gas mixing bottle and heated to 60 C, and the mixture was introduced into the round sleeve of the burner. The biodiesel vaporeair mixture was introduced into the burner through a pipe and burned under the protection of a combustionsupporting gas. The above burner was equipped with an electric heating equipment to prevent condensation of the biodiesel vapor. Particularly, the gas spray needle 1 and the liquid spray needle formed an angle of 90 , and both had an outlet diameter of 0.34 mm. In order to verify the reliability of the system, a Malvern laser particle size analyzer was used to detect the presence of unevaporated droplets at the outlet of the evaporation and atomization system. The result revealed no unevaporated droplets when large amounts of air were employed. These conditions resulted in a good atomization effect, with small droplets easily evaporated. However, small amounts of unevaporated droplets were observed when the amount of atomization air was low. During the test, the fuel injection volume was maintained constant, and the equivalence ratio (F) was changed by controlling the amount of atomization air. Since the gas supply system did not provide a combustion gas, the mixture of biodiesel vapor and air was burned in the external environment. Once stable, the biodiesel laminar flame was detected and analyzed by OH-PLIF. The OH-PLIF system employed herein is shown in Fig. 2. It mainly consisted of a Nd-doped yttrium aluminum garnet (Nd: YAG) laser (Model: LAB-170-10H) with a power of 200 mJ per pulse and a pulse time of 10 ns, a Sirah dye laser (Model: CBST-G-30-EG) with an output wavelength of 283.36 nm was used to excite the OH radicals, and a CCD camera (CCD Type: Imager LX2M 1600 1200 7.4 mm; Intensifier Type: 25 mm V7670U-70-P43) equipped with a ultraviolet (UV) lens and an OH filter. The working environment temperature of the OH-PLIF system was 22 C, and the indoor relative humidity was 62%. The OH-PLIF system can directly provide a two-dimensional (2D) distribution of the OH radical concentration of the
Fig. 1. Scheme of the evaporation and atomization combustion biomass liquid fuel system.
Please cite this article as: Y. Shen et al., Study on the characteristics of evaporationeatomizationecombustion of biodiesel, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2018.08.005
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Fig. 2. Scheme of the OH-PLIF system used herein.
flame during the combustion. The burner was equipped with a flue gas analyzer (MGA5 infrared flue gas analyzer) to measure and analyze the combustion products. Herein, 500 transient pictures were captured at each F. These pictures were subsequently averaged and taken for measurement analysis. 2.2. Experimental materials Two biodiesel samples (Jatropha curcas L. biodiesel and waste oil biodiesel) were used herein. The Jatropha curcas L. biodiesel was made in our labs via transesterification, while the waste oil biodiesel was externally purchased (Manufacture Date: 03/25/2017, Shelf Life: 18 months). The physicochemical properties of both biodiesel samples are shown in Table 1, and the elemental analysis is summarized in Table 2. The theoretical air volume and the F of the biodiesel samples were calculated from the elemental analysis. 2.3. Measurement uncertainties The main source of uncertainties in the measurements [21] was the total flow rate of the premixed unburned gas (UQ) and the calculated flame area image (UA), which was determined by the camera resolution. The uncertainty of the gas flow rate originated from the flow-meter uncertainty (0.5% of reading þ 0.1% full scale) which was estimated to be ca. 2%. The biodiesel mass flows were measured with an accuracy of ca. 5%. Thus, the total flow rate uncertainty of the premixed unburned gas (UQ) was estimated to be ca. 5.4%. The uncertainty derived from the ICCD camera spatial resolution, which was estimated to be ca. 3%. The overall uncertainty was calculated to be within ca. 6%. This overall qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi uncertainty was obtained from the relation U 2Q þ U 2A ) for all the laminar flame recorded at a temperature of 473 K, an F range of 0.9e2.0, and at ambient pressure. Indeed, the fluctuation of the flame position affected the display of the flame structure on the camera during the experiment. This effect modified the position of the flame contours (2e3 pixels), slightly increasing the overall uncertainty. 3. Experimental results and discussion During the test, the fuel injection volume of both biodiesel samples was constant (0.16 mL/min), while the F was changed by controlling the amount of atomization air. The F was selected to range between 0.9 and 2.0. When the F was less than 0.9, the flame was easily blown off, while Fs greater than 2.0 facilitated tempering. Once the flame was stable, for each measurement condition reported in this work, 500 instantaneous images of the OH radicals were systematically recorded by the ICCD camera, and these images were averaged and taken for measurement analysis. We took 5 sets of pictures under each condition for analyze. As shown in Fig. 3, the OH-PLIF average images of the laminar premixed flame generated by both biodiesel samples changed with the F. In the OH-PLIF images, the red and yellow areas corresponded to regions with high concentrations of OH radicals. Thus, the results revealed higher concentrations of OH radicals near the flame front as compared to other areas. The vicinity of the flame front was an area with intense chemical reactions. The distribution of OH radicals at the end of the axial direction of the flame decreased because of the effect of the gas flow speed. The chemical reaction in the flame began at the flame front. Table 1 Physicochemical properties the two biodiesel samples used herein. Physicochemical properties
Waste oil biodiesel
Jatropha curcas L. biodiesel
Density (20 C)/(g/mL) Kinematic viscosity (40 C)/(mm2/s) Closed flash point/ C Ignition point/ C Freezing point/ C Residual carbon/% Calorific value/(kJ/g) Acid value (KOH)/(mg/g) Cetane number
0.875 4.51 164 183 3 0.26 38.98 0.8 51
0.876 4.43 185 196 1 0.063 37.55 0.5 49
Please cite this article as: Y. Shen et al., Study on the characteristics of evaporationeatomizationecombustion of biodiesel, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2018.08.005
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Table 2 Elemental analysis of the two biodiesel samples used herein. Fuel types
C/%
H/%
O/%
N/%
S/%
Jatropha curcas L. biodiesel Waste oil biodiesel
75.75 73.62
12.55 12.69
10.06 12.25
1.270 1.22
0.370 0.22
Fig. 3. OH-PLIF average flame images with different Fs.
