G. De Michele

Quantitative Estimation of Bubble-Size in Pfbc

Abstract This paper presents a study on the quantitative estimation of bubble size in pressurized fluidized bed combustors (PFBC). A generalized correlation of bubble size which fits the experimental data quite well has been developed for PFBC on the basis of a comprehensive analysis of previous work. The correlation takes into account the different flow regimes at different pressures and gas velocities, as well as the special variation of bubble size within the lower pressure range of the bubbling regime. Using this correlation, an ideal operating area of the PFBC can also be defined from the viewpoint of hydrodynamics and mass transfer. In addition, the correlation can be applied to atmospheric operations to overcome problems encountered by the previous correlations under high gas velocities (e.g. turbulent fluidization regime).

Synthesis of ultrafine TiO2 powders by a CW CO2 laser

Abstract A laser-induced process for the synthesis of ultrafine TiO 2 powders has been developed. The work is mainly concerned with the evaluation of the role of the process parameters on production rate and reaction efficiency. The powder characteristics, in particular particle size, crystallinity and composition, have been analyzed through different techniques.

Laser synthesis of vanadium-titanium oxide catalysts

A laser-induced process has been studied and developed for the synthesis either of pure TiO 2 powders for use as a catalyst support or of mixed Ti 1− x V x O 2 oxides to be used directly as catalysts in the selective catalytic reduction of nitrogen oxides with ammonia. The radiation source is a 600 W cw CO 2 laser and the precursors are titanium and vanadium alkoxides whose vapors are carried under the laser beam by controlled flows of an inert gas and of a reaction sensitizer. The study of the process has been mainly devoted to the optimization of the production rate and of the reaction efficiency. The powder characteristics, related to the process parameters, have been investigated through different techniques. The results show that the particles are monodispersed, unaggregated, monocrystalline with anatase structure and their diameters can be as small as 50 A. Vanadium contents up to X = 0.25 have been obtained in Ti 1− x V x O 2 compounds, without V 2 O 5 segregation. Finally, a comparison between the behavior of surface VO x active sites has been carried out in catalysts prepared by chemical impregnation of TiO 2 and in the mixed Ti–V oxide powders.

LDA and PIV velocity measurements in free jets

Abstract Velocity measurements on axisymmetric jets are performed with both laser-Doppler anemometry (LDA) and particle image velocimetry (PIV). Two geometric configurations are investigated: circular and annular jets in still ambient fluid. The results obtained with the two measuring techniques are compared and related to the results of self-preservation theories. To describe mixing properties of the tested geometric configurations, vorticity plots are also calculated from PIV measurements: the results are useful for optimal design of efficient burners.

An X-ray photoelectron spectroscopy study of the vanadia-titania catalysts

Abstract This paper reports an X-ray photoemission study (XPS) of the core levels and the valence band of vanadia-titania catalysts and their precursor oxides. The XPS spectra of the vanadium core levels indicate that surface vanadium atoms are present with different oxidation states. Furthermore, it is demonstrated, for the first time, that the electronic states observed in the XPS valence-band spectra between the Fermi edge and the O 2p bands have mainly a V3d character.

Influence of coal type and operating conditions on the formation of incomplete combustion products. Pilot plant experiments

ABSTRACT This experimental study was mainly aimed to evaluate the influence of the coal type on the production of Polycyclic Aromatic Hydrocarbons (PAHs) and other trace organics from pulverized coal combustion. The behaviour 7 of coals, having different characteristics and geographical origin, was tested in a small scale furnace (50kWt), under relatively good practical combustion conditions. As regards fuel carbon conversion, some observations were also made when the combustion conditions got worse, slightly differing from those set, or becoming definitely poor. A detailed chemical characterization of the Products of Incomplete Combustion (including unburnts, the total Organic Material Extractable by methylenchloride (EOM), PAHs from naphthalene to coronene and n-alkanes (n-A) from C12 to C36) was performed on solid and gaseous emissions. The fluorescence emission spectra of the integral methylenchloride extracts, in the range from 270 to 600 nm, were also studied. The experimental data concerning stack ...

Atomization of Coal-Water Fuels by a Pneumatic Nozzle: Characteristics of the Spray

The main characteristics of CWF (coal-water fuel) sprays produced by an internal mixing twin fluid atomizer are analyzed. Drop size and concentration have been determined by a laser light diffraction method (Malvern) and a high-speed photographic technique. The combined use of different experimental methods, along with the numerical inversion of line-of-sight measurements, allows a reliable characterization of the spray structure along radial and axial coordinates. The mean drop size depends essentially on the dynamic head of the atomizing air, the surface tension, and the coal particle size distribution of the slurry fuel. No clear effect of the CWFs apparent viscosity has been detected. The equation developed to correlate mean drop size takes all relevant factors into account and was derived for a nozzle operating with sonic flow in the exit ports.

Simulation of large non-linear thermo-acoustic vibrations in a pulsating combustor

This paper concerns the modeling and numerical simulation of pulse combustion. As the title highlights, focus is on self-sustained acoustic vibrations, the cause of self-excitation being an unstable feedback loop closed by the combustion process. A Helmholtz-type mechanical-valve pulsating combustor was considered. Numerical models were two-dimensional axis-symmetrical, and efforts were made to model the elementary steps of the unstable feedback loop (valve opening, injection of reactants, turbulent mixing and combustion, etc.) as well as possible, to provide a realistic description of the self-excitation mechanism as a whole. Simulations were carried out the analyse the effects of several design and operation parameters on the feedback loop (from injection of reactants to turbulent combustion rate to history of heat release) and ultimately on the growth rate and frequency of vibrations. Results of simulations were post-processed to extract the time evolution of the total mechanical enery (sum of pressure and kinetic energy) of vibrations. Finite-amplitude self-sustained periodic oscillations in the combustor are completely reproduced and the limit cycle (at which dissipated energy per cycle is equal to amount of mechanical energy furnished by the combustion process) found. The software used was ‘Fluent,” a commerical CFD (Computational Fluid Dynamics) code [13] incorporating a reaction rate model accounting for turbulent mixing. A general purpose CFD code instead of classical linearized models was chosen to preserve the nonlinearity of the original equations, thus allowing us to carry out non-linear instability analysis (not only initial growth rate) and to find the limit cycle of the system.

An XPS study of SO2 on a CaO surface derived from Ca(OH)2

Abstract The adsorption of SO 2 on a CaO surface derived from Ca(OH) 2 has been studied by X-ray photoelectron Spectroscopy. It is shown that SO 2 adsorbs molecularly on the CaO surface forming SO 3 2− and SO 4 2− . The adsorbate SO 3 2− species shift the Ca2 p core levels about 2 eV toward higher binding energy. However, the formation of SO 4 2− species does not induce any further observable changes in the Ca core line spectra. These results are explained and discussed in terms of an electronic charge transfer from the substrate metal atoms to the adsorbed molecules and vice versa.

LDA AND PIV VELOCITY MEASUREMENTS IN FREE JETS

Velocity measurements on axisymmetric jets are performed with both Laser Doppler Anemometry (LDA) and Particle Image Velocimetry (PIV). Two geometrical configurations are investigated: circular and anular jets flowing in free environment. The results obtained with the two measuring techniques are compared between them and with theoretical results. From PIV measurements vorticity plots are also calculated.