Fernando Eduardo Milioli

Numerical study on the influence of various physical parameters over the gas-solid two-phase flow in the 2D riser of a circulating fluidized bed

Abstract A numerical parametric study was performed on the influence of various physical aspects over the hydrodynamics of gas–solid two-phase flow in the riser of a circulating fluidized bed (CFB). The addressed features were the solid phase viscosity, the gas–solid interface momentum transfer correlations, and the coordinate system. An Eulerian continuum formulation was applied for both phases. The resulting two-fluid model and numerical procedure followed a version of the MICEFLOW code. The simulation results are compared to available experimental data. The effects of the concerning features on the flow behavior are shown, mainly regarding cluster evolution. The flow frequency fluctuations typical to CFB risers were observed. It was also observed that for high values of solid phase viscosity there is an accumulation of solids near the outlet forming large clusters. When those clusters fall down, the instantaneous cross-sectional average solid mass velocity becomes negative. For inviscid solid phase, no cluster formation is observed. The results show the incorrectness of assuming symmetry boundary condition at the axis of the riser when cylindrical coordinates are used. Quite different results were obtained for different correlations for gas–solid interface momentum transfer. Finally, a comparison is presented of predictions from the Illinois Institute of Technology (IIT) hydrodynamic models A and B.

Entrainment and elutriation modelling in bubbling fluidized beds

In bubbling fluidized bed combustion and catalytic cracking, elutriation is a major cause of inefficiency, while it is highly desirable, for instance, in sludge incineration. Whether the intention is to quench or promote elutriation, the involved phenomena must be properly known if the process is to be efficiently controlled. In this work, entrainment and elutriation are reviewed and a modelling proposal is developed. The main concern relates to bubbling beds of wide spread particle size distributions under vigorous fluidization. The particles are subdivided into three classes: critical fines, smalls and larges. Critical fines are particles whose terminal velocities are much lower than the gas superficial velocity, and are assumed to be instantaneously entrained at the bed surface. Smalls and larges are particles whose terminal velocities are respectively lower and higher than the gas superficial velocity, and are assumed to be entrained through the bubble wake ejection mechanism. The bed surface entrainment model presents parameters which must be evaluated from empirical data. Comparison to experiment shows that, for UUmf≥10, fwξk should increase with UUmf, and a correlation is proposed to account for this variation. The elutriation model is based on phenomenological observations from experiment, coming out with parameters which must be set for specific situations.

A model for particle size distribution and elutriation in fluidized beds

Abstract Knowledge of particle size distributions is crucial for establishing the utilization of solid carbonaceous and sorbents in fluidized bed combustors. Populational models should consider particle shrinking through disintegration and chemical reaction, and particle removal through elutriation and overflow. A modelling review is carried out and a populational model is constructed. Bed surface entrainment and elutriation models are used to deal with particle carryover. Entrainment is assumed to be accomplished through two different parallel mechanisms: the prompt entrainment of very fine particles, and the entrainment through bubble wake ejections. Attrition is identified as the main mechanism of particle disintegration. Concerning carbonaceous particles, attrition and chemical reaction are treated as parallel shrinking effects. Limestone sorbent particles are assumed to shrink only through attrition. The particle population model is solved simultaneously with the entrainment and the elutriation models. The predictions of the interlinked models are in good agreement with available experiment.

O efeito da granulometria na decrepitação durante a decomposição térmica de calcários e carvão

The use of fluidized bed combustors to burn coal is largely studied to permit the addition of limestone to capture SO2. The particle size for coal and limestone is an important parameter in this process. Thermogravimetry (TG) is used to elucidate the combustion and sulfation processes, but the experimental parameters must be evaluated to be representative in fluidized bed combustors. In the present study the effect of particle size is analyzed in the calcination of limestones and the combustion of coal through the thermogravimetric curve for limestone and derivative thermogravimetric curve for coal. Small peaks representing mass losses between 400 and 500 oC are observed due to the jumping of particles out of the crucible. This effect, recognized as decrepitation is observed for mid-sized particles provoked by the release of water vapor trapped within their lattice.

