CFD Simulation of Moving-Bed Pyrolizer for Sewage Sludge Considering Gas and Tar Behavior

This study focused on the small-scale dual fluidized-bed gasifier for hydrogen (H2) production from sewage sludge. One of the current problems with the pyrolizer is tar condensation. Tar could reduce the efficiency of the H2 yield by adhering and condensing on walls and pipes. It was revealed that more tar can be decomposed with higher reaction temperatures. Therefore, this study aimed to increase the tar decomposition efficiency with raising the heat carriers’ (HCs) temperature and analyzing the temperature distribution in the furnaces using a CFD simulation. The tar decomposition rate in the pyrolizer was +34.4%pt. by 100 °C of the HCs’ temperature rising. It is implied that less tar trouble and a longer lifetime of the H2 production plant could be expected by raising the HCs’ temperature. However, comparing the heat transfer efficiency of the whole system, the lower HC inlet temperature of +7.4%pt., because of the hot gas, which supplies heat to the HCs, required more heat, making the thermal efficiency poorer. In addition, the environmental impact of the AGM was increased by 27.2% with the HCs’ temperature rising to 100 °C. Thus, the heat exchange efficiency of the preheater needs to be improved to raise the HCs’ inlet temperature and reduce the amount of hot gas required.