Production of Synthetic Fuels and Chemicals from Biomass - Biomass Gasification, Pyrolysis and Torrefaction - page 378

11.2.4 Gasification for Syngas Production
The two main routes for production of syngas from biomass or fossil fuel are
low-temperature 1000 C)

( and high-temperature gasification

B,

1200 C).
Low-temperature gasification is typically carried out at temperatures
below 1000 C. In most low-temperature gasifiers, the gasifying medium is
air, which introduces undesired nitrogen in the gas. To avoid this, gasifica-
tion can be carried out indirectly by one of the following means:

(

B.

An oxygen carrier (metal oxide) is used to transfer the oxygen from an
air oxidizer to another reactor, where gasification takes place using the
oxygen from the metal oxide.

A certain amount of combustion in air is allowed in one reactor and heat-
carrier solids carry the combustion heat to a second reactor, where the
heat is used for endothermic gasification.

Dilution of the product gas by nitrogen is avoided by using steam or oxy-
gen instead of using air as the gasifying medium.

Low-temperature gasification produces a number of heavier hydrocarbons
along with carbon monoxide and hydrogen. These heavier hydrocarbons are
further cracked, separated, and used for other applications.
High-temperature gasification is carried out at temperatures above
1200 C, where biomass is converted mainly into hydrogen and carbon mon-
oxide. Primary gasification is often followed by the shift reaction

( Eq. (11.3) ) to adjust the hydrogen-to-carbon monoxide ratio to suit the

downstream application.

11.2.4.1 Shift Reaction
For a reaction like FTS that produces various gaseous and liquid hydrocarbons, a
definite molar ratio of CO and H 2 in the syngas is necessary. This is adjusted
through the shift reaction that converts excess carbon monoxide into hydrogen:

Catalyst

CO H 2 O ! CO 2 1 H 2 2 41 : 1kJ = mol (11.3)

1

The above reaction can be carried out either at higher temperatures
(400

400 C). At high temperatures,
the shift reaction is often catalyzed using oxides of iron and chromium; it is
equilibrium limited. At low temperatures, the shift reaction is kinetically lim-
ited; the catalyst is composed of copper, zinc oxide, and alumina, which help
reduce the CO concentration down to about 1%.

500 C) or at lower temperatures (200

11.2.5 Cleaning and Conditioning of Syngas
Whatever the gasification process is, the product gas must be cleaned before
it is used for synthesis reactions. Special attention must be paid to clean the