Thermodynamic and kinetic evaluation of the reaction between NO (nitric oxide) and char(N) (char bound nitrogen) in coal combustion

Abstract

Using pyridine nitrogen as representative of nitrogen-containing char model, detailed theoretical calculations based on DFT (density functional theory) and conventional TST (transition state theory) are carried out to investigate the thermodynamics and kinetics of the heterogeneous interaction between NO (nitric oxide) and char(N) (char bound nitrogen) during coal combustion. Focus is directed on NO chemisorption of direct nitrogen–nitrogen interaction and N2 desorption from medium chemisorbed surface. It is suggested that side-on chemisorption is a low barrier (2.3 kJ/mol) and high exothermic (178.5 kJ/mol) step, while N-down chemisorption is a high barrier (105.2 kJ/mol) and moderate exothermic (64.4 kJ/mol) step. Worth noticing is that NO chemisorption on char(N) surface is different from that on char surface, largely because there is no unpaired σ electrons located at N atom. After chemisorption, four stepwise reactions with the highest energy barrier of 266.3 kJ/mol are found to produce separated N2. The overall NO reduction rate is expressed as 7.6×105×exp(−9606T). Reaction rate of rate-limiting step involved in N2 desorption is 2 × 100 s−1 at 1000 K, indicating it can tempestuously take place above 1000 K. The calculated results lend credit to previous experimental phenomenon.