Revealing the nitrogen migration mechanism during pyrolysis and steam gasification of biomass: A combined ReaxFF MD and DFT study

The biomass pyrolysis/gasification technology takes advantages of high efficiency and carbon neutrality, which is promising for high value-added products fabrication. However, biomass possesses relatively high amount of N especially for seaweeds and municipal sludge wastes, and the lack of N migration mechanism has resulted in emission of N -containing pollutants and limited its industrial application. In this work, a simulation investigation combining reactive force field molecular dynamic and density functional theory calculations was performed to study the conversion mechanisms of N -containing species in the pyrolysis/gasification of biomass processes. The glutamic acid and cellulose were selected as the model compounds of N -containing and typical compositions of biomass, respectively, and six models containing multiple mass ratios of glutamic acid to cellulose and steam to feedstock were constructed. The results show that the majority of nitrogen in the original model is converted towards NH 3 -N (62%) during glutamic acid pyrolysis by direct cleavage, hydrogen capture, deamination, and decarboxylation. The presence of cellulose effectively inhibits the decomposition of NH 3 at high temperature range, thereby increasing the yield of NH 3 -N (up to 70%) and restraining the conversion of nitrogen into mesomolecules. The introduction of H 2 O provides a substantial amount of center dot H radicals, which facilitates the conversion of nitrogen into NH 3 -N. During DFT calculations, eight reaction pathways for the formation of Ncontaining products were proposed on the basis of distribution of pyrolysis products. The glutamic acid cleavage reaction achieves the lowest energy barrier, and the deamination of glutamic acid towards a ternary cyclic ether is found to be the most feasible pathway for the NH 3 production. This work can provide theoretical support for nitrogen pollutants mitigation during pyrolysis and steam gasification of biomass processes.