Flame Stability of Methane and Syngas Oxy-fuel Steam Flames

Article pubs.acs.org/EF

Flame Stability of Methane and Syngas Oxy-fuel Steam Flames B. K. Dam, N. D. Love,* and A. R. Choudhuri Department of Mechanical Engineering, University of Texas El Paso, 500 West University Avenue, El Paso, Texas 79968, United States ABSTRACT: Oxy-fuel combustion has been used previously in a wide range of industrial applications. Oxy-combustion is carried out by burning a hydrocarbon fuel with oxygen instead of air. Flames burning in this configuration achieve higher flame temperatures, which present opportunities for significant efficiency improvements and direct capture of CO2 from the exhaust stream. In an effort to better understand and characterize the fundamental flame characteristics of oxy-fuel combustion, this research presents the experimental measurements of flame stability of CH4/O2 and syngas (H2−CO)/O2 flames. Effects of the H2 concentration, fuel composition, exhaust gas recirculation ratio, firing inputs, and burner diameters on the flame stability of these fuels are discussed. Effects of exhaust gas recirculation, i.e., CO2 and H2O (steam) acting as diluents on burner operability, are also presented. The roles of firing input on flame stability are then analyzed. For this study, it was observed that many oxyflames did not stabilize without exhaust gas recirculation because of their higher burning velocities. In addition, the stability regime of all compositions was observed to decrease as the burner diameter increased. A flashback model is also presented, using the critical velocity gradient (gF) values for CH4−O2−CO2 flames. The scaling relation [gF = c(SL2/α)] for different burner diameters was obtained for various diameter burners. The paper shows that results correlated linearly with a scaling value of c = 0.0174. CO2 and H2O gases into the feed stream.4,5 For both turbine cycles and pulverized coal-fired plants, several combustor operability issues, such as heat-transfer characteristics, heat loading, flame stability, and flashback, become important with the transition from air to oxygen-based combustion.6,7 A lack of experimental data or proven computer models of oxy-fuel combustors operating on O2, CO2, and H2O in the literature is one of the constraints to building new oxy-combustion-based power plants. To be used effectively and meet sequestration goals, however, the fundamental flame characteristics relevant to oxy-fuel combustion are needed to develop new designs and analyze retrofits on existing power generation plants. The U.S. Department of Energy has investigated the performance of a CO2- and H2O-diluted oxy-fuel combustion system in a high-pressure combustor.8 Jupitar Oxygen Corporation also developed an oxy-fuel system that uses an untempered high-temperature oxy-fuel flame, currently on the market for environmentally sound energy production.7,8 Table 1 presents a list of other studies of oxy-fuel combustion.12−16 Major review works can also be found elsewhere in refs 17−20. On the basis of the works available in the literature and an observed increase in the use of oxy-combustion systems, this study looks to enhance the understanding of flame stability of oxy-fuel flames. Thus, the present study provides the scientific community with flame stability data that can be used for the design of oxy-combustion burners and computational models for methane and coal-based syngas fuels with and without diluents. 1.1. Flame Stability. This paper presents data on flame blowout and flashback. Both phenomena are controlled by

1. INTRODUCTION Oxy-fuel combustion, the burning of a (hydrocarbon or hydrogen) fuel with oxygen in place of air, has been used previously in a wide range of industrial applications. Oxycombustion has potential to offer considerable gains in system efficiency and reduce NOx pollutant emissions if used for power generation. Currently, for many applications using oxygenenhanced combustion, it is not economically attractive because of the oxygen production process; however, future research efforts can offset costs through significant efficiency improvements, fuel savings, and pollutant reductions. To be effective, these systems require innovative efforts in various areas, including materials, combustion, and heat transfer. This paper addresses a research topic in the combustion area. Oxy-fuel combustion with an integrated carbon capture technique is currently being considered as a technology that could substantially reduce NOx and CO2 emissions. Because the fuel, typically a hydrocarbon is burned with oxygen, the products are primarily oxygen, carbon dioxide, and steam. The flue gas with a purity of 95−98 vol % (dry basis) of CO2 can be easily separated by condensing the water from the exhaust stream and eventually stored, thereby significantly reducing greenhouse gas emissions. The additional advantage of this process is the reduction of NOx emissions because of the absence of N2 in the combustion oxidant. Currently, the gas turbine industry has begun to consider implementing an oxyfuel combustion approach on a large scale in some combustors. It is also being considered by the U.S. Department of Energy’s Innovations for Existing Plants Program to meet the goal of capturing 90% CO2 capture without increasing the cost of electricity more than 35%.1−3 In this combustion process, the fuel is burning with oxygen; thus, flame temperatures are significantly higher. The equivalent air-flame temperature is achieved by the addition of recycled © 2012 American Chemical Society

