Experimental and kinetic study on the co-oxidation of pyridine and ammonia as a model compound of coal-ammonia co-firing

Abstract

The co-oxidation of pyridine and ammonia was studied as a model compound to investigate the kinetics of coal-ammonia co-firing. Experiments were conducted in a jet-stirred reactor coupled with synchrotron vacuum ultraviolet photoionization molecular beam mass spectrometer at atmospheric pressure up to 900 K with ammonia to pyridine molar blend ratio of 1:5. Compared with previous pyridine kinetic studies, several new oxidation intermediates were detected during the co-oxidation process, including nitrous acid, methyleneaminoacetonitrile, 2-, and 4-cyanopyridine. The pyridine LTO 3.1 kinetic model, comprising 233 species and 1572 reactions, was developed and used to simulate the reaction process with reasonable predictions, which incorporates the direct interaction between pyridine and NH₂ radical (NH₂+C₅H₅N=C₅H₄N+NH₃) and updates the rate constants of C₅H₅N+OH=C₅H₄N+H₂O, NH₃+NCO=HNCO+NH₂, NH₃+NCO=HOCN+NH₂. The major nitrogen-containing products are HCN, N₂, HNCO, N₂O, pyrrole, and NO. The co-oxidation of pyridine and NH₃ shows a mutual-sensitization effect, promoting the consumption of both pyridine and ammonia. The presence of ammonia boosts pyridine consumption by providing NO and more OH radicals at lower temperatures through the NO-NO₂ looping process (NO+HO₂=NO₂+OH and NO₂+H=NO+OH). The initial reaction temperature of NH₃ is lowered by around 200 K when co-oxidized with pyridine compared with its neat oxidation, as pyridine could supply OH radicals at a lower temperature and trigger the chain-branching reactions. NO_x emissions are also generated at lower temperatures compared with neat pyridine and NH₃ oxidation conditions. N₂O production reaches 367 ppm at 900 K, which is an order of magnitude higher than NO. The results could help better understand the microscopic mechanism of coal-ammonia interactions during the co-firing process, and the design, organization, and optimization of coal-ammonia co-firing applications.