The concept of a ceramic candle filter for high-temperature gas filtration, catalytically activated with nickel using a deposition-precipitation method with urea, was introduced to perform the simultaneous removal of tar and particles from hot biomass gasification gas. Tar cracking over nickel-activated ceramic filter substrates was studied with a synthetic biomass gasification gas (free of H 2 S and dust) under industrial hot gas filtration conditions. Naphthalene was used as the model compound for high-temperature biomass gasification tar. The main parameters investigated were reaction temperature (750-900 °C), gas velocity (2.5-6 cm/s), and catalyst loading (0.5 and 1 wt % nickel). It was found that when typical filtration gas velocities are used, naphthalene can be completely converted mainly to syngas (H 2 and CO) at 800-900 °C over activated filter substrates with a catalyst loading of only 1 wt % nickel.

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The urea method was applied to deposit a nickel-calcium catalyst inside porous filter discs to develop a gas cleaning technique involving the combined removal of tars and particles from hot biomass gasification gas. Some catalytic filter discs were tested in typical filtration conditions for the decomposition of the model tar compound benzene in a simulated biomass gasification gas containing representative amounts of H 2 S. H 2 S-deactivation studies showed a significant improvement in the sulfur resistance of the nickel catalyst on the filter discs by the addition of CaO to the catalyst formulation. The optimisation of the ratio between Ni and CaO and the effect of nickel content in the catalyst formulation on the catalytic performance were investigated.

The urea method has been explored to introduce nickel(II) into the pores of ceramic filter substrates to develop a catalytic filter for the combined removal of tars and particles from hot biomass gasification gas. SEM-EDX characterization showed that the above method gave a fairly uniform spatial distribution of nickel throughout the modified filter substrate. It was observed that urea decomposition products such as NH3 and CO2 limited the incorporation of nickel in the filter substrate due to the complexation of NH3 with Ni(II) and loss of impregnation solution by the discharge of CO2. It was demonstrated that the release of CO2 could be suppressed by increasing the starting pH of the impregnation solution. In addition, the urea method was explored to add calcium(II) to the catalyst formulation but it remained unclear if the calcium precursor coprecipitated with the nickel precursor during the urea decomposition or precipitated in the posttreatment of drying. The resulting catalytic filter substrates were tested in typical filtration conditions for the removal of the tar model compound benzene from a simulated biomass gasification gas. H2S-deactivation studies showed a significant improvement of the sulfur resistance of the nickel catalyst by addition of Ca(II) to the catalyst formulation. The nickel and calcium modified filter substrate exhibited 67% benzene conversion at 900 °C with 100 ppm H2S and 4 cm/s gas velocity, compared to 28% over the pure nickel one under similar reaction conditions.