G.S. Samuelsen

The thermal decomposition of pulverized coal particles

The physical, thermal, and chemical behavior of pulverized coal particles during thermal decomposition are examined for five coal types and two particle sizes for one of the bituminous coals. Particles were injected axially into a lean (35% excess air) methane/air flat flame with a nominal peak temperature of 1750°K. The significant events observed are classified by three time scales. Particles heat to the gas temperature in less than 10 msec, devolatilization occurs between 10 and 75 msec and, under the appropriate conditions, large soot particles are formed WRS and grow for times exceeding 75 msec. The events that accompany devolatilization are dependent upon coal type and particle size. For large bituminous particles (ca., 80 μm) a significant volatile fraction is ejected from the particle as a jet. This volatile jet reacts close to the particle producing a trail of small solid particles. The local heat released during the reaction of the volatiles, in combination with heterogeneous oxidation, increases the particle, temperature and raises it above that of the bulk gas stream. At later times, large soot structures, are formed which are attributed to the agglomeration of small, homogeneously formed soot on the volatile trail structures. Small bituminous particles (ca., 40 μm) burn with a higher intensity (i.e., higher temperature and more rapidly) with few trails and do not produce soot structures probably because of the more diffuse nature of the devolatilization process. Other ranks of coal exhibit different physical, thermal, and chemical behavior. For example, neither the lignites nor the anthracite produce volatile trials. Further, the particle temperature for the lignites is only slightly shifted above the bulk gas temperature in the devolatilization region while anthracite takes 50 msec to reach the bulk gas temperature level. This is attributable to the relatively low heat content of the volatiles in the former case and the low volatile content in the latter. The impact of the above observations on the formation of fuel NO is discussed.

Lean blowout model for a spray-fired swirl-stabilized combustor

In aero-engine applications, the lean blowoff (LBO) limit plays a critical role in the operational envelope of the engine. The geometry of the combustion chamber primary zone plays a critical role in establishing LBO limits. This is especially true for advanced lean burn concepts which introduce the majority of the combustion air in a manner designed to enhance the rate of mixing with the fuel. In the present study, specific mixer hardware has been designed to develop a systematic, statistically sound test matrix to study the effect of mixer components (primary swirl vane, secondary swirl vane, Venturi, and co-and counterswirl) on LBO. A strategy is employed to develop, based on an existing model, a new predictive model for LBO which accounts for a heterogeneous swirl-stabilized reaction and explicitly relates the geometry of the hardware to the LBO limit. The model predicts the LBO fuel/air ratio at three operating temperatures to within 14% of the measured value. The multivariate experiments used to relate LBO to geometry were also further analyzed to establish the main hardware parameters affecting LBO. Specifically, the Venturi and swirl sense (co- versus counter-swirl) were found to impact LBO at lower air inlet temperatures (294 and 366 K). The Venturi and counter-swirl enhance the atomization and mixing processes which are more rate limiting at lower temperatures, and, as a result, improve stability. At a higher inlet air temperature (477 K), the secondary swirl vane angle also plays a role in determining LBO, with larger angles (75°) generating better stability, which is associated with a stronger recirculation zone. The hardware configuration with the best LBO performance over the different conditions was identified (45° primary swirler, 55° secondary swirler, counter-swirl with Venturi).

Active control for gas turbine combustors

Closed-loop feedback control is implemented in two model combustors as a demonstration of the application of feedback control to gas turbine combustion. The first combustor is an axi-symmetric, swirl-stabilized, spray-fired combustor, while the second combustor incorporates discrete wall injection of primary and dilution air, representative of an actual gas turbine combustor. In both combustors, the emission of carbon monoxide and carbon dioxide, the radiative heat flux to the liner associated with soot and combustor stability are monitored in real time and controlled as a function of combustor load. The control input to the system is the nozzle atomizing air flow rate. The emission of carbon monoxide and carbon dioxide, the radiative flux to the liner, and the combustor stability are obtained through non-intrusive radiometric sensors mounted near the combustor exit plane. This information is conveyed to a control computer which invokes an optimization algorithm to minimize the CO and soot radiative flux, while maximizing the CO2 radiative flux. The index of combustion instability (onset of elevated acoustic emission) is, in the present case, a characteristic frequency in the power spectral density of the CO signal. The identical control methodology is applied to the two combustors with satisfactory and promising results that demonstrate the potential of active control to practical systems.

A Detailed Characterization of the Velocity and Thermal Fields in a Model Can Combustor With Wall Jet Injection

This work represents a first step in the establishment of a data base to study the interaction and influence of liquid fuel injection, wall jet interaction, and dome geometry on the fuel air mixing process in a flowfield representative of a practical combustor. In particular, the aerodynamic and thermal fields of a model gas turbine combustor are characterized via detailed spatial maps of velocity and temperature. Measurements are performed at an overall equivalence ratio of 0. 3 with a petroleum JP-4 fuel. The results reveal that the flowfield characteristics are significantly altered in the presence of reaction. Strong on-axis backmixing in the dome region, present in the isothermal flow, is dissipated in the case of reaction. The thermal field exhibits the primary, secondary and dilution zone progression of temperatures characteristic of practical gas turbine combustors. A parametric variation on atomizing air reveals a substantial sensitivity of the mixing in this flow to nozzle performance and spray symmetry.

Droplet sizing interferometry: a comparison of the visibility and phase/Doppler techniques.

