Proceedings of ASME Turbo Expo 2010: Sea and Air Proceedings of ASME Turbo Expo 2010: Power forPower Land,for SeaLand, and Air GT2010 GT 2010 July 14-18, 2010, UK June 14-18, 2010, Glasgow,Glasgow, UK GT2010-22300 GT2010- METHANE OXY-COMBUSTION FOR LOW CO2 CYCLES: MEASUREMENTS AND MODELING OF CO AND O2 EMISSIONS Alberto Amato School of Mechanical Engineering David Scarborough School of Aerospace Engineering Robert Hudak School of Aerospace Engineering Peter A. D’Carlo School of Aerospace Engineering Jerry M. Seitzman School of Aerospace Engineering David R. Noble School of Aerospace Engineering Tim C. Lieuwen School of Aerospace Engineering Georgia Institute of Technology Atlanta, GA, 30332-0150 ABSTRACT Concerns about green house gas emissions have encouraged interest in hydrocarbon combustion techniques that can accommodate carbon dioxide capture and sequestration. Oxy-fuel combustion, where the fuel is combusted in oxygen diluted with steam or CO2, is seen as one promising approach. In this paper we focus on CO2 dilution effects and, in particular, on CO and O2 emissions from these flames. The emissions issue must be considered from a different perspective than conventional power plants as the combustor effluents will be sequestered, and, thus, their interactions with the terrestrial atmosphere are not relevant. Equilibrium CO emissions from CO2 diluted systems are much higher than conventional air systems. In addition, for carbon sequestration application, pipeline specifications impose limitations on CO and O2 levels which also must then be controlled. Equilibrium and kinetic modeling of CH4/O2/CO2 combustion systems was performed in order to analyze CO2 dilution effects upon CO and O2 emissions level. Companion experiments were also performed in an atmospheric pressure, swirl stabilized combustor. These numerical and experimental results quantify the strong dependence of emissions on stoichiometry, CO2 dilution and residence time. hydrocarbon fuels into a high hydrogen mixture, or 2) postcombustion capture of CO2, which can be accomplished with amine membranes (MEA), solvents or condensation processes. This CO2 can then be sequestered in the ocean or deep saline aquifiers, used for enhanced oil recovery (EOR), or for enhanced coal bed methane recovery (ECBM) [5, 6]. The work described here is directed toward post flame capture. In such cases, the combustion process typically involves burning oxygen rather than air, in order to avoid the large volumes of N2 which would otherwise need to be dealt with. The flame temperature is controlled by diluting the oxygen with either steam or CO2. This largely eliminates NOx formation and enables the exhaust stream to be separated into concentrated CO2 and water by a simple condensation process. Several power cycles utilizing oxy-fuel combustion have been proposed that either consist of slight modifications of existing gas turbine combined cycles [7], as completely new concepts (e.g., Graz cycle [8, 9], Matiant cycle [10]), as integration with gasification processes for coal combustion [11-13], or even for internal combustion engines for automotive applications [14]. Key challenges to the economical and technical feasibility of oxy-fuel combustion power cycles concern oxygen production by the air separation unit (ASU), the aerodynamic design of the gas turbine which must be modified because of difference in working fluid properties from that of air, and the combustion process, which is the focus of this study. Using CO2 as a diluent to burn natural gas impacts the flame through changes in: 1) mixture specific heat and adiabatic flame temperature, 2) transport properties (thermal conductivity, mass diffusivity, viscosity) [15] 3) chemical INTRODUCTION Increasing concerns about climate change have promoted interest in zero-CO2 emission hydrocarbon combustion techniques. This can be done by removing carbon in essentially two ways [1-4] 1) pre-combustion capture via reforming of 1 Copyright © 2010 by ASME kinetic rates [16-22], and 4) radiative heat transfer [23-26]. Recently, there has been a significant amount of work on O2/H2O diluted CH4 flames by workers at NETL ([27, 28]), who have reported results of calculations and large scale tests. In addition, recent demonstration tests have been performed by Clean Energy Systems, under support of the US DOE [29], while extensive experimental and numerical studies of oxynatural gas fired furnaces have been conducted at the IFRF in the framework of the OXYFLAM project [30, 31] and at the Swedish Department of Energy and Environment [32]. Our objective in this study is to focus on the O2/CO2 problem. Burning methane in an O2/CO2 diluent raises new combustion opportunities and challenges for both emissions and operability. As operability issues are discussed in a companion paper [33], we focus attention here on emissions. The emissions issue must be considered from a different perspective than conventional power plants as the combustor effluents will be presumably sequestered, and, thus, their interactions with the terrestrial atmosphere are not relevant. However, the system’s emissions are still of interest. For example, it is known that CO emissions are augmented due to higher equilibrium levels as well as slower CO oxidation rates [34]. CO2 competes with O2 for atomic hydrogen, leading to of CO, mainly through the reaction formation CO2+H↔CO+OH [35]. High levels of CO are, in essence, a loss in efficiency, and results in increased fuel costs for the same power output; besides high local CO levels may have corrosion implications. For similar reasons, exhaust O2 emissions are also of interest because of the cost and power consumption associated with generating O2 and possible corrosion in pipeline for CO2 transportation [36]. Diluted oxy-combustion systems have a different degree of freedom than conventional air systems, where variations in stoichiometry and fuel/air ratio are coupled. In these systems, stoichiometry and fuel/air ratio are decoupled. As such, it seems clear that such systems will be operated at or near stoichiometric to minimize both fuel and O2 usage, with an O2/CO2 ratio set by temperature limitations. We see at least three combustion issues which must be addressed for CH4/O2/CO2 systems, summarized in Figure 1, which plots the two key degrees of freedom of this problem: stoichiometry and flame temperature. High stoichiometries and flame temperatures are associated with excessive CO emissions, while low stoichiometries and high flame temperatures with excess O2 emissions. As such, lower temperature considerably broadens the fuel/air ratio sensitivity of these emissions. However, low temperature operation is limited by blowoff. The rest of this paper focuses on measurements and calculations of these CO and O2 emissions. Figure 1. Operational space of a premixed, CH4/O2/CO2 flame in the equivalence ratio – flame temperature space. EQUILIBRIUM TRENDS Equilibrium calculations provide a useful perspective on emissions trends, given a system with a sufficiently long residence time. This section presents example calculations of these equilibrium tendencies, obtained used the GasEq solver [37], at pressure p=1atm or 15atm and at initial temperature Tin=500F(~533K). Figure 2. Dependence of adiabatic flame temperature upon the mole fraction of oxygen in the reactants (Tin = 533K, p = 1atm). First, we consider the dependence of adiabatic flame temperature Tad upon oxygen levels in the reactant stream, shown in Figure 2. As expected, increasing O2 levels in the oxidizer mixture increases adiabatic flame temperature. Figure 3 plots the dependence of equilibrium CO concentration upon flame temperature for CH4/O2/CO2 flames at different equivalence ratios (colored lines). Also shown for reference is a methane/air flame (black lines), where flame temperature is varied with equivalence ratio (bottom loop is lean conditions, top is rich). 2 2 Copyright © 2010 by ASME Copyright © 2010 by ASME These results illustrate that adiabatic flame temperature and stoichiometry exert powerful, but generally opposing influences upon CO and O2 levels. In order to facilitate observing these trends simultaneously, Figure 5 plots iso-contours of O2 and CO levels at p=1atm. This figure illustrates the regions of high and low sensitivity and the opposing trends of O2 and CO in most situations. Figure 3. Dependence of CO equilibrium concentration in wet combustion products on adiabatic flame temperature, pressure and equivalence ratio (Tin = 533K, p = 1atm and 15atm). Figure 5. Dependence of CO and O2 equilibrium concentration (in ppm) in dry combustion products on adiabatic flame temperature and equivalence ratio: solid lines refer to CO while dashed lines to O2 (Tin = 533K, p = 1atm). CHEMICAL KINETICS CO AND O2 CALCULATIONS Equilibrium calculations are useful for providing insight into limiting emissions levels that are approached given a sufficiently long residence time. In reality, the actual combustor residence time is finite and