Received: September 25, 2012 Revised: November 20, 2012 Published: November 26, 2012 523

dx.doi.org/10.1021/ef301574d | Energy Fuels 2013, 27, 523−529

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Table 1. Summary of Previous Studies of Oxy-fuel Combustion reference

combustor/burner

summary

12 8 16

swirl stabilized combustor industrial boiler and aluminum remelt furnace 150 kg/h IHI combustion facility

4

100 kW capacity furnace

3 7

high-pressure combustor industrial boiler and furnace

13 14 15

two different lab-scale combustors circulating fluidized-bed combustor (CFBC) combustion chamber incorporated with five different inlet nozzle arrangements

investigated the effect of CO2 and N2 diluents on flame stability of CH4−O2 combustion studied reducing costs in industrial energy efficiency of oxy-fuel combustion technology measured coal reactivity under both oxy-fuel and air-fuel combustion and predicted gas emissivities in oxy-fuel combustion using computational fluid dynamics (CFD) modeling studied radiation and burnout properties of oxy-fuel (O2/CO2) combustion and found the importance of soot volume fraction while modeling investigated the performance of CO2- and H2O-diluted oxy-fuel combustion systems developed an oxy-fuel system that uses an untempered high-temperature oxy-fuel flame, currently on the market for environmentally sound energy production study on the combustion characteristics and flame length of turbulent non-premixed oxy-fuel flames experimental study of oxy-fuel combustion and sulfur capture in a mini-CFBC experimental study of NO emission characteristics for combustors embedded with flue gas recirculation technology

propagation of a flame front through a flow velocity. A special emphasis has been given on flame flashback for the present study. Flashback is a combustion condition in which the flame propagates upstream against the gas stream into the burner tube. Flashback is a critical issue for premixed combustor designs because it can cause serious hardware damage and possibly results in catastrophic failure of the system. In swirlstabilized lean premixed turbine combustors, the onset of flashback may occur because of (i) boundary layer flame propagation (critical velocity gradient), (ii) turbulent flame propagation in core flow, (iii) combustion instabilities, and (iv) upstream flame propagation induced by combustion-induced vortex breakdown (CIVB). Flashback because of the first two foregoing mechanisms has been studied extensively for pure fuels.9 Furthermore, flashback because of CIVB has been presented in other works.10,11 This paper looks to understand boundary layer flame propagation for fuel mixtures with diluents. Generally, analytical theories and experimental determinations of laminar and turbulent burning velocities model these mechanisms with sufficient precision for design usages. However, effects of composition variations on flashback propensity of fuel blends, such as syngas with oxygen, are largely unknown. For example, the presence of hydrogen in syngas significantly increases the potential for flashback.10,11 Because of the high laminar burning velocity and low lean flammability limit, hydrogen tends to shift the combustor operating conditions toward the flashback regime. Even a small amount of hydrogen in fuel mixtures triggers the onset of flashback by altering the kinetics and thermophysical characteristics of the mixture.10 Motivated by this issue, this study presents flashback propensity of H2−CO, CH4, and mixtures with diluents for oxy-fuel flames.

