Unsaturated Hydrocarbons
The existence of unsaturated hydrocarbons in the raw synthesis gas varies very widely. In the Lurgi dry-bottom process, there will in general be large quantities of aromatics and other unsaturates in the volatiles and tars, though the exact amount also will depend heavily on the coal. For biomass gasification the presence of tars in the gas is also a problem. For high-temperature entrained-flow processes, including oil gasification, the presence of any hydrocarbon other than methane, whether saturated or unsaturated, is minimal.
Removal of hydrocarbons from the product gas from a Lurgi dry-bottom gasifier can be integrated into the design of a Rectisol wash. Kriebel (1989) provides a description of this.
The effect of unsaturated hydrocarbons entering a liquid nitrogen wash together with nitrogen oxides is described in Section 6.9.2.
Care should be taken with oil gasification using naphtha as a medium for soot extraction. Aromatic components in the naphtha dissolve in the water and can be introduced into the gas via the return water to the gas scrubber.
Unsaturated hydrocarbons will be hydrogenated when the gas is treated by a CoMo raw gas shift catalyst (BASF undated).