My name is Cevdet Doğan and I am a PhD student at the Gebze Technical University in the field of air pollution and air pollution control systems. I applied to the BRISK2 project after the recommendation of a colleague who conducted a research stay within the previous BRISK project. After looking for a project partner, I came across with TU Graz research facilities, where several biomass-to-energy research projects are taking place. I thought that it will be a good chance for my future PhD studies to make experiments in a fluidized bed in order to understand its mechanism. I mentioned this possibility to my colleague Ebubekir, who is also a PhD student in the same university and his doctoral thesis is about gasification and mathematical modeling of biomass in a fixed bed downdraft reactor. He decided to join me and we got in contact with Dr. Andrés Anca-Couce, who helped me during the preparation of my proposal. 

In our stay we decided to focus on using a problematic waste source, sewage sludge, as a feedstock and catalyst for gasification in fluidized bed reactors. Therefore, sewage sludge and wood pellets were mixed with different ratios (25/75 and 50/50) and gasified with steam in a bubbling fluidized bed. Then, sewage sludge char produced during pyrolysis and the resulting char after sewage sludge gasification were used as catalysts for tar cracking in a tar reformer downstream. Our main target was investigating whether if gasification of a mixture of sewage sludge and wood is a feasible and sustainable way for disposal of sewage sludge or not.  

The Fluidized bed gasifier (TUG2) at TU Graz was operated at 750 °C, a sigma value (steam to carbon ratio) of 4 and at 1.5 kW of feedstock input. Syngas produced in fluidized bed goes through a filter, optionally through a tar reformer and it finally burns in a flare. The produced tars were collected according to Standard Tar Protocol and were measured gravimetrically. Tar measurements were done both before and after the tar reformer to determine the tar removal efficiency. Moreover, permanent gases in the syngas (CO, CO2, H2 and CH4) were also measured by an ABB gas analyser.  

For the sake of comparison, wood pellets were first gasified at reference conditions (750 °C, sigma value 4, 1.5 kW feedstock). Then mixtures of sewage sludge and wood were gasified and increasing the ratio of sewage sludge lead to a higher tar content. For the mixture with 25% sewage sludge and 75% wood (tar content of around 20 g/Nm3), experiments were conducted with the tar reformer at 800°C using sewage sludge char. The tar concentration after the tar reformer was significantly reduced, with an efficiency of about 80%. This efficiency decreased during time because of deactivation, achieving about 40% after 5 hours. After using steam for reactivation of the used char, the tar removal efficiency was increased to about 80% again. Besides that, the H2/CO ratios increased with the tar reformer from values of around 2 to values close to 3, which can be more suitable for synthesis of chemical products as methane. 

After this experimental campaign the following preliminary results were obtained: 

  • Increasing the percentage of sewage sludge in feedstock composition increases the tar concentration in the syngas.  
  • Char produced from sewage sludge is able to crack tars but it is deactivated after a while (about 300 min). Reactivation by using steam is effective for a short time period. 
  • Using char in the tar reformer improves the permanent gas composition, increasing the H2/CO ratio and calorific value. 

Finally, I would also like to acknowledge with much appreciation the crucial role of the staff of TU Graz, who gave the permission to use all required equipment and the necessary materials to complete our BRISK Project. A special thanks goes to Dr. Andrés Anca-Couce as well as Lukas von Berg and Gonzalo Pradillo, who helped us before and during our experimental work at TU Graz.