Filter blinding phenomenon in pressurised biomass gasification based hot gas filtration

One of the most critical issues in biomass gasification based hot gas filtration is the filter blinding phenomenon at elevated filtration temperatures (above 600 °C) and pressures. The tar-laden gas has a tendency to form coke/soot on the filter. In advanced biomass gasification applications significant benefits could be gained by raising the filtration temperature. This was the main motivation for the study summarised here. The main results considering the critical filter blinding temperature at various process conditions is presented in this paper. A pressurised filtration test rig was built at VTT for studying the filter blinding phenomenon with synthetic gasification gas. The test rig consisted of an electrically heated reactor in which a ceramic fibre candle filter (alumina silicate) was mounted. The synthetic gasification gas applied in test runs consisted of three elements: gasification gas components (N2, CO, CO2, CH4, H2, steam), tar model compound and fly ash. The tar model compound consisting of toluene (90 m-%) and naphthalene (10 m-%) was fed by a liquid chromatography pump into the preheated gasification gas stream before entering the reactor. Real fly ash from CFB gasification test run was fed by a screw feeder and transported to the reactor by means of carrier gas nitrogen. The filter element was periodically cleaned by a reverse nitrogen pulse. The pressure difference across the filter element was measured. The initial test runs were performed with pure nitrogen as gasification gas. In later test runs steam alongside with other gasification gas components were introduced. In all test runs the tar and the fly ash concentration were 100 g/m3n and 9 g/m3n respectively. The filter element was operated in the temperature range of 500-800 °C at pressures 0-2 bar(g). The face velocity through the filter was kept constant at 2.1 cm/s.