so emissions levels do not actually reach their equilibrium levels. As such, kinetic calculations are needed to determine the characteristic times associated with CO and O2 evolution to determine their values as a function of the actual residence time. Calculations were performed using the PREMIX module in CHEMKIN 4.1, using the GRI 3.0 mechanism. The t = 0 point was defined as the point where the heat release rate drops to 10% of its maximum value, as shown in Figure 6. For reference, a typical value of combustor residence time (defined by ratio of combustor volume to burned gas volume flow rate) for an F-class gas turbine is about 40ms. Figure 4. Dependence of O2 equilibrium concentration in dry combustion products on adiabatic flame temperature, pressure and equivalence ratio (Tin = 533K, p = 1atm and 15atm). Note that CO levels are much higher for the CO2 diluted system than the fuel lean air system at a given temperature. They are also lowest at lean conditions and increase monotonically with equivalence ratio. Increasing pressure and decreasing temperature reduce equilibrium levels for lean mixtures, as might be expected from Le Chatelier’s principle. These pressure and temperature sensitivities are much lower for rich mixtures, reflecting the shift in key mechanism controlling equilibrium levels at these high stoichiometries (water gas shift reaction). Similarly, Figure 4 shows that the concentration of O2 increases with flame temperature and decreases with equivalence ratio and pressure. The pressure and temperature sensitivity to fuel/air ratio is inverted from the CO case, reflecting the fact that O2 is a major and trace equilibrium product under lean and rich conditions, respectively, opposite that of CO. CO emissions We first consider CO concentrations. There is a very important difference between the combustion process considered here and lean, premixed combustion that should be emphasized. This can be understood by considering the ratio 3 3 Copyright © 2010 by ASME Copyright © 2010 by ASME between the maximum value of CO and its equilibrium value, as shown in Figure 7. Figure 7. Dependence of CO overshoot upon flame temperature and stoichiometry. Overshoot is defined as the maximum concentration divided by equilibrium concentration. (Tin = 533K, p = 1atm and 15atm). Figure 6. Procedure for defining the end of the flame region (t = 0) for a typical heat release profile from premixed flame calculations. Note the large “CO overshoot” in the flame relative to equilibrium for the fuel lean condition (φ=0.9) at atmospheric pressure. At 1800 K, for example, peak CO levels in the flame exceed equilibrium levels by a factor of nearly 30. Even larger overshoots are present at leaner equivalence ratio. Increasing pressure increases CO overshoot values at lean equivalence ratios further. This overshoot is a major design challenge for lean, premixed systems which must achieve both acceptable CO levels at low power and NOx at higher power. Significant attention is given to designing systems to achieve sufficient residence time for CO burnout, and minimal CO quenching by cooling holes. Moreover, insufficient residence time for CO burnout limits turndown for industrial gas turbines, as low power operation is not possible without CO levels that exceed permitting restrictions. For the oxyfuel system running slightly rich, this sensitivity is not present because of the small value of the “CO overshoot”. While CO levels are substantially higher than lean systems, there is almost no overshoot – typical values are 2 or 3. This implies substantially less sensitivity of exhaust CO levels to combustor design details. In other words, similar CO levels will be experienced regardless of quenching by walls or dilution jets. Another way of looking at these data is to consider the actual value of maximum CO concentrations in the flame, as plotted in Figure 8. This figure shows that the peak CO levels in the flame are much less sensitive to stoichiometry than equilibrium values. Figure 8. Maximum value of CO concentration reached in premixed flame. (Tin = 533K, p = 1atm and 15atm). We next consider post-flame relaxation of CO emissions toward equilibrium. Figure 9 plots the dependence of CO concentration on adiabatic flame temperature, under both equilibrium and fixed residence time conditions, where τres=40ms. Notice the strong influence of flame temperature upon combustion relaxation time. At high temperatures (T >~ 2000K), the fixed residence time and equilibrium values are the same. At lower temperatures, the fixed residence time and equilibrium CO concentrations diverge progressively as the temperature decreases. Note that in all cases, the fixed residence time result is less sensitive to temperature than the equilibrium result. 