Reducing CO/NO and absorbing heavy metals in self-sustained smouldering of high-moisture sludge by regulating inert media with low-cost natural zeolite

Smouldering is a low-energy, low-cost, effective treatment technology for sludge with high moisture. However, combustible gas and pollution in the flue gas limit the low-cost operation. This work proposes a novel method to in-situ reduce gas emissions (CO, NO, VOCs) and absorb heavy metals by regulating inert media with low-cost natural zeolite in self-sustained smouldering of sludge, and the effect of natural zeolite blending ratio on the performance is deeply investigated by fixed-bed and smouldering experiments. Fixed-bed experiments show that adding natural zeolite contributes to the sludge reaction owning to the confined catalysis with porous structure, as observed by the more rapid oxygen consumption, lower CO/NO concentrations. Moreover, smouldering experiments demonstrate that the endothermic dehydroxylation and dehydroxylation processes of the pure natural zeolite decreases the smouldering temperature and the propagation velocity, reduces the pyrolysis layer, but adding natural zeolite significantly reduces the concentrations of CO/NO/VOCs in the flue gas. Furthermore, higher heavy element content in the post-reaction natural zeolite is observed, indicating that the inorganic minerals in natural zeolite can effectively absorb the heavy elements. Taking reaction intensity, CO/NO/VOCs reduction and heavy element absorption into account, adding a small amount of natural zeolite (Sand: Natural-zeolite = 2.90:0.10) may be reasonable with obtaining good performance. Finally, the organic components in condensable liquids of the smouldering flue gas are deeply analyzed, and the main components is 36.7% for amides and 23.41% for nitrogen-containing heterocyclic compounds. This work can provide a possible pathway and useful information for the low-cost application of the sludge smouldring technology.