Insight into the fate of nitrogen during char thermal conversion and the influence mechanism of potassium: A theoretical research
Ji Liu, Yuan‐gu Xia, Huai-de Sun, Bin Hu, Bing Zhang, Qiang Lu
Abstract
Nitrogen oxides (NO ₓ ) are primary pollutants produced during biomass combustion . During the devolatilization stage, char nitrogen (char(N)) is formed. In the subsequent char combustion stage, char(N) can decompose directly into NO x precursors or engage in heterogeneous reactions with O 2 or NO to form NO and N 2 . Nonetheless, a comprehensive understanding of the reaction mechanisms and competitiveness of char(N) migration, especially the influence of the alkali metal potassium (K) present in biomass, remains incomplete. Building on the Zigzag char(N) models, the present study delves into the migration reactions of char(N), assessing their competitive dynamics through the integration of density functional theory, electronic structure analysis, and conventional transition state theory . Furthermore, it examines the impact of K on char(N) conversion. The competitiveness of the heterogeneous reactions follows the sequence: heterogeneous reduction of NO to N 2 > heterogeneous oxidation of char(N) to NO > decomposition of char(N) to NO x precursors. Moreover, the formation of HCN is more favorable than NH₃ production. The successive conversion from char(N) to NO and then to N 2 is the predominant migration route for char(N), with NO generation as the pivotal step. The less preferred char(N) migration route involves decomposition to NH 3 /HCN, followed by oxidation to NO x within the main combustion zone , which cannot be mitigated by char. K can accelerate NO generation and sustain the primacy of the heterogeneous NO reduction, consequently enhancing the oxidation-reduction process of char(N). As a result, K plays a constructive role in managing NO x emissions during the thermal conversion of char.