Gasification of char from dried sewage sludge in fluidized bed: Reaction rate in mixtures of CO2 and H2O

The rate of gasification of char from dried sewage sludge (DSS) with a gas containing CO2 and H2O was studied. The tests were conducted in an atmospheric laboratory fluidized bed (FB), using CO2-H2O-N2 mixtures as fluidizing gas and temperatures in the range of 800-900 °C. The gasification rates were compared with those previously obtained for DSS char using one single reactant (either CO2 or H2O) in the gas, i.e. in mixtures of CO2 and N2 or H2O and N2. It was found that the char gasification rates measured in a mixture containing both reactants, i.e. CO2 and H2O, were well approximated as the sum of the individual rates measured separately with CO2 and H2O. Besides this result, an alternative method to accurately measure the char gasification in an FB using CO2-H2O-N2 mixtures was employed, based on determining the char remaining in the reactor up to a certain time of gasification from the CO and CO2 concentrations in the gas during the combustion of this char. It was shown that this method compares well with the traditional method (based on tracking the CO and CO2 concentrations during gasification) when using a single reactant, but improves significantly the reliability of measurements for tests where both H2O and CO2 react simultaneously with char.