Page 259 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

8.3 Thermochemical Conversion of Fuels 235

x

CH x O y þð1 yÞH 2 O ! y þ 1 H 2 þ CO ðChar gasification) ð8:12Þ
2
If we consider the formula of char containing C, H, O, N, S, and mineral matters,
the chemical reactions are much more complicated. With the presence of steam, the
sulfur content usually is converted into H 2 S. Part of the H 2 S can react with CO 2 to
produce COS.

H 2 S þ CO 2 ! COS þ H 2 O ð8:13Þ

The primary gasification products are synthesis gas also known as syngas, which
is composed of CO, H 2 ,CH 4 , and many others. This syngas has to be cleaned
before H 2 ,CH 4 , and/or CO can be separated as clean fuels.

8.3.2.2 Gasifiers
A gasifier is the main chamber where all the main gasification chemical reactions
take place. Various types of gasifiers have been developed over the last centuries.
Conventional gasifiers (Fig. 8.1) include
• fixed bed gasifiers,
• fluidized bed gasifiers, and
• entrained flow gasifiers.
These gasifiers are widely used in integrated gasification and combined cycle
(IGCC) plants. Most of the coal is gasified in fixed bed gasifiers [36, 42]. Updraft
and downdraft moving bed gasifiers are suitable for smaller scale gasification,
where fluidized bed gasifiers are often used for solid feedstocks other than coal. The
entrained bed gasifiers are most suitable for coal. All these gasifiers have been well
commercialized by different companies. Each has its features aiming at different
applications. They are only briefly introduced as follows in order to focus on the
main scope of this book.
In entrained bed gasifiers, feedstock particles concurrently react with oxidants in
suspended (i.e., entrained) fluid flow mode. Entrained bed gasifiers require pul-
verized feedstocks. The sizes of solid fuel particles have to be less than 1 mm for
effective suspension. The gasification temperature may exceed 1,500 °C and the
residence time is in the order of 1 s only. The units are usually operated at high
pressure (2.94–3.43 MPa). With this high gasification temperature, the syngas
stream is almost free of tars, oils, and phenols. On the other hand, the corresponding
raw syngas exiting the gasifier usually requires significant cooling before it can be
handled by the downstream gas cleaning units. Such a high temperature also
requires expensive burners and sophisticated high-temperature heat exchangers to
cool the syngas.
In a fixed bed gasifier, preheated feedstock is fed from the top of the gasifier. The
feedstock falls through different zones before it reaches the grates. From top to