Rajiv K. Mongia

Use of an optical probe for time-resolved in situ measurement of local air-to-fuel ratio and extent of fuel mixing with applications to low NOx emissions in premixed gas turbines

The lower temperatures associated with lean premixed combustion generally lead to lower NOx emissions; however, the benefit of lean premixed combustion may be lost if the fuel and air are poorly mixed. In this paper, we describe the development of an inexpensive fiber optic probe capable of measuring the extent of mixing. The fuel concentration is determined by laser light absorption at 3.39 μm over a short path length created by using infrared transmitting fiber optics. A hydrogen-piloted, CH4-in-air turbulent flame with a variable fuel injection location is used to vary the degree of mixedness at the burner exit. We use the optical probe to measure the level of mixedness (nonreacting) at the burner exit. The level of mixing and the mean concentration profiles are also measured by using planar laser-initiated Rayleigh scattering. NOx measurements are reported for several mixing distances. We show that at lean conditions (=0.6), incomplete mixing causes a dramatic increase in NOx production because of the exponential temperature dependence of NOx formation about =0.6. We also numerically investigate how the extent of mixing affects NOx production at various equivalence ratios and pressures. Modeling the effect of incomplete mixing on NOx formation is done with a distribution ofconvolved with numerical results from a perfectly stirred reactor in series with a plug flow reactor. The model does an excellent job of predicting the NOx increase caused by incomplete mixing at lean conditions. Model predictions at higher pressures that are typical of gas turbine conditions show good agreement with available data. In particular, for lean premixed combustion, NOx is not a function of pressure if the air and fuel are well mixed.

Catalytic oxidation of natural gas over supported platinum: Flow reactor experiments and detailed numberical modeling

Noble metal (platinum or palladiuni) combustion catalysts have demonstrated low NO2 (nitrogen oxides, consisting of both NO and NO2) emissions in natural-gas-fired turbines. The catalyst permits low temperature combustion below the traditional lean limit. Due to the combined proceses of diffusion and (unknown) surface reaction, the catalyst is typically modeled as a “black box,” often described by a global reaction rate expression. While this approach has been useful for proof-of-concept studies, we expect practical applications to emerge from a greater understanding of the details of the catalytic combustion process. We have constructed a detailed numerical model of the catalytic combustion process based on the wellaccepted CHEMKIN chemical kinetics formalism for detailed gas-phase and surface chemistry. Results from an experimental combustor support the model development. We present measured and modeled axial profiles of fuel conversion for natural-gas combustion over platinum catalysts supported on ceramic honeycomb monoliths, NO emissions are below 1 ppm, and CO is observed at ppm levels. The data are taken at several lean equivalence ratios and flow rates, at atmospheric pressure. Fuedl conveersion rates occur in tworegines: a low, constant conversion rate and a higher conversion rate that inereases linearly with equivalence ratio. Both conversion rates are consistent with kinetically limited processes. The jump from kinetic to mass-diffusion limitation, predicted by most accepted theories of catalytic combustion, is not observed. The agreement of the numerical model with the measured data is good at temperatures below 900 K: above this temperature. the predicted fuel conversion is as much as a factor of 2 lower than the measurements. Carbon monoxide is overpredicted by 2-3 ppm for 0.34. Results from the numerical model indicate that fuel conversion rate has a linear dependence on the fraction of available surface reaction sites.

Real-time optical fuel-to-air ratio sensor for gas turbine combustors

The measurement of the temporal distribution of fuel in gas turbine combustors is important in considering pollution, combustion efficiency and combustor dynamics and acoustics. Much of the previous work in measuring fuel distributions in gas turbine combustors has focused on the spatial aspect of the distribution. The temporal aspect however, has often been overlooked, even though it is just as important. In part, this is due to the challenges of applying real-time diagnostics techniques in a high pressure and high temperature environment.

