Catalytic fast pyrolysis of sewage sludge was conducted considering the effects of pyrolysis temperature, atmospheres and catalysts on carbon and nitrogen distributions using a fixed-bed reactor. H2 atmosphere gave the lowest carbon selectivity of solid products and highest gaseous products yield over HZSM-5 (HZ) at the same temperature compared to Ar and steam. Higher temperature led to an increase in the carbon selectivity of gaseous products and a decrease in the carbon selectivity of solids. H2 increased the carbon yield of light aromatics (LAs), whereas steam reduced the generation of LAs. Over 0.5Ni-HZ, H2 gave the highest LAs carbon yield of 11.4%, compared to 1.6% obtained from noncatalytic fast pyrolysis of SS. Higher temperature, H2 and steam atmospheres, and metal-loaded HZ promoted the transfer of nitrogen into gaseous products. The highest nitrogen yield of NH3 of 55.6% was obtained from sludge pyrolysis over HZ at 700 °C under H2 atmosphere.

Catalytic fast pyrolysis can effectively solve the pollution and treatment problems of sewage sludge to realize the green recycling of biomass waste energy. The study on the mechanism of nitrogen migration is a crucial part of the conversion of sewage sludge. Herein, two amino acids of lysine with linear amines and tryptophan with aromatic nitrogen-containing heterocycles were used as typical model compounds to investigate the pyrolysis mechanism and nitrogen migration behavior. The results revealed that lysine mainly underwent two routes: one was to cyclize itself to lactam and cleavage twice with the gradual dehydrogenation to form stable nitrogen-containing heterocycles; the second was first decarboxylation and then cleavage with gradual dehydrogenation to form nitriles. The first route was easier to proceed at low temperatures, while the second route took up more proportions at high temperatures. Tryptophan was converted gradually to stable indoles after decarboxylation, deamination, dehydrogenation, and denitrification. Based on the full understanding of the nitrogen migration mechanism of lysine and tryptophan, the catalytic denitrification mechanism over dolomite, limonite, and HZSM-5 modified with Ni loading catalysts was also explored.

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