Regulatory effect of CaO on glutamate pyrolysis and NOx precursors formation

Abstract

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 NOx precursors. This study investigated the effect of CaO on NOx precursor formation during glutamate pyrolysis and demonstrated that CaO occupies the carboxyl site of glutamic acid at 320 °C, which delays its decomposition and promotes the conversion of glutamic acid from a five- to a six-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 NH3. At 480–680 °C, CaO interacted with NH3 to form CaCN2. The product of CaCxNy decomposition—CaCx—captured NH3. At high temperatures, CaO inhibited HCN release, which might be related to the reaction between CaO and HCN to form CaCxNy, 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 NOx precursors and the regulation of NOx precursor production.