Steam gasification of biomass for biomethanol production: Model development and analysis

ABSTRACT

Circulating fluidized bed (CFB) gasifiers are advantageous for transforming carbon-based fuels, particularly biomass materials, into product gas, due to a good mixing between solids and gas which improves heat and mass transfer characteristics. In this paper, an Aspen Plus model of integrated biomass gasification–methanol synthesis system was developed to study the influence of working conditions on methanol yield. To enhance the accuracy of the model presented, two FORTRAN subroutines (bed hydrodynamics and reaction rates) were coupled to the computer-based model. Results showed that the presence of steam increases methanol yield, as by increasing the S/B from 0.2 to 1.0 (wt/wt), the methanol yield increases from 2.31 to 9.95 (g/kg biomass) for T_g = 700°C, from 4.02 to 12.01 (g/kg biomass) for T_g = 800°C, from 6.02 to 14.01 (g/kg biomass) for T_g = 900°C. It was also found that the biomass particle size has a minor influence on the methanol yield.

KEYWORDS:

• BPS
• CFB
• gasification
• methanol
• S/B

Nomenclature

a	=	decay constant (dimensionless)
Ar	=	Archimedes number (dimensionless)
B	=	volume fraction occupied by bubbles
dp	=	particle diameter (mm)
G	=	acceleration due to gravity (m/s2)
∆P	=	pressure changes (atm)
U	=	superficial gas velocity (m/s)
Umf	=	minimum fluidization velocity (m/s)
z	=	axial distance from the distributor (m)
zd	=	height of the dense bed in the riser (m)

Greek symbols

	=	volume fraction of bed occupied by bubble
	=	average voidage of bed
	=	cross-sectional average solids holdup
	=	cross-sectional average solids holdup at dense region
	=	cross-sectional average solids holdup at dilute region
	=	gas viscosity (Pa s)
	=	gas density (kg/m3)
	=	particle density (kg/m3)