ReaxFF-based molecular dynamics simulation of the impact of potassium on the formation of NH3 during protein pyrolysis

The study used ReaxFF MD (Reactive Force Field Molecular Dynamics) to investigate the migration behavior and mechanisms of nitrogen in the pyrolysis process of biomass, specifically focusing on the effect of the alkali metal potassium on the main nitrogen-containing substance, proteins. This research primarily centers on the conversion mechanisms of nitrogen in the three-phase products (char, tar, and gas) during the pyrolysis of proteins, as well as the impact on NH3 formation. The findings reveal several key insights: First, the introduction of potassium significantly enhances the cleavage activity of protein macromolecules by promoting the breaking of carbon-carbon and carbon-nitrogen bonds, which accelerates the degradation process of protein molecules and subsequently increases the conversion rate of nitrogen from proteins to the nitrogen in char, tar, and gas. Second, the addition of potassium alters the sources of gaseous nitrogen (gas-N); initially derived mainly from the secondary cracking of tar nitrogen (tar-N), gas-N predominantly originates from the conversion of char nitrogen (char-N) under the influence of potassium. Lastly, the introduction of potassium affects NH3 production by significantly altering the trend of nitrogen-hydrogen bond variation during the pyrolysis of proteins. This is primarily attributed to potassium's facilitation of dehydration reactions, which disrupt nitrogen-hydrogen bonds during protein pyrolysis. Additionally, it allows NH3 produced in the later stages of pyrolysis to participate in the formation of other nitrogen-containing compounds, particularly HCN. Potassium also promotes the dehydrogenation of the large number of HCN molecules generated during protein pyrolysis, forming CN radicals that subsequently react with NH3 to produce new compounds. This is the main reason for the variation in NH3 yield.