Environmental Engineering Reference
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molecular-weight hydrocarbons rich in benzene, toluene and xylene). The presence of tar in
the producer gas is undesirable not only because it is an indicator of low gasification
efficiency, but also due to the fact that it increases the difficulty of syngas cleanup by fouling
and plugging the pipes and tubes of some equipment (Rezaiyan and Cheremisinoff, 2005). It
is thus necessary to develop technical approaches to eliminate tar. Tar removal can be
achieved either by primary method taking place insider the gasifier or by secondary
treatments outside the gasifier (e.g., hot gas cleanup). As a primary method, choosing the
proper configuration of a gasifier can reduce tar formation. Brandt and Larsen (2000)
produced a significantly low tar formation by employing a novel two-stage gasifier composed
of a pyrolysis unit and a gasification unit with a charcoal bed. Nunes et al. (2007) also
observed a reduction in tar formation when the producer gas went through a second-stage bed
packed with char in a downdraft fixed-bed gasifier. Addition of catalysts, such as char,
alkali/alkaline earth metal-based catalysts (e.g. Na, K, and Ca) and transitional metal-based
catalyst (e.g., Ni and Fe), proved to be an effective means to reduce tar formation by
converting tar into combustible gases through steam reforming, CO 2 reforming, thermal
cracking, hydro-reforming/hydro-cracking, and water-gas reactions (Kimura et al, 2006). As
the secondary method for tar removal, hot gas cleanup has attracted increasing attention in
recent years due to the development of integrated gasification combined cycle (IGCC) and
integrated gasification fuel cell (IGFC) technologies. Hot gas cleanup, i.e., catalytic
destruction of tarry products and NH 3 (a contaminant species in the producer gas) at a high
temperature is needed to further increase the overall power generation efficiency of IGCC and
IGFC. The most common catalysts for the decomposition of tar and NH 3 are dolomite (a
calcium magnesium ore, CaMg(CO 3 ) 2 ) and Ni-, Mo-, or Ru-based catalysts (Dayton, 2002),
as well as inexpensive Fe catalysts (such as chars from low rank coals with inherent Fe and
Ca cations and limonite iron ore) (Ohtsuka et al., 2004; Xu et al., 2005).
Recent developments in biomass gasification including some non-conventional
gasification processes are under investigation, such as plasma gasification and supercritical
water gasification. Plasma gasification is a gasification process that decomposes biomass into
basic components, such as hydrogen, carbon monoxide, and carbon dioxide in an oxygen-
starved environment, with the assistance of a plasma torch heating the biomass feedstock to a
temperature of 3000°C or higher (Rezaiyan and Cheremisinoff, 2005). This plasma technique
has high destruction and reduction efficiencies, and it has great application potential for a
wide range of hazardous waste treatment, including both organic and inorganic compounds.
Recently, supercritical water (SCW), highly compressed water at above its critical
temperature of 374°C and critical pressure of 22 MPa, has been chosen as an ideal
gasification medium for biomass or lignocellulosic waste conversion primarily because of its
special properties such as liquid-like density and gas-like diffusivity. More importantly, SCW
has strong solubility for organic compounds, accompanied with its high reactivity (Akiya and
Savage, 2002). A wide range of biomass including some model compounds and
lignocellulosic wastes have been successfully gasified in supercritical water (Xu and Antal,
1998; Yoshida et al., 2004; Hao et al, 2005; Williams and Onwudili, 2006). Compared to
conventional gasification processes, supercritical water gasification (SCWG) has higher
gasification efficiency, hydrogen yield and less tar formation (Xu and Antal, 1998). In
addition, supercritical water gasification can utilize the wet biomass and wastes directly,
eliminating the energy- and capital-intensive drying process. Thus, SCWG is particularly
suitable for gasifying biomass with high moisture content and some waste streams such as
sewage sludge (Xu and Antal, 1998). A recent study by Izumizaki et al. (2005) summarized
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