Gasification of dried sewage sludge in a newly developed three-stage gasifier: Effect of each reactor temperature on the producer gas composition and impurity removal

Gasification of dried sewage sludge was performed in a newly developed three-stage gasifier consisting of an auger as well as fluidized and fixed bed reactors to produce a producer gas with low-levels of contaminants. The auger reactor was designed to generate tar in advance, thereby enhancing the tar destruction in the fluidized bed gasifier and fixed-bed reactor. In the study, the effects of each reactor temperature and activated carbon as an impurity absorbent were mainly investigated. The reaction temperatures of the auger and fixed bed reactors were shown to have a strong influence on the tar removal, while that of the fluidized bed reactor mainly affected the producer gas composition. The three-stage gasifier clearly decreased the tar content in producer gas and condensed tar and increased the H2 production. In an experiment performed with activated carbon at the auger reactor temperature of ∼710 °C and the fluidized bed and fixed bed reactor temperatures of ∼830 °C, the tar and H2 contents in producer gas were only 22 mg/Nm3 and ∼29 vol%, respectively. Activated carbon clearly reduced the NH3 and H2S contents in producer gas. The minimum NH3 and H2S contents were 324 and 346 ppmv, respectively.