3.1. Influence of the F on the flame height Generally, the flame height is determined by visual inspection or by using images taken with a camera. Roper et al. proposed the concept of stoichiometric coefficient flame. According to this concept, the flame height is determined by the distance from the nozzle outlet to the axial position when the ratio of fuel to oxidant is exactly the stoichiometric ratio. The chemiluminescence of the flame is closely related to chemical reactions, and the chemical reactions can be tracked with the OH radicals. Thus, the flame height can be used to calibrate the OHPLIF system, and the calibration results are shown in Fig. 4. As shown in Fig. 4, the flame height increased with F. The air capacity decreased with F, part of the fuel was burned by the outside air via diffusion combustion. As a result, the flame front underwent retrusion and the flame height increased. Waste oil biodiesel generated a flame with lower height than the Jatropha curcas L. biodiesel.
Fig. 4. Flame height as a function of F for the flames generated by two biodiesel samples.
Please cite this article as: Y. Shen et al., Study on the characteristics of evaporationeatomizationecombustion of biodiesel, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2018.08.005
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3.2. Influence of the F on the flame front As shown in Fig. 5, the digital graphic process flow of the flame front information was obtained using the OH-PLIF images of the flame. The process employed herein using digital graphic geometric features was based on gray images. The original images obtained from the OHPLIF system were cut appropriately and processed using MATLAB. These images were gray and binary processed to obtain the binary image. Thus, the flame front area (A) was obtained after a batch processing method including edge extraction, curve-fitting f(x), and calculation of the area after curve-rotation. Since the laminar premixed flame is axisymmetric and the flame front of the 2D images taken using OH-PLIF system was symmetrical on both sides, one side was selected for subsequent research and analysis. The curve f(x) was rotated around the burner in the axial direction to obtain the flame front area (A) as follows [2]:
A ¼ 2p
Zb
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 0 f ðxÞ 1 þ jf ðxÞj2 dx
(1)
a
where (a, b) is the integral interval of the curve f(x). Using the above method, the flame front area for different equivalence ratios of both biodiesel samples was determined, and the results are shown in Fig. 6. The flame front area increased with F to reach a maximum and decreased thereafter. The flame front area of the waste oil biodiesel was smaller than that of the Jatropha curcas L. biodiesel. The flame front area of Jatropha curcas L. biodiesel reached a minimum (0.62 cm2, estimated) for a F of ca. 1.1. Also, when the F was 2.0, the flame front area was estimated to be 5.38 cm2. The flame front area of the waste oil biodiesel reached a minimum (0.80 cm2, estimated) when the F was ca. 1.0. When the F was 2.0, the flame front area was estimated to be 7.97 cm2. Since the airflow decreased with F for a fuel flow constant, part of the fuel required outside air to burn, leading to retro-position of the flame front and larger flame front areas. During lean-fuel combustion, the airflow increased while decreasing the F at constant fuel flow. An increase in the gas mixture flow velocity led to retro-position of the flame front and larger flame front areas. 3.3. Influence of the F or gas mixture heating temperature on the flame speed The laminar flame speed measurements were based on the mass conservation principle between the outlet nozzle and the flame front. The average laminar premixed flame speed on the transverse plane is expressed as [19]:
Su ¼
Q A
(2)
where Q is the total volume flow rate of the fueleair gas mixture and A is the surface area of the flame front. The air used as an atomization medium was controlled by a flowmeter, and the amount of biodiesel evaporated was very low was therefore ignored. Thus, the premixed gas
Fig. 5. Digital graphics process.
Fig. 6. Flame front area as a function of F for the two biodiesel samples used herein.
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flow (Q) was the atomized airflow. The flame front area was also calculated using to the OH-PLIF images, and, finally, the premixed flame speed was obtained using Eq. (2). As shown in Fig. 7, the premixed flame speed reached a maximum with F and decreased thereafter for both biodiesel samples. The flame speed of the waste oil biodiesel was lower than that of Jatropha curcas L. biodiesel. The flame speed of both biodiesel samples reached maxima at F ¼ 1.0 (ca. 27.8 and 35.429 cm/s, respectively). Eq. (3) shows the relationship between the flame speed and different physical parameters [20]:
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi l Tr TB Su ¼ u cp $r$C0 TB T0
(3)
where Su is the flame speed, l is the thermal conductivity of the gas mixture, r is the intensity of the gas mixture, cp is the specific heat of the gas mixture under constant pressure, C0 is the initial concentration of the gas mixture, Tr-TB is the difference between the combustion and the ignition temperatures, TB-T0 is the difference between the ignition and the initial temperatures of the gas mixture, and u is the average chemical reaction speed of the gas mixture in the combustion area. Under lean-fuel conditions, the average chemical reaction speed of the gas mixture was mainly controlled by the concentration of the combusted fuel vapor in combustion. The airflow decreased with F, and, under constant fuel flow, the concentration of the fuel vapor increased in the gas mixture. u increased, and the flame speed also increased as a result. In addition, a decrease in the airflow resulted in larger r and cp for the gas mixture. At this point, u was the dominant factor controlling the laminar flame speed. Thus, the laminar flame speed increased with F during lean-fuel combustion. Under rich-fuel conditions, u was mainly controlled by the air concentration. Under constant fuel flow, the airflow decreased with F, and u decreased while decreasing the air concentration of the gas mixture. In addition, lower airflow values affected u to a lower extent as compared to r and cp. Hence, the laminar flame speed decreased with F during rich-fuel combustion. 3.4. Influence of the F on the distribution of OH radicals in the flame Considering the changes of the OH-PLIF average images for the flames generated by the two biodiesel samples with F (Fig. 3), the variations in the overall intensity of the OH signal of the flame under identical Fs is shown in Fig. 8. As shown in Fig. 8, the OH-PLIF total signal intensity of the flames generated by both biodiesel samples increased with F. The F and the incident laser energy remained unchanged in the experiment, and the test of parameters were consistent with these conditions. The increase in the OH overall signal intensity indicated that the concentration of OH radicals in the flame increased gradually with F. In order to further analyze the distribution of OH radicals in the flame, the OH-PLIF average signal intensity was used herein to analyze the evolution of the OH radical intensity per area of PLIF image with F. According to Fig. 5, a binary image can be obtained by digital graphic processing, and MATLAB can be used to calculate the flame area. The ratio of OH-PLIF overall signal intensity to the flame area was defined as the OH-PLIF average signal intensity. The results are shown in Fig. 9. As shown in Fig. 9, the OH-PLIF average signal intensity for the flames generated by both biodiesel samples reached a maximum with F and decreased thereafter. The OH-PLIF average signal intensity of the waste oil biodiesel reached a maximum at F ¼ 1.2, while the OH-PLIF average signal intensity of the Jatropha curcas L. biodiesel reached a maximum at F ¼ 1.1. Owing to the combustion reaction, the flame may contain radicals, unburned fuel, combustion products, and other products. Therefore, the concentration of OH radical in the combustion field increased with the average OH signal intensity in the flame. At a certain position in the flame field, larger concentrations of OH radicals indicated greater oxidation abilities and higher probabilities of reducing substances being oxidized in this position. 3.5. CO/CO2 emission characteristics in the biodiesel combustion flue gas Fig. 10 shows the CO and CO2 emissions of the flue gasses generated by both biodiesel samples as a function of F. As shown in Fig. 10, the CO emissions were minimized at a certain F and increased thereafter. The opposite trend was found for the CO2 emissions. As shown in Table 2,
Fig. 7. Flame speed as a function of the F for the two biodiesel samples used herein.
