Page 262 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

238 8 Pre-combustion Air Emission Control

The exact syngas properties depend on the type feedstock, gasifier, operating
pressure, temperature, and residence time. In general, higher temperature leads to
greater carbon conversion. However, overheated feedstock may result in ash fusion
and/or ash agglomeration [52]. Most of the commercial gasifiers operate at elevated
pressures (*2.94 MPa) [18] for the equilibrium consideration. However, the
pressure does not alter the syngas composition very much.
Particulate removal
Particulate cleaning of the hot syngas is necessary not only to reduce air emissions,
but also to prevent corrosion and erosion of downstream gas separation compo-
nents. For all gasifiers, char materials along with ash can be removed by water spray
(quenching) followed by carbon scrubber. Around 95 % of the char carbon can be
removed by direct water spray. The residual carbon is handled in the following wet
scrubbers. Because of the cooling of the syngas, the thermal efficiency is greatly
reduced in the entire process if the syngas will be used immediately, like in an
IGCC process.
An alternative approach is hot gas filtration. Particulate filtration at temperatures
above 260 °C is called hot gas filtration; sometimes it can reach 900 °C[23]. This
high temperature demands special filtration materials that can endure the high
temperature as well as the acidic gases in the raw syngas. The filter housing is also
expected to be stable against temperature, pressure, and chemical composition of
gas and dust.
Common materials in hot gas filtration are ceramic and metallic. These materials
allow rigid self-supporting filter elements that can be employed at high tempera-
tures due to their high mechanical strength. The filters can be shaped like candles,
as long as a few meters, or honeycomb structure. High costs and system failure due
to filter clogging are the main challenges to hot gas filtration.

8.3.2.4 Acidic Gas Removal and Sulfur Recovery

Gas separation follows particulate removal. Engineering designs are based on the
principles introduced in Chap. 5, most commonly by adsorption or absorption.
There are many options for CO 2 separation from the syngas too. CO 2 capture and
storage will be introduced in detail in Chap. 12.
H 2 S separation can be achieved by both absorption and adsorption. Physical or
chemical adsorption followed by conventional Claus sulfur recovery units has been
proven successful in petroleum industry. ZnO/CuO, Cr 2 O 3 , and Al 2 O 3 can adsorb
H 2 S components. The simplified adsorption and adsorbent regeneration reactions
are as follows, using ZnO as an example [48].

ZnO þ H 2 S ! ZnS þ H 2 O ðAdsorption, 315 530 CÞ ð8:14Þ

3

ZnS þ O 2 ! ZnO þ SO 2 ðRegeneration, 590 680 CÞ ð8:15Þ
2