During the visit the catalytic activity (and stability) of biomass derived chars for tar conversion during fluidized bed gasification was studied. The experimental approach followed aimed at analysing the performance of non-activated biomass char for tar conversion when the carbon gasification rate is higher than the carbon deposition rate (due to tar conversion) resulting in a negative carbon balance. The objective is to determine if the char deactivation behaviour is straightforwardly related with the carbon balance in the char bed.
The rig of the Technical University of Graz used during the visit consist of a lab-scale fluidized bed gasifier with a continuous fuel feeder connected to a secondary tubular reactor where the syngas produced in the gasifier can be treated. This configuration allows testing the activity of the char with a real tar-containing biomass gasification gas. The temperature profile in the secondary reactor is properly characterized by 16 thermocouples placed along the bed. The installation also has all the necessary apparatus to take tar samples according with the standards. The main limitation of the plant is that the syngas composition can be measured only during tar samplings, limiting the information of the evolution of the char bed activity.
The syngas necessary for the tests was produced in the fluidized bed reactor under steam gasification conditions. Pure steam was used instead of the originally desired air/steam mixture because of some technical difficulties. The evolution of the tar conversion over the char bed has been measured at three different temperatures; 750, 850 and 875 ⁰C. The total duration of the tests ranged from 1 hour up to 6,5 hours of stable operation and the gas residence time on the char bed was 0.2 s approximately. According with the preliminary visual information (the colour of the samples) the char is rapidly deactivated at 750 ⁰C. At 850 ⁰C the char tends to deactivate slowly while at 875 ⁰C the char seems to maintain (or even increase) its original activity during the entire experience. The different tar and char samples will be analysed in Graz, Seville and Berlin in the following months.
The two week stay in Graz has given to our group the opportunity to test the performance of biomass char as a catalyst for tar conversion using a real syngas. The final objective of these experiences is the development of cost-effective devices unlocking the potential of biomass and waste gasification for distributed energy production. Additionally, the visit funded by the BRISK2 project has been an excellent opportunity to share ideas and strengthen the relation between the Technical University of Graz and the University of Seville.