The main alternative energy carriers considered for transportation are electricity and hydrogen. With interest in its practical applications dating back almost 200 years, hydrogen energy is hardly a novel idea. Iceland and Brazil are the only nations where renewable — energy feedstocks are envisioned as the major or sole future source of hydrogen (Solomon & Banerjee, 2006). Fuel-cell vehicles (FCVs) powered by hydrogen are seen by many analysts as an urgent need and as the only viable alternative for the future of transportation (Cropper et al., 2004).

Unlike crude oil or natural gas, reserves of molecular H2 do not exist on earth. Therefore, H2 must be considered more as an energy carrier (like electricity) than as an energy source (Song, 2006). H2 can be derived from existing fuels such as natural gas, methanol or gasoline; however, the best long-term solution is to produce H2 from water by (for example) using heat from solar sources and O2 from the atmosphere.

Today, hydrogen is mainly manufactured by decarbonizing fossil fuels, but in the future it will be possible to produce hydrogen by alternative methods such as water photolysis using semiconductors (Khaselev & Turner, 1998) or by ocean thermal-energy conversion (Avery,

2002) . Such methods are still in the research and development stage and are not yet ready for industrial application.

Hydrogen production from biomass requires multiple reaction steps. The reformation of fuels is followed by two steps in the water-gas shift reaction, a final carbon monoxide purification step and carbon dioxide removal.

Biomass can be thermally processed through gasification or pyrolysis. The main gaseous products resulting from the biomass are expressed by equations (6), (7) and (8) (Kikuchi, 2006).

pyrolysis of biomass — H2 + CO2 + CO + hydrocarbon gases (6)

catalytic steam reforming of biomass — H2 + CO2 + CO (7)

gasification of biomass — H2 + CO2 + CO + N2 (8)

Hydrogen from organic wastes has generally been produced through equations (9), (10) and (11).

In the long run, the methods used for hydrogen production are expected to be specific to the locality. They are expected to include steam reforming of methane and electrolysis when hydropower is available (such as in Brazil, Canada and Scandinavia) (Gummer & Head,

2003) . When hydrogen will become a very common energy source, it will likely be distributed through pipelines. Existing systems, such as the regional H2-distribution network that has been operated for more than 50 years in Germany and the intercontinental liquid-hydrogen transport chain, demonstrate that leak rates of <0.1% can be achieved in industrial applications (Schultz et al., 2003). However, a major threat associated with the hydrogen paradigm is the fact that it is the smallest atom and that leakage is apparently unavoidable. One has to face the possibility that a significant amount of H2 will be released into the stratosphere. Hydrogen is expected to react with ozone following the reaction H2+O3 ^ H2O+O2. This mechanism (reviewed by Kikuchi, 2006) is a potentially dangerous promoter of ozone depletion. Alternatively, hydrogen can be produced from another fuel (e. g., ethanol, biodiesel, gasoline, or synfuel) via onboard reformers (hydrogen fuel processors). This is probably the best solution because synfuel can be produced from local feedstocks through the Fischer-Tropsch process, transported and distributed through existing technologies and infrastructures (Agrawal et al., 2007; Takeshita & Yamaji, 2008). This consideration also applies to biofuels. In addition, the feasibility of cars with onboard reformers has already been proven. The importance of synfuel is expected to increase rapidly because growing reserves of natural gas (or "stranded" gas) are available in remote locations and are considered to be too small for liquefied natural gas (LNG) or pipeline projects.

The biological generation of hydrogen (or biohydrogen) provides a wide range of approaches for generating hydrogen, including direct biophotolysis, indirect biophotolysis, photo-fermentation and dark-fermentation (Lin et al., 2010). Biological hydrogen production processes are found to be more environmentally friendly and less energy intensive as compared to thermochemical and electrochemical processes. There are three types of microorganisms that produce hydrogen, namely cyanobacteria, anaerobic bacteria, and fermentative bacteria (Demirbas, 2008a).

Photosynthetic production of H2 from water is a biological process that can convert sunlight into useful, stored chemical energy. Hydrogen production is a property of many phototrophic organisms and the list of H2 producers includes several hundred species from different genera of both prokaryotes and eukaryotes. The enzyme-mediating H2 production seen in green algae is effected by a reversible hydrogenase that can catalyze ferredoxin oxidation in the absence of ATP (Beer et al., 2009). The enzyme is sensitive to oxidation; however, tolerant allozymes are being selected (Seibert et al., 2001). Hydrogen production has also been obtained from glucose using NADP+-dependent enzymes, glucose-6 phosphate dehydrogenase (G6PDH), 6-phosphogluconate dehydrogenase (6PGDH) and hydrogenase (Heyer & Woodward, 2001).

Carbon monoxide (CO) can be metabolized by a number of naturally occurring microorganisms along with water to produce H2 and CO2 following equation (12), which is the "water-gas shift" reaction, at ambient temperatures.

CO + H2O ^ CO2 + H2 (12)

The biological water-gas shift reaction has been used in the processing of syngas from biomass with the bacterium Rubrivivax gelatinosus (Wolfrum & Watt, 2001).

Nitrogenases can produce hydrogen but require relatively high energy consumption. However, the nitrogenase reaction is essentially irreversible, which allows for hydrogen pressurization. Rhodopseudomonas palustris can drive the nitrogenase reaction using light (Wall, 2004).