Analysis of NO-Formation for Rich / Lean - Staged Combustion

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1 1 Analysis of NO-Formation for Rich / Lean - Staged Combustion P.Frank (a), Y.Tan (a), P.Griebel (b), H.Nannen (b), H.Eickhoff (b) Deutsche Forschungsanstalt für Luft-und Raumfahrt : (a) Institut für Physikalische Chemie der Verbrennung, Pfaffenwaldring 38-40, Stuttgart (b) Institut für Antriebstechnik, Linder Höhe, Köln peter.frank@dlr.de ABSTRACT A combination of well stirred reactor (WSR) and plug flow reactor (PFR) model was used for the simulation of the initial rich part of an experimental RQL-combustion chamber. A propane oxidation mechanism coupled with the NO x -submodel of GRIMech was used mainly as chemical kinetic model. Model calculations were started with this original NO x -submechanism. As this mechanism has been optimized for the NO-formation in methane combustion, it was necessary to make some adjustments for modeling larger aliphatic hydrocarbon combustion. The comparison of the model calculations with measured species concentrations from the initial stage of the combustor with 0.57 < φ < 0.63 at atmospheric pressure shows the correct dependence of the TFN species on the equivalence ratio. Sensitivity analysis reveals strong influence of CH + radical - reactions on the TFN-values and especially of CH + N 2 on NO-formation. Model calculations have been carried out for the conditions at 20 bar in order to see the model prediction at high pressure. INTRODUCTION Reduction of NO x - and other TFN emissions is a major issue in the development of gas turbine combustors. The improvement in the thermodynamic efficiency of gas turbines by increasing combustion pressure and temperature has a promoting effect on NO-formation. Therefore, advanced low emission combustor technologies are required. In preliminary studies 1 of the NO x -reduction potential a 70% reduction potential is assigned to the air-staged or rich-quench-lean combustion (RQL). In principal, the RQL-combustor concept consists of a fuel-rich primary zone, a quenching zone where additional secondary dilution air is injected, and a fuel-lean secondary zone for final combustion of the fuel. In both, the primary and the secondary zone the NO-formation rates are low due to non-stoichiometric conditions and relatively low temperatures 2. Model experiments on nitric oxide formation for rich / lean staged combustion at aeroengine conditions indicate, that about two thirds of the total NO are due to reactions in the rich primary zone. For the lowest total NO emission, two thirds of the TFN- concentrations are due to HCN- and NH 3 - formation in the primary zone. In order to improve the understanding of the parameters influencing the formation of NO and the other TFN-species in the fuel-rich zone of a RQL-combustor, a computational study is performed, based on chemical reactor model and compared with experimental results. EXPERIMENTAL SETUP The experimental device is a RQL-combustion chamber with a rectangular cross-section of about 100 x 255 mm. The preheated air of 850 K is metered by critical nozzles in the inlet and divided in primary and secondary air. The kerosene fuel supply system is watercooled. Fuel preparation is achieved by a double row of twelve airblast nozzles. Swirling of the primary air causes a recirculation in the primary zone, stabilizing the diffusion flame. The liner walls in the primary zone are convectively cooled with the secondary air, liner walls in the secondary zone are film cooled. Two transparent windows which are mounted in the side walls of the quench zone give optical access for the use of non-intrusive measuring techniques like LDV and CARS. Local gas composition is measured by using suction probes. We measured CO 2, CO, H 2,, O 2, NO and UHC. TFN was measured indirectly by a chemiluminescence NO x -analyzer with a heated stainless steel converter. The accuracy of these measurements under fuel-rich conditions was improved by using a converter with sufficient dilution of oxygen. The experimental data were measured in distances between 63 and 93 mm from the plane of fuel injection. The combustor was operated at 1 atm at temperatures which were nearly adiabatic. Runs for equivalence ratios of 1.59, 1.67 and 1.75 were performed, and the results are listed in the following tables:

