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Atomization and combustion of canola methyl ester biofuel spray
Fuel 89 (2010) 3735–3741
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Atomization and combustion of canola methyl ester biofuel spray Jaime A. Erazo Jr., Ramkumar Parthasarathy *, Subramanyam Gollahalli Combustion and Flame Dynamics Laboratory, School of Aerospace and Mechanical Engineering, 865 Asp Ave Room 212, University of Oklahoma, Norman, OK 73019, USA
a r t i c l e
i n f o
Article history: Received 14 July 2009 Received in revised form 10 June 2010 Accepted 15 July 2010 Available online 25 July 2010 Keywords: Biofuel Combustion Spray Emission Drops
a b s t r a c t The spray atomization and combustion characteristics of canola methyl ester (CME) biofuel are compared to those of petroleum based No. 2 diesel fuel in this paper. The spray flame was contained in an optically accessible combustor which was operated at atmospheric pressure with a co-flow of heated air. Fuel was delivered through a swirl-type air-blast atomizer with an injector orifice diameter of 300 lm. A two-component phase Doppler particle analyzer was used to measure the spray droplet size, axial velocity, and radial velocity distributions. Radial and axial distributions of NO, CO, CO2 and O2 concentrations were also obtained. Axial and radial distributions of flame temperature were recorded with a Pt–Pt/13%Rh (type R) thermocouple. The volumetric flow rates of fuel, atomization air and co-flow air were kept constant for both fuels. The droplet Sauter mean diameter (SMD) at the nozzle exit for CME biofuel spray was smaller than that of the No. 2 diesel fuel spray, implying faster vaporization rates for the former. The flame temperature decreased more rapidly for the CME biofuel spray flame than for the No. 2 diesel fuel spray flame in both axial and radial directions. CME biofuel spray flames produced lower in-flame NO and CO peak concentrations than No. 2 diesel fuel spray flames. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Because of the uncertain petroleum prices and the impetus to develop renewable energy sources, biofuels are emerging as alternatives to petroleum fuels with practical applicability to diesel engines, gas turbines, and industrial continuous combustors. Biodiesel fuel has many important advantages over conventional petroleum based fuels. Biodiesel is renewable, carbon-neutral from an environment standpoint, and is sulfur-free. However, one drawback in the use of biodiesel fuels seems to be the increase in NO by 1–14% that has been reported from biodiesel fuelled compression– ignition engines [1–3]. A variety of reasons have been cited for this increase in NO emissions. Increasing iodine number has been correlated with increasing NO emissions from biodiesel fuelled engines [1,4]. Another recent study attributed the increased NO emissions to the increased presence of double bonds in biodiesel fuels [4]. It has also been suggested that the bulk modulus difference between biodiesel and No. 2 diesel fuel causes an advance in the fuel injection when using biodiesel [4–6], resulting in higher temperatures and higher NO. However, the results of experiments with continuous combustion systems such as gas turbine combustors and oil furnaces show the opposite effect: NOx seems to be lowered when certain biofuels are substituted for petroleum fuels, either in the pure form or as blends [7–9].
The laser imaging studies by Dec [10] have revealed that the mechanisms and processes in the combustion of a fuel spray in a diesel engine significantly differ from the earlier model proposed by Faeth [11] that was also applicable to continuous spray combustors. Therefore, the NO emission increases observed in biodiesel fuelled engines may not occur in continuous combustors such as gas turbines. To understand this discrepancy, studies on flame structure of sprays, in a more controlled environment than the complex thermo-chemical environment existing in engines are needed; this idea formed the basis of the present study. In this paper, combustion characteristics of canola methyl ester (CME) biodiesel were documented in a continuous combustor setup. In a companion project, biodiesel combustion in a laminar flame was studied to isolate fuel chemistry effects [12], the results of which provide baseline data for comparison. The specific goal of this paper was to investigate the differences in the combustion and emission characteristics between No. 2 diesel and CME spray flames. Parameters, such as air-preheat temperature, atomization air, and global equivalence ratio, were controlled to provide direct comparison. In-flame temperature, in-flame concentrations of NO, CO, CO2 and O2, and spray droplet size and mean droplet axial/radial velocities were measured.
2. Experimental apparatus * Corresponding author. E-mail addresses: [email protected] (J.A. Erazo Jr.), [email protected] (R. Parthasarathy), [email protected] (S. Gollahalli). 0016-2361/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2010.07.022
The experiments were conducted in a large, steel combustion chamber, shown in Fig. 1. A preheated, air co-flow system was used
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J.A. Erazo Jr. et al. / Fuel 89 (2010) 3735–3741
1 – Fuel Inlet 2 – Air Inlet 3 – Settling Chamber with Marbles 4 – Screen 5 – Air Co- Flow Inlet 6 – Flame Chamber 7 – Injector 8 – Set Screw
10 1 – Air Filter and Rotameter 2 – Co Flow Air Heater 3 – Fuel Rotameter 4 – Air Rotameter 5 – Settling Chamber 6 – Flame Chamber 7 – Fuel Tank 8 – Nitrogen Tank 9 – Steel Chamber 10 – Exhaust Vent
9
51.8 cm 7
6
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Not to Scale
4
7
6
3
8
5
20.3 cm
5
8
2
1
2
3
4
1
Fig. 1. Schematic drawing of combustion chamber. Fig. 2. Schematic drawing of air and fuel tubing.
