Gasification reactivity of char from dried sewage sludge in a fluidized bed

The gasification reactivity of char from dried sewage sludge (DSS) applicable to fluidized bed gasification (FBG) was determined. The char was generated by devolatilizing the DSS with nitrogen at the selected bed temperature and was subsequently gasified by switching the fluidization agent to mixtures of CO2 and N2 (CO2 reactivity tests) and steam and N2 (H2O reactivity tests).. The tests were conducted in the temperature range of 800–900 °C at atmospheric pressure, using partial pressure of the main reactant in the mixture (CO2 or H2O) in the range of 0.10–0.30 bar. Expressions for the intrinsic reactivity (free of diffusion effects) as a function of temperature, partial pressure of gas reactant (CO2 or H2O) and degree of conversion were obtained for each reaction. For the whole range of conversion it was found that the char reactivity in an H2O–N2 mixture was roughly three times higher than that in a mixture with the corresponding partial pressure of CO2. The reactivity was only influenced by particle size greater than 1.2 mm in the tests with steam at 900 °C. It was demonstrated that the method of char preparation greatly influences the reactivity, highlighting the importance of generating the char in conditions similar to that in FBG.