Carbon dioxide removal in indirect gasification - SGC

SGC Rapport 2013:277
circulating fluidized bed gasifiers where the bed material exits the gasifier with the
gas stream; the fluid bed material is then separated from the gas stream and
brought back into the gasifier. As mentioned above, indirect gasification can be of
any kind of gasifier but is usually a variation on the fluid bed theme, Figure 6 show
the two types of fluidized bed gasifiers used.
Figure 6. The different categories of fluidized bed gasification, bubbling bed to the
left and circulating fluidized bed to the right, adapted from [18].
In the bubbling bed gasifier, the fuel is fed into or just above the bed and the oxidant
(oxygen, air, steam) is introduced from below with speeds of about 1-3 m/s
through the bed, resulting in bubbles up through the bed. The most important factor
is the velocity of the oxidant, which decides the size and speed of the bubbles
and in turn the heat transfer and mixing in the system. The gas exiting the top of
the gasifier is cleaned from sand and ash using a cyclone filter. The operating
temperature is in the 650ºC to 950ºC range.
In circulating fluidized bed gasifiers a higher gas velocity is used, 5-10 m/s,
which suspends particles in the entire reactor and pulls parts of the sand and
charcoal out of the reactor with the exiting gas stream. A cyclone filter separates
and returns the particles to the bed; the gas is removed from the top of the cyclone.
The temperature is usually in the 800ºC to 1,000ºC range. Both types of
fluidized bed gasifiers are operated at both atmospheric pressure and under
higher pressures, 35 bar (bubbling bed) and 20 bar (circulating bed) [18].
Fluid beds allow higher gas throughput than fixed bed gasifiers, with the highest
gas throughput for the circulating bed. This result in good mixing, optimized kinetics,
good gas/particle contact, high heat transfer rates and long residence times.
These factors help in obtaining a high conversion of charcoal and a high yield.
Aside from that, the tar content of the exiting gas is approximately 10 g/Nm 3 , which
is considered low, but not as low as in downdraft fixed bed gasification and entrained
flow gasification. The use of a bed material also gives the opportunity to
introduce catalysts into the bed to e.g. lower the tar content of the gas. The disadvantage
of this kind of gasifiers is the higher amount of particulate matter in the
gas, there is also a risk of bed agglomeration due to the high alkaline content of
the biomass [18].
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