Tars in the product gas can be tolerated in some systems where the gas is used as a fuel in applications, such as burners. However, in most of applications, tars in the raw product gases, even at low concentrations, can create major handling and disposal problems. Two basic approaches have been used to remove tars from product gas streams: (1) physical removal technologies similar to those used for particulate removal, such as wet scrubbers, ESP; and 2) catalytic and thermal tar-reduction methods where tars are converted to permanent gases. The catalytic approaches can potentially destroy tars in either vaporized or condensed state. The second approach is discussed below.

10.5.2.1 Thermal Cracking

By increasing the gasifier temperature all organic compounds will crack to smaller hydrocarbons (~ 1,200 °C). Oxygen or air can be added to the gasifier to allow partial combustion of the tar to raise its temperature. Using electrical arc plasma for tar cracking is another option. It is a simple technique but it produces gas with lower energy content.

10.5.2.2 Catalytic Cracking

This technique can be applied in the gasifier or in a secondary reactor. The gasifier is commercially used in many plants for the removal of undesired elements from product gas. This method is explained in more detail in Sect. 4. For most syngas applications, the optimization of operating conditions, catalytic gasifier material or additives combined with secondary hot gas cleaning (catalytic cracking) are the most preferred methods.