- Email: [email protected]
Clean combustion in gas turbine engines using Butyl Nonanoate biofuel
Fuel 116 (2014) 522–528
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Clean combustion in gas turbine engines using Butyl Nonanoate biofuel Ahmed E.E. Khalil, Ashwani K. Gupta ⇑,1 Department of Mechanical Engineering, University of Maryland, College Park, MD 20742, USA
h i g h l i g h t s Examined Butyl Nonanoate (BN) biofuel for gas turbine applications. BN produced 15% larger droplet sizes compared to JP8 under similar condition. BN provided 15% reduction in NO emission compared to JP8 in the distributed combustor. CO emission was also lower compared to JP8, and contrary to HRJ biofuel. No start-up issues or combustion instabilities observed with the BN biofuel.
a r t i c l e
i n f o
Article history: Received 18 June 2013 Received in revised form 7 August 2013 Accepted 8 August 2013 Available online 23 August 2013 Keywords: Biofuels Colorless distributed combustion (CDC) Ultra-low NOx and CO emission Gas turbine combustion Fuel flexibility
a b s t r a c t Developing new and renewable biofuels for ultra-low emission gas turbine combustors is much desired to secure future power needs. Several fuels are being developed to replace fossil fuels with minimal carbon footprint and pollutants emission. Ease of transition between fossil fuels and biofuels is also desired for fuel flexibility in engine operation. In this paper, the performance of Butyl Nonanoate biofuel is evaluated and compared to both Hydrogenated Renewable Jet (HRJ) fuel and JP-8. The atomization characteristics are examined using Phase Doppler Particle Analyzer (PDPA) for favorable condition of fuel introduction to the combustor. The Biofuel provided 10–15% larger size droplets compared to JP-8 for a given atomizer and operational pressure. The combustion behavior of biofuel was also examined in a swirling colorless distributed combustor with focus on pollutants emission under high intensity simulated gas turbine conditions. The liquid fuels were directly fed into the path of the air stream prior to combustion without using any atomizer or spray device. The biofuel provided 6 PPM of NO and 35 PPM of CO at an energy release intensity of 27 MW/m3-atm at 0.6 equivalence ratio. The recorded emissions are about 15% lower as compared to JP-8 for equivalence ratio higher than 0.65. For lower equivalence ratios, NO emissions were identical. The Butyl Nonanoate biofuel also exhibited lower NO emissions than HRJ biofuel. The CO emissions were lower with Butyl Nonanoate as compared to JP-8 (this is in contrast to HRJ biofuel which showed an increase in CO compared to JP-8). The examined biofuel did not show any instability with smooth transition between different fuels. These results provide promising behavior of Butyl Nonanoate biofuel for use in future energy needs without any modifications to the combustor injectors, while maintaining high performance. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Rapid depletion of fossil fuels and widespread concerns on energy security have motivated scientists and engineers to develop new fuels that are derived from environmental friendly sources to support the current and future energy needs in a sustainable fashion. Several authors have discussed the viability of such biofuels [1–4]. The current interest on alternative jet fuels for commercial and military application stems from multiple factors including sustained energy availability, high petroleum fuel costs, price vol⇑ Corresponding author. Tel.: +1 301 405 5276; fax: +1 301 314 9477. 1
E-mail address: [email protected] (A.K. Gupta). Distinguished University Professor.
0016-2361/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2013.08.022
atility, lack of energy diversity, global climate impacts, and potential air quality. Civil and military jet aircraft require a near-term fuel replacement to conventional petroleum based jet fuel that is a ‘‘drop-in’’ hydrocarbon substitute that can function within the existing aircraft infrastructure while meeting rigorous safety and performance standards. Such drop-in alternative jet fuel pathways can be grouped into five broad categories: jet fuel produced from unconventional sources of petroleum such as oil sands, very heavy oils, and oil shale; synthetic jet fuel from thermochemical processes involving natural gas, coal, and/or lignocellulosic biomass such as Fischer– Tropsch (F–T) synthesis and pyrolysis; advanced fermentation, catalytic, and other means of converting sugars to jet fuel; hydro-processing of conventional oils to synthetic jet fuel; and conversion of
A.E.E. Khalil, A.K. Gupta / Fuel 116 (2014) 522–528
Fig. 1. Droplet size measurement using PDPA.
calorific liquids from micro-organisms to synthetic jet fuel [5]. Through both ground and flight tests, the USAF has already approved a 50% blend of F–T fuels with conventional jet fuels. It is expected that the certification of a 50% blend of hydro-processed renewable oils with conventional jet fuel should occur in the near future [6]. Commercial and military aviation have set ambitious alternative fuel and environmental targets for the next half century. The International Air Transport Association has set a goal of 10% alternative fuel use by 2017, carbon neutral growth in 2020, and a 50% decrease in aviation CO2 emissions by 2050, relative to 2005 levels. In a detailed research on the viability of different alternative jet fuels, it was concluded that switchgrass F–T fuel and camelina HRJ have the potential to reduce life cycle greenhouse gas emissions by 60–80% [5]. Synthetic isoparaffin-rich fuels produced by hydro-processing camelina oil show great promise as ‘‘drop-in’’ alternatives to petroleum jet and diesel fuel. Recent evaluations of HRJ fuel derived from camelina oil indicate the fuel performed just as well as petroleum based fuel but with lower exhaust and GHG emissions. Using updated estimates of camelina cultivation and commercial scale estimates of oil recovery and refining requirements a life cycle GHG savings of 75% and 80% was estimated for camelina-derived HRJ and Green Diesel (GD) relative to their petroleum fuel counterparts. Using data from recent field trials, GHG savings of >67% were achieved [7]. Also reduction in NOx emissions can be achieved via biofuels. On the other hand, CO emissions depend on the engine type [4]. The use of biofuels have also been extended to furnaces with emission reduction of both NO and CO [8]. A study on biofuel using T63 test engine revealed that there was minimal changes in CO emission with slight increase in THC (10%) at idle with biodiesel concentrations of 20%. This increase was attributed to incomplete combustion of biodiesel at the low power setting. At the higher power conditions, there was little or no change in CO and THC. The addition of biodiesel had negligible effects on oxides of nitrogen (NOx) emissions for all the test cases. This was expected, because the primary route for NOx formation is via thermal NOx, and the relative combustion temperature was not changed during the blends tests. Only slight reductions in oxides of sulfur (SOx) emissions were observed as the biodiesel concentration was increased, which was likely a dilution effect, because the biodiesel did not contain sulfur [9]. The quest for developing biofuels from untraditional sources as a new energy source is a topic of ongoing work at several institutions and organizations. In that sense, Butyl Nonanoate is a Air/Fuel Inlet
