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Steam Gasification and Reforming Technologies
have higher temperatures and are more suitable for high-rank coals. The flow rate in
fluidized bed reactor is higher than that in fixed-bed reactor. The conversion per pass is
usually low due to elutriation of carbonaceous materials. The mixing in the reactor is
high giving more uniform temperature. Fluidized bed is most useful for raw materials
such as biomass which form highly corrosive ash that can damage the walls of slagging
gasifiers. The fluidized bed reactor is generally operated under “bubbling fluidized bed”
conditions. A modeling and experimental validation of biomass–steam gasification in
the bubbling fluidized bed reactor is given by Gopalakrishnan [82]. His analysis indicated that for steam gasification of biomass, an increase in temperature in such a reactor
increases the production of hydrogen and carbon monoxide and decreases the production of carbon dioxide and methane. An increase in steam-to-biomass ratio increases
the production of carbon dioxide, decreases the production of hydrogen and carbon
monoxide, and has no effect on the production of methane.
4.6.1.2.2 CFB Reactor
One way to improve conversion in fluidized bed reactor is to recycle solids back into
the reactor. In this type of reactor, the solids coming out of reactor are separated
from gas and recycled back into the reactor. This reactor thus provides more flexibility on the solids residence time within the reactor. The solids recycling also provides
better solids mixing and uniform temperature distribution.
A variation of single CFB was examined by Matsuoka et al. [83] who examined
a circulating dual bubbling fluidized bed system. In this system, two bubbling fluidized beds were used as a gasifier and a combustor. The gasifier and combustor had
identical inner diameters (80 mm), and the static bed heights of the bed material in
the gasifier and combustor were 270 and 150 mm, respectively. The inner diameter
of the riser was 18 mm and its height from the top of the gasifier to the cyclone was
about 1800 mm. Porous γ-alumina particles with a diameter of 75–150 μm were used
as refractory materials. The system was used to treat sawdust at temperatures ranging from 773 to 1073 K. The data were obtained at different steam-to-biomass ratios
and different set of residence times. The system was found to be very efficient, and
higher carbon conversion and hydrogen yield were achieved in this system compared
to those obtained in conventional CFB.
Another variation of the dual fluidized bed steam gasification process was developed by Pfeifer et al. [81] at the Vienna University of Technology. This system is graphically depicted in Figure 4.5 later in the chapter. In this process, heat for the gasification
reactor is provided by circulating bed material. This system was a further development
of the so-called fast internally CFB (FICFB) technology [75–83,178–181,187]. In this
technology, biomass enters a bubbling fluidized bed gasifier in which drying, thermal
degasification, and partially heterogeneous char gasification take place at temperatures
of about 850°C–900°C. Residual biomass char leaves the gasifier together with the
bed material through an inclined, steam fluidized chute toward the combustion reactor. The combustion reactor serves for heating up the bed material and is designed
as highly expanded fluidized bed (riser). Air is used as the fluidization agent in the
riser. The circulating rate can be adjusted easily by changing the amount of primary
and secondary air in the combustion chamber. After particle separation from the flue
gas in a cyclone, the hot bed material flows back to the gasifier via a loop seal. The