| Summary: | This work is devoted to the experimental study of biomass gasification in a pilot-scale
circulating fluidized bed, and development of an equilibrium model of the process based on
Gibbs free-energy minimization. Biomass gasification has considerable potential for reducing
greenhouse gas emissions. In the present study, six types of sawdust were gasified in a pilotscale
air-blown circulating fluidized bed gasifier to produce low-calorific-value gases.
The pilot gasifier employs a riser 6.5 m high and 0.1 m in diameter, a high-temperature
cyclone for solids recycle and a ceramic fibre filter unit for gas cleaning. The riser temperature
was maintained at 970-1120 K (700-850°C), while the sawdust feed rate varied from 16-45 kg/h,
corresponding to a superficial gas velocity of 4-10 m/s. It was found that gas composition and
heating value depended heavily on the air or O/C ratio, and to a lesser extent on operating
temperature. The higher heating value of the product gas decreased from 5.6 to 2.1 MJ/Nm³ as
the stoichiometric air ratio increased from 0.22 to 0.54. The gas heating value was increased by
increasing the overall suspension density in the riser. Fly ash re-injection and steam injection led
to increases in gas heating value for the same Q/C molar ratio.
Tar yield from biomass gasification was found to decrease drastically from 15 to 0.54
g/Nm³ as the average suspension temperature increased from 970 to 1090 K. Elevating the
operating temperature provides the simplest solution for tar removal in the absence of any
catalyst. Secondary air had only a very limited effect on tar removal with the total air ratio
maintained constant. A nickel-based, catalyst proved to be effective in reducing the tar yield and
in adjusting the gas composition. The cold gas efficiency decreased with increasing air ratio (or O/C molar ratio), though the
carbon conversion increased. The cold gas efficiency provides a better criterion for evaluating
the gasification process than the carbon conversion. Experimental data showed that the
gasification efficiency can be maximized within an optimum range of air ratio (a = 0.30-0.35, or
O/C = 1.5-1.7), while keeping the tar yield acceptably low.
A non-stoichiometric equilibrium model based on Gibbs free energy minimization was
developed for biomass gasification. Five elements (C, H, O, N and S) and 44 species were
considered in the model. Both pure equilibrium and situations where kinetic factors cause a
partial approach to equilibrium are considered. The equilibrium model predicts that the product
gas composition from gasification of woody biomass (e.g. sawdust) depends primarily on the air
ratio. An air ratio of 0.2-0.3 is predicted to be most favourable for producing CO-rich gas, while
temperatures of 1200-1400 K and an air ratio of 0.15-0.25 are predicted to be optimum for H₂
production. The predicted cold gas efficiency reached a maximum at an air ratio of about 0.25.
The model successfully predicts the onset of carbon formation in a C-H-O-dominated system
when the relative abundance of carbon exceeds a certain level. When a system is C-saturated, the
gas composition is insensitive to the elemental abundance of carbon in the total feed streams.
The equilibrium model successfully predicts the limiting behaviour of the system with
changes in different operating parameters and provides an in-depth understanding of the
underlying thermodynamic principles governing biomass gasification. The model was modified
to take non-equilibrium factors into account. The modified model successfully predicts product
gas compositions, heating value, gas yield and cold gas efficiency in good qualitative agreement
with the experimental data.
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