Daniel J. Maloney

Assessment of Rich-Burn, Quick-Mix, Lean-Burn Trapped Vortex Combustor for Stationary Gas Turbines

This paper describes the evaluation of an alternative combustion approach to achieve low emissions for a wide range of fuel types. This approach combines the potential advantages of a staged rich-burn, quick-mix, lean-burn (RQL) combustor with the revolutionary trapped vortex combustor (TVC) concept. Although RQL combustors have been proposed for low-Btu fuels, this paper considers the application of an RQL combustor for high-Btu natural gas applications. This paper will describe the RQL/TVC concept and experimental results conducted at 10 atm (1013 kPa or 147 psia) and an inlet-air temperature of 644 K (700°F). The results from a simple network reactor model using detailed kinetics are compared to the experimental observations. Neglecting mixing limitations, the simplified model suggests that NOx and CO performance below 10 parts per million could be achieved in an RQL approach. The CO levels predicted by the model are reasonably close to the experimental results over a wide range of operating conditions. The predicted NOx levels are reasonably close for some operating conditions; however, as the rich-stage equivalence ratio increases, the discrepancy between the experiment and the model increases. Mixing limitations are critical in any RQL combustor, and the mixing limitations for this RQL/TVC design are discussed.

Heat capacity and thermal conductivity considerations for coal particles during the early stages of rapid heating

Abstract Heating and cooling transients for a number of individual coal particles in the 100-μm size range were measured under rapid heating conditions (10 4 –10 5 K/s heating rate). In addition to temperature measurements, each particle was fully characterized with respect to external surface area, volume, mass, and density prior to heating. Measured temperatures were compared with model predictions and a sensitivity analysis was performed to critically evaluate model assumptions regarding particle thermal properties. Simulations using temperature-dependent heat capacity and thermal conductivity correlations routinely applied to coal severely under predicted the particle temperature rise during the early stages of heating. Simulations using constant room temperature values for heat capacity and thermal conductivity showed excellent agreement with measurements during the early stages of heating. Increases in coal heat capacity and thermal conductivity reported in the literature are observed under slow heating conditions and result from bond breaking and structural changes which lead to an increase in vibrational modes of freedom in the coal structure. Results of the present study suggest that under rapid heating conditions the coal structure is frozen and that these vibrational modes only become accessible at higher temperatures or longer soak times. These considerations are important if one desires to accurately model the combustion behavior of coals.

Measurements of heat capacities, temperatures, and absorptivities of single particles in an electrodynamic balance

A rapid measurement technique to determine heat capacities, temperatures, and absorptivities of single submillimeter size (50–200 μm diam) particles in an electrodynamic balance (EDB) is described. Accurate estimates of heat capacities and absorptivities are made for single carbon particles at temperatures ranging from 800 to 1200 K with measurement times of less than 0.5 s. Measurements are made by isolating and holding charged particles at the null position in an EDB. Particles are heated using the output from a well‐characterized CO2 laser which is modulated to provide heating pulses of 3‐ms duration at a repetition rate of 100 Hz. The resulting periodic temperature oscillations are monitored continuously using a single wavelength high‐speed optical pyrometer. Heat capacities and absorptivities are determined based on comparisons of the recorded heating and cooling profiles with theoretical predictions obtained from numerical solution of the Fourier equation. Experimentally determined heat capacities f...

Measurements and analysis of temperature histories and size changes for single carbon and coal particles during the early stages of heating and devolatilization