Figure 1. Schematic drawing of the tubular burner. 2.2. Steam Generation System. A steam generation system consisted of a laboratory-scale electric boiler and a water reservoir. The boiler was synchronized with a pump connected to the reservoir to keep the boiler exit steam pressure constant. The steam piping from the boiler to burner facility was insulated to avoid steam condensation in the line. A steam pressure regulator valve was installed in the line, right before the burner entrance, to maintain a constant steam pressure at the downstream of the line. This unit was designed to offer highly sensitive response to reduced pressure changes and ensure desired flow conditions without an appreciable pressure drop. 2.3. Setup and Components. Figures 2 and 3 show the complete experimental setup used for oxy-fuel flame stability measurements. The fuels used for these experiments were methane and syngas (CO−H2), which were burned with oxidants O2 and recirculated CO2 and H2O. Research-grade fuel and oxidant were delivered to the burners from pressurized tanks. Manual precision metering valves in conjunction with low-torque, quarter-turn plug valves were used to control and meter fuel and oxidant flow rates. A bank of digital mass flow meters was used to measure mass flow rates of fuels and oxidant composition. Prior to each experiment mass, flow meters were calibrated using a laser-based mass flow meter calibrator. In the burner system, premixed fuel, oxidant, and recirculated gases entered into the manifold through four alternate injection holes. A laboratory-scale boiler system and pump produced superheated steam. Steam was then premixed with fuel and oxygen and sent to the burner. To maintain superheated steam throughout the line up to the burner exit, heavy insulated high-temperature heater tape along with fiberglass

2. EXPERIMENTAL SECTION 2.1. Development of the Burner System. A burner facility was designed and developed to investigate the flame stability characteristics using different diameter burners. A laboratory-scale burner was designed and fabricated on the basis of the capability of withstanding the higher oxy-fuel flame temperatures, which were calculated to potentially reach as high as 3600 K. A finite element analysis of the designed burner, under thermal stress, was carried out using NASTRAN 6.1. The purpose of this numerical analysis was to ensure that the burner could withstand the high thermal load. Figure 1 presents the final design of the tubular burner used for the experiments. The tubular burner system consists of three primary components: (a) platform, (b) mixing manifold, and (c) burner tube assembly. The tube assembly section merges with the adapters to accommodate tubes of different diameters. 524


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safety. The resulting flame was analyzed with the use of a highresolution digital video camera. Measurement of the steam flow rate from the boiler was calibrated using a graduated cylinder. Steam was captured into a flask through a bypass numbered needle valve and allowed to condense in the cylinder. The subsequent weight of the condensate was measured using a high-precision digital scale. This value was then used to determine the recirculation ratio. A robust test matrix of oxy-fuel was developed for investigating the effect of recirculating CO2 and H2O on flame stability using different burner diameters. The equivalence ratio was set at a constant value of Φ = 1 for all of the experiments without considering CO2 in the oxidizer to draw a stability limit [Φ = (F/O2)act)/(F/O2)stoic)]. Furthermore, the recirculation ratio, which is presented in many figures in this paper, was calculated using eq 1. This ratio is used in this study to quickly gauge the amount of CO2 from the exhaust stream that is premixed with the oxidizer. recirculation ratio =

ṁ CO2 ṁ CO2 + ṁ O2 + ṁ H2O

(1)

At the beginning of the experiment, the flame was ignited at a stable condition. During an experiment, the firing input and equivalence ratio were kept constant, while the amount of diluents (CO2 and H2O) was varied until the flame propagated back into the tube or experienced blowout. A high-speed direct video imaging system was used to confirm the flashback condition, which was defined at the point when the flame entered the burner (Figure 4).

Figure 2. Schematic diagram of the experimental setup.

Figure 4. (Top) Typical blowout sequence and (bottom) flame flashback sequence of a CH4−O2 flame at Φ = 1. The critical boundary velocity gradient of different compositions of hydrocarbon fuel blends and syngas was calculated using eq 2, with more information found in refs 9 and 10 Figure 3. Experimental setup used for measurements.

gF =

insulation was used for keeping lines continuously heated. A temperature controller was used to monitor and control the required temperature in the steam line up to the burner exit. The combination of these components allowed for the analysis of the effect of steam recirculation in the burner. After the premixing section, the fuel and oxygen mixture was ignited with an external ignition source. A flame arrestor device was positioned in the line before the manifold for

