Newly developed detailed urea decomposition mechanism by marine engine urea-SCR system crystallization test and DFT calculations

The problem of urea crystallization in selective catalytic reduction (SCR) systems has been a concern for a long time, especially in vehicle engine after-treatment. Compared with the vehicle engine SCR system, the layout space of the marine engine SCR system is more spacious, but the impingement of urea-water solution (UWS) on the wall as well as the formation of urea deposits still cannot be avoided in many cases. In this paper, the urea deposits in the SCR systems of 10 MW marine two-stroke and 140 kW marine four-stroke engines are taken as the research objects. The research results showed that the composition of urea deposits is affected by the composition of exhaust gas. Besides biuret, ammelide, cyanuric acid (cya) and its homologues, the urea deposits contain non-urea decomposition by-products such as particulate matter (PM) and sulfate. In addition, most of the existing urea decomposition mechanisms are obtained by inferring the intermediate reaction pathways through ther-mogravimetric analysis (TGA), which has the disadvantage of kinetic parameters being inaccurate calculation due to oversimplified reactions, and cannot explain the coexistence phenomenon of cya and its homologues in the urea deposits of marine SCR systems. Therefore, based on density functional theory (DFT), a new detailed urea decomposition mechanism is proposed in this paper, including 15 species and 23 reactions. This mechanism introduces homologues of urea, biuret, and triuret, complementing the reaction pathways for forming cya and its homologue, and is more reliable in theory. On this basis, the correctness of the proposed urea decomposition mechanism is validated by using the TGA experimental data of urea deposits and the urea decomposition kinetic model, and machine learning.