Please cite this article as: Y. Shen et al., Study on the characteristics of evaporationeatomizationecombustion of biodiesel, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2018.08.005
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Fig. 8. Evolution of the OH-PLIF overall signal intensity of the flame with F.
Fig. 9. Evolution of the OH-PLIF average signal intensity of the flame with F.
Fig. 10. Evolution of the CO/CO2 emissions with F.
elemental analysis revealed higher carbon contents for the Jatropha curcas L. biodiesel as compared to the waste oil biodiesel. These results can explain the higher CO/CO2 emissions of the Jatropha curcas L. biodiesel combustion flue gas. To further study the relationship between the CO/CO2 emissions and the OH radicals, the ratio of CO/CO2 emissions [CO]/[CO2] ([] stands for the concentration of each substance) was used to explore the relationship with the OH-PLIF signal intensity. As shown in Fig. 11, [CO]/ [CO2] was minimized at F ¼ 1.1 and increased thereafter for both biodiesel samples. Please cite this article as: Y. Shen et al., Study on the characteristics of evaporationeatomizationecombustion of biodiesel, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2018.08.005
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Fig. 11. Evolution of the [CO]/[CO2] ratio with F.
During the combustion of the two biodiesel samples, the generation of CO was mainly controlled by the following elementary reaction [21]: *OH þ CO 4 H* þ CO2
(4)
Therefore, if the concentration of OH radicals increases in the flame, the elementary reaction (4) is produced in the forward direction, promoting the oxidation of large amounts of CO to CO2. As a result, the CO emissions decreased and the production of CO2 increased, resulting in lower [CO]/[CO2] ratios. As shown in Fig. 9, the OH-PLIF average signal intensity for the flames generated by both biodiesel samples reached a maximum with F and decreased thereafter. These results revealed that the evolution of the OH-PLIF average signal intensity was related to that of [CO]/[CO2]. Under the same equivalence ratio, [CO]/[CO2] decreased with the OH-PLIF average signal intensity and vice versa. However, this result needs to be further studied in terms of the control of CO emissions through the OH radical intensity. 3.6. NO emission characteristics of the biodiesel combustion flue gas Fig. 12 shows the NO emissions generated by both biodiesel samples as a function of F. As shown in Fig. 12, the NO emissions decreased with F, with Jatropha curcas L. biodiesel showing higher emissions than the waste oil biodiesel. According to the currently known pathways leading to NO, the main source of NO is high-temperature NO (i.e., thermal NO), which is formed by the splitting of N2 in air. As suggested by the extended Zelkovas mechanism [22,23], the rate of the thermal NO formation reactions is highly temperature-dependent, with higher flame temperatures resulting in higher NO emissions. Although the OH radicals can promote the formation of NO, thermal NO production is mainly controlled by the flame temperature. The adiabatic temperature of the flame decreased with F, and the amount of heat produced also decreased with this parameter. Therefore, it is impossible to provide energy to produce thermal NO steadily (i.e., NO production is suppressed). We used a thermocouple to measure the maximum temperature in the flame under different conditions, and the result are shown in Fig. 13. The flame temperature reached 1800 K at relatively low F values, which
Fig. 12. Evolution of the NO emissions with F.
Please cite this article as: Y. Shen et al., Study on the characteristics of evaporationeatomizationecombustion of biodiesel, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2018.08.005
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Fig. 13. Maximum temperature in flame as a function of F.
