Combustion of straight algae oil in a swirl-stabilized burner using a novel twin-fluid injector

Fuel 241 (2019) 176–187

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Full Length Article

Combustion of straight algae oil in a swirl-stabilized burner using a novel twin-fluid injector

T

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Oladapo S. Akinyemia, Lulin Jianga,b, , Rafael Hernandezb,c, Carl McIntyrec, Williams Holmesb,c a

Department of Mechanical Engineering, University of Louisiana at Lafayette, LA 70503, USA Energy Institute of Louisiana, University of Louisiana at Lafayette, LA 70503, USA c Department of Chemical Engineering, University of Louisiana at Lafayette, LA 70503, USA b

A R T I C LE I N FO

A B S T R A C T

Keywords: Algae oil Property characterization Swirl-burst injector Lean-premixed combustion Ultra-low emissions

The current study investigates the combustion performance of straight algae oil (AO) in a 7-kW lab-scale gas turbine burner enabled by a novel twin-fluid injector, named Swirl-burst (SB) injector. The chemical structure (fatty acid profile), the physical, and chemical properties of AO are acquired to understand the combustibility of the oil as a potential biofuel. Effects of equivalence ratio (ER) and atomizing air to liquid mass ratio (ALR) across the injector on the global combustion characteristics are investigated at a constant heat release rate for the oil. The features of interest include visual flame images, product gas temperature, emissions of carbon monoxide (CO), and nitrogen oxides (NOx) at the combustor exit. Results show that mono-unsaturated fatty acid is predominant in the composition of the oil, suggesting possibly short ignition delay. AO has a heating value comparable to that of diesel but with a high kinematic viscosity (approximately 16 times more viscous than diesel). Clean combustion highly depends on fine sprays that lead to fast fuel pre-vaporization, thorough fuel-air mixing, and thus clean premixed combustion. Conventional injectors such as air blast (AB) atomizers cannot finely atomize viscous oils for clean combustion due to the low viscosity tolerance. Fortunately, with the SB injection, clean, complete, and lean-premixed combustion of straight AO has been successfully achieved without fuel preheating at most of the investigated cases, reasonably reflecting the fine atomization capability of the SB injector even for the viscous oil. The blue flames, overlapping temperature profiles, and low emissions of CO and NOx consistently show the clean lean-premixed AO flames. Stable and clean flames are obtained at ERs of 0.60–0.75 (with the optimum ER identified at 0.65, in terms of flame stability and low emissions), and a blowout limit at the ER of about 0.55. At the ER of 0.65, clean and lean-premixed flames are also acquired for all the tested ALRs (2.0–5.0). Increase in ALR varies the SAA and spray behavior of the SB injection as well as the aerodynamic interaction between fuel and oxidizer. The increasing ALR results in dominantly blue flames and decrease in CO and NOx concentrations, less than 10 ppm at ALRs > 2.5, due to finer atomization and better fuel-air mixing at the higher ALRs, and thus enhanced premixed combustion. Overall, clean and stable leanpremixed combustion of straight AO is achieved without fuel preheating using the novel SB injector despite the high fuel viscosity. The SB injection potentially enables AO itself as a cost-effective and near-zero-emission biofuel.

1. Introduction Biodiesel has been a significant interest of recent biofuel research to combat greenhouse gas emissions [1,2] due to its closed carbon cycle and “drop-in” property to potentially replace fossil fuels without major modification of current technology [3,4]. Biodiesels are mostly derived from source oils such as soybean oil, sunflower oil, palm oil and rapeseed oil [4]. However, the sustainable production of these source oils for biodiesel generation has been a major concern because of large-area

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plantation requirement [4]. Recently, algae because of its high biomass yield per unit of light and area, fast growth cycle, high oil content, minimal agricultural land use, and inedibility to avoid competition between food and energy production markets, has received much attention as a biomass source for biofuel production [3,5,6]. Algae culture, used to produce the oil, plays the role of the photosynthetic organism that captures carbon dioxide (CO2). The oil serves as biofuel with zero or negative net carbon balance, rendering algae oil (AO) a third-generation source oil of biodiesel [3,7–11].

Corresponding author at: 241 E Lewis St, Rougeau Hall Rm 320, Lafayette, LA 70503, USA. E-mail address: [email protected] (L. Jiang).

https://doi.org/10.1016/j.fuel.2018.12.006 Received 12 July 2018; Received in revised form 15 September 2018; Accepted 3 December 2018 0016-2361/ © 2018 Elsevier Ltd. All rights reserved.

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Nomenclature AA AB AC ADB ALR AO CO CO2 D ER FAME FB H

HRR ID ISN LPM MFC MFM NOx PA PIV ppm PS SAA SB Slpm SN VO

Atomizing air Air blast average number of carbon atoms Average number of double bonds Atomizing air to liquid mass ratio Algae oil Carbon monoxide Carbon dioxide Diameter (mm) Equivalence ratio Fatty acid methyl ester Flow blurring Gap between liquid tube tip and the injector exit orifice (mm)

Heat release rate Ignition delay Injector swirl number Lean-premixed Mass flow controller Mass flow meter Nitrogen oxides Primary air Particle Image Velocimetry Parts per million Pressure swirl Swirling atomizing air Swirl burst Standard liters per minute Swirl number Vegetable oil

