The two main routes for production of syngas from biomass or fossil fuel are low-temperature (~<1000 °C) and high-temperature gasification (~>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, gasification can be carried out indirectly by one of the following means:

• 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 combustion reaction in air is carried out in one reactor and heat-carrier solids carry the heat to a second reactor, where this heat is then used in gasification.

• Dilution of the product gas by nitrogen is avoided by the use of steam or oxygen 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 monoxide. Primary gasification is often followed by the shift reaction, as described in the next section, to adjust the hydrogen-to-carbon monoxide ratio to suit the downstream application.

In any case, the product gas must be cleaned before it is used for synthesis reactions. Special attention must be paid to clean the syngas of tar and other

catalyst-poisoning elements before it is used for Fischer-Tropsch synthesis, which uses iron — or cobalt-based catalysts.

Shift Reaction

For a reaction like Fischer-Tropsch synthesis that produces various gaseous and liquid hydrocarbons, a definite molar ratio of CO and H2 in the syngas is neces­sary. This is done through the shift reaction that converts excess carbon mon­oxide into hydrogen:

The reaction can be carried out either at higher temperatures (400-500 °C) or at lower temperatures (200-400 °C). For 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 limited; the catalyst is composed of copper, zinc oxide, and alumina, which help reduce the CO con­centration down to about 1%.