to deliver combustion air obtained from the lab supply line. Heating was accomplished using a 10 kW electrical resistance heater in conjunction with a temperature controller. A settling chamber was used to provide a uniform flow of air surrounding the spray. The flame was contained in a stainless steel test unit with Vycor glass windows for optical accessibility. The fuel tank was pressurized with nitrogen, while the atomization air was supplied from an air cylinder. An air-blast atomizer with an injector diameter of 300 lm was used to produce the spray. A schematic diagram of the air/fuel tubing is presented in Fig. 2. The spray droplet size and velocity distributions were measured using a 2-channel phase Doppler particle analyzer (PDPA) [13,14]. The source of light was an argon-ion laser operated at 225 mW, which was split into the green (514 nm) and blue (488 nm) light. Bragg cells were used to frequency shift one beam of each color to facilitate the measurement of reversed flows. The receiving optics was set-up in the off-axis (30°) forward-scatter mode. The diameter-measurement system was calibrated with a mono-disperse droplet generator. In general, approximately 10,000 data points were collected at each measurement location in the spray flame and averaged. At certain locations in the spray flame, such as at the edges and far downstream of the injector, it was not possible to collect such a high number of data points within a reasonable amount of time. On an average, approximately 120 s were needed to collect the data points at a given location. The PDPA transmitting and receiving optics were mounted on three-way traverses to move the measuring volume to different locations in the spray flame. The flame was almost symmetric about the vertical axis, therefore, only half-width profiles are presented. Species concentration profiles of CO, CO2, NO, and O2 were measured using a portable gas analyzer. CO and CO2 concentrations
were measured using a non-disperse infrared detector (NDIR) based on the attenuation of the infrared wavelength beam specific to the species [15], while NOx and O2 concentrations were measured using electro-chemical detectors [16]. The samples were collected using a 1 mm diameter orifice, uncooled quartz probe. An inline filter and a condenser were used to filter the moisture and particles before passing the flue gases through the analyzers. The probe was mounted on a two-way traverse. Access into the flame was accomplished by using two custom cut Vycor glass pieces which provided a narrow slot through which the probe was inserted. A thin ceramic gasket material was used to cover the excess slot area. The flame temperature was measured using a silicacoated type R thermocouple (Pt/Pt–Rh 13%), with a 0.35 mm bead diameter. Data acquisition was accomplished using LabView software and a personal computer. All thermocouple data were corrected for radiation and convection errors. Radial profiles of temperature and species concentration profiles were recorded at axial locations at 25%, 50%, and 75% of the visible flame length from the burner exit; the flame length was recorded by a digital camera with a long-time exposure (1 s) in background lighting. The properties of the fuels are provided in Table 1. Test conditions including the fuel and atomization air flow rates are presented in Table 2. A global equivalence ratio of nominal value 0.68 was used to simulate lean burning combustors. All test conditions were held constant with the exception of the air-preheat temperature. CME has a higher initial boiling point than No. 2 diesel fuel (Table 1). Preheating the fuel would aid the atomization and evaporation processes; however, overheating could result in the fuel coking and clogging the atomizer tip. Therefore, the fuel was not preheated. The heat release and pollutant emissions are dependent on the vaporization and modes of combustion of the drops in the spray. In order to minimize the influence of the differ-
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J.A. Erazo Jr. et al. / Fuel 89 (2010) 3735–3741 Table 1 Physical and chemical properties of No. 2 diesel fuel and canola methyl ester fuel. Fuel
Molecular formula
Density (kg/m3)
Boiling point (°C)
Viscosity (cSt)
Heating value (MJ/kg)
Iodine number
No. 2 diesel fuel Canola methyl ester
C16H34 C19H36O2
850 881
150–350 340–405
2.63 at 40 °C 4.37 at 40 °C
42.6 37.4
8.6 115
Table 2 Fuel and air flow rates and temperature settings.
No. 2 diesel fuel Canola methyl ester
Atomization air flow rate (l/min)
Co-flow air flow rate (l/min)
Co-flow air temperature (°C)
Fuel flow rate (ml/min)
6.32 6.32
58 58
100 232
4.6 4.6
ence in boiling points on the combustion characteristics, the co-flow air temperature was adjusted to be a constant fraction (two-thirds) of the initial boiling point in both spray flames. The uncertainties in the measurements were computed following standard procedures and are shown as uncertainty bars in the figures displaying measurements. More details of the set-up and procedure are presented by Erazo [17]. 3. Results and discussion The spray droplet Sauter mean diameter (SMD) profiles for No. 2 diesel fuel and CME fuel are presented in Fig. 3. The refractive index of the drops changes due to the heat transfer in the flame; the change in refractive index was estimated to be 6.8% [18]. The maximum estimated error in the SMD measurement due to the change in refractive index was 5% for the 30-lm drops and 8% for the 60-lm drops [19]. The drop diameter calibration curve slope was set at the initial refractive index value, as suggested by Schneider and Hirleman [19]. In general, the droplet size increased with increasing radial distance from the injector at all axial locations. Due to the swirl imparted by the injector, the large drops were thrown to the spray edge, and took longer to evaporate and burn, resulting in an increase in SMD in the outer edge of the spray, similar to observations made in other investigations [20–22]. The drop sizes in both spray flames are comparable in the near-injector region. Farther downstream of the injector, the drop sizes were comparable for both sprays. At an axial distance of 3 cm from the injector, the SMD of CME was larger than that of the diesel spray flame; this could be due to the smaller drops of CME evaporating faster than the diesel drops [23], leaving the larger drops to remain at this axial location.