523
chemical that has not been used as a biofuel before. It is characterized by a lower calorific value, higher viscosity and density. This fuel has been processed at Michigan State University for use as a new biofuel. The combustion characteristics of this novel fuel will be evaluated with view to its use in gas turbines (both stationary and aviation). The most important parameters are the ability of the fuel to be atomized (which depend on surface tension, viscosity, and density), along with the pollutants emission resulting from the combustion of this fuel. The combustion characteristics of this Butyl Nonanoate fuel will be compared to standard JP-8 to determine its viability for use in gas turbine applications. The performance of Butyl Nonanoate will be evaluated and compared to JP-8 and HRJ fuels in terms of droplet formation and pollutants emission. These fuels will be examined in a colorless distributed combustor [10,11] for performance evaluation. Colorless distributed combustion (CDC), which is based on the principles of High Temperature Air Combustion (HiTAC [12]), have been shown to provide ultra low emissions with high efficiency under different configurations [13–16]. Fuel flexibility have also been demonstrated using CDC combustors [17,18]. The examined fuels included both gaseous and liquid fuels with no change to the combustor geometry or fuel introduction method [18]. The resultant emissions from different fuels using this combustor were found to be lower than the regulations set by Environmental Protection Agency (EPA) [19], outlining the potential application of this combustor design. 2. Experimental facility The atomization characteristics of biofuel were first evaluated using a standard atomizer. The droplets diameter was measured using a phase Doppler particle analyser and an average was calculated to evaluate the atomization of the novel Butyl Nonanoate biofuel. Fig. 1 shows the experimental setup used for droplet diameter evaluation. The pollutants emission from the biofuel was also measured where the fuel was fed to a swirling distributed combustor operating at a nominal heat load of 6.25 kW and energy release intensity of 36 MW/m3-atm. Different liquid fuels were injected in a preheated air stream (for simulating gas turbine combustor intake with elevated temperature after air exiting the compressor) upstream the combustor. The combustion chamber utilized was a cylindrical chamber; air was injected tangentially at half the height of chamber for all the cases investigated here. For enhancing the residence time of reactants in the combustor, a tube was extended inside the combustor for exiting the product gases from the combustor. Fig. 2 shows a schematic diagram of the combustor used. More detailed description of the combustor geometry is given in Ref. [11]. For combustion, air was supplied using an air compressor. Air flow rates were measured using choked flow orifice systems. Precision stainless steel orifices were used to choke the flow of the gases and upstream pressure was controlled to supply the required mass flow rate of gases. The upstream pressure was controlled using pressure regulators to maintain a steady pressure and avoid Air/Fuel Inlet
Exit
Fig. 2. Schematic of the combustor used.
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Fig. 3. A schematic of the experimental combustion facility.
oscillations in pressure due to compressor turning on and off periodically. The upstream pressure for air was maintained between ±0.2 psi of the desired pressure. Upstream pressure was measured using a piezo-resistive digital pressure gauge with accuracy of ±0.25 psi. It was ensured that the downstream pressure was lower than the pressure required for choking the flow based on the upstream pressure. Liquid fuels were supplied to the combustor using a compressed fuel tank that delivers specific liquid fuel flow rate at a specific upstream pressure of a thin fuel introduction tube (inner diameter of 0.01 in.). This fuel delivery system was calibrated for every fuel to ensure that the delivered flow rates are adequate for the examined condition. The fuel was directly introduced into the preheated air stream at an upstream location of the combustor to support near premixed combustion mode. Fig. 3 shows the experimental setup used for fuel introduction and emission measurement. Detailed investigations on the overall emissions from the combustor as well as visible emissions have been performed for the various experimental conditions. A Horiba PG-250 gas analyzer was used for emissions measurements. The concentration of NO was measured using a NO–NOx chemiluminescent method; CO concentration was measured using the non-dispersive infrared method and O2 concentration (used to correct the NO and CO emissions at standard 15% oxygen concentration) was measured using galvanic cell method. The gas analyser was continuously calibrated against calibration gases. For NO emissions below 5 PPM, a high accuracy thermo-electron chemiluminescence NO–NO2–NOx low source analyzer was also used (Model 42C) to support the meaTable 1 Biofuels examined. Case no.
Fuel
ATP-JP8 ATP-BN ATP-HRJ
JP-8 Butyl Nonanoate (BN) Hydrogenated Renewable Jet fuel (HRJ)
surements. During a single experiment, measurements were repeated at least three times for each configuration and the uncertainty was estimated to be about ±0.5 PPM for NO and ±10% for CO emission. The experiments were repeated at least three times to ensure good repeatability of the experimental data. The recorded data was then corrected to 15% O2 concentration and averaged. For imaging the OH chemiluminescent intensity distribution, an ICCD camera coupled to a narrow band filter having center wavelength of 307 nm wavelength was used. The resulting OH intensity distributions were then normalized to provide the same scale. The inlet air temperature to the combustor was preheated to 600 K to simulate elevated air temperatures at inlet to the combustor (simulating outlet temperature from a characteristic gas turbine compressor). The preheating of the inlet air affects the combustion kinetics and pollutants emission. In general, higher air inlet temperature increases the flame temperature, which substantially decreases CO emission and slightly increases NOx levels [11]. 3. Experimental The performance of Butyl Nonanoate biofuel was examined and compared to JP-8 in terms of droplet size atomization and emissions of NO and CO. The emissions were compared to that from JP-8 and HRJ fuels in the same combustor under identical
Table 2 Properties of biofuels. Property
Butyl JP-8 Nonanoate (BN)
Calorific value (kJ/kg) Density (kg/m3) Kinematic viscosity (mm2/s) Surface tension (N/m) Cetane number
35,677 849–855 1.989 @ 23.6 °C 0.029 @ 25 °C 51.5
HRJ
43,000 44,300 795 751 1.692 @ 20 °C 1.1 @ 40 °C 0.026 @ 25 °C 0.0264 @ 25 °C 44.1 58
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Fig. 4. PDPA arrangement with spray nozzle.
Table 3 Average diameters for Butyl Nonanoate and JP8.
Number average Area average Volume average Volume/area average
Table 4 Average diameters for fuel mixture and JP8.
