High temperature gas filtration with ceramic candles and ashes characterisation during steam–oxygen blown gasification of biomass

Two experimental campaigns were performed with the aim to study the effect of two different bed materials, magnesite and olivine during steam–O2 biomass gasification. The test-rig consists of a 100 kWth 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-Schumalith1 (DS3) and Dia-Schumalith1 N (DSN1), which operated at 800 °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.