4 4 Copyright © 2010 by ASME Copyright © 2010 by ASME Figure 9. Dependence of CO concentration in wet combustion products at τres=40ms upon adiabatic flame temperature (Tin = 533K, p = 1atm). Figure 11. Dependence of CO concentration in wet combustion products upon adiabatic flame temperature (Tin = 533K, p=15atm). To illustrate the effect of pressure, Figure 10 plots the CO concentrations at fixed residence time and different pressures. As it can be observed, CO concentrations at 15atm are considerably lower than those at 1atm especially in lean conditions, where CO concentrations are lower. This is due to the combination of two effects, (i) the lower CO equilibrium concentration (as described in the previous section) and (ii) the faster CO relaxation kinetics at higher pressures. The latter effect can be seen from Figure 11, which compares the fixed residence time and equilibrium results at 15atm. Notice that the two curves diverge from each other at temperatures much lower than in the 1atm case, meaning that equilibrium values are reached faster at high pressure. Figure 12. Dependence of O2 concentration in wet combustion products at τres=40ms upon adiabatic flame temperature. (Tin = 533K, p = 1atm). O2 emissions We next consider the trends for O2 concentrations. We start in Figure 12 showing the dependence of O2 concentration on adiabatic flame temperature both at equilibrium and at τres = 40ms. Similar conclusions as discussed for CO (see Figure 9) can be seen here as well. An important point that should be reemphasized is the substantially reduced temperature sensitivity of the fixed residence time results relative to the equilibrium, even more the case here than for CO. To illustrate, consider φ=1.05. Equilibrium considerations suggest that O2 emissions can be substantially reduced by decreasing flame temperature. For example, there is a two order of magnitude difference between levels at T = 1800 and 2200 K. The important dependence of relaxation times upon temperature substantially changes the fixed residence time result. Note that the difference between O2 emissions at fixed residence time for these two temperatures changes from a 100x reduction to a 2x reduction. Figure 10. Dependence of CO concentration at τres =40ms in dry combustion products upon adiabatic flame temperature at two pressures. (Tin = 533K, p = 1atm and 15atm). 5 5 Copyright © 2010 by ASME Copyright © 2010 by ASME EXPERIMENTAL FACILITY Parallel measurements were obtained to study emissions trends in a swirling, atmospheric combustor shown in Figure 15. This system is a duplicated from an experimental rig at Sandia National Laboratories [38]. The facility consists of a swirler/nozzle, combustor, and exhaust sections. Premixed gas, consisting of CH4 and O2/CO2 mixture flows through a swirler/nozzle section. The nozzle is an annular tube with inner diameter of 28mm. The center body has an outer diameter of 20 mm. The overall flow area remains constant at 3.0 cm2 inside the nozzle. Tests were performed with a six-vane, 45o swirler, which is located in the annulus between the centerbody and nozzle wall. The theoretical swirl number is 0.85. The fuel is injected 150 cm upstream of the combustor to achieve a premixed condition. The combustor consists of a 508 mm (20 inches) long quartz tube, with a 115 and 120 mm inner and outer diameter, respectively. It rests in a circular groove in a base plate. The O2, CO2, and fuel flow rates are measured with mass flow meters. Reactant mixtures were preheated to 500F (~ 533K). Particular attention was paid in order to eliminate air leakages, which is an important “contaminant” in the low O2 exhaust. The combustor was insulated with ceramic cloth and sealed at the bottom and top edges of the quartz tube using an 8” steel pipe. The 8” pipe features flanges on either end for attachment of the combustor base plate and an exhaust plate to allow for sealing of the quartz from the atmosphere. Without these sealing provisions, we found that atmospheric air is pulled into the quartz tube, leading to inaccurate O2 measurements. To illustrate pressure effects, Figure 13 compares the temperature dependence of O2 concentration at fixed residence time at two pressures. Again, similar results