Effect of ash on the performance of iron-based oxygen carrier in the chemical looping gasification of municipal sludge

As sludge ash is rich in alkali metals, the oxygen carrier (OC) performance in the multi-cycle chemical looping gasification (CLG) of dried sludge is affected by simultaneously cycling and accompanying ash. Fe-based OC was used in a lab-scale fluidized-bed reactor to investigate the multi-cycle CLG of dried sludge and to study the effect of accompanied ash on OC particle. The synthesis gas, carbon conversion rate, and reaction rate were monitored over several cycles. Physicochemical characterization of the OC was accomplished to determine its effective components and the alkali metal deposition from ash. The results showed that as the number of cycles increased, the mole fractions of H2 and CO2 in synthesis gas decreased and increased, respectively, both eventually stabilizing. The carbon conversion rate reached its peak in fifth cycle. The gasification rate of sludge reached its maximum value in the 14th cycle, slowing down and then stabilizing thereafter. The OC was loaded with more alkali metal during the cycle, whose activity was enhanced at the beginning. However, once the cycle number became higher than 25, ash spheroidization and sintering occurred on its surface. Accordingly, the OCs particle size and mass became a little larger for the accumulation of unreacted lattice oxygen.