Two experimental campaigns were performed with the aim to study the effect of two different bed materials, magnesite and olivine during steam-O-2 biomass gasification. The test-rig consists of a 100 kW(th) atmospheric circulating fluidized-bed gasifier and a high temperature filter unit which contains 3 rigid ceramic candles with an outer diameter of 60 mm, 10 mm wall thickness and a length of 1520 mm. Tests were performed with different fuels (A-wood, B-wood, miscanthus and straw). Two types of filter elements were used, Dia-Schumalith(1) (DS3) and Dia-Schumalith(1) N (DSN1), which operated at 800 degrees C for 58 h and 50 h, respectively. The filtration performance was studied through continuous observation of the increasing pressure drop during the build-up of the dust cake. Gas face velocities ranged between 2.5 and 5 cm s(-1). DSN1 elements showed longer steady filtration compared to DS3 candles with filtration efficiencies equal to 100%. Formation of calcium and potassium silicates resulted from filter cake analyses. The filtration process influenced gas and tar composition of the incoming gas flow. Hydrogen content increased about 10% (dry basis) and the heavier tar compounds appeared to be broken into lighter chains, such as naphthalene whose concentration increased. (C) 2011 Elsevier Ltd. All rights reserved.