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Modeling Homogeneous Combustion in Bubbling Beds Burning Liquid Fuels

[+] Author Affiliations
Tiziano Faravelli, Alessio Frassoldati, Eliseo Ranzi

Politecnico di Milano, Milano, MI, Italy

Francesco Miccio

Istituto di Ricerche sulla Combustione - CNR, Napoli, NA, Italy

Michele Miccio

Università di Salerno, Fisciano, SA, Italy

Paper No. FBC2003-133, pp. 533-540; 8 pages
doi:10.1115/FBC2003-133
From:
  • 17th International Conference on Fluidized Bed Combustion
  • 17th International Conference on Fluidized Bed Combustion
  • Jacksonville, Florida, USA, May 18–21, 2003
  • Conference Sponsors: Advanced Energy Systems
  • ISBN: 0-7918-3680-0 | eISBN: 0-7918-3675-4
  • Copyright © 2003 by ASME

abstract

This paper presents a first implementation of a model for the description of homogeneous combustion of different fuels in fluidized bed combustors (FBC) at temperatures lower than the classical value for solid fuels, i.e. 850°C. Model construction is based on a key feature of the bubbling fluidized bed: a fuel-rich (endogenous) bubble is generated at the fuel injection point, travels inside the bed at constant pressure and undergoes chemical conversion in presence of mass transfer with the emulsion phase and of coalescence with air (exogenous) bubbles formed at the distributor and, possibly, with other endogenous bubbles. The model couples a fluid-dynamic sub-model based on the two phases theory of fluidization with a sub-model of gas phase oxidation. To this end, model development takes full advantage of a detailed chemical kinetics scheme, which includes both the low and high temperature mechanisms of hydrocarbon oxidation and accounts for about 200 molecular and radical species involved in more than 5000 reactions. Simple hypotheses are made to set-up and close mass balances of the various species as well as enthalpy balances in the bed. First, conversion and oxidation of gaseous fuels (e.g. methane) have been calculated as a test case for the model; then, n-dodecane has been taken into consideration to simply represent a diesel fuel by means of a pure hydrocarbon. Model predictions qualitatively agree with some evidences coming from experimental data reported in the literature. The fate of hydrocarbon species is extremely sensitive to temperature changes and oxygen availability in the rising bubble. A preliminary model validation has been attempted against the results of experiments carried out on a pre-pilot, bubbling combustor fired with underbed injection of a diesel fuel. In particular, model results confirm the trends that the heat release either in the bed or in the freeboard experimentally shows as a function of bed temperature. At lower emulsion phase temperatures many combustible species leave unburned the bed, post-combustion occurs past the bed and freeboard temperature considerably increases; as it is well known, this is an undesirable feature from the viewpoints of practical application and emission control.

Copyright © 2003 by ASME
Topics: Combustion , Fuels , Modeling

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