Biomedical Engineering Reference
In-Depth Information
TABLE 8.3
Comparison of Hydrodynamic Operating Conditions of a
Commercial Transport Gasifier and CFB of Fluid Catalyst Cracking Units
Reactor type
Transport
(Smith et al.,
2002)
CFB (Petersen
and Werther,
2005)
Fluid catalytic
cracker (Zhu and
Venderbosch, 2005)
Parameter
Particle size (
μ
m)
200
350
180
230
20
150
Riser velocity (m/s)
12
18
3.5
5.0
6
28
Circulation rate (kg/m 2 s)
9.2 a
730
3400
2.5
400
1200
Riser temperature ( C)
910
1010
800
900
500
550
Riser pressure (bar)
140
270 psig 1 bar
150
300 kPa
Operation as
KBR gasifier
CFB gasifier
FCC cracker
a Computed from comparable units. KBR - Kellon, Brown and Root
circulation rates, velocities, and riser densities considerably higher than those
of a conventional CFB. This results in higher throughput, better mixing, and
higher mass and heat-transfer rates. The fuel particles are also very fine
(Basu, 2006) and as such it requires a pulverizer or a hammer mill. A com-
parison of typical hydrodynamic operating conditions in a transport gasifier,
CFB, and fluid catalytic cracking unit is given in Table 8.3 .
A transport gasifier consists of a mixing zone, a riser, a gas
solid separa-
tor, a standpipe, and a J-leg. Coal, sorbent (for sulfur capture), and air are
injected into the reactor's mixing zone. The gas
solid disengager removes
the larger carried-over particles, and the separated solids return to the mixing
section through the J-valve located at the base of the standpipe ( Figure 8.11 ).
Most of the remaining finer particles are removed by a cyclone located
downstream of the disengager from which the gas exits the reactor. The reac-
tor can use either air or oxygen as the gasification medium.
Use of oxygen as the gasifying medium avoids nitrogen, the diluting
agent in the product gas. For this purpose, air is more suitable for power gen-
eration, while oxygen is more suitable for chemicals production. The trans-
port gasifier has proved to be effective for gasification of coal, but it is yet
to be proven for biomass.
8.3.2.2 Twin Reactor System
One of the major problems in air gasification of coal or biomass is the dilution
of its product gas by the nitrogen in the air. This air is essential for the
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