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8.3 Thermochemical Conversion of Fuels 237

cleaning units. The temperature of the syngas leaving the top is about 400–500 °C.
The feedstock particle size is normally in the order of tens millimeters and the
residence time is about 15–30 min.
In a fluidized bed gasifier, both the feedstock and the oxidants can enter from the
bottom of the gasifier. And they travel upward together. As a result of effective
mixing, the temperature within the air-based gasifier is relatively uniform and it is
in the range of 900–1,050 °C to minimize ash melting. In order to maintain the
fluidity of the particles, the feedstock particles are smaller than those in a fixed bed
gasifier. It is in the range of 0.5–5 mm. And the residence time of feedstock in the
gasifier is typically 10–100 s, which is much shorter than a fixed bed gasifier [19].
Right at the exit of a fluidized bed gasifier, an optional cyclone is usually used to
recycle the large unspent feedstock. Large particles will be recycled for further
gasification. Smaller ones penetrating through the cyclone become the particulate
matter in the syngas stream. As we learned in Sect. 6.4, a cyclone is effective in
separating particles of a few micrometers. Therefore, particulate matter in the
syngas stream is primarily in the micron range or smaller. Most of these particles
can be separated using electrostatic precipitators or filters.

8.3.2.3 Syngas Cleaning and Separation

Raw syngas compounds depend on the feedstock and the gasifier. Among various
carbonaceous feedstocks, coal, petroleum coke, and petroleum residues have been
used for gasification. Recently, biomass has been tested too as a feedstock.
A sample raw syngas compounds produced from coal, pet coke, and petroleum
residues are listed in Table 8.2. Most raw syngases contain considerable amount of
CO, H 2 ,CO 2 ,H 2 S and COS, and particulate matter. CO 2 ,H 2 S and COS are also
collectively called acidic gases because they can be converted easily into acids with
moisture. CO 2 is also considered as a greenhouse gas.

Table 8.2 Pre-combustion
Gas Mole fraction (%)
syngas after water gas shift
reaction H 2 55.5
CO 2 37.7
N 2 3.9
CO 1.7
H 2 O 0.14
H 2 S 0.4
Others 0.66
Source Ramdin et al. [40]