2 2 Table 1: Experimental results for the mixture of φ = 1.75 x NO HCN+NH 3 +NO 2 UHC T mm ppm ppm ppm K Table 2: Experimental results for the mixture of φ = 1.67 x NO HCN+NH 3 +NO 2 UHC T mm ppm ppm ppm K Table 3: Experimental results for the mixture of φ = 1.59 x NO HCN+NH 3 +NO 2 UHC T mm ppm ppm ppm K More details about the experimental results will be presented in a future paper 3. MODELING The primary zone of the combustor was approximated by a combination of a well-stirred reactor (WSR) and a plug flow reactor (PFR). Due to the recirculation of the primary air in the first part of the primary zone, the mixing in this region is very intensive. Therefore, this zone can be simulated by a WSR-model. The volume of this WSR was chosen to be 40% of the total primary zone volume. We deliberately limited our modeling efforts to the TFN species and, to a lesser degree, the unburned hydrocarbons, which refer in this work the concentration of methane, acetylene and ethylene. The input gas composition for the PFR was the exit gas composition of the WSR. We used the PSR 4 code of Sandia National Laboratories to perform the calculation for the WSR. The SENKIN 5 code was used to simulate the PFR where the temperature was assumed to be constant. In fact, due to experimental limitations, the temperature in the whole reactor is assumed to be constant, that is, we used the same temperature for the WSR section and the PFR section. Since we do not have a model specifically for kerosene, the reaction mechanism used for the computation is the NO x code from GRI 6 coupled with the propane model 7 from the Laboratoire de Combustion et Système Réactif of Orléans, France. The model comprises 82 species and 467 reactions. We have also performed several computations by using a decane model (also from Orléans), and we found that the results did not differ considerably. In order to simplify the data processing task, we chose to use the propane mechanism coupled with an NO x sub-model.

3 AAA AAA AAA AAA AAA AAA AAA AAA AAA AAA AAA AAA AAA AA AA AA AA AA AA AA A A A A A A A A A A A A A A AA AA AA AA AA AA AA AA AA AA AA AA AA 3 The computation showed that this model considerably overestimates the concentration of NO and underestimates that of RTFN (Rest of TFN refers, in this work, to the total concentrations of HCN, NH 3 and NO 2 ). We then began to modify the NO x part of the model to see whether we could improve the model's prediction. By using sensitivity analysis (fig. 1) and reaction path flux analysis, we modified the following reactions (Table 4, the rate constants are in mol, cm 3, s, K and cal, and given for T=2250K, the values in the last column were obtained by fitting the data present in the NIST database 8 ): Table 4: Reactions modified in this work Reaction GRI mechanism Our value NIST Database N + OH NO + H 5.71E E13 N/A N + OH NH + O N/A 1.19E E11 NH + H 2 O HNO + H E E E11 CN + OH NCO + H 4.00E E E13 HCN + O NCO + H 2.57E E E12 HCN + OH HOCN + H 3.54E E E11 CH + N 2 HCN + N 1.45E E E10 CH + NO HCN + O 5.0E13 4.0E14 1.0E14 HOCN + H H + HNCO 6.47E E14 N/A Normalized sensitivity coefficient for NO in WSR φ=1.59 CH+NO=HCN+O Reaction CH+N2=HCN+N N+NO=O+N2 A A A AA N+OH=NO+H -0,3-0,2-0,1 0 0,1 0,2 0,3 Sensitivity coefficient With these modifications, the model's prediction on NO is much closer to the experimental data (Fig. 2), and showed the correct dependance of NO and HCN concentrations on equivalence ratio (fig 3 and 4), though it continues to overestimate NO concentration and underestimate that of RTFN.

4 4 The reaction path flux analysis in the WSR showed that, at 1 atm, NO is formed mostly through reaction N + OH -> NO + H, the second most important reaction for NO formation is H + HNO -> H 2 + NO. Due to the very rich condition, the Zeldovich reactions N + O 2 -> NO + O and N 2 + O -> NO + N are not important. The main consumption reactions for NO are CH + NO -> HCN + O and H + NO -> HNO. NO and TFN profiles in the PFR P=1atm, φ=1.75 3,50E-04 3,00E-04 2,50E-04 NO, GRI NO, modified 2,00E-04 1,50E-04 1,00E-04 5,00E-05 exp data 0,00E NO and TFN profiles in the PFR P=1atm, φ= E-03 RTFN, GRI 1.00E-05 RTFN, modified 1.00E E-07

