Direct pyrolysis of catalysed raw sewage sludge unlocks superior energy recovery: a kinetic evidence

TL;DR

This study shows that adding low-cost catalysts during the pyrolysis of raw sewage sludge improves energy recovery by reducing the energy needed for decomposition.

Contribution

The novel contribution is the identification of CaO as an effective low-cost catalyst for raw sewage sludge pyrolysis, supported by kinetic evidence.

Findings

Raw sewage sludge pyrolysis shows a three-phase activation energy profile.

CaO significantly reduces activation energy in the medium conversion range.

Al2O3 increases activation energy in initial stages, possibly promoting condensation.

Abstract

The present study investigates the pyrolysis kinetics of raw (undigested) sewage sludge and evaluates the effect of three low-cost catalysts (CaO, MgO, and Al2O3). The activation energies (Ea) were determined by means of the Flynn–Wall–Ozawa method. The raw sludge displayed a three-phase Ea profile, which suggests sequential decomposition of heterogeneous biopolymers. Among the catalysts, CaO exhibited the most significant reduction in Ea within the medium conversion range (α ~ 0.4–0.7). The effect of MgO was found to be reaction stage-dependent, while Al2O3 was observed to increase Ea in the initial stages, thus potentially promoting condensation reactions. The findings provide a robust foundation for the direct pyrolysis of raw sludge with low-cost catalysts, a process that has been demonstrated to enhance energy recovery in a single step, thereby circumventing the losses that are typically associated with prior anaerobic digestion.

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