There are two basic procedures for transforming solid biomass into liquid or gaseous biofuels. The first is to transform it by microbiological fermentation (Gavrilescu & Chisti,

2005) (i. e., to convert the polysaccharides into alcohols such as bioethanol or biobutanol) or to convert raw plant biomass by anaerobic fermentation into biomethane (Demirbas & Balat,

2006) . One of the main drawbacks of the anaerobic system is that it is slow (because of the small amount of energy that is available to the organisms). Therefore, the amount of methane that can be produced is limited and this technology is only sustainable under selected scenarios (Asam et al., 2011). However, the introduction of even a small installation for transforming agricultural and human wastes into methane can have an enormous effect on the living standards of small communities (Arthur et al., 2011; Parker, 2002).

The second procedure aims to thermochemically convert the total biomass into a synthesis gas of high calorific value (also called syngas, i. e., H2 + CO) with subsequent production of various liquid and gaseous fuels (Tijmensen et al., 2002). The production of syngas is a potential area for large-scale CO2 conversion and utilization. The reforming of CO2 to CH4 has been extensively studied and reported on in the literature (Song & Pan, 2004). The catalytic reduction of CO2 to form methanol (or even CH4) using renewable energy sources could become a viable alternative to scarce or expensive fossil resources.

Biodiesel from plant oils and bioalcohol from sugar use only a portion of the total biomass. Next-generation processes are being developed to convert biomass to syngas (Baker & Keisler, 2011; Fagernas et al., 2010) that can then be converted into fuels or chemicals by a synthetic process (the so-called Fischer-Tropsch, or FT, process).

CH0.8 + 0.7O2 ^ CO + 0.4H2O (1)

CH0.8 + H2O ^ CO + 1.4H2 (2)

CO + H2O ^ CO2 + H2 (3)

(2n + 1)H2 + nCO ^ CnH2n+2 + nH2O (4)

Considering that coal inputs supply a 0.8 to 1 ratio of H/C, the whole FT process can be briefly written as follows. The partial oxidation of coal by oxygen gives equation (1). The interaction of water with carbon monoxide through "steam reforming" produces equation (2). Subsequently, the H/CO ratio is improved by "shifting" (transferring) the oxygen from the molecular water to CO, producing an additional hydrogen and carbon dioxide following equation (3). After removing the sulfur and carbon dioxide contaminants, the syngas is reacted over a catalyst in the FT reactor to produce high-quality clean fuels following the formula (4) (Greyvenstein et al., 2008).

Biomass is more reactive than coal and (depending on the technology) is usually gasified at temperatures of between 550 °C and 1,500 °C and at pressures varying between 4 and 30 bars (Damartzis & Zabaniotou 2011; Leibold et al., 2008; Steinberg, 2006). Typically, biomass is burned in an electrically heated furnace consisting of several multiple-tube units that can be heated separately up to 1,350 °C (Theis et al., 2006). Alternatively, the conversion of fossil fuel or biomass can be performed in hydrogen plasma. The temperature induced by an electric arc in hydrogen plasma is very high (~1,500 °C); therefore, this technology produces hydrogen and CO gas with a conversion rate of near 100% (Steinberg, 2006). FT synthesis generates intermediate products for synthetic fuels (Liu et al., 2007). The thermal efficiency of producing electricity and hydrogen through hydrogen plasma and carbon fuel cells varies from 87% to 92%, depending on the type of fuel and the biomass feedstock. This is more than twice as efficient as a conventional steam plant that burns coal and generates power with a ~38% efficiency. In addition, coupling hydrogen plasma and carbon fuel-cell technologies allows for a 75% reduction in CO2 emission per unit of electricity (Steinberg, 2006).

Because FT produces predominantly linear hydrocarbon chains, this process is currently attracting considerable interest. FT has already been applied on a commercial scale by Sasol, Petro S. A. and Shell, mainly to produce transportation fuels and chemicals (the feedstock being coal or natural gas). This fuel option has several notable advantages. First, the FT process can produce hydrocarbons of different lengths (typically <C15, Liu et al., 2007) from any carbonaceous feedstocks; these hydrocarbons can then be refined to easily transportable liquid fuels. Secondly, because of their functional similarities to conventional refinery products, the synthetic fuels (synfuels) produced by the FT process (i) can be handled by existing transportation systems; (ii) can be stored in refueling infrastructure for petroleum products; (ii) are largely compatible with current vehicles; and (iv) can be blended with current petroleum fuels (Tijmensen et al., 2002). Thirdly, synfuels are of high quality (this is especially true for FT diesel), have a very high cetane number and are free of sulfur, nitrogen, aromatics, and other contaminants typically found in petroleum products. The principal drawbacks of the FT process are that the capital cost of FT-conversion plants is relatively high and that the energy efficiency for the production of FT liquids by conventional techniques is lower than the energy efficiency for the production of alternative fuels (Takeshita & Yamaji, 2008).