Energy Recovery from Municipal Sewage Sludge: Combustion Kinetics in a Varied Oxygen–Carbon Dioxide Atmosphere

Abstract

Energy from municipal sewage sludge can be obtained by applying a thermal conversion method. In this study, the combustion kinetics of municipal sewage sludge were analyzed in an O2/CO2 atmosphere. Studies were conducted in different gaseous atmospheres consisting of varying proportions of oxygen and carbon dioxide. The participation of oxygen was as follows: 20, 40, 60, 80 and 100% vol. The experimental temperatures varied from 873 to 1273 K. The experimentally obtained results helped establish the basic kinetic parameters, such as the reaction order n, factor Ko and activation energy Ea of sludge grains. The values of the activation energy Ea and Ko were, respectively, 46 kJ/mol and 0.0127 mg/m2sPa. They show that the limiting factor of combustion is the diffusion of oxygen and that combustion reactions take place in the outer layer of the unreacted core. Therefore, sludge is promising for energy recovery because it has quite a high net calorific value (NCV) and a high gross calorific value (GCV). The GCV was 14.1 MJ/kg and the NCV was 12.8 MJ/kg. The experimental studies with a wide range of process parameters helped to create an array of apparent reaction rates as a function of the temperature and oxygen concentration, showing the significant effect of oxygen on the apparent reaction rate, in contrast to the effect of temperature.