Investigation of N Migration during Municipal Sludge/Coal Co-Pyrolysis via ReaxFF Molecular Dynamics

In this work, the N migration behavior and mechanism during the municipal sludge/coal co-pyrolysis were studied employing the reactive force field molecular dynamics (ReaxFF MD) method. To verify the accuracy of this method implementation, the individual pyrolysis properties of sludge and coal obtained from simulations were compared to those obtained from isothermal thermogravimetric experiments. The co-pyrolysis ReaxFF MD simulation results highlighted a negative synergistic effect on the release of N during the initial stage of sludge/coal co-pyrolysis. In addition, the implementation of the atomic labeling method found that the release of sludge-N was promoted, while the release of coal-N was inhibited during the initial stage of sludge/coal co-pyrolysis. The acceleration of sludge-N release was attributed to the generation of OH radicals from coal, which attacked the weak bonds in the sludge. During the secondary reaction stage of co-pyrolysis, a positive synergy related to the condensation of N-containing tar was observed. This was a result of the increased reactivity of the sludge-N-containing tar and the reduction of H and OH radicals. This, in turn, inhibited the stabilization and decomposition reactions of the sludge-N-containing tar during co-pyrolysis. As a result, there was more condensation of sludge-N-containing tar into char. Furthermore, the emission of HCN was positively impacted by the synergistic effect during co-pyrolysis. In this regard, the amount of HCN generated from sludge-N increased, while that generated from coal-N remained constant. By investigating and categorizing the pathways of HCN generated from sludge-N, it was found that the addition of coal had a significant impact on the migration and transformation of sludge-N. On the one hand, the H radicals derived from coal promoted the cleavage of N-heterocycles from sludge. On the other hand, some NHi derived from sludge-N adsorbed on coal-char, altering the generation pathway of HCN from sludge-N.