Symbol
JP8 (lm)
Butyl Nonanoate (lm)
Difference (%)
D10 D20 D30 D32
37.07 40.42 43.51 50.44
42.2 46.95 50.42 59.04
13 15 15 17
operational conditions. Table 1 shows the experimental investigations designation with respect to each fuel. Table 2 gives the properties of the fuels examined. Properties of Butyl Nonanoate were measured at Michigan state university, while properties of Hydrogenated renewable jet fuel were provided by the supplier. Properties of JP-8 were gathered from the literature. 4. Results and discussion 4.1. Droplet size Liquid fuel atomization is of critical importance for combustion, as larger droplet size will require more time to evaporate and mix before ignition, which leads to improper mixing and onset of concentrated thin reaction zone flames. These flames should be avoided for seeking distributed reactions with near zero emissions. Comparing the properties of Butyl Nonanoate to standard JP8 properties, it was found that Butyl Nonanoate has higher surface tension, viscosity and density, leading to bigger droplet diameter compared to JP8 for a given set of conditions. To evaluate the size average and size distribution difference between both fuels, a standard spray nozzle was installed in the measuring volume of a Phase Doppler Particle Analyzer (PDPA). The liquid fuel tank was pressurized to an upstream pressure of 40 PSI to ensure same operational conditions for all of the experiments. Both fuels were examined multiple times successively and separately. For each experiment, 10,000 droplets were measured to obtain the droplet size distribution. Average diameters based on number, area, volume and volume to area ratio were calculated from the obtained histogram and are used to evaluate the average droplet size diameter for the fuels. Fig. 4 shows the nozzle used with measurement volume.
Number average Area average Volume average Volume/area average
Symbol
JP8 (lm)
50% Butyl Nonanoate–50% JP8 (lm)
Difference (%)
D10 D20 D30 D32
34.67 39.43 43.47 52.83
36.19 40.89 44.95 54.32
4.3 3.6 3.4 2.8
The obtained data on size distribution for both fuels showed that Butyl Nonanoate had a 10–15% bigger droplet size compared to JP8. For instance, the average number droplet diameter (D10) for Butyl Nonanoate was about 42.2 lm compared to 37.07 lm for JP8. Table 3 gives a sample of the averaged diameters for both JP8 and Butyl Nonanoate. Fig. 5 shows a sample size distribution for both the fuels. The reported values are with the probe volume corrected to accommodate for Gaussian distribution of the laser beam power. The results obtained outline the difference between the average droplets sizes produced, wherein Butyl Nonanoate provided 10– 15% bigger droplet size compared to JP8. A mixture of both fuels should result in a smaller difference compared to JP8. Such smaller size difference can be tolerated in actual combustion systems. The obtained data on size distribution for 50% Butyl Nonanoate 50% JP8 mixture as compared to JP8 showed that fuel mixture had a 3–4% bigger droplet size compared to JP8. For instance, the average number droplet diameter (D10) for the mixture was about 36.19 lm compared to 34.67 lm for JP8. Table 4 gives a sample of the averaged diameters for both JP8 and Butyl Nonanoate–JP8 mixture. Fig. 6 shows a sample size distribution for both the fuels. The results obtained show the differences between the average droplet size produced, with Butyl Nonanoate having 13–17% bigger droplet size compared to JP8. A mixture of these two fuels (50–50% by volume) showed only about 3–4% difference in size. Such smaller size difference could be tolerated in actual combustion systems.
4.2. Pollutants emission The pollutants emission from the biofuel was measured using swirling distributed combustor operating at a nominal heat load
Fig. 5. Number diameter distribution for JP8 (left) and Butyl Nonanoate (right).
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Fig. 6. Number diameter distribution for JP8 (left) and fuel mixture (right).
Table 5 Experimental parameters.
of 6.25 kW and energy release intensity of 36 MW/m3-atm. Different liquid fuels were injected in a preheated air stream (for simulating gas turbine combustor intake with elevated temperature after air exiting the compressor) upstream the combustor. Emissions of NO and CO was measured for the different fuels examined. For all of the fuels examined, the heat load was kept constant at 6.25 kW (having energy release intensity (HRI) of 36 MW/m3-atm) at an equivalence ratio of 0.8. Air flow rate was adjusted to furnish the combustor with the adequate air requirement for each fuel used. Fuel flow rate was varied to allow for change in equivalence ratio that ranged from 0.8 to 0.5 and the subsequent overall emissions measured. Table 5 gives the experimental conditions for the conducted experiments. Fig. 7 shows the NO emission levels resulting from operating the combustor with different fuels for the novel premixed combustion configuration. Obtained data show the performance of Butyl Nonanoate as compared to JP8. Butyl Nonanoate produced less NO emission at higher equivalence ratios. At lower equivalence ratios, the emissions from Butyl Nonanoate and JP8 were similar. Emissions less than 8 PPM of NO were achieved at an equivalence ratio of 0.6, with lower NO emission at lower equivalence ratios. These emissions are substantially lower than the current EPA regulations of 42 PPM for units larger than 250 MW operating on fuels other than natural gas [19]. The CO emissions were also obtained for the above fuels and the results are shown in Fig. 8. For different fuels, CO emission de-
creased with decrease in equivalence ratio until a minimum was reached, and then it started to increase again. The decrease in CO emission is attributed to excess oxygen that helps in complete conversion of CO to CO2, while the increase near the extinction limit is attributed to lower temperatures and weaker flames found near the extinction limit. Comparing CO emissions of Butyl Nonanoate with JP8, one can see the beneficial aspects of using Butyl Nonanoate as a fuel. CO emissions were similar at high equivalence ratio, however, near the lean flammability limit, Butyl Nonanoate produced less CO emissions. Butyl Nonanoate combustion emitted 32 PPM of CO at an equivalence ratio of 0.6, while JP8 fuel combustion resulted in about 40 PPM at the same equivalence ratio. Emissions from Butyl Nonanoate were also compared with Hydrogenated Renewable Jet (HRJ) biofuel representing a certified biofuel that is currently used in both military and civilian applications. NO and CO emissions for the three fuels are shown in Fig. 9. One can see that HRJ produces less NO emissions as one would expect from biofuels. However, the reduction in NO emissions through the use of Butyl Nonanoate was higher especially at higher equivalence ratios. HRJ also produced higher CO emissions as compared to both JP8 and Butyl Nonanoate. The radical intensity distribution of OH chemiluminescence was acquired and the results are shown in Fig. 10. OH chemiluminescence distribution for both JP8 and Butyl Nonanoate show that the reaction zone was found around the entry jet, particularly at high equivalence ratios. This suggests the use of multiport fuel injection into the combustor. In the case of Butyl Nonanoate, the maximum reaction intensity was located downstream as compared to JP8 indicating a slower reaction process. Also, the overall reaction intensity was decreased compared to JP8 (22,500 a.u. as compared to 30,000 a.u.). Less intense reaction can be related to the lower NO emissions emitted by Butyl Nonanoate. Also, the delayed reaction gives more time for favorable mixture preparation (mixing with hot recirculated reactive species) prior to combustion.