Fine droplets result in rapid fuel vaporization, better fuel–air mixing, and thus clean premixed flames. Hence, FB injection results in clean combustion of different fuels with widely varied viscosities [17–19,21]. These fuels include kerosene, diesel, biodiesel, viscous straight VO, and glycerol without fuel-preheating, signifying that the cost of biofuel production can be significantly reduced by not converting the source oils such as VO, AO, nor post-processing glycerol. Prior studies have also explored the spray characteristics of the FB injectors [20,22,23]. Jiang and Agrawal (2015) investigated the FB atomization in the near field of the nozzle exit for water and highly viscous glycerol using spatially-resolved high-speed imaging and Particle Imaging Velocimetry (PIV) [20,23]. Results showed that water had been atomized into mostly fine droplets at the nozzle exit, from the bubble bursting of the bubbly two-phase flow characterized as the primary breakup in the FB, with relatively larger ones at the spray periphery [20]. Interestingly, both fine droplets and thin ligaments were observed from the primary atomization of viscous glycerol which has about 200 times higher viscosity than that of diesel [23]. The larger droplets/ligaments underwent secondary breakup by Rayleigh-Taylor’s instabilities between the liquid phase and high-velocity atomizing air [20,23]. The secondary atomization of viscous liquids such as glycerol is more significant and was accomplished at around 30 mm downstream of the injector exit, compared to that of around 4 mm for water spray from an FB atomizer with an exit orifice diameter, D, of 1.5 mm [20,23]. Consequently, FB

Fine atomization of liquid fuels significantly affects fuel pre-vaporization, fuel–air mixing, and thus the subsequent flame propagation and emissions. Conventional injectors, such as air blast (AB) and pressure swirl (PS) injectors, are effective for low-viscosity liquid fuels. They first generate jets or sheets that gradually disintegrate into fine droplets by shear layer instabilities between the high-velocity atomizing gas and liquid phases [12]. However, the high viscosity and surface tension of source oils suppress the shear layer instabilities, thus hindering the generation of fine sprays for efficient and clean combustion using the existing injectors [12]. Hence, transesterification is necessitated to modify the physical properties of viscous bio-oil (algae oil) or vegetable oil (VO) to produce biodiesel that has similar properties to diesel [2,13–15]. However, the transesterification process requires significant energy input and results in abundant glycerol-byproduct that is extremely viscous and difficult to dispose of [13,15]. Thus, the conventional injection technology contributes to the high production cost of biofuels and limits their widespread applications. Fortunately, viscous liquids such as VO (the source oil) and glycerol (the viscous byproduct of biodiesel production) have been finely atomized using a flow blurring (FB) injection concept reported by Gañán Calvo (2005) [16], and hence efficiently and cleanly burned [17–19]. The FB injector forms a stable internal two-phase flow, and generates fine sprays immediately at the injector exit rather than a typical liquid jet or sheet from conventional injectors, as shown in Fig. 1(a-b) [20].

Fig. 1. Water spray images for (a) AB injector (Model: Delavan 30609-2) and (b) FB injector at the liquid flow rate of 8 mlpm and ALR of 3.5 [20]. 177

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sprays of viscous fuels experienced elongated fuel pre-vaporization and mixing zone, hence longer and further lifted flames that were susceptible to blow-out, offsetting flame stability and sustainability [17–19]. In addition, larger droplets were mainly detected at the FB spray periphery [20,23]. Diffusion/incomplete burning of the big drops for viscous fuels including straight VO and glycerol resulted in high CO emissions close to the combustor wall, though mainly clean and lean premixed combustion was achieved by the FB injector without fuel preheating [17–19]. The prior results of FB injection prompted the design of a novel twin-fluid injector to overcome the above challenges while using its advantages by integrating swirling atomizing air (SAA) with the fine FB injection. The SAA aims to rapidly disintegrate the larger droplets and/ or ligaments into finer droplets, hence improving fuel pre-vaporization and fuel-air mixing, to produce stable and clean premixed combustion. Swirling flow provides an aerodynamic flame stabilization in reacting systems with the flows characterized by the swirl number (SN), a dimensionless variable for comparing axial flux of tangential to axial flux of axial momentum [24]. Prior research has shown that swirling of combustion air can improve fuel-air mixing and enhance flame stability [25–30]. Based on the advantages of swirling flow, our group recently designed a novel injector - the Swirl-Burst (SB) injector–incorporating a swirling path for atomizing air to aerodynamically strengthen the secondary atomization. Preliminary study using the SB injector showed that the injector successfully generated the SAA, finer droplets at axial locations in the near field of the injector exit, wider spray angle with earlier atomization completion compared to the FB nozzle hence, enhanced secondary atomization [31]. The present study aims to use the novel SB injector to achieve stable and clean combustion of straight AO without fuel preheating. The work characterizes relevant properties of algae oil and investigates the effect of equivalence ratio (ER) and atomizing air-to-liquid mass ratio (ALR), through the SB injector, on the global combustion characteristics of straight AO.

⎡ 2 1− SN = ⎢ 3 ⎢1 − ⎢ ⎣

dh 3 dt d 2

( ) ⎤⎥ tan(α) ( ) ⎥⎥⎦ h

dt

(1)

where dh is the hub diameter, dt is the tip diameter of the swirl, and α is the vane angle of swirl, defined as the angle between the tangent to the tip of the curved vane and the axial plane. The SB fuel injector consists of a central liquid-fuel port, and an annular channel around the liquidfuel port that the atomizing air flows through. Liquid fuel enters the injector through a sidewise inlet channel on an injector holder, and the atomizing air is supplied through the bottom of the holder. The SB concept incorporates the FB atomization with SAA. The flow blurring concept is ensured by (1) having a gap of H = 0.25D between the liquid fuel tube tip and the injector exit orifice, and (2) an identical inside diameter D = 1.5 mm for both the internal liquid tubing and the injector exit orifice. In the FB atomization, the atomizing air (AA) bifurcates as it reaches the gap H. A small amount of AA flows back into the liquid tubing to form internal bubble flow that results in the FB primary atomization [16], while the major portion that has a high momentum leaves through the exit to assist with the secondary atomization in a weak co-flow pattern with the spray [34]. In contrast, the SB concept geometrically guides most of the AA leaving the exit in a swirling flow, using the high momentum of the main part of AA to trigger vigorous shearing interaction between the liquid phase and the swirling air. This further enhances the secondary disintegration of droplets/ligaments in the near region of injector exit. In the current study, the SAA leaves the injector exit through a swirl path generated with an axial curved vane at 70-degree angle with respect to the axial plane. This gives an injector-swirl-number (ISN), defined as the magnitude of the swirl of the atomizing air generated by the injector, of approximately 2.4. 2.2. Preliminary comparison of spray diagnostics for the SB and FB injection