4V πr 3

(2)

where V is the experimentally measured volumetric flow rate at flashback conditions and r is the tube radius. The volumetric flow rate for a particular tube diameter and mixture composition at which the flame propagates down the burner tube was defined as the critical flow rate V. To draw the flashback propensity map, the critical volumetric flow rate for flashback at different mixture compositions in cylindrical 525


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tubes of various diameters was measured. The critical volumetric flow rate was defined in eq 3.10,11

gF = c

SL 2 α

(3)

If laminar flame speed (SL) and thermal diffusivity (α) are known, it is possible to estimate the boundary layer flashback propensity of a fuel mixture, provided that the value of c is available. However, boundary layer flashback data for fuel blends are scarce, and the value of c is not readily available for various fuel mixtures. Therefore, for this study, c is experimentally determined by calculating SL and α from the CHEMKIN kinetic code using the GRI 3.0 mechanism; this has been performed previously to estimate this parameter.10 After measurements, the experimental uncertainties were calculated using a Student’s t distribution with a 95% confidence interval. The random error associated with the measurements was very low and considered negligible for all measurements. Therefore, experimental uncertainties are based on the bias error associated with the instrumentation, which is less than ±1% of the mean value.

Figure 6. Stability limits including blowout (BO) and flashback (FB) of CH4−O2−CO2 flames for different firing inputs and burner diameters.

conditions because of a higher adiabatic flame temperature. The stability regime was also observed to narrow as the burner diameter increased because the unburned gas velocity in the upstream is decreased as the burner diameter increased. The flashback and blowout limits were also plotted against the recirculation ratio of CO2 with respect to the theoretical amount of CO2 production obtained from a thermodynamic calculation (Figure 7). The purpose of this graph was to estimate the maximum allowable recirculation of CO2 from the exhaust stream.

3. RESULTS AND DISCUSSION 3.1. Visual Observation and Qualification of the Experimental Setup. Flame images at a typical blowout and flashback condition are presented in Figure 4. For blowout, the flame was initially attached to the tip of the burner, followed by flame detachment from the tip of the burner before blowout. For a typical flashback sequence, flashback was defined at the condition when the flame initially moved inside the burner. Initially, a benchmarking test was performed for the critical velocity gradients (gF) for a natural gas composition of 81.8% CH4, 17.7% C2H6, and 0.5% N2 to reproduce the data provided by Lewis and von Elbe.9 Figure 5 presents the

Figure 7. Stability limits of CH4−O2−CO2 plotted as a ratio of CO2 recirculated to the amount calculated using equilibrium chemistry. Figure 5. Comparison between measurements of current research and data from ref 9.

Figure 8 presents the effects of recirculated steam (H2O) on the stability regime of CH4−O2 flames. Experiments were performed with an increase of steam at a fixed amount of CO2 in the fuel mixture. An almost equal amount of steam could be recirculated potentially until the flame was extinguished. The effect of steam was primarily observed for lower burner diameters, causing the flame to extinguish. Figure 8 demonstrates that, at these conditions, the large latent heat of water quenched the flames, ceasing chemical reactions. Figure 9 shows the stability of CH4−O2−N2 flames and plotted recirculation ratio of N2 in the oxidizer by mass against the burner diameter. The effects of N2 diluents follow a similar trend to that of CO2 diluents. A comparison study between the effects of CO2 and N2 diluents on oxy-fuel (CH4−O2) flames is

measured gF values in the current research group study plotted with data from ref 9. The measurements from the current study agree well with the previously reported data. 3.2. Flame Stability. 3.2.1. Effect of Recirculation of CO2 and H2O. Figure 6 shows the stability map of CH4−O2−CO2 flames with an initial condition of 0.5, 1.0 and1.5 kW firing rate using different burner diameters. The filled and unfilled symbols represent the flashback and blowout conditions for each burner diameter, respectively. The flashback and blowout behaviors are significantly controlled by the percentage of diluents (CO2 and H2O) in the mixtures. It is interesting to note that CH4−O2 flames did not stabilize at this equivalence ratio without recirculated gases, even with lower firing input 526


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Figure 8. Stability limits of CH4−O2−CO2 and CH4−O2−CO2−H2O flames. The filled and empty symbols represent a firing input of 1.5 and 1.0 kW, respectively. Triangles represent CO2 diluent flashback limits, while diamonds represent extinction limits using CO2 and H2O.

