Tar removal from gasification gas

This paper presents a summary of studies carried out at VTT Energy on minimisation of tar formation in gasifiers, tar removal by water scrubbing and catalytic tar removal. Tar is characterised in this context as light and heavy tars. In a pressurised bubbling fluidised-bed gasifier, when using fine grained biomass feedstocks, heavy tar formation can be markedly reduced by using bituminous coal as additional feedstock. Use of dolomite as bed material did not affect the tar content of gas in the pressurised process, while in atmospheric conditions used in the circulated fluidised-bed gasifier the tar content of gas decreased. Downdraft gas scrubbing by water removed the water-soluble tar components, but was ineffective in removing aromatic compounds. Contaminated wastewater was formed. Effective catalysts for tar removal were carbonate rocks and nickel catalysts. The nickel monolith catalyst at 900 degreesC and at 5 bar pressure used for dust-containing feed gas gave almost 100 % conversion for tars. No deactivation was observed during a 500 h long-term test. The carbonate rock catalysts were active only if calcined. Thus, in conditions typical of an IGCC process, catalysts operating temperatures close to 1 000 degreesC are required. At atmospheric pressure, effective tar removal can be achieved with carbonate rock catalysts at around 900 degreesC.