2. Design of the novel swirl burst injector and preliminary results

A high spatial resolution Particle Image Velocimetry (PIV) system was used in the preliminary experiments to investigate the effectiveness of the novel SB design, in terms of successful generation of SAA, and improved atomization compared to the FB injection. Water was investigated as the working fluid. The investigation was conducted to compare the SB injector with SN of 2.4 and the FB injector with identical D of 1.5 mm and H of 0.375 mm [16,31]. A dual head Nd-YAG laser, from Litron Lasers (Model No.: LPU550 156 mJ/pulse),

2.1. Swirl burst injector design Fig. 2 shows the design concept and the working principle of the novel SB injector. The nozzle design incorporates a swirl path onto the FB injection based on the geometric approximation of SN given by Eq. (1) [32,33].

(a)

(b)

dh dt

Top View

SB Concept

Fig. 2. (a) Design of SB Concept (b) Working principle of SB injector. 178

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2.3. Preliminary combustion comparison using the SB and FB injectors

illuminated the droplets and froze the fast-moving droplets with a short laser pulse duration of 4 ns. Tests were carried out at the same liquid flow rate of 12 ml per minute, and ALR of 1.5 for both injectors. Spray images and droplet velocity profiles were acquired in the near-field of injector exit, with a field of view (FOV) of 10 mm × 10 mm and at a spatial resolution of 7.14 μm per pixel for both injectors. Results showed that, compared to the FB injector, the SB atomizer generated a wider spray and less bright dots as shown in Fig. 3(a) and (b). Larger and brighter white dots represent larger droplets that scatter more light into the camera sensor. Hence, less large drops were generated within the SB injector near region. The relatively larger drops were nearly absent after 2 mm downstream of the SB injector exit, in Fig. 3b, suggesting that the secondary breakup has been likely accomplished at almost 2 mm, i.e. 1.33 D, away from the injector exit. In contrast, few larger and bright drops were observed at the downstream location of 4 mm in the FB spray. The visual spray images qualitatively suggested that the SAA resulted in finer spray in the injector near field and faster completion of atomization. Also, time-averaged droplet axial velocity computed from 500 instantaneous velocity fields at different axial locations was used to quantitatively characterize the near-field spray features for both injectors as shown in Fig. 4 [31]. Prior investigation in the near-injectorexit of FB sprays (water and glycerol) by Jiang and Agrawal (2015) [20,23] has shown that droplet axial velocity dominates the spray flow field. Significantly, the axial velocity variation reflects size change in droplets as finer drops streamline with the high-velocity atomizing air and accelerate, signifying an ongoing secondary atomization process [20,23]. For the FB spray, in Fig. 4(a), the droplet velocity increased along the flow direction signifying an ongoing atomization process. The velocity discrepancy between two axial planes decreased at further downstream, indicating that the secondary breakup gradually approached completion at around 4–6 mm, i.e. 2.67–4.0 D, downstream of the atomizer exit. Interestingly, for the SB spray, the average droplet axial velocity remained constant after about 2 mm downstream of the injector exit as suggested by the overlapping velocity profile, shown in Fig. 4(b). Consistent with the spray images, the velocity profiles quantitatively showed that the SB injector achieved shorter atomization completion length, less than half of the FB breakup length, signifying the effectiveness of the SB injector design [31].

The combustion of VO was also carried out to compare the combustion performance of the SB and FB injectors with the same injector diameter given above, and at the same flow rates in a 7 kW swirl-stabilized lab-scale gas turbine burner [31]. Results showed that both injectors achieved steady clean flames for straight vegetable oil (VO) as evident in the blue flames in Fig. 5 and the low emissions of CO around 5–6 ppm at the combustor exit with slightly lower values for the SB injector. The dark zone upstream of the flame represents the fuel prevaporization and fuel-air mixing region. The dark zone of FB spray combustion, in Fig. 5(a), was about 6.5 cm long starting from the injector exit. In comparison, the fuel pre-vaporization zone for the SB spray, in Fig. 5(b), was around 3 cm, about half of that in the FB flame. The less lifted VO flame obtained using the SB atomizer was attributed to (1) effective generation of SAA which enhanced the secondary atomization in terms of finer droplets immediately at the injector exit, as well as the accelerated completion of atomization [31]. These small drops underwent faster fuel vaporization and brought about the lesslifted flame; (2) the fine droplets streamlined with the SAA and traveled through a longer swirling droplet path, yielding the shorter axial distance for fuel pre-vaporization; (3) evident from the wider SB spray angle in Fig. 3(b), the SAA also drove the droplets closer to the outer recirculation zone, where the hot gas products were transported upstream from the reaction zone to further speed up fuel evaporation; (4) the SAA combined with the PA/swirl jet through the combustor swircic interaction, enhancing convective heat transfer and hot products delivery to the flame upstream region and aiding fuel evaporation. Overlifted flames suffer the possibility of blow-off, especially for long operation period in real applications, leading to flame instabilities. Less lifted SB flame could better sustain the flames with sufficient thermal feedback to rapidly vaporize incoming droplets and thus achieve clean and complete lean premixed (LPM) combustion even for the viscous straight VO. Compared to the FB spray flame, the nearly halved flame lift-off height using the SB injector again indicated the effective design of the novel SB atomizer, in consistent with the preliminary spray investigation. The promising preliminary results motivated the investigation of the combustion characteristics of straight algae oil (AO) as a potential biofuel using the novel SB injector in a swirl-stabilized gas turbine burner in the present study.