Figure 11. Stability limits of different compositions of syngas−O2− CO2 flames.

flames. However, the presence of CO2 is necessary for stabilizing flames at higher diameter (ranges from 6 to 20 mm) burners. Initially, blowout points of each fuel composition were different at smaller diameter burners, but those points merged together at larger diameters. Figure 12 shows the

Figure 9. Stability limits of CH4−O2−N2 flames.

shown in Figure 10. There is no significant difference between the effects of CO2 and N2 diluents on the flame stability limit. Figure 12. Comparison of stability limits for CH4−O2−CO2 and H2/ CO−O2−CO2 flames.

comparison study of CH4−O2−CO2 and H2−CO-O2−CO2 flames on the stability limit. There is no significant difference between CH4−O2−CO2 and H2/CO−O2−CO2 flames on the stability limit. 3.2.3. Effect of Firing Input. The effects of firing input on the flame stability limit was also investigated and plotted. The burner diameter is plotted against the recirculation ratio of CO2 in the oxidizer by mass (Figure 13). It is expected that there is a higher concentration of CO2 in the fuel mixture in the case of smaller firing input conditions until blowout. This is due to the small bulk velocity in the low firing input condition. Experimental results show this behavior at smaller diameter burners (ranges from 3 to 9 mm). However, at a 13 mm diameter burner, the firing input does not show a significant effect on flame stability. The required amount of CO2 recirculated in the fuel mixtures for stabilizing the flame is significant compared to the amount of the fuel−oxidizer mixture as the burner diameter increases. Figure 14 shows the comparison between stability limits between CH4−O2−CO2 and 10% H2−90% CO−O2−CO2 flames at different firing input conditions. The 10% H2−90% CO−O2 flames have a wider

Figure 10. Stability limits of CH4−O2−CO2 and CH4−O2−CO2−N2 flames.

3.2.2. Effect of the Fuel Composition. Figure 11 shows the stability limit of syngas (H2−CO) mixtures diluted with CO2 at different H2 concentrations. It is interesting to note that the flames of a 10% H2−90% CO mixture stabilized without CO2 in the fuel mixture using the 3 mm diameter burner because of a lower adiabatic flame temperature compared to CH4−O2 527


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in %F in the mixture. The 20 mm burner had significantly lower gF values compared to the 3 and 6 mm burner tubes. The cooling of a partially entered flame front because of the extended surface area of a large burner tube determined the propensity of flashback. Experimental results also show that the increased concentration of recirculated gas in the fuel mixture lowered the critical velocity gradients of CH4−O2−CO2 (Figure 16).

Figure 13. Stability limit of a 10% H2−90% CO mixture at different firing input conditions.

Figure 16. Critical velocity gradients of CH4−O2−CO2 at different burner diameters and recirculation ratios.

Equation 3 shows a generally accepted scaling relation between gF, SL, and α. It is important to note that the scaling constant, c, in eq 3 is related to the burner dimension. Figure 17 Figure 14. Comparison of stability limits at different firing inputs for CH4−O2−CO2 and 10% H2−90% CO−O2−CO2 flames.

stability regime than CH4−O2 flames at smaller diameter burners. This effect was due to the reheating caused by the presence of a partially entered flame front, which increased the propensity of flashback. 3.3. Flashback Modeling. Figure 15 shows the critical velocity gradients of CH4−O2−CO2 mixtures at different burner diameters and mixture ratios (%F). The flashback behavior of CH4−O2−CO2 changed linearly with the increase

Figure 17. Scaling of gF and SL2/α (for the CH4−O2−CO2 mixture).

shows the gF values of the CH4−O2−CO2 mixture measured at different burner diameters and plotted against the scaling ratio SL2/α. The critical velocity gradient correlates linearly with a c value of 0.0174.

