Here, water or an appropriate scrubbing liquid is sprayed on the gas. Solid particles and tar droplets collide with the drops, forming larger droplets because of coalescence. These larger droplets are easily separated from the gas by a

demisterlike cyclone. The gas needs to be cooled until it is below 100 °C before cleaning. The tar-laden scrubbing liquid may be fed back into the gasifier or its combustion section. Alternatively, it may be regenerated by stripping the tar away.

Some commercial methods, such as the OLGA and TARWTC technologies, use proprietary oil as the scrubbing liquid. The tar liquid is then reinjected into the gasifier for further conversion (Knoef, 2005, p. 196). Scrubbers have a high (>90%) collection efficiency, but the efficiency drops sharply below 1-micron­sized particles. They consume a large amount of fan power owing to the large (~50-inch water gauge) pressure drop across the scrubber. While their operating cost is high, their capital cost is much less than that for ESPs.

A system with a tar removal scrubber produces cleaned gas with a lower outlet temperature and a higher energy content, but it contains tars that are more difficult to remove. The main challenge of tar removal is the formation of “tar balls,” which are long-chained hydrocarbons that have a tendency to agglomer­ate and stick together, fouling equipment in the initial stages of tar condensing and collecting.

The tar-laden stripper gas, if fed into the gasifier, lowers its dewpoint well below that of water. This allows condensation of the tar, while flue gas contain­ing tar vapor can be recycled back to the combustion section of the gasifier for combustion.