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The fluidized-bed combustion of large coal char particles was studied with a focus on the burning rate and the primary CO/CO2 ratio at the particle surface. To this end, single particle laboratory-scale experiments were carried out using a low-attrition and high-reactivity coal char. A previously proposed indirect experimental technique was applied, which was modified to cover a wider range of experimental conditions. The experiments were conducted at different bed temperatures (800-900 °C), fluidization velocities (0.3 and 0.53 m/s), inlet oxygen concentrations (0.5%-8.0%), and inert bed particle size ranges (100-212, 500-600, and 900-1000 μm). The actual sphericity and temperature of the particle were also measured during selected experiments and used in the calculations. Results showed that, under the experimental conditions investigated, carbon was completely oxidized to CO2 within the particle boundary layer. The experiments confirmed that the char particles burned under boundary layer diffusion control in the temperature range of 800-900 °C. It was demonstrated that single particle burning rate experiments with a high-reactivity char can be used to estimate the particle Sherwood number in fluidized beds, but only if char attrition can be assumed to be negligible. © 2011 American Chemical Society. Source

Branca C.,CNR Institute for Research on Combustion | Di Blasi C.,University of Naples Federico II
Fuel Processing Technology

The combustion behavior of dry distiller's grains with solubles (DDGS) and the corresponding pyrolysis char using a thermogravimetric system is studied. Comparison with beech wood indicates that DDGS devolatilization occurs with a slower rate and the char consists of two fractions with different reactivity. Thermogravimetric curves, obtained with heating rates between 2.5 and 20 K/min, are used to develop a multi-step global reaction mechanism. The description of DDGS devolatilization requires four reactions, representative of the evolution of lumped classes of components, with activation energies of 95, 107, 106 and 100 kJ/mol, which are lower than those typically associated with wood devolatilization. DDGS char combustion is well described by two reaction steps with activation energies of 137 and 153 kJ/mol that again barely touch the lower limit of the typical range of values reported for lignocellulosic chars. © 2014 Elsevier B.V. All rights reserved. Source

Di Blasi C.,University of Naples Federico II | Branca C.,CNR Institute for Research on Combustion

A detailed mathematical model, comprehensive of the main chemical and physical processes, is proposed for the open-core downdraft gasification of wood pellets, which permits a dual air entry: from the top section (primary air) and at a certain height of the packed bed (secondary air). A transition is simulated from a single, top-stabilized front (zero percentage of secondary air), to a double front stabilization (percentages of secondary air up to 60-70%) and finally to a single, forced center-stabilized front at the position of secondary air injection. For sufficiently high percentages of secondary air, following the complete or partial separation between the zone of primary wood degradation and a high-temperature, oxygen-rich zone and higher temperatures along the char bed, tar and char conversion is highly enhanced. Good agreement is obtained between model predictions and measurements for a pilot-scale plant. © 2010 Elsevier Ltd. All rights reserved. Source

Scala F.,CNR Institute for Research on Combustion
Chemical Engineering Science

The problem of mass transfer between active solid particles and a fluid in multiparticle systems is examined with a focus on the stagnant and the low Reynolds number cases. This problem has attracted significant attention with regard to operation of fixed and fluidized beds. It is recognized that different Sherwood numbers can be defined depending on the choice of the reference concentration difference (driving force). An effective Sh has often been used to analyze experimental data based on an overall concentration difference across the bed. A local Sh can also be introduced based on a concentration difference close to the active particle. However, the use of these two different Sherwood numbers implies a different implementation of the mass balance equations.The mass balance equations are here analytically solved both under stagnant and non-stagnant conditions in a multiparticle system under suitable simplifying assumptions. Equations for the effective and local Sherwood numbers are derived for the general case and for the asymptotic limits. Allowance is given for the variation of bed voidage and volume fraction of active particles in the bed. It is shown that the local Sh only depends on geometrical and fluid-dynamics considerations and accordingly has a general validity. On the contrary, the effective Sh also depends on the assumptions made in deriving the mass balance equations across the bed (e.g., fluid plug flow). The use of the local Sh is therefore suggested.Results show that for Re→0 the limiting value of the local Sh is always a finite number, while the effective Sh tends to zero linearly with Re. It is shown that this result is simply a consequence of the plug flow assumption made in the bed mass balance. The general expressions derived here compare very well to experimental trends for the cases of large Reynolds numbers and of few isolated active spheres immersed in a bed of inert particles, where most of the reported experimental data gather. Unfortunately, for the most controversial case of very low Re in beds made entirely of active particles no reliable data appears to be available to check the accuracy of the expressions. © 2013 Elsevier Ltd. Source

Scala F.,CNR Institute for Research on Combustion
Combustion and Flame

In this paper we address the calculation of the mass transfer coefficient around a burning carbon particle in an atmosphere of O2, N2, CO2, CO, and H2O. The complete set of Stefan-Maxwell equations is analytically solved under the assumption of no homogeneous reaction in the boundary layer. An expression linking the oxygen concentration and the oxygen flux at the particle surface (as a function of the bulk gas composition) is derived which can be used to calculate the mass transfer coefficient. A very simple approximate explicit expression is also given for the mass transfer coefficient, that is shown to be valid in the low oxygen flux limit or when the primary combustion product is CO2. The results are given in terms of a correction factor to the equimolar counter-diffusion mass transfer coefficient, which is typically available in the literature for specific geometries and/or fluid-dynamic conditions. The significance of the correction factor and the accuracy of the different available expressions is illustrated for several cases of practical interest. Results show that under typical combustion conditions the use of the equimolar counter-diffusion mass transfer coefficient can lead to errors up to 10%. Larger errors are possible in oxygen-enriched conditions, while the error is generally low in oxy-combustion. © 2009 The Combustion Institute. Source

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