4. SUMMARY AND CONCLUSION Oxy-fuel flame stabilities of CH4−O2−CO2 and H2/CO−O2− CO2 flames burning with steam have been analyzed, and factors affecting their behaviors have been explored. A broad range of variables that potentially affect the flame stability have been investigated. Effects of CO2 and steam on the flame stability of CH4−O2 mixtures showed that the flashback and blowout points of the flame were controlled by the amount recirculated in the fuel

Figure 15. Critical velocity gradients of CH4−O2−CO2 mixtures for different fuel percentages in the mixture measured at different burner diameters. 528


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mixture. It was observed that CH4−O2 flames did not stabilize without recirculated gases even with lower firing inputs because of the higher adiabatic flame temperature of the mixture. In addition, the stability regime of all compositions was observed to decrease as the burner diameter increased. Furthermore, it was found that more diluents in the fuel mixtures were necessary to stabilize the flame as the burner diameter increased to balance between the flame speed and unburned gas velocity. A comparison of CH4−O2−CO2 and H2/CO−O2−CO2 flames was also presented. Stability limits of syngas mixtures diluted with CO2 at different H2 concentrations did not stabilize at small burner diameters (3 mm) without the presence of recirculated gases in the fuel mixture. For larger burner diameter burners (6−20 mm), recirculated components were also necessary. Another important finding was that, for 10% H2−90% CO−O2 flames, a wider stability regime exists compared to CH4−O2 flames because of the reheating caused by the presence of a partially entered flame front, which increases the propensity of flashback. A flashback model was also presented using gF values of CH4−O2−CO2 mixtures measured using different burner diameters and plotted against the scaling ratio SL2/α. Results correlated linearly with a scaling value, c, of 0.0174 with a R2 of 0.97.