Fig. 7. NO emissions for Butyl Nonanoate and JP8.
Fig. 8. CO emissions for Butyl Nonanoate and JP8.
Case no.
Fuel
Equivalence ratio
Flow rates (ml/min)
ATP-JP8 ATP-BN
JP-8 Butyl Nonanoate (BN) Hydrogenated Renewable Jet fuel (HRJ)
0.5–0.84 0.49–0.86
6.9–11.7 8.2–14.4
0.56–0.82
8–12.3
ATP-HRJ
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527
Fig. 9. NO and CO emissions for Butyl Nonanoate, JP8, and HRJ.
4.3. Butyl Nonanoate as a novel biofuel Butyl Nonanoate have demonstrated stable combustion with ultra-low emissions for gas turbine applications. Butyl Nonanoate is characterized by a lower heating value as compared to JP-8 (17% lower) which means larger fuel flow and fuel storage will be required for the same output. This is somewhat mitigated by the fact that Butyl Nonanoate is about 7% more dense than JP-8, which means more fuel can be stored in a given tank leading to a net decrease in the output due to the lower heating value of Butyl Nonanoate. On the other hand, the lower heating value of Butyl Nonanoate will not be a problem in applications where full fuel storage capacity is not required, such as short flights and shorter range aircrafts. In the current combustor, as described earlier, liquid fuels vaporize in the inlet air injector prior to being introduced to the combustor. However, this is not necessarily the case in different applications where atomizers are used. Butyl Nonanoate produced droplets that are 13–17% larger than JP-8 which might lead to combustion and performance problem as a ‘‘drop in’’ fuel. In such appli-
cations, a blend of Butyl Nonanoate and JP-8 will be more appropriate as the droplets will be only 3–4% larger than those formed with JP-8. Such small variation in size can be tolerated in most combustion systems.
5. Conclusions Results obtained with Butyl Nonanoate have shown good potential of this biofuel for power generation and other application. Butyl Nonanoate was somewhat harder to atomize as compared to JP-8 to result in a larger droplet size and size distribution. Such increase in average droplet size can be mitigated by mixing Butyl Nonanoate and JP-8. However, the recorded pollutants emissions from the swirling colorless distributed combustor using Butyl Nonanoate were found to be lower compared to JP8. Lower NO and CO emissions were shown especially at higher equivalence ratios. Even when compared to certified biofuel (Hydrogenated Renewable Jet fuel), Butyl Nonanoate showed lower NO and CO emissions. OH chemiluminescence showed that the resultant reaction from both fuels (JP8 and Butyl Nonanoate) share the same
Fig. 10. OH chemiluminescence for JP8 and Butyl Nonanoate flames.
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general characteristic with minor differences in maximum intensity value and location. The combustor did not show any instability or fluctuation while running on any of those fuels thus providing the potential of Butyl Nonanoate as a future biofuel for use as a fuel in the CDC combustor previously shown to give ultra-low emissions using other petroleum based fuels. Acknowledgments This research was supported by DLA and ONR (program managers Dr. Gabriel D. Roy and Dr. Clifford Bedford) and their support is gratefully acknowledged. References [1] Hileman JI, Stratton RW, Donohoo PE. Energy content and alternative jet fuel viability. J Propul Power 2010;26(6):1184–95. [2] Boateng AA, Mullen CA, Goldberg NM. Producing stable pyrolysis liquids from the oil-seed presscakes of mustard family plants Pennycress (Thlaspi arvense L.) and Camelina (Camelina sativa). Fuels & Energy 2010;24(12):6624–32. [3] Rye L, Blakey S, Wilson CW. Sustainability of supply or the planet: a review of potential drop-in alternative. Energy Environ Sci 2010;3(1):17–27. [4] Blakey S, Rye L, Wilson CW. Aviation gas turbine alternative fuels: a review. Proc Combust Inst 2011;33:2863–85. [5] Carter NA, Stratton RW, Bredehoeft, MK, Hileman, JI. Energy and environmental viability of select alternative jet fuel pathways. In: 47th AIAA/ ASME/SAE/ASEE joint propulsion conference, San Diego, CA, 2011. [6] Corporan E, Dewitt MJ, Klingshirn CD, Striebich R, Cheng M. Emissions characteristics of military helicopter engines with JP-8 and Fischer–Tropsch fuels. J Propul Power 2010;26(2):317–24.
[7] Shonnard DR, Williams L, Kalnes TN. Camelina-derived jet fuel and diesel: sustainable advanced biofuels. Environ Progr & Sustain Energy 2010;29(3):382–92. [8] Tran VH, Morton CD, Parthasarathy RN, Gollahalli SR. Combustion characteristics of petroleum and biofuel spray flames in a furnace. In: 51st AIAA aerospace sciences meeting, Grapevince (Dallas/Ft. Worth Region), TX, 2013. [9] Corporan E, Reich R, Monroig O, DeWitt MJ, Larson V, Aulich T, et al. Impacts of biodiesel on pollutant emissions of a JP-8-fueled turbine engine. J Air & Waste Manage Assoc 2005;55(7):940–9. [10] Khalil AEE, Gupta AK. Swirling distributed combustion for clean energy conversion in gas turbine applications. J. Appl Energy 2011;88:3685–93. [11] Khalil AEE, Gupta AK. Distributed swirl combustion for gas turbine application. J. Appl Energy 2011;88:4898–907. [12] Tsuji H, Gupta AK, Hasegawa T, Katsuki M, Kishimoto K, Morita M. High temperature air combustion: from energy conservation to pollution reduction. Boca Raton, Florida: CRC Press; 2003. [13] Arghode VK, Gupta AK. Effect of flow field for colorless distributed combustion (CDC) for gas turbine combustion. J. Appl Energy 2010;87:1631–40. [14] Arghode VK, Gupta AK. Investigation of forward flow distributed combustion for gas turbine application. J. Appl Energy 2010;88:29–40. [15] Khalil AEE, Arghode VK, Gupta AK. Novel mixing for ultra-high thermal intensity distributed combustor. J. Appl Energy 2013;105:327–34. [16] Khalil AEE, Gupta AK, Bryden MK, Lee SC. Mixture preparation effects on distributed combustion for gas turbine applications. J Energy Resour Technol 2012;134(3):032201. [17] Arghode VK, Khalil AEE, Gupta AK. Fuel dilution and liquid fuel operational effects on ultra-high thermal intensity distributed combustor. J Appl Energy 2012;95:132–8. [18] Khalil AEE, Gupta AK. Fuel flexible distributed combustion with swirl for gas turbine applications. J. Applied Energy 2013;109:267–74. [19] Environmental Protection Agency. Standards of performance for stationary combustion turbines; final rule. Fed Reg 2006;71(129):38481–506.
