Discussions on the heat transfer performance of the indirect pyrolysis plant using CFD modeling

Pyrolysis is a pathway for hydrogen reproduction from sewage sludge. The critical problem in the indirect pyrolysis process is the lower heat transfer performance in the furnace. The circulation of heat media controls the heat transfer in the pyrolizer (e.g., heat carriers (HCs) in the form of alumina balls), which is essential in determining the heat transfer performance and ultimately determining the hydrogen yield. The temperature of the HCs and amount of hydrogen yield are low because the required temperature in the vessel is not satisfied. This study investigated the relationship between the operating conditions and HC temperature profile to determine the optimum conditions for sufficient HC heating. We discuss the HC performance aspects to achieve a more effective operation using low inputs. Using a computational fluid dynamics simulation, we assessed the effect of the inlet gas temperature, inlet gas flow rate, and furnace structure on the heat transfer performance. We validated our model using the test results from a demonstration plant. We observed that by increasing the hot gas inlet temperature by 51 °C, the heat loss was reduced by 2,170 MJ/h, and the thermal efficiency of the entire plant was improved by 5.46%. When the number of hot-gas inlets was split, the efficiency did not change, but the temperature dispersion of the HCs was small. This implied that tar generation, including that of liquid substances, was prevented. At the pyrolysis temperature of 600°C, the cold gas efficiency was 81.2 LHV%.