No. 2 Diesel Fuel Vf = 4.6 ml/min Vaa = 6.32 l/min Vcf = 58 l/min Tcf = 100 C
60
0.5 cm 1 cm 2 cm 3 cm 3.5 cm
40
20
0
0
80
Axial Distances Downstream of Nozzle
Sauter Mean Diameter (micron)
Sauter Mean Diameter (micron)
80
The axial and radial components of velocity of the No. 2 diesel and CME fuel droplets at varying axial locations in the spray flame are presented in Figs. 4 and 5 respectively. Similar trends are observed in both fuel sprays. The mean droplet axial velocity peaked at the centerline and decreased with increasing radial distance from the injector. The mean axial component of droplet velocity decreased with increasing axial distance from the injector as the spray width increased. This behavior is similar to the gas velocity profile in a jet. The mean axial velocity of the CME drops was higher than that of the diesel drops near the spray edge due to the smaller SMD at these locations. The axial momentum injected was constant for both fuels; the smaller SMD drops have less slip, and therefore higher velocity near the edge. The mean radial component of droplet velocity, on the other hand, increased with increasing radial distance from the injector axis. The swirl effect coupled with the preferential combustion of smaller droplets depleted the number of smaller droplets at the spray edges leaving only larger droplets with more momentum. The radial velocity of the CME spray drops was smaller than the radial velocity of the diesel spray drops because of their smaller size in the near-injector region. A comparison of the in-flame temperature profiles (Fig. 6) of the two spray flames indicates that in the near-injector region (25% flame height) the flame temperatures were similar, but in the far-injector region the CME spray flame had lower temperatures. In some places, the CME in-flame temperature was lower by as much as 200 K than in the No. 2 diesel spray flame. The in-flame concentration profiles of CO, CO2, O2 and NO at different axial locations (25%, 50%, and 75% of visible flame height) are presented in Figs. 7 and 8. The NO concentration measurements are plotted together with the in-flame temperatures to eval-
0.5 1 1.5 Radial Position (cm)
2
CME Fuel Vf = 4.6 ml/min Vaa = 6.32 l/min Vcf = 58 l/min Tcf = 232 C
Axial Distances Downstream of Nozzle
60
0.5 cm 1 cm 2 cm 3 cm
40
20
0
0
0.5 1 1.5 Radial Position (cm)
Fig. 3. Sauter mean diameter profiles for No. 2 diesel fuel and CME fuel spray flames.
2
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Mean Droplet Axial Velocity (m/s)
18
No. 2 Diesel Fuel Vf = 4.6 ml/min Vaa = 6.32 l/min Vcf = 58 l/min Tcf = 100 C
15
Axial Distances Downstream of Nozzle
0.5 cm 1 cm 2 cm 3 cm 3.5 cm
12 9 6 3 0 0
0.25 0.5 0.75 1 1.25 1.5 1.75 Radial Position (cm)
18 Mean Droplet Axial Velocity (m/s)
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15
CME Fuel Vf = 4.6 ml/min Vaa = 6.32 l/min Vcf = 58 l/min Tcf = 232 C
Axial Distances Downstream of Nozzle
0.5 cm 1 cm 2 cm 3 cm
12 9 6 3 0 0
2
0.25 0.5 0.75
1
1.25 1.5 1.75
2
Radial Position (cm)
5
Mean Droplet Radial Velocity (m/s)
Mean Droplet Radial Velocity (m/s)
Fig. 4. Mean droplet axial velocity profiles for No. 2 diesel fuel and CME fuel spray flames.
4 3 2 1 0 Axial Distances Downstream of Nozzle
-1
0.5 cm 1 cm 2 cm 3 cm 3.5 cm
No. 2 Diesel Fuel Vf = 4.6 ml/min Vaa = 6.32 l/min Vcf = 58 l/min Tcf = 100 C
-2 -3 0
0.25 0.5 0.75 1 1.25 1.5 1.75 Radial Position (cm)
5 4 3 2 1 0 -1 -2
CME Fuel Vf = 4.6 ml/min Vaa = 6.32 l/min Vcf = 58 l/min Tcf = 232 C
-3 0
2
Axial Distances Downstream of Nozzle
0.5 cm 1 cm 2 cm 3 cm
0.25 0.5 0.75 1 1.25 1.5 1.75 Radial Position (cm)
2
Fig. 5. Mean radial droplet velocity profiles for No. 2 diesel fuel and CME fuel spray flames.
2000 1800 1600 No. 2 Diesel Fuel Vf = 4.6 ml/min Vaa = 6.32 l/min Vcf = 58 l/min Tcf = 100 C
1400 1200
2200
Axial Distances Downstream of Nozzle 25% Flame Length 50% Flame Length 75% Flame Length
Flame Temperature (K)
Flame Temperature (K)
2200
0
0.5 1 1.5 Radial Position (cm)
2
Axial Distances Downstream of Nozzle 25% Flame Length 50% Flame Length 75% Flame Length
2000 1800 1600 CME Fuel Vf = 4.6 ml/min Vaa = 6.32 l/min Vcf = 58 l/min Tcf = 232 C
1400 1200
0
0.5 1 1.5 Radial Position (cm)
2
Fig. 6. In-flame temperature profiles for No.2 diesel fuel and CME fuel spray flames.
uate the significance of NO production by the Zeldovich mechanism, which is closely correlated with flame temperature. In the diesel spray flame, the concentrations of CO, CO2 and NO peaked at 0.75 cm in the radial direction, while the O2 concentration was at a minimum at this location. The in-flame temperature profile also correlated well with the in-flame concentration profiles of NO, with profiles having peaks at this same location. This behavior
indicates that NO formation in these flames is strongly driven by the Zeldovich mechanism. The in-flame concentration trends imply that the reaction front of the spray flame at this flame height is at approximately 0.75 cm from the flame axis in the radial direction. This behavior was observed at all the flame heights. The CME in-flame concentration profiles displayed trends different from those observed in the diesel spray flame. A clear-cut
J.A. Erazo Jr. et al. / Fuel 89 (2010) 3735–3741
3739
Fig. 7. In-flame concentration profiles of CO, CO2, O2 and NO of No. 2 diesel fuel spray flame.
