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RESEARCH PAPERS

Modeling Homogeneous Combustion in Bubbling Beds Burning Liquid Fuels

[+] Author and Article Information
Tiziano Faravelli

 Dipartimento di Chimica Industriale ed Ingegneria, Chimica “Giulio Natta,” Politecnico di Milano, Piazza Leonardo da Vinci, 32 20133 Milano MI, Italytiziano.faravelli@polimi.it

Alessio Frassoldati

 Dipartimento di Chimica Industriale ed Ingegneria, Chimica “Giulio Natta,” Politecnico di Milano, Piazza Leonardo da Vinci, 32 20133 Milano MI, Italytiziano.faravelli@polimi.it

Eliseo Ranzi

 Dipartimento di Chimica Industriale ed Ingegneria, Chimica “Giulio Natta,” Politecnico di Milano, Piazza Leonardo da Vinci, 32 20133 Milano MI, Italytiziano.faravelli@polimi.it

Miccio Francesco

 Istituto di Ricerche sulla Combustione–CNR, Via Metastasio, 17, 80125 Napoli NA, Italymiccio@irc.na.cnr.it

Miccio Michele1

Dipartimento di Ingegneria Chimica ed Alimentare, Università di Salerno, Via Ponte don Melillo, 84084 Fisciano SA, Italymmiccio@unisa.it

1

Corresponding author.

J. Energy Resour. Technol 129(1), 33-41 (Feb 21, 2006) (9 pages) doi:10.1115/1.2424957 History: Received August 16, 2004; Revised February 21, 2006

This paper introduces a model for the description of the homogeneous combustion of various fuels in fluidized bed combustors (FBC) at temperatures lower than the classical value for solid fuels, i.e., 850°C. The model construction is based on a key bubbling fluidized bed feature: A fuel-rich (endogenous) bubble is generated at the fuel injection point, travels inside the bed at constant pressure, and undergoes chemical conversion in the 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 submodel based on two-phase fluidization theory with a submodel of gas phase oxidation. To this end, the model development takes full advantage of a detailed chemical kinetic 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 for the various species as well as enthalpy balances in the bed. First, the conversion and oxidation of gaseous fuels (e.g., methane) were calculated as a test case for the model; then, n-dodecane was taken into consideration to give a simple representation of diesel fuel using a pure hydrocarbon. The model predictions qualitatively agree with some of the evidence from the experimental data reported in the literature. The fate of hydrocarbon species is extremely sensitive to temperature change and oxygen availability in the rising bubble. A preliminary model validation was attempted with results of experiments carried out on a prepilot, bubbling combustor fired by underbed injection of a diesel fuel. Specifically, the model results confirm that heat release both in the bed and in the freeboard is a function of bed temperature. At lower emulsion phase temperatures many combustible species leave the bed unburned, while post-combustion occurs after the bed and freeboard temperature considerably increases. This is a well-recognized undesirable feature from the viewpoint of practical application and emission control.

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Copyright © 2007 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematic representation of (a) the bubbling bed; (b) the emulsion phase control volume with mass transfer fluxes

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Figure 2

Theoretical temperature and main composition profiles of the fuel bubble rising in the reactor. Bubble initial conditions: Tin=500K, db,in=3.10−2m, 14% nC12H26 in air; Φin=14, U−Umf=0.5m∕s. (a) Temperature evolution. Dashed line refers to the emulsion phase. (Te=1000K) (b) Time evolution of selected species at Φout=2.

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Figure 3

Effect of the emulsion temperature (Te) on the bubble temperature for premixed combustion of three different fuels in stoichiometric conditions

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Figure 4

Theoretical temperature and main composition profiles of the fuel bubble rising in the reactor. Tin=500K, db,in=3.10−2m, 14% nC12H26 in air, Φin=14, Φout=2, U−Umf=0.5m∕s. (a) Temperature evolution. Dashed line refers to the emulsion phase (Te=900K). (b) Evolution of selected species. (Te=900K)(c) Temperature evolution. Dashed line refers to the emulsion phase (Te=1200K). (d) Evolution of selected species. (Te=1200K).

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Figure 5

Cumulative distribution of the heat release between the bed and the freeboard as a function of the emulsion temperature. db,in=2cm; Tin=500K; Φin=2.5; Φout=1; U−Umf=0.5m∕s.

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Figure 6

Temperature increase ΔT in the freeboard as a function of the emulsion temperature. (a) Experimental ΔT in the freeboard of FBC370 pre-pilot facility (8): ▴ nozzle size=2mm; ∎ nozzle size=4mm. (b) Trends of calculated (adiabatic) and experimental ΔT: db,in=2cm; Tin=500K; Φin=2.5; Φout=1; U−Umf=0.5m∕s.

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