Fig. 3. Water spray image from (a) FB injector and (b) SB injector at a liquid flow rate of 12 mlpm and ALR of 1.5 [31]. 179

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Fig. 4. Radial profile of time-average droplet axial velocity for (a) FB injector (b) SB injector at different axial locations [31].

using shear rates starting from dγ/dt = 0.01 s−1 to 100 s−1 at 10 points/decade. The surface tension measurement of the algae oil is achieved using a force tensiometer (Model No.: KSV Sigma 702) from Biolin scientific with an uncertainty of ± 0.001 mN/m. Four measurements, using a correction factor by Zuidema and Waters [37], are taken in this experiment to evaluate the surface tension of algae oil. The density of the oil at 25 °C is obtained using a 50 cubic-centimeter pycnometer, and an OHAUS TP2000 analytical mass balance with an uncertainty of ± 0.03 g. A known volume of oil is weighed to obtain the density from the ratio of mass to its volume. The algae oil density is obtained by averaging ten runs of the density measurements. The calorific value of the oil is determined using a bomb calorimeter (Parr 6200 Calorimeter Model No: A1290DDEB) with an accuracy of ± 0.1% of the reading and in accordance with the standard procedure of the American Society for Testing and Materials (ASTM) D 4809.

3.2. Experimental setup for combustion of straight algae oil Fig. 6 illustrates the schematic of the experimental setup of the 7kW lab-scale gas turbine burner with the flow lines used in this study. Compressed air is supplied from an Ingersoll Rand air compressor (Model number UP6S-20-200 with 200 psi capacity) and regulated to the desired pressure with a pressure regulator. After passing through a filter and water trap to eliminate in-line moisture, the air flow is split into two supply lines, namely primary air (PA) as the main oxidizer and atomizing air for fuel injection. The primary air flow rate is controlled by a needle valve and measured using a 0–500 standard liters per minute (slpm) mass flow meter (MFM, Omega Model No.: FMA5542A) with an accuracy of ± 1% of the full scale. The atomizing air flow rate is controlled and measured using a 0–50 slpm mass flow controller (MFC, Omega Model No: FMA 2609A) with an uncertainty of ± 0.3% of the reading. The primary air passes through a plenum partially filled with marbles to break down the vortical structure of the air and ensure laminar flow into the mixing chamber. Natural gas, measured with a multi-gas MFC (Omega Model No: FMA 2609A), is delivered to the mixing chamber to premix with PA. The mixture enters the open-end quartz combustor, 45 cm in length and 7.62 cm diameter, through a swirler with an axial curved vane angle of 60-degrees with respect to the axial plane giving a combustor SN of approximately 1.5. The combustion of natural gas is used to preheat the combustor before switching completely to the liquid fuel. Liquid fuel is delivered by a Cole-Parmer gear pump (Model 75211-70) with an accuracy of ± 1% of the reading. It is worth mentioning that the gear pump is not a positive

Fig. 5. Visual flame images of straight VO at HRR of 6.7 kW and ALR = 4.0 using (a) FB (b) SB injector [31].

3. Experimental setup 3.1. Algae oil characterization setup Pure algae oil is obtained commercially from Thrive® Algae Oil company. The properties of the AO characterized in this study include the fatty acid profile, kinematic viscosity, surface tension, density, and calorific value. The fatty acid methyl esters (FAMEs) yield of the oil are extracted using standard methods given in [35]. The FAME profile is obtained using an Agilent 6890N gas chromatograph equipped with a mass spectral detector (GC–MS) using procedure illustrated in Revellame et al. (2009) [36]. The kinematic viscosity of algae oil is characterized using an Anton Paar’s MCR 302 rheometer. An oil sample is loaded onto a stage and compressed between two parallel 20 mm diameter plates to a gap of 0.5 mm. A second sample is compressed between the parallel plates to a gap of 0.4 mm for more detailed result and repeatability. This is followed with a detailed flow curve estimation to obtain the viscosity. The flow curves for the algae oil are estimated

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Table 1 Fatty acid composition of pure algae oil. Fatty acid

Carbon length with double bond

% Weight Composition

Myristic Myristoleic Palmitic Palmitoleic Stearic Oleic Vaccenic Linoleic Arachidic Gadoleic (Cis-11-Eicosenoic) Docosahexaenoic Nervonic

C14:0 C14:1 C16:0 C16:1 C18:0 C18:1 C18:1 C18:2 C20:0 C20:1 C22:6 C24:1

0.33 0.15 0.91 0.65 8.99 50.2 33.6 2.35 0.08 0.84 0.68 1.26

that gives a stable, clean, lean-premixed, and efficient combustion. All the experiments are conducted at a constant heat release rate of 6.9 kW, signifying the liquid fuel flow rate of 11.3 ml per minute (mlpm), for AO. Equivalence ratio is controlled by varying the total combustion air (PA + AA) with a constant AA flow rate of 37.8 slpm, i.e. constant ALR of 4.34. The investigated equivalence ratios are in the range of 0.55–0.80 with increments of 0.05. At the constant HRR and ER of 0.65, ALR effect is explored in the range of 2.0–5.0 with increments of 0.5 by adjusting AA and PA composition but keeping the total air at 166 slpm. For all the tested ALRs, the pressures (gauge pressures) in the AA and fuel lines at the immediate upstream of the injector are measured using a pressure transducer (Omega PX 309-050G5V) with an accuracy of ± 0.25% of the full scale.