reaction front was not observed in the CME spray flame. At 25% flame height, the CO concentration peaked at approximately 1.25 cm in the radial direction. The O2 concentration reached a minimum at the centerline and increased in the radial direction, whereas the CO2 concentration peaked at the centerline and decreased in the radial direction. The NO and in-flame temperature profiles did not display any correlation. At all flame heights, the CME spray flame produced less NO than the No. 2 diesel fuel spray flame. This behavior was seen at other flame heights, but in a less pronounced fashion. From these measurements, distinctly different combustion processes can be discerned in the two spray flames. The No. 2 diesel fuel burned in a heterogeneous combustion environment, where fuel droplets of varying size were evaporating and burning. Soot formation and oxidation was evident from the luminous, yellow flame produced. This combustion mode resulted in an increase in-flame temperature and NO formation, as seen in the in-flame measurements. In contrast, increased droplet evaporation, less luminosity, and reduced in-flame temperatures in the CME spray
flame are all indicative of significant homogenous gas phase reactions. The difference is due to the fuel-bound oxygen present in the ester functional group of the CME fuel molecule, which aids the oxidation process and suppresses soot precursors [24]. The lower NO concentrations in the CME spray flame are opposite to the observations of increased NO emissions in biodiesel fuelled, intermittent combustion, compression–ignition engines reported in various studies [1–3]; however, recent spray flame studies under continuous combustion conditions similar to those simulated in this paper, have reported decreases in NOx emissions [7–9] in agreement with our work. This disagreement between the compression–ignition engine and other spray flame studies, at a first glance, may be attributed to the large differences in variability of pressure, temperature, and residence time between compression–ignition engines and continuous combustors. According to the long-accepted Faeth’s model [11] for spray combustion, both spray flames (intermittent or continuous) exhibit similar behavior; therefore, it was initially difficult to rationalize these observations. Besides, we were sur-
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Fig. 8. In-flame concentration profiles of CO, CO2, O2 and NO of CME fuel spray flame.
prised to see that NOx emissions were higher for biofuels than for the diesel fuel when their vapors were burned in gas-flame burners [12], in agreement with the observations made in CI engines. A much closer examination of the more recent, laser-diagnostics based combustion model of Dec [10], reveals that most of the fuel injected into a compression–ignition engine burns in the homogeneous gas-flame mode, and hence accounts for the congruence of observations between the gas burners and CI engines, and variance from those in continuous spray combustors. 4. Summary and conclusions Spray flames of No. 2 diesel fuel and CME fuel were studied under conditions simulating continuous combustors. Droplet size, velocity, in-flame temperature, and in-flame species concentration profiles were obtained for the two flames. The CME spray flame displayed higher rates of droplet evaporation compared to the No. 2 diesel spray. The No. 2 diesel fuel spray produced larger droplets. The smaller drops in the CME spray flame had higher mean axial velocities and lower radial velocities in the far-injector region. The CME spray flame produced less NO at all flame heights compared to the No. 2 diesel flame. Also, the CME spray flame was
up to 200 K cooler than the No. 2 diesel spray flame in the far-burner region. Overall, the No. 2 diesel spray flame operated mostly in the heterogeneous combustion mode, in contrast to more homogenous, gas phase combustion demonstrated by the CME fuel spray flame. The CME spray flame behavior was in agreement with other biodiesel spray flame studies under similar continuous combustion conditions. Also, the variation between CI engine and continues spray combustion in the NOx production of biofuels compared to petroleum fuel can be explained by the differences of the dominant combustion mode. Acknowledgments This work was supported by a grant from the Oklahoma Bioenergy Center. The first author would also like to thank the US Department of Education for funding received through a GAANN fellowship. References [1] McCormick RL, Graboski MS, Alleman TL, Herring AM. Impact of source material and chemical structure on emissions of criteria pollutants from a heavy-duty engine. Environ Sci Technol 2001;35:1742–7.
J.A. Erazo Jr. et al. / Fuel 89 (2010) 3735–3741 [2] Lin Y, Wu YG, Chang CT. Combustion characteristics of waste-oil produced biodiesel/diesel fuel blends. Fuel 2007;86:1772–80. [3] Tat ME, Van Gerpen JH. Fuel property effects on biodiesel. ASAE annual international meeting, paper no. 036034; 2003. [4] Ban-Weiss G, Chen JY, Buchholz BA, Dibble RW. A numerical investigation into the anomalous slight NOx increase when burning biodiesel: a new (old) theory. Am Chem Soc, Fuel Chem Div 2006;51(1):24–30. [5] Szybist JP, Boehman AL, Taylor JD, McCormick RL. Evaluation of formulation strategies to eliminate the biodiesel NOx effect. Fuel Process Technol 2005;86:1109–26. [6] Mueller CJ, Boehman AL, Martin GC. An experimental investigation of the origin of increased NOx emissions when fueling a heavy-duty compression– ignition engine with soy biodiesel. SAE paper no. 2009-01-1792; 2009. [7] Li L. Experimental study of biodiesel spray and combustion characteristics. SAE power train and fluid systems conference and exhibit, paper 2006-01-3250; October 2006. [8] Hashimoto N, Ozawa Y, Mori N, Yuri I, Hamamatsu T. Fundamental combustion characteristics of palm methyl ester as alternative fuel for gas turbines. Fuel 2008;87:3373–8. [9] Habib Z, Parthasarathy RN, Gollahalli SR. Effects of biodiesel on the performance and emission characteristics of a small-scale gas turbine. 47th AIAA aerospace sciences meeting and exhibit, AIAA 2009-0827, Orlando; 2009. [10] Dec JE. A conceptual model of DI diesel engine combustion based on laser sheet imaging. SAE paper no. 910224; 1997. [11] Faeth GM. Current status of droplet and liquid combustion. Prog Energy Combust Sci 1977;3:191–224. [12] Love N, Parthasarathy RN, Gollahalli SR. Rapid characterization of radiation and pollutant emissions of biodiesel and hydrocarbon liquid fuels. J Energy Res Technol 2009;131:012202-1–2-8. [13] Bachalo WD, Houser MJ. Phase/doppler spray analyzer for simultaneous measurements of drop size and velocity distributions. Opt Eng 1984;23:583–90.
