煤粉掺氢燃烧中焦炭及NOx转化机理研究OA

[Objective]In actual coal-hydrogen co-firing processes,the rapid combustion of hydrogen forms a localized hydrogen-rich atmosphere around char particles,and the intrinsic mechanism by which this pre-adsorbed hydrogen affects char-nitrogen oxidation remains largely unexplored.This paper aims to thoroughly investigate the microscopic oxidation mechanism of nitrogen-containing coke and the conversion pathways of nitrogen oxides(NOx)within the complex microenvironment of hydrogen-blended pulverized coal combustion,and facilitate the clean,efficient,and low-carbon utilization of coal resources.[Methods]Density functional theory(DFT)calculations were rigorously conducted using the M06-2X exchange-correlation functional coupled with a 6-311G(d)basis set,a method that has been proven highly reliable for thermochemical kinetics of main-group elements.A zigzag edge model comprising seven carbon rings with a substituted pyridine nitrogen was established to effectively represent the typical local edge structures of pure coal char,alongside a correspondingly constructed pre-hydrogenated char model.Linear synchronous transit(LST)combined with quadratic synchronous transit(QST)methods were applied for locating transition states,validated by intrinsic reaction coordinate(IRC)and vibrational frequency analyses.Electrostatic potential(ESP)analysis was performed to visually evaluate and predict the initial adsorption behavior and preferential reactive sites of oxygen molecules.Subsequently,three representative competitive oxidation pathways resulting in different gas-phase products were systematically evaluated,identifying the respective transition states,intermediate complexes,and intrinsic reaction energy barriers to assess both kinetic and thermodynamic feasibilities.[Results]ESP visualizations reveal that the pre-hydrogenated coke model exhibits highly polarized surface features due to the uneven charge distribution caused by the synergistic effect of hydrogenation and nitrogen doping.The pre-adsorbed hydrogen atoms act as distinct electrophilic targets with significant positive charges,while the pyridine nitrogen atom serves as an electron-rich nucleophilic center.Reaction coordinate analysis demonstrates that the initial cleavage energy barrier of N—C bonds in the pure char skeleton is 237.4 kJ/mol.In contrast,this critical rate-determining step(RDS)barrier increases to 274.63 kJ/mol in the pre-hydrogenated char model,proving that surface hydrogenation substantially increases the ring-opening kinetic resistance of nitrogen-containing heterocycles.A comprehensive comparison among the three oxidation pathways on the pre-hydrogenated char highlights striking differences.Pathway three,which leads to NO2 formation,requires overcoming an exceptionally high energy barrier of 334.19 kJ/mol during the adsorption of the second O2 molecule,rendering it kinetically unfavorable and negligible.Conversely,pathway one(producing NO and H2O)and pathway two(producing NO and CO)emerge as the primary feasible competing routes.Both pathways share an identical kinetic bottleneck:the cleavage of the N—C bond within the nitrogen-containing heterocycle,which requires 274.63 kJ/mol.Thermodynamically,pathway one presents an overwhelming advantage,releasing a total energy of 692.76 kJ/mol compared to the 453.86 kJ/mol released by pathway two.However,once the shared RDS is overcome,pathway two exhibits a slight kinetic superiority during the desorption phases.[Conclusion]The oxidation of char in a hydrogen-doped microenvironment is fundamentally a high-temperature process rigidly controlled by the initial N—C bond cleavage.NO is conclusively the dominant nitrogen oxide product,whereas NO2 formation is negligible under these constraints.By shedding light on the molecular-level pathways,this theoretical study bridges the gap between microscopic kinetics and macroscopic observations.The meticulously extracted intrinsic kinetic parameters offer reliable foundational data necessary to refine computational fluid dynamics(CFD)models for large-scale coal-hydrogen co-firing,ultimately aiding in the precise control and mitigation of NOx emissions in power plants.