Fig. 6. Schematics of the experimental setup.

displacement pump and it is prone to backflow in the presence of a back pressure. In the SB injection of the current study, the atomizing air flow bifurcates as shown in Fig. 2. A small amount of atomizing air penetrates into the injector internal liquid tubing and hence generates back pressure. Hence, multipoint calibration is carried out for the pump using a graduated cylinder and a stopwatch in the presence of various AA flow rates to ensure that the actual liquid fuel flow is used for the investigated heat release rate (HRR). Results showed that the back pressure had a negligible effect on the AO fuel flow rate likely due to the high viscosity of the AO resisting a backflow and the low-pressure requirement of the injector. The global combustion characteristics including flame images, product gas temperatures, emissions of carbon monoxide (CO) and nitrogen oxides (NOx), and surface temperatures of combustor outside wall are investigated to understand the AO combustion using the novel SB injector design. A digital camera (Sony Alpha A3000 with 18–55 mm lens) is used to acquire visual flame images at the same camera settings. The images are captured at an exposure time of 0.01 s, which is much larger than the time-scale of combustion reactions that involve hundreds of species and elementary reactions in chemical kinetics. For example, the characteristic chemical time of OH-radical is in the order of 10-4 s for oxidation of CH4 [38]. The content of the images is therefore the average of the fast-changing chemical kinetics and various flame stages within the much larger camera exposure time [39]. Thus, each visual flame image is an ensemble flame image of a time averaging process [39]. Product gases passing through an in-line filter are sampled continuously at 3 cm upstream of the combustor exit plane, and at 7 different radial locations using the ENERACTM M500 emission analyzer with an uncertainty of ± 2 parts per million (ppm) of the reading for CO and NOx emissions. Product gas temperatures at the combustor exit are also measured using the analyzer equipped with a thermocouple with an uncertainty of ± 1.1 °C of the reading. The surface temperatures of the combustor’s outside-wall are acquired using a high-temperature surface thermocouple probe (Omega HPS-HT-K-12-SMP-M) with an uncertainty of ± 2.2 °C of the reading. The present study investigates the effect of equivalence ratio and ALR on the combustion performance and emission characteristics of straight AO using the novel SB injector in the open-ended combustion chamber. The work aims to obtain the blow-out limit for the AO using the current experimental setup, and the effective range of ER and ALR

4. Results and discussion 4.1. Characterization of chemical structure and fuel properties of algae oil The fatty acid composition of the pure algae oil investigated in this study is given in Table 1. Saturated fatty acids (fatty acids with no carbon to carbon double bonds in their structure) and poly-unsaturated fatty acids (fatty acids with 2 or more double bonds) account for about 10.3% and 3% of the algae oil composition respectively. The rest of the compounds represent mono-unsaturated fatty acid (fatty acids with a single carbon-carbon double bond) and accounts for about 86.7%. Structural indices developed based on fatty acid composition have been used to correlate ignition behavior of VOs [40,41]. The indices are the average number of carbon atoms (AC) and the average number of double bonds (ADB) in the fatty acid composition of the oil. A high correlation was found between ADB and ignition delay (ID) for different vegetable oils [41], where an increase in the number of double bonds in the fatty acid profile of a VO prolongs the ID of the oil. The high fraction of mono-unsaturated fatty acid in the algae oil promotes shorter ID of the oil as the ADB is low, rendering algae oil a potentially desirable biofuel compared to straight vegetable oils in combustion engines [42]. Also, unlike VOs, algae oil does not require large land mass to grow nor promote competition with food industries. Table 2 shows the chemical and physical properties of AO compared to those of diesel and VO (soybean). The kinematic viscosity of AO at room temperature is approximately 16 times higher than that of diesel and slightly higher than the viscosity of VO. The high kinematic viscosity of AO may be attributed to the high mass fraction of the monounsaturated fatty acid present in the oil [43]. As aforementioned, conventional fuel injectors such as AB or PS injectors disintegrate fuel liquid jet/film into ligaments and ultimately finer droplets with long breakup length (∼cm) by shear layer instabilities between the highvelocity air and the liquid jet [12]. However, the high viscosity suppresses the instabilities between the two phases, signifying that these existing fuel atomizers cannot finely atomize viscous oils such as VO 181

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resulting in fast fuel pre-vaporization and better fuel-air mixing, hence clean and premixed combustion. Jiang and Agrawal (2015) [23] showed that the FB injector generated both fine droplets and thin ligaments immediately at the injector exit for much more viscous glycerol (∼200 times more viscous than diesel). The preliminary water spray diagnostics of the current study indicated that the SB injector, integrating the SAA and FB concept, has further improved the atomization capability of the FB injection in terms of finer drops in the near field of injector exit, and nearly halved the spray breakup length. Hence, it is reasonable to infer that the SB injection generated fine AO spray; (2) the droplets of the SB spray streamline with the SAA and travels through a longer swirl path, within a short axial distance, giving the droplets sufficient time to vaporize before entering the reaction zone. The fuel vapor mixes more homogenously with the oxidizer and thus completely and cleanly burns at lean-premixed mode emitting the blue chemiluminescence [38]. The blue color is more dominant at lower ER, i.e. higher PA flow rates through the combustor swirler. The higher swirl jet of PA might further enhance the secondary liquid breakup through aerodynamic interaction and strengthened fuel–air mixing, thus result in more thorough combustion of CH* with completely blue color. The flame of the lowest ER of 0.55 in Fig. 7(a) is highly unstable and tends to blow off, implying the blow-out limit of straight AO is at ER of about 0.55 for the current experiment setup. This can be ascribed to the high turbulence and fluctuation by the reactant (mainly PA) flow velocity being greatly higher than the flame speed. The instability of the flame hindered the emission measurements at this case, as the CO emissions are high beyond the measurement range of the current study’s gas analyzer (0–200 ppm). The AO flame at ER of 0.60 is lifted to about y = 6 cm and more turbulent compared to higher ER flames, shown as the asymmetric flame of Fig. 7(b), again owing to the increased PA flow velocity. Further increase in ERs results in orange sooty color in the flames, as seen at the tip of the flame at ER = 0.75, due to higher reaction temperature at the fuel-richer condition, facilitating local soot formation from fuel pyrolysis, and the diffusion combustion of more larger drops existing at lower PA flow. Though orange color is present at ER of 0.75, the dominant blue flame and the downstream dark zone signify the thermally homogenous mixture of gas products from nearly complete lean-premixed combustion of AO at ER of 0.75. Orange sooty flame appeared at the highest ER of 0.80, shown in Fig. 7(f). Two main reasons can explain this phenomenon: (1)

Table 2 Physical and chemical properties of diesel, vegetable oil (soybean), and pure algae oil.