5 5 RTFN profiles in the PFR P=1atm 1.00E-03 RTFN, 1.59 RTFN, E E-06 RTFN, E E-08 An interesting point is that, we observed in the PFR a quick increase in the NO concentration and a decrease in that of RTFN in the first few milliseconds. NO then passed through a peak concentration to slowly decrease while RTFN behaved just contrarily. Just like in the WSR, reaction N + OH -> NO + H is responsible for NO formation in this part of the reactor, as this is clearly shown in fig.5. Influence of certain reactions on NO profile in the PFR 1.30E E E-04 NO, modified NO, w/o N+OH NO, w/o H+HNO 9.00E E E E Independant of the model which we used during our calculation, we found that in the PFR section, the hydrocarbons were consumed immediately and that at the exit of the PFR, no hydrocarbons were reported by the models. This is quite different from the experimental results where one can see a non negligible amount of hydrocarbons. We also made several calculations by assuming the whole reactor as a well-stirred reactor. The results of the calculation are shown in the table 5:

6 6 Table 5: NO and RTFN concentration calculated with WSR model Equivalance ratio NO ppm RTFN UHC ppm ppm From this table, we can see that a much better agreement with the experimental data can be obtained with this assumption. Also to be noted is that the unburned hydrocarbons reported by the models, though still quite low compared to the experimental results, were better in line with experimental results. This shows us that an exact knowledge of the reactor characteristic is essential for this kind of modeling. Since most gas turbines operate at high pressure, it is of interest to see model prediction under such conditions. For this, we put the two models through a test at 20 atm. The results (Fig. 6 and 7) showed that there is a strong decrease in NO concentration while RTFN concentration remains almost unchanged. 1.60E-04 NO profiles in the PFR P=20atm, φ= E-04 NO, GRI NO, modified 1.20E E E E-05 RTFN profiles in the PFR P=20atm, φ= E-05 RTFN, GRI 3.00E-05 RTFN, modified 2.50E E E E E E+00

7 7 The important point is that at 20 atm, the difference between the two models is much less pronounced. At the exit of the WSR, there is merely a 2-fold difference in the NO concentration between the two models compared to 4 times at 1 atm. In the PFR, after 6 milliseconds, the two models virtually give the same results. This could indicate that the reactions we have modified are not very influent at higher pressure and that the reaction system reached equilibrium state rapidly in the PFR section. In fact, at 20 atm, the most important reaction for NO formation switched from N + OH -> NO + H to H + HNO -> H 2 + NO whose rate constant was not modified. The discrepancy between the model prediction and the experimental data can arise mainly from following factors: 1. The NO x submodel has been validated only under low-pressure flame condition, therefore it may not be suitable under such relatively high pressure, very rich condition. 2. The assumption of constant temperature throughout the reactor is arguable and can be an important source of uncertainty. 3. Quite a few key reactions of the NO x submodel are still not well known, for example, CH + NO -> HCN + O. Even the very important reaction N + OH -> NO + H presents some uncertainties at high temperature. The pressure dependance of reactions such as H + NO -> HNO is also unknown. In this work, the low pressure limit rate constant was used, which is surely not adequate. CONCLUSION A series of experiments have been carried out in the primary zone of a RQL-combustor at 1 atm in the rich regime. TFN species along with major species have been measured. The experimental results then served as reference data for the modeling work which was carried out by assuming the reactor either as a combination of well-stirred reactor followd by a plug-flow reactor or solely as a well-stirred reactor. The modeling results showed the correct dependance of TFN species on equivalence ratios in both cases, and a better agreement with experimental data was obtained by assuming the whole reactor as well-stirred. REFERENCES 1 BRITE EURAM : Low Emission Combustor Technology, AERO/ Task 3, Technical Report : Summary of Pollutant Reduction Methods, N. K. Rizk and H. C. Mongia, ASME, 90-GT-87, P. Griebel, C. Hassa, Experimental Investigation of an Atmospheric Rectangular Rich Quench Lean Combustor Sector for Aeroengines, in preparation for ASME P. Glarborg, R. J. Kee, J. F. Grcar and J. A. Miller, PSR: A FORTRAN Program for Modeling Well-Stirred Reactors, SANDIA REPORT, SAND , A. E. Lutz, R. J. Kee and J. A. Miller, SENKIN: A Fortran Program for Predicting Homogeneous Gas Phase Chemical Kinetics With Sensitivity Analysis, Sandia Report, SAND , C. T. Bowman, R. K. Hanson, D. F. Davidson, W. C. Gardiner, V. Lissianski, Jr., G. P. Smith, D. M. Golden, M. Frenklach and M. Goldenberg, GRI-Mech, 2.11, 1996, 7 Y. Tan, Ph. Dagaut, M. Cathonnet and J.-C. Boettner, Combust. Sci. and Tech., 102, 21 (1994). 8 W. G. Mallard, NIST Chemical Kinetics Database, version 6.0.