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[14] Sankar SV, Bachalo WD. Response characteristics of the phase Doppler particle analyzer for sizing spherical particles larger than the light wavelength. Appl Opt 1991;30:1487–96. [15] Yi SH, Park YW, Han SO, Min N, Kim ES, Ahn TW. Novel NDIR CO2 sensor for indoor air quality monitoring. In: 13th International conference on solid-state sensors, actuators and microsystems, Seoul, South Korea; 2005. [16] Nova Analytical Systems Inc. Instruction manual for NOVA model 7466K portable engine exhaust analyzer. New York: Nova Analytical Systems Inc. [17] Erazo Jr JA. Effects of droplet size and fuel iodine number on biodiesel spray flames. MS thesis. Norman, Oklahoma: School of Aerospace and Mechanical Engineering, University of Oklahoma. [18] Lipkin MR, Kurtz SS. Temperature coefficient of density and refractive index for hydrocarbons in the liquid state. Ind Eng Chem 1941;13:291–5. [19] Schneider M, Hirleman ED. Influence of internal refractive index gradients on size measurements of spherically symmetric particles by phase Doppler anemometry. Appl Opt 1994;33:2379–87. [20] Gupta AK, Presser C, Hodges JT, Avedisian CT. Role of combustion on droplet transport in pressure-atomized spray flames. J Propul Power 1996;12:543–53. [21] Marchine T, Allouis C, Amoresano A, Beretta F. Experimental investigation of a pressure swirl atomizer spray. J Propul Power 2007;23:1096–101. [22] Lee SG. Geometrical effects on spray characteristics of air-pressurized swirl flows. J Mech Sci Technol 2008;22:1633–9. [23] Morin C, Chauveau C, Dagaut P, Gokalp I, Cathonnet M. Vaporization and oxidation of liquid fuel droplets at high temperature and high pressure: application to n-alkanes and vegetable oil methyl esters. Combust Sci Technol 2004;176:499–529. [24] Kitamura T, Ito T, Senda J, Fujimoto H. Detailed chemical kinetic modeling of diesel spray combustion with oxygenated fuels. Society of automotive engineers, SAE 2001 World congress, paper no. 2001-01-1262.
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Atomization and combustion of canola methyl ester biofuel spray Jaime A. Erazo Jr., Ramkumar Parthasarathy *, Subramanyam Gollahalli Combustion and Flame Dynamics Laboratory, School of Aerospace and Mechanical Engineering, 865 Asp Ave Room 212, University of Oklahoma, Norman, OK 73019, USA
a r t i c l e
i n f o
Article history: Received 14 July 2009 Received in revised form 10 June 2010 Accepted 15 July 2010 Available online 25 July 2010 Keywords: Biofuel Combustion Spray Emission Drops
a b s t r a c t The spray atomization and combustion characteristics of canola methyl ester (CME) biofuel are compared to those of petroleum based No. 2 diesel fuel in this paper. The spray flame was contained in an optically accessible combustor which was operated at atmospheric pressure with a co-flow of heated air. Fuel was delivered through a swirl-type air-blast atomizer with an injector orifice diameter of 300 lm. A two-component phase Doppler particle analyzer was used to measure the spray droplet size, axial velocity, and radial velocity distributions. Radial and axial distributions of NO, CO, CO2 and O2 concentrations were also obtained. Axial and radial distributions of flame temperature were recorded with a Pt–Pt/13%Rh (type R) thermocouple. The volumetric flow rates of fuel, atomization air and co-flow air were kept constant for both fuels. The droplet Sauter mean diameter (SMD) at the nozzle exit for CME biofuel spray was smaller than that of the No. 2 diesel fuel spray, implying faster vaporization rates for the former. The flame temperature decreased more rapidly for the CME biofuel spray flame than for the No. 2 diesel fuel spray flame in both axial and radial directions. CME biofuel spray flames produced lower in-flame NO and CO peak concentrations than No. 2 diesel fuel spray flames. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Because of the uncertain petroleum prices and the impetus to develop renewable energy sources, biofuels are emerging as alternatives to petroleum fuels with practical applicability to diesel engines, gas turbines, and industrial continuous combustors. Biodiesel fuel has many important advantages over conventional petroleum based fuels. Biodiesel is renewable, carbon-neutral from an environment standpoint, and is sulfur-free. However, one drawback in the use of biodiesel fuels seems to be the increase in NO by 1–14% that has been reported from biodiesel fuelled compression– ignition engines [1–3]. A variety of reasons have been cited for this increase in NO emissions. Increasing iodine number has been correlated with increasing NO emissions from biodiesel fuelled engines [1,4]. Another recent study attributed the increased NO emissions to the increased presence of double bonds in biodiesel fuels [4]. It has also been suggested that the bulk modulus difference between biodiesel and No. 2 diesel fuel causes an advance in the fuel injection when using biodiesel [4–6], resulting in higher temperatures and higher NO. However, the results of experiments with continuous combustion systems such as gas turbine combustors and oil furnaces show the opposite effect: NOx seems to be lowered when certain biofuels are substituted for petroleum fuels, either in the pure form or as blends [7–9].
The laser imaging studies by Dec [10] have revealed that the mechanisms and processes in the combustion of a fuel spray in a diesel engine significantly differ from the earlier model proposed by Faeth [11] that was also applicable to continuous spray combustors. Therefore, the NO emission increases observed in biodiesel fuelled engines may not occur in continuous combustors such as gas turbines. To understand this discrepancy, studies on flame structure of sprays, in a more controlled environment than the complex thermo-chemical environment existing in engines are needed; this idea formed the basis of the present study. In this paper, combustion characteristics of canola methyl ester (CME) biodiesel were documented in a continuous combustor setup. In a companion project, biodiesel combustion in a laminar flame was studied to isolate fuel chemistry effects [12], the results of which provide baseline data for comparison. The specific goal of this paper was to investigate the differences in the combustion and emission characteristics between No. 2 diesel and CME spray flames. Parameters, such as air-preheat temperature, atomization air, and global equivalence ratio, were controlled to provide direct comparison. In-flame temperature, in-flame concentrations of NO, CO, CO2 and O2, and spray droplet size and mean droplet axial/radial velocities were measured.