Improving the determination of bubble size histograms by wavelet de-noising techniques

Intrusive phase detection probes are widely used in association with numerical processing techniques to obtain bubble size distributions in two-phase bubbly flows. Under these circumstances, a numerical problem must be solved which consists in determining bubble size histograms from measured chord or pierced length histograms. This can be done by inverting a Fredholm integral operator of the first kind, which is known to be often ill conditioned, i.e. the solution will be extremely sensitive to small changes or errors in the input data. In practical situations, the ill conditioned nature of the problem may be critical, particularly because the signals delivered by phase detection probes always have noise and, also, when the construction of the chord histogram is based on restrictive assumptions such as the definition of threshold levels. These two issues are addressed in this work. Specifically, the application of a numerical de-noising technique based on orthogonal wavelet decomposition is described and compared with the classical Fourier de-noising technique. In addition, a new method is proposed for the extraction of pierced lengths based on the instantaneous frequency of the phase signal. Numerical experiments with artificial signals were carried out, aiming to test the proposed methodology and to delimit its range of applicability with respect to maximum allowable noise levels. Results confirm the advantage of the wavelet de-noising technique, mostly due to its ability of removing noise without a significant distortion of the edges of the signal from which the pierced lengths are determined.

Thermal decomposition kinetics of Brazilian limestones: effect of CO2 partial pressure

The influence of the partial pressure of carbon dioxide (CO2) on the thermal decomposition process of a calcite (CI) and a dolomite (DP) is investigated in this paper using a thermogravimetric analyser. The tests were non-isothermal at five different heating rates in dynamic atmosphere of air with 0% and 15% carbon dioxide (CO2). In the atmosphere without CO2, the average activation energies (E α) were 197.4 kJ mol−1 and 188.1 kJ mol−1 for CI and DP, respectively. For the DP with 15% CO2, two decomposition steps were observed, indicating a change of mechanism. The values of E α for 15% CO2 were 378.7 kJ mol−1 for the CI, and 299.8 kJ mol−1 (first decomposition) and 453.4 kJ mol−1 (second decomposition) for the DP, showing that the determination of E α for DP should in this case be considered separately in those two distinct regions. The results obtained in this study are relevant to understanding the behaviour changes in the thermal decomposition of limestones with CO2 partial pressure when applied to technologies, such as carbon capture and storage (CCS), in which carbon dioxide is present in high concentrations.

Estudo termogravimétrico do efeito da temperatura e da atmosfera na absorção de dióxido de enxofre por calcário

Thermogravimetry was applied to investigate the effects of temperature and atmosphere on conversion of sulfur dioxide (SO2) absorbed by limestone. Ranges of temperature and particle size were studied, typical of fluidized-bed coal combustion. Isothermal experiments were performed at different temperatures (between 750 and 950 oC) under local atmospheric pressure (~ 697 mmHg) in dynamic atmospheres of air and nitrogen. The maximum conversion was 29% higher in nitrogen atmosphere than in air atmosphere. The optimum conversion temperature was found at 831 oC in air atmosphere and at 894 oC in nitrogen atmosphere.

Metodologia para o estudo da porosidade de dolomita em ensaio de sulfatação interrompida

The aim of this work is to propose a methodology to evaluate the evolution of the pore blockage of limestone during the sulfation reaction. The experiments were performed for a national limestone (dolomite) with average particle size of 545 μm in interrupted sulfation tests were conducted at seven different times and at three different temperatures of the process. The empirical data were obtained from porosimetry tests to establish BET surface area, volume and average size of pore and distribution of pore sizes of the sulfated samples. Thermogravimetric tests were performed to evaluate the preparation methodology of the samples used in the porosimetry tests.

DETERMINAÇÃO DOS PARÂMETROS DE ARRHENIUS DA REAÇÃO DE SORÇÃO DO DIÓXIDO DE ENXOFRE POR CALCÁRIO

Sulfur emission in coal power generation is a matter of great environmental concern and limestone sorbents are widely used for reducing such emissions. Thermogravimetry was applied to determine the effects of the type of limestone (calcite and dolomite), particle size (530 and 650 µm) and atmosphere (air and nitrogen) on the kinetics of SO2 sorption by limestone. Isothermal experiments were performed for different temperatures (650 to 950 oC), at local atmospheric pressure. The apparent activation energies, as indicated by the slope of the Arrhenius plot, resulted between 3.03 and 4.45 kJ mol-1 for the calcite, and 11.24 kJ mol-1 for the dolomite.

On the statistical steady state gas-solid flow in a riser as predicted through a two-fluid simulation

This work is concerned with the extremely high computational costs of the two-fluid simulations of gas-solid flows in risers. In a previous article [1] a procedure was proposed to speed up the simulations towards the desired statistical steady state flow regime. In this continuing article the concern is turned to the time extent that a simulation must advance inside the statistical steady state regime so that suitable time averaged predictions can be made. An analysis is carried out using the results of a transient two-fluid simulation of a riser flow performed inside the statistical steady state regime. Time averaged results were produced considering different time averaging intervals of 5, 10, 15 and 20 seconds. Both the transient behavior of the predictions and the time averaged results are discussed. For the present case, it was found that 10 seconds of fluidization taken inside the statistical steady state regime are enough for a reasonable qualitative description of the average flow.