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Clean combustion in gas turbine engines using Butyl Nonanoate biofuel Ahmed E.E. Khalil, Ashwani K. Gupta ⇑,1 Department of Mechanical Engineering, University of Maryland, College Park, MD 20742, USA
h i g h l i g h t s Examined Butyl Nonanoate (BN) biofuel for gas turbine applications. BN produced 15% larger droplet sizes compared to JP8 under similar condition. BN provided 15% reduction in NO emission compared to JP8 in the distributed combustor. CO emission was also lower compared to JP8, and contrary to HRJ biofuel. No start-up issues or combustion instabilities observed with the BN biofuel.
a r t i c l e
i n f o
Article history: Received 18 June 2013 Received in revised form 7 August 2013 Accepted 8 August 2013 Available online 23 August 2013 Keywords: Biofuels Colorless distributed combustion (CDC) Ultra-low NOx and CO emission Gas turbine combustion Fuel flexibility
a b s t r a c t Developing new and renewable biofuels for ultra-low emission gas turbine combustors is much desired to secure future power needs. Several fuels are being developed to replace fossil fuels with minimal carbon footprint and pollutants emission. Ease of transition between fossil fuels and biofuels is also desired for fuel flexibility in engine operation. In this paper, the performance of Butyl Nonanoate biofuel is evaluated and compared to both Hydrogenated Renewable Jet (HRJ) fuel and JP-8. The atomization characteristics are examined using Phase Doppler Particle Analyzer (PDPA) for favorable condition of fuel introduction to the combustor. The Biofuel provided 10–15% larger size droplets compared to JP-8 for a given atomizer and operational pressure. The combustion behavior of biofuel was also examined in a swirling colorless distributed combustor with focus on pollutants emission under high intensity simulated gas turbine conditions. The liquid fuels were directly fed into the path of the air stream prior to combustion without using any atomizer or spray device. The biofuel provided 6 PPM of NO and 35 PPM of CO at an energy release intensity of 27 MW/m3-atm at 0.6 equivalence ratio. The recorded emissions are about 15% lower as compared to JP-8 for equivalence ratio higher than 0.65. For lower equivalence ratios, NO emissions were identical. The Butyl Nonanoate biofuel also exhibited lower NO emissions than HRJ biofuel. The CO emissions were lower with Butyl Nonanoate as compared to JP-8 (this is in contrast to HRJ biofuel which showed an increase in CO compared to JP-8). The examined biofuel did not show any instability with smooth transition between different fuels. These results provide promising behavior of Butyl Nonanoate biofuel for use in future energy needs without any modifications to the combustor injectors, while maintaining high performance. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Rapid depletion of fossil fuels and widespread concerns on energy security have motivated scientists and engineers to develop new fuels that are derived from environmental friendly sources to support the current and future energy needs in a sustainable fashion. Several authors have discussed the viability of such biofuels [1–4]. The current interest on alternative jet fuels for commercial and military application stems from multiple factors including sustained energy availability, high petroleum fuel costs, price vol⇑ Corresponding author. Tel.: +1 301 405 5276; fax: +1 301 314 9477. 1
E-mail address: [email protected] (A.K. Gupta). Distinguished University Professor.
0016-2361/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2013.08.022
atility, lack of energy diversity, global climate impacts, and potential air quality. Civil and military jet aircraft require a near-term fuel replacement to conventional petroleum based jet fuel that is a ‘‘drop-in’’ hydrocarbon substitute that can function within the existing aircraft infrastructure while meeting rigorous safety and performance standards. Such drop-in alternative jet fuel pathways can be grouped into five broad categories: jet fuel produced from unconventional sources of petroleum such as oil sands, very heavy oils, and oil shale; synthetic jet fuel from thermochemical processes involving natural gas, coal, and/or lignocellulosic biomass such as Fischer– Tropsch (F–T) synthesis and pyrolysis; advanced fermentation, catalytic, and other means of converting sugars to jet fuel; hydro-processing of conventional oils to synthetic jet fuel; and conversion of
A.E.E. Khalil, A.K. Gupta / Fuel 116 (2014) 522–528
Fig. 1. Droplet size measurement using PDPA.
calorific liquids from micro-organisms to synthetic jet fuel [5]. Through both ground and flight tests, the USAF has already approved a 50% blend of F–T fuels with conventional jet fuels. It is expected that the certification of a 50% blend of hydro-processed renewable oils with conventional jet fuel should occur in the near future [6]. Commercial and military aviation have set ambitious alternative fuel and environmental targets for the next half century. The International Air Transport Association has set a goal of 10% alternative fuel use by 2017, carbon neutral growth in 2020, and a 50% decrease in aviation CO2 emissions by 2050, relative to 2005 levels. In a detailed research on the viability of different alternative jet fuels, it was concluded that switchgrass F–T fuel and camelina HRJ have the potential to reduce life cycle greenhouse gas emissions by 60–80% [5]. Synthetic isoparaffin-rich fuels produced by hydro-processing camelina oil show great promise as ‘‘drop-in’’ alternatives to petroleum jet and diesel fuel. Recent evaluations of HRJ fuel derived from camelina oil indicate the fuel performed just as well as petroleum based fuel but with lower exhaust and GHG emissions. Using updated estimates of camelina cultivation and commercial scale estimates of oil recovery and refining requirements a life cycle GHG savings of 75% and 80% was estimated for camelina-derived HRJ and Green Diesel (GD) relative to their petroleum fuel counterparts. Using data from recent field trials, GHG savings of >67% were achieved [7]. Also reduction in NOx emissions can be achieved via biofuels. On the other hand, CO emissions depend on the engine type [4]. The use of biofuels have also been extended to furnaces with emission reduction of both NO and CO [8]. A study on biofuel using T63 test engine revealed that there was minimal changes in CO emission with slight increase in THC (10%) at idle with biodiesel concentrations of 20%. This increase was attributed to incomplete combustion of biodiesel at the low power setting. At the higher power conditions, there was little or no change in CO and THC. The addition of biodiesel had negligible effects on oxides of nitrogen (NOx) emissions for all the test cases. This was expected, because the primary route for NOx formation is via thermal NOx, and the relative combustion temperature was not changed during the blends tests. Only slight reductions in oxides of sulfur (SOx) emissions were observed as the biodiesel concentration was increased, which was likely a dilution effect, because the biodiesel did not contain sulfur [9]. The quest for developing biofuels from untraditional sources as a new energy source is a topic of ongoing work at several institutions and organizations. In that sense, Butyl Nonanoate is a Air/Fuel Inlet