is beneficial to the generation of thermal NO. When F increased from 0.9 to 1.3, the NO emission decreased significantly. A further increase in F resulted in the adiabatic temperature of the flame not reaching the temperature required for thermal NO generation. The main NO produced was prompt NO. The Fenimore mechanism also suggested that OH radicals could promote the production of prompt NO. As shown in Fig. 9, the OH-PLIF average signal intensity decreased with F for F values higher than 1.2, and the decreasing range is relatively slow. However, the reduction in NO emissions was relatively slow when F was greater than 1.3, indicating that there was a certain relationship between the NO emissions and the presence of OH radicals. Under similar equivalence ratios, the NO emissions decreased while decreasing the OH-PLIF average signal intensity. 4. Conclusions In this paper, a biodiesel laminar premixed flame was generated using an evaporation and atomization combustion biomass liquid fuel system, and the OH signal of the biodiesel laminar premixed flame was studied at different Fs using OH-PLIF 2D measurements. The main conclusions of this study were: 1) When F increased, the height of the biodiesel laminar premixed flame increased, with the waste oil biodiesel showing lower heights than the Jatropha curcas L. biodiesel. The flame front area reached a maximum with F and decreased thereafter. The flame front area of the waste oil biodiesel was lower than that of the Jatropha curcas L. biodiesel. The premixed flame speed also reached a maximum with F and decreased thereafter, with the waste oil biodiesel showing lower premixed flame speeds than the Jatropha curcas L. biodiesel. 2) The OH-PLIF overall signal intensity of the flames generated by both biodiesel samples increased with F, while the OH-PLIF average signal intensity reached a maximum with F and decreased thereafter. The OH-PLIF average signal intensity of the waste oil biodiesel reached a maximum at F ¼ 1.2, while the Jatropha curcas L. biodiesel reached a maximum signal at F ¼ 1.1. 3) The CO emissions were minimized at a certain F value and increased thereafter, and the opposite trend was found for the CO2 emissions. The CO/CO2 emissions of the Jatropha curcas L. biodiesel combustion flue gas were higher than those of waste oil biodiesel. 4) The generation of CO was mainly controlled by the following elementary reaction: OH þ CO 4 H þ CO2, and higher OH radical intensities suppressed CO production. Under the same F, [CO]/[CO2] decreased with the OH-PLIF average signal intensity and vice versa. 5) The NO emissions decreased with F, with the Jatropha curcas L. biodiesel showing higher emissions than the waste oil biodiesel. Acknowledgments Q6
This work was supported by the National Natural Science Fund of China [Grant number: 51766007]. References [1] V. Mahendra Reddy, et al., Combustion characteristics of biodiesel fuel in high recirculation conditions, Fuel Process. Technol. 118 (1) (2014) 310e317. [2] Yi Wu, et al., Laminar flame speed of lignocellulosic biomass-derived oxygenates and blends of gasoline/oxygenates, Fuel 202 (2017). rcio De Almeida DAgosto, [3] Ma et al., Comparative study of emissions from stationary engines using biodiesel made from soybean oil, palm oil and waste frying oil, Renew. Sustain. Energy Rev. 70 (2017) 1376e1392. [4] Wilson Merchan-Merchan, H.O.T. Ware, Study of carbon and carbonemetal particulates in a canola methyl ester air-flame, Combust. Flame 162 (1) (2015) 216e225. [5] G.J. Xu, et al., The production and affect factors of biodiesel carbonyl pollutants in the premixed flame conditions, Kung Cheng Je Wu LI Hsueh Pao J. Eng. Thermophys. 32 (12) (2011) 2137e2141. [6] Ahmed E.E. Khalil, A.K. Gupta, Hydroxyl radical distribution in distributed reaction combustion condition, Fuel 122 (15) (2014) 28e35. [7] Kazuhiro Yamamoto, S. Isii, M. Ohnishi, Local flame structure and turbulent burning velocity by joint PLIF imaging, Proc. Combust. Inst. 33 (1) (2011) 1285e1292. [8] Z.S. Li, B. Li, Z.W. Sun, et al., Turbulence and combustion interaction: high resolution local flame front structure visualization using simultaneous single-shot PLIF imaging of CH, OH, and CH 2 O in a piloted premixed jet flame, Combust. Flame 157 (6) (2010) 1087e1096. [9] Li Yang, et al., Premixed jet flame characteristics of syngas using OH planar laser induced fluorescence, Sci. Bull. 56 (26) (2011) 2862e2868. [10] R. Yuan, et al., Reaction zone visualisation in swirling spray n-heptane flames, Proc. Combust. Inst. 35 (2) (2014) 1649e1656. [11] D. Alviso, et al., Experimental and numerical studies of biodiesel combustion mechanisms using a laminar counterflow spray premixed flame, Fuel 153 (2015) 154e165.
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[12] Meng Zhang, et al., Measurement on instantaneous flame front structure of turbulent premixed CH4/H2/air flames, Exp. Therm. Fluid Sci. 52 (1) (2014) 288e296. [13] A. Matynia, et al., Absolute OH concentration profiles measurements in high pressure counterflow flames by coupling LIF, PLIF, and absorption techniques, Appl. Phys. B Laser Opt. 108 (2) (2012) 393e405. [14] N. Love, R.N. Parthasarathy, S.R. Gollahalli, Concentration measurements of CH and OH radicals in laminar biofuel flames, Int. J. Green Energy 8 (1) (2011) 113e120. [15] D. Alviso, et al., Experimental and numerical studies of biodiesel combustion mechanisms using a laminar counterflow spray premixed flame, Fuel 153 (2015) 154e165. [16] Mohmadrais Usmangani Parasara, F.N. Adroja, An Experimental Investigation of Palm Oil Blend with Diesel Fuel on Engine Performance and Emission of a Diesel Engine: a Review, 2015. [17] Nadir Yilmaz, F.M. Vigil, Potential use of a blend of diesel, biodiesel, alcohols and vegetable oil in compression ignition engines, Fuel 124 (15) (2014) 168e172. [18] S. Muthuraman, The performance of four stroke surface ignition ceramic heater C.I. Engine using ethanol-diesel blend. 3.2, 2014, p. 38. [19] J. Natarajan, T. Lieuwen, J. Seitzman, Laminar flame speeds of H2/CO mixtures: effect of CO2, dilution, preheat temperature, and pressure, Combust. Flame 151 (1e2) (2007) 104e119. [20] Kefa Cen, Qiang Yao, Zhongyang Luo, Advanced Combustion. Hangzhou, China, 2002. [21] Chun Zou, et al., Characteristics and mechanistic analysis of CO formation in MILD regime with simultaneously diluted and preheated oxidant and fuel, Fuel 130 (5) (2014) 10e18. [22] Ahmed O. Said, A.E.E. Khalil, A.K. Gupta, Dual location fuel injection effects on emissions and NO/OH chemiluminescence in a high intensity combustor, J. Energy Resour. Technol. 138 (4) (2016). [23] H. Hiroyasu, T. Kadota, Models for combustion and formation of nitric oxide and soot in direct injection diesel engines. SAE Paper 760129, Mathemat. Models 6 (1) (1976) 327e335.
