Value of biohydrogen as a fuel resource

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Despite its clean and green nature, most hydrogen is currently produced primarily from non-renewable sources, such as natural gas, oil and coal. Fig. 23.4 shows the ratio of sources used in worldwide production of hydrogen (US DOE Website, 2004a). Hydrogen production is 500 billion nm3/year and electrolysis, the only non-fossil fuel source, comprises only 4% of total hydrogen production at 20 billion nm3/year. Unfortunately, hydrogen production via electrolysis and thermal decomposition of water is more costly than methods that utilize fossil fuels. One commonly used method is steam reforming of natural gas (US DOE Website, 2004b). Natural gas is a finite resource, so new sources of hydrogen need to be found.

Biological production of hydrogen, using microorganisms, is an exciting new area of technology that offers the potential production of usable hydrogen from a variety of renewable resources. Table 23.5 shows a comparison of the energy costs of different hydrogen generation processes including biological as compared with alcohol and gasoline processes (Das and Veziroglu, 2001). This comparison indicates that hydrogen production from coal is the cheapest process. Fermentative hydrogen production, which has a conversion efficiency of 10%, is economically less attractive when compared with conventional fuels or hydrogen from coal. However, costs

Fig. 23.4 Sources used in the worldwide production of hydrogen. Table 23.5 Unit cost of energy obtained by different processes

□ Electrolysis

Fig. 23.4 Sources used in the worldwide production of hydrogen. Table 23.5 Unit cost of energy obtained by different processes

Type of energy

Unit cost of energy content of fuel (US$/MJ)

Hydrogen from fermentation Hydrogen from coal Hydrogen from electrolysis Hydrogen from thermal decomposition

of natural gas

Ethanol Gasoline

6.30

can be lowered if the conversion efficiency is increased. For example, if the fermentative hydrogen conversion ratio from sucrose is 75%, hydrogen yields correspond to 6.0 mol/mol sucrose. Assuming an overall fuel cell efficiency of 90%, Gibbs free energy of hydrogen as 56.7 kcal/mol and the lower heating values (LHVs) of hydrogen and sucrose as 58.3 and 1234 kcal/ mol, respectively, the following adjusted energy costs can be calculated (Kumar and Das, 2000; Das and Veziroglu, 2001):

Energy recovery from substrate = (LHV of H2 x H2 yield)/LHV of sucrose = (58.3 x 6.0)/1234 = 28.3%

Final conversion efficiency = Gibbs free energy for H2 x H2 yield x efficiency of fuel cell/LHV of sucrose = (56.7 x 6.0 x 0.90)/1234 = 24.8%

Based on this approximate energy cost calculation, fermentative anaerobic hydrogen production looks attractive. The anaerobic fermentation producing hydrogen is actually more cost-effective than indicated, and has considerable potential as an environmentally friendly process, since the process utilizes waste materials of low or negative value.

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