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(4) Andersson, K. Fundamental oxy-fuel combustion research carried out within the ENCAP project. Proceedings of the Oxy-fuel Workshop; Cottbus, Germany, Nov 29−30, 2005. (5) Anderson, R.; Brandt, H.; Mueggenburg, H.; Taylor, J.; Viteri, F. A power plant concept which minimizes the cost of carbon dioxide sequestration and eliminates the emission of atmospheric pollutants. Proceedings of the 4th International Conference on Greenhouse Gas Control Technologies; Interlaken, Switzerland, Aug 30−Sept 2, 1998. (6) Ditaranto, M.; Oppelt, T.; Lode, B.; Sannumand, I. Stability and heat radiation properties methane flames in CO2 diluted oxy-fuel atmospheres combustion. Proceedings of the 1st International Conference on Sustainable Fossil Fuels for Future Energy; Rome, Italy, July 6−10, 2009. (7) Ochs, T.; Gross, A.; Patrick, B.; Oryshchyn, D.; Summers, C.; Turner, P. The Next Generation of Oxy-Fuel Boiler Systems; Albany Research Center, United States Department of Energy: Albany, OR, 2005; Department of Energy Technical Report DOE/ARC-2005-058. (8) Gerdemann, S.; Summers, C.; Oryshchyn, D.; Patrick, B.; Ochs, T. Developments in Integrated Pollutant Removal for Low-Emission OxyFuel Combustion; Albany Research Center, United States Department of Energy: Albany, OR, 2005; Department of Energy Technical Report DOE/ARC-2005-026. (9) Lewis, B.; von Elbe, G. Combustion, Flames, and Explosion of Gases, 3rd ed.; Academic Press: New York, 1987. (10) Dam, B.; Love, N.; Choudhuri, A. Flashback propensity of syngas fuels. Fuel 2011, 90 (2), 618−625. (11) Dam, B.; Corona, G.; Hayder, M.; Choudhuri, A. Effects of syngas composition on combustion induced vortex breakdown (CIVB) flashback in a swirl stabilized combustor. Fuel 2011, 90 (11), 3274−3284. (12) Amato, A.; Hudak, B.; Carlo, P.; Noble, D.; Scarborough, D.; Seitzmann, J.; Lieuwen, T. Methane oxy-combustion for low CO2 cycles: Blowoff measurements and analysis. J. Eng. Gas Turbines Power 2011, 133, 061503-1−061503-9. (13) Kim, H. K.; Kim, Y.; Lee, S. M.; Ahn, K. Y. Studies on combustion characteristics and flame length of turbulent oxy-fuel flames. Energy Fuels 2007, 21, 1459−1467. (14) Jia, L.; Tan, Y.; Wang, C.; Anthony, E. Experimental study of oxy-fuel combustion and sulfur capture in a mini-CFBC. Energy Fuels 2007, 21, 3160−3164. (15) Kim, H. K.; Kim, Y.; Lee, S. M.; Ahn, K. Y. NO reduction in 0.03−0.2 MW oxy-fuel combustor using flue gas recirculation technology. Proc. Combust. Inst. 2007, 31, 3377−3384. (16) Rathnam, R.; Wall, T.; Elliott, L.; Liu, Y.; Moghtaderi, B. Pulverized coal pyrolysis and char combustion characteristics in simulated air (O2/N2) and oxy-fuel (O2/CO2) conditions. Proceedings of the 1st Oxy-fuel Combustion Conference; Cottbus, Germany, Sept 8− 11, 2009. (17) Habib, M.; Badr, H.; Ahmed, S.; Ben-Mansour, R.; Mezghani, K.; Imashuku, S.; Shao-Horn, Y.; Mancini, N.; Mitsos, A.; Kirchen, P.; Ghoneim, A. A review of recent developments in carbon capture utilizing oxy-fuel combustion in conventional and ion transport membrane systems. Int. J. Energy Res. 2010, 35, 741−764. (18) Scheffknecht, G.; Al-Makhadmeh, L.; Schnell, U.; Maier, J. Oxyfuel coal combustionA review of the current state of the art. Int. J. Greenhouse Gas Control 2011, 5S, S16−S35. (19) Wall, T.; Liu, Y.; Spero, C.; Elliot, L.; Khare, S.; Rathnam, R.; Zeenathal, F.; Moghtaderi, B.; Buhre, B.; Sheng, C.; Gupta, R.; Yamada, T.; Makino, K.; Yu, J. An overview on oxyfuel coal combustionState of the art research and technology development. Chem. Eng. Res. Des. 2009, 87, 1003−1016. (20) Chen, L.; Yong, S.; Ghoniem, G. Oxy-fuel combustion of pulverized coal: Characterization, fundamentals, stabilization, and CFD modeling. Prog. Energy Combust. Sci. 2012, 38, 156−214.

AUTHOR INFORMATION

Corresponding Author

*Telephone: 915-747-8981. Fax: 915-747-5019. E-mail: [email protected]. Notes

Disclosure: Any opinions, findings, conclusions, or recommendations expressed herein are those of the authors and do not necessarily reflect the views of the U.S. Department of Energy. The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is supported by the U.S. Department of Energy, under Award DE-FE-0002402 (Project Manager Anthony Zammerilli).

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NOMENCLATURE %F = fuel mixture ratio gF = critical velocity gradient (s−1) SL = laminar flame speed (m/s) Φ = equivalence ratio α = thermal diffusivity (m2/s)

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REFERENCES

(1) United States Department of Energy. Innovations for Existing Plants/CO2 Emissions Control, Oxy-Fuel Combustion; National Energy Technology Laboratory (NETL): Washington, D.C., 2008; http:// www.netl.doe.gov/technologies/coalpower (accessed March 2008). (2) United States Department of Energy. Carbon Sequestration: OxyFuel Combustion; National Energy Technology Laboratory (NETL): Washington, D.C., 2009; http://www.netl.doe.gov/technologies/ coalpower (accessed Oct 2009). (3) United States Department of Energy. Carbon Sequestration: Technologies; National Energy Technology Laboratory (NETL): Washington, D.C., 2007; http://www.netl.doe.gov/technologies/ carbon_seq/ 2007Roadmap (accessed Feb 2007). 529


Flame Stability of Methane and Syngas Oxy-fuel Steam Flames

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