Pyrolysis of sewage sludge in a fixed and a bubbling fluidized bed – Estimation and experimental validation of the pyrolysis time

•Pyrolysis of sewage sludge in a fixed and a fluidized bed is studied.•The evolution of the mass released by the sample during pyrolysis was measured.•The effect of the bed temperature and gas velocity on the pyrolysis was analyzed.•Sewage sludge pyrolysis time was experimentally obtained for each operating condition.•A model based on first order kinetics was proposed to estimate the pyrolysis time. Pyrolysis of sewage sludge was studied experimentally in a stainless-steel reactor operated as a fixed or fluidized bed. A novel measuring technique, consisting of measuring the mass of the whole reactor and the sample on a scale, was applied. The scale was capable of measuring the whole mass of the reactor with enough accuracy to detect the mass released by the sewage sludge sample during its pyrolysis. This original measuring technique permitted the measurement of the evolution over time of the mass of sewage sludge supplied to the bed in batch during its pyrolysis while moving freely in the bed. From the measurement of the mass of the solid residue remaining in the reactor, the pyrolysis time of the sewage sludge sample can be obtained accurately for each operating condition. Different operating conditions were selected to analyze the evolution with time of the sample mass during the pyrolysis process, including the bed temperature and the velocity of the Nitrogen used as inert gas. An increase of the velocity of Nitrogen from that of a fixed bed (0.8Umf) to that of a low velocity bubbling fluidized bed (2.5Umf) accelerates remarkably the pyrolysis process, i.e. reduces the pyrolysis time, however increasing the Nitrogen velocity further has a slight effect on the characteristic velocity of the pyrolysis process. The pyrolysis process of sewage sludge can also be accelerated by increasing the bed temperature, even though the effect of the temperature is lower than that of the Nitrogen velocity. Furthermore, a mathematical model based on a first order apparent kinetics for the pyrolysis of sewage sludge was proposed. The model was employed to estimate the pyrolysis time for each operating condition, obtaining a proper agreement with the experimental measurements.