After leaving the gasifier the product gas has to be cleaned (purified) and upgraded (conditioned).

Purification eliminates substances negatively affecting downstream processes. Especially catalysts are very sensitiv to small amounts of sulfur, halogens, nitrogen components, metals, dust and organic compounds. Particles can be removed from the gas by using cyclones, barrier filters, electrostatic precipitators or scrubbers. Tars or hydrocarbons (methane, ethylene, benzene and toluene) can be removed for example at temperatures above 1000°C or with the help of catalysts. In entrained flow gasifierers all organic compounds are thermally destructed during the process. Inorganic components lice sulfur can be removed by wet scrubbing (absorption by physical, chemical or solid solvents - e.g. rectisol, amins or zinc oxide).

Conditioning removes undesired gas components like CO₂ and adjust gas components (e.g. H₂) to the wanted ratios. After finishing cleaning and upgrading the syngas is ready for liquefaction.