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Water for Energy and Fuel Production
temperature increased from 600°C to 800°C, hydrogen production increased from
53 to 73 vol% in reaction time of 2–6 s. She indicated that hydrogen productions can
be obtained from biomass such as bio-nutshell, olive husk, tea waste, crop straw,
black liquor, MSW, crop grain residue, pulp and paper waste, petroleum-based plastic waste, and manure slurry.
An extensive amount of work on SCW gasification of organic wastes has been
reported in the literature [129,130,139]. The studies have shown that the gasification generally produces hydrogen and carbon dioxide mixture with simultaneous
decontamination of wastes, particularly at higher temperatures. The homogeneous
solution of waste and water makes it easy to pump to the high-pressure reactor without pretreatment. Guo et al. [130] presented an excellent review of SCW gasification
of biomass and organic wastes. They as well as Lu et al. [133] showed the equilibrium effects of temperature, pressure, and feed concentration of wood sawdust on
hydrogen, carbon dioxide, carbon monoxide, and methane concentrations in SCW.
The typical effects of temperature on product gas composition are illustrated in
Table 10.2. The data showed that equilibrium favors the productions of hydrogen
and carbon dioxide at high temperatures. The study also showed that an increase in
pressure significantly decreased the product concentration of carbon monoxide and
slightly decreased the product concentration of the hydrogen. The pressure change
had very little effect on the product concentrations of carbon dioxide and methane.
The complex effect of pressure on the product distribution was believed to be due
to the complex interplay between hydrolysis and water–gas shift reactions. Besides
temperature and pressure, other parameters that affected the gas yield were feedstock concentration, oxidant, reaction time, feedstock composition, inorganic impurities in the feedstock, and biomass particle size. Guo et al. [131] also concluded that
alkali such as NaOH, KOH, Na 2 CO 3 , K 2 CO 3 , and Ca (OH) 2 ; activated carbon; metal
oxides and metals such as noble metal catalysts (Ru/alpha-alumina > Ru/carbon > Rh/
carbon  >  Pt/alpha-alumina, Pd/carbon, Pd/alpha-alumina); as well as Ni catalysts
and metal oxides such as CeO 2 particles, nano-CeO 2 , and nano-(CeZr) x O 2 enhanced
taBle 10.2
equilibrium Gas yield for 5 wt% sawdust in sCW at 25 mPa Pressure
temperature (°C)
Gas yield (mol/kg)
Carbon
methane/
hydrogen
dioxide
methane
Carbon monoxide
hydrogen
400
13
24
20
10 −3
1.54
500
40
31
10
2.5 × 10 −3
0.25
600
80
40
~1
3.1 × 10 −3
0.0125
700
89
43
0
1.2 × 10 −3
0.0
800
89
43
0
0.5 × 10 −3
0.0
Source: Guo, L., Cao, C., and Lu, Y., “Supercritical water gasification of biomass and organic wastes,” in
Momba, M. and Bux, F. (eds.), Biomass, 165–182, 2010. With permission.
Note: These data are the best estimates from the graphical data presented.