2. Experimental apparatus * Corresponding author. E-mail addresses: [email protected] (J.A. Erazo Jr.), [email protected] (R. Parthasarathy), [email protected] (S. Gollahalli). 0016-2361/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2010.07.022
The experiments were conducted in a large, steel combustion chamber, shown in Fig. 1. A preheated, air co-flow system was used
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J.A. Erazo Jr. et al. / Fuel 89 (2010) 3735–3741
1 – Fuel Inlet 2 – Air Inlet 3 – Settling Chamber with Marbles 4 – Screen 5 – Air Co- Flow Inlet 6 – Flame Chamber 7 – Injector 8 – Set Screw
10 1 – Air Filter and Rotameter 2 – Co Flow Air Heater 3 – Fuel Rotameter 4 – Air Rotameter 5 – Settling Chamber 6 – Flame Chamber 7 – Fuel Tank 8 – Nitrogen Tank 9 – Steel Chamber 10 – Exhaust Vent
9
51.8 cm 7
6
Not to Scale
Not to Scale
4
7
6
3
8
5
20.3 cm
5
8
2
1
2
3
4
1
Fig. 1. Schematic drawing of combustion chamber. Fig. 2. Schematic drawing of air and fuel tubing.
to deliver combustion air obtained from the lab supply line. Heating was accomplished using a 10 kW electrical resistance heater in conjunction with a temperature controller. A settling chamber was used to provide a uniform flow of air surrounding the spray. The flame was contained in a stainless steel test unit with Vycor glass windows for optical accessibility. The fuel tank was pressurized with nitrogen, while the atomization air was supplied from an air cylinder. An air-blast atomizer with an injector diameter of 300 lm was used to produce the spray. A schematic diagram of the air/fuel tubing is presented in Fig. 2. The spray droplet size and velocity distributions were measured using a 2-channel phase Doppler particle analyzer (PDPA) [13,14]. The source of light was an argon-ion laser operated at 225 mW, which was split into the green (514 nm) and blue (488 nm) light. Bragg cells were used to frequency shift one beam of each color to facilitate the measurement of reversed flows. The receiving optics was set-up in the off-axis (30°) forward-scatter mode. The diameter-measurement system was calibrated with a mono-disperse droplet generator. In general, approximately 10,000 data points were collected at each measurement location in the spray flame and averaged. At certain locations in the spray flame, such as at the edges and far downstream of the injector, it was not possible to collect such a high number of data points within a reasonable amount of time. On an average, approximately 120 s were needed to collect the data points at a given location. The PDPA transmitting and receiving optics were mounted on three-way traverses to move the measuring volume to different locations in the spray flame. The flame was almost symmetric about the vertical axis, therefore, only half-width profiles are presented. Species concentration profiles of CO, CO2, NO, and O2 were measured using a portable gas analyzer. CO and CO2 concentrations
were measured using a non-disperse infrared detector (NDIR) based on the attenuation of the infrared wavelength beam specific to the species [15], while NOx and O2 concentrations were measured using electro-chemical detectors [16]. The samples were collected using a 1 mm diameter orifice, uncooled quartz probe. An inline filter and a condenser were used to filter the moisture and particles before passing the flue gases through the analyzers. The probe was mounted on a two-way traverse. Access into the flame was accomplished by using two custom cut Vycor glass pieces which provided a narrow slot through which the probe was inserted. A thin ceramic gasket material was used to cover the excess slot area. The flame temperature was measured using a silicacoated type R thermocouple (Pt/Pt–Rh 13%), with a 0.35 mm bead diameter. Data acquisition was accomplished using LabView software and a personal computer. All thermocouple data were corrected for radiation and convection errors. Radial profiles of temperature and species concentration profiles were recorded at axial locations at 25%, 50%, and 75% of the visible flame length from the burner exit; the flame length was recorded by a digital camera with a long-time exposure (1 s) in background lighting. The properties of the fuels are provided in Table 1. Test conditions including the fuel and atomization air flow rates are presented in Table 2. A global equivalence ratio of nominal value 0.68 was used to simulate lean burning combustors. All test conditions were held constant with the exception of the air-preheat temperature. CME has a higher initial boiling point than No. 2 diesel fuel (Table 1). Preheating the fuel would aid the atomization and evaporation processes; however, overheating could result in the fuel coking and clogging the atomizer tip. Therefore, the fuel was not preheated. The heat release and pollutant emissions are dependent on the vaporization and modes of combustion of the drops in the spray. In order to minimize the influence of the differ-
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J.A. Erazo Jr. et al. / Fuel 89 (2010) 3735–3741 Table 1 Physical and chemical properties of No. 2 diesel fuel and canola methyl ester fuel. Fuel
Molecular formula
Density (kg/m3)
Boiling point (°C)
Viscosity (cSt)
Heating value (MJ/kg)
Iodine number
No. 2 diesel fuel Canola methyl ester
C16H34 C19H36O2
850 881
150–350 340–405
2.63 at 40 °C 4.37 at 40 °C
42.6 37.4
8.6 115
Table 2 Fuel and air flow rates and temperature settings.
