Study on the reaction mechanism of nitrogen-containing gases from polyurethane pyrolysis with carbonate molten salts

• The molten salt oxidation (MSO) behavior of polyurethane (PU) in carbonate salt has been firstly investigated. • The E a of MSO processes of PU are 161.48 kJ mol −1 and 419.72 kJ mol −1 , respectively. • NO 2 and NO could be neutralized by alkaline molten carbonate salt into nitrate and nitrite. • The presence of Li + and nitrates in the melt can promote the production of O 2 2– to oxidize PU. Polyurethane (PU) is extensively utilized in industrial and consumer applications. However, the direct pyrolysis of PU will generate nitrogen-containing tail gases such as NO and NO 2 . In this study, alkaline carbonate molten salts are innovatively used to neutralize the nitrogen-containing gases produced during the pyrolysis of PU while pyrolyzing it. The results of TG tests show that the thermal decomposition of PU can be divided into two processes: functional group cracking (502.97–640.30 K) and carbonization (640.30–765.44 K), with activation energies of 161.48 kJ mol −1 and 419.72 kJ mol −1 respectively. The carbonization process generates reactive carbon intermediates, which then participate in the reduction of NO 2 to yield NO and CO. The presence of carbonates can effectively absorb NO 2 and NO tail gases and generate nitrates and nitrites in the molten salts. The existence of nitrates also enables the generation of O 2 2− , an oxidizing ion, in the molten salts, which promotes the further decomposition of residues. EM-EDS and XRD analyses confirm molten carbonate salts effectively intercept nitrogenous gases, with HSC simulations validating this capture mechanism. Therefore, carbonate molten salts can adsorb nitrogen-containing tail gases while catalyzing the further oxidation of residues. Molten salt oxidation is an advanced means suitable for the safe and green treatment of PU.