I am a PhD student at the Technische Universität Berlin in the field of thermochemical energy conversion processes. On the recommendation of one of my colleges, who made a research stay within the previous BRISK project, I decided to get in touch with TU Graz, since I could have the possibility to work with a continuous fluidized bed gasifier (TUG2) utilizing a catalytic tar cracking reactor with different chars produced at TU Berlin.   

As a first step, the experimental project was planned in collaboration with my hosts in order to maximize the scientific output that could be obtained and consider all the possibilities in terms of equipment utilization. The main objective was to address tar cracking over a char bed (heterogeneous cracking) as a producer gas conditioning system in biomass gasification. For this reason, two different chars with different physical and chemical properties were produced through pyrolysis at TU Berlin before the visit.  

On the first day, the gasifier (TUG2), tar protocol setup and gas analyser were already prepared in order to perform tests efficiently. The gasifying conditions (750 °C and steam to carbon ration of 2 employing wood pellets as a fuel) were selected with the aim of producing gas with high tar content. To be aware of the total amount of tars generated with the selected parameters, a reference experiment was conducted without tar reformer at the begin and end of the campaign, obtaining a tar content of around 20 g/Nm3. The obtained values were in accordance to experiments previously conducted with similar conditions.  

Once we had the starting setpoint, further experiments were performed with the tar cracking reactor at temperatures around 800 °C employing a real producer gas. However, due to the small particle size from both chars (obtained from beech wood chips) and the high flow from the produced gas, the valves situated after the tar reformer got several times blocked. After a few tests, the setup could be changed in such a way as to avoid the produced gas going through these valves (Figure 1). 

Further tar cracking experiments were performed with both chars in the tar reformer under the same conditions. First of all, two different measurements (with tar sampling, Figure 2) with one hour difference between them were conducted with CHAR-01. The results showed catalytic cracking but also degradation of the char inside the reactor after 1 hour, being the gravimetric tar content of 9 and 14 g/Nm3 respectively.  After that, a new experiment was conducted with CHAR-02, where tar cracking was also observed, however, no significant difference was recognized with respect to the behaviour of CHAR-01,  as the measured tar content was as for CHAR-01 of around 9 g/Nm3 (Figure 3). 

Based on the outcomes of these experiments, it was decided to pre-activate the char with the purpose of improving tar cracking. This pre-activation was made with CHAR-01 letting steam go through the tar reformer during 1 hour before the test. In a last experiment employing a real producer gas tar cracking was significantly enhanced (>90% of tars were cracked), obtaining a gravimetric tar content of 0.8 g/Nm3 (Figure 4).  

These preliminary results contribute to gain a better understanding on the heterogeneous tar cracking process and point out further investigation paths. 

Finally, I would like to thank my host Andrés Anca Couce as well as Lukas von Berg, Cevdet Dogan and Gonzalo Pradillo for the welcome and the efficient work at TU Graz. It was really nice to share scientific work with you.