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Branca C.,CNR Institute for Research on Combustion | Di Blasi C.,University of Naples Federico II
Fuel Processing Technology | Year: 2014

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.


Raganati F.,University of Naples Federico II | Ammendola P.,CNR Institute for Research on Combustion | Chirone R.,CNR Institute for Research on Combustion
Applied Energy | Year: 2014

Among all the CCS strategies, post-combustion capture provides a near-term solution for stationary fossil fuel-fired power plants, eliminating the need for substantial modifications to existing combustion processes and facilities. In this respect, adsorption using solid sorbents has the potential, in terms of energy saving, to complement or replace the current absorption technology. Therefore, the design of highly specific CO2 adsorbents materials is requested. In this framework, great interest is focused on nanomaterials, whose chemico-physical properties can be tuned at the molecular level. As regards the handling of such materials, sound-assisted fluidization is one of the best technological options to improve the gas-solid contact by promoting a smooth fluidization regime. The present work is focused on the CO2 capture by sound-assisted fluidized bed of fine activated carbon. Tests have been performed in a laboratory scale experimental set-up at ambient temperature and pressure, pointing out the effect of CO2 partial pressure, superficial gas velocity, sound intensity and frequency. Effectiveness of CO2 adsorption has been assessed in terms of the moles of CO2 adsorbed per unit mass of adsorbent, the breakthrough time and the fraction of bed utilized at breakpoint. The results show, on one hand, the capability of the sound in enhancing the adsorption process and, on the other hand, confirm that sound assisted fluidization of fine solid sorbents is a valid alternative to the fixed bed technology, which require also an additional previous step of pelletization. © 2013 Elsevier Ltd.


Di Blasi C.,University of Naples Federico II | Branca C.,CNR Institute for Research on Combustion
Fuel | Year: 2013

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.


Miccio F.,CNR Institute for Research on Combustion
Applied Thermal Engineering | Year: 2013

The present article deals with the integration of a fluidized bed combustor and a Stirling engine for cogeneration purposes. An experimental study was carried out, proving the ability of the bed to exchange heat at high rate with an immersed coil that realistically emulates the heat exchanger of a small Stirling engine. The heat transfer coefficient attains values up to 280 W m -2 K-1. No dirtying of the immersed surface occurred during a combustion test of biomass. The paper also reports on a newly developed mathematical model of a fluidized bed combustor coupled with a Stirling engine for co-generation purposes. It consists of four fundamental blocks describing i) the heat transfer, ii) the fluidized bed combustion, iii) the heat recovery, and iv) the Stirling engine. The model produces as relevant outputs the bed temperature, the mechanical power and the efficiency of the Stirling engine, at changing the operating conditions and geometrical parameters of the system. A slow dynamic response is predicted, that it is significantly improved by adopting an efficient control strategy. On the whole, the model results indicate that the proposed "integrated system" is of interest for micro-scale cogeneration from biomass fuels. © 2012 Elsevier Ltd. All rights reserved.


Scala F.,CNR Institute for Research on Combustion
Chemical Engineering Science | Year: 2013

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.


Scala F.,CNR Institute for Research on Combustion
Energy and Fuels | Year: 2011

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.


Scala F.,CNR Institute for Research on Combustion
Proceedings of the Combustion Institute | Year: 2015

Gasification of a lignite char with either CO2 or H2O at atmospheric pressure was studied in a lab-scale fluidized bed apparatus at different bed temperatures (775-900 °C) and gas concentrations. CO2 concentrations in the range 20-100% and H2O concentrations in the range 10-70% were used in the batch experiments. The carbon conversion rate was measured by following the outlet CO and CO2 concentrations with time. A predictive kinetic model for both CO2 and H2O gasification of the lignite char was developed from the experimental results, that was able to correctly predict the evolution of carbon conversion versus time. Both gasification reactions kinetics followed a Langmuir-Hinshelwood (L-H)-type equation. The activation energy of the CO2 gasification reaction was larger than that of steam gasification, indicating a larger reactivity of the lignite char towards H2O in the investigated temperature range, as expected. Interestingly, the structure-related parameters of the kinetic expressions of the two gasification reactions were the same, suggesting that CO2 and H2O are likely to attack the same carbon surface sites on this char. © 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.


Scala F.,CNR Institute for Research on Combustion
Combustion and Flame | Year: 2010

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.


Sanchirico R.,CNR Institute for Research on Combustion
AIChE Journal | Year: 2012

The minimum number of thermoanalytical experiments that should be considered with the aim of performing a complete and reliable kinetic analysis when using semiempirical models is the problem of concern. It is shown by means of a series of numerical experiments that three differential scanning calorimetric (DSC) dynamic runs performed at different heating rates provide a complete and reliable kinetic analysis while the topological structure of two DSC curves is enough to determine the semiempirical model (among reaction order and Sestack-Bergrenn) that better describe the experimental data. The procedure is analyzed by means of the simulation of a complex kinetic scheme and the results verified considering the thermal decomposition of cumene hydroperoxide. The proposed approach can be easily generalized to different kinetic models and is provided with a possible criterion devoted to identify autocatalytic processes by means of dynamical DSC experiments. © 2011 American Institute of Chemical Engineers (AIChE).


Scala F.,CNR Institute for Research on Combustion
Proceedings of the Combustion Institute | Year: 2015

Gasification of a lignite char with either CO2 or H2O at atmospheric pressure was studied in a lab-scale fluidized bed apparatus at different bed temperatures (775-900 °C) and gas concentrations. CO2 concentrations in the range 20-100% and H2O concentrations in the range 10-70% were used in the batch experiments. The carbon conversion rate was measured by following the outlet CO and CO2 concentrations with time. A predictive kinetic model for both CO2 and H2O gasification of the lignite char was developed from the experimental results, that was able to correctly predict the evolution of carbon conversion versus time. Both gasification reactions kinetics followed a Langmuir-Hinshelwood (L-H)-type equation. The activation energy of the CO2 gasification reaction was larger than that of steam gasification, indicating a larger reactivity of the lignite char towards H2O in the investigated temperature range, as expected. Interestingly, the structure-related parameters of the kinetic expressions of the two gasification reactions were the same, suggesting that CO2 and H2O are likely to attack the same carbon surface sites on this char. © 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

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