Biomass gasification involves pyrolysis and partial oxidation in a well-controlled oxidizing envi­ronment. It leads to products, such as H2, CO, CO2, H2O, and hydrocarbon species. The heat required for biomass drying, heating and pyrolysis is provided by the partial oxidation of biomass. Gasification is deemed as the most promising technology for producing renewable and carbon — free energy, as it provides tremendous flexibility with regards to feedstock and the fuels produced. In general, the gasification process converts low value biomass to a gaseous mixture containing syngas (mixture of H2 and CO) and varying amounts of CH4, and CO2. It can also produce hydrocarbons, particularly in the lower temperature range. The oxidizing agents can be pure O2, air, steam, CO2 or their mixtures. The syngas composition can be varied by using air and steam as the gasification agent (Rapagna, 2000). Moreover, the presence of CO2 can be used to increase H2 and CO contents, as it transforms char, tar and CH4 into H2 and/or CO in the presence of a catalyst such as Ni/Al (Ollero, 2003). Table 2.1 from (Jones et al., 2003; Giles, 2003) lists the representative compositions and properties of syngas used in various Integrated Gasification Combined Cycle (IGCC) facilities. As indicated, syngas has a wide composition range due to a large variety of source materials and processing techniques.

Numerous studies have been reported in recent years, dealing with the type of reactors used for gasification, thermo-chemical processes involved, and various gaseous and liquid fuels produced during gasification. Wang et al. (2008) and Gill et al. (2000) provide reviews of work on biomass gasification. As discussed in these reviews, significant advances have been reported in biomass gasification technology and syngas utilization. The syngas can be used to generate heat and power, for example, in an IGCC facility (Rodrigues et al., 2003), produce H2 (Watanabe, 2002), and synthesize other chemicals and liquid fuels such as F-T fuels (Tijmensen, 2002). Gill etal. (2000) summarize the various routes for the utilization of syngas, including the production of F-T and other transportation fuels. As discussed by Gill et al. (2000), the global reactions associated with syngas formation from biomass (CH„) include:

Reaction (2.1) corresponds to syngas formation in the presence of O2, while reaction (2.2) is the well-known water-gas-shift-reaction and reaction (2.3) is associated with the steam reforming of methane. Reactions (2.2) and (2.3) are used to control the H2/CO ratio. The production of F-T fuels from syngas involves a series of reactions in the presence of a catalyst. The global reactions for this process can be written as:

nCO + (2n + 1)H2 ^ CnH2n+2 + nH2O (Paraffins) (2.4)

nCO + 2nH2 ^ CnH2n + nH2O (Olefins) (2.5)

The first step during F-T formation is the conversion of syngas into -CH2- alkyl radicals and H2O. The — CH2- alkyl radicals then combine in a catalyst reaction to produce synthetic paraffin and olefin hydrocarbon (HC) fuels of various chain lengths. The amount and type of fuels formed are determined by parameters such as temperature, pressure, H2/CO ratio, and the type of catalyst. In general, F-T fuels can be produced from a variety of solid, liquid, and gaseous sources, and further processed to yield clean transportation fuels with desired specifications. Gill et al. (2000) provide an overview of technologies, including Biomass-to-Liquid (BTL) and Coal-to-Liquid (CTL) Gas-to-Liquid (GTL) processes, for producing various fuels through gasification and F-T processes.

Regardless of feedstock or process, F-T fuels have a number of desirable properties. For example, F-T diesel fuels can be produced with a high cetane number, with ultra-low sulfur and aromatic content, with the consequence of improved engine performance, significantly lower particulate mass (PM) emissions and favorable NOX/PM trade-off. However, these fuels generally have poor lubricity and lower volumetric energy density. These shortcomings can be alleviated by blending these fuels with petro-fuels. Thus, the biomass gasification can be used to produce syngas and subsequently clean drop-in transportation fuels. The effects of F-T fuel properties on engine performance and emissions have been reported by a number of investigations (Abu-Jrai et al., 2006; Schaberg et al, 2005; Wu et al., 2007). Gill et al. (2000) illustrate the improved HC/NOX tradeoff achieved with advanced injection timing using the GTL fuel compared to petro-diesel and rapeseed methyl ester (RME) biodiesel fuels.