J. De Ruyck

Kinetic modeling of the decomposition and flames of hydrazine

Abstract A detailed N/H reaction mechanism has been developed and validated by comparing modeling results with measurements of hydrazine pyrolysis in shock waves, and in hydrazine decomposition flames at low and atmospheric pressures. The mechanism consists of 51 reactions for 11 species. Rate constants for several decomposition reactions have been estimated employing updated thermodynamic data. Analysis of the reactions abstracting an H atom from NH 3 , NH 2 , NH and N 2 H 4 from 1000 to 2000 K demonstrates that the Evans-Polanyi correlation holds for the radicals H, NH, and NH 2 . Probably it is also valid for the radicals N, NNH and N 2 H 3 . Several rate constants were estimated with this assumption. No further adjustment of the mechanism was attempted. The modeling correctly reproduces the experimental rate of decomposition of hydrazine and also the product distribution. The initial decomposition of N 2 H 4 into two NH 2 radicals and the subsequent reaction N 2 H 4 + NH 2 → NH 3 + N 2 H 3 mainly govern the decomposition of hydrazine in dilute mixtures and together with the reaction NH 2 + NH 2 → N 2 H 2 + H 2 control the propagation speed of a hydrazine flame. The computed speeds of such decomposition flames agree well with low-pressure and atmospheric pressure experiments for pure hydrazine and its mixtures with Ar, N 2 , H 2 O and NH 3 . Also the concentration profiles of major and minor species in low-pressure hydrazine flames are well reproduced. A sensitivity analysis identifies the critical reactions in particular experimental conditions. The choice of rate constants for key reactions and further development of the mechanism is discussed.

Probe sampling measurements and modeling of nitric oxide formation in methane-air flames

Probe sampling measurements of the concentrations of O2, CO2, CO, and NO in the postflame zone of the methane-air flames are reported. A heat flux method was used for stabilization of nonstretched flames on a perforated plate burner at 1 atm. Major attention in this work has been paid to the identification of possible uncertainties and errors of the measurements. Radial and axial profiles of the concentrations of stable species and NO in the postflame zone were used to evaluate the influence of the ambient air en-trainment. flame expansion, and downstream heat losses. In the core region of the flames, the radial profiles of the major species and NO are flat from moderately lean to moderately rich mixtures. In the very lean mixtures an ambient air dilutes the burnt gases, while in the very rich mixtures it causes oxidation of the combustion products. The buoyancy and radial flame expansion can significantly modify observed concentration gradients in the postflame region. In the methane-air mixtures, the NO concentrations measured at a fixed distance from the burner as a function of stoichiometric ratio were obtained. These dependencies clearly possess two maxima: in stoichiometric mixtures due to thermal-NO mechanism and in rich mixtures at equivalence ratio around 1·3 due to prompt-NO mechanism. The numerical predictions of the concentrations of O2 CO2, CO, and NO in the postflame zone are in a good agreement with the experiment when downstream heat losses to the environment are taken into account.

Temperature-dependent rate constant for the reaction NNH + O → NH + NO

Abstract The rate constant of NNH + O → NH + NO has been derived by comparing experiments in low-pressure (0.05 and 0.103 bar) hydrogen-air flames at ∼1200 K [1] with modeling that uses an updated detailed H/N/O kinetic scheme. To match calculations with experimental [NO] profiles along these flames, the currently adopted rate constant of this reaction has to be reduced significantly (by ∼50%). Combination of the adjusted rate constant with measurements in the range 1800 to 2500 K strongly suggests a non-zero activation energy for the reaction. The rate constant derived from 1200 to 2500 K is k 1 = (2 ± 1)× 10 14 exp (−16.8 ± 4.2 kJ/mol/RT) cm 3 /mol/s. The sum of the rate constants of every channel of the reaction NH + NO → Products, including that of the reverse Reaction −1 proposed in the present work, is in good agreement with available experimental data.

Kinetic Modeling of the Thermal Decomposition of Ammonia

A detailed N/H reaction mechanism has been developed and validated in comparison with experimental data for ammonia pyrolysis in shock waves (Davidson et al., Int. J. Chem. Kinet, 1990, 22:513). It has been shown that incorporation of the reactions with N2H3 and N2H4 into the mechanism significantly influences calculated rise-time and peak concentrations of the NH and NH2 radicals if the currently adopted rate constant of the reaction is employed. A sensitivity analysis reveals which reactions are critical for the quality of the modeling in particular experimental conditions. The choice of the rate constants for these reactions is discussed. It has been found that only significant decrease of the reaction (22) rate constant can improve the agreement between the modeling and experimental data. The best fit in the range 2200 - 2800 K is met with the rate constant k22 = 1.0E+11 T0.5 exp(-21600/RT).