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Contents lists available at ScienceDirect
Journal of the Energy Institute journal homepage: http://www.journals.elsevier.com/journal-of-the-energyinstitute
Study on the characteristics of evaporationeatomizationecombustion of biodiesel Q1 Q8
Yicheng Shen a, b, Fashe Li a, b, *, Zuowen Liu a, b, Huage Wang a, b, Jiaxu Shen a, b a b
State Key Laboratory of Complex Nonferrous Metal Resources Cleaning Utilization in Yunnan Province, Kunming, 650093, China Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming, 650093, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 May 2018 Received in revised form 6 August 2018 Accepted 29 August 2018 Available online xxx
A great deal of research is being carried out on renewable diesel fuels. The number of raw materials (especially waste, animal, and vegetable oils), production technologies, and additives of biodiesel is increasing. In our work, a evaporationeatomizationecombustion system consisting of a biomass liquid fuel was designed to produce a laminar premixed flame for studying the combustioneemission characteristics of biodiesel. The combustion characteristics of biodiesel including flame height, flame front area, flame speed, and OH total signal intensity were studied by planar laser-induced fluorescence of OH (OH-PLIF). The emission characteristics of biodiesel (CO, CO2, and NO) were studied with a flue gas analyzer. The experimental results showed that the flame height, flame front area, flame speed, and the OH total signal intensity changed with the equivalence ratio (F). The relationship between the OH radical intensity and the emission of CO/CO2 was obtained from the OH-PLIF average signal intensity. The [CO]/ [CO2] ratio decreased with the OH-PLIF average signal intensity. Finally, we obtained the relationship between the OH-PLIF average signal intensity and the NO emissions. © 2018 Energy Institute. Published by Elsevier Ltd. All rights reserved.
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Keywords: Laminar premixed Combustion flame OH-PLIF Equivalence ratio [CO]/[CO2] NO
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1. Introduction Fossil fuels have remained the main source of energy for our societal development for a long time. However, the increasing demand for energy is putting additional pressure on fossil fuel resources, leading to early depletion [1]. The ever-increasing stringent emissions regulations, rising oil prices, and the finite supply of fossil fuels have made it necessary to search for sustainable and environmentally friendly energy sources [2]. As a renewable biofuel, biodiesel can be a promising alternative for meeting energy demands [3]. Among the production routes currently available for transforming oils or fats into biodiesel, transesterification has become increasingly advanced. Biodiesel is a liquid with composition and properties similar to those of fossil/mineral diesel. However, unlike fossil-derived diesel, biodiesel is an oxygenated fuel and does not contain aromatics and cycloalkanes. In addition, when mixed with regular diesel, biodiesel can effectively reduce CO, HC, and soot emissions from diesel engines and reduce greenhouse gas emissions [4,5]. Thus, in order to use fuels more efficiently and rationally and to reduce pollutant emissions, it is necessary to understand the combustion characteristics of these liquids. Biodiesel combustion research includes combustion mechanism, combustion stability, and emission characteristics, among other aspects. OH radicals are important combustion intermediates that allow identifying combustion reaction areas more accurately. These species are commonly used to study flame characteristics such as structure, propagation velocity, turbulence, heat release rates, local extinguishing, and OH radical concentration distributions [6e10]. The distribution of OH radicals as determined by planar laser-induced fluorescence (PLIF) can be used to study flame characteristics and combustion mechanisms [11,12]. When burned, hydrocarbon fuels readily generate OH radicals [13]. Since biodiesel fuels contain hydrocarbons, it is desirable to study the combustion of biodiesel by OH-PLIF. Basic research on the combustion characteristics of biodiesel fuels from the perspective of free radical evolution is scarce in the literature. Love Norman et al. [14] used PLIF to measure the concentrations of CH and OH radicals in laminar flames using soy methyl ester, canola methyl ester, and methyl stearate. Alviso et al. [15] used experimental and numerical methods to study biodiesel combustion mechanisms
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* Corresponding author. State Key Laboratory of Complex Nonferrous Metal Resources Cleaning Utilization in Yunnan Province, Kunming, 650093, China. E-mail address: [email protected] (F. Li). https://doi.org/10.1016/j.joei.2018.08.005 1743-9671/© 2018 Energy Institute. Published by Elsevier Ltd. All rights reserved.
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using a laminar counter-flow spray premixed flame. These authors used OH-PLIF to experimentally analyze the structure of flames generated by biodiesel and MD. The combustion characteristics of several fuels in engines (e.g., neat biodiesel, biodiesel, diesel-biodiesel mixtures, and ethanol blended fuels) has been widely studied [16e18]. Therefore, in order to study the characteristics of biodiesel and its combustion of from the perspective of OH radical evolution, a biodiesel laminar premixed flame was produced by using an evaporation and atomization combustion biomass liquid fuel system by OH-PLIF. These results can provide reference values for the characteristics of biodiesel flames and the relationship between OH radicals and combustion emissions. 2. Experimental setup and procedures 2.1. Experimental facility In this study, a biodiesel laminar premixed flame was produced by using an evaporation and atomization combustion biomass liquid fuel system. As shown in Fig. 1, this system included a gas supply carrier system, a fuel supply system, an evaporation and atomization system, and a burner. The fuel supply system comprised a fuel syringe pump and a liquid heater. The gas carrier system was formed by an air path (composed of an air compressor and a flow-meter), an oxygen gas path (composed of an oxygen cylinder and a flow-meter), a gas mixing bottle, and a gas heater (1, Fig. 1). The gas carrier system included an air cylinder, a flow-meter, and a gas heater (2, Fig. 1). The evaporation and atomization system included liquid and gas spray (1 and 2) needles, an evaporation and atomization chamber, a ceramic heater, and a thermocouple. The burner outlet was a stainless-steel tube with an internal diameter of 8 mm. In this system, refined biodiesel was used as the combustion agent, while air was used as the atomization medium. During the test, the air was first heated to 200 C in the gas supply system and injected into the evaporation and atomization chamber through the gas spray needle 2 in order to preheat the system. The gas spray needle 2 has a large diameter of 2 mm. At the same time, a ceramic heater was wrapped outside the chamber as auxiliary heating and insulation material up to 200 C. Second, the biodiesel was heated to 60 C in the fuel supply system and injected into evaporation and atomization chamber through the liquid spray needle. Meanwhile, the air was heated to 200 C in the gas carrier system and injected into the evaporation and atomization chamber through the gas spray needle 1, and the air pressure was adjusted to 0.7 MPa. The biodiesel was sprayed, atomized into small droplets with air, and rapidly evaporated at high temperature after which it entered into the burner. Finally, the air and oxygen were mixed in the gas mixing bottle and heated to 60 C, and the mixture was introduced into the round sleeve of the burner. The biodiesel vaporeair mixture was introduced into the burner through a pipe and burned under the protection of a combustionsupporting gas. The above burner was equipped with an electric heating equipment to prevent condensation of the biodiesel vapor. Particularly, the gas spray needle 1 and the liquid spray needle formed an angle of 90 , and both had an outlet diameter of 0.34 mm. In order to verify the reliability of the system, a Malvern laser particle size analyzer was used to detect the presence of unevaporated droplets at the outlet of the evaporation and atomization system. The result revealed no unevaporated droplets when large amounts of air were employed. These conditions resulted in a good atomization effect, with small droplets easily evaporated. However, small amounts of unevaporated droplets were observed when the amount of atomization air was low. During the test, the fuel injection volume was maintained constant, and the equivalence ratio (F) was changed by controlling the amount of atomization air. Since the gas supply system did not provide a combustion gas, the mixture of biodiesel vapor and air was burned in the external environment. Once stable, the biodiesel laminar flame was detected and analyzed by OH-PLIF. The OH-PLIF system employed herein is shown in Fig. 2. It mainly consisted of a Nd-doped yttrium aluminum garnet (Nd: YAG) laser (Model: LAB-170-10H) with a power of 200 mJ per pulse and a pulse time of 10 ns, a Sirah dye laser (Model: CBST-G-30-EG) with an output wavelength of 283.36 nm was used to excite the OH radicals, and a CCD camera (CCD Type: Imager LX2M 1600 1200 7.4 mm; Intensifier Type: 25 mm V7670U-70-P43) equipped with a ultraviolet (UV) lens and an OH filter. The working environment temperature of the OH-PLIF system was 22 C, and the indoor relative humidity was 62%. The OH-PLIF system can directly provide a two-dimensional (2D) distribution of the OH radical concentration of the
Fig. 1. Scheme of the evaporation and atomization combustion biomass liquid fuel system.