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chemical that has not been used as a biofuel before. It is characterized by a lower calorific value, higher viscosity and density. This fuel has been processed at Michigan State University for use as a new biofuel. The combustion characteristics of this novel fuel will be evaluated with view to its use in gas turbines (both stationary and aviation). The most important parameters are the ability of the fuel to be atomized (which depend on surface tension, viscosity, and density), along with the pollutants emission resulting from the combustion of this fuel. The combustion characteristics of this Butyl Nonanoate fuel will be compared to standard JP-8 to determine its viability for use in gas turbine applications. The performance of Butyl Nonanoate will be evaluated and compared to JP-8 and HRJ fuels in terms of droplet formation and pollutants emission. These fuels will be examined in a colorless distributed combustor [10,11] for performance evaluation. Colorless distributed combustion (CDC), which is based on the principles of High Temperature Air Combustion (HiTAC [12]), have been shown to provide ultra low emissions with high efficiency under different configurations [13–16]. Fuel flexibility have also been demonstrated using CDC combustors [17,18]. The examined fuels included both gaseous and liquid fuels with no change to the combustor geometry or fuel introduction method [18]. The resultant emissions from different fuels using this combustor were found to be lower than the regulations set by Environmental Protection Agency (EPA) [19], outlining the potential application of this combustor design. 2. Experimental facility The atomization characteristics of biofuel were first evaluated using a standard atomizer. The droplets diameter was measured using a phase Doppler particle analyser and an average was calculated to evaluate the atomization of the novel Butyl Nonanoate biofuel. Fig. 1 shows the experimental setup used for droplet diameter evaluation. The pollutants emission from the biofuel was also measured where the fuel was fed to a swirling distributed combustor operating at a nominal heat load of 6.25 kW and energy release intensity of 36 MW/m3-atm. Different liquid fuels were injected in a preheated air stream (for simulating gas turbine combustor intake with elevated temperature after air exiting the compressor) upstream the combustor. The combustion chamber utilized was a cylindrical chamber; air was injected tangentially at half the height of chamber for all the cases investigated here. For enhancing the residence time of reactants in the combustor, a tube was extended inside the combustor for exiting the product gases from the combustor. Fig. 2 shows a schematic diagram of the combustor used. More detailed description of the combustor geometry is given in Ref. [11]. For combustion, air was supplied using an air compressor. Air flow rates were measured using choked flow orifice systems. Precision stainless steel orifices were used to choke the flow of the gases and upstream pressure was controlled to supply the required mass flow rate of gases. The upstream pressure was controlled using pressure regulators to maintain a steady pressure and avoid Air/Fuel Inlet
Exit
Fig. 2. Schematic of the combustor used.
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Fig. 3. A schematic of the experimental combustion facility.
oscillations in pressure due to compressor turning on and off periodically. The upstream pressure for air was maintained between ±0.2 psi of the desired pressure. Upstream pressure was measured using a piezo-resistive digital pressure gauge with accuracy of ±0.25 psi. It was ensured that the downstream pressure was lower than the pressure required for choking the flow based on the upstream pressure. Liquid fuels were supplied to the combustor using a compressed fuel tank that delivers specific liquid fuel flow rate at a specific upstream pressure of a thin fuel introduction tube (inner diameter of 0.01 in.). This fuel delivery system was calibrated for every fuel to ensure that the delivered flow rates are adequate for the examined condition. The fuel was directly introduced into the preheated air stream at an upstream location of the combustor to support near premixed combustion mode. Fig. 3 shows the experimental setup used for fuel introduction and emission measurement. Detailed investigations on the overall emissions from the combustor as well as visible emissions have been performed for the various experimental conditions. A Horiba PG-250 gas analyzer was used for emissions measurements. The concentration of NO was measured using a NO–NOx chemiluminescent method; CO concentration was measured using the non-dispersive infrared method and O2 concentration (used to correct the NO and CO emissions at standard 15% oxygen concentration) was measured using galvanic cell method. The gas analyser was continuously calibrated against calibration gases. For NO emissions below 5 PPM, a high accuracy thermo-electron chemiluminescence NO–NO2–NOx low source analyzer was also used (Model 42C) to support the meaTable 1 Biofuels examined. Case no.
Fuel
ATP-JP8 ATP-BN ATP-HRJ
JP-8 Butyl Nonanoate (BN) Hydrogenated Renewable Jet fuel (HRJ)
surements. During a single experiment, measurements were repeated at least three times for each configuration and the uncertainty was estimated to be about ±0.5 PPM for NO and ±10% for CO emission. The experiments were repeated at least three times to ensure good repeatability of the experimental data. The recorded data was then corrected to 15% O2 concentration and averaged. For imaging the OH chemiluminescent intensity distribution, an ICCD camera coupled to a narrow band filter having center wavelength of 307 nm wavelength was used. The resulting OH intensity distributions were then normalized to provide the same scale. The inlet air temperature to the combustor was preheated to 600 K to simulate elevated air temperatures at inlet to the combustor (simulating outlet temperature from a characteristic gas turbine compressor). The preheating of the inlet air affects the combustion kinetics and pollutants emission. In general, higher air inlet temperature increases the flame temperature, which substantially decreases CO emission and slightly increases NOx levels [11]. 3. Experimental The performance of Butyl Nonanoate biofuel was examined and compared to JP-8 in terms of droplet size atomization and emissions of NO and CO. The emissions were compared to that from JP-8 and HRJ fuels in the same combustor under identical
Table 2 Properties of biofuels. Property
Butyl JP-8 Nonanoate (BN)
Calorific value (kJ/kg) Density (kg/m3) Kinematic viscosity (mm2/s) Surface tension (N/m) Cetane number
35,677 849–855 1.989 @ 23.6 °C 0.029 @ 25 °C 51.5
HRJ
43,000 44,300 795 751 1.692 @ 20 °C 1.1 @ 40 °C 0.026 @ 25 °C 0.0264 @ 25 °C 44.1 58
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Fig. 4. PDPA arrangement with spray nozzle.
Table 3 Average diameters for Butyl Nonanoate and JP8.
Number average Area average Volume average Volume/area average
Table 4 Average diameters for fuel mixture and JP8.