1 2 3 4 5

Property

Diesel [18]

VO [18]

Algae oil

Density (kg/m3) at 25 °C Kinematic viscosity (mm2/s) at 25 °C Surface tension (mN/m) at 25 °C Calorific value (MJ/kg) Auto-ignition temperature (oC)

834.0 ± 9.2 3.88 ± 0.02

925.0 ± 8.3 53.74 ± 0.22

917 ± 4.2 64.2 ± 0.6

28.2 ± 0.6

30.1 ± 0.6

28.7 ± 0.12

44.6 260

37 406

40 –

and AO, let alone cleanly combust the oils. Therefore, for instance, an AB injector (Model: Delavan 30609–2) resulted in sooty flames of nonpreheated straight VO with high CO and NOx emissions beyond the maximum measurement limit (2000 ppm of CO and NOx) of the gas analyzer (NOVA model 376WP) [44]. Furthermore, the AO possesses calorific value comparable to that of diesel, as shown in Table 2. Besides, the possibly shorter ID, closed-carbon cycle, the domestic availability, and fast growth rate [3,5,6,41] together render AO a desirable biofuel, if fine atomization of straight AO can be technically viable. Fortunately, as will be illustrated in the subsequent sections, clean premixed combustion of straight AO has been successfully achieved without fuel preheating in the present study owing to the novel SB atomization that has fine atomization capability and high viscosity tolerance.

4.2. Effect of equivalence ratio (ER) on AO combustion Fig. 7 illustrates the visual flame images of the combustion of AO at the different ERs investigated. The images are used to qualitatively assess the cleanness of the flames related to chemiluminescence from the flame color [38]. At ERs of 0.55–0.75, the upstream dark regions of fuel pre-vaporization and fuel-air mixing zones are followed by the dominantly blue flames, indicating that lean-premixed and clean combustion of the hydrocarbons (CH* chemiluminescence) [38] has been achieved for viscous straight AO atomized at room temperature. This can be attributed to (1) fine SB atomization even for the viscous AO,

Fig. 7. Visual flame images of straight AO at equivalence ratios of (a) 0.55 (b) 0.60 (c) 0.65 (d) 0.70 (e) 0.75 and (f) 0.80. 182

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turbulence and hence the asymmetric flame. Regardless of the discrepancy, the CO emissions are ultra-low overall, within the range of mainly 6–14 ppm for all the investigated ERs. This indicates that the new SB injector enabled the fine atomization and thus clean and leanpremixed combustion of the viscous AO without fuel nor air preheating. In addition, it is worth mentioning that the uniform radial profile of CO emissions for ERs of 0.65–0.75 validates the homogenous thermal mixing of gas products as the result of the complete combustion. This again reflects the further improved atomization by the SB injector, compared to the FB atomizer. The FB injection resulted in around 2–3 times higher CO concentrations close to the combustor wall than the spray center for viscous straight VO, due to incomplete and/or diffusion burning of larger droplets formed at the FB spray periphery [18,20,23]. Next, increase in ER leads to increased NOx emissions, illustrated in Fig. 10(b). Again, the higher reaction temperature at increased ER accounts for the thermal NOx formation for the tested fuel not bounded with nitrogen. Low NOx emissions are obtained, within a range of 0–13 ppm, at the investigated ERs, with the lowest value of 6 ppm for ER of 0.65. Overall, the novel SB injector has achieved stable, clean and complete lean-premixed combustion of non-preheated straight AO, with the optimum ER of 0.65 in terms of CO and NOx emissions.

high reaction temperature at the high ER promotes fuel pyrolysis and thus forms more soot [38], and (2) some relatively large fuel droplets at lower PA enter the combustion zone and burn in diffusion mode yielding soot. In summary, stable and clean AO flames are acquired at ERs of 0.60–0.75 using the novel SB injector as shown in Fig. 7(b-e). The study also investigates the effect of ER on the flame structure involving mean flame lift-off height and flame length from the visual flame images. Fig. 8 depicts the plot of the flame lift-off height and flame length at the investigated ERs. The lifted flames are stabilized at almost the same axial plane, around 5 cm downstream of the injector exit, for ERs of 0.60 to 0.75 with a slight decrease at the higher ER. A nearly attached flame with lift-off height of around 1 cm is observed for the sooty flame at ER of 0.8. The dramatic decrease in lift-off height can be explained by the higher flame temperature of the richer flame that provides more thermal feedback to the incoming fuel droplets and thus quickens the ignition of the fuel. The flames are elongated with the increasing ER. The difference in flame length is the complicated mutual effect of PA flow rates, atomization, reaction zone temperatures and thermal feedback from the flame to fuel pre-vaporization zone, fuel-air mixing, and the fuel oxidation at different ERs. At the same ALR, the shorter flame at lower ER is likely because of (1) rapid fuel pre-vaporization of relatively fine drops from enhanced secondary atomization of the fuel by the higher PA, and (2) faster fuel oxidation from sufficient oxidizer and better fuel-air mixing at higher swirling PA flow. Fig. 9(a) and (b) respectively depict the radial profiles of product gas temperature (uncorrected) at the combustor exit plane and the axial profile of surface temperature along the combustor outer wall. The temperatures are highest at the midsection of the combustor but lower at the walls because of the heat loss to the surroundings through thermal radiation and convection. The nearly overlapping profiles of the temperatures again signify that complete lean-premixed combustion of the straight AO has been achieved for all tested ERs at the same input HRR of 6.9 kW using the SB injector. The symmetric product-gas temperature profile indicates that uniform fuel-air mixing has been achieved by the SB injector, hence, a uniform temperature distribution as the result of homogenous thermal mixing of combustion products [45]. The surface temperature, shown in Fig. 9(b), increases along the flow direction up to a high-temperature zone and drops further downstream as the combustion approaches completion. The high-temperature region signifies local HRR exceeding the local heat loss, reflecting that the primary reaction zone is within 15 cm and 20 cm downstream of the injector exit plane, respectively for ERs of 0.65 and 0.75, consistent with the flame height acquired from the visual flame images. The higher temperature obtained at ER of 0.75 simply indicates the higher flame temperature at increased ER [38]. The matching surface temperatures at the downstream location of the combustor (30–40 cm) consistently validate the complete lean-premixed combustion of pure AO at ERs of 0.65 and 0.75 using the SB injector. Overall, regardless of the variations in the near field fuel atomization and vaporization, fuel-air mixing, and chemical reactions, the novel SB injection has achieved stable, clean and complete lean-premixed combustion of straight AO without fuel preheating at ERs of 0.60–0.75. Next, Fig. 10(a) and (b) present the transverse profiles of CO and NOx emissions at the combustor exit respectively for the stable and clean flames at ERs of 0.60–0.75. For all the ER cases, CO emissions are less than 15 ppm, which is low for non-preheated viscous oil, compared to the CO emissions (200–600 ppm) obtained from non-preheated VOs using conventional injectors [46,47]. Evidently, CO emissions peak at the highest ER of 0.75, which is typical with increase in ER. Also, more CO might be emitted from local diffusion burning of larger fuel drops at lower PA flow rate as discussed above. The minimum CO concentrations of 6–8 ppm are obtained at ERs of 0.65 and 0.70. Interestingly, the lower ER of 0.60 generated slightly higher CO emissions, which can be attributed to the shorter residence time for complete oxidation of CO because of the increased combustion flow velocity. The asymmetry of the CO emissions at ER of 0.60 is again due to the relatively higher