Abstract A novel system is described for monitoring rapid changes in particle size and temperature during coal devolatilization at heating rates representative of high-intensity combustion environments. The system incorporates an electrodynamic balance and a pulsed radiation source to isolate and rapidly heat individual particles. A high-speed two-dimensional photodiode array and a single wavelength radiation pyrometer are applied to record changes in particle size and temperature. Dynamics of volatile evolution and particle swelling are also recorded using high-speed cinematography. Measurements of temperature and size changes are reported for carbon spheres and a HVA bituminous coal at heating rates on the order of 105 K/s. Measured temperature histories are compared with theoretical estimates of the temperature response of radiatively heated coal and carbon particles. The analysis considers the development of the transient temperature distribution inside the particles by numerical solution of the Fourier equation. Measurements and model predictions for 135-μm-diameter carbon spheres were in excellent agreement using property data correlations commonly applied in modeling coal devolatilization and combustion behavior. Model predictions for coal particles, however, significantly underestimated (on the order of 50%) the observed heating rates for 115-μm-diameter coal particles using the same property correlations. It is concluded that coal particles heat significantly faster than is predicted using commonly employed approaches to model heat transfer with assumptions routinely applied to coal. Potential reasons for this may include inadequate understanding of relevant coal thermodynamic and heat transfer properties as well as failure to account for particle shape factors. Heat transfer analyses employing spherical particle assumptions and commonly used coal property correlations can lead to large errors in predicted temperature histories and associated devolatilization rates.

An automated imaging and control system for the continuous determination of size and relative mass of single compositionally dynamic droplets

A novel droplet imaging system (DIS) has been developed which, when interfaced with an electrodynamic balance (EDB), offers unique capabilities for characterizing size, mass, density, and composition of single, compositionally dynamic droplets. The DIS employs a standard closed‐circuit video camera as a droplet sensor. Real‐time hardware image processing techniques are employed to extract cross‐sectional area and position information from the video signals. The position information is used to generate a control signal which is proportional to the droplet mass. The DIS capabilities are demonstrated for evaporating water and coal‐water slurry droplets. The results presented clearly demonstrate that this system can be applied to easily detect compositional variations from one droplet to another. Accurate estimates of droplet composition can be obtained with little knowledge of the starting composition. The DIS has a wide range of applications and is particularly valuable when applied to the study of multicom...

Statistical kinetics for pulverized coal combustion

Coal is a heterogeneous substance whose structure and properties are highly variable on the length scale of the particle sizes used in suspension-fired combustion systems. For certain applications the statistical variations among particles can play an important role. In this paper, three specialized, single-particle techniques are applied to quantify the variations in combustion reactivity and char particle density within pulverized char particle populations. Reactivity variations are investigated through captive particle imaging experiments and entrained flow reactor experiments employing single-particle optical diagnostics. Single-particle density variations are determined directly by a novel technique based on an electrodynamic microbalance equipped with an automated video imaging and image processing system. From these data, a coal-general statistical kinetic model is developed and validated against a large set of single-particle temperature measurements for ten coals of various rank burning in three different combustion environments. The model incorporates a single empirical parameter describing the heterogeneity in reactivity and can adequately describe the entire database using a single coal-independent value of this parameter. The use of the model is demonstrated in a series of numerical simulations of complete burnout process for size-classified and polydisperse fuel samples. The simulations show that incorporating statistical kinetics has an important effect on burnout predictions in certain cases, the importance increasing with decreases in temperature, mean reactivity, and breadth of the particle size distribution.

Evaporation, Agglomeration, and explosive boiling characteristics of coal-water fuels under intense heating conditions

Unique capabilities have been developed to monitor the dynamics behavior of fuel droplets during rapid heating. These capabilities have been applied to study the evaporation, explosive boiling, and secondary atomization behavior of CWF droplets. The objective was to determine the energy flux (heating rate) requirements for explosive boiling and to evaluate the potential for secondary atomization in practical combustion systems. Single CWF droplets were suspended electrodynamically and irradiated from two sides with well-characterized heating pulses. The dynamic response of the droplets was recorded using high-speed cinematography. Explosive boiling thresholds were measured as a function of CWF droplet size and composition. Heat flux intensities were varied from 0.3 to 10 kW/cm2 (equivalent heating rates of 104 to 106 K/s), and heating times were varied from 0.2 to 8.0 ms. Results of this study suggest that fuel additives play a vital role in establishing conditions necessary for internal superheating and explosive boiling. A mechanism is proposed which involves elimination of the water-air interface, inhibition of surface evaporation, and the formation of a fused surface layer during the rapid heating process. In order to achieve fine atomization, droplets must be heated volumetrically. The energy requirements for explosive boiling were proportional to the mass for droplets with equivalent coal loadings and scaled with the weighted average heat capacity for droplets with differing composition. The time domain over which explosive boiling was observed spanned from about 2 to 8 ms. For longer times (lower heating intensities) heat losses due to droplet evaporation prevented the internal superheating required for explosive boiling. At shorter times, the required heat fluxes were well beyond practical technological limitations. The energy fluxes required to cause explosive boiling and secondary atomization are on the order of 1 kW/cm2 for droplets ranging from 75 to 160 μm in diameter (50 percent by mass coal). This may be considered as a target level for advanced combustor designs.