Broadening the capabilities of pinch analysis through virtual heat exchanger networks

Abstract The present paper proposes a technique to enlarge the capabilities of pinch analysis. Conventional pinch analysis is limited to heat exchange between streams of unchanging composition, whereas in reality, much heat exchange occurs in components such as chemical reactors, mixers etc. This can be overcome by introducing virtual heat exchangers that convert the considered components into equivalent heaters and coolers where the stream compositions remain unchanged. In this way, they are automatically taken into consideration in the current pinch analysis packages, which may lead to different and better optimisation. A client–server interface program has been written to introduce virtual heat exchangers after completion of the process analysis through Aspen Plus and before entering the pinch analyser Super Target. The application of virtual heat exchangers is next illustrated through a simple example for methanol synthesis.

Efficient CO2 capture through a combined steam and CO2 gas turbine cycle

Abstract A major issue in defining CO2 mitigation strategies is the dominant role of the fossil fuels which will certainly last for at least another fifty years. In the long term the total capture and disposal of CO2 is to be considered as the ultimate solution and different routes for total CO2 capture are now considered. The present paper proposes the use of a highly efficient combined steam/CO2 gas turbine cycle for an economically viable CO2 capture. The proposed cycle is close to the STIG and HAT combined cycles. The cycle behaviour is analysed and the main technological issues are assessed. A project for the proof of concept of the cycle has been defined and submitted to the European Communities (Joule program).

Exergetic optimization of intercooled reheat chemically recuperated gas turbine

The capabilities of the chemical recuperation of exhaust and compressor intercooling heat from gas turbines have been investigated in this paper considering methanol as primary fuel for the Reheat InterCooled (intercooler heat is recovered) Aeroderivative Gas Turbine. The model includes an exergy analysis method and a simplified blade cooling model. The location of the intercooler, the reheat combustion chamber as well as the overall pressure ratio have been optimized. Comparisons are made with the Chemically Recuperated Gas Turbine CRGT cycles using methane as a primary fuel.

Composite curve theory with inclusion of chemical reactions

Abstract The present paper proposes a way to introduce chemical reactions in composite curves, in such a way that the irreversibility’s generated by the chemical reaction are expressed as equivalent thermal diffusion. The heat of reaction is considered to be released at a “reversible reaction temperature” and the diffusion between this reversible temperature and the actual temperature generates the actual exergy destruction. Corresponding hot and cold curve contributions result in the composite curves, which visualizes the exergy destruction and the chemical pinches which are present in the exchanger and reactor network. The concept is illustrated through an application on chemical recuperation in a gas turbine cycle.

Exergy analysis and design of mixed CO2/steam gas turbine cycles

Abstract The capturing and disposal of CO 2 from power plant exhaust gases is a possible route for reducing CO 2 emissions. The present paper investigates the full recirculation of exhaust gases in a gas turbine cycle, combined with the injection of steam or water. Such recirculation leads to an exhaust gas with very high CO 2 concentration (95% or more). Different regenerative cycle layouts are proposed and analyzed for efficiency, exergy destruction and technical feasibility. Pinch Technology methods are next applied to find the best configuration for heat regeneration and injection of water. From this analysis, dual pressure evaporation with water injection in the intercooler emerges as an interesting option.

PROBE SAMPLING MEASUREMENTS OF NO IN CH4+O2+N2 FLAMES DOPED WITH NH3

ABSTRACT Probe sampling measurements of the concentrations of nitric oxide in the post-flame zone of methane + oxygen + nitrogen flames doped with ammonia (0.5% of the fuel) are reported. The goal of this work was to analyze formation of NOx from fuel-N under well-controlled conditions. A Heat Flux method was used for stabilization of non-stretched flames on a perforated plate burner at atmospheric pressure. Dilution ratios of oxygen, O2/(O2 + N2), were varied from 0.16 to 0.209. The concentrations of O2, CO, CO2 and NOx were measured by means of a non-cooled quartz probe at different axial distances from the burner. Measured burning velocities for these flames and concentrations of the major species (O2, CO, CO2) agree well with those of the flames of methane + oxygen + nitrogen within an experimental accuracy. The concentrations of NOx in the post-flame zone have a maximum near the stoichiometry. These measurements were compared to predictions of detailed kinetic models and to similar experiments in flames of methane + oxygen + carbon dioxide doped with ammonia. In (CH4 + NH3) + O2 + CO2 mixtures the modeling over-predicts the measured concentrations of NOx however the experimental trends are well reproduced. In (CH4 + NH3) + O2 + N2 mixtures the plots of the concentrations of NOx in the post-flame zone as a function of the stoichiometric ratio differ qualitatively from that in (CH4 + NH3) + O2 + CO2 mixtures. The modeling is in satisfactory agreement with the experiments in lean flames, while in rich flames it is not.