Co-combustion kinetics and synergistic characteristics of sewage sludge and spent coffee grounds

The combustion characteristics of sewage sludge (SS), spent coffee grounds (SCG), and their blends were studied. The TGA characteristics of the blends combustion at different heating rates (10, 15, and 20 K/min) with temperatures ranging from 298 K to 1273 K have been investigated. Results indicated that the comprehensive combustion of the SS significantly improved by blending with SCG. The ignition and peak temperatures were reached at lower temperatures as the mass percentage of SS increased, which agreed with the combustion index. The integral method-the Coats-Redfern equation-was adopted to calculate kinetic equations and activation energy. Three reaction models in the diffusion mechanism well reflected the combustion of SS and blends in the second stage, but no single mechanism was suitable in the third stage. The synergistic effects between SCG and SS were investigated and the results showed that the positive effects were enhanced by the heating rates but the higher mass ratio of SCG has negative effects. Therefore, the percentage of SCG should be operated from the perspective of positive synergy. Although there is a gap between the thermogravimetric analysis and the boiler process, these experimental results can explain and predict the combustion behavior of blends in practical applications.