Unveiling the mechanism of NOx precursor formation during sewage sludge pyrolysis: Effects of carbohydrate–protein interactions

Municipal sewage sludge poses environmental and health risks, and thus, requires proper disposal using pyrolytic techniques. However, these techniques are hindered by the production of NH 3 and HCN (NO x precursors) by the sludge. Hence, a comprehensive understanding of NH 3 and HCN formation during sludge pyrolysis is required to minimize its NO x footprint. As sludge N mainly occurs in proteins forms, its transformations can be modeled using amino acids. Here, we aimed to the study the mechanism by which carbohydrates influence the formation of NO x precursors during sludge protein pyrolysis at different temperatures using glutamic acid , tyrosine, and histidine as protein models and cellulose and lignin as carbohydrate models. During pyrolysis, the release of NH 3 and HCN was promoted by high temperatures and inhibited by carbohydrates. Despite this inhibitory effect, the results suggested that the release of NH 3 should be considered for samples rich in aliphatic amino acids. For glutamic acid and tyrosine, NO x precursor formation was inhibited by N fixation in coke under the action of volatiles produced during carbohydrate pyrolysis. For glutamic acid, adding cellulose and lignin increased the coke-N content by 25.32% and 44.73% at 700 ℃. For histidine, this effect was ascribed to the ring-opening reactions induced by the free radical products of carbohydrate decomposition and the enhanced transfer of N-containing compounds to tar after ring-opening recombination. Further, heterocyclic-N within tar increased to 69.12% due to lignin-histidine interactions at 700 ℃. The results of this study can assist in regulating sludge protein pyrolysis intended for minimizing the production of NO x precursors.