Alkali metal-driven release behaviors of volatiles during sewage sludge pyrolysis

The increasing fraction of alkali metals generally in the form of chlorides contained in municipal sewage sludge influences the pyrolysis characteristics and pollutant emission during sludge pyrolysis, while studies on the effect of alkali metals are still limited. This work systematically investigated the effect of alkali metals addition on the release behaviors of the volatile products during sludge pyrolysis as well as the related kinetics, performed on an advanced thermogravimetric analyzer equipped with Fourier transform infrared spectroscopy and mass spectrometry (TG-FTIR-MS). The results indicated that the alkali metal chlorides reduced the release orders of volatile products during pyrolysis by various degrees depending on the additions and pyrolysis temperature. Over 340 °C, both NaCl and KCl addition improved the total absorbance of the volatile products during pyrolysis, while the loss of alkali metal-containing chlorides occurred. When the addition was more than 30 wt%, the release magnitude of alkanes and alkenes as well as contaminants including nitrogen-/sulfur-/chlorine-derived inorganics and aromatics with nitrogen substituted was larger during NaCl-added sludge pyrolysis than that during KCl-added sludge pyrolysis. It was anticipated that the sludge with NaCl addition of 30 wt% and KCl addition of 10 wt% showed the optimal pyrolysis performance in terms of value-added products recovery and pollutants mitigation. The kinetics revealed that the thermal reactivity of alkali metals via chlorides dominated the interactions with sludge matrix such as dehydroxylation and decarboxylation. The obtained results proved that the presence of alkali metals could mitigate the emission of pollutant during sludge pyrolysis, but the reduced fractions of valuable commodities and the release of chlorinated compounds should be considered in practice.