Optical probe for in-situ measurements of air-to-fuel ratio in low emission engines

AIAA, Aerospace Sciences Meeting and Exhibit, 34th, Reno, NV, Jan. 15-18, 1996 The lower temperatures associated with lean premixed combustion generally lead to lower NO(x) emissions; however, the NO(x) emissions also depend on how well the air and fuel are mixed. In this paper, we describe the development of an inexpensive fiber optic probe capable of measuring the extent of turbulent mixing. The fuel concentration is determined by laser absorption at 3.39 microns over a short path length created by infrared transmitting fiber optics. We use this probe to show that NO(x) formation depends significantly on the extent of fuel-air mixing as well as the overall stoichiometry. A hydrogen-piloted, CH4-in-air turbulent flame with a variable fuel injection location is used to vary the degree of mixedness at the burner exit. The level of mixing and the mean concentration profiles are also measured by using Planar Laser Initiated Rayleigh Scattering. We show that at lean conditions, incomplete mixing causes an increase in NO(x) production because of the unfavorable temperature dependence of NO(x) formation at Phi = 0.6. We also show that the optical probe is capable of measuring the extent of mixing of the fuel-air mixture. (Author)

Catalytic Combustion of Natural Gas Over Supported Platinum: Flow Reactor Experiments and Detailed Numerical Modeling

The use of a noble-metal combustion catalyst such as platinum or palladium in a natural-gas fired turbine can lower NOx (nitrogen oxides, consisting of both NO and NO2) emissions for two reasons. First, most of the combustion occurs on the catalyst surface; surface production of NOx is low or nonexistent. Second, the catalyst permits low temperature combustion below the traditional lean limit, thus inhibiting NOx formation routes in the gas phase. Due to the complexity of the catalytic combustion process, the catalyst has traditionally been modeled as a “black box” that produces a desired amount of fuel conversion. While this approach has been useful for proof-of-concept studies, we expect practical applications to emerge from a greater understanding of the details of the catalytic combustion process.We have constructed a numerical model of catalytic combustion based on the well-accepted CHEMKIN chemical kinetics formalism for gas-phase and surface chemistry. To support the model development, we built a research combustor. We present measured and modeled axial profiles of temperature, fuel conversion, and pollutant emissions for natural-gas combustion over platinum catalysts supported on ceramic honeycomb monoliths. NOx emissions are below 1 ppm, and CO is observed at ppm levels. The data are taken at several lean equivalence ratios and flow rates. Fuel conversion rates occur in two regimes: a low, constant conversion rate and a higher conversion rate that increases linearly with equivalence ratio.The agreement of the numerical model with the measured data is good at temperatures below 900 K; above this temperature, fuel conversion is underpredicted by as much as a factor of two. The predicted surface ignition temperatures agree well with the measured values. Results from the numerical model indicate that the fractional conversion rate of fuel has a linear dependence on the fraction of available surface reaction sites.Copyright

Fast Response Extraction Probe for Measurement of Air-Fuel Ratio Fluctuations in Lean Premixed Combustors

The extent of air and fuel mixing prior to combustion in lean premixed combustion has been shown to drastically affect combustor performance, both in terms of emissions and stability. Standard extraction probes are often used for measuring the spatial (average over time) distribution of fuel concentration. However, the temporal fluctuations in fuel mole fraction, which are averaged by conventional extraction probes, have been also shown to drastically affect combustor performance. Several methods have been developed to measure the fluctuations in fuel mole fraction both temporally and spatially. These techniques are often difficult or impossible to apply to an operating combustor at high pressures and temperatures.In this paper, we describe a Fast Response Extraction Probe (FREP) which is capable of measuring temporal mole fraction fluctuations up to frequencies of 1 kHz. A short, capillary tube is inserted into the premixing passage (which is at high pressure) for sampling the flow. The capillary tube is connected to a small gas cell at atmospheric pressure. Light from a 3.39 μm He-Ne laser is passed through the gas cell. By measuring the absorption of laser light, the concentration of hydrocarbons can be determined. The temporal response of the system is dependent on the geometry of the gas cell and sampling tube and the pressure drop through the sampling tube. A model for predicting the time response of the FREP is presented and compared to a laboratory scale experiment. The FREP was also tested in a high-pressure combustion rig at United Technologies Research Center. It is shown that the FREP is capable of measuring fuel-air fluctuations at gas turbine conditions. It is also shown that, at the conditions studied, there is a relationship between pressure oscillations and fuel mole fraction oscillations.Copyright