Symbol
JP8 (lm)
Butyl Nonanoate (lm)
Difference (%)
D10 D20 D30 D32
37.07 40.42 43.51 50.44
42.2 46.95 50.42 59.04
13 15 15 17
operational conditions. Table 1 shows the experimental investigations designation with respect to each fuel. Table 2 gives the properties of the fuels examined. Properties of Butyl Nonanoate were measured at Michigan state university, while properties of Hydrogenated renewable jet fuel were provided by the supplier. Properties of JP-8 were gathered from the literature. 4. Results and discussion 4.1. Droplet size Liquid fuel atomization is of critical importance for combustion, as larger droplet size will require more time to evaporate and mix before ignition, which leads to improper mixing and onset of concentrated thin reaction zone flames. These flames should be avoided for seeking distributed reactions with near zero emissions. Comparing the properties of Butyl Nonanoate to standard JP8 properties, it was found that Butyl Nonanoate has higher surface tension, viscosity and density, leading to bigger droplet diameter compared to JP8 for a given set of conditions. To evaluate the size average and size distribution difference between both fuels, a standard spray nozzle was installed in the measuring volume of a Phase Doppler Particle Analyzer (PDPA). The liquid fuel tank was pressurized to an upstream pressure of 40 PSI to ensure same operational conditions for all of the experiments. Both fuels were examined multiple times successively and separately. For each experiment, 10,000 droplets were measured to obtain the droplet size distribution. Average diameters based on number, area, volume and volume to area ratio were calculated from the obtained histogram and are used to evaluate the average droplet size diameter for the fuels. Fig. 4 shows the nozzle used with measurement volume.
Number average Area average Volume average Volume/area average
Symbol
JP8 (lm)
50% Butyl Nonanoate–50% JP8 (lm)
Difference (%)
D10 D20 D30 D32
34.67 39.43 43.47 52.83
36.19 40.89 44.95 54.32
4.3 3.6 3.4 2.8
The obtained data on size distribution for both fuels showed that Butyl Nonanoate had a 10–15% bigger droplet size compared to JP8. For instance, the average number droplet diameter (D10) for Butyl Nonanoate was about 42.2 lm compared to 37.07 lm for JP8. Table 3 gives a sample of the averaged diameters for both JP8 and Butyl Nonanoate. Fig. 5 shows a sample size distribution for both the fuels. The reported values are with the probe volume corrected to accommodate for Gaussian distribution of the laser beam power. The results obtained outline the difference between the average droplets sizes produced, wherein Butyl Nonanoate provided 10– 15% bigger droplet size compared to JP8. A mixture of both fuels should result in a smaller difference compared to JP8. Such smaller size difference can be tolerated in actual combustion systems. The obtained data on size distribution for 50% Butyl Nonanoate 50% JP8 mixture as compared to JP8 showed that fuel mixture had a 3–4% bigger droplet size compared to JP8. For instance, the average number droplet diameter (D10) for the mixture was about 36.19 lm compared to 34.67 lm for JP8. Table 4 gives a sample of the averaged diameters for both JP8 and Butyl Nonanoate–JP8 mixture. Fig. 6 shows a sample size distribution for both the fuels. The results obtained show the differences between the average droplet size produced, with Butyl Nonanoate having 13–17% bigger droplet size compared to JP8. A mixture of these two fuels (50–50% by volume) showed only about 3–4% difference in size. Such smaller size difference could be tolerated in actual combustion systems.
4.2. Pollutants emission The pollutants emission from the biofuel was measured using swirling distributed combustor operating at a nominal heat load
Fig. 5. Number diameter distribution for JP8 (left) and Butyl Nonanoate (right).
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Fig. 6. Number diameter distribution for JP8 (left) and fuel mixture (right).
Table 5 Experimental parameters.
of 6.25 kW and energy release intensity of 36 MW/m3-atm. Different liquid fuels were injected in a preheated air stream (for simulating gas turbine combustor intake with elevated temperature after air exiting the compressor) upstream the combustor. Emissions of NO and CO was measured for the different fuels examined. For all of the fuels examined, the heat load was kept constant at 6.25 kW (having energy release intensity (HRI) of 36 MW/m3-atm) at an equivalence ratio of 0.8. Air flow rate was adjusted to furnish the combustor with the adequate air requirement for each fuel used. Fuel flow rate was varied to allow for change in equivalence ratio that ranged from 0.8 to 0.5 and the subsequent overall emissions measured. Table 5 gives the experimental conditions for the conducted experiments. Fig. 7 shows the NO emission levels resulting from operating the combustor with different fuels for the novel premixed combustion configuration. Obtained data show the performance of Butyl Nonanoate as compared to JP8. Butyl Nonanoate produced less NO emission at higher equivalence ratios. At lower equivalence ratios, the emissions from Butyl Nonanoate and JP8 were similar. Emissions less than 8 PPM of NO were achieved at an equivalence ratio of 0.6, with lower NO emission at lower equivalence ratios. These emissions are substantially lower than the current EPA regulations of 42 PPM for units larger than 250 MW operating on fuels other than natural gas [19]. The CO emissions were also obtained for the above fuels and the results are shown in Fig. 8. For different fuels, CO emission de-
creased with decrease in equivalence ratio until a minimum was reached, and then it started to increase again. The decrease in CO emission is attributed to excess oxygen that helps in complete conversion of CO to CO2, while the increase near the extinction limit is attributed to lower temperatures and weaker flames found near the extinction limit. Comparing CO emissions of Butyl Nonanoate with JP8, one can see the beneficial aspects of using Butyl Nonanoate as a fuel. CO emissions were similar at high equivalence ratio, however, near the lean flammability limit, Butyl Nonanoate produced less CO emissions. Butyl Nonanoate combustion emitted 32 PPM of CO at an equivalence ratio of 0.6, while JP8 fuel combustion resulted in about 40 PPM at the same equivalence ratio. Emissions from Butyl Nonanoate were also compared with Hydrogenated Renewable Jet (HRJ) biofuel representing a certified biofuel that is currently used in both military and civilian applications. NO and CO emissions for the three fuels are shown in Fig. 9. One can see that HRJ produces less NO emissions as one would expect from biofuels. However, the reduction in NO emissions through the use of Butyl Nonanoate was higher especially at higher equivalence ratios. HRJ also produced higher CO emissions as compared to both JP8 and Butyl Nonanoate. The radical intensity distribution of OH chemiluminescence was acquired and the results are shown in Fig. 10. OH chemiluminescence distribution for both JP8 and Butyl Nonanoate show that the reaction zone was found around the entry jet, particularly at high equivalence ratios. This suggests the use of multiport fuel injection into the combustor. In the case of Butyl Nonanoate, the maximum reaction intensity was located downstream as compared to JP8 indicating a slower reaction process. Also, the overall reaction intensity was decreased compared to JP8 (22,500 a.u. as compared to 30,000 a.u.). Less intense reaction can be related to the lower NO emissions emitted by Butyl Nonanoate. Also, the delayed reaction gives more time for favorable mixture preparation (mixing with hot recirculated reactive species) prior to combustion.
Fig. 7. NO emissions for Butyl Nonanoate and JP8.
