9 ORGANIC IMPURITIES
Organic compounds can only be present in product gas generated gasification processes operated at temperatures below 1000 ̊C, such as in fixed or fluidized gasification processes. The main organic compound is methane. Other organic compounds are the aliphatic C2+ hydrocarbon higher than methane, with the most important representative being ethylene (C2H4), the mono aromatic compounds benzene and toluene, and “tars”. At the high temperatures of entrained flow gasification all organic material is completely destructed and converted into H2 and CO (with CO2).
Methane, the C2+ hydrocarbons and benzene are not considered as impurities in most product gas applications. Complete removal of the tars or at least the larger part of them, however, is necessary. A lot of research and development is ongoing on tar removal and a good overview is given in reference [21], therefore, only the main concepts are discussed here:
High Temperature Destruction
Upon increasing the temperature of the raw product gas by oxygen injection all organic compounds are ‘cracked’ and destructed. The effect is similar to direct gasification at a high temperature (i.e. an entrained flow gasifier can be used as tar cracker). As the methane, C2+ hydrocarbons, and benzene are destructed, which is unnecessary or even undesired, this approach is not preferred.
Catalytic Destruction
Catalytic tar reduction is typically performed in the raw gas after gasifier at elevated temperatures (400-900 ̊C) in most cases with nickel-based catalysts. With this method a large part of the tars can be destructed, while the methane and benzene remain intact for the major part. Major disadvantage of this concept is the sensitivity of the catalysts for (inorganic) impurities in the raw product gas.
Physical Removal
The most promising tar removal concepts are based on physical removal with organic washing liquids. In the FICFB plant in Gusssing biodiesel is used to clean the product gas to meet gas engine specifications (tar dewpointof ̴40 ̊C) [8]. ECN has developed the OLGA process that is capable of removing all aromatic organic components to very low levels (i.e. tar dewpoints below 15 ̊C). Additional advantage is that the tars are removed prior to water condensation to prevent pollution of process water and related costs for wastewater treatment.