Biohydrogen Generation and Other Methods - Hydrogen Generation, Storage, and Utilization - page 59

was strongly influenced. The content of H 2 and CO 2 increased, while that of
CO decreased. A drastic reduction in the content of organic compounds could
also be observed. Because the char yields remained almost constant com-
pared to an equivalent no catalytic thermal run, the increase in the content
of hydrogen was probably due to the influence of the catalyst on the water
gas shift reaction. Dolomite, Ni-based catalysts and alkaline metal oxides
are widely used as gasification catalysts [22] The yields of hydrogen from
biomass with the use of dolomite in the fluidized-bed gasifier and the use of
nickel-based catalysts in the fixed bed reactor downstream from the gasifier
were investigated by Lv et al. [51]. They obtained a maximum hydrogen
yield (130.28 g H 2 ·kg -1 biomass) over the temperature range of 925-1125 K.
K 2 CO 3 catalyst shows a destructive effect on the organic compounds, and H 2
and CO 2 form at the end of the catalytic steam reforming process [52]. The
catalytic steam gasification of biomass in a lab-scale fixed bed reactor was
carried out in order to evaluate the effects of particle size at different bed
temperatures on the gasification performance [53]. With decreasing particle
size, the dry gas yield, carbon conversion efficiency and H 2 yield increased,
and the content of char and tar decreased.

4.3.3 Hydrogen from Biomass via Supercritical Water
(Fluid-Gas) Extraction

The supercritical fluid extraction (SFE) is a separation technology that uses
supercritical fluid solvent. Fluids cannot be liquefied above the critical tem-
perature, regardless of the pressure applied, but may reach the density close
to the liquid state. Every fluid is characterized by a critical point, which is
defined in terms of the critical temperature and critical pressure. Water is a
supercritical fluid above 647.2 K and 22.1 MPa [54, 55].
Supercritical water (SCW) possesses properties very different from those
of liquid water. The dielectric constant of SCW is much lower, and the
number of hydrogen bonds is much lower and their strength is weaker. As a
result, high temperature water behaves like many organic solvents so that
organic compounds have complete miscibility with SCW. Moreover, gases
are also miscible in SCW, thus a SCW reaction environment provides an
opportunity to conduct chemistry in a single fluid phase that would otherwise
occur in a multiphase system under conventional conditions [56].
The biomass gasification in SCW is a complex process, but the overall
chemical conversion can be represented by the simplified net reaction:

CH O + − ( 2 y ) H O CO → + − + ( 2 y x / ) 2 H ,

2

(4.9)

x y 2 2