Design of Biomass Gasifiers - Biomass Gasification, Pyrolysis and Torrefaction - page 302

Magnesite (MgO) was successfully used in the first biomass-based inte-
grated gasification combined cycle plant in V¨rnamo, Sweden (St˚hl et al.,
2001).
Tar is a mixture of higher-molecular-weight (higher than benzene) chemi-
cal compounds that condense on downstream metal surfaces at lower tem-
peratures. It can plug the passage and/or make the gas unsuitable for use.
The bed materials, besides serving as a heat carrier, can catalyze the gasifica-
tion reaction by increasing the gas yield and reducing the tar. Bed materials
that act as a catalyst for tar reduction are an attractive option. Some are
listed here (Pfeifer et al., 2005; Ross et al., 2005):

Olivine
Activated clay (commercial)
Acidified bentonite
Raw bentonite
House brick clay

Common house brick clay can be effectively used in a CFB gasifier to
reduce tar emission and enhance hydrogen production. The alkalis deposited
on the bed materials from biomass may potentially behave as catalysts if
their agglomerating effect can be managed (Ross et al., 2005).
Tar production can be reduced using olivine. The Fe content of oliv-
ine is catalytically active, and that helps with tar reforming (Hofbauer,
2002). Nickel-impregnated olivine gives even better tar reduction
because nickel is active for the steam tar reforming reaction (Pfeifer
et al., 2005).
Bingyan et al. (1994) reported using ash from the fuel itself (sawmill
dust) as the bed material in a CFB gasifier. This riser is reportedly operated
at a very low velocity of 1.4 m/s, which is 3.5 times the terminal velocity of
the biomass particles. Chen et al. (2005) tried to operate a 1-MWe CFB gas-
ifier with rice husk alone, but the system had difficulty with fluidization in
the loop seal because of the low sphericity of the husk ash; however, the
main riser reportedly operated in the fast bed regime without major

difficulty.

8.11 DESIGN OPTIMIZATION
Design optimization generally starts after the preliminary design is complete
and actual project execution is set to begin. It has two aspects: (i) process
and (ii) engineering.
Process optimization tells the designer if the preliminary design will give
the best performance in terms of efficiency and gas yield, and how this is
related to the operation and design parameters. Commercial simulation pro-
grams (mathematical models) or computational fluid dynamics codes are the