4.3. Effect of atomizing air to liquid mass ratio (ALR) on AO combustion The current study also investigates the effect of ALR through the SB injector at a constant total air flow rate of 166 slpm, HRR of 6.9 kW, and the optimum ER of 0.65. The flame images of the combustion of straight AO at ALRs of 2.0–5.0 using the SB injector are shown in Fig. 11. The flame color becomes mainly blue with the increase in ALR, again indicating clean, lean-premixed and, complete combustion of the hydrocarbons (CH* chemiluminescence) [38]. This can be first attributed to the improved primary and secondary atomization by the higher AA flow rates at increased ALRs yielding finer droplets [12,22]. The AA at increased flow rate carries higher pressure, shown in Fig. 12, and hence improves penetration of AA into the liquid tube tip inside the injector to boost the bubbly internal two-phase mixing [34]. Consequently, the higher AA flow rate generates higher pressure-drop at the injector exit, resulting in more vigorous primary breakup by bubble bursting to generate a finer fuel spray. Besides, a major portion of the AA leaves the injector exit in a swirling flow with higher momentum at higher ALRs. This improves the shearing interaction between the SAA and the liquid parts, formed by the primary atomization, to yield finer

Fig. 8. Flame lift-off height and flame length of straight AO at the investigated ERs. 183

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Fig. 9. (a) Radial profile of combustion gas temperature at the combustor exit, y = 42 cm (b) axial profile of combustor outer wall surface temperature at the investigated ERs.

sprays from the enhanced secondary breakup. The finer spray hence leads to faster fuel pre-vaporization, enhanced fuel-air mixing, and clean premixed flames. Second, the increase in ALR achieved by adjusting the AA flow rate varies the SAA and spray behavior of the SB injection, including the droplet velocity, trajectory, evaporation, and aerodynamic interaction among fuel, AA and PA, together contributing to the variations in the flames. The slight orange color, peculiar to soot, at the tip of the flame in Fig. 11(a) can be attributed to large droplets formed at the low ALR entering the reaction zone and burning incompletely and/or in diffusion mode. Fig. 13 depicts the plot of the flame lift-off height and flame length at the investigated ALRs from the visual flame images in Fig. 11(a-g). The flame lift-off height, i.e. the length of the dark region upstream of the flame, increases while the flame length decreases with increase in ALR. The more lifted flames at higher ALRs can be mainly ascribed to the increased AA flow velocity that results in higher droplet velocity and farther fuel penetration. The flame lengths become shorter with the increase in ALR again due to improved atomization leading to faster fuel evaporation and fuel-air mixing, hence rapid fuel oxidation. The elongated flames at low ALRs reflect the longer residence time needed for combustion of larger fuel drops generated at the low ALRs. Despite the wide range of ALRs used in this study, stable, clean, and mainly lean-premixed flames are

obtained, indicating the efficacy of the SB injector in the combustion of viscous straight AO. Next, thermal characteristics and emissions are investigated to quantitatively explore the ALR effect on the combustion of straight AO using the SB injector. Fig. 14(a) illustrates the radial temperature profile of product gas at about 3 cm upstream of the combustor exit. The product gas temperature is typically high at the midsection of the combustor and low at the wall due to heat loss. The product gas temperature slightly decreases with increase in ALR with the maximum discrepancy of approximately 10% between the lowest and the highest tested ALRs. For the complete combustion of AO for all the ALRs at the same HRR, the more compact flames of higher ALRs yield slightly higher local temperature in the reaction zone. Hence, more heat is lost to the surroundings through the combustor wall due to thermal radiation and convection, bringing about relatively cooler exhaust at the combustor exit. Fig. 14(b) depicts the axial profile of the outer surface temperature of the quartz burner. The outer wall temperature increases in the flow direction up to a peak in the reaction zone and thereafter decreases reflecting the typical flame structure evolution within the combustor. Compared to the low ALR cases, the surface temperatures near the dump plane (y = 0 cm) are higher at the high ALRs. Consistently, the shorter flames at higher ALRs with hotter reaction zones

Fig. 10. Radial profile of (a) CO (b) NOx emissions of AO combustion acquired at axial location, y = 42 cm at the investigated ERs. 184

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Fig. 11. Visual flame images of AO combustion at ALRs of (a) 2.0 (b) 2.5 (c) 3.0 (d) 3.5 (e) 4.0 (f) 4.5 and (g) 5.0.

