Regulatory effect of CaO on glutamate pyrolysis and NOx precursors formation

Pyrolysis is an effective method for safely disposing of sewage sludge via conversion into a usable energy source. However, municipal sludge has a high protein content, which is a major source of NO x precursors. This study investigated the effect of CaO on NO x precursor formation during glutamate pyrolysis and demonstrated that CaO occupies the carbo x yl site of glutamic acid at 320 °C, which delays its decomposition and promotes the conversion of glutamic acid from a five- to a si x -membered ring. At 400 °C, heterocyclic nitrogen in the tar peaked from 4.87 to 53.87% after adding CaO. As the temperature increases to 480 °C, CaO might promote the conversion of heterocyclic nitrogen into tricyclic piperazine dione in the tar, which subsequently decomposed to produce HCN in large quantities. At 480 °C, CaO contributed to the decrease in pyrrole, pyridine, and quaternary nitrogen in the semi-coke from 24.81%, 12.93%, and 7.43% to 1.91%, 5.97%, and 1.43%, respectively and promoted HCN hydrolysis to form NH 3 . At 480–680 °C, CaO interacted with NH 3 to form CaCN 2 . The product of CaC x N y decomposition—CaC x —captured NH 3 . At high temperatures, CaO inhibited HCN release, which might be related to the reaction between CaO and HCN to form CaC x N y , subsequently reconverted into CaO at temperatures above 680 °C. These findings provide novel insights into the influence of CaO on amino acid pyrolysis to produce NO x precursors and the regulation of NO x precursor production.