The methods to remove tars from product gas are in principle thermal, by for example by partial oxidation or direct thermal exposure or catalytic. In these processes, tars decompose to form additional product gas. Tar destruction can be accomplished thermally only at above about 1200°C or with catalysts at moderate temperatures of 750-900°C. These approaches have the potential to increase conversion efficiencies while simultaneously eliminate the need for collection and disposal of tars. The catalytic cracking of tars can be very effective, up to >99%.

6.4.2.2.1 Thermal processes for tar destruction

While catalysts facilitate tar destruction at intermediate temperatures, tars can also be cracked thermally at higher temperatures, typically above 1000°C. The minimum temperature required for efficient tar destruction is not well characterized; it depends on the types of tars formed in the gasifier. Thus, thermal destruction of the oxygenated tars from updraft gasifiers might be treated at lower temperatures than the refractory ones from high temperature reactors. Apart from economical and material problems, thermal decomposition at high temperatures can lead to soot formation, which can be even more troublesome than the aromatics. The difficulties of achieving complete thermal cracking, in parallel with operational and economic considerations, often make thermal cracking less attractive in large-scale gasification systems.

6.4.2.2.2 Catalytic processes for tar destruction

This approach has the potential advantage that tars can be rapidly destroyed as they are formed or just thereafter, eliminating downstream problems. The tars are cracked to smaller molecules on the catalyst surface, and the mechanisms of tar destruction are reasonably well documented. The ideal catalyst may be characterized as follows: effective in the removal of tars, resistant to deactivation, easily regenerated, strong and inexpensive. Additionally, they may be capable of reforming methane thus providing a suitable syngas ratio (if the desired product is syngas) (Sutton, 2001).

Tar decomposition catalysts used in biomass conversion can be divided into two different types depending where in process the catalyst performs; primary and secondary catalysts. The primary catalysts are added directly into the biomass prior to gasification whilst the secondary catalysts are placed in a secondary reactor downstream from the gasifier.

The primary catalysts are added directly to the biomass prior to its gasification. The addition can be done either by wet impregnation of the biomass material or by dry mixing of the catalyst. These catalysts may catalyze the gasification reactions, but their main purpose is to reduce the tar content of the product gas. They operate at the same conditions as the gasifier, they are usually non-renewable and consist of low-cost disposable materials. Normally, they have little effect on the methane and C2-3 hydrocarbon conversion. It has been noticed that the primary catalyst alternative tend to be more problematic with respect catalyst deactivation, erosion and elutriation. Therefore, more emphasis has been put on studying and investigating the separate secondary catalytic bed alternatives.

The secondary catalysts are placed in a secondary reactor downstream the gasifier. Since the catalytic bed operates independently of the gasifier, these catalysts can operate under different process conditions than those of the gasification unit. This type of catalyst is also active in hydrocarbon reforming and often in methane reforming.

Additional advantages with this alternative are that the pyrolysis reactions do not continue in the catalytic reactor and that it is easier to arrange for a good catalyst-gas contact in a separate catalytic reactor. Particles and other impurities may also be removed before catalyst exposure

Figure 6.12.

and the temperature and other process conditions in the catalyst reactor may as mentioned, be controlled separately from those of the gasifier.

Several different materials have been investigated and classified as potential catalyst. Fig­ure 6.12 illustrates the different types of catalytic materials (Abu El-Rub, 2004). The catalytic materials most comprehensively studied are dolomites, both as primary and secondary catalysts, nickel-based, mainly as secondary catalyst and alkali metals, mainly as primary catalyst.