Fig. 8. CO emissions for Butyl Nonanoate and JP8.
Case no.
Fuel
Equivalence ratio
Flow rates (ml/min)
ATP-JP8 ATP-BN
JP-8 Butyl Nonanoate (BN) Hydrogenated Renewable Jet fuel (HRJ)
0.5–0.84 0.49–0.86
6.9–11.7 8.2–14.4
0.56–0.82
8–12.3
ATP-HRJ
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527
Fig. 9. NO and CO emissions for Butyl Nonanoate, JP8, and HRJ.
4.3. Butyl Nonanoate as a novel biofuel Butyl Nonanoate have demonstrated stable combustion with ultra-low emissions for gas turbine applications. Butyl Nonanoate is characterized by a lower heating value as compared to JP-8 (17% lower) which means larger fuel flow and fuel storage will be required for the same output. This is somewhat mitigated by the fact that Butyl Nonanoate is about 7% more dense than JP-8, which means more fuel can be stored in a given tank leading to a net decrease in the output due to the lower heating value of Butyl Nonanoate. On the other hand, the lower heating value of Butyl Nonanoate will not be a problem in applications where full fuel storage capacity is not required, such as short flights and shorter range aircrafts. In the current combustor, as described earlier, liquid fuels vaporize in the inlet air injector prior to being introduced to the combustor. However, this is not necessarily the case in different applications where atomizers are used. Butyl Nonanoate produced droplets that are 13–17% larger than JP-8 which might lead to combustion and performance problem as a ‘‘drop in’’ fuel. In such appli-
cations, a blend of Butyl Nonanoate and JP-8 will be more appropriate as the droplets will be only 3–4% larger than those formed with JP-8. Such small variation in size can be tolerated in most combustion systems.
5. Conclusions Results obtained with Butyl Nonanoate have shown good potential of this biofuel for power generation and other application. Butyl Nonanoate was somewhat harder to atomize as compared to JP-8 to result in a larger droplet size and size distribution. Such increase in average droplet size can be mitigated by mixing Butyl Nonanoate and JP-8. However, the recorded pollutants emissions from the swirling colorless distributed combustor using Butyl Nonanoate were found to be lower compared to JP8. Lower NO and CO emissions were shown especially at higher equivalence ratios. Even when compared to certified biofuel (Hydrogenated Renewable Jet fuel), Butyl Nonanoate showed lower NO and CO emissions. OH chemiluminescence showed that the resultant reaction from both fuels (JP8 and Butyl Nonanoate) share the same
Fig. 10. OH chemiluminescence for JP8 and Butyl Nonanoate flames.
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general characteristic with minor differences in maximum intensity value and location. The combustor did not show any instability or fluctuation while running on any of those fuels thus providing the potential of Butyl Nonanoate as a future biofuel for use as a fuel in the CDC combustor previously shown to give ultra-low emissions using other petroleum based fuels. Acknowledgments This research was supported by DLA and ONR (program managers Dr. Gabriel D. Roy and Dr. Clifford Bedford) and their support is gratefully acknowledged. References [1] Hileman JI, Stratton RW, Donohoo PE. Energy content and alternative jet fuel viability. J Propul Power 2010;26(6):1184–95. [2] Boateng AA, Mullen CA, Goldberg NM. Producing stable pyrolysis liquids from the oil-seed presscakes of mustard family plants Pennycress (Thlaspi arvense L.) and Camelina (Camelina sativa). Fuels & Energy 2010;24(12):6624–32. [3] Rye L, Blakey S, Wilson CW. Sustainability of supply or the planet: a review of potential drop-in alternative. Energy Environ Sci 2010;3(1):17–27. [4] Blakey S, Rye L, Wilson CW. Aviation gas turbine alternative fuels: a review. Proc Combust Inst 2011;33:2863–85. [5] Carter NA, Stratton RW, Bredehoeft, MK, Hileman, JI. Energy and environmental viability of select alternative jet fuel pathways. In: 47th AIAA/ ASME/SAE/ASEE joint propulsion conference, San Diego, CA, 2011. [6] Corporan E, Dewitt MJ, Klingshirn CD, Striebich R, Cheng M. Emissions characteristics of military helicopter engines with JP-8 and Fischer–Tropsch fuels. J Propul Power 2010;26(2):317–24.
[7] Shonnard DR, Williams L, Kalnes TN. Camelina-derived jet fuel and diesel: sustainable advanced biofuels. Environ Progr & Sustain Energy 2010;29(3):382–92. [8] Tran VH, Morton CD, Parthasarathy RN, Gollahalli SR. Combustion characteristics of petroleum and biofuel spray flames in a furnace. In: 51st AIAA aerospace sciences meeting, Grapevince (Dallas/Ft. Worth Region), TX, 2013. [9] Corporan E, Reich R, Monroig O, DeWitt MJ, Larson V, Aulich T, et al. Impacts of biodiesel on pollutant emissions of a JP-8-fueled turbine engine. J Air & Waste Manage Assoc 2005;55(7):940–9. [10] Khalil AEE, Gupta AK. Swirling distributed combustion for clean energy conversion in gas turbine applications. J. Appl Energy 2011;88:3685–93. [11] Khalil AEE, Gupta AK. Distributed swirl combustion for gas turbine application. J. Appl Energy 2011;88:4898–907. [12] Tsuji H, Gupta AK, Hasegawa T, Katsuki M, Kishimoto K, Morita M. High temperature air combustion: from energy conservation to pollution reduction. Boca Raton, Florida: CRC Press; 2003. [13] Arghode VK, Gupta AK. Effect of flow field for colorless distributed combustion (CDC) for gas turbine combustion. J. Appl Energy 2010;87:1631–40. [14] Arghode VK, Gupta AK. Investigation of forward flow distributed combustion for gas turbine application. J. Appl Energy 2010;88:29–40. [15] Khalil AEE, Arghode VK, Gupta AK. Novel mixing for ultra-high thermal intensity distributed combustor. J. Appl Energy 2013;105:327–34. [16] Khalil AEE, Gupta AK, Bryden MK, Lee SC. Mixture preparation effects on distributed combustion for gas turbine applications. J Energy Resour Technol 2012;134(3):032201. [17] Arghode VK, Khalil AEE, Gupta AK. Fuel dilution and liquid fuel operational effects on ultra-high thermal intensity distributed combustor. J Appl Energy 2012;95:132–8. [18] Khalil AEE, Gupta AK. Fuel flexible distributed combustion with swirl for gas turbine applications. J. Applied Energy 2013;109:267–74. [19] Environmental Protection Agency. Standards of performance for stationary combustion turbines; final rule. Fed Reg 2006;71(129):38481–506.