PCDD/Fs Inhibition by Sludge Decomposition Gases: Effects of Sludge Dosage, Treatment Temperature and Oxygen Content

ABSTRACT

Sludge decomposition gases (SDG) have been identified as potential PCDD/F-inhibitors, since they contain high concentrations of NH₃ and SO₂ as well as various unidentified N- and S-compounds. The effects of sludge dosage, its treatment temperature and oxygen content on PCDD/Fs suppression were observed during test runs using a model fly ash (MFA) system, generating high chlorinated PCDD/Fs. The experiments were carried out in two parts: a first set with SDG of undefined composition, and a second with injection of NH₃ and SO₂ combined in three different concentration ratios. It was observed that the SDG evolving from dried sludge (DS) could suppress the formation of 2,3,7,8-substituted PCDD/Fs even with very high efficiency (> 99%), both in weight units and in I-TEQ units. Such suppression was somewhat stronger for PCDD than PCDF. It was also found that 300°C might be a suitable sludge decomposition temperature when using SDG as suppressant in PCDD/Fs inhibition. Moreover, the oxygen content only slightly influenced upon the suppression capabilities. The results revealed that SO₂ was more effective than NH₃ in suppressing PCDD/Fs formation, with suppression efficiencies of 61.9% and 38.6%, respectively. Remarkably, the formation of OCDD/Fs was inhibited least and even further formation could occur when both NH₃ and SO₂ were injected together as simulated flue gas. The results exposed that NH₃ and SO₂ might be the primary PCDD/Fs suppressants resulting from mild thermal decomposition of sludge. However, further parameters should be evaluated and adjusted before SDG could be used in large-scale applications.