Experimental investigation and modeling of gasification of sewage sludge in the circulating fluidized bed

Experiments of sewage sludge gasification were performed in a circulating fluidized bed of pilot plant scale (15 m height, 0.1 m i.d.). For the examination of the influence of the air ratio, gasification temperature, feeding height and fluidization velocity several screening tests were conducted. To understand better the results from the screening experiments, axial profiles of the gas composition were measured. As the most influencing factor for the heating value of the gasification gas the air ratio was found. Additionally, a low feeding height is recommended for good gas quality. While feeding into the lower dense zone of a circulating fluidized bed (CFB), mixing of the fuel particles is better. With low feeding ports, high velocities are attainable and therefore high fuel throughput can be achieved. In a second step a model of the CFB gasifier was developed. The fluid dynamics of a CFB were included as well as the complete reaction network of gasification. With the measured axial profiles of gas composition during pyrolysis, and gasification with air and a CO2/N-2-mixture kinetic rate expressions for sewage sludge gasification under fluidized-bed conditions were determined which may now be used for reactor scale-up calculations. (c) 2004 Elsevier B.V. All rights reserved.