Molecular Dynamics Simulation Of Biomass Tar Cracking Based On ReaxFF Force Field

Under the goal of"carbon peak"and"carbon neutrality",biomass energy is one of the most promising energy sources in China,because the net emission of CO₂in the process of utilization is zero.The tar generated in the process of biomass energy thermal conversion and utilization is limited to its extensive commercial application.It is of great significance to deeply understand the pyrolysis and conversion mechanism of biomass tar for effective tar removal.The process of tar cracking is accompanied by the generation and interaction of numerous unstable free radicals,as well as the conversion of a large number of aromatic isomers,making it difficult to conduct in-depth research on the occurrence and evolution of biomass tar.Therefore,based on Reax FF reaction force field,molecular dynamics simulation of biomass tar cracking process was carried out in this thesis,so as to reveal the pyrolysis mechanism of biomass tar from the molecular level and provide theoretical basis for effective removal of biomass tar.The specific work of this thesis is as follows:（1）The main components of complex biomass tar are toluene,phenol and naphthalene,and the molecular dynamics simulation of the pyrolysis process of three tar model compounds is carried out in combination with density functional theory（DFT）and Reax FF reaction force field.The results showed that the Mayer bond order of C-H bond was smaller than that of C-C bond,which was easy to break.The Mulliken net charge number of hydrogen in the side chain structure of aromatic ring was larger than that of hydrogen in benzene ring,which showed stronger activity.The thermal stability of the three tar model compounds was in descending order:naphthalene>phenol>toluene.Phenol containing highly active phenol hydroxyl group was easily decomposed into gas products,while naphthalene was easily formed into heavy tar with macromolecular through double and triple rearrangement reactions.The apparent activation energies of toluene,phenol and naphthalene were 314.03kJ/mol,362.39 kJ/mol and 396.74 kJ/mol,respectively.The initial decomposition path of toluene was that the C-H bond of benzene ring connected with methyl group broke to generate benzyl group,the initial decomposition path of phenol was that the O-H bond on phenol hydroxyl group broke to generate phenoxy group,and the initial decomposition path of naphthalene was that the benzene ring C-H bond broke to generate naphthol group.Active free radicals can accelerate the tar pyrolysis process.（2）The molecular dynamics of the steam reforming reactions of toluene,phenol and naphthalene were simulated under different water-carbon molar ratios（S/C=0～5）.The results showed that water molecules promoted the transformation of toluene,phenol and naphthalene molecules and the formation of intermediate products.The introduction of water molecules adjusted the composition distribution of the pyrolysis products of toluene,phenol and naphthalene.Under the reforming condition,the occurrence ratio of C element in the products of C_1-4 increased by 33.6%,27.5% and52.5%,respectively.OH radical attack was the main ring-opening reaction mechanism of aromatic rings under reforming conditions,OH radicals will preferentially attack o-carbon or p-carbon on toluene and phenol,and for naphthalene molecules,OH radicals will preferentially attack the C-atom at the second position.The water molecule provided the H source and promoted the formation of the aldehyde group of the CO precursor,increasing the yield of H₂ and CO in the gas product.（3）The pyrolysis,catalytic pyrolysis and catalytic reforming processes of biomass tar（toluene/phenol/naphthalene mixture system）were simulated.The simulation results of biomass tar pyrolysis show that high temperature can promote the secondary reaction of intermediate products in the tar system and inhibit the formation of C₁₂₊ compounds.The molecular orbital energy difference between phenol and naphthalene is small,and there is strong interaction,which easily leads to chemical bond rearrangement.The nucleophilic property of phenol is beneficial to destroy the stable conjugated structure of naphthalene molecules,reduce the energy barrier of naphthalene aromatic ring opening reaction,and inhibit the generation of polycyclic aromatic hydrocarbons in the mixed system.The simulation results of catalytic cracking of biomass tar show that the catalytic capacity of iron-based catalysts for tar decreases with the increase of iron valence state,and the catalytic cracking capacity of Fe（111）catalyst is strong because of the low coordination number of Fe atoms at the metal active site.The H₂ and CH₄ yield of Fe（111）catalyst is higher than that of Fe₂O₃（111）and Fe₃O₄（111）catalyst,and the lattice oxygen in Fe₂O₃（111）and Fe₃O₄（111）catalyst will release continuously into reactive oxygen species with the progress of reaction,which is more conducive to the generation of CO and CO₂.The simulation results of catalytic reforming of biomass tar show that Fe in Fe（111）catalyst can be oxidized to FeOx and oxygen atoms in water molecules through steam oxidation to some extent compensate for the consumption of lattice oxygen in Fe₂O₃（111）and Fe₃O₄（111）catalysts.Lattice oxygen oxidizes carbon deposited on adjacent Fe atoms into CO to maintain the reactivity of catalyst surface.With the addition of water molecules,the highest water molecule consumption rate of Fe（111）catalyst system is 71.4% under the condition of catalytic reforming,which not only reduces carbon accumulation,but also significantly increases the production of H₂ and CO.