as in the CO case occur (see Figure 10): there are significant differences between the two pressures due to faster relaxation times and lower equilibrium values at high pressure. The latter effect is illustrated in Figure 14 (as done in Figure 11), where the fixed residence time and equilibrium results at 15atm are shown as a function of adiabatic flame temperature. Notice that the two curves diverge from each other at temperatures much lower than in the 1atm case, meaning that equilibrium values are reached faster at high pressure. Figure 13. Dependence of O2 concentration at τres=40ms in dry combustion products upon adiabatic flame temperature for different pressures (Tin = 533K, p = 1atm and 15atm). Figure 15. Combustor configuration used for emissions sampling. Assembly on left and cut-away view of model on right (dimensions in mm). Figure 14. Dependence of O2 concentration at τres =40ms in dry combustion products upon adiabatic flame temperature for different pressures (Tin = 533K, p = 15atm). O2 emissions were obtained with an Advanced Instruments Inc. GPR-1200 Portable ppm Oxygen Analyzer. This O2 analyzer has the ability to auto range to four different scales, 010 ppm, 0-100 ppm, 0-1000 ppm, and 0-25%, allowing for the 6 6 Copyright © 2010 by ASME Copyright © 2010 by ASME full spectrum of O2 emissions of interest to be measured with the same device. The sensor was zeroed with pure nitrogen and calibrated at 800 ppm oxygen in a nitrogen bath calibration gas. CO emissions were obtained with a LAND Lancom Series II Combustion Products Analyzer. This analyzer was zeroed with air and calibrated with 3000 ppm and 30,000 ppm CO in nitrogen bath calibration gases. The combustion gases were sampled with a water quenched probe illustrated in Figure 16. The probe was designed using three concentric tubes with 1/8”, 1/4”, and 3/8” outer diameters. Assembly of the probe was performed such that the incoming cooling water would flow between the 3/8” and 1/4” tubes and then stagnate at the probe tip and return the heated water between the 1/4” and 1/8” tubes. This design allows the emissions sample to cool very rapidly in order to freeze the O2 and CO reactions. The sample was then filtered through a coalescing filter to extract any water prior to the vacuum pump used to draw out the emissions sample. After the vacuum pump, the sample was then run to either the O2 sensor or CO sensor. Care was taken to use all stainless steel tubing and to minimize leaks or losses in order to acquire quality O2 measurements. Calculations using Henry’s law [39] show that, assuming an average temperature of 350K inside the probe, up to 0.3% of the oxygen can dissolve in the condensing water which would not then be measured by the O2 sensor. 2000, 2050, and 2100 K). No measurable sensitivity to stoichiometry at a fixed adiabatic flame temperature was observed. Note the slightly higher temperature loss relative to the CH4/air case, probably reflecting increased radiation emissions from carbon dioxide. In addition, axial traverses were performed for a CH4/CO2/O2 flame at φ = 1.10 and Tad = 1900K, as reported in Figure 18. The axial temperature gradient revealed an expected trend, with temperature decreasing with downstream distance. Figure 17. Difference ∆T between measured flame temperature T and adiabatic flame temperature Tad for CH4/Air and CH4/CO2/O2 flames. Figure 16. Internal assembly of water cooled sample probe (a) and sample probe assembly with water inlet and outlet on right (b). Flame temperature measurements were obtained using a grounded, K-type thermocouple from Omega Engineering, model number KMQXL-125G-24. The sheath material of this thermocouple is Nickel-Chrome based, providing good performance in heavily oxidizing environments. Radiation losses were minimized by inserting the 0.125” diameter thermocouple into a 0.145” diameter ceramic sleeve. Figure 17 plots measured temperatures for CH4/Air and CH4/CO2/O2 flames, where ∆T represents the difference between the measured flame temperature T and the theoretical adiabatic flame temperature Tad. This difference ranges between 300 and 400K. The two curves “with ceramic” and “without ceramic” illustrate the effect of radiation losses from the thermocouple tip. The same measurements (with ceramic) were repeated for CH4/CO2/O2 flames, as reported in Figure 17: two fuel-air ratios (φ = 0.90 and 1.10) were tested for different theoretical adiabatic flame temperatures (Tad = 1900, 1950, Figure 18. Temperature measurements for CH4/CO2/O2 flame at φ = 1.10 and Tad = 1900K. Axial position is measured starting from the dump plane. 