Measurement and dynamic simulation of particle trajectories in an electrodynamic balance: Characterization of particle drag force coefficient/mass ratios

Complementary measurement and simulation methods are described that enable rapid characterization of drag force coefficient/mass (Cd/m) ratios of individual particles in the 20 to 200 μm size range. Individual particles are suspended in a charge trap known as an electro‐dynamic balance (EDB). A step change is applied to the EDB end cap voltage to stimulate a dynamic response of the particle from an initial steady state. The resulting transient response is measured by means of a high speed, two‐dimensional diode array and imaging system which provides an analog output indicating particle position along the EDB center axis at frame rates of 6200 per second. A particle dynamic model (PDM) is developed to simulate trajectories of particles in the EDB. The PDM is a force balance simulation which accounts for field forces, gravitational forces, and drag forces acting on a particle. Simulations are performed using particle Cd/m ratio as a fit parameter to match model outputs with measurements. Data are presented...

A new approach to determine external surface and volume of irregularly shaped particles

A new approach to characterize fine particles is described that enables rapid determination of external surface area and volume. Individual particles are isolated and held in an electrodynamic balance. The particles are backlit to produce a high contrast shadow image that is projected onto a video camera detector. The resulting video signals are sent to a video imaging system that digitizes the video camera output in real time. Each video image field is reduced to a set of 20 to 30 number pairs that can be used to reconstruct the image to determine particle length, width, cross-sectional area, and perimeter. Surface areas and volumes are obtained by rotating particles using a set of directed gas jets and recording image data for successive video fields as a function of rotation angle. Particle perimeters and radii of rotation are determined for each video field and these data are numerically integrated to yield surface area and volume. Data acquisition times of less than 5 s are required for complete part...

Pressure Effect on NOx and CO Emissions in Industrial Gas Turbines

In order to determine the effect of pressure on emissions and stability limit, an experimental and modeling study has been performed jointly by UTRC and DOE-FETC. Experiments have been performed at lean conditions in 100–400 psi range with two different nozzles. Measured NOx and CO concentrations have been modeled with a PSR Network using detailed chemistry. Good agreement between the data and model predictions over a wide range of conditions indicate the consistency and reliability of the measured data and validity of the modeling approach.Experiments were conducted at the DOE-FETC facility in Morgantown. A simple refractory combustor liner with a fuel-air-premixing nozzle was used to map stability margins, emission levels of NOx, CO and combustion efficiency. Each experimental nozzle had a centerbody and wall pilot for flame stabilization. Data was collected at four different pressures of 100, 200, 300 and 400 psi, and at different diffusion pilot and moisture levels. The premixing nozzle hardware could be easily lit and operated over a broad range of flame temperatures with minimal combustion generated noise. Two different nozzles designed at UTRC were used to determine pressure and nozzle effects.Computations were made for comparison with the experiments. GRI Mech 2.11 kinetics and thermodynamic database was used for modeling the flame chemistry. A Perfectly Stirred Reactor (PSR) network code developed internally at UTRC was used to create a network of PSRs to simulate the flame and combustor. A total of 10 to 15 reactors were used in the network. Residence time varied with the flow rates (air was fixed while fuel flow rate was varied in order to obtain the required equivalence ratio, ϕ).Good agreement between the measured and modeled NOx (5–10%) was obtained, but the agreement for CO (model predictions are higher by 30–50%) was not as good as for NOx. The experimental data and the modeling predictions indicate that the NOx emission functionality with pressure is dependent on both equivalence ratio and absolute pressure. The CO levels tend to go down with increase in pressure as P−0.5, at different equivalence ratios, consistent with an equilibrium analysis.Copyright