Fig. 12. Pressure drop across the liquid and AA flow lines of the SB injector at the investigated ALRs.

Fig. 13. Flame lift-off height and flame length of straight AO at the investigated ALRs.

enhance thermal feedback to the upstream dark region. Furthermore, the higher SAA flow, at the high ALRs, improves the aerodynamic interaction between the SAA and the swirl jet of the PA. This strengthens the corner recirculation close to the dump plane, transporting more hot combustion products upstream and increasing the local temperature. This consequently results in higher heat loss to the surrounding validating the slightly lower product gas temperatures obtained at the combustor exit at the high ALRs. The nearly overlapping surface temperature values at the axial location (y = 40 cm) consistently signify complete combustion of AO at all ALRs. Fig. 15 shows the radial profiles of the CO and NOx concentrations at the combustor exit from straight AO combustion for different ALRs. Again, compared to (200–600 ppm) from preheated VO combustion in conventional engines [46,47], low emissions of CO and NOx (< 20 ppm) are achieved at the midsection of the combustor for all the tested ALRs, owing to the fine SB atomization. CO concentrations, shown in Fig. 15(a), decrease with increase in ALR, mainly attributed to finer sprays at higher ALRs and enhanced fuel-air mixing by the stronger SAA interaction with PA and fuel. At low ALRs (2.0 and 2.5),

the high CO emission values near the walls are mainly due to diffusion and/or incomplete combustion of large droplets that migrate to the low-temperature wall region. The uniform radial CO profiles at ALRs > 2.5 at the combustor exit further suggest the homogenous combustion products from the complete combustion of AO enabled by the fine SB injection. NOx emissions, shown in Fig. 15(b), increases with the decrease in ALR. Thermal NOx mechanism can explain the trend for the fuel without nitrogen element. As discussed above, at the low ALRs, larger droplets are formed and burnt in diffusion mode, i.e. with a local equivalence ratio of 1, yielding high local temperature favored by thermal NOx. In a word, ultra-low emissions of CO and NOx, less than about 10 ppm, are obtained for ALRs > 2.5. Thus, signifying the fine atomization ability and high viscosity tolerance of the novel SB injector to enable clean combustion of viscous non-preheated straight AO.

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Fig. 14. Temperature distribution of (a) product gas acquired at axial location, y = 42 cm (b) combustor outer wall surface for AO combustion at the investigated ALRs.

Fig. 15. Radial profile of (a) CO (b) NOx emissions of AO combustion acquired at axial location, y = 42 cm at the investigated ALRs.

with the increase in ER due to the mutual effect of flow rates, reaction temperature, and aerodynamic interaction of reactants. Radial profiles of the product gas temperature at the combustor exit for ERs of 0.60–0.75 overlaps one another, quantitatively signifying that clean, complete, and lean-premixed combustion of viscous straight AO has been achieved by the SB injection. For all the investigated ERs with stable flames, low emissions of CO and NOx have been achieved in the range of 6–14 ppm and 0–13 ppm respectively, regardless of the increasing trend at the higher ER. This consistently shows the enhanced atomization and combustion capability of the SB injector for the highly viscous oil compared to conventional injectors such as AB atomizers with low-viscosity tolerance. The optimum equivalence ratio, in terms of flame stability and low emissions, is identified at ER of 0.65 with CO of 6–8 ppm and NOx of 6 ppm. Investigation of the effect of ALR on straight AO combustion indicates that increase in ALR yields dominantly blue flames and lowers the emissions due to finer droplets yielded at higher ALRs by the enhanced primary and secondary breakup. More compact flames are obtained at higher ALRs as the result of finer sprays, quick fuel pre-evaporation, improved fuel-air mixing, and faster fuel oxidation, consistently evident from the product gas temperature and surface temperatures of the combustor outer wall. At ALRs > 2.5, the uniform

5. Conclusions The current study characterizes the chemical structure and properties of viscous AO, a source oil for biodiesel, and investigates the global combustion characteristics of straight AO as a potential biofuel without fuel preheating nor chemical conversion into low-viscosity biodiesel. The fatty acid profile shows that mono-unsaturated fatty acid dominates the fatty acid composition of the oil suggesting possibly short ignition delay. The oil is approximately 16 times more viscous than diesel but has a calorific value close to that of diesel. The possibly short ignition delay and the comparable heating value to diesel potentially render straight AO itself as a desired biofuel. On the other hand, clean combustion highly depends on spray fineness. Existing injectors have limited atomization capability and low viscosity tolerance. Fortunately, in the current study, the clean lean-premixed flame of straight AO has been achieved using the novel SB injector. Visual images illustrate dominantly blue flames in the range of equivalence ratios investigated, indicative of clean lean-premixed combustion. A blow-out limit is identified at ER of around 0.55 for the current experimental setup, with stable and clean flames at ER of 0.60–0.75. For ERs of 0.60 to 0.75, flame lift-off heights are stabilized at almost the same axial location of y = 5 cm. The flames are elongated 186

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and nearly overlapping low CO emissions again substantiate the homogenous gas products of complete combustion due to the fine SB atomization. Overall, despite the noticeable difference for flames with varying ALR and ER, clean, stable, and lean-premixed flames of straight AO are acquired with the novel SB injector. The novel SB injection potentially enables straight AO itself to be a cost-effective and nearzero-emission renewable fuel without the need for conversion into biodiesel that yields unwanted byproducts.

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