7 7 Copyright © 2010 by ASME Copyright © 2010 by ASME COMPARISON OF EXPERIMENTAL AND NUMERICAL RESULTS Comparison of the 1-D model results described earlier and experimental data for this swirling burner is not straightforward, primarily because of uncertainties associated with residence time (because of flow recirculation in the vortex breakdown bubble). In addition, heat losses cause the actual flame temperature to be significantly lower than the theoretical adiabatic flame temperature, as already shown by the data reported in Figure 17 and Figure 18. This effect was partially compensated for by using the actual measured temperature at the probe locations. Finally, CO and O2 levels are both quite sensitive to variations in stoichiometry and temperature in parameter space regions where their concentrations are low, implying that small uncertainties in temperature and flow rate can lead to significant variations in predicted O2 and CO levels. All the data reported here were measured at a fixed probe location, 46 cm from the base plate, in the combustor center. The flow rates were adjusted at each flame temperature in order to give the same average burned combustor velocity of 10m/s, based upon a calculated adiabatic flame temperature. As such, this probe location was estimated to correspond approximately to a bulk residence time of ~46ms. visualization of various trends, Figure 20 replots these data as a function of equivalence ratio. Figure 20. Comparison between calculated and experimental data for CO concentration in dry products: CO levels are represented as function of equivalence ratio at different flame temperatures (Tin = 533K, p = 1atm). Figure 21. Comparison between calculated and experimental data for O2 concentration in dry products: O2 levels are represented as function of equivalence ratio at different flame temperatures (Tin = 533K, p = 1atm). Figure 19. Comparison between calculated and experimental data for CO concentration in dry products: CO levels are represented as function of flame temperature at different equivalence ratio (Tin = 533K, p = 1atm). Figure 21 plots the dependence of the measured O2 emissions on adiabatic flame temperatures at different equivalence ratios. Similar to CO emissions, the measured O2 emissions disagree with the numerical model primarily in the regions of high sensitivity (where values are kinetically controlled, rather than by equilibrium) described previously. However, the same qualitative trends are observed both in numerical and in experimental results; i.e., the O2 emissions monotonically rise with flame temperature and decrease with stoichiometry. To facilitate the visualization of these trends, Figure 22 plots these same experimental data as a function of equivalence ratio at different flame temperatures. Figure 19 plots the dependence of the measured CO emissions on temperature for different equivalence ratios. It can be observed that the measured CO emissions are in good agreement with the model at high equivalence ratios. Near stoichiometric conditions, where the sensitivity to experimental uncertainties is greater, the difference between numerical and experimental values is larger. However, the same qualitative trends are observed both for numerical and experimental measurements; i.e., the CO emissions monotonically rise with flame temperature and stoichiometry. To facilitate the 8 8 Copyright © 2010 by ASME Copyright © 2010 by ASME 4. 5. 6. 7. Figure 22. Comparison between calculated and experimental data for O2 concentration in dry products: O2 levels are represented as function of equivalence ratio at different flame temperatures (Tin = 533K, p = 1atm). 8. CONCLUDING REMARKS Emissions concerns associated with high CO and O2 levels place limitations on the flame temperature and feasible stoichiometries of operation of premixed methane oxy-fueled systems. Equilibrium CO emissions are higher than in air due to higher CO2 levels. In addition, at a fixed residence time, CO emissions are also higher because of slower burnout of the intermediate CO formed in the flame. Equilibrium CO emissions are an exponentially increasing function of flame temperature. However, this temperature sensitivity is substantially reduced at a fixed residence time. Pressure lowers CO emissions because of faster chemical kinetics and decreased equilibrium levels. 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