Fluidization of forest biomass-sand mixtures: experimental evaluation of minimum fluidization velocity and CFD modeling

Abstract

An experimental fluid-dynamic study of sand-forest biomass mixture fluidization was conducted. Different proportions of biomass/sand were tested. Sawdust from the Abra Ancha sawmill, located in the town of Aluminé, Argentina, was adopted as the forest biomass material. In parallel, computational fluid dynamics (CFD) simulations of the fluidization process of these mixtures were performed to obtain exhaustive knowledge of their fluid-dynamic behavior. As a result, sand incorporation, even at low concentrations, reduces the cohesion and bonding forces between biomass particles and decreases the segregation and preferential channel formation in sawdust, thus improving fluidization. Satisfactory qualitative results were obtained in relation to the agitation state, bed expansion, and formation of bubbles in the different fluidized mixtures analyzed. The occurrence of two different fluidization regimes was confirmed in both the biomass and biomass-sand mixtures. This phenomenon allows the definition of the incipient ($U_{if}$) and complete ($U_{cf}$) fluidization velocities. The potential of ANSYS-Fluent software as a simulation tool of sand-sawdust mixture fluidization was verified, and a suitable simulation methodology for predicting the minimum fluidization velocity was developed. This study constitutes the first and most indispensable stage to achieve complete simulation, including chemical reactions, of a sawdust fluidized bed gasification reactor.

Keywords:

• Fluidized bed
• minimum fluidization velocity
• binary mixture
• biomass
• sawdust

Nomenclature

Ar	=	$d_p^3{\rho }_g\left({\rho }_s-{\rho }_g\right)g/{\mu }^2,$ Archimedes number
$d_p$	=	particle diameter (mm)
g	=	acceleration of gravity (m s−2)
$U_{cf}$	=	complete fluidization velocity (m s−1)
$U_f,U_{pk}$	=	minimum fluidization velocity of fluid and packed components in single component fluidized beds, in Chiba et al.’s correlation (Table 11) (m.s−1)
$U_{if}$	=	incipient fluidization velocity (m s−1)
$U_{la},U_{sm}$	=	minimum fluidization velocity velocity of larger and smaller component in single component fluidized bed, in Rowe and Nienow’s correlation (Table 11) (m.s−1)
$U_{mf}$	=	minimum fluidization velocity (m s−1)
$U_s$	=	superficial gas velocity (m s−1)
Re	=	$d_{p}U{\rho }_g/\mu ,$ Reynolds number
$x$	=	mass fraction
$x_f$	=	mass fraction of fluid component, in Chiba et al.’s correlation
$x_{la}$	=	mass fraction of larger component, in Rowe and Nienow’s correlation

Greek letters

φ	=	sphericity
ρ	=	density (kg m−3)
µ	=	viscosity of the fluidizing gas (Pa s)
$\Delta P$	=	pressure drop (Pa)

Subscripts

bm	=	biomass (sawdust)
g	=	gas
in	=	inert material (sand)
mf	=	minimum fluidization

Acknowledgments

The authors wish to thank the support of the following Argentine institutions: ANPCyT – MINCyT (PICT No. 2014-2078), University of Comahue, Argentina, PIN 0222 and PIN 0223. Florencia Toschi had a Doctoral Fellowship from CONICET. Lic. Lucas Cavaliere (PROBIEN, CONICET-UNCo) is acknowledged for his technical assistance. Germán Mazza is a research member of CONICET, Argentina.

Additional information

Funding

This is supported by Universidad Nacional del Comahue [PIN 0222,PIN 0223] and ANPCyT – MINCyT, Argentina [PICT No. 2014-2078].