No. 2 diesel fuel Canola methyl ester
Atomization air flow rate (l/min)
Co-flow air flow rate (l/min)
Co-flow air temperature (°C)
Fuel flow rate (ml/min)
6.32 6.32
58 58
100 232
4.6 4.6
ence in boiling points on the combustion characteristics, the co-flow air temperature was adjusted to be a constant fraction (two-thirds) of the initial boiling point in both spray flames. The uncertainties in the measurements were computed following standard procedures and are shown as uncertainty bars in the figures displaying measurements. More details of the set-up and procedure are presented by Erazo [17]. 3. Results and discussion The spray droplet Sauter mean diameter (SMD) profiles for No. 2 diesel fuel and CME fuel are presented in Fig. 3. The refractive index of the drops changes due to the heat transfer in the flame; the change in refractive index was estimated to be 6.8% [18]. The maximum estimated error in the SMD measurement due to the change in refractive index was 5% for the 30-lm drops and 8% for the 60-lm drops [19]. The drop diameter calibration curve slope was set at the initial refractive index value, as suggested by Schneider and Hirleman [19]. In general, the droplet size increased with increasing radial distance from the injector at all axial locations. Due to the swirl imparted by the injector, the large drops were thrown to the spray edge, and took longer to evaporate and burn, resulting in an increase in SMD in the outer edge of the spray, similar to observations made in other investigations [20–22]. The drop sizes in both spray flames are comparable in the near-injector region. Farther downstream of the injector, the drop sizes were comparable for both sprays. At an axial distance of 3 cm from the injector, the SMD of CME was larger than that of the diesel spray flame; this could be due to the smaller drops of CME evaporating faster than the diesel drops [23], leaving the larger drops to remain at this axial location.
No. 2 Diesel Fuel Vf = 4.6 ml/min Vaa = 6.32 l/min Vcf = 58 l/min Tcf = 100 C
60
0.5 cm 1 cm 2 cm 3 cm 3.5 cm
40
20
0
0
80
Axial Distances Downstream of Nozzle
Sauter Mean Diameter (micron)
Sauter Mean Diameter (micron)
80
The axial and radial components of velocity of the No. 2 diesel and CME fuel droplets at varying axial locations in the spray flame are presented in Figs. 4 and 5 respectively. Similar trends are observed in both fuel sprays. The mean droplet axial velocity peaked at the centerline and decreased with increasing radial distance from the injector. The mean axial component of droplet velocity decreased with increasing axial distance from the injector as the spray width increased. This behavior is similar to the gas velocity profile in a jet. The mean axial velocity of the CME drops was higher than that of the diesel drops near the spray edge due to the smaller SMD at these locations. The axial momentum injected was constant for both fuels; the smaller SMD drops have less slip, and therefore higher velocity near the edge. The mean radial component of droplet velocity, on the other hand, increased with increasing radial distance from the injector axis. The swirl effect coupled with the preferential combustion of smaller droplets depleted the number of smaller droplets at the spray edges leaving only larger droplets with more momentum. The radial velocity of the CME spray drops was smaller than the radial velocity of the diesel spray drops because of their smaller size in the near-injector region. A comparison of the in-flame temperature profiles (Fig. 6) of the two spray flames indicates that in the near-injector region (25% flame height) the flame temperatures were similar, but in the far-injector region the CME spray flame had lower temperatures. In some places, the CME in-flame temperature was lower by as much as 200 K than in the No. 2 diesel spray flame. The in-flame concentration profiles of CO, CO2, O2 and NO at different axial locations (25%, 50%, and 75% of visible flame height) are presented in Figs. 7 and 8. The NO concentration measurements are plotted together with the in-flame temperatures to eval-
0.5 1 1.5 Radial Position (cm)
2
CME Fuel Vf = 4.6 ml/min Vaa = 6.32 l/min Vcf = 58 l/min Tcf = 232 C
Axial Distances Downstream of Nozzle
60
0.5 cm 1 cm 2 cm 3 cm
40
20
0
0
0.5 1 1.5 Radial Position (cm)
Fig. 3. Sauter mean diameter profiles for No. 2 diesel fuel and CME fuel spray flames.
2
J.A. Erazo Jr. et al. / Fuel 89 (2010) 3735–3741
Mean Droplet Axial Velocity (m/s)
18
No. 2 Diesel Fuel Vf = 4.6 ml/min Vaa = 6.32 l/min Vcf = 58 l/min Tcf = 100 C
15
Axial Distances Downstream of Nozzle
0.5 cm 1 cm 2 cm 3 cm 3.5 cm
12 9 6 3 0 0
0.25 0.5 0.75 1 1.25 1.5 1.75 Radial Position (cm)
18 Mean Droplet Axial Velocity (m/s)
3738
15
CME Fuel Vf = 4.6 ml/min Vaa = 6.32 l/min Vcf = 58 l/min Tcf = 232 C
Axial Distances Downstream of Nozzle
0.5 cm 1 cm 2 cm 3 cm
12 9 6 3 0 0
2
0.25 0.5 0.75
1
1.25 1.5 1.75
2
Radial Position (cm)
5
Mean Droplet Radial Velocity (m/s)
Mean Droplet Radial Velocity (m/s)
Fig. 4. Mean droplet axial velocity profiles for No. 2 diesel fuel and CME fuel spray flames.
4 3 2 1 0 Axial Distances Downstream of Nozzle
-1
0.5 cm 1 cm 2 cm 3 cm 3.5 cm
No. 2 Diesel Fuel Vf = 4.6 ml/min Vaa = 6.32 l/min Vcf = 58 l/min Tcf = 100 C
-2 -3 0
0.25 0.5 0.75 1 1.25 1.5 1.75 Radial Position (cm)
5 4 3 2 1 0 -1 -2
CME Fuel Vf = 4.6 ml/min Vaa = 6.32 l/min Vcf = 58 l/min Tcf = 232 C
-3 0
2
Axial Distances Downstream of Nozzle
0.5 cm 1 cm 2 cm 3 cm
0.25 0.5 0.75 1 1.25 1.5 1.75 Radial Position (cm)
2
Fig. 5. Mean radial droplet velocity profiles for No. 2 diesel fuel and CME fuel spray flames.