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Fig. 2. Scheme of the OH-PLIF system used herein.
flame during the combustion. The burner was equipped with a flue gas analyzer (MGA5 infrared flue gas analyzer) to measure and analyze the combustion products. Herein, 500 transient pictures were captured at each F. These pictures were subsequently averaged and taken for measurement analysis. 2.2. Experimental materials Two biodiesel samples (Jatropha curcas L. biodiesel and waste oil biodiesel) were used herein. The Jatropha curcas L. biodiesel was made in our labs via transesterification, while the waste oil biodiesel was externally purchased (Manufacture Date: 03/25/2017, Shelf Life: 18 months). The physicochemical properties of both biodiesel samples are shown in Table 1, and the elemental analysis is summarized in Table 2. The theoretical air volume and the F of the biodiesel samples were calculated from the elemental analysis. 2.3. Measurement uncertainties The main source of uncertainties in the measurements [21] was the total flow rate of the premixed unburned gas (UQ) and the calculated flame area image (UA), which was determined by the camera resolution. The uncertainty of the gas flow rate originated from the flow-meter uncertainty (0.5% of reading þ 0.1% full scale) which was estimated to be ca. 2%. The biodiesel mass flows were measured with an accuracy of ca. 5%. Thus, the total flow rate uncertainty of the premixed unburned gas (UQ) was estimated to be ca. 5.4%. The uncertainty derived from the ICCD camera spatial resolution, which was estimated to be ca. 3%. The overall uncertainty was calculated to be within ca. 6%. This overall qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi uncertainty was obtained from the relation U 2Q þ U 2A ) for all the laminar flame recorded at a temperature of 473 K, an F range of 0.9e2.0, and at ambient pressure. Indeed, the fluctuation of the flame position affected the display of the flame structure on the camera during the experiment. This effect modified the position of the flame contours (2e3 pixels), slightly increasing the overall uncertainty. 3. Experimental results and discussion During the test, the fuel injection volume of both biodiesel samples was constant (0.16 mL/min), while the F was changed by controlling the amount of atomization air. The F was selected to range between 0.9 and 2.0. When the F was less than 0.9, the flame was easily blown off, while Fs greater than 2.0 facilitated tempering. Once the flame was stable, for each measurement condition reported in this work, 500 instantaneous images of the OH radicals were systematically recorded by the ICCD camera, and these images were averaged and taken for measurement analysis. We took 5 sets of pictures under each condition for analyze. As shown in Fig. 3, the OH-PLIF average images of the laminar premixed flame generated by both biodiesel samples changed with the F. In the OH-PLIF images, the red and yellow areas corresponded to regions with high concentrations of OH radicals. Thus, the results revealed higher concentrations of OH radicals near the flame front as compared to other areas. The vicinity of the flame front was an area with intense chemical reactions. The distribution of OH radicals at the end of the axial direction of the flame decreased because of the effect of the gas flow speed. The chemical reaction in the flame began at the flame front. Table 1 Physicochemical properties the two biodiesel samples used herein. Physicochemical properties
Waste oil biodiesel
Jatropha curcas L. biodiesel
Density (20 C)/(g/mL) Kinematic viscosity (40 C)/(mm2/s) Closed flash point/ C Ignition point/ C Freezing point/ C Residual carbon/% Calorific value/(kJ/g) Acid value (KOH)/(mg/g) Cetane number
0.875 4.51 164 183 3 0.26 38.98 0.8 51
0.876 4.43 185 196 1 0.063 37.55 0.5 49
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Table 2 Elemental analysis of the two biodiesel samples used herein. Fuel types
C/%
H/%
O/%
N/%
S/%
Jatropha curcas L. biodiesel Waste oil biodiesel
75.75 73.62
12.55 12.69
10.06 12.25
1.270 1.22
0.370 0.22
Fig. 3. OH-PLIF average flame images with different Fs.
3.1. Influence of the F on the flame height Generally, the flame height is determined by visual inspection or by using images taken with a camera. Roper et al. proposed the concept of stoichiometric coefficient flame. According to this concept, the flame height is determined by the distance from the nozzle outlet to the axial position when the ratio of fuel to oxidant is exactly the stoichiometric ratio. The chemiluminescence of the flame is closely related to chemical reactions, and the chemical reactions can be tracked with the OH radicals. Thus, the flame height can be used to calibrate the OHPLIF system, and the calibration results are shown in Fig. 4. As shown in Fig. 4, the flame height increased with F. The air capacity decreased with F, part of the fuel was burned by the outside air via diffusion combustion. As a result, the flame front underwent retrusion and the flame height increased. Waste oil biodiesel generated a flame with lower height than the Jatropha curcas L. biodiesel.
