Density functional theory study of the heterogenous interaction between char-bound nitrogen and CO2 during oxy-fuel coal combustion

Abstract

In order to have a deep insight into the NO formation mechanism during oxy-fuel combustion, density functional theory (DFT) was used to investigate the interaction between char(N) and CO2 at molecular level. The geometric structures, orbital electron distribution properties and reaction paths were calculated and optimized. The outer orbital electron properties of char(N) and CO2 indicate that CO2 acts as the oxidizer, which tends to attract electrons from char(N) during char(N)-CO2 interaction. The geometries and Mulliken properties of three initial chemisorption intermediates indicate that char(N) may mainly undergo two oxidation/gasification paths: one is carbon atom oxidation by CO2 to form CO via initial chemisorption structure of IM1. The other is nitrogen atom combination with oxygen atom and further to be oxidized to form NO via initial chemisorption structure of IM2. The initial chemisorption reactions of CO2 on char(N) surface are exothermic reactions and take place spontaneously at the operation temperature during oxy-fuel combustion. According to the reaction energy barriers and kinetic analysis, path 1 and path 2 have the comparative probability during char(N)-CO2 interaction to produce CO and NO. Nevertheless, NO produced in path 2 will further be in-situ reduced by CO over char surface, which will lead to low NO concentration in flue gas during oxy-fuel combustion.