2000 1800 1600 No. 2 Diesel Fuel Vf = 4.6 ml/min Vaa = 6.32 l/min Vcf = 58 l/min Tcf = 100 C
1400 1200
2200
Axial Distances Downstream of Nozzle 25% Flame Length 50% Flame Length 75% Flame Length
Flame Temperature (K)
Flame Temperature (K)
2200
0
0.5 1 1.5 Radial Position (cm)
2
Axial Distances Downstream of Nozzle 25% Flame Length 50% Flame Length 75% Flame Length
2000 1800 1600 CME Fuel Vf = 4.6 ml/min Vaa = 6.32 l/min Vcf = 58 l/min Tcf = 232 C
1400 1200
0
0.5 1 1.5 Radial Position (cm)
2
Fig. 6. In-flame temperature profiles for No.2 diesel fuel and CME fuel spray flames.
uate the significance of NO production by the Zeldovich mechanism, which is closely correlated with flame temperature. In the diesel spray flame, the concentrations of CO, CO2 and NO peaked at 0.75 cm in the radial direction, while the O2 concentration was at a minimum at this location. The in-flame temperature profile also correlated well with the in-flame concentration profiles of NO, with profiles having peaks at this same location. This behavior
indicates that NO formation in these flames is strongly driven by the Zeldovich mechanism. The in-flame concentration trends imply that the reaction front of the spray flame at this flame height is at approximately 0.75 cm from the flame axis in the radial direction. This behavior was observed at all the flame heights. The CME in-flame concentration profiles displayed trends different from those observed in the diesel spray flame. A clear-cut
J.A. Erazo Jr. et al. / Fuel 89 (2010) 3735–3741
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Fig. 7. In-flame concentration profiles of CO, CO2, O2 and NO of No. 2 diesel fuel spray flame.
reaction front was not observed in the CME spray flame. At 25% flame height, the CO concentration peaked at approximately 1.25 cm in the radial direction. The O2 concentration reached a minimum at the centerline and increased in the radial direction, whereas the CO2 concentration peaked at the centerline and decreased in the radial direction. The NO and in-flame temperature profiles did not display any correlation. At all flame heights, the CME spray flame produced less NO than the No. 2 diesel fuel spray flame. This behavior was seen at other flame heights, but in a less pronounced fashion. From these measurements, distinctly different combustion processes can be discerned in the two spray flames. The No. 2 diesel fuel burned in a heterogeneous combustion environment, where fuel droplets of varying size were evaporating and burning. Soot formation and oxidation was evident from the luminous, yellow flame produced. This combustion mode resulted in an increase in-flame temperature and NO formation, as seen in the in-flame measurements. In contrast, increased droplet evaporation, less luminosity, and reduced in-flame temperatures in the CME spray
flame are all indicative of significant homogenous gas phase reactions. The difference is due to the fuel-bound oxygen present in the ester functional group of the CME fuel molecule, which aids the oxidation process and suppresses soot precursors [24]. The lower NO concentrations in the CME spray flame are opposite to the observations of increased NO emissions in biodiesel fuelled, intermittent combustion, compression–ignition engines reported in various studies [1–3]; however, recent spray flame studies under continuous combustion conditions similar to those simulated in this paper, have reported decreases in NOx emissions [7–9] in agreement with our work. This disagreement between the compression–ignition engine and other spray flame studies, at a first glance, may be attributed to the large differences in variability of pressure, temperature, and residence time between compression–ignition engines and continuous combustors. According to the long-accepted Faeth’s model [11] for spray combustion, both spray flames (intermittent or continuous) exhibit similar behavior; therefore, it was initially difficult to rationalize these observations. Besides, we were sur-
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J.A. Erazo Jr. et al. / Fuel 89 (2010) 3735–3741
Fig. 8. In-flame concentration profiles of CO, CO2, O2 and NO of CME fuel spray flame.
prised to see that NOx emissions were higher for biofuels than for the diesel fuel when their vapors were burned in gas-flame burners [12], in agreement with the observations made in CI engines. A much closer examination of the more recent, laser-diagnostics based combustion model of Dec [10], reveals that most of the fuel injected into a compression–ignition engine burns in the homogeneous gas-flame mode, and hence accounts for the congruence of observations between the gas burners and CI engines, and variance from those in continuous spray combustors. 4. Summary and conclusions Spray flames of No. 2 diesel fuel and CME fuel were studied under conditions simulating continuous combustors. Droplet size, velocity, in-flame temperature, and in-flame species concentration profiles were obtained for the two flames. The CME spray flame displayed higher rates of droplet evaporation compared to the No. 2 diesel spray. The No. 2 diesel fuel spray produced larger droplets. The smaller drops in the CME spray flame had higher mean axial velocities and lower radial velocities in the far-injector region. The CME spray flame produced less NO at all flame heights compared to the No. 2 diesel flame. Also, the CME spray flame was
up to 200 K cooler than the No. 2 diesel spray flame in the far-burner region. Overall, the No. 2 diesel spray flame operated mostly in the heterogeneous combustion mode, in contrast to more homogenous, gas phase combustion demonstrated by the CME fuel spray flame. The CME spray flame behavior was in agreement with other biodiesel spray flame studies under similar continuous combustion conditions. Also, the variation between CI engine and continues spray combustion in the NOx production of biofuels compared to petroleum fuel can be explained by the differences of the dominant combustion mode. Acknowledgments This work was supported by a grant from the Oklahoma Bioenergy Center. The first author would also like to thank the US Department of Education for funding received through a GAANN fellowship. References [1] McCormick RL, Graboski MS, Alleman TL, Herring AM. Impact of source material and chemical structure on emissions of criteria pollutants from a heavy-duty engine. Environ Sci Technol 2001;35:1742–7.
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