45471-AC9
Study of Hydrogen/Syngas Combustion and NO Emissions at High Pressures and Temperatures
The laminar flame speeds of Hydrogen/syngas mixtures of a wide range of equivalence ratio were measured experimentally up to 25 atm. The experiments were conduced in a pressure-release type high pressure combustion chamber. The measurement system was validated using laminar burning velocities of methane-air flames. A comparison with the previous experimental data showed an excellent agreement and demonstrated the accuracy and reliability of the present experimental system. Moreover, in order to accurately measure the laminar speeds utilizing propagating spherical flames in a closed chamber, different effects such ignition, unsteadiness, compression, and stretch on the accuracy of flame speed determination were systematically investigated. New methods to obtain more accurate flame speeds in a broader experimental range by correcting for these effects were developed. For example, the Flow-Corrected Flame Speed (FCFS) method and the Compression-Corrected Flame Speed (CCFS) developed in the current study significantly improved accuracy of flame speed measurements. With the help of these methods, accurate flame speeds of hydrogen/syngas mixtures as well as all other hydrocarbon fuels could be obtained from experimental measurements.
The effects of H2/CO ratio, CO2 dilution and pressure on the laminar flame speeds were systematically investigated. Moreover, Markstein lengths were extracted from the flame speed data to indicate the flame response to flame stretch and flame instability behavior at different pressures. At one atmosphere, the present measured data are in good agreement with available literature data. At high pressures, helium was used as a diluent to suppress the development of hydrodynamic and thermal-diffusive instabilities, and the measured flame speeds are much lower than their counterparts at normal atmosphere. CO2 dilution decreases the laminar burning velocities of the syngas flames at a given equivalence ratio. The chemical effect of CO2 dilution becomes dominant over thermal and radiation effects at higher CO2 addition. The present flame speed data at high pressures are expected to be valuable for further validation of chemical kinetic models.
Experimental measurements showed all of the existing kinetic models failed to predict the mass burning rates at high pressures. Specifically, the mass burning rate of hydrogen/air mixtures exhibited a non-monotonic pressure dependence for low flame temperature conditions. At lower pressures (less than about 10 atm), the mass burning rate increases with pressure. At higher pressures (above 10 atm), the mass burning rate decreases with pressure. Recent chemical mechanisms for hydrogen combustion predict that for even higher pressures still the mass burning rate will once again increase with pressure. The experimental values for the pressure dependence of burning rate were compared to predictions using recently published hydrogen chemical mechanisms. The comparisons revealed large discrepancies among the model predictions and between the model predictions and the experimental data at high pressure, low flame temperature conditions. The discrepancies reach up to a factor of 2 at very rich conditions. The sensitivity of the burning rate predictions to rate parameters increases dramatically as the extended second limit moves into the core of the reaction zone, since the limit divides two distinct kinetic regimes. The amplified sensitivities are the most probable cause of model disagreement, since small differences in rate parameters result in large differences in model predictions. Uncertainty estimates for the predictions based on the uncertainties in elementary rates yield unacceptably high values for the uncertainty bounds of the models. Therefore, the modeling issues will likely have to be solved through optimizing select rates to achieve agreement with the experimental data. Preliminary attempts using A-factor optimization proved ineffective. We are in the process of developing more advanced approaches, which allow for variation of the temperature exponents and activation energies.
NO emissions of lean syngas-air premixed flames were numerically investigated in a counterflow configuration and the contributions of major NO formation routes were characterized with varying H2 concentration in the H2/CO fuel. It is found that the addition of hydrogen can significantly reduces the total NO emissions in the syngas flames if the maximum flame temperature is kept constant. This is because that the N2O intermediate route becomes progressively less important with the increase of hydrogen content in the fuel. It is also found that at a fixed flame temperature the NO formation can be effectively reduced by diluting the syngas flames using H2O and CO2 instead of N2, which is due to the distinct chemical effects of H2O, CO2, and N2 in the flames. This study shows that the syngas is a promising fuel for the low emission gas turbine combustion.
