Biohydrogen Generation and Other Methods - Hydrogen Generation, Storage, and Utilization - page 60

where x and y are the elemental molar ratios of H/C and O/C in biomass,
respectively. The reaction product is syngas, whose quality depends on x and
y . The reaction (Eq. 4.9) is an endothermic. It is known from the reaction
(Eq. 4.9) that water is not only the solvent but also a reactant, and the hydro-
gen in the water is released by the gasification reaction [57].
Compared with other biomass thermochemical gasifications, such as air
gasification or steam gasification, the SCW gasification has high gasification
efficiency at lower temperature and can deal directly with wet biomass
without drying [58]. Hydrogen production by biomass gasification in SCW
is a promising technology for utilizing high moisture content biomass.
Another advantage of SCW reforming is that the H 2 is produced at a high
pressure, which can be stored directly, thus avoiding the large energy expen-
ditures associated with its compression. The cost of hydrogen production
from SCW gasification of wet biomass was several times higher than the
current price of hydrogen from steam methane reforming. Biomass is gas-
ified in supercritical water at a series of temperatures and pressures during
different resident times to form a product gas composed of H 2 , CO 2 , CO,
CH 4 , and a small amount of C 2 H 4 and C 2 H 6 [36, 59]. SCW is a promising
reforming media for the direct production of hydrogen at 875-1075 K tem-
peratures with a short reaction time (2-6 seconds). As the temperature is
increased from 875 to 1075 K, the H 2 yield increases from 53% to 73% by
volume, respectively [59, 60]. Only a small amount of hydrogen is formed
at low temperatures, indicating that direct reformation reaction of ethanol as
a model compound in SCW is favored at high temperatures (>975 K) [59].
With an increase in the temperature, the hydrogen and carbon dioxide yields
increase, while the methane yield decreases.
4.3.4 Comparison of Thermochemical Processes
In general, the gasification temperature is higher than that of pyrolysis and
the yield of hydrogen from the gasification is higher than that of the pyroly-
sis. The yield of hydrogen from conventional pyrolysis of corncob increases
from 33% to 40% with increasing of temperature from 775 to 1025 K. The
yields of hydrogen from steam gasification increase from 29% to 45% for
(W/S) = 1 and from 29% to 47% for (W/S) = 2 with increasing of tempera-
ture from 975 to 1225 K [45]. Hydrogen yields and energy contents, com-
pared with biomass energy contents obtained from processes with biomass,
are shown in Table 4.6 [61].
Demirbas investigated the yield of hydrogen from supercritical fluid
extraction (SFE), pyrolysis, and steam gasification of wheat straw and olive
waste at different temperatures [45]. The highest yields (% dry and ash free