273
Fuel Production by Supercritical Water
Li et al. [116] investigated coal gasification in the temperature range of
650°C–800°C and pressure 23–27 MPa, K 2 CO 3 and Raney Ni as catalysts, and H 2 O 2
as oxidant. Most experiments were performed with inlet slurry containing 16.5 wt%
coal and 1.5 wt% CMC (carboxy methyl cellulose). The results showed that high
temperature favors the gasification of coal in SCW, whereas pressure has a little
effect on the gasification results. An optimum flow rate needs to be found to get the
best results. Both gasification and carbon gasification efficiencies were improved by
the catalysts; K 2 CO 3 performed better than Raney Ni. Less char and tar were formed
in the presence of catalysts. An increase in feed concentration decreased the hydrogen and gasification efficiencies. SCW desulfurizes the coal and the solid particles
remained had less carbon and hydrogen than original coal. The data of Li et al. [116]
indicate that for the entire range they studied, 90% of the gas-phase concentration
was for hydrogen (60%) and carbon dioxide (30%).
Vostrikov et al. [84] examined coal gasification in the temperature range of
500°C–750°C, pressure of 30 MPa, and reaction time of 60–720 s with and without
CO 2 . Once again, the main gaseous products were CH 4 , CO, CO 2 , and H 2 . Within
the range of operating conditions examined, best carbon conversion was obtained at
750°C. The results show a significant temperature dependence on product compositions for temperatures below 650°C. BTx (benzene, toluene, and xylene), methane,
and carbon dioxide were the main products below 650°C–700°C. Similar results
were obtained by Cheng et al. [83] who studied gasification of lignite coals in the
temperature range of 350°C–550°C and reaction time of 0–60 min in N 2 atmosphere.
These data along with the data described earlier clearly indicate that product distribution during coal gasification in SCW will depend on the nature of coal along with
all the operating parameters.
Battelle Pacific Northwest National Laboratory demonstrated that various alkali
carbonate and Ni catalysts can convert wet biomass to methane-rich gas at temperatures between 400°C and 450°C and pressure as high as 34.5 MPa. Yu et al. [120]
found that glucose at low concentration (0.1 M) can be completely gasified in 20 s
at 600°C and 34.5 MPa, with major products being hydrogen and carbon dioxide.
Higher concentration of glucose, however, reduces the product concentration
of hydrogen and carbon dioxide and increases the concentration of methane. xu
et al. [129] showed that a wide range of carbons effectively catalyze the gasification
of glucose in SCW at 600°C and 34.5 MPa pressure, with nearly 100% carbon gasification efficiency. The available surface area of carbon did not affect the effectiveness
of the catalyst. For concentrated organic feeds in water, in the presence of a catalyst, the temperature above 600°C is needed to achieve high gasification efficiencies.
Mass transfer resistances at high concentration (if any) can affect the equilibrium
of water–gas shift reaction. In the presence of coconut shell, activated carbon, cellobiose, and various whole biomass feeds, as well as depithed bagasse liquid extract
and sewage sludge were completely gasified. There was some deactivation of carbon
catalyst after 4–6 h of operation.
Demirbas [2,6,131] examined the decomposition of olive husk, cotton cocoon
shell, and tea waste by water under both sub- and supercritical conditions. He also
observed an increase in hydrogen production with temperature, particularly for temperatures higher than the supercritical temperature. Demirbas [131] observed that as
Fuel Production by Supercritical Water
Li et al. [116] investigated coal gasification in the temperature range of
650°C–800°C and pressure 23–27 MPa, K 2 CO 3 and Raney Ni as catalysts, and H 2 O 2
as oxidant. Most experiments were performed with inlet slurry containing 16.5 wt%
coal and 1.5 wt% CMC (carboxy methyl cellulose). The results showed that high
temperature favors the gasification of coal in SCW, whereas pressure has a little
effect on the gasification results. An optimum flow rate needs to be found to get the
best results. Both gasification and carbon gasification efficiencies were improved by
the catalysts; K 2 CO 3 performed better than Raney Ni. Less char and tar were formed
in the presence of catalysts. An increase in feed concentration decreased the hydrogen and gasification efficiencies. SCW desulfurizes the coal and the solid particles
remained had less carbon and hydrogen than original coal. The data of Li et al. [116]
indicate that for the entire range they studied, 90% of the gas-phase concentration
was for hydrogen (60%) and carbon dioxide (30%).
Vostrikov et al. [84] examined coal gasification in the temperature range of
500°C–750°C, pressure of 30 MPa, and reaction time of 60–720 s with and without
CO 2 . Once again, the main gaseous products were CH 4 , CO, CO 2 , and H 2 . Within
the range of operating conditions examined, best carbon conversion was obtained at
750°C. The results show a significant temperature dependence on product compositions for temperatures below 650°C. BTx (benzene, toluene, and xylene), methane,
and carbon dioxide were the main products below 650°C–700°C. Similar results
were obtained by Cheng et al. [83] who studied gasification of lignite coals in the
temperature range of 350°C–550°C and reaction time of 0–60 min in N 2 atmosphere.
These data along with the data described earlier clearly indicate that product distribution during coal gasification in SCW will depend on the nature of coal along with
all the operating parameters.
Battelle Pacific Northwest National Laboratory demonstrated that various alkali
carbonate and Ni catalysts can convert wet biomass to methane-rich gas at temperatures between 400°C and 450°C and pressure as high as 34.5 MPa. Yu et al. [120]
found that glucose at low concentration (0.1 M) can be completely gasified in 20 s
at 600°C and 34.5 MPa, with major products being hydrogen and carbon dioxide.
Higher concentration of glucose, however, reduces the product concentration
of hydrogen and carbon dioxide and increases the concentration of methane. xu
et al. [129] showed that a wide range of carbons effectively catalyze the gasification
of glucose in SCW at 600°C and 34.5 MPa pressure, with nearly 100% carbon gasification efficiency. The available surface area of carbon did not affect the effectiveness
of the catalyst. For concentrated organic feeds in water, in the presence of a catalyst, the temperature above 600°C is needed to achieve high gasification efficiencies.
Mass transfer resistances at high concentration (if any) can affect the equilibrium
of water–gas shift reaction. In the presence of coconut shell, activated carbon, cellobiose, and various whole biomass feeds, as well as depithed bagasse liquid extract
and sewage sludge were completely gasified. There was some deactivation of carbon
catalyst after 4–6 h of operation.
Demirbas [2,6,131] examined the decomposition of olive husk, cotton cocoon
shell, and tea waste by water under both sub- and supercritical conditions. He also
observed an increase in hydrogen production with temperature, particularly for temperatures higher than the supercritical temperature. Demirbas [131] observed that as