Nonisothermal torrefaction kinetics of sewage sludge using the simplified distributed activation energy model

• Torrefaction characteristics of the sewage sludge were investigated.
• Distributed activation energy models were used to evaluate torrefaction kinetics.
• Gauss-Peak function was employed to describe activation energy distribution.
• Kinetic model developed had a good predicting for conversion degree of the sample.

Sewage sludge can be converted into bio-material or bio-fuel by means of thermal–chemical technique. Torrefaction characteristics of municipal sewage sludge were investigated based on a thermogravimetric analysis technique in nitrogen atmosphere. The whole torrefaction process could be divided into two regions, which referred to coexistence and devolatilization. Moreover, the torrefaction kinetics of sewage sludge was evaluated based on the distributed activation energy models with differential and integral methods. There was a linear dependency between activation energy and pre-exponential factor at different conversion levels. The Gauss-Peak function was employed to depict the distribution curves of the activation energies for both methods. The distributed activation energy model developed by the integral method predicted accurately the conversion degree of the sewage sludge torrefaction, whereas the model developed by the differential method only had a good prediction for high conversion degrees.