A molecular investigation on the mechanism of co-pyrolysis of ammonia and biodiesel surrogates

Abstract

As renewable energy sources with great potential to reduce carbon footprints and pollutant emissions, ammonia and biodiesel both have garnered substantial research interest. This study aimed to investigate the detailed mechanism of co-pyrolysis of ammonia and biodiesel surrogates, including the pyrolysis of different biodiesel surrogates, the effect of biodiesel decomposition on ammonia reactions, nitric oxide (NO) generation during the ammonia-biodiesel reaction process, and the effect of ammonia on soot formation during biodiesel pyrolysis. Using ReaxFF-based molecular dynamics simulations, the results revealed that the presence of ester groups in biodiesel lowers the activation energy of the reaction compared to alkanes. Meanwhile, biodiesel structures with shorter chain lengths, isomerisation, and carbon-carbon double bond effectively lower the activation energy. Time evolutions of the main pyrolysis products of methyl butanoate (MB), ethyl propionate (EP), and methyl crotonate (MC), as well as their detailed decomposition pathways, were produced. The coexistence of biodiesel and ammonia can promote the decomposition of ammonia, with MC containing carbon-carbon double bonds providing the most abundant free radical environment for ammonia decomposition. High temperature promotes the occurrence of the reaction, with MC, MB, and EP producing NO in the order of MC > MB > EP. Ammonia addition reduces soot production in the pyrolysis of different biodiesel surrogates, with MC exhibiting the most significant effect. The roles of oxygen-containing and nitrogen-containing species in soot suppression and their synergistic impact were identified at the atomic-scale. The insights into the detailed reaction mechanism of the co-pyrolysis of ammonia and biodiesel obtained in this study can be used to guide the development of ammonia-biodiesel co-firing technology.