Fig. 4. Flame height as a function of F for the flames generated by two biodiesel samples.
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3.2. Influence of the F on the flame front As shown in Fig. 5, the digital graphic process flow of the flame front information was obtained using the OH-PLIF images of the flame. The process employed herein using digital graphic geometric features was based on gray images. The original images obtained from the OHPLIF system were cut appropriately and processed using MATLAB. These images were gray and binary processed to obtain the binary image. Thus, the flame front area (A) was obtained after a batch processing method including edge extraction, curve-fitting f(x), and calculation of the area after curve-rotation. Since the laminar premixed flame is axisymmetric and the flame front of the 2D images taken using OH-PLIF system was symmetrical on both sides, one side was selected for subsequent research and analysis. The curve f(x) was rotated around the burner in the axial direction to obtain the flame front area (A) as follows [2]:
A ¼ 2p
Zb
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 0 f ðxÞ 1 þ jf ðxÞj2 dx
(1)
a
where (a, b) is the integral interval of the curve f(x). Using the above method, the flame front area for different equivalence ratios of both biodiesel samples was determined, and the results are shown in Fig. 6. The flame front area increased with F to reach a maximum and decreased thereafter. The flame front area of the waste oil biodiesel was smaller than that of the Jatropha curcas L. biodiesel. The flame front area of Jatropha curcas L. biodiesel reached a minimum (0.62 cm2, estimated) for a F of ca. 1.1. Also, when the F was 2.0, the flame front area was estimated to be 5.38 cm2. The flame front area of the waste oil biodiesel reached a minimum (0.80 cm2, estimated) when the F was ca. 1.0. When the F was 2.0, the flame front area was estimated to be 7.97 cm2. Since the airflow decreased with F for a fuel flow constant, part of the fuel required outside air to burn, leading to retro-position of the flame front and larger flame front areas. During lean-fuel combustion, the airflow increased while decreasing the F at constant fuel flow. An increase in the gas mixture flow velocity led to retro-position of the flame front and larger flame front areas. 3.3. Influence of the F or gas mixture heating temperature on the flame speed The laminar flame speed measurements were based on the mass conservation principle between the outlet nozzle and the flame front. The average laminar premixed flame speed on the transverse plane is expressed as [19]:
Su ¼
Q A
(2)
where Q is the total volume flow rate of the fueleair gas mixture and A is the surface area of the flame front. The air used as an atomization medium was controlled by a flowmeter, and the amount of biodiesel evaporated was very low was therefore ignored. Thus, the premixed gas
Fig. 5. Digital graphics process.
Fig. 6. Flame front area as a function of F for the two biodiesel samples used herein.
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flow (Q) was the atomized airflow. The flame front area was also calculated using to the OH-PLIF images, and, finally, the premixed flame speed was obtained using Eq. (2). As shown in Fig. 7, the premixed flame speed reached a maximum with F and decreased thereafter for both biodiesel samples. The flame speed of the waste oil biodiesel was lower than that of Jatropha curcas L. biodiesel. The flame speed of both biodiesel samples reached maxima at F ¼ 1.0 (ca. 27.8 and 35.429 cm/s, respectively). Eq. (3) shows the relationship between the flame speed and different physical parameters [20]:
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi l Tr TB Su ¼ u cp $r$C0 TB T0
(3)
where Su is the flame speed, l is the thermal conductivity of the gas mixture, r is the intensity of the gas mixture, cp is the specific heat of the gas mixture under constant pressure, C0 is the initial concentration of the gas mixture, Tr-TB is the difference between the combustion and the ignition temperatures, TB-T0 is the difference between the ignition and the initial temperatures of the gas mixture, and u is the average chemical reaction speed of the gas mixture in the combustion area. Under lean-fuel conditions, the average chemical reaction speed of the gas mixture was mainly controlled by the concentration of the combusted fuel vapor in combustion. The airflow decreased with F, and, under constant fuel flow, the concentration of the fuel vapor increased in the gas mixture. u increased, and the flame speed also increased as a result. In addition, a decrease in the airflow resulted in larger r and cp for the gas mixture. At this point, u was the dominant factor controlling the laminar flame speed. Thus, the laminar flame speed increased with F during lean-fuel combustion. Under rich-fuel conditions, u was mainly controlled by the air concentration. Under constant fuel flow, the airflow decreased with F, and u decreased while decreasing the air concentration of the gas mixture. In addition, lower airflow values affected u to a lower extent as compared to r and cp. Hence, the laminar flame speed decreased with F during rich-fuel combustion. 3.4. Influence of the F on the distribution of OH radicals in the flame Considering the changes of the OH-PLIF average images for the flames generated by the two biodiesel samples with F (Fig. 3), the variations in the overall intensity of the OH signal of the flame under identical Fs is shown in Fig. 8. As shown in Fig. 8, the OH-PLIF total signal intensity of the flames generated by both biodiesel samples increased with F. The F and the incident laser energy remained unchanged in the experiment, and the test of parameters were consistent with these conditions. The increase in the OH overall signal intensity indicated that the concentration of OH radicals in the flame increased gradually with F. In order to further analyze the distribution of OH radicals in the flame, the OH-PLIF average signal intensity was used herein to analyze the evolution of the OH radical intensity per area of PLIF image with F. According to Fig. 5, a binary image can be obtained by digital graphic processing, and MATLAB can be used to calculate the flame area. The ratio of OH-PLIF overall signal intensity to the flame area was defined as the OH-PLIF average signal intensity. The results are shown in Fig. 9. As shown in Fig. 9, the OH-PLIF average signal intensity for the flames generated by both biodiesel samples reached a maximum with F and decreased thereafter. The OH-PLIF average signal intensity of the waste oil biodiesel reached a maximum at F ¼ 1.2, while the OH-PLIF average signal intensity of the Jatropha curcas L. biodiesel reached a maximum at F ¼ 1.1. Owing to the combustion reaction, the flame may contain radicals, unburned fuel, combustion products, and other products. Therefore, the concentration of OH radical in the combustion field increased with the average OH signal intensity in the flame. At a certain position in the flame field, larger concentrations of OH radicals indicated greater oxidation abilities and higher probabilities of reducing substances being oxidized in this position. 3.5. CO/CO2 emission characteristics in the biodiesel combustion flue gas Fig. 10 shows the CO and CO2 emissions of the flue gasses generated by both biodiesel samples as a function of F. As shown in Fig. 10, the CO emissions were minimized at a certain F and increased thereafter. The opposite trend was found for the CO2 emissions. As shown in Table 2,
Fig. 7. Flame speed as a function of the F for the two biodiesel samples used herein.