Spatially resolved measurements of droplet size and velocity are desirable to aid in matching fuel injectors to combustor flow fields and to support development of two-phase-flow modeling. Interferometric laser-based techniques have been available since the early 1970s. Successful application to practical sprays, however, has been hampered by numerous difficulties. In this paper, two interferometric techniques (visibility/intensity validation and phase/Doppler) are critically examined in characterizing the spray of an air-assist nozzle with Sauter mean diameter < 35 microm. The two techniques are compared to each other and evaluated against a Malvern diffraction unit. With the use of a rotating grating for frequency shifting, the interferometric techniques compare well with each other and to the diffraction method. Due to its broadened size and velocity ranges, the phase/Doppler technique is more easily applied to the spray than is visibility/intensity validation. The consistency of the interferometric results raises questions with regard to the use of Malverns most frequently applied distribution model.

Observations of chemical effects accompanying pulverized coal thermal decomposition

Abstract A thermal decomposition reactor is used to simulate the environment of a practical coal burner and assess the influence of fuel properties on the chemical behaviour that accompanies the thermal decomposition of pulverized coal particles. The present study complements and extends previous work that explored the physical and thermal behaviour. The rate of component burnout is determined for hydrogen, nitrogen and carbon as well as the evolution of NO and HCN, and the efficiency of fuel nitrogen conversion. The results demonstrate a significant dependency on coal particle size and coal type. Modelling demonstrates that the penetration of oxygen into the fuel-rich devolatilization cloud depends on particle size and coal properties and, as a result, controls to a large extent the evolution and transformation of the chemical constituents associated with the parent fuel.

Soot morphology in a liquid-fueled, swirl-stabilized combustor

Abstract The morphology of particulate soot formed in a liquid-fueled, complex-flow (i.e., turbulent, recirculating) combustor has been studied using thermophoretic sampling and transmission electron microscopy. Soot size information was obtained via computer-aided image analysis. Particle morphology was observed to be similar to that found in other combustion devices (i.e., nearly spherical primary particles fused into aggregate chains and clusters). Axial regions where soot nucleation, growth, agglomeration, and oxidation occur were identified. Primary particle size was observed to increase with combustor height, but the largest primary size reached is small compared to that observed in laminar diffusion flames. From calculated estimates of the specific soot surface growth and oxidation rates, the growth rate was found to be lower and the oxidation rate comparable to those for laminar diffusion flames. Aggregate sizes were also observed to increase with combustor axial location, and were found to be distributed in a lognormal manner during the particle inception and growth stages. Fractal dimensions for characteristic aggregate populations were determined to be around 1.8, and were independent of combustor axial location. The results suggest that cluster-to-cluster aggregation—and not surface growth—is the dominant soot aggregate growth mechanism in the complex-flow reactor. Comparisons with more traditional methods for soot size determination were made with moderate success.

Impact of Ethane and Propane Variation in Natural Gas on the Performance of a Model Gas Turbine Combustor

In the area of stationary power generation, there exists a growing interest in understanding the role that gaseous fuel composition plays on the performance of natural gas-fired gas turbine systems. In this study, an atmospherically fired model gas turbine combustor with a fuel flexible fuel/air premixer is employed to investigate the impact of significant amounts of ethane and propane addition into a baseline natural gas fuel supply. The impacts of these various fuel compositions, in terms of the emissions of NO x and CO, and the coupled impact of the degree of fuel/air mixing, are captured explicitly for the present system by means of a statistically oriented testing methodology. These explicit expressions are also compared to emissions maps that encompass and expand beyond the statistically based test matrix to verify the validity of the employed statistical approach.

A comparison of spatially-resolved drop size and drop velocity measurements in an isothermal chamber and a swirl-stabilized combustor

A liquid fuel air-assist atomizer is characterized in both an isothermal spray chamber and a swirl-stabillized combustor. The objective is to asses the extent to which the isothermal characterization represents atomizer performance under reacting conditions and in the presence of strong, aerodynamic swirl. The technique employed for the point measurements of droplet size and droplet velocity is phase Doppler interferometry. In the isothermal chamber, comparative measurements are conducted using laser diffraction (Malvern). To explain the differences in spray behavior observed in the combustor, the droplet measurements are complemented with laser anemometry measurements of the axial and azimuthal velocity fields, and thermocouple measurements of the temperature field. The results show that, except for initial Sauter mean diameter (SMD), the spray behavior in the combustor is substantially different than that observed in the isothermal chamber. In particular, (1) the spray field in the combustor has a hollow-cone structure, (2) the radial spread of the spray widens, (3) the number density decreases by at least an order of magnitude, and (4) the SMD at points displanced from the centerline is reduced. These differences are explained by the presence of heat release and the aerodynamically induced recirculation. In addition, the droplets and fuel vapor in the widened spray boundary are found to be important to combustor stability and performance. The results establish the importance and potential of characterizing sprays within the practical, operating environment.

Response of a Model Gas Turbine Combustor to Variation in Gaseous Fuel Composition

The effect of fuel composition on performance is evaluated on a model gas turbine combustor designed to mimic key features of practical devices. A flexible fuel injection system is utilized to control the placement of the fuel in the device to allow exploration and evaluation of fuel distribution effects in addition to chemistry effects. Gas blends reflecting the extremes in compositions found in the U.S. are considered. The results illustrate that, for the conditions and configuration studied, both fuel chemistry and fuel air mixing play a role in the performance of the device. While chemistry appears to be the predominant factor in stability, a role is noted in emissions performance as well. It is also found that changes in fuel distribution associated with changes in fuel momentum for fixed firing rate also have an impact on emissions. For the system considered, a strategy for sustaining optimal performance while fuel composition changes is illustrated.