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Fig. 8. Evolution of the OH-PLIF overall signal intensity of the flame with F.
Fig. 9. Evolution of the OH-PLIF average signal intensity of the flame with F.
Fig. 10. Evolution of the CO/CO2 emissions with F.
elemental analysis revealed higher carbon contents for the Jatropha curcas L. biodiesel as compared to the waste oil biodiesel. These results can explain the higher CO/CO2 emissions of the Jatropha curcas L. biodiesel combustion flue gas. To further study the relationship between the CO/CO2 emissions and the OH radicals, the ratio of CO/CO2 emissions [CO]/[CO2] ([] stands for the concentration of each substance) was used to explore the relationship with the OH-PLIF signal intensity. As shown in Fig. 11, [CO]/ [CO2] was minimized at F ¼ 1.1 and increased thereafter for both biodiesel samples. Please cite this article as: Y. Shen et al., Study on the characteristics of evaporationeatomizationecombustion of biodiesel, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2018.08.005
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Fig. 11. Evolution of the [CO]/[CO2] ratio with F.
During the combustion of the two biodiesel samples, the generation of CO was mainly controlled by the following elementary reaction [21]: *OH þ CO 4 H* þ CO2
(4)
Therefore, if the concentration of OH radicals increases in the flame, the elementary reaction (4) is produced in the forward direction, promoting the oxidation of large amounts of CO to CO2. As a result, the CO emissions decreased and the production of CO2 increased, resulting in lower [CO]/[CO2] ratios. As shown in Fig. 9, the OH-PLIF average signal intensity for the flames generated by both biodiesel samples reached a maximum with F and decreased thereafter. These results revealed that the evolution of the OH-PLIF average signal intensity was related to that of [CO]/[CO2]. Under the same equivalence ratio, [CO]/[CO2] decreased with the OH-PLIF average signal intensity and vice versa. However, this result needs to be further studied in terms of the control of CO emissions through the OH radical intensity. 3.6. NO emission characteristics of the biodiesel combustion flue gas Fig. 12 shows the NO emissions generated by both biodiesel samples as a function of F. As shown in Fig. 12, the NO emissions decreased with F, with Jatropha curcas L. biodiesel showing higher emissions than the waste oil biodiesel. According to the currently known pathways leading to NO, the main source of NO is high-temperature NO (i.e., thermal NO), which is formed by the splitting of N2 in air. As suggested by the extended Zelkovas mechanism [22,23], the rate of the thermal NO formation reactions is highly temperature-dependent, with higher flame temperatures resulting in higher NO emissions. Although the OH radicals can promote the formation of NO, thermal NO production is mainly controlled by the flame temperature. The adiabatic temperature of the flame decreased with F, and the amount of heat produced also decreased with this parameter. Therefore, it is impossible to provide energy to produce thermal NO steadily (i.e., NO production is suppressed). We used a thermocouple to measure the maximum temperature in the flame under different conditions, and the result are shown in Fig. 13. The flame temperature reached 1800 K at relatively low F values, which
Fig. 12. Evolution of the NO emissions with F.
Please cite this article as: Y. Shen et al., Study on the characteristics of evaporationeatomizationecombustion of biodiesel, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2018.08.005
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Fig. 13. Maximum temperature in flame as a function of F.
is beneficial to the generation of thermal NO. When F increased from 0.9 to 1.3, the NO emission decreased significantly. A further increase in F resulted in the adiabatic temperature of the flame not reaching the temperature required for thermal NO generation. The main NO produced was prompt NO. The Fenimore mechanism also suggested that OH radicals could promote the production of prompt NO. As shown in Fig. 9, the OH-PLIF average signal intensity decreased with F for F values higher than 1.2, and the decreasing range is relatively slow. However, the reduction in NO emissions was relatively slow when F was greater than 1.3, indicating that there was a certain relationship between the NO emissions and the presence of OH radicals. Under similar equivalence ratios, the NO emissions decreased while decreasing the OH-PLIF average signal intensity. 4. Conclusions In this paper, a biodiesel laminar premixed flame was generated using an evaporation and atomization combustion biomass liquid fuel system, and the OH signal of the biodiesel laminar premixed flame was studied at different Fs using OH-PLIF 2D measurements. The main conclusions of this study were: 1) When F increased, the height of the biodiesel laminar premixed flame increased, with the waste oil biodiesel showing lower heights than the Jatropha curcas L. biodiesel. The flame front area reached a maximum with F and decreased thereafter. The flame front area of the waste oil biodiesel was lower than that of the Jatropha curcas L. biodiesel. The premixed flame speed also reached a maximum with F and decreased thereafter, with the waste oil biodiesel showing lower premixed flame speeds than the Jatropha curcas L. biodiesel. 2) The OH-PLIF overall signal intensity of the flames generated by both biodiesel samples increased with F, while the OH-PLIF average signal intensity reached a maximum with F and decreased thereafter. The OH-PLIF average signal intensity of the waste oil biodiesel reached a maximum at F ¼ 1.2, while the Jatropha curcas L. biodiesel reached a maximum signal at F ¼ 1.1. 3) The CO emissions were minimized at a certain F value and increased thereafter, and the opposite trend was found for the CO2 emissions. The CO/CO2 emissions of the Jatropha curcas L. biodiesel combustion flue gas were higher than those of waste oil biodiesel. 4) The generation of CO was mainly controlled by the following elementary reaction: OH þ CO 4 H þ CO2, and higher OH radical intensities suppressed CO production. Under the same F, [CO]/[CO2] decreased with the OH-PLIF average signal intensity and vice versa. 5) The NO emissions decreased with F, with the Jatropha curcas L. biodiesel showing higher emissions than the waste oil biodiesel. Acknowledgments Q6
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Please cite this article as: Y. Shen et al., Study on the characteristics of evaporationeatomizationecombustion of biodiesel, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2018.08.005